Photosynthesis is the process of treating water and carbon dioxide with or without light energy. It is characteristic of plants. Consider further what constitutes the dark and light phase of photosynthesis.Dark and light phase of photosynthesis

General information

The organ of photosynthesis in higher plants is a leaf. Chloroplasts act as organoids. Photosynthetic pigments are present in the membranes of their thylakoids. They are carotenoids and chlorophylls. The latter exist in several forms (a, c, b, d). The main of them is considered a-chlorophyll. In its molecule, a porphyrin “head” with a magnesium atom located in the center, as well as a fitl “tail” is released. The first element is presented in the form of a flat structure. The “head” is hydrophilic, therefore it is located on the part of the membrane that is directed towards the aquatic environment. Fitnik “tail” is hydrophobic. Due to this, it retains the chlorophyll molecule in the membrane. Chlorophyll absorbs blue-violet and red light. They also reflect green, due to which the plants have a characteristic color for them. In the membranes of tilaktoidov chlorophyll molecules are organized into photosystems. Systems 1 and 2 are characteristic of blue-green algae and plants. Photosynthetic bacteria have only the first. The second system can decompose H2  Oh, give off oxygen.

Light phase of photosynthesis

The processes occurring in plants are complex and multi-stage. In particular, there are two groups of reactions. They are the dark and light phase of photosynthesis. The latter proceeds with the participation of the enzyme ATP, electron-transporting proteins, and chlorophyll. The light phase of photosynthesis occurs in the membranes of tilaktoidov. Chlorophyll electrons are excited and leave the molecule. After that, they fall on the outer surface of the membrane of tilaktoida. She, in turn, charges negatively. After oxidation begins the restoration of chlorophyll molecules. They take electrons from water that is present in the intralacoid space. Thus, the light phase of photosynthesis proceeds in the membrane during disintegration (photolysis): H2  O + Q light → H + + HE -

Hydroxyl ions are converted to reactive radicals, giving away their electrons:

HE - → • HE + e -

• OH radicals combine to form free oxygen and water:

At the same time, oxygen is removed to the surrounding (external) environment, and inside the tilactoid there is an accumulation of protons in a special “reservoir”. As a result, where the light phase of photosynthesis proceeds, the membrane of the thylactoid due to H + on the one hand receives a positive charge. Along with this, due to electrons, it is charged negatively.

ADH phosphorylation

Where the light phase of photosynthesis proceeds, there is a potential difference between the inner and outer surfaces of the membrane. When it reaches 200 mV, protons are pushed through the channels of ATP synthetase. Thus, the light phase of photosynthesis occurs in the membrane during the phosphorylation of ADP to ATP. At the same time, atomic hydrogen is directed to the reduction of a special carrier of nicotinamide adenine dinucleotide phosphate NADP + to NADP • H2:

2H + + 2e - + NADF → NADF • N2

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

  1. ATP synthesis.
  2. Education NADF • N2 .
  3. The formation of oxygen.

The light phase of photosynthesis is accompanied by the release of the latter into the atmosphere. NADPH • H2 and ATP move into the stroma of the chloroplast. This completes the light phase of photosynthesis.

Another group of reactions

For the dark phase of photosynthesis, no light energy is needed. It goes to the stroma of the chloroplast. Reactions are presented in the form of a chain of successively occurring transformations of carbon dioxide coming from the air. As a result, glucose and other organic substances are formed. The first reaction is fixation. Ribulozobifosfat (five-carbon sugar) riBF acts as a carbon dioxide acceptor. The catalyst in the reaction is ribulozobifosfat-carboxylase (enzyme). As a result of carboxylation of RibP, a six-carbon unstable compound is formed. It almost instantly breaks down into two molecules of FGK (phosphoglyceric acid). After this, there is a cycle of reactions, where it is transformed into glucose through several intermediate products. They use the energy of NADP • N2  and ATP, which were converted when the light phase of photosynthesis was going on. The cycle of these reactions is referred to as the “Calvin cycle”. It can be represented as follows:

In addition to glucose, other monomers of organic (complex) compounds are formed during photosynthesis. These include, in particular, fatty acids, glycerol, amino acid nucleotides.

C3 reaction

They are a type of photosynthesis in which three-carbon compounds are formed as the first product. That he described above as the cycle of Calvin. The characteristic features of C3 photosynthesis are:

  1. RibF is an acceptor for carbon dioxide.
  2. The carboxylation reaction is catalyzed by ribof-carboxylase.
  3. A six-carbon substance is formed, which subsequently decomposes into 2 FGKs.

Phosphoglyceric acid is reduced to TF (triosophosphates). Some of them are directed to the regeneration of ribulozobifosfat, and the rest is converted into glucose.

C4 reaction

This type of photosynthesis is characterized by the appearance of four carbon compounds as the first product. In 1965, it was revealed that C4 substances appear first in some plants. For example, it was found for millet, sorghum, sugarcane, corn. These cultures began to be called C4-plants. In the following, 1966, Slack and Hatch (Australian scientists) revealed that they almost completely lack photorespiration. It was also found that such C4 plants are much more efficient in absorbing carbon dioxide. As a result, the path of carbon transformation in such cultures was called the Hatch-Slack path.


The value of photosynthesis is very large. Thanks to him, huge amounts of carbon dioxide (billions of tons) are absorbed from the atmosphere annually. Instead, it produces no less oxygen. Photosynthesis acts as the main source of formation of organic compounds. Oxygen is involved in the formation of the ozone layer, which protects living organisms from exposure to short-wave UV radiation. In the process of photosynthesis, the leaf absorbs only 1% of the total energy of the light falling on it. Its productivity is within 1 g of an organic compound per 1 sq. Km. m surface per hour.