Photosynthesis table light and dark phase processes. The light phase of photosynthesis. Conditions required for photosynthesis

Photosynthesis is the conversion of light energy into chemical bond energy. organic compounds.

Photosynthesis is characteristic of plants, including all algae, a number of prokaryotes, including cyanobacteria, and some unicellular eukaryotes.

In most cases, photosynthesis produces oxygen (O2) as a by-product. However, this is not always the case as there are several different pathways for photosynthesis. In the case of oxygen release, its source is water, from which hydrogen atoms are split off for the needs of photosynthesis.

Photosynthesis consists of many reactions in which various pigments, enzymes, coenzymes, etc. participate. The main pigments are chlorophylls, in addition to them, carotenoids and phycobilins.

In nature, two ways of plant photosynthesis are common: C 3 and C 4. Other organisms have their own specific reactions. What unites these different processes under the term “photosynthesis” is that in all of them, in total, the conversion of photon energy into a chemical bond occurs. For comparison: during chemosynthesis, energy is converted chemical bond some compounds (inorganic) to others - organic.

There are two phases of photosynthesis - light and dark. The first depends on the light radiation (hν), which is necessary for the reactions to proceed. The dark phase is light independent.

In plants, photosynthesis takes place in chloroplasts. As a result of all reactions, primary organic substances are formed, from which carbohydrates, amino acids, fatty acids, etc. are then synthesized. Usually, the total reaction of photosynthesis is written in relation to glucose - the most common product of photosynthesis:

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

The oxygen atoms that make up the O 2 molecule are not taken from carbon dioxide, but from water. Carbon dioxide is a source of carbon which is more important. Due to its binding, plants have the opportunity to synthesize organic matter.

The chemical reaction presented above is a generalized and total. It is far from the essence of the process. So glucose is not formed from six individual molecules of carbon dioxide. The binding of CO 2 occurs in one molecule, which first attaches to an already existing five-carbon sugar.

Prokaryotes have their own characteristics of photosynthesis. So in bacteria, the main pigment is bacteriochlorophyll, and oxygen is not released, since hydrogen is not taken from water, but often from hydrogen sulfide or other substances. In blue-green algae, the main pigment is chlorophyll, and oxygen is released during photosynthesis.

Light phase of photosynthesis

In the light phase of photosynthesis, ATP and NADP·H 2 are synthesized due to radiant energy. It happens on the thylakoids of chloroplasts, where pigments and enzymes form complex complexes for the functioning of electrochemical circuits, through which electrons and partly hydrogen protons are transferred.

The electrons end up at the coenzyme NADP, which, being negatively charged, attracts some of the protons to itself and turns into NADP H 2 . Also, the accumulation of protons on one side of the thylakoid membrane and electrons on the other creates an electrochemical gradient, the potential of which is used by the enzyme ATP synthetase to synthesize ATP from ADP and phosphoric acid.

The main pigments of photosynthesis are various chlorophylls. Their molecules capture the radiation of certain, partly different spectra of light. In this case, some electrons of chlorophyll molecules move to a higher energy level. This is an unstable state, and, in theory, electrons, by means of the same radiation, should give the energy received from outside into space and return to the previous level. However, in photosynthetic cells, excited electrons are captured by acceptors and, with a gradual decrease in their energy, are transferred along the chain of carriers.

On thylakoid membranes, there are two types of photosystems that emit electrons when exposed to light. Photosystems are a complex complex of mostly chlorophyll pigments with a reaction center from which electrons are torn off. In the photosystem sunlight catches a lot of molecules, but all the energy is collected in the reaction center.

The electrons of photosystem I, having passed through the chain of carriers, restore NADP.

The energy of the electrons detached from photosystem II is used to synthesize ATP. And the electrons of photosystem II fill the electron holes of photosystem I.

The holes of the second photosystem are filled with electrons formed as a result of water photolysis. Photolysis also occurs with the participation of light and consists in the decomposition of H 2 O into protons, electrons and oxygen. It is as a result of the photolysis of water that free oxygen is formed. Protons are involved in the creation of an electrochemical gradient and the reduction of NADP. Electrons are received by the chlorophyll of photosystem II.

Approximate summary equation of the light phase of photosynthesis:

H 2 O + NADP + 2ADP + 2P → ½O 2 + NADP H 2 + 2ATP

Cyclic electron transport

The so-called non-cyclic light phase of photosynthesis. Is there some more cyclic electron transport when NADP reduction does not occur. In this case, electrons from photosystem I go to the carrier chain, where ATP is synthesized. That is, this electron transport chain receives electrons from photosystem I, not II. The first photosystem, as it were, implements a cycle: the emitted electrons return to it. On the way, they spend part of their energy on the synthesis of ATP.

Photophosphorylation and oxidative phosphorylation

The light phase of photosynthesis can be compared with the stage of cellular respiration - oxidative phosphorylation, which occurs on the mitochondrial cristae. There, too, ATP synthesis occurs due to the transfer of electrons and protons along the carrier chain. However, in the case of photosynthesis, energy is stored in ATP not for the needs of the cell, but mainly for the needs of the dark phase of photosynthesis. And if during respiration organic substances serve as the initial source of energy, then during photosynthesis it is sunlight. The synthesis of ATP during photosynthesis is called photophosphorylation rather than oxidative phosphorylation.

