EntranceDark phase of photosynthesis. Dark and light phase of photosynthesis
- synthesis organic matter from carbon dioxide and water with the mandatory 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.
Oxygen is removed in external environment, and protons accumulate inside the thylakoid in a “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”. Characteristics 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 in soil minerals, improve soil fertility, promote cleaning Wastewater and 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”
NADH - the basis of energy and life
In its ordinary sense, biological life can be defined as the ability to generate energy within a cell. This energy is high-energy phosphate bonds chemical substances, synthesized in the body. The most important high-energy compounds are adenosine triphosphate (ATP), guanosine triphosphate (GTP), creatine phosphoric acid, nicotinamide dinucleotide phosphate (NAD(H) and NADP(H)), phosphorylated carbohydrates.
Nicotinamide adenine dinucleotide (NADH) is a coenzyme present in all living cells and is part of the dehydrogenase group of enzymes that catalyze redox reactions; performs the function of a carrier of electrons and hydrogen, which it receives from oxidizable substances. The reduced form (NADH) is capable of transferring them to other substances.
How to improve performance
What is NADH? Many people call it “an abbreviation for life.” And indeed it is. NADH (nicotinamide adenine dinucleotide coenzyme) is found in all living cells and is a vital element through which energy is produced inside cells. NADH is involved in the production of ATP (ATP). NAD(H), as a universal energy molecule, unlike ATP, can constantly unload mitochondria from excessive accumulation of lactate towards the formation of pyruvate from it, due to stimulation of the pyruvate dehydrogenase complex, which is sensitive specifically to the NAD(H)/NAD ratio.
Chronic Fatigue Syndrome: Focus on Mitochondria
A number of clinical studies have shown the effectiveness of NADH drugs in CFS. The daily dose was usually 50 mg. The most powerful effect occurred after 2-4 weeks of treatment. Fatigue decreased by 37-52%. In addition, such an objective cognitive parameter as concentration of attention improved.
NADH in the treatment of chronic fatigue syndrome
NADH (vitamin B3 coenzyme), present in all living cells, is part of the dehydrogenase group of enzymes that catalyze redox reactions; performs the function of a carrier of electrons and hydrogen, which it receives from oxidizable substances. It is a reserve source of energy in cells. It takes part in almost all energy production reactions, ensuring cell respiration. By influencing the corresponding processes in the brain, the vitamin B3 coenzyme can prevent the death of nerve cells during hypoxia or age-related changes. Takes part in detoxification processes in the liver. IN Lately Its property has been established to block lactate dehydrogenase and, thereby, limit ischemic and/or hypoxic damage to the myocardium. Studies of the effectiveness of oral administration in the treatment of chronic fatigue syndrome have confirmed its activating effect on people’s condition.
NADH in sports and medicine: review of foreign literature
We wrote about NADH (nicotinamide adenine dinucleotide phosphate) in previous articles. Now we want to provide information from English-language sources about the role and significance of this substance in energy metabolism in the body, its effect on the nervous system, and its role in the development of a number of pathological situations and prospects for use in medicine and sports. (Download monograph on NADH).
Herbalife Quickspark CoEnzyme 1 (NADH) ATP Energy
Natural Energy at a Cellular Level
Quickspark is a product of the company Herbalife. It is a stable form of Vitamin B3 CoEnzyme1. CoEnzyme1 was found in 1906 in Austria by a scientist called Professor George Birkmayer. CoEnzyme1 was developed for medical purposes and used in the second world war
NADH (Enada)
Nicotinamide adenine dinucleotide (NADH) is a substance that helps the functionality of enzymes in the body. NADH plays a role in the production of energy and helps produce L-dopa, which the body turns into the neurotransmitter dopamine. NADH is being evaluated for many conditions and may be helpful for enhancing mental functionality and memory.
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. Chemical reactions The dark phases 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:
- ATP synthesis;
- creation of NADP H2;
- 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
| Comparison criteria | Light phase | Dark phase |
| sunlight | Required | Not required |
| Place of reaction | Chloroplast grana | Chloroplast stroma |
| Dependency on energy source | Depends on sunlight | Depends on ATP and NADP H2 formed in the light phase and on the amount of CO2 from the atmosphere |
| Starting materials | Chlorophyll, electron transport proteins, ATP synthetase | Carbon dioxide |
| The essence of the phase and what is formed | Free O2 is released, ATP and NADP H2 are formed | Formation of natural sugar (glucose) and absorption of CO2 from the atmosphere |
Photosynthesis - video
Dark reactions that occur in the stroma do not require light. The reduction of CO 2 occurs due to energy (ATP) and reducing force (NADPH 2) generated during light reactions. Dark reactions are controlled by enzymes. The sequence of these reactions was determined in the USA by Calvin, Benson and Bassem between 1946 and 1953; in 1961, Calvin was awarded the Nobel Prize for this work.
