How photosynthesis occurs. Photosynthesis

Photosynthesis occurs in plants (mainly in their leaves) in the light. This is a process in which the organic substance glucose (one of the types of sugars) is formed from carbon dioxide and water. Next, glucose in the cells is converted into a more complex substance, starch. Both glucose and starch are carbohydrates.

The process of photosynthesis not only produces organic matter, but also produces oxygen as a by-product.

Carbon dioxide and water are inorganic substances, while glucose and starch are organic. Therefore, it is often said that photosynthesis is the process of formation of organic substances from inorganic substances in the light. Only plants, some single-celled eukaryotes, and some bacteria are capable of photosynthesis. There is no such process in the cells of animals and fungi, so they are forced to absorb organic substances from the environment. In this regard, plants are called autotrophs, and animals and fungi are called heterotrophs.

The process of photosynthesis in plants occurs in chloroplasts, which contain the green pigment chlorophyll.

So, for photosynthesis to occur, you need:

chlorophyll,

carbon dioxide.

During the process of photosynthesis the following are formed:

organic matter,

oxygen.

Plants are adapted to capture light. In many herbaceous plants, the leaves are collected in a so-called basal rosette, when the leaves do not shade each other. Trees are characterized by a leaf mosaic, in which the leaves grow in such a way as to shade each other as little as possible. In plants, leaf blades can turn towards the light due to the bending of the leaf petioles. With all this, there are shade-loving plants that can only grow in the shade.

Water for photosynthesis enters the leaves from the roots along the stem. Therefore, it is important that the plant receives enough moisture. With a lack of water and certain minerals, the process of photosynthesis is inhibited.

Carbon dioxide for photosynthesis is taken directly from the air by the leaves. Oxygen, which is produced by the plant during photosynthesis, on the contrary, is released into the air. Gas exchange is facilitated by intercellular spaces (spaces between cells).

Organic substances formed during photosynthesis are partly used in the leaves themselves, but mainly flow into all other organs and are converted into other organic substances, used in energy metabolism, and converted into reserve nutrients.

As the name implies, photosynthesis is essentially the natural synthesis of organic substances, converting CO2 from the atmosphere and water into glucose and free oxygen.

This requires the presence of solar energy.

The chemical equation for the process of photosynthesis can generally be represented as follows:

Photosynthesis has two phases: dark and light. The chemical reactions of the dark phase of photosynthesis differ significantly from the reactions of the light phase, but the dark and light phases of photosynthesis depend on each other.

The light phase can occur in plant leaves exclusively in sunlight. For dark, the presence of carbon dioxide is necessary, which is why the plant must constantly absorb it from the atmosphere. All comparative characteristics of the dark and light phases of photosynthesis will be provided below. For this purpose, a comparative table “Phases of Photosynthesis” was created.

Light phase of photosynthesis

The main processes in the light phase of photosynthesis occur in the thylakoid membranes. It involves chlorophyll, electron transport proteins, ATP synthetase (an enzyme that accelerates the reaction) and sunlight.

Further, the reaction mechanism can be described as follows: when sunlight hits the green leaves of plants, chlorophyll electrons (negative charge) are excited in their structure, which, having passed into an active state, leave the pigment molecule and end up on the outside of the thylakoid, the membrane of which is also negatively charged. At the same time, chlorophyll molecules are oxidized and the already oxidized ones are reduced, thus taking electrons from the water that is in the leaf structure.

This process leads to the fact that water molecules disintegrate, and the ions created as a result of photolysis of water give up their electrons and turn into OH radicals that are capable of carrying out further reactions. These reactive OH radicals then combine to create full-fledged water molecules and oxygen. In this case, free oxygen escapes into the external environment.

As a result of all these reactions and transformations, the leaf thylakoid membrane on one side is charged positively (due to the H+ ion), and on the other - negatively (due to electrons). When the difference between these charges on the two sides of the membrane reaches more than 200 mV, protons pass through special channels of the ATP synthetase enzyme and due to this, ADP is converted to ATP (as a result of the phosphorylation process). And atomic hydrogen, which is released from water, restores the specific carrier NADP+ to NADP·H2. As we can see, as a result of the light phase of photosynthesis, three main processes occur:

1. ATP synthesis;
2. creation of NADP H2;
3. formation of free oxygen.

The latter is released into the atmosphere, and NADP H2 and ATP take part in the dark phase of photosynthesis.

