Where the dark phase of photosynthesis occurs. Light phase of photosynthesis. During which phase of photosynthesis does ATP synthesis occur?

- synthesis of organic substances from carbon dioxide and water with the obligatory use of light energy:

6CO 2 + 6H 2 O + Q light → C 6 H 12 O 6 + 6O 2.

In higher plants, the organ of photosynthesis is the leaf, and the organelles of photosynthesis are the chloroplasts (structure of chloroplasts - lecture No. 7). The membranes of chloroplast thylakoids contain photosynthetic pigments: chlorophylls and carotenoids. There are several different types of chlorophyll ( a, b, c, d), the main one is chlorophyll a. In the chlorophyll molecule, a porphyrin “head” with a magnesium atom in the center and a phytol “tail” can be distinguished. The porphyrin “head” is a flat structure, is hydrophilic and therefore lies on the surface of the membrane that faces the aqueous environment of the stroma. The phytol “tail” is hydrophobic and due to this retains the chlorophyll molecule in the membrane.

Chlorophylls absorb red and blue-violet light, reflect green light and therefore give plants their characteristic green color. Chlorophyll molecules in thylakoid membranes are organized into photosystems. Plants and blue-green algae have photosystem-1 and photosystem-2, while photosynthetic bacteria have photosystem-1. Only photosystem-2 can decompose water to release oxygen and take electrons from the hydrogen of water.

Photosynthesis is a complex multi-step process; photosynthesis reactions are divided into two groups: reactions light phase and reactions dark phase.

Light phase

This phase occurs only in the presence of light in thylakoid membranes with the participation of chlorophyll, electron transport proteins and the enzyme ATP synthetase. Under the influence of a quantum of light, chlorophyll electrons are excited, leave the molecule and enter the outer side of the thylakoid membrane, which ultimately becomes negatively charged. Oxidized chlorophyll molecules are reduced, taking electrons from water located in the intrathylakoid space. This leads to the breakdown or photolysis of water:

H 2 O + Q light → H + + OH - .

Hydroxyl ions give up their electrons, becoming reactive radicals.OH:

OH - → .OH + e - .

OH radicals combine to form water and free oxygen:

4NO. → 2H 2 O + O 2.

In this case, oxygen is removed to the external environment, and protons accumulate inside the thylakoid in the “proton reservoir”. As a result, the thylakoid membrane, on the one hand, is charged positively due to H +, and on the other, due to electrons, it is charged negatively. When the potential difference between the outer and inner sides of the thylakoid membrane reaches 200 mV, protons are pushed through the ATP synthetase channels and ADP is phosphorylated to ATP; Atomic hydrogen is used to restore the specific carrier NADP + (nicotinamide adenine dinucleotide phosphate) to NADPH 2:

2H + + 2e - + NADP → NADPH 2.

Thus, during the light phase, photolysis of water occurs, which is accompanied by three the most important processes: 1) ATP synthesis; 2) the formation of NADPH 2; 3) the formation of oxygen. Oxygen diffuses into the atmosphere, ATP and NADPH 2 are transported into the stroma of the chloroplast and participate in the processes of the dark phase.

1 - chloroplast stroma; 2 - grana thylakoid.

Dark phase

This phase occurs in the stroma of the chloroplast. Its reactions do not require light energy, so they occur not only in the light, but also in the dark. Dark phase reactions are a chain of successive transformations of carbon dioxide (coming from the air), leading to the formation of glucose and other organic substances.

The first reaction in this chain is the fixation of carbon dioxide; The carbon dioxide acceptor is a five-carbon sugar. ribulose biphosphate(RiBF); enzyme catalyzes the reaction Ribulose biphosphate carboxylase(RiBP carboxylase). As a result of carboxylation of ribulose bisphosphate, an unstable six-carbon compound is formed, which immediately breaks down into two molecules phosphoglyceric acid(FGK). A cycle of reactions then occurs in which phosphoglyceric acid is converted through a series of intermediates to glucose. These reactions use the energy of ATP and NADPH 2 formed in the light phase; The cycle of these reactions is called the “Calvin cycle”:

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

In addition to glucose, other monomers of complex organic compounds are formed during photosynthesis - amino acids, glycerol and fatty acids, nucleotides. Currently, there are two types of photosynthesis: C 3 - and C 4 photosynthesis.

C 3-photosynthesis

This is a type of photosynthesis in which the first product is three-carbon (C3) compounds. C 3 photosynthesis was discovered before C 4 photosynthesis (M. Calvin). It is C 3 photosynthesis that is described above, under the heading “Dark phase”. Characteristic features of C 3 photosynthesis: 1) the carbon dioxide acceptor is RiBP, 2) the carboxylation reaction of RiBP is catalyzed by RiBP carboxylase, 3) as a result of carboxylation of RiBP, a six-carbon compound is formed, which decomposes into two PGAs. FGK is restored to triose phosphates(TF). Some of the TF is used for the regeneration of RiBP, and some is converted into glucose.

