Photosynthesis is the conversion of light energy into the energy of chemical bonds 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 byproduct. 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. are involved. The main pigments are chlorophylls, in addition to them - carotenoids and phycobilins.

In nature, two pathways 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, the energy of photons is converted into a chemical bond. 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 light radiation (hν), which is necessary for reactions to occur. The dark phase is light-independent.

In plants, photosynthesis occurs 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 glucose - the most common product of photosynthesis:

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

The oxygen atoms included in the O 2 molecule are taken not from carbon dioxide, but from water. Carbon dioxide - source of carbon, which is more important. Thanks to its binding, plants have the opportunity to synthesize organic matter.

Presented above chemical reaction there is generalized and summary. It is far from the essence of the process. So glucose is not formed from six separate molecules of carbon dioxide. CO 2 binding occurs one molecule at a time, which first attaches to an 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's happening on chloroplast thylakoids, where pigments and enzymes form complex complexes for the functioning of electrochemical circuits through which electrons and partly hydrogen protons are transmitted.

The electrons ultimately end up with the coenzyme NADP, which, when charged negatively, attracts some protons 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, through the same radiation, should give into space the energy received from outside 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 a chain of carriers.

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

Electrons from photosystem I, passing through the chain of transporters, reduce NADP.

The energy of electrons released from photosystem II is used for the synthesis of ATP. And the electrons of photosystem II themselves fill the electron holes of photosystem I.

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

An approximate summary equation for the light phase of photosynthesis:

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

Cyclic electron transport

The so-called non-cyclical light phase of photosynthesis. There are more cyclic electron transport when NADP reduction does not occur. In this case, electrons from photosystem I go to the transporter chain, where ATP synthesis occurs. That is, this electron transport chain receives electrons from photosystem I, not II. The first photosystem, as it were, implements a cycle: the electrons emitted by it are returned to it. Along the way, they spend part of their energy on ATP synthesis.

Photophosphorylation and oxidative phosphorylation

The light phase of photosynthesis can be compared with the stage of cellular respiration - oxidative phosphorylation, which occurs on the cristae of mitochondria. ATP synthesis also occurs there due to the transfer of electrons and protons through a chain of carriers. 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 the initial source of energy is organic substances, 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, and Bassem. The reaction cycle they discovered was later called the Calvin cycle, or C 3 photosynthesis. In certain groups of plants, a modified photosynthetic pathway is observed - C 4, also called the Hatch-Slack cycle.

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

The reduction of CO 2 occurs due to the energy of ATP and the reducing force 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 occurs 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-bisphosphate) – 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, breaks down into two three-carbon molecules phosphoglyceric acid (PGA)- the first product of photosynthesis. PGA is also called phosphoglycerate.

RiBP + CO 2 + H 2 O → 2FGK

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

Three-carbon sugar (glyceraldehyde phosphate) is formed from PGA triose phosphate (TP), already including an aldehyde group (-CHO):

FHA (3-acid) → TF (3-sugar)

This reaction requires the energy of ATP and the reducing power of NADP H2. TF is the first carbohydrate of photosynthesis.

After this, most of the triose phosphate is spent on the regeneration of ribulose biphosphate (RiBP), which is again used to fix CO 2. Regeneration includes a series of ATP-consuming reactions involving sugar phosphates with a number of carbon atoms from 3 to 7.

This cycle of RiBF is the Calvin cycle.

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 → 2TP

In this case, 6 molecules of RiBP participate in the binding and 12 molecules of PGA are formed, which are converted into 12 TF, of which 10 molecules remain in the cycle and are converted into 6 molecules of RiBP. Since TP is a three-carbon sugar, and RiBP is a five-carbon one, then in relation to carbon atoms we have: 10 * 3 = 6 * 5. The number of carbon atoms providing the cycle does not change, all necessary RiBP is regenerated. And six carbon dioxide molecules entering the cycle are spent on the formation of two triose phosphate molecules leaving the cycle.

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

The calculation is based on two triose phosphate molecules leaving the cycle, since the subsequently formed glucose molecule includes 6 carbon atoms.

Triose phosphate (TP) is the final product of the Calvin cycle, but it can hardly be called the final product of photosynthesis, since it almost does not accumulate, but, reacting with other substances, is converted into glucose, sucrose, starch, fats, fatty acids, and amino acids. In addition to TF, FGK 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.

Six-carbon sugar is formed from FHA by stepwise enzymatic catalysis fructose 6-phosphate, which turns into glucose. In plants, glucose can polymerize into starch and cellulose. Carbohydrate synthesis is similar to the reverse process of glycolysis.

