Lecture 6. ENERGY OF LIFE. 2. ORGANICS
FROM CARBON DIOXIDE: PHOTOSYNTHESIS AND CHEMOSYNTHESIS

Photosynthesis

Using glucose as an example, we looked at how organic molecules in living organisms are broken down into carbon dioxide and water for energy. Now consider the reverse process - how these organic substances (the same glucose) are formed from carbon dioxide and water, i.e. photosynthesis. In fact, there are other, less common options for organic biosynthesis, which we will consider below. However, the main one is photosynthesis, as a result of which 150 billion tons of sugars are produced annually on Earth.

We have already recalled the total reaction of photosynthesis:

CO 2 + H 2 O \u003d (CH 2 O) + O 2.

When we breathe, we break down glucose to produce a certain amount (about 30 pieces) of ATP molecules. It is logical to assume that for the synthesis of glucose, you need to spend a number of ATP molecules. Moreover, if we take into account the far from one hundred percent efficiency of biochemical reactions, most likely for the synthesis of one glucose molecule, several more ATP molecules will have to be spent. It would be natural to assume that, like breathing, the process of organic synthesis will consist of two parts - the one that deals with ATP, and the one that deals with glucose, i.e. In other words, ATP synthesis as a universal energy resource must first occur somewhere, and only then - glucose synthesis due to the energy of this ATP. Both of these processes do take place.

Since we are talking about increasing, not decreasing, the amount of organic matter, we will take the energy for producing ATP not from the breakdown of organic matter, but from another source. In the most common case, the source will be sunlight.

Even at the beginning of photosynthesis research, it was shown that there is a group of reactions that depend on illumination and do not depend on temperature, but there is a group of reactions that, on the contrary, does not depend on illumination and depends on temperature. The first one was named light stage of photosynthesis, the second - dark stage of photosynthesis. This should not be understood in the sense that one goes by day and the other by night. Both sets of reactions take place simultaneously, it's just that one needs light and the other doesn't. It is quite natural for realizable tasks that the light phase of photosynthesis resembles oxidative phosphorylation, and the dark phase is a cycle somewhat similar to the Krebs cycle.

To get acquainted with the light phase of photosynthesis, we need to consider such a chemical phenomenon as pigments. What are pigments? These are colored substances. Why are some substances colored while most substances are colorless? What does our vision of a certain color mean? This means that light comes from matter, in which the ratio of photons with different wavelengths differs from daylight white light. As you know, white light is a mixture of photons, literally all the colors of the rainbow. The coloration of light refers to the predominance of certain wavelengths over others. We consider substances in daylight. Accordingly, if we see a substance colored, it means that it selectively absorbs photons with certain wavelengths. Having no rest mass, the absorbed photons cease to exist. Where does their energy go? It goes to excite the molecule, to transfer it to a new, more energetically saturated state.

To have the ability to absorb light and pass into an energetically saturated state, the molecule must be a system in which such a state is possible. Most organic pigments are substances with regular alternation of double and single bonds between carbons, i.e. with conjugated double bonds. These bonds form resonant systems in which the electrons involved in the formation of double bonds (formed by orbitals not involved in sp 2 -hybridization), can move throughout the system and be in several energy states. The number of such states and the energy required for the transition of an electron from one to another are strictly fixed for each molecule. This follows from quantum physics, the science most difficult to understand for an unprepared person, which we are. Therefore, let's take it on faith, trusting the critical properties of the scientific community, which at one time accepted quantum theory not without resistance, but its huge successes dispelled all doubts.

The energy that distinguishes the states of an electron in resonant systems is such that it closely corresponds to the energy of photons of one or another wavelength within the visible part of the spectrum. Therefore, resonant systems will absorb those photons whose energy is equal to or slightly greater than the transfer of their electrons to one of the more energetically saturated states. (Since the energy of a photon is extremely rarely exactly equal to the excitation energy of an electron, the remainder of the photon's energy, after the bulk of it has been given to the electron, is converted into heat.) That is why substances with resonant systems usually have a color, that is, they are pigments.

Let's look at the molecules of some important pigments for our case. Let’s start with the most important pigment, chlorophyll.

As in the case of heme, which is attached to hemoglobin and cytochrome molecules, we see an openwork and almost symmetrical organic structure that includes several double bonds - porphyrin ring. In its center there is also a metal atom, but not iron, as in the case of heme, but magnesium. It is bonded to four nitrogen atoms (magnesium and a porphyrin ring form complex). We may well expect such a molecule to be colored, and we will not be mistaken. This molecule absorbs photons in the violet and blue, and then in the red part of the spectrum, and does not interact with photons in the green and yellow part of the spectrum. Therefore, chlorophyll and plants look green - they simply cannot take advantage of the green rays in any way and leave them to walk around in the wide world (thus making it greener).

A long hydrocarbon tail is attached to the porphyrin ring in the chlorophyll molecule. On fig. 6.1 it looks a bit like an anchor chain. He is such. Having no electronegative atoms, this part of the molecule is non-polar and therefore hydrophobic. With the help of it, chlorophyll anchors in the hydrophobic middle part of the phospholipid membrane.

Chlorophyll in plants is represented by two forms -a And b. In green plants, about a quarter of chlorophyll is represented by the second form. b. It differs in that one methyl group along the edge of the porphyrin ring is CH 3 replaced by a group CH2OH . This is sufficient to shift the absorption spectrum of the molecule. These spectra are shown in the figure:

During the light phase of photosynthesis, the energy of absorbed photons sunlight is converted into an excited state of the electrons of the chlorophyll molecule and is subsequently used for the synthesis of ATP - we have already seen how living systems can tame excited electrons, handling them deftly and profitably. The figure above shows a graph of the efficiency of photosynthesis as a function of the wavelength of light that is exposed to the plant.

Carotenoids have a slightly different structure - red and yellow pigments. (It is carotenoids that color carrots and mountain ash, they are also vitamin A.) But they also have a system of conjugated double bonds, somewhat simpler.:

Carotenoids are also involved in photosynthesis, but as auxiliary molecules.

We again need to make a spatial disclaimer. Just as cellular respiration takes place in mitochondria, photosynthesis takes place in chloroplasts. Chloroplasts are organelles similar to mitochondria, but they are larger and have a more developed internal structure; filled with flat bubbles - thylakoids, which are collected in piles - grana.

Photosynthesis pigments are located on the inner side of the thylakoid membrane. They are organized in photosystems- whole antenna fields for capturing light - each system contains 250-400 molecules of different pigments. But among them, one molecule of chlorophyll is of fundamental importance. but- it is called reaction center photosystems. All other pigment molecules are called antenna molecules. All pigments in the photosystem are capable of transferring excited state energy to each other. The photon energy absorbed by one or another pigment molecule is transferred to the neighboring molecule until it reaches the reaction center. When the resonant system of the reaction center goes into an excited state, it transfers two excited electrons to the acceptor molecule and thereby oxidizes and acquires a positive charge.

Plants have two photosystems - 1 and 2. The molecules of their reaction centers are somewhat different - the first has a maximum absorption of light at a wavelength of 700 nm, the second - 680 nm (a reservation was made in order to clarify the images in the diagrams), they are designated P700 and P680 . (The differences in absorption optima are due to slight differences in pigment structure.) Typically, these two systems work in conjunction, like a two-piece conveyor called non-cyclic photophosphorylation.

Another diagram:

The production cycle starts with photosystem 2. The following happens to it:

1) antenna molecules trap a photon and transfer excitation to the P680 active center molecule;

2) the excited P680 molecule donates two electrons to the cofactor Q (very similar to the one that participates in the electron transport chain in mitochondria), while it is oxidized and acquires a positive charge;

3) under the action of certain enzymes containing manganese, the oxidized P680 molecule is reduced, taking away two electrons from the water molecule. In this case, water dissociates into protons and molecular oxygen. To create one oxygen molecule, two P680 molecules must be restored, having lost a total of four electrons, and four protons are formed.

Note that this is where oxygen is produced during photosynthesis. Since it is formed by the splitting of water molecules by the action of light, this process is called photolysis of water;

4) these protons are formed in the inner space of the thylakoid, where an excess concentration of protons is created in comparison with the surrounding space (i.e., a more acidic environment). Thus, our old acquaintances are formed - the proton gradient and the membrane potential. We already know how all this will be used: ATP synthetase will release protons in pairs and synthesize ATP from ADP. Let us pay attention to one apparent difference from mitochondria - during oxidative phosphorylation in mitochondria, protons are pumped out of the space limited by the inner mitochondrial membrane and enter back through ATP synthetase. In our case, protons are pumped into the inner space of the thylakoid and exit from there through ATP synthetase. However, the internal space of the thylakoid corresponds to the space between the two membranes of the chloroplast - these are, as it were, folded folds (similar to mitochondrial cristae) of the inner membrane;

5) meanwhile, two electrons that arrived at the cofactor Q , are passed down the protein chain, which is very similar to the electron transport chain. It also involves quinones, cytochromes - proteins containing heme in a complex with an iron atom, proteins containing iron and sulfur, chlorophyll and plastocyanin - an enzyme containing copper. And the passage of electrons along it is also accompanied by the transport of protons against the concentration gradient through the thylakoid membrane, which again pours water on the mill of ATP synthetase;

6) in the end, electrons come from plastocyanin to the reaction center of photosystem 1 - the P700 molecule.

In photosystem 1, the following happens:

1) antenna molecules catch a photon and transfer energy to the resonant system of the P700 reaction center, which is excited and donates two electrons to the acceptor iron-containing protein (P430). As in the case photosystem 2, P700 is thereby oxidized and acquires a positive charge;

2) this molecule is restored and loses its charge, having received two “calmed down” (but not to the initial state - their energy has not yet been fully used up!) Electrons that initially came from photosystem 2. In this case, there is no need for photolysis and it does not occur;

3) P430 donates electrons to another iron-containing protein called ferrodoxin;

4) having received electrons, this protein restores the NADP + coenzyme to NADP-H. This coenzyme is a phosphorylated NAD. The process occurs on the outer membrane of the thylakoid. It requires a proton, which is taken from the interior of the chloroplast, external to the thylakoid. Thus, the proton gradient only increases.