1 - chloroplast; 2 - peroxisome; 3 - mitochondria.

This is a light-dependent absorption of oxygen and release of carbon dioxide. At the beginning of the last century, it was established that oxygen suppresses photosynthesis. As it turned out, for RiBP carboxylase the substrate can be not only carbon dioxide, but also oxygen:

O 2 + RiBP → phosphoglycolate (2C) + PGA (3C).

The enzyme is called RiBP oxygenase. Oxygen is a competitive inhibitor of carbon dioxide fixation. The phosphate group is split off and the phosphoglycolate becomes glycolate, which the plant must utilize. It enters peroxisomes, where it is oxidized to glycine. Glycine enters the mitochondria, where it is oxidized to serine, with the loss of already fixed carbon in the form of CO 2. As a result, two glycolate molecules (2C + 2C) are converted into one PGA (3C) and CO 2. Photorespiration leads to a decrease in the yield of C3 plants by 30-40% ( With 3 plants- plants characterized by C 3 photosynthesis).

C 4 photosynthesis is photosynthesis in which the first product is four-carbon (C 4) compounds. In 1965, it was found that in some plants (sugar cane, corn, sorghum, millet) the first products of photosynthesis are four-carbon acids. These plants were called With 4 plants. In 1966, Australian scientists Hatch and Slack showed that C4 plants have virtually no photorespiration and absorb carbon dioxide much more efficiently. The pathway of carbon transformations in C 4 plants began to be called by Hatch-Slack.

C 4 plants are characterized by a special anatomical structure of the leaf. All vascular bundles are surrounded by a double layer of cells: the outer layer is mesophyll cells, the inner layer is sheath cells. Carbon dioxide is fixed in the cytoplasm of mesophyll cells, the acceptor is phosphoenolpyruvate(PEP, 3C), as a result of carboxylation of PEP, oxaloacetate (4C) is formed. The process is catalyzed PEP carboxylase. Unlike RiBP carboxylase, PEP carboxylase has a greater affinity for CO 2 and, most importantly, does not interact with O 2 . Mesophyll chloroplasts have many grains where light phase reactions actively occur. Dark phase reactions occur in the chloroplasts of the sheath cells.

Oxaloacetate (4C) is converted to malate, which is transported through plasmodesmata into the sheath cells. Here it is decarboxylated and dehydrogenated to form pyruvate, CO 2 and NADPH 2 .

Pyruvate returns to the mesophyll cells and is regenerated using the energy of ATP in PEP. CO 2 is again fixed by RiBP carboxylase to form PGA. PEP regeneration requires ATP energy, so it requires almost twice as much energy as C 3 photosynthesis.

The meaning of photosynthesis

Thanks to photosynthesis, billions of tons of carbon dioxide are absorbed from the atmosphere every year and billions of tons of oxygen are released; photosynthesis is the main source of the formation of organic substances. Oxygen forms the ozone layer, which protects living organisms from short-wave ultraviolet radiation.

During photosynthesis, a green leaf uses only about 1% of the solar energy falling on it; productivity is about 1 g of organic matter per 1 m2 of surface per hour.

Chemosynthesis

The synthesis of organic compounds from carbon dioxide and water, carried out not due to the energy of light, but due to the energy of oxidation of inorganic substances, is called chemosynthesis. Chemosynthetic organisms include some types of bacteria.

Nitrifying bacteria ammonia is oxidized to nitrous and then to nitric acid (NH 3 → HNO 2 → HNO 3).

Iron bacteria convert ferrous iron into oxide iron (Fe 2+ → Fe 3+).

Sulfur bacteria oxidize hydrogen sulfide to sulfur or sulfuric acid (H 2 S + ½O 2 → S + H 2 O, H 2 S + 2O 2 → H 2 SO 4).

As a result of oxidation reactions of inorganic substances, energy is released, which is stored by bacteria in the form of high-energy ATP bonds. ATP is used for the synthesis of organic substances, which proceeds similarly to the reactions of the dark phase of photosynthesis.

Chemosynthetic bacteria contribute to the accumulation of minerals in the soil, improve soil fertility, promote wastewater treatment, etc.