Photorespiration

Oxygen inhibits photosynthesis. The more O 2 in environment, the less efficient the process of CO 2 binding. The fact is that the enzyme ribulose biphosphate 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 split off from it, and it turns into glycolic acid (glycolate). To “recycle” it, oxygen is again needed. Therefore, the more oxygen in the atmosphere, the more it will stimulate photorespiration and the more oxygen the plant will require to get rid of reaction products.

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

During photorespiration, carbon atoms from glycolate are returned 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) → FHA (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, and carbon dioxide is released.

Oxygen is required during the conversion of glycolate to glyoxylate and glycine to serine.

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

Photorespiration is characteristic mainly of plants with the C 3 type of photosynthesis. It can be considered harmful, since energy is wasted on converting glycolate into PGA. Apparently photorespiration arose due to the fact that ancient plants were not prepared for a large amount of oxygen in the atmosphere. Initially, their evolution took place in an atmosphere rich in carbon dioxide, and it was this that mainly captured reaction center rubisco enzyme.

C 4 photosynthesis, or the Hatch-Slack cycle

If during C 3 -photosynthesis the first product of the dark phase is phosphoglyceric acid, which contains three carbon atoms, then during 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 and corn.

C4 plants absorb carbon monoxide more efficiently and have almost no photorespiration.

Plants in which the dark phase of photosynthesis proceeds along the C4 pathway have a special leaf structure. In it, the vascular bundles are surrounded by a double layer of cells. The inner layer is the lining of the conductive bundle. The outer layer is mesophyll cells. The chloroplasts of the cell layers are different from each other.

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

In the chloroplasts of the vascular bundle cells, grana are almost undeveloped, 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 to organic acids, in this form is transported to the sheath cells, released and further bound in the same way as in C 3 plants. That is, the C 4 path complements, rather than replaces C 3 .

In the mesophyll, CO2 combines with phosphoenolpyruvate (PEP) to form oxaloacetate (an acid) containing four carbon atoms:

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

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

The separated CO 2 in the chloroplasts of the sheath cells goes to the usual C 3 pathway of the dark phase of photosynthesis, i.e., to the Calvin cycle.

Photosynthesis via the Hatch-Slack pathway requires more energy.

It is believed that the C4 pathway arose later in evolution than the C3 pathway and is largely an adaptation against photorespiration.

I.2 Photosynthesis, necessary conditions for it

Photosynthesis in green plants is the process of converting light into chemical energy from organic compounds synthesized from carbon dioxide and water. The process of photosynthesis is a chain of redox reactions, the totality of which is divided into two phases - light and dark.

1. Light phase. This phase is characterized by the fact that the energy solar radiation, absorbed by the pigments of the chloroplast system, is converted into electrochemical.

When light acts on the chloroplast, an electron flow begins through a system of carriers - complex organic compounds built into the thylakoid membranes. The transfer of electrons along the ETC is associated with the active flow of protons through the thylakoid membrane from the stroma into the thylakoid. In the thylakoid space, the concentration of protons increases due to the splitting of water molecules and as a result of the oxidation of the electron carrier plastoquinone on the inner side of the membrane. When protons go back along the gradient from the thylakoid space to the stroma, ATP is synthesized on the outer surface of the thylakoid with the participation of the enzyme ATP synthetase from ADP and phosphoric acid, i.e. photosynthetic phosphorylation occurs with the storage of energy in ATP, which then passes into the stroma of the chloroplast.

The transfer of electrons ends as follows. Having reached the outer surface of the thylakoid membrane, a pair of electrons follows with a hydrogen ion located in the stroma. Both electrons and the hydrogen ion are attached to the hydrogen carrier molecule – NADP+ (nicatinamide adenine dinucleotide phosphate), which is converted into its reduced form

NADP H+H+:

NADP++2Н++2е-→NADP H+H+

Consequently, electrons activated by light energy are used to attach the hydrogen atom to its carrier, i.e., to reduce NADP+ V NADP H+H+, which from the outer surface of the photosynthetic membrane passes into the stroma.

In chlorophyll molecules that have lost their electrons, the resulting electron “holes” act as a strong oxidizing agent and strip electrons from water molecules. Through a series of carriers, these electrons are transferred to the chlorophyll molecule and fill the “hole”. Photooxidation (photolysis) of water occurs inside the thylakoid, as a result of which free oxygen is released and hydrogen ions accumulate

2H2O→4H++4e-+O2

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, and ATP and NADP H2 are transported into the plastid matrix and participate in the dark phase process.