Dark phase of photosynthesis

For the first time the dark phase of photosynthesis was studied in detail by Calvin, Benson, Bassem. The cycle of reactions discovered by them was later called the Calvin cycle, or C 3 -photosynthesis. In certain groups of plants, a modified photosynthesis pathway is observed - C 4, also called the Hatch-Slack cycle.

In the dark reactions of photosynthesis, CO 2 is fixed. The dark phase takes place in the stroma of the chloroplast.

Recovery of CO 2 occurs due to the energy of ATP and the reducing power of NADP·H 2 formed in light reactions. Without them, carbon fixation does not occur. Therefore, although the dark phase does not directly depend on light, it usually also proceeds in light.

Calvin cycle

The first reaction of the dark phase is the addition of CO 2 ( carboxylatione) to 1,5-ribulose biphosphate ( ribulose 1,5-diphosphate) – RiBF. The latter is a doubly phosphorylated ribose. This reaction is catalyzed by the enzyme ribulose-1,5-diphosphate carboxylase, also called rubisco.

As a result of carboxylation, an unstable six-carbon compound is formed, which, as a result of hydrolysis, decomposes into two three-carbon molecules phosphoglyceric acid (PGA) is the first product of photosynthesis. FHA is also called phosphoglycerate.

RiBP + CO 2 + H 2 O → 2FGK

FHA contains three carbon atoms, one of which is part of the acid carboxyl group(-COOH):

FHA is converted into a three-carbon sugar (glyceraldehyde phosphate) triose phosphate (TF), which already includes an aldehyde group (-CHO):

FHA (3-acid) → TF (3-sugar)

This reaction consumes the energy of ATP and the reducing power of NADP · H 2 . TF is the first carbohydrate of photosynthesis.

After that, most of the triose phosphate is spent on the regeneration of ribulose bisphosphate (RiBP), which is again used to bind CO 2 . Regeneration involves a series of ATP-consuming reactions involving sugar phosphates with 3 to 7 carbon atoms.

It is in this cycle of RiBF that the Calvin cycle is concluded.

A smaller part of the TF formed in it leaves the Calvin cycle. In terms of 6 bound molecules of carbon dioxide, the yield is 2 molecules of triose phosphate. The total reaction of the cycle with input and output products:

6CO 2 + 6H 2 O → 2TF

At the same time, 6 RiBP molecules participate in the binding and 12 FHA molecules are formed, which are converted into 12 TF, of which 10 molecules remain in the cycle and are converted into 6 RiBP molecules. Since TF is a three-carbon sugar, and RiBP is a five-carbon one, in relation to carbon atoms we have: 10 * 3 = 6 * 5. The number of carbon atoms that provide the cycle does not change, all the necessary RiBP is regenerated. And six molecules of carbon dioxide included in the cycle are spent on the formation of two molecules of triose phosphate leaving the cycle.

The Calvin cycle, based on 6 bound CO 2 molecules, consumes 18 ATP molecules and 12 NADP · H 2 molecules, which were synthesized in the reactions of the light phase of photosynthesis.

The calculation is carried out for two triose phosphate molecules leaving the cycle, since the glucose molecule formed later includes 6 carbon atoms.

Triose phosphate (TF) is the end product of the Calvin cycle, but it can hardly be called the end product of photosynthesis, since it almost does not accumulate, but, reacting with other substances, turns into glucose, sucrose, starch, fats, fatty acids, amino acids. In addition to TF, FHA plays an important role. However, such reactions occur not only in photosynthetic organisms. In this sense, the dark phase of photosynthesis is the same as the Calvin cycle.

PHA is converted into a six-carbon sugar by stepwise enzymatic catalysis. fructose-6-phosphate, which turns into glucose. In plants, glucose can be polymerized into starch and cellulose. The synthesis of carbohydrates is similar to the reverse process of glycolysis.

photorespiration

Oxygen inhibits photosynthesis. The more O 2 in the environment, the less efficient the CO 2 sequestration process. The fact is that the enzyme ribulose bisphosphate carboxylase (rubisco) can react not only with carbon dioxide, but also with oxygen. In this case, the dark reactions are somewhat different.

Phosphoglycolate is phosphoglycolic acid. The phosphate group is immediately cleaved from it, and it turns into glycolic acid (glycolate). For its "utilization" oxygen is needed again. Therefore, the more oxygen in the atmosphere, the more it will stimulate photorespiration and the more oxygen the plant will need to get rid of the reaction products.

Photorespiration is the light-dependent consumption of oxygen and the release of carbon dioxide. That is, the exchange of gases occurs as during respiration, but takes place in chloroplasts and depends on light radiation. Photorespiration depends on light only because ribulose biphosphate is formed only during photosynthesis.

During photorespiration, carbon atoms are returned from glycolate to the Calvin cycle in the form of phosphoglyceric acid (phosphoglycerate).