Does the last step remind you of anything? Yes, it is reminiscent of how NAD-H oxidized to NAD+ and donated electrons through the electron transport chain. Only here everything happens in reverse order. There, NAD-H transferred energy to the electron, which lost it, passing through the electron transport chain. And here, on the contrary, an electron, excited by the energy of sunlight accumulated by two successively coupled photosystems, transfers it to NADP +, restoring it to NADP-H.

Indeed, the entire light phase of photosynthesis is similar to oxidative phosphorylation in mitochondria in that during it, electrons are transferred along a similar chain of proteins, as a result of which an excess concentration of protons is created in some space limited by the membrane - in this case, the internal space of the thylakoid - potential difference. The emerging potential energy of electrostatic forces is used to synthesize ATP due to the movement of protons along the gradient, carried out by ATP synthetase. The difference from oxidative phosphorylation is that if a reduced NAD-H molecule was used to excite electrons, then light is used for this, and NADP +, on the contrary, is reduced and used in the dark stage of photosynthesis (and can be further used in those or mitochondria). In general, it turns out that protons are formed in the inner space of the thylakoid during the photolysis of water, pumped there during the operation of photosystem 2, and scooped from the outer space of the thylakoid to reduce NADP+ to NADP-H, through which hydrogen enters the carbohydrates synthesized during photosynthesis.

Here, the diagram more or less shows all the main processes of the light stage of photosynthesis:

However, photosystem 1 can also work autonomously. In this case, a bypass path for electron transfer from an excited reaction center is used - namely, the same electron transfer chain that leads from photosystem 2. Electrons pass through it, causing coupled transport of protons from the external environment of the thylakoid to the internal, which increases the proton gradient, and return back to the reaction center of photosystem 1 - Р700. Thus, here the light seems to turn the wheel of the proton pump, without oxidizing water and without restoring NADP. It is called cyclic photophosphorylation. It can go in parallel with non-cyclic. In addition, it is used by some photosynthetic bacteria that do not release oxygen during photosynthesis.

The result of the light phase of photosynthesis during non-cyclic photophosphorylation (and this is the main option) can be written as the following reaction:

2NADP + 2ADP + 2F- + 2 H 2 O + 4 hv \u003d 2NADP-H + 2ATP + O 2.

Here hv - symbol the energy of one photon, F is the symbol for the remainder of phosphoric acid from the solution. It is approximate because, as in oxidative phosphorylation, the amount of ATP synthesized by ATP synthetase is not strictly dependent on the number of electrons passed through the protein chain in photosystem II.

Our approximate gesheft as a result of the light phase of photosynthesis, the full scheme of which is shown in Fig. 6.6, - one ATP and one reduced coenzyme (which, as we remember, when breathing "costs" 2.5 ATP) per two photons, that is, almost two ATP per quantum of energy borrowed from one absorbed photon. Not bad!

So, we have considered where energy (i.e. ATP) is taken from during photosynthesis. It remains to consider how organic matter is made using this energy.

Plants use three variants of such production. Consider the most common of them, which is also used by blue-green algae and photosynthetic and even chemosynthetic bacteria - the Calvin cycle. This is another closed cycle of interconversion of organic substances one into another under the action of special enzymes, similar to the Krebs cycle. And by the way, one more Nobel Prize, 1961 - Melvin Calvin, who discovered it.

The cycle begins with a sugar that has a chain of five carbon atoms and carries two phosphate groups - ribulose-1,5-bisphosphate (and ends with it). The process begins when a special enzyme - ribulose bisphosphate carboxylase - attaches a CO 2 molecule to it. The six-carbon molecule formed for a short time immediately decomposes into two molecules of glycerate-3-phosphate (aka 3-phosphoglycerate, we have already met with this substance in glycolysis). Each of them contains three carbon atoms (which is why the Calvin cycle is also called C 3-way carbon dioxide fixation).

In fact, the fixation of carbon dioxide in organic matter is carried out by this enzyme - ribulose biphosphate carboxylase. This is a surprisingly slow enzyme - it carboxylates only three molecules of ribulose-1,5-bisphosphate per second. For an enzyme, this is very little! Therefore, this enzyme itself requires a lot. It is fixed on the surface of thylakoid membranes and makes up about 50% of all chloroplast proteins. It is known to be the most abundant protein in the world (think why).

Glycerate-3-phosphate with the expenditure of one ATP molecule is phosphorylated to diphosphoglycerate. He, in turn, dephosphorylated to glyceraldehyde-3-phosphate, and during this reaction one molecule of reduced NADP-H is oxidized to NADP+. Another waste of energy!

The resulting compound, glyceraldehyde-3-phosphate, is our old friend. It is formed during the breakdown of glucose during glycolysis, namely, the breakdown of fructose-1,6-bisphosphate. From it, in the course of enzymatic reactions that take place without energy expenditure, glucose can be obtained. Some of the reactions of glycolysis are irreversible (namely, those that dephosphorylate ATP), so other reactions and other mediators are involved.

It would seem that this is the whole photosynthesis. But in order for it to continue, we need to somehow regenerate ribulose-1,5-bisphosphate - the main substrate of the fixative carbon dioxide enzyme. Therefore, for every 12 molecules of glyceraldehyde-3-phosphate formed, only two go to the synthesis of glucose, and 10 go to the reduction of six molecules of ribulose-1,5-bisphosphate. This process involves 12 x 3 \u003d 6 x 5 \u003d 30 carbon atoms, which rearrange from 10 three-carbon molecules to 6 five-carbon ones. At the same time, at the input we have 10 phosphate groups (one for each molecule of glyceraldehyde-3-phosphate), and at the output we should have 12 of them. However, six ATP molecules are additionally spent for this entire part of the process.

If we subtract the substances regenerating during the cycle (which are not additionally synthesized and not spent), then the total equation for fixing carbon dioxide is obtained as follows:

6CO2 + 12NADP-N +18 ATP = 1 glucose + 12NADP+ + 18ADP + 18P-+ 6 H2O

(here F is a free phosphorus group).

We get the cost of 12 reduced coenzymes and 18 ATP per molecule of glucose. If we remember the “price” of the reduced coenzyme in the Electron Transport Chain Company at 2.5 ATP molecules, then obtaining one glucose molecule - a single intercellular currency - costs us, in a single cellular currency, 48 ATP. When it was split, we received only about 30 ATP. It seems that the difference in the rate of buying and selling is called "margin". In this case, it is rather big! About 1/3 of the energy is lost due to the efficiency of biochemical processes. (In engineering, this would be downright huge efficiency.)

As we can see, photosynthesis in general is a bit like cellular respiration turned inside out. There, in the course of a closed-loop interconversion of small organic substances, some of them were consumed with the release of carbon dioxide and coenzymes were reduced, which were then oxidized, donating electrons to the electron transport chain, from where they ultimately went to molecular oxygen to form water. Here, the process begins with the removal of electrons from water with the formation of molecular oxygen, from there they (having received energy from light) enter the electron transport chain and ultimately go to the reduction of coenzymes. The reduced coenzymes and carbon dioxide, on the other hand, enter into a cyclic interconversion of organic substances, in which they are synthesized with the consumption of ATP. Even parts of the outer space in relation to the organelle turned out to be inside out and became the inner space of the thylakoid.

However, this most popular version of photosynthesis has a pitfall. Ribulose bisphosphate carboxylase is designed in such a way that it is able to convert ribulose-1,5-bisphosphate not only into two molecules of glycerate-3-phosphate that we (i.e. plants) want, but also to do the exact opposite thing - simply oxidize it with oxygen to one molecule glycerate-3-phosphate with the elimination of a carbon dioxide molecule.

Phosphoglycolic acid is then converted to glycolic acid and oxidized with oxygen to two more molecules of carbon dioxide (this occurs in special cell organelles - pyroxisomes, which for this purpose are closely adjacent to plastids). Instead of fixing carbon dioxide in an organic molecule, we, on the contrary, produce it from organic molecule. This process, since it consists in the consumption of oxygen with the release of carbon dioxide, is called photorespiration, but unlike real breathing, it does not store any useful energy. The desirable process (fixation of carbon dioxide) is catalyzed by ribulose bisphosphate carboxylase at high concentrations of carbon dioxide and low concentrations of oxygen, while the undesirable process (removal of carbon dioxide) is catalyzed by ribulose biphosphate carboxylase at low concentrations of carbon dioxide and high concentrations of oxygen, but these conditions prevail in the atmosphere and mesophyll cells. plant tissue in which photosynthesis takes place.

As a result, up to half of the newly fixed carbon is lost due to photorespiration. To get around this hurdle, a workaround for CO 2 fixation has been developed by many unrelated plants. It's called the C4 path. With it, carbon dioxide is fixed twice - first on the phosphoenolpyruvate molecule with the formation of malic acid, or malate (in other plants - aspartic acid), which has 4 carbon atoms.

This process is catalyzed by the enzyme phosphoenolpyruvate carboxylase, which is not fixed on the membrane, but is dissolved in the cytoplasm of mesophilic cells. In addition, it does not use the CO 2 molecule as such, but its hydrated form - the ion carbonic acid CO 3 -, which is in equilibrium with CO 2 when it is dissolved in water. Then malic acid migrates to other cells (the lining of the vascular bundles), where a carbon dioxide molecule is again split off from it, and immediately, as if nothing had happened, enters the Calvin cycle. The resulting pyruvate returns to the mesophilic cells, where it is phosphorylated with the consumption of ATP and converted to phosphoenolpyruvate, which thereby regenerates - and everything repeats in a cycle. The whole trick is that in the lining cells, where so much oxygen does not penetrate, an increased concentration of carbon dioxide will be created so that ribulose biphosphate carboxylase will catalyze the desired reaction. Note that, by engaging the C4 pathway, we are forced to spend an additional ATP molecule in order to phosphorylate pyruvate.

Let us only draw your attention to the fact that pyruvate and malic acid have already met us in the Krebs cycle, i.e., some part of this good old cycle was involved to “save” the dark stage of photosynthesis from photorespiration. A typical example of how things are in biochemistry.