Calvin's experiments
Calvin's work was based on the use of the radioactive isotope of carbon 14 C (half-life 5570 years, see Appendix 1.3), which became available to researchers only in 1945. In addition, Calvin used paper chromatography, which at that time was relatively new, not yet a little common method. Cultures of unicellular green algae Chlorella (Chlorella) were grown in a special apparatus (Fig. 9.17). The culture was kept at 14 CO 2 for various periods of time, then the cells were quickly fixed by pouring the suspension into hot methanol. Soluble photosynthetic products were extracted, concentrated and separated using two-dimensional paper chromatography(Fig. 9.18 and Appendix 1.8.2). The goal was to trace the path by which labeled carbon passes (through a series of intermediate products) into the final products of photosynthesis. The position of radioactive compounds on paper was determined using autoradiography: for this purpose, photographic film sensitive to 14 C radiation was placed on the chromatogram, and it was exposed, i.e., blackened, in those places where there were radioactive substances(Fig. 9.18). Within just one minute of incubation with 14 CO 2, many sugars and organic acids, including various amino acids. However, Calvin was able, using very short exposures - for 5 seconds or less - to identify the first product of photosynthesis and establish that it was an acid containing three carbon atoms, namely phosphoglyceric acid(FGK). He then figured out the entire chain of intermediates through which the fixed carbon is transmitted; these stages will be discussed later. Since then these reactions have been called Calvin cycle(or the Calvin-Benson-Bassem cycle).
Rice. 9.18. A. Fixation of 14 CO 2 in algae under short-term illumination. Determination of fixation products using paper chromatography and autoradiography. B. Autoradiographs of photosynthesis products obtained after short-term illumination of algae in the presence of 14 CO 2
9.18. What are the advantages of using long-lived radioactive isotopes in biological research?
9.19. What benefits can you get by taking chlorella instead of a higher plant?
9.20. Why is the Calvin apparatus vessel flat and not spherical?
Carbon Pathway Stages
Carbon dioxide fixation:
The CO 2 acceptor is a five-carbon sugar (pentose) ribulose bisphosphate(i.e., ribulose with two phosphate groups; this compound was previously called ribulose diphosphate). The addition of CO 2 to a particular substance is called carboxylation, and the enzyme that catalyzes such a reaction is carboxylase. The resulting six-carbon product is unstable and immediately breaks down into two molecules phosphoglyceric acid(FGK), which is the first product of photosynthesis. The enzyme ribulose bisphosphate carboxylase is found in large quantities in the stroma of chloroplasts - it is in fact the most abundant protein in the world.
Recovery phase:
FHA contains three carbon atoms and has an acidic carboxyl group (-COOH). TP is triose phosphate, or glyceraldehyde phosphate (three-carbon sugar); it has an aldehyde group (-CHO).
To remove oxygen from PGA (i.e., to restore it), the reducing force of NADPH 2 and the energy of ATP are used. The reaction proceeds in two stages: first, part of the ATP formed during light reactions is consumed, and then all of the NADP·H 2, also obtained in light, is used. The overall result is the reduction of the carboxyl group of the acid (-COOH) to aldehyde group(-SNO). The reaction product is triose phosphate, that is, a three-carbon sugar with a phosphate group attached to it. There is more in this connection chemical energy, than in FHA, and it is the first carbohydrate that is formed during photosynthesis.
Regeneration of the acceptor for CO 2 - ribulose bisphosphate. Part of the triose phosphate (TP) must be spent on the regeneration of ribulose bisphosphate, which is used in the first reaction. This process is a complex cycle that involves sugar phosphates with 3, 4, 5, 6, 7 carbon atoms. This is where the rest of the ATP is used up. All dark reactions are summarized in Fig. 9.19. In this figure, the Calvin cycle is depicted as a “black box”, into which CO 2 and H 2 O enter on one side, and triose phosphate exits on the other side. As can be seen from this diagram, the ATP residue is used to phosphorylate ribulose bisphosphate, but the details of this complex chain of reactions are not shown.
From Fig. 9.19 we can derive the following summary equation:
It is important to note here that the formation of two molecules of triose phosphate requires six molecules of CO 2. The equation can be simplified by dividing all coefficients by 6:
9.21. Redraw the figure. 9.19, indicating only the number of carbon atoms participating in the reactions; for example, instead of 6 RiBF write "6 × 5C", etc.