2.Dark phase photosynthesis occurs in the chloroplast matrix both in the light and in the dark and represents a series of sequential transformations of CO2 coming from the air. Dark phase reactions are carried out using the energy of ATP and NADP H2 and the use of five-carbon sugars present in plastids, one of which, ribulose diphosphate, is a CO2 acceptor. Enzymes combine five-carbon sugars with carbon dioxide in the air. In this case, compounds are formed that are successively reduced to a six-carbon glucose molecule.

Total reaction of photosynthesis

6СО2+6Н2 light energy С6Н12О6+6О2

Chlorophyll

During the process of photosynthesis, in addition to monosaccharides (glucose, etc.), which are converted into starch and stored by the plant, monomers of other organic compounds are synthesized - amino acids, glycerol and fatty acids. Thus, thanks to photosynthesis, plant cells, or more precisely, chlorophyll-containing cells, provide themselves and all living things on Earth with the necessary organic substances and oxygen.

I.3 Cell division

Three methods of division of eukaryotic cells have been described: amitosis (direct division), mitosis (indirect division) and meiosis (reduction division).

Amitosis- a relatively rare method of cell division. In amitosis, the interphase nucleus is divided by constriction, and uniform distribution of the hereditary material is not ensured. Often the nucleus divides without subsequent separation of the cytoplasm and binucleate cells are formed. A cell that has undergone amitosis is subsequently unable to enter into the normal mitotic cycle. Therefore, amitosis occurs, as a rule, in cells and tissues doomed to death.

Mitosis. Mitosis, or indirect division, is the main method of division of eukaryotic cells. Mitosis is the division of the nucleus, which leads to the formation of two daughter nuclei, each of which has exactly the same set of chromosomes as was in the parent nucleus.

In the continuous process of mitotic division, there are four phases: prophase, metaphase, anaphase and telophase.

Prophase– the longest phase of mitosis, when the entire structure of the nucleus undergoes restructuring for division. In prophase, chromosomes shorten and thicken due to their spiralization. At this time, the chromosomes are double (doubling occurs in the S-period of interphase) and consist of two chromatids connected to each other in the region of the primary constriction by a special structure - the centromere. Simultaneously with the thickening of the chromosomes, the nucleolus disappears and the nuclear membrane fragments (breaks up into separate tanks). After the breakdown of the nuclear membrane, the chromosomes lie freely and randomly in the cytoplasm. The formation of the achromatic spindle begins - the fission spindle, which represents a system of threads coming from the poles of the cell. The spindle filaments have a diameter of about 25 nm. These are bundles of microtubules consisting of subunits of the protein tubulin. Microtubules begin to form from the centrioles or from the chromosomes (in plant cells).

Metaphase. In metaphase, the formation of the division spindle is completed, which consists of two types of microtubules: chromosomal, which bind to the centromeres of the chromosomes, and centrosomal (polar), which stretch from pole to pole of the cell. Each double chromosome is attached to the spindle microtubules. Chromosomes seem to be pushed out by microtubules into the equator region of the cell, i.e. located on equal distance from the poles. They lie in the same plane and form the so-called equatorial or metaphase plate. In metaphase, the double structure of chromosomes is clearly visible, connected only at the centromere. It is during this period that it is easy to count the number of chromosomes and study their morphological features.

Anaphase begins with division of the centromere. Each chromatid of one chromosome becomes an independent chromosome. Contraction of the pulling filaments of the achromatin spindle carries them to the opposite poles of the cell. As a result, each pole of the cell has the same number of chromosomes as there were in the mother cell, and their set is the same.

Telophase – last phase of mitosis. Chromosomes despiral and become poorly visible. At each pole, a nuclear envelope is recreated around the chromosomes. Nucleoli are formed, the spindle disappears. In the resulting nuclei, each chromosome now consists of only one chromatid, rather than two.

Biochemical functions

Transfer of hydride ions H– (hydrogen atom and electron) in redox reactions

Thanks to the transfer of hydride ions, the vitamin provides the following tasks:

1. Metabolism of proteins, fats and carbohydrates. Since NAD and NADP serve as coenzymes of most dehydrogenases, they participate in the reactions

• during the synthesis and oxidation of fatty acids,
• during the synthesis of cholesterol,
• metabolism of glutamic acid and other amino acids,
• carbohydrate metabolism: pentose phosphate pathway, glycolysis,
• oxidative decarboxylation of pyruvic acid,
• tricarboxylic acid cycle.

2. NADH does regulating function, since it is an inhibitor of certain oxidation reactions, for example, in the tricarboxylic acid cycle.

3. Protection of hereditary information– NAD is a substrate of poly-ADP-ribosylation during the process of cross-linking chromosomal breaks and DNA repair, which slows down necrobiosis and cell apoptosis.