2 Glycolate (C 2) → 2 Glyoxylate (C 2) → 2 Glycine (C 2) - CO 2 → Serine (C 3) → Hydroxypyruvate (C 3) → Glycerate (C 3) → FGK (C 3)

As you can see, the return is not complete, since one carbon atom is lost when two molecules of glycine are converted into one molecule of the amino acid serine, while carbon dioxide is released.

Oxygen is needed at the stages of conversion of glycolate to glyoxylate and glycine to serine.

The conversion of glycolate to glyoxylate and then to glycine occurs in peroxisomes, and serine is synthesized in mitochondria. Serine again enters the peroxisomes, where it first produces hydroxypyruvate, and then glycerate. Glycerate already enters the chloroplasts, where FHA is synthesized from it.

Photorespiration is typical mainly for plants with C3-type photosynthesis. It can be considered harmful, since energy is wasted on the conversion of glycolate into FHA. Apparently, photorespiration arose due to the fact that ancient plants were not ready for a large amount of oxygen in the atmosphere. Initially, their evolution took place in an atmosphere rich in carbon dioxide, and it was he who mainly captured the reaction center of the rubisco enzyme.

C 4 -photosynthesis, or the Hatch-Slack cycle

If in C 3 photosynthesis the first product of the dark phase is phosphoglyceric acid, which includes three carbon atoms, then in the C 4 pathway, the first products are acids containing four carbon atoms: malic, oxaloacetic, aspartic.

C 4 -photosynthesis is observed in many tropical plants, for example, sugar cane, corn.

C 4 -plants absorb carbon monoxide more efficiently, they have almost no photorespiration.

Plants in which the dark phase of photosynthesis proceeds along the C 4 pathway have a special leaf structure. In it, the conducting bundles are surrounded by a double layer of cells. Inner layer- lining of the conducting beam. outer layer mesophyll cells. Chloroplast cell layers differ from each other.

Mesophilic chloroplasts are characterized by large grains, high activity of photosystems, absence of the enzyme RiBP carboxylase (rubisco) and starch. That is, the chloroplasts of these cells are adapted mainly for the light phase of photosynthesis.

In the chloroplasts of the cells of the conducting bundle, the grana are almost not developed, but the concentration of RiBP carboxylase is high. These chloroplasts are adapted for the dark phase of photosynthesis.

Carbon dioxide first enters the mesophyll cells, binds with organic acids, is transported in this form to the sheath cells, is released, and then binds in the same way as in C3 plants. That is, the C 4 -path complements rather than replaces C 3 .

In the mesophyll, CO 2 is added to phosphoenolpyruvate (PEP) to form oxaloacetate (acid), which includes four carbon atoms:

The reaction takes place with the participation of the PEP-carboxylase enzyme, which has a higher affinity for CO 2 than rubisco. In addition, PEP-carboxylase does not interact with oxygen, and therefore is not spent on photorespiration. Thus, the advantage of C 4 photosynthesis lies in the more efficient fixation of carbon dioxide, an increase in its concentration in the lining cells and, consequently, more effective work RiBP-carboxylase, which is almost not consumed for photorespiration.

Oxaloacetate is converted into a 4-carbon dicarboxylic acid (malate or aspartate), which is transported to the chloroplasts of the cells lining the vascular bundles. Here, the acid is decarboxylated (removal of CO2), oxidized (removal of hydrogen) and converted to pyruvate. Hydrogen restores NADP. Pyruvate returns to the mesophyll, where PEP is regenerated from it with the consumption of ATP.

The torn off CO 2 in the chloroplasts of the lining cells goes to the usual C 3 path of the dark phase of photosynthesis, i.e., to the Calvin cycle.

Photosynthesis along the Hatch-Slack pathway requires more energy.

It is believed that the C 4 pathway evolved later than the C 3 pathway and is in many ways an adaptation against photorespiration.

• proceeds only with the participation of sunlight;
• in prokaryotes, the light phase proceeds in the cytoplasm; in eukaryotes, reactions occur on the membranes of the gran chloroplasts, where chlorophyll is located;
• due to the energy of sunlight, the formation of ATP molecules (adenosine triphosphate) occurs, in which it is stored.

Reactions taking place in the light phase

A necessary condition for the light phase of photosynthesis to begin is the presence of sunlight. It all starts with the fact that a photon of light hits chlorophyll (in chloroplasts) and translates its molecules into an excited state. This happens because an electron in the composition of the pigment, having caught a photon of light, goes to a higher energy level.

Then this electron, passing through the chain of carriers (they are proteins sitting in the chloroplast membranes), gives off excess energy to the ATP synthesis reaction.

ATP is a very convenient energy storage molecule. It belongs to high-energy compounds - these are substances, during the hydrolysis of which a large amount of energy is released.

The ATP molecule is also convenient in that it is possible to extract energy from it in two stages: to separate one phosphoric acid residue at a time, each time receiving a portion of energy. It goes further to any needs of the cell and the organism as a whole.