Basic information about the process of photosynthesis is summarized in Table. 9.6.
| Light reactions | Dark reactions | |
| Localization in chloroplasts | Thylakoids | Stroma |
| Reactions | Photochemical, i.e. require light. Light energy causes the transfer of electrons from electron "donors" to their "acceptors" in either a non-cyclic or cyclic path. Two photosystems are involved - Ι and ΙΙ. They contain chlorophyll molecules, which, when absorbing light energy, emit electrons. Water serves as an electron donor for the non-cyclic pathway. Electron transfer leads to the formation of ATP (photophosphorylation) and NADPH 2 (see also Table 9.5). | They don't require light. CO 2 is fixed when it binds to a five-carbon acceptor, ribulose bisphosphate (RiBP); in this case, two molecules of the three-carbon compound phosphoglyceric acid (PGA), the first product of photosynthesis, are formed. A number of reactions occur, collectively called the Calvin cycle; in this case, the acceptor for CO 2 -RiBP is regenerated, and FGA is reduced, turning into sugar (see also Fig. 9.19). |
| Combined equations |
Oxygen is the most important component of the existence of all life on Earth. Surprisingly, this element on our planet, although its concentration in the air, according to some scientists, is inexorably decreasing, is a replenishable reserve. Even more striking is the fact that it is synthesized from more than available resources- water, sunlight and carbon dioxide. And plants carry out this wonderful process.
Of course, we are talking about photosynthesis - an amazing creation of nature. Despite the fact that scientists have thoroughly studied this issue, repeating the stages of photosynthesis in laboratory conditions unrealistic to this day.
This process is usually divided into two stages:
- Light phase of photosynthesis.
- Dark phase of photosynthesis.
From their name it is quite clear that the first part of the process takes place in the light, that is, with the participation of sunlight. It occurs only in the green leaves of plants, since they contain chloroplasts - special elements in the membranes of which ATP is synthesized - a molecule in which energy is stored.
When photons of sunlight hit the leaves of plants containing chlorophyll, the energy from sunlight is converted into the energy molecules ATP, already mentioned above. In addition, due to the abstraction of two hydrogen atoms from a water molecule (which also occurs with the help of sunlight), a NADP molecule is formed. A decomposed water molecule, devoid of two hydrogen atoms, remains with free oxygen, which enters the atmosphere. Thus, the products of photosynthesis in the light phase are:
- oxygen;
- energy molecule ATP;
- atomic hydrogen NADP H2.
It is curious that the formation of oxygen in this process is not the final goal. Rather, it is a side effect. Next, the dark phase of photosynthesis occurs, or chemosynthesis, in which the products of the first phase are directly involved. Let's take a closer look at it.
Indeed, the purpose of the process is not to produce oxygen. The dark phase of photosynthesis occurs in another part of the leaf - in the stroma of its chloroplasts. At the end of the light phase, the plant manages to stock up on an impressive amount of energy molecules - ATP and NADP H2, therefore, the participation of light is no longer necessary. It is with the help of these molecules that organic elements are synthesized. It is logical that the task of the energy molecule ATP is to supply energy for the implementation of synthesis processes, while the role of NADP H2 is restoration.
At the beginning of this phase, the reducing agent molecule is oxidized, due to which two hydrogen atoms disappear, which gives the output pure molecule NADP. At the same time, ATP gives up the phosphoric acid residue, turning into ADP. These two processes occur in the leaf matrix. The newly obtained molecules then return to the edges of the leaves, which makes it possible to repeat the entire process of the light phase. However, this is not the key; we only outlined the cyclicity and sequence of operations occurring in the leaves.
The final product of this phase is glucose - organic compound, classified as simple sugars. Melvin Calvin was the first to describe in detail the synthesis of this molecule. It turned out that both molecules considered within the light phase - energetic and reducing - are involved in synthesis processes. In addition, the important elements for the formation of simple sugars are 6 molecules of carbon dioxide (CO2), 24 hydrogen atoms, 6 water molecules:
6СО2 + 24Н + ATP С6Н12О6 + 6Н2O.
The dark phase of photosynthesis is important for plants because, in addition to glucose, various amino acids, nucleotides, fatty acids and glycerol are formed during this period.
Photosynthesis is a highly unique natural process. Not only is it the key to maintaining a constant level of oxygen in the atmosphere, but it also represents the perfection of nature when organic elements are created from inorganic elements.