The C 4 bypass route is effective at high temperatures but ineffective at low temperatures. Therefore, the proportion of plants that use it increases towards the south.

There is also the so-called "path of Crassula" - it is implemented in the family of Crassula and cactus. These are really very thick plants that grow where it is hot and there is little water. Saving water, during a hot day they close their stomata (these are the holes through which gases enter the leaves) and therefore cannot absorb CO 2. They fix CO 2 only at night, during which malic acid is stored in large quantities. During the day, with closed stomata, it decarboxylates, and the regenerated CO 2 enters the Calvin cycle (although it belongs to the dark phase of photosynthesis). So these plants also use the bypass C4 pathway, fixing carbon dioxide twice, but their primary fixation is separated from the Calvin cycle not in space (in different cells), as in the previous version, but in time.

We deliberately consider these subtleties, which you may not really need, in order to demonstrate the relationship of biochemistry with ecology - the science of the interactions of organisms with the external environment and with each other.

Thus, the dark stage of photosynthesis, i.e., the synthesis of organic matter, exists in several variants. The light phase is organized in the same way in all green plants and in cyanobacteria (blue-green algae). However, in another type of photosynthetic bacteria, or phototrophic bacteria, which are not cyanobacteria, - purple And green bacteria, other types of the light stage of photosynthesis are also realized. These two types of phototrophic bacteria differ in the structure of their chlorophylls and their set. Moreover, the purple (or brown, yellow) color of purple bacteria is due, as in higher plants, to carotenoids. The most interesting thing is that the chlorophyll of purple bacteria is able to absorb photons and carry out photosynthesis in the invisible infrared part of the spectrum. This is very important at depths where visible light does not penetrate. The inner space of cells of phototrophic bacteria is filled with photosynthetic membrane structures, in some cases resembling thylakoids.

General Equation photosynthesis in phototrophic bacteria remains almost the same as in green plants:

CO 2 + H 2 X \u003d (CH 2 O) + 2X.

Only oxygen is replaced by X, in this case H2X - this is not water, but any substance that can be oxidized with the transfer of an electron to the photosystem and at the same time donate a proton. Such a substance can be hydrogen sulfide, thiosulfate, molecular hydrogen (in this case X = 0) and organic compounds.

Green and purple bacteria have only one type of photosystem. They can carry out both cyclic photophosphorylation, which does not require an exogenous electron and hydrogen donor, or non-cyclic, which requires such a donor. Why did plants and cyanobacteria need the combined work of two photosystems? The fact is that for the synthesis of organics in the Calvin cycle, not only energy is needed, which can come in the form of ATP, but also reduced NADP coenzymes as a donor of not only energy, but also hydrogen. In order to transfer an electron to a state with such a high energy, which will be enough to reduce the NADP + molecule to NADP-H, the sequential use of two photosystems is necessary. The energy of two photons also turned out to be enough to take away electrons from the oxygen atom in the composition of water.

It is noteworthy that in the coupled pair of two photosystems, which was first invented by cyanobacteria (blue-green algae), photosystem 1 comes from the photosystem of green bacteria, and photosystem 2 from the photosystem of purple bacteria. By combining these two ready-made mechanisms, cyanobacteria were able to oxidatively photolyze water and reduce NADP+. Bacteria easily exchange genetic material, and such a combination of two unrelated evolutionary lines is not something exceptional for them. Plants have inherited a paired photosystem from blue-green algae. How - we will see this in Lecture 8.

The most common variant of photosynthesis in phototrophic bacteria is when a combination of hydrogen with an element from the same oxygen group, sulfur, is used instead of water. Phototrophic sulfur bacteria, which implemented this option, absorb hydrogen sulfide, and emit sulfur.

Sulfur bacteria is part of the purple and almost all green bacteria. Where should such bacteria live? Apparently, in areas of active volcanism. Volcanoes emit a lot of sulfur, mainly in its combination with oxygen (sulfur dioxide SO 3 ) and hydrogen (hydrogen sulfide H 2 S ). Yes, you can’t really live in the crater of an active volcano. However, near it, as well as at the foot of extinct volcanoes, there are always places for the outflow of volcanic gases - fumaroles. Usually they are located in fissures of igneous rocks, which correspond to depressions in the surface, where water accumulates accordingly. This water is saturated with hydrogen sulfide, which is a favorable environment for photosynthetic sulfur bacteria.

In what form is sulfur released? All sulfur phototrophic bacteria oxidize reduced sulfur compounds to mineral sulfur - solid. In some bacteria, sulfur accumulates inside the cells in the form of solid particles. As bacteria die, they are released into environment. Others are able to release sulfur immediately into the environment. Many green and purple sulfur bacteria are capable of oxidizing sulfur further, up to sulfates, but it is hydrogen sulfide and some other compounds of reduced sulfur with hydrogen that are used as a substrate for the light stage of photosynthesis.

However, phototrophic sulfur bacteria are found not only in fumaroles - they can appear wherever hydrogen sulfide is found. And it is often formed during the anaerobic decomposition of organic matter by other bacteria. In particular, they develop, sometimes in large numbers, in the bottom layer of ponds, lakes and seas. Most phototrophic bacteria are strict (obligate) anaerobes. However, among them there are also facultative aerobes that can exist and grow in the presence of oxygen.

In the above equation, X can be equal to zero. Such photosynthetic bacteria consume pure molecular hydrogen. The reaction center of the photosystem takes two electrons from a hydrogen atom and turns it into two protons. Bacteria that use hydrogen as a reducing agent are less common than sulfur bacteria.

Most phototrophic bacteria are capable of photooxidizing organic substances (here X is an organic radical), but this can hardly be called photosynthesis, since organic substances are spent here more than they are formed.

We should not forget about the existence of cyclic photophosphorylation - a process that does not require donor molecules of either protons or electrons. It can be assumed that this was the historically first operating scheme of the light stage of photosynthesis, since it is the simplest, including only one photosystem and does not require additional reducing agents. In the course of cyclic photophosphorylation, not very much ATP is formed, and in its classic case, NADP + is not restored at all (but in some phototrophic bacteria it can be restored). Surely, being "invented", cyclic phosphorylation served only as some energy help to its carriers. But since the whole mechanism works on creating a difference in the concentration of protons inside and outside a certain membrane space, it turned out to be convenient to enhance this gradient by oxidizing a certain hydrogen-containing substance - molecular hydrogen, water or hydrogen sulfide.

Finally, quite recently a completely different system of photosynthesis was discovered in halobacteria- microorganisms that develop in concentrated salt solutions and stain them red. In fact, they belong to archeobacteria - special microorganisms that, in many ways, are as distant from bacteria as they are from eukaryotes. The color is due to the pigment retinaldehyde, which belongs to the class of carotenoids. This pigment is related to the light-sensitive pigment responsible for our vision. It is attached to the protein bacteriopsin as a coenzyme. This protein spans the cell membrane with seven alpha helices. The green photon energy detaches retinaldehyde from bacteriopsin. In this case, bacteriopsin works as a proton pump and pushes a proton through the membrane. After that, retinaldehyde can reassociate with bacteriopsin. We see the same principle again - creating a proton gradient and a membrane potential for ATP synthesis. Moreover, the proton gradient is created by the photosynthetic protein itself. In this case, as in cyclic phosphorylation, no additional substance is restored. This seems to be the simplest of the photosynthesis pathways currently in existence.

What can we conclude? Different photosynthetic systems may have been invented repeatedly and based on different key pigments. The tandem of two photosystems based on chlorophyll that we have considered is one of many options and, apparently, the most effective. Both photosystems were invented by phototrophic bacteria, united by cyanobacteria (blue-green algae) and inherited by plants (we will see how exactly).

It should be noted that not all phototrophic bacteria are autotrophs in full sense of this word, that is, they are capable of developing on purely mineral media. Most of them still need some ready-made organic substances, so carbon dioxide photofixation is just an additional source of carbon for them.

This is exactly the case with halobacteria. Moreover, they have another striking feature - they are not able to absorb sugars and actually "eat" only amino acids from exogenous organic matter. Perhaps this is one of the aspects of the archaism of these amazing microorganisms.

Chemosynthesis

The synthesis of organics can occur not only due to sunlight, but also due to a resource, the development of which does not require such advanced antenna technology as photosystems based on pigments of a complex structure, due to the energy stored in the chemical bonds of inorganic substances. This so-called chemosynthesis.

Organisms that are capable of chemosynthesis and do not require an external source of organic matter are called chemoautotrophs. Chemoautotrophs are found only among bacteria, and in modern world the diversity of chemosynthetic bacteria is small. They were opened at the end XIX in. domestic microbiologist S. N. Vinogradsky. However, as in the case of green and purple bacteria, many bacteria capable of chemosynthesis still need certain organic substances and cannot be formally classified as autotrophs. At the same time, it is clear that the very ability to chemosynthesis is fundamental, which can serve as the basis for the formation of chemoautotrophy.

Considering the options for bacterial photosynthesis, we touched on volcanism, which is directly related to this topic. Indeed, the same substances that phototrophic bacteria used as electron donors for photosynthesis can be used by chemoautotrophs to obtain energy by oxidizing them without using light energy. Chemoautotrophic bacteria can use as a source of energy, i.e. as reducing agents:

1) sulfur compounds;

2) hydrogen;

3) nitrogen compounds;

4) iron compounds;

and presumably:

5) manganese carbonate MnCO3 to manganese oxide Mn 2 O 3 ;

6) trivalent antimony oxide Sb2O3 , oxidizing it to pentavalent Sb2O5.