Dark phase of photosynthesis

The dark and light phases of photosynthesis are characterized by large energy expenditures on the part of the plant, but the dark phase proceeds faster and requires less energy. Dark phase reactions do not require sunlight, so they can occur both day and night.

All the main processes of this phase occur in the stroma of the plant chloroplast and represent a unique chain of successive transformations of carbon dioxide from the atmosphere. The first reaction in such a chain is the fixation of carbon dioxide. To make it happen more smoothly and faster, nature provided the enzyme RiBP-carboxylase, which catalyzes the fixation of CO2.

Next, a whole cycle of reactions occurs, the completion of which is the conversion of phosphoglyceric acid into glucose (natural sugar). All these reactions use the energy of ATP and NADP H2, which were created in the light phase of photosynthesis. In addition to glucose, photosynthesis also produces other substances. Among them are various amino acids, fatty acids, glycerol, and nucleotides.

Phases of photosynthesis: comparison table

Photosynthesis - video

There are three types of plastids:

• chloroplasts- green, function - photosynthesis
• chromoplasts- red and yellow, are dilapidated chloroplasts, can give bright colors to petals and fruits.
• leucoplasts- colorless, function - storage of substances.

The structure of chloroplasts

Covered with two membranes. The outer membrane is smooth, the inner one has outgrowths inward - thylakoids. Stacks of short thylakoids are called grains, they increase the area of ​​the inner membrane in order to accommodate as many photosynthetic enzymes as possible.

The internal environment of the chloroplast is called the stroma. It contains circular DNA and ribosomes, due to which chloroplasts independently make part of their proteins, which is why they are called semi-autonomous organelles. (It is believed that plastids were previously free bacteria that were absorbed by a large cell, but not digested.)

Photosynthesis (simple)

In the green leaves in the light
In chloroplasts using chlorophyll
From carbon dioxide and water
Glucose and oxygen are synthesized.

Photosynthesis (medium difficulty)

1. Light phase.
Occurs in the light in the grana of chloroplasts. Under the influence of light, decomposition (photolysis) of water occurs, producing oxygen, which is released, as well as hydrogen atoms (NADP-H) and ATP energy, which are used in the next stage.

2. Dark phase.
Occurs both in light and in darkness (light is not needed), in the stroma of chloroplasts. From carbon dioxide obtained from the environment and hydrogen atoms obtained in the previous stage, glucose is synthesized using the energy of ATP obtained in the previous stage.

Choose one, the most correct option. Cellular organelle containing a DNA molecule
1) ribosome
2) chloroplast
3) cell center
4) Golgi complex

Answer

Choose one, the most correct option. In the synthesis of what substance do hydrogen atoms participate in the dark phase of photosynthesis?
1) NADP-2H
2) glucose
3) ATP
4) water

Answer

Choose one, the most correct option. Which cellular organelle contains DNA?
1) vacuole
2) ribosome
3) chloroplast
4) lysosome

Answer

Choose one, the most correct option. In cells, primary glucose synthesis occurs in
1) mitochondria
2) endoplasmic reticulum
3) Golgi complex
4) chloroplasts

Answer

Choose one, the most correct option. Oxygen molecules during photosynthesis are formed due to the decomposition of molecules
1) carbon dioxide
2) glucose
3) ATP
4) water

Answer

Choose one, the most correct option. The process of photosynthesis should be considered as one of the important links in the carbon cycle in the biosphere, since during its
1) plants absorb carbon from inanimate nature into living matter
2) plants release oxygen into the atmosphere
3) organisms release carbon dioxide during respiration
4) industrial production replenishes the atmosphere with carbon dioxide

Answer

Choose one, the most correct option. Are the following statements about photosynthesis correct? A) In the light phase, the energy of light is converted into the energy of chemical bonds of glucose. B) Dark phase reactions occur on thylakoid membranes, into which carbon dioxide molecules enter.
1) only A is correct
2) only B is correct
3) both judgments are correct
4) both judgments are incorrect

Answer

CHLOROPLAST
1. All of the following characteristics, except two, can be used to describe the structure and functions of the chloroplast. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) is a double-membrane organelle
2) has its own closed DNA molecule
3) is a semi-autonomous organelle
4) forms the spindle
5) filled with cell sap with sucrose