4. Free Radical Protection– NADPH is an essential component of the cell’s antioxidant system.

5. NADPH is involved in the reactions of resynthesis of tetrahydrofolic acid from dihydrofolic acid, for example after the synthesis of thymidyl monophosphate.

Hypovitaminosis

Cause

Nutritional deficiency of niacin and tryptophan. Hartnup syndrome.

Clinical picture

It is manifested by the disease pellagra (Italian: pelle agra - rough skin). Appears as three D syndrome:

• dementia(nervous and mental disorders, dementia),
• dermatitis(photodermatitis),
• diarrhea(weakness, indigestion, loss of appetite).

If left untreated, the disease is fatal. Children with hypovitaminosis experience slow growth, weight loss, and anemia.

Antivitamins

Phtivazid, tubazid, niazid are medications used to treat tuberculosis.

Dosage forms

Nicotinamide and nicotinic acid.

Vitamin B5 (pantothenic acid)

Sources

Any food products, especially legumes, yeast, animal products.

Daily requirement

Structure

The vitamin exists only in the form of pantothenic acid; it contains β-alanine and pantoic acid (2,4-dihydroxy-3,3-dimethylbutyric).

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The structure of pantothenic acid

Its coenzyme forms are coenzyme A(coenzyme A, HS-CoA) and 4-phosphopantetheine.

The structure of the coenzyme form of vitamin B5 - coenzyme A

Biochemical functions

Coenzyme form of the vitamin coenzyme A is not tightly bound to any enzyme, it moves between different enzymes, providing transfer of acyl(including acetyl) groups:

• in reactions of energetic oxidation of glucose and amino acid radicals, for example, in the work of the enzymes pyruvate dehydrogenase, α-ketoglutarate dehydrogenase in the tricarboxylic acid cycle),
• as a carrier of acyl groups during the oxidation of fatty acids and in fatty acid synthesis reactions
• in the reactions of the synthesis of acetylcholine and glycosaminoglycans, the formation of hippuric acid and bile acids.

Hypovitaminosis

Cause

Nutritional deficiency.

Clinical picture

Appears as pediolalgia(erythromelalgia) - damage to the small arteries of the distal parts of the lower extremities, the symptom is burning in the feet. The experiment shows graying of hair, damage to the skin and gastrointestinal tract, dysfunction nervous system, adrenal dystrophy, hepatic steatosis, apathy, depression, muscle weakness, convulsions.

But since the vitamin is found in all foods, hypovitaminosis is very rare.

Dosage forms

Calcium pantothenate, coenzyme A.

Vitamin B6 (pyridoxine, anti-dermatitis)

Sources

The vitamin is rich in cereals, legumes, yeast, liver, kidneys, meat, and is also synthesized by intestinal bacteria.

Daily requirement

Structure

The vitamin exists in the form of pyridoxine. Its coenzyme forms are pyridoxal phosphate and pyridoxamine phosphate.

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Structural formula of substances

What is the structural formula

It has two varieties: planar (2D) and spatial (3D) (Fig. 1).

Structure of the oxidized forms of NAD and NADP

When depicting a structural formula, intramolecular bonds are usually denoted by dashes (primes).

Rice. 1. Structural formula ethyl alcohol: a) planar; b) spatial.

Planar structural formulas may be depicted differently.

Highlight a short graphic formula, in which the bonds of atoms with hydrogen are not indicated:

CH3 - CH2 - OH(ethanol);

a skeletal graphic formula, which is most often used when depicting the structure of organic compounds; it not only does not indicate the bonds of carbon with hydrogen, but also does not indicate the bonds connecting carbon atoms to each other and other atoms:

for organic compounds of the aromatic series, special structural formulas are used, depicting the benzene ring in the form of a hexagon:

Examples of problem solving

Adenosine triphosphoric acid (ATP) is a universal source and main energy accumulator in living cells. ATP is found in all plant and animal cells. The amount of ATP is on average 0.04% (of the wet weight of the cell), the largest amount of ATP (0.2-0.5%) is found in skeletal muscles.

In a cell, an ATP molecule is used up within one minute of its formation. In humans, an amount of ATP equal to body weight is produced and destroyed every 24 hours.

ATP is a mononucleotide consisting of nitrogenous base residues (adenine), ribose and three phosphoric acid residues. Since ATP contains not one, but three phosphoric acid residues, it belongs to ribonucleoside triphosphates.

Most of the work that happens in cells uses the energy of ATP hydrolysis.