The so-called colorless sulfur bacteria develop in hydrogen sulfide sources, including hot ones (some have a temperature optimum of about 50 ° C), and even in sources that are weak (up to one-normal, pH = 0) sulfuric acid or saturated saline solution. Some of these bacteria are found in soil, in sulfur deposits, and in some decaying rocks (contributing to their so-called sulfuric weathering). Naturally, different types of these bacteria are adapted to different conditions. Many of them are not only capable of oxidizing one particular sulfur compound, but successively increasing its degree of oxidation, i.e., oxidizing hydrogen sulfide (H 2 S ) to molecular sulfur ( S ), and molecular sulfur to thiosulfate ( S 2 O 3 - ), thiosulfate to sulfite ( SO 3 - ), sulfite - to sulfate, i.e. sulfuric acid ( SO 4 - ). In this case, the oxidation state of sulfur increases from –2 to +6. It is not surprising that such an element as sulfur was chosen for chemosynthesis, the degree of oxidation of which can vary over such a wide range.

Some are able to oxidize sulfur even from insoluble heavy metal sulfides. Such bacteria are used to develop depleted deposits of these metals. Water with bacteria is passed through crushed ore, represented by sulfides, and it is collected, enriched with sulfates of the corresponding metals.

As we know, all we need from any source of energy is ATP. Obtaining ATP based on the reduction of sulfur can go in two ways.

The most striking way is almost straight. It is realized at least during the oxidation of sulfite. Sulfite reacts with AMP to form adenosine phosphosulfate (APS). It is in this reaction that the oxidation state of sulfur changes from +4 to +6, and the released electrons are transferred to the electron transport chain for oxidative phosphorylation. The API molecule, in turn, replaces the sulfate group with a free phosphoric acid residue from the solution to form ADP, while the sulfate is released into solution. (Just in case, we recall that each such reaction is catalyzed by a special enzyme.) ADP already contains one macroergic bond. The enzyme adenylate kinase converts two ADP molecules into one ATP molecule and one AMP molecule. We see here the simplest of all the ways we have considered for the synthesis of ATP - in just three stages. The enzyme catalyzes the connection of a direct source of energy - sulfur compounds - with AMP, and the next enzyme - the replacement of residues of one acid with another to form ATP. The electrons taken away from sulfur can be sent to the transfer chain without AMP phosphorylation - in this case, the oxidation of sulfur is carried out directly by one of the cytochromes.

As you can see, both processes involve oxidative phosphorylation, which requires free oxygen. Therefore, chemosynthetic bacteria are, as a rule, obligate aerobes.

This example shows us that: 1) the ways of obtaining ATP during chemosynthesis are diverse and 2) some of them are very simple; they may have evolved first.

By the way, the efficiency of sulfur-based chemosynthesis is low - it uses from 3 to 30% of the energy contained in the reduced forms of sulfur.

In order to oxidize sulfur and extract energy from this alone without resorting to additional sources of it, modern chemosynthetic bacteria need a strong oxidizing agent, and this is oxygen. This is either the molecular oxygen of the air, or the oxygen of nitrates ( NO 3 - ). As you know, nitrates, i.e. saltpeter, are a very good oxidizing agent and are used in the manufacture of gunpowder.

Bacteria that use hydrogen oxidation as their sole source of energy, hydrogen bacteria, live in soil and water bodies. Hydrogen oxidation occurs through cytochromes using the electron transport chain, i.e., using molecular oxygen as an electron acceptor. Thus, for the life of these bacteria, the presence of not only hydrogen, but also oxygen in the environment is necessary - in fact, they live on an explosive mixture and use the energy that could be released as a result of the combustion of hydrogen. This is quite a lot of energy, and they use it quite efficiently - up to 30%. The general equation of hydrogen chemosynthesis is such that for six molecules of oxidized hydrogen there is one CO 2 molecule fixed in synthesized organic compounds.

It is curious that the hydrogen used by hydrogen bacteria is released as a by-product of vital activity by other bacteria - ordinary heterotrophic ones, which use ready-made organic matter as an energy source. The simultaneous presence of hydrogen and oxygen is again a very rare ecological situation. Perhaps that is why all hydrogen bacteria can assimilate ready-made biological organic substances.

Nitrogen-based chemosynthesis is carried out nitrifying bacteria. As you know, nitrogen, like sulfur, is an element that easily changes its oxidation state. There are two groups of nitrifying bacteria. One restores ammonium ( NH 4+ ) to nitrites (NO 2- ), while the oxidation state of nitrogen changes from –3 to +3. The second group oxidizes nitrites to nitrates ( NO 3 - ), increasing the oxidation state of nitrogen to +5. All nitrifying bacteria are obligate aerobes. The electrons from the nitrogen are transferred to the electron transport chain via a flavoprotein (containing flavin) or via cytochromes.

There are also bacteria capable of oxidizing ferrous iron to ferric iron. Of these, the ability for autotrophic existence has been proven only for a few species that are simultaneously sulfur bacteria and are capable of oxidizing molecular sulfur and its various compounds with oxygen and heavy metals. The general equation of chemosynthesis in this case looks like this:

4Fe 2+ SO 4 + H 2 SO 4 + O 2 = 2Fe 3+ 2 (SO 4) 3 + 2H 2 O.

Such bacteria living in swamps form swamp iron deposits.

All considered chemoautotrophs obtain energy by oxidizing inorganic substances and store it in the form of ATP molecules. The energy stored in ATP is used by them to fix carbon dioxide and build biological organic molecules. To do this, they all use the Calvin cycle we have already considered. Recall, however, that in this cycle, in addition to ATP, NADP-H is also needed. At the same time, the energy gain from the oxidation of all substances used for chemosynthesis is not enough to restore NADP-H from NADP+. Therefore, its recovery takes place as a separate process with the expenditure of part of the ATP obtained during chemosynthesis.

So, chemosynthesis is a tempting opportunity to use the energy of inorganic compounds of elements that easily change the degree of their oxidation, to obtain ATP and the synthesis of organic substances by fixing carbon dioxide. Let us note, however, four circumstances.

1. Most of the known cases of chemoautotrophy require free oxygen as an oxidizing agent, in rare cases it is replaced by nitrate oxygen. And as you remember, oxygen in the atmosphere is a product of photosynthesis. All this means that from the point of view of the geochemical cycle of substances, chemosynthesis on Earth is now secondary to photosynthesis.

2. Substances such as ammonia, hydrogen sulfide and hydrogen are often themselves formed as a result of the vital activity of bacteria, although they are completely different, which use such an effective resource as ready-made organic matter to obtain energy and build the substance of their body. Thus, in many cases, due to chemoautotrophs, the total amount of organic matter does not increase. They are simply elements of the general chain of its splitting, which includes the set microorganisms - just at this stage, local resynthesis of organic matter from CO 2 is added due to the energy of some intermediate compounds formed in the process of its global decomposition.

3. The currently dominant type of chemosynthesis on the planet is the oxidation of hydrogen sulfide of volcanic origin.

4. Air oxygen easily oxidizes hydrogen sulfide "on its own", without the help of microorganisms. Therefore, these two gases almost never occur together. For example, the deep layers of the soil are characterized by a reducing environment, there is methane and hydrogen sulfide, but there is no oxygen. The reducing environment is replaced by an oxidizing one, where oxygen is present, but there is no hydrogen sulfide - in a very narrow layer, both gases are present here - literally a few millimeters. It is and only there that soil chemosynthetic sulfur bacteria can develop. (Even more exotic is the simultaneous presence of oxygen and hydrogen.)

However, there are places on the planet where both gases - hydrogen sulfide and oxygen - are present in sufficient concentrations at the same time. And even at the moment, a large amount of organic matter is formed there as a result of chemosynthesis based on sulfur compounds of course of volcanic origin. Let's find out where volcanism comes from. Have you heard of continental drift? Who has not heard, remember the world map and pay attention to the fact that if Africa is moved to the west, its outlines will fit very well into the shores of both Americas. Yes, the continents are moving slowly! Africa and the Americas have split and are drifting apart. Asia and North America are sailing towards each other. India relatively recently broke away from Africa, rushed to the northeast and crashed into Asia. As a result, the Himalayas and Tibet grew at the site of the collision, and the recent earthquake in Altai was due to the fact that it still cannot stop. The crust under the oceans is much thinner than under the continents. Continents float on it like ice floes. When continents advance on the ocean, as, for example, Eurasia and America on the Pacific, subduction occurs - the continents crush under themselves the earth's crust, it plunges into the mantle and melts. It is in subduction zones - for example, throughout the periphery of the Pacific Ocean - that volcanism takes place, which is quite easy to observe in the form of volcanoes and hot springs rich in sulfur, in which we find chemosynthetic bacteria. Where the continents separate and the ocean opens up, like the Atlantic, for example, the continents pull the oceanic crust behind them. As a result, there is a crack in the middle of the ocean - rift zone, along which molten magma rises from the mantle, solidifies and forms a new oceanic crust. This is an area hidden from our eyes, but much more powerful volcanism. On the sides of the crack, underwater volcanic mountains grow, and the crack itself still looks like a depression between two mountain ranges. This is called the mid-ocean ridge. There are many outflows of volcanic gases rich in sulfur compounds and carbon dioxide. They got the name black smokers. Why smokers and why blacks? Sulfur compounds with metals - sulfides - are usually painted black. (By the way, who knows why the Sea is Black? Because at a certain depth its water is saturated with hydrogen sulfide and all metal objects turn black there.) The sources of the rift zone throw out a lot of sulfides, dissolved and suspended in hot water - such jets remotely resemble puffs of black smoke, and the precipitated sulfides form bizarre buildings several tens of meters high around the springs.

There is no active chemosynthesis in the Black Sea, since there is practically no oxygen at that depth - all this is because its configuration contributes to stagnant water. And in the rift zones of the oceans, water is mobile and there is oxygen. It is also important that the black smoker heats the water and thereby sets it in motion, which promotes gas exchange. Therefore, there is an intense chemosynthesis around black smokers, during which large amounts of carbon dioxide are fixed and converted into biological organic molecules.

This resource does not go unnoticed by marine life, so thriving communities of marine organisms form around black smokers. They are based on chemosynthetic bacteria that cover these same sulfide buildings of black smokers with an even layer.

