Photosynthesis - what is it? Stages of photosynthesis. Conditions of photosynthesis

Have you ever wondered how many living organisms on the planet ?! And after all, they all need to breathe oxygen in order to generate energy and breathe out carbon dioxide. It is carbon dioxide - the main reason for such a phenomenon, as the stuffiness in the room. It takes place when there are a lot of people in it, and the room is not ventilated for a long time. In addition, poisonous substances are filled with air production facilities, private road and public transport.

In view of the above, a quite logical question arises: how come we are not yet choked if everything living is a source of toxic carbon dioxide? The savior of all living beings in this situation is photosynthesis. What is this process and what is its necessity?

Its result is the adjustment of the balance of carbon dioxide and the saturation of air with oxygen. Such a process is known only to representatives of the world of flora, that is, to plants, as it occurs only in their cells.

By itself, photosynthesis is an extremely complicated procedure, depending on certain conditions and occurring in several stages.

Definition of concept

According to the scientific definition, organic substances in the process of photosynthesis are transformed into organic ones at the cellular level in autotrophic organisms due to the action of sunlight.

In a more understandable language, photosynthesis is a process in which the following occurs:

1. The plant is saturated with moisture. The source of moisture can be water from the ground or moist tropical air.
2. There is a reaction of chlorophyll (a special substance that is contained in the plant) to the effect of solar energy.
3. Formation of food necessary for the representatives of the flora, which they can not independently produce is heterotrophic, and they themselves are its producer. In other words, plants eat what they produce themselves. This is the result of photosynthesis.

Stage One

Virtually every plant contains a green substance, through which it can absorb light. This substance is nothing more than chlorophyll. His whereabouts are chloroplasts. But the chloroplasts are located in the stem part of the plant and its fruits. But photosynthesis of leaf is especially common in nature. Since the latter is rather simple in its structure and has a relatively large surface, which means that the amount of energy needed for the savior process will be much greater.

When light is absorbed by chlorophyll, the latter is in a state of excitation and transmits its energy messages to other organic molecules of the plant. The greatest amount of this energy goes to participants in the process of photosynthesis.

Stage Two

The formation of photosynthesis in the second stage does not require the obligatory participation of light. It consists in the formation of chemical bonds using poisonous carbon dioxide, formed from air masses and water. Also, a number of substances are synthesized, which ensure the vital activity of representatives of the flora. Such are starch, glucose.

In plants, such organic elements act as a source of nutrition for individual parts of the plant, while ensuring the normal course of life processes. Such substances are obtained and representatives of the fauna that eat plants for food. The human body is saturated with these substances through food, which is included in the daily diet.

What? Where? When?

To organic substances turned into organic, it is necessary to ensure the appropriate conditions for photosynthesis. For the process under consideration, light is needed first. It's about artificial, and sunlight. In nature, usually the activity of plants is characterized by intensity in spring and summer, that is, when there is a need for a large amount of solar energy. What can not be said about the autumn season, when there is less light, the day is shorter. As a result, the foliage turns yellow, and then completely falls off. But as soon as the first spring rays of the sun shine, the green grass will rise, then chlorophyll will resume its activity, and active production of oxygen and other nutrients, which are of vital importance, will begin.

The conditions of photosynthesis include not only the presence of illumination. Moisture too should be enough. After all, the plant first absorbs moisture, and then a reaction begins with the participation of solar energy. The result of this process are the food products of plants.

In the presence of a green substance, photosynthesis occurs. What is chlorophyll, we already told above. They act as a kind of conductor between light or solar energy and the plant itself, ensuring the proper flow of their life and activity. Green substances have the ability to absorb a lot of sunlight.

Oxygen plays an important role. To make the process of photosynthesis successful, plants need a lot of it, since it contains only 0.03% of carbonic acid. Hence, from 6,000 m ^{3 of} air, 6 m ^{3 of} acid can be obtained. It is the latter substance - the main source material for glucose, which, in turn, is a substance necessary for life.

There are two stages of photosynthesis. The first is light, the second is dark.

What is the mechanism of the flow of the light stage

The light stage of photosynthesis has another name - photochemical. The main participants at this stage are:

• energy of sun;
• A variety of pigments.