In this case, when the terminal residue of phosphoric acid is eliminated, ATP transforms into ADP (adenosine diphosphoric acid), and when the second phosphoric acid residue is eliminated, it turns into AMP (adenosine monophosphoric acid).

The free energy yield upon elimination of both the terminal and second residues of phosphoric acid is about 30.6 kJ/mol. The elimination of the third phosphate group is accompanied by the release of only 13.8 kJ/mol.

The bonds between the terminal and second, second and first phosphoric acid residues are called macroergic(high energy).

ATP reserves are constantly replenished.

Biological functions.

In the cells of all organisms, ATP synthesis occurs in the process phosphorylation, i.e. addition of phosphoric acid to ADF. Phosphorylation occurs with varying intensity during respiration (mitochondria), glycolysis (cytoplasm), and photosynthesis (chloroplasts).

ATP is the main link between processes accompanied by the release and accumulation of energy, and processes occurring with energy expenditure.

In addition, ATP, along with other ribonucleoside triphosphates (GTP, CTP, UTP), is a substrate for RNA synthesis.

In addition to ATP, there are other molecules with macroergic bonds - UTP (uridine triphosphoric acid), GTP (guanosine triphosphoric acid), CTP (cytidine triphosphoric acid), the energy of which is used for the biosynthesis of protein (GTP), polysaccharides (UTP), phospholipids (CTP). But all of them are formed due to the energy of ATP.

In addition to mononucleotides, dinucleotides (NAD+, NADP+, FAD), belonging to the group of coenzymes ( organic molecules, retaining contact with the enzyme only during the reaction).

NAD+ (nicotinamide adenine dinucleotide), NADP+ (nicotinamide adenine dinucleotide phosphate) are dinucleotides containing two nitrogenous bases - adenine and nicotinic acid amide - a derivative of vitamin PP), two ribose residues and two phosphoric acid residues (Fig. .). If ATP is a universal source of energy, then NAD+ and NADP+ are universal acceptors, and their restored forms are NADH And NADPH – universal donors reduction equivalents (two electrons and one proton).

The nitrogen atom included in the nicotinic acid amide residue is tetravalent and carries a positive charge ( NAD+). This nitrogenous base readily accepts two electrons and one proton (i.e.

is reduced) in those reactions in which, with the participation of dehydrogenase enzymes, two hydrogen atoms are removed from the substrate (the second proton goes into solution):

Substrate-H2 + NAD+ substrate + NADH + H+

In reverse reactions, enzymes oxidize NADH or NADPH, reduce substrates by adding hydrogen atoms to them (the second proton comes from the solution).

FAD – flavin adenine dinucleotide– a derivative of vitamin B2 (riboflavin) is also a cofactor for dehydrogenases, but FAD adds two protons and two electrons, reducing to FADN2.

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Nucleoside cyclophosphates (cAMP and cGMP) as secondary messengers in the regulation of cell metabolism.

Nucleoside cyclophosphates include nucleotides in which one molecule of phosphoric acid simultaneously esterifies two hydroxyl groups of a carbohydrate residue.

Almost all cells contain two nucleoside cyclophosphates - adenosine 3′,5′-cyclophosphate (cAMP) and guanosine 3′,5′-cyclophosphate (cGMP). They are secondary messengers (messengers) in transmitting a hormonal signal into the cell.

6. Structure of dinucleotides: FAD, NAD+, its phosphate NADP+.

Their participation in redox reactions.

The most important representatives of this group of compounds are nicotinamide adenine dinucleotide (NAD, or in Russian literature NAD) and its phosphate (NADP, or NADP). These compounds play an important role as coenzymes in many redox reactions.

In accordance with this, they can exist both in oxidized (NAD +, NADP +) and reduced (NADH, NADPH) forms.

The structural fragment of NAD+ and NADP+ is a nicotinamide residue in the form of a pyridinium cation. As part of NADH and NADPH, this fragment is converted into a 1,4-dihydropyridine residue.

During biological dehydrogenation, the substrate loses two hydrogen atoms, i.e.

two protons and two electrons (2H+, 2e) or a proton and a hydride ion (H+ and H-). The NAD+ coenzyme is usually considered as an acceptor of the H- hydride ion (although it has not been definitively established whether the transfer of a hydrogen atom to this coenzyme occurs simultaneously with the electron transfer or whether these processes occur separately).

As a result of reduction by addition of a hydride ion to NAD+, the pyridinium ring is converted into a 1,4-dihydropyridine fragment.

This process is reversible.

In the oxidation reaction, the aromatic pyridinium ring is converted to a non-aromatic 1,4-dihydropyridine ring. Due to the loss of aromaticity, the energy of NADH increases compared to NAD+. In this way, NADH stores energy, which is then used in other biochemical processes that require energy.