Answer

2. Select three features of the structure and functions of chloroplasts
1) internal membranes form cristae
2) many reactions occur in grains
3) glucose synthesis occurs in them
4) are the site of lipid synthesis
5) consist of two different particles
6) double-membrane organelles

Answer

3. Choose three correct answers out of six and write down the numbers under which they are indicated. The following processes occur in the chloroplasts of plant cells:
1) hydrolysis of polysaccharides
2) breakdown of pyruvic acid
3) photolysis of water
4) breakdown of fats into fatty acids and glycerol
5) synthesis of carbohydrates
6) ATP synthesis

Answer

CHLOROPLASTS EXCEPT
1. The following terms, except two, are used to describe plastids. Identify two terms that “drop out” from the general list and write down the numbers under which they are indicated in the table.
1) pigment
2) glycocalyx
3) grana
4) crista
5) thylakoid

Answer

2. All but two of the following characteristics can be used to describe chloroplasts. Identify two characteristics that “fall out” from the general list and write down the numbers under which they are indicated.
1) double-membrane organelles
2) use light energy to create organic substances
3) internal membranes form cristae
4) glucose synthesis occurs on the cristae membranes
5) the starting materials for carbohydrate synthesis are carbon dioxide and water

Answer

STROMA - THYLAKOID
Establish a correspondence between the processes and their localization in chloroplasts: 1) stroma, 2) thylakoid. Write numbers 1 and 2 in the order corresponding to the letters.
A) use of ATP
B) photolysis of water
B) stimulation of chlorophyll
D) formation of pentose
D) electron transfer along the enzyme chain

Answer

1. The features listed below, except two, are used to describe the structure and functions of the cell organelle depicted. Identify two characteristics that “fall out” from the general list and write down the numbers under which they are indicated.

2) accumulates ATP molecules
3) provides photosynthesis

5) has semi-autonomy

Answer

2. All of the characteristics listed below, except two, can be used to describe the cell organelle shown in the figure. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) single-membrane organelle
2) consists of cristae and chromatin
3) contains circular DNA
4) synthesizes its own protein
5) capable of division

Answer

The features listed below, except two, are used to describe the structure and functions of the cell organelle depicted. Identify two characteristics that “fall out” from the general list and write down the numbers under which they are indicated.
1) breaks down biopolymers into monomers
2) accumulates ATP molecules
3) provides photosynthesis
4) refers to double-membrane organelles
5) has semi-autonomy

Answer

LIGHT
1. Choose two correct answers out of five and write down the numbers under which they are indicated. During the light phase of photosynthesis in the cell
1) oxygen is formed as a result of the decomposition of water molecules
2) carbohydrates are synthesized from carbon dioxide and water
3) polymerization of glucose molecules occurs to form starch
4) ATP molecules are synthesized
5) the energy of ATP molecules is spent on the synthesis of carbohydrates

Answer

2. Identify three correct statements from the general list, and write down the numbers under which they are indicated in the table. During the light phase of photosynthesis occurs
1) photolysis of water


4) hydrogen connection with the NADP+ transporter

Answer

LIGHT EXCEPT
1. All of the signs below, except two, can be used to determine the processes of the light phase of photosynthesis. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) photolysis of water
2) reduction of carbon dioxide to glucose
3) synthesis of ATP molecules using the energy of sunlight
4) formation of molecular oxygen
5) use of the energy of ATP molecules for the synthesis of carbohydrates

Answer

2. All of the characteristics listed below, except two, can be used to describe the light phase of photosynthesis. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) a by-product is formed - oxygen
2) occurs in the stroma of the chloroplast
3) binding of carbon dioxide
4) ATP synthesis
5) photolysis of water

Answer

3. All of the characteristics listed below, except two, are used to describe the stage of photosynthesis shown in the figure. Identify two characteristics that “fall out” from the general list and write down the numbers under which they are indicated. At this stage
1) glucose synthesis occurs
2) the Calvin cycle begins
3) ATP is synthesized
4) photolysis of water occurs
5) hydrogen combines with NADP

Answer

DARK
Choose three options. The dark phase of photosynthesis is characterized by
1) the occurrence of processes on the internal membranes of chloroplasts
2) glucose synthesis
3) fixation of carbon dioxide
4) the course of processes in the stroma of chloroplasts
5) the presence of photolysis of water
6) ATP formation

Answer

DARK EXCEPT
1. The concepts listed below, except two, are used to describe the dark phase of photosynthesis. Identify two concepts that “fall out” from the general list and write down the numbers under which they are indicated.