In the rift zone of the Pacific Ocean, on the periphery of black smokers, there are colonies of absolutely amazing animals - vestimentifer. They were discovered only about 20 years ago, now a dozen or two species are known. They are something like worms from 15-30 cm to 2.5 m long, living in tubes, through the open end of which a crown of scarlet tentacles protrudes. They belong to a special family of polychaete annelids - sibaglids, although they differ greatly from other annelids in body structure; this family was previously even considered a separate type - pogonophores.

They have a well-developed circulatory system, but no mouth or intestines. Along the body they have the so-called trophosome (in Greek trophos - food, soma - body) - a strand consisting of special cells and blood vessels. Inside the cells are chemosynthetic sulfur bacteria - only one species (out of about two hundred in the external environment of smokers). They oxidize hydrogen sulfide to sulfuric acid (which is neutralized by carbonates). Vestimentifera self-digest some of their cells and thus feed.

Here is the external and internal structure vestimentifers:

(and even development cycle)

The question is, how does hydrogen sulfide get into the trophosome? It is transported there by blood hemoglobin along with oxygen. Oxygen binds to heme, hydrogen sulfide to the protein part of hemoglobin. Red (from hemoglobin) tentacles serve as gills - they absorb oxygen and hydrogen sulfide. Thus, vestimentifera exist due to symbiosis - mutually beneficial cohabitation with organisms of a different type. And they build their body from organics obtained as a result of chemosynthesis (but using chemosynthetic oxygen). Crabs, shrimps, barnacles, bivalves, octopuses, fish, etc. live in vestimentifer colonies due to chemosynthetic organics (mostly simply feeding on vestimentifers).

And mind you, no plants! Only bacteria and animals. Recall that at these depths, sunlight is completely absent.

All this is adjacent to the practically lifeless ocean depths, where photosynthetic organic matter coming from the ocean surface almost does not reach, since almost all of it is utilized by microorganisms along the way. There, the bottom biomass is only 0.1–0.2 g/m2 (I have not come across an estimate of the biomass density near smokers, but it is several orders of magnitude higher).

Such a riot of life is possible because due to convection mixing in black smokers there is a fairly wide zone of waters in which both hydrogen sulfide and oxygen are present at the same time. (While the zone of simultaneous presence of these gases in the soil is only a few millimeters.) By the way, with a lack of molecular oxygen, the same bacteria can use it from nitrates, reducing them to nitrites.

Geologists have long found mysterious pipes in deposits of silver, copper and zinc ores, which were formed 350 million years ago. The deposits were formed from sulfides of the rift zone. Vestimentifera already existed then. For comparison, dinosaurs became extinct 65 million years ago.

Let's take one step back. Somewhat earlier vestimentiferans were discovered by their relatives - pogonophores - mainly deep-sea marine organisms of a similar structure. Instead of a trophosome, they have a so-called median canal - something like an intestine closed at both ends. Symbiotic bacteria also live in it, but not chemosynthetic, but methanotrophic. They "eat" methane ( CH 4 ). What do we know about methane? It is one of the main components of natural gas. Apparently, pogonophores live in areas where underwater oil and gas deposits are located and can point to them.

What is typical in the rift zone Atlantic Ocean no vestimentifer. Most likely, they simply did not have time to get there during the existence of this ocean. But there, as in pacific ocean, there are:

1) shrimp, in which hydrogen sulfide symbiotic bacteria live on the surface of the mouth limbs;

2) bivalves, in which they live in the gills;

3) bright red polychaete worms, in which they live on the surface of the body (moreover, the worm can somehow absorb them through the surface).

As stated earlier, all organisms in the black smoker community are made from organics derived from volcanic carbon dioxide through the energy of volcanic sulfur compounds. However, since all of them (including bacteria) used free oxygen as an oxidant, it still cannot be said that they exist independently of photosynthesis. De facto, chemosynthesis and photosynthesis have been invested in the life of these ecosystems on a parity basis. The bowels of the Earth delivered a reducing agent to these ecosystems, and the Sun (through photosynthetic plants) - an oxidizing agent. It should be noted that the source of the oxidizing agent is younger than the source of the reducing agent. The Sun's energy comes from the fusion of helium from hydrogen. The energy of the chemical compounds of the Earth's interior was stored in them, roughly speaking, during the formation of the Earth, and it was formed from cosmic gas and dust simultaneously with the Sun, as part of the Solar System as a whole. The sun is a second generation star, therefore, solar system, including the earth, was formed as a result of the condensation of matter ejected during the explosions of supernovae of the first generation.

The history of the evolution of biological processes of matter and energy metabolism on Earth

We, as animals, surrounded mainly by animals and plants, should not forget about the variety of possible options for organizing the metabolism of matter and energy in living beings. All of them (and even probably some that we don’t know about) were realized in prokaryotes, while eukaryotes inherited only two of them - “animal” and “vegetable”. In general, the “economic” side of life has, like Marxism (although this joke is hardly relevant now), it has three sources: energy, carbon and electrons (i.e., a substance used as a reducing agent; of the elements, hydrogen is most often an electron donor) .

According to the source of energy, all living beings are divided into phototrophs - using light energy, and chemotrophs - using energy. chemical bonds.

According to the source of carbon, they are divided into autotrophs - using carbon dioxide, and heterotrophs - using organic substances.

According to the source of electrons, they are divided into organotrophs - organics using hydrogen, and lithotrophs - using inorganic substances - derivatives of the lithosphere. It can be molecular hydrogen, ammonia, hydrogen sulfide, sulfur, carbon monoxide, iron compounds, etc.

Tell me, who are you and I according to this tripartite classification? We do not use the energy of light Directly, we use the energy of nutrients. So we are chemotrophs. Where do we get carbon to build the molecules of our body? Also from food, so we are heterotrophs. Where do we get electrons from? What substance are we oxidizing? Probably the most non-trivial question. Let's guess. It has been said more than once that oxygen is a strong oxidizing agent. You can guess that oxidation occurs where oxygen is used. Where is it used? In all variants: both in everyday life and in biochemical, this process is called respiration. Recall the generalized breathing equation:

(CH 2 O) + O 2 \u003d CO 2 + H 2 O.

We see that oxygen has taken hydrogen from the carbohydrate, while oxidizing it. And here the organic matter of food serves as the initial source of electrons.

Therefore, we animals are chemoorganoheterotrophs.

What about plants? They are phototrophs, that's for sure. They are autotrophs, we learned that too. And what do they ultimately oxidize? Let's now recall the photosynthesis equation again. Actually, you don’t need to remember anything, just rearrange the right and left parts of the previous equation:

CO 2 + H 2 O \u003d (CH 2 O) + O 2.

The great oxidizing agent itself, oxygen, is oxidized here. In molecular oxygen, its oxidation state is 0, in all substances on the left it is -2. And carbon is reduced (which is oxidized in the reverse reaction). It is found in inorganic matter - carbon dioxide. Recall, however, that photosynthesis occurs in two stages, and oxygen is formed during the photolysis of water, when electrons are detached from the water molecule. Water is also an inorganic compound, so plants are photoautolithotrophs.

Our lecture is devoted to obtaining energy and fixing carbon from carbon dioxide into organic matter. But in biological organics there are other important elements. Many of them, such as phosphorus, sulfur, are available in water-soluble substances. Nitrogen is another matter. It is also available in water-soluble substances such as ammonium salts, nitrites and nitrates. However, almost all of them in the modern world (with the exception of volcanic products) are themselves of biogenic origin, and abiogenic nitrogen exists only in molecular form. Therefore, the fixation of atmospheric nitrogen is an important problem in itself. Only bacteria, including cyanobacteria, can solve it. We will not burden you with biochemical schemes of nitrogen fixation. (Note that in all such schemes all the most important characters- enzymes - always remain behind the scenes due to their extraordinary complexity: only the interconversions of substrates and products of enzymatic reactions are drawn in the schemes.)

Let's make a small mental digression into the problem of the origin of life. Who do you think came first - autotrophs or heterotrophs? A simple thought may come to mind that since autotrophs create organic substances, and heterotrophs only eat them, life should have begun with autotrophs, since heterotrophs, if they appeared first, simply would have nothing to “eat”. This idea is completely wrong. It is an absurdity the principle of actualism- reconstruction of situations of the past on the basis of what is now. Heterotrophs should have appeared before autotrophs, since they can simply be arranged much more simply - after all, obtaining energy by destroying complex molecules is easier than building these complex molecules from simple ones, while receiving energy from some other source. The principle of "break - do not build" is absolutely universal, since it is a fairly accurate reflection of the second law of thermodynamics.

We have no reason to assume that life originally arose as something immediately very complex, so we must consider the emergence and evolution of life as a path from simple to complex. Where did life even come from? In general, if we discard the fabulous options, then the only thing that comes to mind for both us and serious scientists is that the very first living systems self-organized from some kind of “inanimate” organic matter, which should have been quite a lot for this. According to modern scientific data, it was so: on the surface of the Earth at that time there were a lot of fairly complex organic compounds that appeared extrabiologically. Here is the "food" for the first heterotrophs! But it should have ended fairly quickly. Heterotrophs could eat each other for some time, but with all such processes there are inevitable losses of matter and energy. Their reserves in the biosphere had to be replenished somehow. Here the situation was already saved by the appearance of autotrophs.

Surely they were not photoautotrophs. Photosynthesis is too complicated. All scientists agree that the first autotrophs were chemoautotrophs. We have already seen that the chemical ways of extracting energy from inorganic substances, even now, are diverse. It is quite obvious that at the dawn of the formation of life this diversity was even greater, as was the diversity of chemical situations. At that time, volcanism and bombardment by cosmic bodies were much more active, there was no free oxygen in the atmosphere in significant concentrations, which made it possible for a variety of inorganic reducing agents (ammonia, hydrogen, etc.) to exist on the earth's surface, and in the end there was organic matter of abiogenic origin. The atmosphere then had a reducing character and the oxidizing agent was in short supply. All this should have provoked organisms that originally emerged from abiogenic organics precisely as consumers of this very organics, to switch to a wide variety of inorganic energy sources.

But chemoautotrophs also have a very complex biochemistry. Any autotrophy known to us requires systems associated with the creation and use of the difference in the concentration of protons on the sides of the membrane, primarily the electron transport chain and ATP synthetase. How did all this come about? On this score, there is a very plausible, albeit rather unexpected, theory about what stages the development of the original life on our planet went through.