Typical examples of biochemical reactions involving NAD+ are the oxidation of alcohol groups into aldehyde groups (for example, the conversion of ethanol into ethanal), and with the participation of NADH, the reduction of carbonyl groups into alcohol groups (the conversion of pyruvic acid into lactic acid).

Ethanol oxidation reaction involving the coenzyme NAD+:

During oxidation, the substrate loses two hydrogen atoms, i.e.

two protons and two electrons. The coenzyme NAD+, having accepted two electrons and a proton, is reduced to NADH and the aromaticity is disrupted. This reaction is reversible.

When the oxidized form of the coenzyme passes into the reduced form, the energy released during the oxidation of the substrate accumulates. The energy accumulated by the reduced form is then spent in other endergonic processes involving these coenzymes.

FAD - flavin adenine dinucleotide- a coenzyme that takes part in many redox biochemical processes.

FAD exists in two forms - oxidized and reduced, its biochemical function, as a rule, is to transition between these forms.

FAD can be reduced to FADH2, in which case it accepts two hydrogen atoms.

The FADH2 molecule is an energy carrier, and the reduced coenzyme can be used as a substrate in the oxidative phosphorylation reaction in mitochondria.

The FADH2 molecule is oxidized to FAD, releasing energy equivalent (stored in the form) to two moles of ATP.

The main source of reduced FAD in eukaryotes is the Krebs cycle and lipid β-oxidation. In the Krebs cycle, FAD is a prosthetic group of the enzyme succinate dehydrogenase, which oxidizes succinate to fumarate; in β-lipid oxidation, FAD is a coenzyme of acyl-CoA dehydrogenase.

FAD is formed from riboflavin; many oxidoreductases, called flavoproteins, use FAD as a prosthetic group in electron transfer reactions for their work.

Primary structure nucleic acids: nucleotide composition of RNA and DNA, phosphodiester bond. Hydrolysis of nucleic acids.

In polynucleotide chains, the nucleotide units are linked through a phosphate group. The phosphate group forms two ester bonds: with C-3′ of the previous and with C-5′ of the subsequent nucleotide units (Fig. 1). The backbone of the chain consists of alternating pentose and phosphate residues, and the heterocyclic bases are the "side" groups attached to the pentose residues.

A nucleotide with a free 5'-OH group is called 5'-terminal, and a nucleotide with a free 3'-OH group is called 3'-terminal.

Rice. 1. General principle structure of the polynucleotide chain

Figure 2 shows the structure of an arbitrary section of a DNA chain, including four nucleic bases. It is easy to imagine how many combinations can be obtained by varying the sequence of four nucleotide residues.

The principle of constructing the RNA chain is the same as that of DNA, with two exceptions: the pentose residue in RNA is D-ribose, and the set of heterocyclic bases uses uracil rather than thymine.

The primary structure of nucleic acids is determined by the sequence of nucleotide units linked covalent bonds into a continuous polynucleotide chain.

To make it easier to write the primary structure, there are several abbreviations.

One of them is to use the previously given abbreviated names of nucleosides. For example, shown in Fig. 2 DNA chain fragment can be written as d(ApCpGpTp...) or d(A-C-G-T...). Often the letter d is omitted if it is obvious that we're talking about about DNA.

7. Structure of the enzyme.

Primary structure of a DNA strand section

An important characteristic of nucleic acids is the nucleotide composition, that is, the set and quantitative ratio of nucleotide components. The nucleotide composition is determined, as a rule, by studying the products of hydrolytic cleavage of nucleic acids.

DNA and RNA differ in their behavior under conditions of alkaline and acid hydrolysis.

DNA is resistant to hydrolysis in an alkaline environment. RNA is easily hydrolyzed under mild conditions in an alkaline environment to nucleotides, which, in turn, are capable of splitting off a phosphoric acid residue in an alkaline environment to form nucleosides. Nucleosides in acidic environment hydrolyze to heterocyclic bases and carbohydrates.

The concept of the secondary structure of DNA. Complementarity of nucleic bases. Hydrogen bonds in complementary pairs of nucleic bases.

Secondary structure refers to the spatial organization of a polynucleotide chain.

According to the Watson-Crick model, the DNA molecule consists of two polynucleotide chains, right-handedly twisted around a common axis to form a double helix. The purine and pyrimidine bases are directed toward the inside of the helix. Hydrogen bonds occur between the purine base of one chain and the pyrimidine base of the other chain. These bases form complementary pairs.