With the first component everything is clear, this is sunlight. And that's what pigments are, not everyone knows. They are green, yellow, red or blue. To green belong chlorophyll groups "A" and "B", to yellow and red / blue - phycobilins respectively. Photochemical activity among participants in this stage of the process is manifested only by chlorophylls "A". The rest belongs to a complementary role, the essence of which is the collection of light quanta and their transportation to the photochemical center.

Since chlorophyll is endowed with the ability to effectively absorb solar energy with a certain wavelength, the following photochemical systems were identified:

- Photochemical center 1 (green substances of group "A") - the composition includes pigment 700, absorbing light rays, the length of which is approximately 700 nm. This pigment has a fundamental role in creating products of the light stage of photosynthesis.

- Photochemical center 2 (green substances of group "B") - the composition includes pigment 680, absorbing light rays, the length of which is 680 nm. He owns the role of the second plan, consisting in the function of replenishment of electrons lost by the photochemical center 1. It is achieved due to the hydrolysis of the liquid.

For 350-400 molecules of pigments, which concentrate light streams in photosystems 1 and 2, there is only one molecule of the pigment, which is the active photochemically - chlorophyll of group "A".

What's happening?

1. The light energy absorbed by the plant affects the pigment 700 contained therein, which passes from the usual state to the excitation state. The pigment loses an electron, resulting in the formation of a so-called electron hole. Further, the pigment molecule, which has lost the electron, can act as its acceptor, that is, the party that receives the electron, and return its shape.

2. The process of decomposition of the liquid in the photochemical center of the light-absorbing pigment 680 of the photosystem 2. When water is decomposed, electrons are formed which are initially accepted by a substance such as cytochrome C550 and are designated by the letter Q. Then from cytochrome, electrons enter the vector chain and are transported to the photochemical center 1 for Replenishment of the electron hole, which was the result of the penetration of light quanta and the reconstruction process of the pigment 700.

There are cases when such a molecule gets back an electron identical to the former. This will result in the release of the energy of light in the form of heat. But almost always an electron having a negative charge connects to special iron-sulfur proteins and is transported along one of the chains to the pigment 700 or falls into another chain of vectors and reconnects with a constant acceptor.

In the first variant, cyclic transport of an electron of a closed type takes place, while in the second case, a noncyclic transport takes place.

Both processes fall in the first stage of photosynthesis under the catalysis by the same chain of electron carriers. But it is worth noting that with cyclophosphorylation of the cyclic type, the initial and at the same time the final point of transportation is chlophyll, while non-cyclic transportation implies the transition of the green substance of group "B" to chlorophyll "A".

Features of cyclic transportation

Phosphorylation of cyclic is also called photosynthetic. As a result of this process, ATP molecules are formed. This transportation is based on the return through several successive stages of electrons in the excited state to the pigment 700, thereby releasing the energy that takes part in the phosphorylating enzyme system for further accumulation in the phosphate bonds of ATP. That is, energy does not dissipate.

Phosphorylation cyclic is the primary reaction of photosynthesis, which is based on the technology of the formation of chemical energy on the membrane surfaces of the chloroplast tilaktoid due to the use of solar energy.

Without photosynthetic phosphorylation, assimilation reactions in the dark phase of photosynthesis are impossible.

Nuances of transportation of non-cyclic type

The process consists in the restoration of NADP + and the formation of NADP * H. The mechanism is based on electron transfer to ferredoxin, its reduction reaction and subsequent transition to NADP + with further reduction to NADP * H.

As a result, electrons that lost the pigment 700 are replenished by the electrons of water, which decomposes under the light rays in the photosystem 2.

The non-cyclic path of electrons, the flow of which also implies light photosynthesis, is realized through the interaction of both photosystems with each other, and their electronic transport chains connect them. Light energy directs the flow of electrons back. During transport from the photochemical center 1 to the center 2, the electrons lose some of their energy due to accumulation as a proton potential on the membrane surface of the tilaktoid.

In the dark phase of photosynthesis, the process of creating a proton-type potential in the electron transport chain and its operation for the formation of ATP in chloroplasts is almost completely identical with the same process in the mitochondria. But the features are still present. Tylactoids in this situation are the mitochondria turned on the wrong side. This is the main reason for the fact that electrons and protons move through the membrane in the opposite direction relative to the flow of transport in the mitochondrial membrane. Electrons are transported to the outside, and protons accumulate in the inner part of the tilaktoid matrix. The latter takes only a positive charge, and the outer membrane of the tilaktoid is negative. It follows that the path of the proton-type gradient is the opposite of its path in the mitochondria.