1. invention of glycolysis. The only universal and at the same time very inefficient mechanism for obtaining energy in living beings is glycolysis. Apparently, the first living creatures existed due to the fact that they received a certain amount of ATP through oxidation processes similar to modern glycolysis of the abiogenic organic matter present in the hydrogen-rich environment (from which they themselves organized themselves), in other words, due to fermentation. During these processes, electrons and hydrogen are transferred from one organic molecule to another via NADH or NADP-H. As a rule, reduced molecules are used to build living matter, while oxidized ones are released into the environment in the form of “production waste”. Such molecules are usually organic acids (lactic, acetic, formic, propionic, butyric, succinic - all of these variants are found in modern bacteria).

2. Invention of the proton pump. As a consequence of this primary chemical activity of life, the environment was steadily acidified. It can be assumed that at some stage in the development of life, the waters of the Earth rich in organic matter - at least the soils saturated with them, or even the entire World Ocean - literally turned sour. Acidification of the aquatic environment required the development of systems for the active pumping of protons from cells in order to maintain their internal environment. Such pumping was carried out with the consumption of ATP special protein pumps that penetrate the cell membrane.

Living organisms at this stage continued to be heterotrophs.

3. Invention of the electron transport chain. Abiogenic organic matter was a non-renewable resource. It remained less and less, it became more and more difficult to extract ATP by glycolysis. And resistance to progressive acidification through proton pumps required more and more ATP. To solve the acidification problem in a different way, systems of membrane-bound proteins were invented that carried out transmembrane transport of protons against a concentration gradient due to the energy of redox reactions associated with the transfer of electrons from some substances in the environment in excess to others, but without the mediation of NAD. -H or NADP-H. Such substances were organic acids and inorganic substances accumulated in the medium. The systems of transmembrane proton export that were discussed were the prototype of the electron transport chain. Bacteria living in an acidic environment still use the electron transport chain to maintain a less acidic internal environment. Due to the emergence of the electron transport chain, ATP was saved, so the carriers of this chain received an undoubted advantage over those organisms that did not have it.

4. Invention of ATP synthetase. Systems for exporting protons across the membrane using redox reactions gradually improved and eventually outperformed ATP-dependent membrane pumps in efficiency. This made it possible to reverse the work of the latter. Now they, on the contrary, launched protons into the cell, while synthesizing ATP from ADP. This is how ATP synthetase arose, using the difference in proton concentration for ATP synthesis. As mentioned above, the action of ATP synthetase is reversible: at high ATP concentrations and a small potential difference on both sides of the mambrane, on the contrary, it creates a proton concentration gradient. It is precisely as a proton pump that ATP synthetase (and this protein complex is present in all modern living beings without exception) works in anaerobic bacteria.

The creation of ATP synthetase was a major breakthrough. At this stage, organisms solved both the problem of maintaining the internal environment and the problem of obtaining energy, for the first time becoming autotrophs from heterotrophs, namely chemoautotrophs. Like modern chemoautotrophs, they obtained energy through redox reactions using the electron transport chain. However, in addition to energy, life requires the synthesis of reduced organics. Its abiogenic reserves were practically exhausted by that time. For the synthesis of such organicsde novostrong hydrogen donors are needed, such as the reduced coenzyme NADP-H. The restoration of this coenzyme can proceed, like the synthesis of ATP, due to the difference in proton concentration by reversing the electron transport chain and the work of the enzyme, similar to modern NAD-H-dehydrogenase, which then worked in the opposite direction - restored NAD-H from NAD +.

We draw your attention to the fact that these organisms were anaerobic chemotrophs, which are extremely rare in the modern world. In the absence of such a strong oxidizing agent as oxygen, most likely the first schemes of chemosynthesis were based on redox reactions with a slight energy gain. The idea behind using the proton gradient was that the small gains from many of these reactions were summed up in it and could be used in energy-intensive reactions such as the reduction of NADP-H.

5. Invention of photosynthesis and photosystem 1. As you can see, many prerequisites for photosynthesis already existed by that time - ATP synthetase, an electron transport chain, and biochemical pathways for organic synthesis using NAD-H were invented. There was only one step left before photosynthesis - the appearance of pigments capable of capturing the energy of photons and transferring it to the system of redox reactions associated with the electron transport chain. The antenna systems of modern photosynthetics are very complex, the first ones must have been quite simple. We have already considered a simple mechanism for the use of light energy by halobacteria. There is an opinion that the very first antennas capable of capturing the energy of photons were all the same old acquaintances of ours - the nitrogenous bases of nucleic acids. As you remember, there also exists a resonant system of alternating double and single bonds, although not on such an impressive scale as in chlorophylls.

Probably, photosystem 1 was the first to have survived to this day, which led to the appearance of green sulfur bacteria. It is possible, again, that historically, cyclic photophosphorylation, which does not require external oxidizing agents and reducing agents, was the first to arise. However, the most important was the ability acquired by this photosystem to directly reduce NADP + to NADP-H due to the energy of sunlight, taking an electron, for example, from hydrogen sulfide and oxidizing it to atomic sulfur, like in modern green sulfur bacteria. Let's pay attention to the fact that sulfur plays important role in the proteins of the photosystem 1.

By the way, this happened 3-4 billion years ago, i.e., only a billion years after the formation of the Earth. The time of chemoautotrophs has passed, the time of photoaphtotrophs has begun.

6. Invention of water photolysis. The appearance of free oxygen. The problem with early photosynthetics was the lack of good inorganic reducing agents. Water is a “very bad” reducing agent, but it is available in unlimited quantities. The combination of two photosystems inherited from green (photosystem 1) and purple (photosystem 2) sulfur bacteria into one conjugated system, which occurred in blue-green algae (cyanobacteria), made it possible, by combining the energy of two successively captured photons, to oxidize water, taking electrons from oxygen atoms. This was an important breakthrough in the energy of the first organisms, which had truly monstrous consequences. With the union of the two photosystems in the ancestors of cyanobacteria, or blue-green algae, organisms appeared with minimal requirements for chemicals environment. This led to the emergence of a large amount of biogenic restored organic matter - life began to flourish. However, on earth's surface such a terrible poison as free oxygen began to appear.

At first, all the oxygen released during photosynthesis went to the oxidation of ferrous ions, which were abundant in the oceans, to trivalent, which began to precipitate in the form of iron oxides. This process began 2.7 billion years ago and ended about 2 billion years ago. During all these 700 million years (recall that dinosaurs died out only 65 million years ago), modern type photosynthesis existed on Earth, accompanied by water photolysis, free oxygen was formed, but it was absent in the atmosphere. This means that breathing has not yet been invented on Earth. And this means, again, that there were no prerequisites for the existence of effective heterotrophs on the planet. There could be no talk not only of "animals", but also of aerobic bacteria, which in the modern world play such an important role in the breakdown of biogenic organic matter. We can say that all this time on Earth there was a kind of golden age, an earthly paradise in which no one ate anyone (and did not even eat corpses). It was inhabited by the most perfect and truly sinless living beings, "feeding" on sunlight, water, carbon dioxide and nitrogen in the air. These were cyanobacteria, or blue-green algae (the same ones that still exist perfectly today). As the most autonomous living beings, they are more perfect than plants, since, like many bacteria, they are able to fix atmospheric nitrogen. (Plants do not know how to do this and are forced to use oxidized nitrate nitrogen or reduced ammonium nitrogen, which is now of biogenic origin.) Blue-green algae lived and flourished in the form of colonies in shallow waters. These colonies were more or less spherical and grew from the surface. Small particles of soil enriched with ferric iron settled on them, which eventually buried dead cells inside the colony. In the absence of "animals", the age of an individual colony could be very large. Such colonies have been preserved as fossils called stromatolites(translated from Greek as "layered stones"), stones that have a structure of concentric layers, often enriched with iron.

7. The invention of breathing. However, the "sinlessness" of cyanobacteria was apparent. By releasing such a strong oxidizing agent as oxygen during the photolysis of water, they gradually poisoned the World Ocean and prepared the collapse of their peaceful paradise reign, which quickly gave way to that hell of hungry demons familiar to us, where living beings continuously devour each other. (This is all a metaphor, of course. But recently a biology textbook published by the Orthodox Church was published, which says that before the fall of Adam, female mosquitoes may have fed on the nectar of flowers, which supposedly could contain hemoglobin. Now this is no longer funny, this is an attempt to return us at the time of the wildest superstitions by raising children with lies and crazy fantasies.) About 2 billion years ago, the ferrous iron in the ocean ended and oxygen began to flow into the atmosphere. It reached its present content in the atmosphere between 1.5 and 0.5 billion years ago. The appearance of oxygen required a restructuring of the entire biochemistry of almost all living beings then, since it literally poisoned many enzymes (or rather, coenzymes). At the same time, a powerful oxidizing agent appeared in the medium, which was adapted as an acceptor of electrons in electron transport chains, thereby significantly increasing their efficiency. This is how cellular respiration arose. Many modern purple bacteria can switch from photosynthesis to respiration using the same electron transport chains.

Only at this stage did it become possible for the appearance of heterotrophs using processes more efficient than glycolysis, and much more efficient (remember - 18 times!). The renaissance of heterotrophs has begun. Do you know what currently exists great amount aerobic bacteria. All of them come from photosynthetic bacteria that have lost the ability to photosynthesize, but retained the electron transport chain. Even our E. coli comes from purple bacteria! The prerequisites for the emergence of organisms that live due to the effective oxidation of organic matter produced by autotrophs have arisen. Thus, the undivided reign of autotrophs was put to an end. It is important that along with the cessation of ferric iron deposits at that time, oil accumulation was catastrophically reduced. If earlier biogenic organic matter was formed so much that its excess after long chemical transformations was deposited in the bowels in the form of oil, which served as a bonus for such distant descendants of organisms of those days as we are with you, then with the advent of breathing, there was a rush demand for this organic matter, which a new aggressive consumer began to take directly from the manufacturer.