Water splitting

light phase Photosynthesis allows you to get energy from sunlight. It goes not only to the formation of ATP, but also to the splitting of water:

This process is also called photolysis (photo - light, lysis - split). As you can see, as a result, oxygen is released, which is allowed to breathe for all animals and plants.

The protons are used to form NADP-H, which will be used in the dark phase as a source of the same protons.

And the electrons formed during the photolysis of water will compensate for the loss of chlorophyll at the very beginning of the chain. Thus, everything falls into place and the system is again ready to absorb another photon of light.

Light phase value

Plants are autotrophs - organisms that are able to obtain energy not from the breakdown of ready-made substances, but to create it on their own, using only light, carbon dioxide and water. That is why they are producers in the food chain. Animals, unlike plants, cannot perform photosynthesis in their cells.

The mechanism of photosynthesis - video

With or without light energy. It is characteristic of plants. Let us further consider what the dark and light phases of photosynthesis are.

General information

The organ of photosynthesis in higher plants is the leaf. Chloroplasts act as organelles. The membranes of their thylakoids contain photosynthetic pigments. They are carotenoids and chlorophylls. The latter exist in several forms (a, c, b, d). The main one is a-chlorophyll. Its molecule contains a porphyrin "head" with a magnesium atom located in the center, as well as a phytol "tail". The first element is presented as a flat structure. The "head" is hydrophilic, therefore it is located on that part of the membrane that is directed towards the aquatic environment. Phytol "tail" is hydrophobic. Due to this, it keeps the chlorophyll molecule in the membrane. Chlorophyll absorbs blue-violet and red light. They also reflect green, giving the plants their characteristic color. In thylactic membranes, chlorophyll molecules are organized into photosystems. Blue-green algae and plants are characterized by systems 1 and 2. Photosynthetic bacteria have only the first. The second system can decompose H 2 O and release oxygen.

Light phase of photosynthesis

The processes occurring in plants are complex and multi-staged. In particular, two groups of reactions are distinguished. They are the dark and light phases of photosynthesis. The latter proceeds with the participation of the ATP enzyme, electron transport proteins, and chlorophyll. The light phase of photosynthesis occurs in the membranes of the thylactoids. Chlorophyll electrons are excited and leave the molecule. After that, they fall on the outer surface of the thylactic membrane. She, in turn, is charged negatively. After oxidation, the restoration of chlorophyll molecules begins. They take electrons from the water that is present in the intralakoid space. Thus, the light phase of photosynthesis proceeds in the membrane during decay (photolysis): H 2 O + Q light → H + + OH -

Hydroxyl ions are converted into reactive radicals by donating their electrons:

OH - → .OH + e -

OH radicals combine and form free oxygen and water:

4NO. → 2H 2 O + O 2.

In this case, oxygen is removed into the surrounding (external) medium, and protons are accumulated inside the thylactoid in a special "reservoir". As a result, where the light phase of photosynthesis proceeds, the thylactic membrane receives a positive charge due to H + on the one hand. At the same time, due to electrons, it is charged negatively.

Phosphyrylation of ADP

Where the light phase of photosynthesis proceeds, there is a potential difference between the inner and outer surfaces of the membrane. When it reaches 200 mV, protons are pushed through the channels of ATP synthetase. Thus, the light phase of photosynthesis occurs in the membrane when ADP is phosphorylated to ATP. In this case, atomic hydrogen is directed to the reduction of a special carrier of nicotinamide adenine dinucleotide phosphate NADP+ to NADP.H2:

2H + + 2e - + NADP → NADP.H 2

The light phase of photosynthesis thus involves the photolysis of water. It, in turn, is accompanied by three major reactions:

1. Synthesis of ATP.
2. Education NADP.H 2 .
3. The formation of oxygen.

The light phase of photosynthesis is accompanied by the release of the latter into the atmosphere. NADP.H2 and ATP move into the stroma of the chloroplast. This completes the light phase of photosynthesis.

Another group of reactions

The dark phase of photosynthesis does not require light energy. It goes in the stroma of the chloroplast. The reactions are presented as a chain of sequential transformations of carbon dioxide coming from the air. As a result, glucose and other organic substances are formed. The first reaction is fixation. RiBF acts as a carbon dioxide acceptor. The catalyst in the reaction is ribulose bisphosphate carboxylase (enzyme). As a result of carboxylation of RiBP, a six-carbon unstable compound is formed. It almost instantly breaks down into two molecules of FHA (phosphoglyceric acid). This is followed by a cycle of reactions, where it is transformed into glucose through several intermediate products. They use the energies of NADP.H 2 and ATP, which were converted when the light phase of photosynthesis was going on. The cycle of these reactions is called the "Calvin cycle". It can be represented as follows:

6CO 2 + 24H+ + ATP → C 6 H 12 O 6 + 6H 2 O

In addition to glucose, other monomers of organic (complex) compounds are formed during photosynthesis. These include, in particular, fatty acids, glycerol, amino acids, nucleotides.