Hydrogen bonds are formed between the amino group of one base and the carbonyl group of another -NH...O=C-, as well as between the amide and imine nitrogen atoms -NH...N.

For example, as shown below, two hydrogen bonds are formed between adenine and thymine, and these bases form a complementary pair, i.e.

That is, adenine in one chain will correspond to thymine in another chain. Another pair of complementary bases is guanine and cytosine, between which three hydrogen bonds occur.

Hydrogen bonds between complementary bases are one of the types of interactions that stabilize the double helix. The two strands of DNA that form a double helix are not identical, but are complementary to each other.

This means that the primary structure, i.e. nucleotide sequence, one chain determines primary structure the second chain (Fig. 3).

Rice. 3. Complementarity of polynucleotide chains in the DNA double helix

The complementarity of the chains and the sequence of units are chemical basis The most important function of DNA is the storage and transmission of hereditary information.

In stabilizing the DNA molecule, along with hydrogen bonds, acting across the helix, an important role is played by intermolecular interactions directed along the helix between neighboring spatially close nitrogenous bases.

Since these interactions are directed along the stack nitrogenous bases DNA molecules, they are called stacking interactions. Thus, the interactions of nitrogenous bases with each other bind the double helix of the DNA molecule both along and across its axis.

Strong stacking interactions always strengthen hydrogen bonds between bases, promoting helix compaction.

As a result, water molecules from the surrounding solution bind mainly to the pentose phosphate backbone of DNA, the polar groups of which are located on the surface of the helix. When the stacking interaction is weakened, water molecules, penetrating inside the helix, interact competitively with the polar groups of the bases, initiate destabilization and contribute to further disintegration of the double helix. All this indicates the dynamism of the secondary structure of DNA under the influence of the components of the surrounding solution.

4. Secondary structure of the RNA molecule

9. Medicines based on modified nucleic bases (fluorouracil, mercaptopurine): structure and mechanism of action.

As medicines in oncology, synthetic derivatives of the pyrimidine and purine series are used, which are similar in structure to natural metabolites (in this case, nucleic bases), but not completely identical to them, i.e.

which are antimetabolites. For example, 5-fluorouracil acts as an antagonist of uracil and thymine, and 6-mercaptopurine acts as an antagonist of adenine.

By competing with metabolites, they disrupt the synthesis of nucleic acids in the body at different stages.

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

Light 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 decomposition and photolysis of water: H 2 O+ Q light →H + +OH - . Hydroxide ions give up their electrons, turning into reactive radicals ∙OH: OH - →∙OH+e - . ∙OH radicals combine to form water and free oxygen: 4HO∙→ 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 + to NADP∙H 2: 2H + +2 e - + NADP→ NADP∙H 2. Thus, in the light phase, photolysis of water occurs, which is accompanied by three important processes: 1) ATP synthesis; 2) formation of NADP∙H 2; 3) formation of oxygen. Oxygen diffuses into the atmosphere, ATP and NADP∙H 2 are transported into the stroma of the chloroplast and participate in the processes of the dark phase.

Dark phase. Occurs in the stroma of the chloroplast. Its reactions require light energy, so they occur not only in the light, but also in the dark. The dark phase reactions represent a chain of successive transformations of carbon dioxide (from the air), leading to the formation of glucose and other organic substances. First, CO 2 fixation occurs, the acceptor is the sugar ribulose biphosphate, catalyzed by ribulose biphosphate carboxylase. As a result of carboxylation of ribulose biphosphate, an unstable six-carbon compound is formed, which immediately breaks down into two molecules of phosphoglyceric acid. Then a cycle of reactions occurs in which, through a series of intermediate products, PGA is converted into glucose. The energy of ATP and NADPH 2 formed in the light phase is used. (Calvin Cycle).

23. Co2 assimilation reactions in the dark phase of photosynthesis.

The Calvin cycle is the main pathway for CO 2 assimilation. Decarboxylation phase - carbon dioxide binds with ribulose biphosphate to form two molecules of phosphoglycerate. This reaction is catalyzed by ribulose biphosphate carbosylase.

Rice. 12. Scheme of electron transfer of chlorophyll during ATP synthesis (phosphorylation). During non-cyclic phosphorylation, ATP is synthesized due to the energy of PS II, and NADP 2H is synthesized due to PS I. During cyclic phosphorylation, only PS I works and ATP synthesis occurs

Under the influence of light, PS I and PS II chlorophyll molecules are excited, which, absorbing light quanta, emit excited electrons. Due to the energy of electrons flying along the chain of enzymes from PS II to PS I, ATP molecules are synthesized. This process is called phosphorylation and occurs in the presence of the enzyme ATP synthetase.