2) photolysis
3) oxidation of NADP 2H
4) grana
5) stroma

Answer

2. All of the characteristics listed below, except two, are used to describe the dark phase of photosynthesis. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) oxygen formation
2) carbon dioxide fixation
3) use of ATP energy
4) glucose synthesis
5) stimulation of chlorophyll

Answer

LIGHT - DARK
1. Establish a correspondence between the process of photosynthesis and the phase in which it occurs: 1) light, 2) dark. Write numbers 1 and 2 in the correct order.
A) formation of NADP-2H molecules
B) release of oxygen
B) monosaccharide synthesis
D) synthesis of ATP molecules
D) addition of carbon dioxide to carbohydrate

Answer

2. Establish a correspondence between the characteristic and the phase of photosynthesis: 1) light, 2) dark. Write numbers 1 and 2 in the correct order.
A) photolysis of water
B) carbon dioxide fixation
B) splitting of ATP molecules
D) excitation of chlorophyll by light quanta
D) glucose synthesis

Answer

3. Establish a correspondence between the process of photosynthesis and the phase in which it occurs: 1) light, 2) dark. Write numbers 1 and 2 in the correct order.
A) formation of NADP*2H molecules
B) release of oxygen
B) glucose synthesis
D) synthesis of ATP molecules
D) reduction of carbon dioxide

Answer

4. Establish a correspondence between the processes and the phase of photosynthesis: 1) light, 2) dark. Write numbers 1 and 2 in the order corresponding to the letters.
A) polymerization of glucose
B) carbon dioxide binding
B) ATP synthesis
D) photolysis of water
D) formation of hydrogen atoms
E) glucose synthesis

Answer

5. Establish a correspondence between the phases of photosynthesis and their characteristics: 1) light, 2) dark. Write numbers 1 and 2 in the order corresponding to the letters.
A) photolysis of water occurs
B) ATP is formed
B) oxygen is released into the atmosphere
D) proceeds with the expenditure of ATP energy
D) reactions can occur both in light and in darkness

Answer

6 Sat. Establish a correspondence between the phases of photosynthesis and their characteristics: 1) light, 2) dark. Write numbers 1 and 2 in the order corresponding to the letters.
A) restoration of NADP+
B) transport of hydrogen ions across the membrane
B) occurs in the grana of chloroplasts
D) carbohydrate molecules are synthesized
D) chlorophyll electrons move to a higher energy level
E) ATP energy is consumed

Answer

FORMING 7:
A) movement of excited electrons
B) conversion of NADP-2R to NADP+
B) oxidation of NADPH
D) molecular oxygen is formed
D) processes occur in the stroma of the chloroplast

SUBSEQUENCE
1. Establish the correct sequence of processes occurring during photosynthesis. Write down the numbers under which they are indicated in the table.
1) Use of carbon dioxide
2) Oxygen formation
3) Carbohydrate synthesis
4) Synthesis of ATP molecules
5) Excitation of chlorophyll

Answer

2. Establish the correct sequence of photosynthesis processes.
1) conversion of solar energy into ATP energy
2) formation of excited electrons of chlorophyll
3) carbon dioxide fixation
4) formation of starch
5) conversion of ATP energy into glucose energy

Answer

3. Establish the sequence of processes occurring during photosynthesis. Write down the corresponding sequence of numbers.
1) carbon dioxide fixation
2) ATP breakdown and energy release
3) glucose synthesis
4) synthesis of ATP molecules
5) stimulation of chlorophyll

Answer

PHOTOSYNTHESIS
Select cell organelles and their structures involved in the process of photosynthesis.
1) lysosomes
2) chloroplasts
3) thylakoids
4) grains
5) vacuoles
6) ribosomes

Answer

PHOTOSYNTHESIS EXCEPT
All but two of the following characteristics can be used to describe the process of photosynthesis. Identify two characteristics that “drop out” from the general list, and write down the numbers under which they are indicated in your answer.
1) Light energy is used to carry out the process.
2) The process occurs in the presence of enzymes.
3) The central role in the process belongs to the chlorophyll molecule.
4) The process is accompanied by the breakdown of the glucose molecule.
5) The process cannot occur in prokaryotic cells.