C3 reactions

They are a type of photosynthesis in which three-carbon compounds are formed as the first product. It is he who is described above as the Calvin cycle. As characteristic features C3 photosynthesis are:

1. RiBP is an acceptor for carbon dioxide.
2. The carboxylation reaction is catalyzed by RiBP carboxylase.
3. A six-carbon substance is formed, which subsequently decomposes into 2 FHAs.

Phosphoglyceric acid is reduced to TF (triose phosphates). Some of them are sent to the regeneration of ribulose biphosphate, and the rest is converted into glucose.

C4 reactions

This type of photosynthesis is characterized by the appearance of four-carbon compounds as the first product. In 1965, it was found that C4 substances appear first in some plants. For example, this has been established for millet, sorghum, sugarcane, corn. These cultures became known as C4 plants. The following year, 1966, Slack and Hatch (Australian scientists) found that they almost completely lack photorespiration. It has also been found that such C4 plants are much more efficient at absorbing carbon dioxide. As a result, the carbon transformation pathway in such cultures has been referred to as the Hatch-Slack pathway.

Conclusion

The importance of photosynthesis is very great. Thanks to him, carbon dioxide is absorbed from the atmosphere every year in huge volumes (billions of tons). Instead, less oxygen is released. Photosynthesis acts as the main source of the formation of organic compounds. Oxygen is involved in the formation of the ozone layer, which protects living organisms from the effects of short-wave UV radiation. During photosynthesis, a leaf absorbs only 1% of all the energy of light falling on it. Its productivity is within 1 g of organic compound per 1 sq. m surface per hour.

Basic concepts and key terms: photosynthesis. Chlorophyll. light phase. dark phase.

Remember! What is plastic exchange?

Think!

The green color is quite often mentioned in the verses of poets. So, Bogdan-Igor Anto-nich has the lines: "... poetry seething and wise, like greens", "... a blizzard of greens, a fire of greens",

"...vegetable rivers rises green flood." Green is the color of renewal, a symbol of youth, tranquility, the color of nature.

Why are plants green?

What are the conditions for photosynthesis?

Photosynthesis (from the Greek photo - light, synthesis - combination) is an extremely complex set of plastic exchange processes. Scientists distinguish three types of photosynthesis: oxygenic (with the release of molecular oxygen in plants and cyanobacteria), anoxic (with the participation of bacteriochlorophyll under anaerobic conditions without oxygen release in photobacteria) and chlorophyll-free (with the participation of bacteriorhodopsins in archaea). At a depth of 2.4 km, green sulfur bacteria GSB1 were found, which use the weak rays of black smokers instead of sunlight. But, as K. Swenson wrote in a monograph on cells: "The primary source of energy for wildlife is the energy of visible light."

The most common in living nature is oxygen photosynthesis, which requires light energy, carbon dioxide, water, enzymes and chlorophyll. Light for photosynthesis is absorbed by chlorophyll, water is delivered to the cells through the pores of the cell wall, carbon dioxide enters the cells by diffusion.

The main photosynthetic pigments are chlorophylls. Chlorophils (from the Greek chloros - green and phylon - leaf) are green pigments of plants, with the participation of which photosynthesis occurs. The green color of chlorophyll is a device for absorbing blue rays and partially red ones. And green rays are reflected from the body of plants, fall on the retina of the human eye, irritate the cones and cause color visual sensations. That's why plants are green!

In addition to chlorophylls, plants have auxiliary carotenoids, cyanobacteria and red algae have phycobilins. Green

and purple bacteria contain bacteriochlorophylls that absorb blue, violet, and even infrared rays.

Photosynthesis occurs in higher plants, algae, cyanobacteria, some archaea, that is, in organisms known as photo-autotrophs. Photosynthesis in plants is carried out in chloroplasts, in cyanobacteria and photobacteria - on internal invaginations of membranes with photopigments.

So, PHOTOSYNTHESIS is the process of formation of organic compounds from inorganic ones using light energy and with the participation of photosynthetic pigments.

What are the features of the light and dark phases of photosynthesis?

In the process of photosynthesis, two stages are distinguished - the light and dark phases (Fig. 49).

The light phase of photosynthesis occurs in the grana of chloroplasts with the participation of light. This stage begins from the moment of absorption of light quanta by the chlorophyll molecule. In this case, the electrons of the magnesium atom in the chlorophyll molecule move to a higher energy level, accumulating potential energy. A significant part of the excited electrons transfers it to others chemical compounds for the formation of ATP and the reduction of NADP (nicotinamide adenine dinucleotide phosphate). This compound with such a long name is the universal biological carrier of hydrogen in the cell. Under the influence of light, the process of decomposition of water - photolysis occurs. This produces electrons (e“), protons (H +) and, as a by-product, molecular oxygen. Hydrogen protons H +, attaching electrons with high energy level, are converted into atomic hydrogen, which is used to reduce NADP+ to NADP. N. Thus, the main processes of the light phase are: 1) photolysis of water (splitting of water under the action of light with the formation of oxygen); 2) reduction of NADP (addition of a hydrogen atom to NADP); 3) photophosphorylation (formation of ATP from ADP).