ADP + Fn ATP

where Fn is inorganic phosphate H3 PO4.

PS II electrons that have lost energy fall on PS I, which is their final acceptor.

Electrons leaving the chlorophyll of the PS I photosystem upon absorption of light also enter the electron transport chain. But due to their energy, the H+ protons combine with the NADP+ transporter.

The electrons that PS I loses are replenished with electrons from PS II, and the electrons given up by a water molecule during photolysis replenish the electrons lost by PS II chlorophyll, and it is also restored:

chlorophyll+2 + 2 ē chlorophyll.

There are special proton channels on the membrane, through which at a certain moment hydrogen ions can pass from the H+ reservoir of the thylakoid to the stroma of the chloroplast. The channels are connected to the enzyme ATP synthetase. When excited electrons of PS I, moving along the membrane, reach the proton channel, it opens and hydrogen ions rush into it. This process is associated with ATP synthesis and occurs synchronously.

On the outside of the thylakoid membrane, i.e., in the stroma of the chloroplast, molecules of the hydrogen carrier NADP+ accumulate in the oxidized state. They accept electrons from PS I, due to which they combine with hydrogen ions H+ and form NADP 2H:

NADP+ + 2H+ + 2 ē NADP · 2H. (4)

The synthesis of ATP and NADP 2H occurs on thylakoid membranes and is associated with the transfer of excited electrons along the electron transport chain. Thus, the energy of the sun is converted into the energy of excited electrons, and is then stored during the synthesis process in ATP and NADP 2H molecules.

The overall equation for the reactions of the light phase is:

H2 O + NADP+ + 2ADP + 2Pn NADP · 2H + 2ATP + 1/2 O2.

Dark phase

The reactions of the dark phase (Fig. 13) occur in the stroma of chloroplasts, where molecules of NADP 2H and ATP synthesized in the light phase and carbon dioxide from the atmosphere enter. The sequence of cyclic reactions of this phase was described by the American biochemist Melvin Calvin and was called the Calvin cycle. Here, CO2 molecules are bound, compounds are activated by ATP (phosphorylation), carbon is reduced by hydrogen from NADP 2H, and glucose is synthesized. The source of energy is ATP molecules synthesized at the first stage.

Rice. 13. General scheme dark reactions of photosynthesis. Calvin cycle

In the stroma of the chloroplast, a five-carbon carbohydrate (pentose) is constantly present, associated with two phosphoric acid residues - ribulose diphosphate.

The resulting six-carbon compound is unstable and immediately breaks down into two triose phosphates.

C5 -carbohydrate-2P + CO2 C6 -carbohydrate-2P 2C3 -P

2C3 P + 2ATP 2ADP + 2C3 ~2P.

Rice. 14. Carbon fixation, phosphorylation and reduction

After this, triose diphosphates are reduced by NADP 2H molecules:

2C3 ~2P + 2NADP · 2H 2C3 + 2NADP+ + 2P.

Two triose molecules combine to form glucose, which can further be converted into sucrose, starch and other polysaccharides:

2C3 C6 H12 O6.

Some triose molecules can be used for the synthesis of amino acids, glycerol, and higher fatty acids.

Partial trioses continue to participate in cyclic reactions and are converted back into pentose, which closes the cycle.

Six molecules of each substance participate simultaneously in the reaction. Thus, to synthesize one completely new glucose molecule, the cycle must be repeated six times, i.e., six molecules of CO2 must be absorbed.

The released ATP and NADP+ molecules return to the thylakoid membranes to participate in light reactions.

The influence of various factors on the rate of photosynthesis

The intensity of photosynthesis depends on a number of factors. Firstly, it is the wavelength of the light.

The process is most intense in the ultraviolet and red parts of the spectrum. In addition, the rate of photosynthesis depends on the degree of illumination, and up to a certain value it increases in proportion to illumination, but then no longer depends on it (Fig. 15).

Rice. 15. The influence of various factors on the rate of photosynthesis

Another factor is the concentration of carbon dioxide. The higher the amount of carbon dioxide, the more intense the photosynthesis process. Under normal conditions, the lack of carbon dioxide is the main limiting factor, since in atmospheric air contains a small percentage of it. However, in greenhouse conditions this deficiency can be eliminated, which has a beneficial effect on the intensity of plant growth and development.

An important factor is temperature.

All photosynthetic reactions are catalyzed by enzymes, for which the optimal temperature is 25–30 °C. At low temperatures, the rate of enzyme action decreases sharply.

Water is also an important factor affecting photosynthesis. However, it is impossible to quantify this factor, since it is involved in many other metabolic processes.