8. Emergence of mitochondria and plastids. About 1.5 billion years ago, some aerobic bacteria began to live in the cells of primitive (and initially anaerobic!) eukaryotes and eventually turned into mitochondria. From that moment on, the appearance of animals, originally unicellular, became possible. All modern eukaryotes have mitochondria, and all these mitochondria are related to each other and have clearly been "domesticated" only once. Primary anaerobic eukaryotes deprived of mitochondria have not survived to this day. Later, some cyanobacteria, also moving to life inside eukaryotic cells, turned into algal plastids, and this happened at least three times in different algae. Plants subsequently evolved from green algae. In all cases, plastids were acquired by cells that already had mitochondria. These cells "were able to breathe", but they were not able to synthesize organic matter, i.e. they were animal cells. Thus, plants, that is, eukaryotes capable of photosynthesis, having plastids and mitochondria, originated from animals (of course, this happened at the unicellular level).

We see that development, or evolution life on Earth (we will discuss the meaning of the word "evolution" in more detail in the 15th lecture), was very uneven. Periods of hundreds of millions of years, when nothing fundamentally new happened, were replaced by rapid constructive breakthroughs, as a result of which the face of the Earth was radically transformed. Each of these breakthroughs was accompanied by the invention of a way to overcome some kind of deficiency - first it was a deficiency of reducing agents, and then a deficiency of oxidizing agents. Each of these "inventions" had to wait for hundreds of millions or billions of years, which only says that they happened by chance - any "purposeful" engineering activity would be much more effective. As a result, life on earth has learned to manage with the most scarce resources - water, carbon dioxide, atmospheric nitrogen, and the main source of energy - non-renewable (!), But sunlight has become practically inexhaustible. It is possible that the face of the Earth has yet to change, and it is possible that now as a result of "intelligent" human activity. But how expedient it will turn out to be and whether a person and life in the forms known to us will lead to death - this is another question.

Lecture 6 ends. The first was devoted to the definition of life, and the other five - all kinds of chemistry. And it is right! Let us recall the definition of life we ​​settled on then: a set of self-sustaining open systems that exist, in the form of special structures, due to the constant flow of matter and the influx of energy and are capable of more or less accurate self-reproduction.

Most of what is said here is realized at the chemical level we have considered. Nucleic acids capable of template biosynthesis are responsible for more or less accurate self-reproduction. The flow of matter and energy is realized through enzymatic reactions involving fairly simple organic acids, specific nucleotides, intermediary coenzymes, and more complex protein-pigment systems, such as photosystems and the electron transport chain. Perhaps, only the reservation “in the form of special structures” goes beyond the framework of the chemistry we have considered. In two similar cases, you and I already needed special structures - a space bounded by a membrane, according to different sides which creates a difference in the concentration of protons - the inner space of the mitochondria in the process of oxidative phosphorylation and the inner space of the thylakoid in the case of the light stage of photosynthesis. In fact, at least the outer membrane of the cell remained behind the scenes of our consideration all the time, which delimited our chemical reactor with a finely tuned concentration various substances from outside space.

Thus, we have considered almost the entire essence of life, which we need to supplement with its structural organization. We will move on to this in the next lecture, because, in fact, all biology beyond biochemistry is the science of biological structures. We will start with structures at the so-called cellular level.

The history of the discovery of an amazing and such a vitally important phenomenon as photosynthesis is rooted deep in the past. More than four centuries ago, in 1600, the Belgian scientist Jan Van - Helmont set the simplest experiment. He placed a willow branch in a bag containing 80 kg of earth. The scientist recorded the initial weight of the willow, and then for five years watered the plant exclusively with rainwater. What was the surprise of Jan Van - Helmont when he re-weighed the willow. The weight of the plant increased by 65 kg, and the mass of the earth decreased by only 50 grams! Where did the plant get 64 kg 950 g of nutrients for the scientist remained a mystery!

The next significant experiment on the path to the discovery of photosynthesis belonged to the English chemist Joseph Priestley. The scientist put a mouse under the cap, and after five hours the rodent died. When Priestley placed a sprig of mint with the mouse and also covered the rodent with a cap, the mouse remained alive. This experiment led the scientist to the idea that there is a process opposite to breathing. Jan Ingenhaus in 1779 established the fact that only the green parts of plants are capable of releasing oxygen. Three years later, the Swiss scientist Jean Senebier proved that carbon dioxide, under the influence of sunlight, decomposes in the green organelles of plants. Just five years later, the French scientist Jacques Bussingault, conducting laboratory research, discovered the fact that the absorption of water by plants also occurs during the synthesis of organic substances. A landmark discovery in 1864 was made by the German botanist Julius Sachs. He was able to prove that the volume of carbon dioxide consumed and oxygen released occurs in a ratio of 1: 1.

Photosynthesis is one of the most important biological processes

talking scientific language, photosynthesis (from other Greek φῶς - light and σύνθεσις - connection, binding) is a process in which organic substances are formed from carbon dioxide and water in the light. The main role in this process belongs to photosynthetic segments.

Speaking figuratively, the leaf of a plant can be compared with a laboratory, the windows of which face the sunny side. It is in it that the formation of organic substances occurs. This process is the basis for the existence of all life on Earth.

Many will reasonably ask the question: what do people who live in the city breathe, where not only trees, and you can’t find blades of grass during the day with fire. The answer is very simple. The fact is that land plants account for only 20% of the oxygen released by plants. Algae play a major role in the production of oxygen into the atmosphere. They account for 80% of the oxygen produced. In the language of numbers, both plants and algae release 145 billion tons (!) of oxygen into the atmosphere every year! No wonder the world's oceans are called the "lungs of the planet."

The general formula for photosynthesis is as follows:

Water + Carbon Dioxide + Light → Carbohydrates + Oxygen

Why do plants need photosynthesis?

As we have understood, photosynthesis is a necessary condition for the existence of man on Earth. However, this is not the only reason why photosynthetic organisms actively produce oxygen into the atmosphere. The fact is that both algae and plants annually form more than 100 billion organic substances (!), which form the basis of their life activity. Remembering the experiment of Jan Van Helmont, we understand that photosynthesis is the basis of plant nutrition. It has been scientifically proven that 95% of the crop is determined by organic substances obtained by the plant in the process of photosynthesis, and 5% - those mineral fertilizers that the gardener introduces into the soil.

Modern summer residents focus on the soil nutrition of plants, forgetting about its air nutrition. It is not known what kind of harvest gardeners could get if they were attentive to the process of photosynthesis.

However, neither plants nor algae could produce oxygen and carbohydrates so actively if they did not have an amazing green pigment - chlorophyll.

The secret of the green pigment

The main difference between plant cells and cells of other living organisms is the presence of chlorophyll. By the way, it is he who is the culprit of the fact that the leaves of plants are painted precisely in green color. This complex organic compound has one amazing property: it can absorb sunlight! Thanks to chlorophyll, the process of photosynthesis becomes possible.

Two stages of photosynthesis

talking plain language Photosynthesis is a process in which water and carbon dioxide absorbed by a plant in the presence of light with the help of chlorophyll form sugar and oxygen. Thus, inorganic substances are miraculously transformed into organic ones. The resulting sugar is the energy source of plants.

Photosynthesis has two stages: light and dark.

Light phase of photosynthesis

Occurs on thylakoid membranes.

Thylakoid are structures bounded by a membrane. They are located in the stroma of the chloroplast.

The order of events of the light stage of photosynthesis:

1. Light hits the chlorophyll molecule, which is then absorbed by the green pigment and brings it into an excited state. The electron included in the molecule passes to more high level, is involved in the synthesis process.
2. There is a splitting of water, during which protons under the influence of electrons turn into hydrogen atoms. Subsequently, they are spent on the synthesis of carbohydrates.
3. At the final stage of the light stage, ATP (adenosine triphosphate) is synthesized. This is an organic substance that plays the role of a universal energy accumulator in biological systems.

Dark phase of photosynthesis

The site of the dark phase is the stroma of chloroplasts. It is during the dark phase that oxygen is released and glucose is synthesized. Many will think that this phase got such a name because the processes taking place within this stage are carried out exclusively at night. Actually, this is not entirely true. Glucose synthesis occurs around the clock. The point is that it is this stage light energy is no longer consumed, which means that it is simply not needed.

Importance of photosynthesis for plants

We have already identified the fact that plants need photosynthesis no less than we do. It is very easy to talk about the scale of photosynthesis in the language of numbers. Scientists have calculated that only land plants store as much solar energy as 100 megacities could use up within 100 years!

Plant respiration is a process opposite to photosynthesis. The meaning of plant respiration is to release energy in the process of photosynthesis and direct it to the needs of plants. In simple terms, harvest is the difference between photosynthesis and respiration. The more photosynthesis and the lower respiration, the greater the harvest, and vice versa!

Photosynthesis is the amazing process that makes life possible on Earth!

Photosynthesis- synthesis of organic compounds from inorganic ones due to light energy (hv). The overall photosynthesis equation is:

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

Photosynthesis proceeds with the participation of photosynthetic pigments, which have the unique property of converting sunlight energy into chemical bond energy in the form of ATP. Photosynthetic pigments are protein-like substances. The most important of these is the pigment chlorophyll. In eukaryotes, photosynthetic pigments are embedded in the inner membrane of plastids; in prokaryotes, they are embedded in invaginations of the cytoplasmic membrane.

The structure of the chloroplast is very similar to that of the mitochondria. The inner membrane of the grana thylakoids contains photosynthetic pigments, as well as electron transport chain proteins and ATP synthetase enzyme molecules.

The process of photosynthesis consists of two phases: light and dark.

light phase Photosynthesis takes place only in the presence of light in the thylakoid grana membrane. In this phase, the absorption of light quanta by chlorophyll, the formation of an ATP molecule, and the photolysis of water occur.

Under the action of a light quantum (hv), chlorophyll loses electrons, passing into an excited state:

Chl → Chl + e —

These electrons are transferred by carriers to the outer, i.e. the surface of the thylakoid membrane facing the matrix, where they accumulate.