So, the light phase is a set of processes that ensure the formation of molecular oxygen, atomic hydrogen and ATP due to light energy.

The dark phase of photosynthesis occurs in the stroma of chloroplasts. Its processes do not depend on light and can proceed both in the light and in the dark, depending on the needs of the cell for glucose. The basis of the dark phase is a cyclic reaction called the carbon dioxide fixation cycle, or Calvin cycle. This process was first studied by the American biochemist Melvin Calvin (1911 - 1997), laureate Nobel Prize in Chemistry (1961). In the dark phase, glucose is synthesized from carbon dioxide, hydrogen from NADP and the energy of ATP. CO2 fixation reactions are catalyzed by ribulose bisphosphate carboxylase (Rubisco), the most common enzyme on Earth.

So, the dark phase is a set of cyclic reactions that, thanks to the chemical energy of ATP, provide the formation of glucose using carbon dioxide, which is a source of carbon, and water, a source of hydrogen.

What is the planetary role of photosynthesis?

The importance of photosynthesis for the biosphere cannot be overestimated. It is due to this process that the light energy of the Sun is converted by photo-autotrophs into the chemical energy of carbohydrates, which in general give the primary organic matter. Food chains begin with it, along which energy is transferred to heterotrophic organisms. Plants serve as food for herbivores, which receive the necessary nutrients through this. Then herbivores become food for predators, they also need energy, without which life is impossible.

Only a small part of the Sun's energy is captured by plants and used for photosynthesis. The energy of the Sun is mainly used to evaporate and maintain the temperature regime of the earth's surface. So, only about 40 - 50% of the solar energy penetrates into the biosphere, and only 1 - 2% of the solar energy is converted into synthesized organic matter.

Green plants and cyanobacteria affect the gas composition of the atmosphere. All oxygen in the modern atmosphere is a product of photosynthesis. The formation of the atmosphere completely changed the state of the earth's surface, made possible the emergence of aerobic respiration. Later in the process of evolution, after the formation of the ozone layer, living organisms made landfall. In addition, photosynthesis prevents the accumulation of CO 2 and protects the planet from overheating.

So, photosynthesis is of planetary importance, ensuring the existence of the living nature of planet Earth.

ACTIVITY Match task

Using the table, compare photosynthesis with aerobic respiration and draw a conclusion about the relationship between plastic and energy metabolism.

COMPARATIVE CHARACTERISTICS OF PHOTOSYNTHESIS AND AEROBIC RESPIRIT

Knowledge Application Task

Recognize and name the levels of organization of the process of photosynthesis in plants. Name the adaptations of a plant organism for photosynthesis at different levels of its organization.

ATTITUDE Biology + Literature

K. A. Timiryazev (1843 - 1920), one of the most famous researchers of photosynthesis, wrote: “A microscopic green grain of chlorophyll is a focus, a point in the world space, into which the energy of the Sun flows from one end, and all manifestations of life originate from the other on the ground. It is the real Prometheus, who stole fire from the sky. The ray of sun stolen by him burns both in the shimmering abyss and in the dazzling spark of electricity. The ray of the sun sets in motion the flywheel of a giant steam engine, and the artist's brush, and the poet's pen. Apply your knowledge and prove the statement that the ray of the Sun sets the poet's pen in motion.

	Tasks for self-control
	1. What is photosynthesis? 2. What is chlorophyll? 3. What is the light phase of photosynthesis? 4. What is the dark phase of photosynthesis? 5. What is primary organic matter? 6. How does photosynthesis determine the aerobic respiration of organisms?
	7. What are the conditions for photosynthesis? 8. What are the features of the light and dark phases of photosynthesis? 9. What is the planetary role of photosynthesis?
	10. What are the similarities and differences between photosynthesis and aerobic respiration?

This is textbook material.

Every living thing on the planet needs food or energy to survive. Some organisms feed on other creatures, while others can produce their own nutrients. They make their own food, glucose, in a process called photosynthesis.

Photosynthesis and respiration are interconnected. The result of photosynthesis is glucose, which is stored as chemical energy in the body. This stored chemical energy comes from the conversion of inorganic carbon (carbon dioxide) into organic carbon. The process of breathing releases stored chemical energy.

In addition to the products they produce, plants also need carbon, hydrogen, and oxygen to survive. Water absorbed from the soil provides hydrogen and oxygen. During photosynthesis, carbon and water are used to synthesize food. Plants also need nitrates to make amino acids (an amino acid is an ingredient for making protein). In addition to this, they need magnesium to produce chlorophyll.

The note: Living things that depend on other foods are called. Herbivores such as cows, as well as insect-eating plants, are examples of heterotrophs. Living things that produce their own food are called. Green plants and algae are examples of autotrophs.

In this article, you will learn more about how photosynthesis occurs in plants and the conditions necessary for this process.

Definition of photosynthesis

Photosynthesis is the chemical process by which plants, some and algae produce glucose and oxygen from carbon dioxide and water, using only light as an energy source.

This process is extremely important for life on Earth, because it releases oxygen, on which all life depends.