Answer

Analyze the table. Fill in the blank cells of the table using the concepts and terms given in the list. For each lettered cell, select the appropriate term from the list provided.
1) thylakoid membranes
2) light phase
3) fixation of inorganic carbon
4) photosynthesis of water
5) dark phase
6) cell cytoplasm

Answer

Analyze the table “Reactions of Photosynthesis”. For each letter, select the corresponding term from the list provided.
1) oxidative phosphorylation
2) oxidation of NADP-2H
3) thylakoid membranes
4) glycolysis
5) addition of carbon dioxide to pentose
6) oxygen formation
7) formation of ribulose diphosphate and glucose
8) synthesis of 38 ATP

Answer

Insert into the text “Synthesis of organic substances in a plant” the missing terms from the proposed list, using numerical notations. Write down the selected numbers in the order corresponding to the letters. Plants store the energy necessary for their existence in the form of organic substances. These substances are synthesized during __________ (A). This process occurs in leaf cells in __________ (B) - special green plastids. They contain a special green substance – __________ (B). A prerequisite for the formation of organic substances in addition to water and carbon dioxide is __________ (D).
List of terms:
1) breathing
2) evaporation
3) leukoplast
4) food
5) light
6) photosynthesis
7) chloroplast
8) chlorophyll

Answer

Establish a correspondence between the stages of the process and the processes: 1) photosynthesis, 2) protein biosynthesis. Write numbers 1 and 2 in the correct order.
A) release of free oxygen
B) formation of peptide bonds between amino acids
B) synthesis of mRNA on DNA
D) translation process
D) restoration of carbohydrates
E) conversion of NADP+ to NADP 2H

Answer

© D.V. Pozdnyakov, 2009-2019

Plants obtain water and minerals from their roots. The leaves provide organic nutrition to the plants. Unlike roots, they are not in the soil, but in the air, therefore they provide not soil, but air nutrition.

From the history of studying aerial nutrition of plants

Knowledge about plant nutrition accumulated gradually. About 350 years ago, the Dutch scientist Jan Helmont first experimented with the study of plant nutrition. He grew willow in a clay pot filled with soil, adding only water. The scientist carefully weighed the fallen leaves. After five years, the mass of the willow together with fallen leaves increased by 74.5 kg, and the mass of the soil decreased by only 57 g. Based on this, Helmont came to the conclusion that all substances in the plant are formed not from soil, but from water. The opinion that the plant increases in size only due to water persisted until the end of the 18th century.

In 1771, the English chemist Joseph Priestley studied carbon dioxide, or, as he called it, “spoiled air” and made a remarkable discovery. If you light a candle and cover it with a glass cover, then after it burns a little, it will go out. A mouse under such a hood begins to suffocate. However, if you place a mint branch under the cap with the mouse, the mouse does not suffocate and continues to live. This means that plants “correct” the air spoiled by the breathing of animals, that is, they convert carbon dioxide into oxygen.

In 1862, the German botanist Julius Sachs proved through experiments that green plants not only produce oxygen, but also create organic substances that serve as food for all other organisms.

Photosynthesis

The main difference between green plants and other living organisms is the presence in their cells of chloroplasts containing chlorophyll. Chlorophyll has the property of capturing solar rays, the energy of which is necessary for the creation of organic substances. The process of formation of organic matter from carbon dioxide and water using solar energy is called photosynthesis (Greek pbo1os light). During the process of photosynthesis, not only organic substances - sugars - are formed, but oxygen is also released.

Schematically, the process of photosynthesis can be depicted as follows:

Water is absorbed by the roots and moves through the conductive system of the roots and stem to the leaves. Carbon dioxide is a component of air. It enters the leaves through open stomata. The absorption of carbon dioxide is facilitated by the structure of the leaf: the flat surface of the leaf blades, which increases the area of ​​contact with air, and the presence of a large number of stomata in the skin.

Sugars formed as a result of photosynthesis are converted into starch. Starch is an organic substance that does not dissolve in water. Kgo can be easily detected using an iodine solution.

Evidence of starch formation in leaves exposed to light

Let us prove that in the green leaves of plants starch is formed from carbon dioxide and water. To do this, consider an experiment that was once carried out by Julius Sachs.

A houseplant (geranium or primrose) is kept in the dark for two days so that all the starch is used up for vital processes. Then several leaves are covered on both sides with black paper so that only part of them is covered. During the day, the plant is exposed to light, and at night it is additionally illuminated using a table lamp.