At the same time, photolysis of water occurs inside the thylakoids, i.e. its decomposition under the influence of light

2H 2 O → O 2 + 4H + + 4e -

The resulting electrons are transferred by carriers to the chlorophyll molecules and restore them: the chlorophyll molecules return to a stable state.

Hydrogen protons, formed during the photolysis of water, accumulate inside the thylakoid, creating an H + -reservoir. As a result, the inner surface of the thylakoid membrane is charged positively (due to H +), and the outer surface is negatively charged (due to e -). As oppositely charged particles accumulate on both sides of the membrane, the potential difference increases. When the critical value of the potential difference is reached, the force electric field begins to push protons through the ATP synthetase channel. The energy released in this case is used to phosphorylate ADP molecules:

ADP + F → ATP

The formation of ATP during photosynthesis under the influence of light energy is called photophosphorylation.

Hydrogen ions, once on the outer surface of the thylakoid membrane, meet electrons there and form atomic hydrogen, which binds to the hydrogen carrier molecule NADP (nicotinamide adenine dinucleotide phosphate):

2H + + 4e - + NADP + → NADP H 2

Thus, during the light phase of photosynthesis, three processes occur: the formation of oxygen due to the decomposition of water, the synthesis of ATP, the formation of hydrogen atoms in the form of NADP H 2 . Oxygen diffuses into the atmosphere, ATP and NADP H 2 are involved in the processes of the dark phase.

dark phase photosynthesis takes place in the chloroplast matrix both in light and in the dark and is a series of successive transformations of CO 2 coming from the air in the Calvin cycle. The reactions of the dark phase are carried out due to the energy of ATP. In the Calvin cycle, CO 2 bonds with hydrogen from NADP H 2 to form glucose.

In the process of photosynthesis, in addition to monosaccharides (glucose, etc.), monomers of other organic compounds are synthesized - amino acids, glycerol and fatty acids. Thus, thanks to photosynthesis, plants provide themselves and all life on Earth with the necessary organic substances and oxygen.

Comparative characteristics photosynthesis and respiration of eukaryotes is given in the table:

Comparative characteristics of photosynthesis and respiration of eukaryotes

sign	Photosynthesis	Breath
Reaction equation	6CO 2 + 6H 2 O + Light energy → C 6 H 12 O 6 + 6O 2	C 6 H 12 O 6 + 6O 2 → 6H 2 O + Energy (ATP)
starting materials	carbon dioxide, water
reaction products	organic matter, oxygen	carbon dioxide, water
Significance in the cycle of substances	Synthesis of organic substances from inorganic	Decomposition of organic substances to inorganic
Energy transformation	The conversion of light energy into the energy of chemical bonds of organic substances	The conversion of the energy of chemical bonds of organic substances into the energy of macroergic bonds of ATP
Milestones	Light and dark phase (including the Calvin cycle)	Incomplete oxidation (glycolysis) and complete oxidation (including the Krebs cycle)
Place of the process	Chloroplast	Hyaloplasm (incomplete oxidation) and mitochondria (complete oxidation)

How is the energy of sunlight in the light and dark phases of photosynthesis converted into the energy of chemical bonds of glucose? Explain the answer.

Answer

In the light phase of photosynthesis, the energy of sunlight is converted into the energy of excited electrons, and then the energy of excited electrons is converted into the energy of ATP and NADP-H2. In the dark phase of photosynthesis, the energy of ATP and NADP-H2 is converted into the energy of glucose chemical bonds.

What happens during the light phase of photosynthesis?

Answer

Chlorophyll electrons, excited by the energy of light, go along the electron transport chains, their energy is stored in ATP and NADP-H2. Photolysis of water occurs, oxygen is released.

What are the main processes that take place during the dark phase of photosynthesis?

Answer

From carbon dioxide obtained from the atmosphere and hydrogen obtained in the light phase, glucose is formed due to the energy of ATP obtained in the light phase.

What is the function of chlorophyll in a plant cell?

Answer

Chlorophyll is involved in the process of photosynthesis: in the light phase, chlorophyll absorbs light, the chlorophyll electron receives light energy, breaks off and goes along the electron transport chain.

What role do chlorophyll electrons play in photosynthesis?

Answer

Chlorophyll electrons, excited by sunlight, pass through electron transport chains and give up their energy to the formation of ATP and NADP-H2.

At what stage of photosynthesis is free oxygen produced?

Answer

In the light phase, during the photolysis of water.

During what phase of photosynthesis does ATP synthesis occur?

Answer

light phase.

What is the source of oxygen during photosynthesis?

Answer

Water (oxygen is released during the photolysis of water).

The rate of photosynthesis depends on limiting (limiting) factors, among which are light, carbon dioxide concentration, temperature. Why are these factors limiting for photosynthesis reactions?

Answer

Light is necessary for the excitation of chlorophyll, it supplies energy for the process of photosynthesis. Carbon dioxide is needed in the dark phase of photosynthesis; glucose is synthesized from it. A change in temperature leads to the denaturation of enzymes, the reactions of photosynthesis slow down.

In what metabolic reactions in plants is carbon dioxide the initial substance for the synthesis of carbohydrates?

Answer

in the reactions of photosynthesis.

In the leaves of plants, the process of photosynthesis proceeds intensively. Does it occur in mature and unripe fruits? Explain the answer.

Answer

Photosynthesis takes place in the green parts of plants exposed to light. Thus, photosynthesis occurs in the skin of green fruits. Inside the fruit and in the skin of ripe (not green) fruits, photosynthesis does not occur.

Table of contents of the subject "Transfer of Substances in a Bacterial Cell. Nutrient Substrates of Bacteria. Energy Metabolism of Bacteria.":
1. Active transfer of substances in a bacterial cell. Transport of substances due to phosphorylation. Isolation of substances from a bacterial cell.
2. Enzyme. bacterial enzymes. Regulatory (allosteric) enzymes. effector enzymes. Determination of the enzymatic activity of bacteria.
3. Nutrient substrates for bacteria. Carbon. Autotrophy. Heterotrophy. Nitrogen. Use of inorganic nitrogen. Assimilation processes in the cell.
4. Dissimilation processes. The use of organic nitrogen in the cell. Ammonification of organic compounds.
5. Phosphorus. Sulfur. Oxygen. Obligate (strict) aerobes. Obligate (strict) anaerobes. facultative anaerobes. Aerotolerant bacteria. microaerophilic bacteria.
6. Growth factors of bacteria. Auxotrophs. Prototrophs. Classification of factors stimulating the growth of bacteria. Trigger factors for bacterial growth.
7. Energy metabolism of bacteria. Identification scheme for an unknown bacterium. exergonic reactions.
8. Synthesis (regeneration) of ATP. Obtaining energy in the process of photosynthesis. phototrophic bacteria. photosynthesis reactions. Stages of photosynthesis. Light and dark phase of photosynthesis.
9. Obtaining energy from the oxidation of chemical compounds. Chemotrophic bacteria. Energy production by substrate phosphorylation. Fermentation.
10. Alcoholic fermentation. Homofermentative lactic acid fermentation. heterofermentative fermentation. Formic acid fermentation.

Synthesis (regeneration) of ATP. Obtaining energy in the process of photosynthesis. phototrophic bacteria. photosynthesis reactions. Stages of photosynthesis. Light and dark phase of photosynthesis.

ATP synthesis It is carried out in three ways: photosynthetic phosphorylation, oxidative phosphorylation (associated with the transport of electrons along the respiratory chain) and substrate phosphorylation.

In the first two processes, the transformation of the energy received with the flow of electrons into energy phosphoester bonds of ATP carries out a special enzyme - ATP synthetase. This enzyme is present in all membranes involved in energy conversion (membranes of bacteria, mitochondria and chloroplasts). ATP synthetase catalyzes the addition of inorganic phosphate (Pn) to ADP, the formation of which is carried out by adenylate kinase (AMP + ATP = 2 ADP). ATP synthetase activity can be detected by the reverse reaction of ATP hydrolysis: ATP + H20 \u003d ADP + Fn + H +. Due to the reversibility of the phosphorylation reaction, the accumulated ATP can be used to create a proton gradient that provides energy for the movement of flagella and osmotic work. Energy is also used for reverse electron transfer, which is necessary for the reduction of nicotinamide adenine dinucleotide (NAD) when bacteria use inorganic electron donors (SO3, NO3, Fe2+, etc.).

Obtaining energy in the process of photosynthesis. phototrophic bacteria. photosynthesis reactions.

Obtaining energy in the process of photosynthesis. The main source of energy for life on Earth is the Sun, but only a few are able to directly utilize the energy of insolation in the world of bacteria. phototrophic bacteria[from Greek. photos, light, + trophe, food]. Photosynthetic bacteria, like plants, convert the energy of visible light into a proton potential for energy-converting membrane. Subsequently, using ATP synthetase energy is stored in ATP. The main feature that distinguishes photosynthetic reactions in purple and green bacteria from those in plants and cyanobacteria is the absence of oxygen release (since they use not water, but H2S or organic substances as an electron donor). In bacteria, an analogue of the chloroplasts of plant cells - chromatophores containing chlorophyll and carotenoid pigments.

Thus, under photosynthesis understand the transformation of light energy into biochemically available energy (proton gradient on the membrane of thylakoids and chloroplasts, ATP) and the restorative power of NADPH+, as well as the synthesis of cellular components associated with this, taking place in the cells of phototrophic organisms. Photosynthesis reactions flow into two stages (light and dark phases).

Light phase of photosynthesis. Under the influence of photons, the electron of the chromatophore is activated, then it returns to its original state. This releases energy that is used to create a proton gradient, and then the synthesis of ATP and the reduction of nicotinamide adenine dinucleotide phosphate (NADP) to NADPH +. The latter can occur due to the reverse transport of electrons with the consumption of ATP.

Dark phase of photosynthesis. The resulting macroergic compounds are used for the assimilation reduction of CO2 into glucose. Glucose contains a significant amount of energy (about 690 kcal / mol), which is used by heterotrophic bacteria, decomposing glucose and storing energy in the universal store - ATP.