Go to lectures No. 11“The concept of metabolism. Biosynthesis of proteins"

Go to lectures No. 13“Methods of division of eukaryotic cells: mitosis, meiosis, amitosis”

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

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

Photosynthesis occurs with the participation of photosynthetic pigments, which have the unique property of converting the energy of sunlight into chemical bond energy in the form of ATP. Photosynthetic pigments are protein-like substances. The most important of them 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 the structure of the mitochondrion. 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 occurs only in the light in the grana thylakoid membrane. In this phase, chlorophyll absorbs light quanta, produces an ATP molecule, and photolysis of water.

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

Chl → Chl + e -

These electrons are transferred by carriers to the outside, 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 chlorophyll molecules and restore them: the chlorophyll molecules return to a stable state.

Hydrogen protons formed during 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 charged negatively (due to e -). As oppositely charged particles accumulate on both sides of the membrane, the potential difference increases. When the potential difference reaches a critical value, the electric field force begins to push protons through the ATP synthetase channel. The energy released in this case is used to phosphorylate ADP molecules:

ADP + P → 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, and the formation of hydrogen atoms in the form of NADP H2. Oxygen diffuses into the atmosphere, ATP and NADP H2 participate in the processes of the dark phase.

Dark phase photosynthesis occurs in the chloroplast matrix both in the light and in the dark and represents a series of sequential transformations of CO 2 coming from the air in the Calvin cycle. Dark phase reactions are carried out using 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 living things on Earth with the necessary organic substances and oxygen.

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

How to explain such a complex process as photosynthesis briefly and clearly? Plants are the only living organisms that can produce their own food. How do they do it? For growth and receive all the necessary substances from environment: carbon dioxide - from the air, water and - from the soil. They also need energy, which they get from the sun's rays. This energy triggers certain chemical reactions during which carbon dioxide and water are converted into glucose (food) and is photosynthesis. The essence of the process can be explained briefly and clearly even to school-age children.

"Together with the Light"

The word "photosynthesis" comes from two Greek words - "photo" and "synthesis", the combination of which means "together with light." The solar energy is converted into chemical energy. Chemical equation of photosynthesis:

6CO 2 + 12H 2 O + light = C 6 H 12 O 6 + 6O 2 + 6H 2 O.

This means that 6 molecules of carbon dioxide and twelve molecules of water are used (along with sunlight) to produce glucose, resulting in six molecules of oxygen and six molecules of water. If you represent this as a verbal equation, you get the following:

Water + sun => glucose + oxygen + water.

The sun is a very powerful source of energy. People always try to use it to generate electricity, insulate houses, heat water, and so on. Plants “figured out” how to use solar energy millions of years ago because it was necessary for their survival. Photosynthesis can be briefly and clearly explained this way: plants use the light energy of the sun and convert it into chemical energy, the result of which is sugar (glucose), the excess of which is stored as starch in the leaves, roots, stems and seeds of the plant. The sun's energy is transferred to plants, as well as to the animals that eat these plants. When a plant needs nutrients for growth and other life processes, these reserves are very useful.

How do plants absorb energy from the sun?

Talking about photosynthesis briefly and clearly, it is worth addressing the question of how plants manage to absorb solar energy. This occurs due to the special structure of the leaves, which includes green cells - chloroplasts, which contain a special substance called chlorophyll. This is what gives the leaves their green color and is responsible for absorbing energy from sunlight.

Why are most leaves wide and flat?

Photosynthesis occurs in the leaves of plants. The amazing fact is that plants are very well adapted to capture sunlight and absorb carbon dioxide. Thanks to the wide surface, much more light will be captured. It is for this reason that solar panels, which are sometimes installed on the roofs of houses, are also wide and flat. The larger the surface, the better the absorption.

What else is important for plants?

Like people, plants also need beneficial nutrients to stay healthy, grow, and perform their vital functions well. They obtain minerals dissolved in water from the soil through their roots. If the soil lacks mineral nutrients, the plant will not develop normally. Farmers often test the soil to ensure it has enough nutrients for crops to grow. Otherwise, resort to the use of fertilizers containing essential minerals for plant nutrition and growth.

Why is photosynthesis so important?

To explain photosynthesis briefly and clearly for children, it is worth telling that this process is one of the most important chemical reactions in the world. What reasons are there for such a loud statement? First, photosynthesis feeds plants, which in turn feed every other living thing on the planet, including animals and humans. Secondly, as a result of photosynthesis, oxygen necessary for respiration is released into the atmosphere. All living things inhale oxygen and exhale carbon dioxide. Fortunately, plants do the opposite, so they are very important for humans and animals, as they give them the ability to breathe.