After a day, the leaves under study are cut off. To find out in which part of the leaf starch is formed, the leaves are boiled in water (to swell the starch grains) and then kept in hot alcohol (the chlorophyll dissolves and the leaf becomes discolored). Then the leaves are washed in water and treated with a weak solution of iodine. Thus, areas of leaves that have been exposed to light acquire a blue color from the action of iodine. This means that starch was formed in the cells of the illuminated part of the leaf. Therefore, photosynthesis occurs only in light.

Evidence for the need for carbon dioxide for photosynthesis

To prove that carbon dioxide is necessary for the formation of starch in the leaves, the houseplant is also first kept in the dark. One of the leaves is then placed in a flask with a small amount of lime water. The flask is closed with a cotton swab. The plant is exposed to light. Carbon dioxide is absorbed by lime water, so it will not be in the flask. The leaf is cut off and, just as in the previous experiment, examined for the presence of starch. It is kept in hot water and alcohol and treated with iodine solution. However, in this case, the result of the experiment will be different: the leaf does not turn blue, because it does not contain starch. Therefore, for the formation of starch, in addition to light and water, carbon dioxide is needed.

Thus, we answered the question of what food the plant receives from the air. Experience has shown that it is carbon dioxide. It is necessary for the formation of organic matter.

Organisms that independently create organic substances to build their body are called autotrophamnes (Greek autos - itself, trophe - food).

Evidence of oxygen production during photosynthesis

To prove that during photosynthesis, plants release oxygen into the external environment, consider an experiment with the aquatic plant Elodea. Elodea shoots are dipped into a vessel with water and covered with a funnel on top. Place a test tube filled with water at the end of the funnel. The plant is exposed to light for two to three days. In the light, elodea produces gas bubbles. They accumulate at the top of the test tube, displacing water. In order to find out what kind of gas it is, the test tube is carefully removed and a smoldering splinter is introduced into it. The splinter flashes brightly. This means that oxygen has accumulated in the flask, supporting combustion.

The cosmic role of plants

Plants containing chlorophyll are able to absorb solar energy. Therefore K.A. Timiryazev called their role on Earth cosmic. Some of the solar energy stored in organic matter can be stored for a long time. Coal, peat, oil are formed by substances that in ancient geological times were created by green plants and absorbed the energy of the Sun. By burning natural combustible materials, a person releases energy stored millions of years ago by green plants.

Non-chlorophyll photosynthesis

Spatial localization

Plant photosynthesis occurs in chloroplasts: isolated double-membrane organelles of the cell. Chloroplasts can be found in the cells of fruits and stems, but the main organ of photosynthesis, anatomically adapted for its conduct, is the leaf. In the leaf, the palisade parenchyma tissue is richest in chloroplasts. In some succulents with degenerate leaves (such as cacti), the main photosynthetic activity is associated with the stem.

Light for photosynthesis is more fully captured due to the flat leaf shape, which provides a high surface to volume ratio. Water is delivered from the root through a developed network of vessels (leaf veins). Carbon dioxide enters partly by diffusion through the cuticle and epidermis, but most of it diffuses into the leaf through the stomata and through the leaf through the intercellular space. Plants that carry out CAM photosynthesis have developed special mechanisms for the active assimilation of carbon dioxide.

The internal space of the chloroplast is filled with colorless contents (stroma) and is penetrated by membranes (lamellae), which, when connected to each other, form thylakoids, which in turn are grouped into stacks called grana. The intrathylakoid space is separated and does not communicate with the rest of the stroma; it is also assumed that the internal space of all thylakoids communicates with each other. The light stages of photosynthesis are confined to membranes; autotrophic fixation of CO 2 occurs in the stroma.

Chloroplasts have their own DNA, RNA, ribosomes (70s type), and protein synthesis occurs (although this process is controlled from the nucleus). They are not synthesized again, but are formed by dividing the previous ones. All this made it possible to consider them the descendants of free cyanobacteria that became part of the eukaryotic cell during the process of symbiogenesis.

Photosystem I

Light-harvesting complex I contains approximately 200 chlorophyll molecules.

In the reaction center of the first photosystem there is a dimer of chlorophyll a with an absorption maximum at 700 nm (P700). After excitation by a light quantum, it restores the primary acceptor - chlorophyll a, which restores the secondary acceptor (vitamin K 1 or phylloquinone), after which the electron is transferred to ferredoxin, which reduces NADP using the enzyme ferredoxin-NADP reductase.