The next feature is a large pH level in the potential of protons.

The third feature is the presence in the tilaktoid chain of only two conjugation sites and as a consequence the ratio of the ATP molecule to the protons is 1: 3.

Conclusion

In the first stage, photosynthesis is the interaction of light energy (artificial and non-artificial) with the plant. React to the rays of green substances - chlorophyll, most of which is contained in the leaves.

The formation of ATP and NADP * H is the result of this reaction. These products are necessary for the passage of dark reactions. Hence, the light stage is an obligatory process, without which the second stage will not take place - the dark one.

Dark stage: essence and features

Dark photosynthesis and its reactions are a procedure of carbon dioxide in substances of organic origin with the production of carbohydrates. Such reactions occur in the stroma of the chloroplast and the products of the first stage of photosynthesis, light, take an active part in them.

The mechanism of the dark stage of photosynthesis is based on the process of assimilation of carbon dioxide (also called photochemical carboxylation, the Calvin cycle), which is characterized by cyclicity. It consists of three phases:

1. Carboxylation is the addition of CO ₂ .
2. Recovery phase.
3. The phase of regeneration of ribulosodiphosphate.

Ribulophosphate, a sugar with five carbon atoms, is amenable to phosphorylation by ATP, resulting in the formation of ribulosodiphosphate, which is further subjected to carboxylation by combining with a CO ₂ product with six carbons that are instantly decomposed by reaction with a water molecule, creating two molecular acid particles of a phosphoglycerol . Then this acid undergoes a full recovery course when carrying out the enzymatic reaction, for which the presence of ATP and NADPH is necessary to form a sugar with three carbons - three-carbon sugar, triose or aldehyde of phosphoglycerol. When two such trioses condense, a hexose molecule is obtained that can become an integral part of the starch molecule and be debugged in reserve.

This phase is completed by the fact that during the process of photosynthesis, one CO ₂ molecule is absorbed and three ATP molecules and four N atoms are used. The hexose phosphate is reacted to the reactions of the pentose phosphate cycle, as a result of which regeneration of ribulose phosphate occurs, which can again reunite with another molecule of carbonic acid.

The reactions of carboxylation, reduction, regeneration can not be called specific only for the cell in which photosynthesis takes place. What is the "uniform" flow of processes, too, can not be said, since the difference still exists - in the recovery process, NADP * H is used, and not NAD * N.

The addition of CO _{2 by} ribulosodiphosphate undergoes the catalysis provided by ribulose diphosphate carboxylase. The reaction product is 3-phosphoglycerate, which is reduced by NADP * H2 and ATP to glyceraldehyde-3-phosphate. The reduction process is catalyzed by glyceraldehyde-3-phosphate dehydrogenase. The latter is easily converted to dihydroxyacetone phosphate. Fructose-bisphosphate is formed. Some of its molecules participate in the regenerating process of ribulosodiphosphate, closing the cycle, and the second part is used to create carbohydrate stores in photosynthetic cells, that is, photosynthesis of carbohydrates takes place.

The energy of light is necessary for phosphorylation and synthesis of substances of organic origin, and the energy of oxidation of organic substances is necessary for oxidative phosphorylation. That is why vegetation provides life to animals and other organisms that are heterotrophic.

Photosynthesis in a plant cell occurs in this way. Its product is carbohydrates, which are necessary for the creation of carbon skeletons of many substances of representatives of the world of flora, which are of organic origin.

Substances of the organo-nitrogen type are assimilated in photosynthetic organisms due to the reduction of inorganic nitrates, and sulfur - due to the reduction of sulfates to sulfhydryl groups of amino acids. Provides the formation of proteins, nucleic acids, lipids, carbohydrates, cofactors namely photosynthesis. What is "assorted" substances is vitally important for plants, it has already been emphasized, but not a word was said about the products of secondary synthesis, which are valuable medicinal substances (flavonoids, alkaloids, terpenes, polyphenols, steroids, organic acids and others). Consequently, without exaggeration, we can say that photosynthesis is the pledge of the life of plants, animals and people.