Amazing process

Plants, it turns out, also know how to breathe, but, unlike people and animals, they absorb carbon dioxide from the air, not oxygen. Plants drink too. That's why you need to water them, otherwise they will die. With the help of the root system, water and nutrients are transported to all parts of the plant body, and carbon dioxide is absorbed through small holes on the leaves. Trigger to start chemical reaction is sunlight. All metabolic products obtained are used by plants for nutrition, oxygen is released into the atmosphere. This is how you can briefly and clearly explain how the process of photosynthesis occurs.

Photosynthesis: light and dark phases of photosynthesis

The process under consideration consists of two main parts. There are two phases of photosynthesis (description and table below). The first is called the light phase. It occurs only in the presence of light in thylakoid membranes with the participation of chlorophyll, electron transport proteins and the enzyme ATP synthetase. What else does photosynthesis hide? Light and replace each other as day and night progress (Calvin cycles). During the dark phase, the production of that same glucose, food for plants, occurs. This process is also called a light-independent reaction.

Conclusion

From all of the above, the following conclusions can be drawn:

• Photosynthesis is a process that produces energy from the sun.
• Light energy from the sun is converted into chemical energy by chlorophyll.
• Chlorophyll gives plants their green color.
• Photosynthesis occurs in the chloroplasts of plant leaf cells.
• Carbon dioxide and water are necessary for photosynthesis.
• Carbon dioxide enters the plant through tiny holes, stomata, and oxygen exits through them.
• Water is absorbed into the plant through its roots.
• Without photosynthesis there would be no food in the world.

With or without the use of light energy. It is characteristic of plants. Let us next 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. Photosynthetic pigments are present in the membranes of their thylakoids. 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 aqueous environment. The phytol "tail" is hydrophobic. Due to this, it retains the chlorophyll molecule in the membrane. Chlorophylls absorb blue-violet and red light. They also reflect green, giving plants their characteristic color. In thylactoid 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-stage. In particular, two groups of reactions are distinguished. They are the dark and light phases of photosynthesis. The latter occurs with the participation of the enzyme ATP, electron transfer proteins, and chlorophyll. The light phase of photosynthesis occurs in thylactoid membranes. Chlorophyll electrons become excited and leave the molecule. After this, they end up on the outer surface of the thylactoid membrane. It, in turn, becomes negatively charged. After oxidation, the reduction of chlorophyll molecules begins. They take electrons from water, which is present in the intralacoid space. Thus, the light phase of photosynthesis occurs in the membrane during decay (photolysis): H 2 O + Q light → H + + OH -

Hydroxyl ions turn into reactive radicals, donating their electrons:

OH - → .OH + e -

OH radicals combine to form free oxygen and water:

4NO. → 2H 2 O + O 2.

In this case, oxygen is removed into the surrounding (external) environment, and protons accumulate inside the thylactoid in a special “reservoir”. As a result, where the light phase of photosynthesis occurs, the thylactoid membrane receives a positive charge due to H + on one side. At the same time, due to electrons, it is charged negatively.

Phosphyrylation of ADP

Where the light phase of photosynthesis occurs, there is a potential difference between the inner and outer surfaces of the membrane. When it reaches 200 mV, protons begin to be 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 sent to restore the special carrier nicotinamide adenine dinucleotide phosphate NADP+ to NADP.H2:

2Н + + 2е — + NADP → NADP.Н 2

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

1. ATP synthesis.
2. Formation of NADP.H 2.
3. 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 in the form of 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. Ribulose biphosphate (five-carbon sugar) RiBP acts as a carbon dioxide acceptor. The catalyst in the reaction is ribulose biphosphate 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 PGA (phosphoglyceric acid). After this, a cycle of reactions occurs where it is transformed into glucose through several intermediate products. They use the energy of NADP.H 2 and ATP, which were converted during the light phase of photosynthesis. 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 and nucleotides.

C3 reactions

They are a type of photosynthesis that produces three-carbon compounds as the first product. It is this that is described above as the Calvin cycle. The characteristic features of 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 breaks down into 2 FHA.

Phosphoglyceric acid is reduced to TP (triose phosphates). Some of them are used for 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 discovered that C4 substances appear first in some plants. For example, this has been established for millet, sorghum, sugar cane, and corn. These crops became known as C4 plants. The next year, 1966, Slack and Hatch (Australian scientists) discovered that they almost completely lack photorespiration. It was also found that such C4 plants absorb carbon dioxide much more efficiently. As a result, the pathway of carbon transformation in such crops began to be called the Hatch-Slack pathway.

Conclusion

The importance of photosynthesis is very great. Thanks to it, carbon dioxide is absorbed from the atmosphere in huge volumes (billions of tons) every year. Instead, no 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 the total energy of light falling on it. Its productivity is within 1 g organic compound per 1 sq. m of surface per hour.