The plastocyanin protein, reduced in the b 6 f complex, is transported to the reaction center of the first photosystem from the side of the intrathylakoid space and transfers an electron to the oxidized P700.

Cyclic and pseudocyclic electron transport

In addition to the complete non-cyclic electron path described above, a cyclic and pseudo-cyclic path has been discovered.

The essence of the cyclic pathway is that ferredoxin, instead of NADP, reduces plastoquinone, which transfers it back to the b 6 f complex. This results in a larger proton gradient and more ATP, but no NADPH.

In the pseudocyclic pathway, ferredoxin reduces oxygen, which is further converted into water and can be used in photosystem II. In this case, NADPH is also not formed.

Dark stage

In the dark stage, with the participation of ATP and NADPH, CO 2 is reduced to glucose (C 6 H 12 O 6). Although light is not required for this process, it is involved in its regulation.

C 3 photosynthesis, Calvin cycle

The third stage involves 5 PHA molecules, which, through the formation of 4-, 5-, 6- and 7-carbon compounds, are combined into 3 5-carbon ribulose-1,5-biphosphate, which requires 3ATP.

Finally, two PHAs are required for glucose synthesis. To form one of its molecules, 6 cycle revolutions, 6 CO 2, 12 NADPH and 18 ATP are required.

C 4 photosynthesis

Main articles: Hatch-Slack-Karpilov cycle, C4 photosynthesis

At a low concentration of CO 2 dissolved in the stroma, ribulose biphosphate carboxylase catalyzes the oxidation reaction of ribulose-1,5-biphosphate and its breakdown into 3-phosphoglyceric acid and phosphoglycolic acid, which is forced to be used in the process of photorespiration.

To increase CO2 concentration, type 4 C plants changed their leaf anatomy. The Calvin cycle is localized in the sheath cells of the vascular bundle; in the mesophyll cells, under the action of PEP carboxylase, phosphoenolpyruvate is carboxylated to form oxaloacetic acid, which is converted into malate or aspartate and transported to the sheath cells, where it is decarboxylated to form pyruvate, which is returned to the mesophyll cells.

With 4, photosynthesis is practically not accompanied by losses of ribulose-1,5-biphosphate from the Calvin cycle, and therefore is more efficient. However, it requires not 18, but 30 ATP for the synthesis of 1 glucose molecule. This is justified in the tropics, where the hot climate requires keeping the stomata closed, which prevents the entry of CO 2 into the leaf, as well as with a ruderal life strategy.

photosynthesis itself

Later it was found that in addition to releasing oxygen, plants absorb carbon dioxide and, with the participation of water, synthesize organic matter in the light. Based on the law of conservation of energy, Robert Mayer postulated that plants convert the energy of sunlight into the energy of chemical bonds. W. Pfeffer called this process photosynthesis.

Chlorophylls were first isolated by P. J. Peltier and J. Caventou. M. S. Tsvet managed to separate the pigments and study them separately using the chromatography method he created. The absorption spectra of chlorophyll were studied by K. A. Timiryazev, who, developing Mayer’s principles, showed that it is the absorbed rays that make it possible to increase the energy of the system, creating high-energy C-C bonds instead of weak C-O and O-H bonds (before that it was believed that in photosynthesis uses yellow rays that are not absorbed by leaf pigments). This was done thanks to the method he created for accounting for photosynthesis based on absorbed CO 2: during experiments on illuminating a plant with light of different wavelengths (different colors), it turned out that the intensity of photosynthesis coincides with the absorption spectrum of chlorophyll.

The redox nature of photosynthesis (both oxygenic and anoxygenic) was postulated by Cornelis van Niel. This meant that oxygen in photosynthesis is formed entirely from water, which was experimentally confirmed by A.P. Vinogradov in experiments with an isotope label. Robert Hill found that the process of water oxidation (and oxygen release) and CO 2 assimilation can be separated. W. D. Arnon established the mechanism of the light stages of photosynthesis, and the essence of the CO 2 assimilation process was revealed by Melvin Calvin using carbon isotopes in the late 1940s, for which he was awarded the Nobel Prize.

Other facts

see also

Literature

• Hall D., Rao K. Photosynthesis: Transl. from English - M.: Mir, 1983.
• Plant physiology / ed. prof. Ermakova I. P. - M.: Academy, 2007
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