Why do plants need glucose (food)?

Just like humans and other living things, plants also need food to stay alive. The value of glucose for plants is as follows:

• The glucose obtained from photosynthesis is used during respiration to release the energy the plant needs for other vital processes.
• Plant cells also convert some of the glucose into starch, which is used as needed. For this reason, dead plants are used as biomass because they store chemical energy.
• Glucose is also needed to produce other chemicals such as proteins, fats and plant sugars needed for growth and other essential processes.

Phases of photosynthesis

The process of photosynthesis is divided into two phases: light and dark.

Light phase of photosynthesis

As the name suggests, light phases need sunlight. In light-dependent reactions, the energy of sunlight is absorbed by chlorophyll and converted into stored chemical energy in the form of the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy molecule ATP (adenosine triphosphate). Light phases occur in thylakoid membranes within the chloroplast.

Dark phase of photosynthesis or Calvin cycle

In the dark phase or the Calvin cycle, excited electrons from the light phase provide energy for the formation of carbohydrates from carbon dioxide molecules. The light-independent phases are sometimes called the Calvin cycle because of the cyclic nature of the process.

Although the dark phases do not use light as a reactant (and as a result can occur day or night), they require the products of light-dependent reactions to function. The light-independent molecules depend on the energy carrier molecules ATP and NADPH to create new carbohydrate molecules. After the transfer of energy to the molecules, the energy carriers return to the light phases to obtain more energetic electrons. In addition, several dark phase enzymes are activated by light.

Diagram of the phases of photosynthesis

The note: This means that the dark phases will not continue if the plants are deprived of light for too long, as they use the products of the light phases.

The structure of plant leaves

We cannot fully understand photosynthesis without knowing more about leaf structure. The leaf is adapted to play a vital role in the process of photosynthesis.

The external structure of the leaves

• Area

One of the most important features of plants is the large surface area of ​​the leaves. Most green plants have broad, flat and open leaves that are capable of capturing as much solar energy (sunlight) as is needed for photosynthesis.

• Central vein and petiole

The midrib and petiole join together and form the base of the leaf. The petiole positions the leaf in such a way that it receives as much light as possible.

• leaf blade

Simple leaves have one leaf blade, while compound leaves have several. The leaf blade is one of the most important components of the leaf, which is directly involved in the process of photosynthesis.

• veins

A network of veins in leaves carries water from the stems to the leaves. The released glucose is also sent to other parts of the plant from the leaves through the veins. In addition, these parts of the leaf support and hold the leaf plate flat for greater sunlight capture. The arrangement of veins (venation) depends on the type of plant.

• leaf base

The base of the leaf is its lowest part, which is articulated with the stem. Often, at the base of the leaf there is a pair of stipules.

• leaf edge

Depending on the type of plant, the leaf edge may have various shapes, including: entire, serrated, serrate, notched, crenate, etc.

• Leaf tip

Like the edge of the sheet, the top is various shapes, including: sharp, round, blunt, elongated, retracted, etc.

The internal structure of the leaves

Below is a close diagram of the internal structure of leaf tissues:

• Cuticle

The cuticle acts as the main, protective layer on the surface of the plant. As a rule, it is thicker on the top of the leaf. The cuticle is covered with a wax-like substance that protects the plant from water.

• Epidermis

The epidermis is a layer of cells that is the integumentary tissue of the leaf. Its main function is to protect the internal tissues of the leaf from dehydration, mechanical damage and infections. It also regulates the process of gas exchange and transpiration.

• Mesophyll

The mesophyll is the main tissue of the plant. This is where the process of photosynthesis takes place. In most plants, the mesophyll is divided into two layers: the upper one is palisade and the lower one is spongy.

• Protective cells

Guard cells are specialized cells in the leaf epidermis that are used to control gas exchange. They perform a protective function for the stomata. The stomatal pores become large when water is freely available, otherwise the protective cells become lethargic.

• Stoma

Photosynthesis depends on the penetration of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), obtained as a by-product of photosynthesis, exits the plant through the stomata. When the stomata are open, water is lost through evaporation and must be replenished through the flow of transpiration by water taken up by the roots. Plants are forced to balance the amount of CO2 absorbed from the air and the loss of water through the stomatal pores.

Conditions required for photosynthesis

The following are the conditions that plants need to carry out the process of photosynthesis:

• Carbon dioxide. A colorless, odorless natural gas found in the air and has the scientific designation CO2. It is formed during the combustion of carbon and organic compounds, and also occurs during respiration.
• Water. transparent liquid Chemical substance odorless and tasteless (under normal conditions).
• Light. Although artificial light is also suitable for plants, natural sunlight generally creates the best conditions for photosynthesis because it contains natural ultraviolet radiation, which has positive influence on plants.
• Chlorophyll. It is a green pigment found in the leaves of plants.
• Nutrients and minerals. Chemicals and organic compounds that plant roots absorb from the soil.

What is formed as a result of photosynthesis?

• Glucose;
• Oxygen.

(Light energy is shown in parentheses because it is not a substance)

The note: Plants take in CO2 from the air through their leaves, and water from the soil through their roots. Light energy comes from the Sun. The resulting oxygen is released into the air from the leaves. The resulting glucose can be converted into other substances, such as starch, which is used as an energy store.

If the factors that promote photosynthesis are absent or present in insufficient quantities, this can negatively affect the plant. For example, less light creates favorable conditions for insects that eat the leaves of a plant, while a lack of water slows it down.

Where does photosynthesis take place?

Photosynthesis takes place inside plant cells, in small plastids called chloroplasts. Chloroplasts (mostly found in the mesophyll layer) contain a green substance called chlorophyll. Below are other parts of the cell that work with the chloroplast to carry out photosynthesis.

The structure of a plant cell

Functions of plant cell parts

• : provides structural and mechanical support, protects cells from bacteria, fixes and defines the shape of the cell, controls the rate and direction of growth, and gives shape to plants.
• : provides a platform for most of the chemical processes controlled by enzymes.
• : acts as a barrier, controlling the movement of substances into and out of the cell.
• : as described above, they contain chlorophyll, a green substance that absorbs light energy during photosynthesis.
• : a cavity within the cell cytoplasm that stores water.
• : contains a genetic mark (DNA) that controls the activity of the cell.

Chlorophyll absorbs the light energy needed for photosynthesis. It is important to note that not all color wavelengths of light are absorbed. Plants mainly absorb red and blue wavelengths - they do not absorb light in the green range.

Carbon dioxide during photosynthesis

Plants take in carbon dioxide from the air through their leaves. Carbon dioxide seeps through a small hole at the bottom of the leaf - the stomata.

The underside of the leaf has loosely spaced cells to allow carbon dioxide to reach other cells in the leaf. It also allows the oxygen produced by photosynthesis to easily leave the leaf.

Carbon dioxide is present in the air we breathe in very low concentrations and is a necessary factor in the dark phase of photosynthesis.

Light in the process of photosynthesis

The sheet usually has a large surface area, so it can absorb a lot of light. Its upper surface is protected from water loss, disease and weather by a waxy layer (cuticle). The top of the sheet is where the light falls. This layer of mesophyll is called the palisade. It is adapted to absorb a large amount of light, because it contains many chloroplasts.

In the light phases, the process of photosynthesis increases with more light. More chlorophyll molecules are ionized and more ATP and NADPH are generated if light photons are focused on a green leaf. Although light is extremely important in the light phases, it should be noted that too much of it can damage chlorophyll and reduce the process of photosynthesis.

Light phases are not too dependent on temperature, water or carbon dioxide, although they are all needed to complete the photosynthesis process.

Water during photosynthesis

Plants get the water they need for photosynthesis through their roots. They have root hairs that grow in the soil. The roots are characterized by a large surface area and thin walls, which allows water to easily pass through them.

The image shows plants and their cells with enough water (left) and its lack (right).

The note: Root cells do not contain chloroplasts because they are usually in the dark and cannot photosynthesize.

If the plant does not absorb enough water, it will wilt. Without water, the plant will not be able to photosynthesize fast enough, and may even die.

What is the importance of water for plants?

• Provides dissolved minerals that support plant health;
• Is the medium for transportation;
• Supports stability and uprightness;
• Cools and saturates with moisture;
• Allows for various chemical reactions in plant cells.

Importance of photosynthesis in nature

The biochemical process of photosynthesis uses the energy of sunlight to convert water and carbon dioxide into oxygen and glucose. Glucose is used as building blocks in plants for tissue growth. Thus, photosynthesis is the way in which roots, stems, leaves, flowers and fruits are formed. Without the process of photosynthesis, plants cannot grow or reproduce.

• Producers

Because of their photosynthetic ability, plants are known as producers and serve as the backbone of almost every food chain on Earth. (Algae are the plant's equivalent). All the food we eat comes from organisms that are photosynthetic. We eat these plants directly, or we eat animals such as cows or pigs that consume plant foods.

• Basis of the food chain

Within aquatic systems, plants and algae also form the basis of the food chain. Algae serve as food for, which, in turn, act as a food source for larger organisms. Without photosynthesis in the aquatic environment, life would be impossible.

• Removal of carbon dioxide

Photosynthesis converts carbon dioxide into oxygen. During photosynthesis, carbon dioxide from the atmosphere enters the plant and is then released as oxygen. In today's world where carbon dioxide levels are rising at an alarming rate, any process that removes carbon dioxide from the atmosphere is environmentally important.

• Nutrient cycling

Plants and other photosynthetic organisms play a vital role in nutrient cycling. Nitrogen in the air is fixed in plant tissues and becomes available for making proteins. Trace elements found in the soil can also be incorporated into plant tissue and made available to herbivores further up the food chain.

• photosynthetic addiction

Photosynthesis depends on the intensity and quality of light. At the equator, where sunlight is plentiful all year round and water is not the limiting factor, plants have high growth rates and can become quite large. Conversely, photosynthesis is less common in the deeper parts of the ocean, because light does not penetrate these layers, and as a result, this ecosystem is more barren.