Photosynthesis is a process of processing water and carbon dioxide using light energy or without it. It is characteristic of plants. Let us further consider what the dark and light phase of photosynthesis are.
The organ of photosynthesis in higher plants is a leaf. Organoids are chloroplasts. In the membranes of their thylakoids, photosynthetic pigments are present. They are carotenoids and chlorophylls. The latter exist in several forms (a, c, b, d). The most important of them is a-chlorophyll. In its molecule, a porphyrin "head" is distinguished with a magnesium atom located in the center, and also a phytol "tail". The first element is presented in the form of a flat structure. The "head" is hydrophilic, so it is located on that part of the membrane that is directed towards the aquatic environment. The phytol "tail" is hydrophobic. Due to this, it retains the chlorophyll molecule in the membrane. Chlorophylls absorb blue-violet and red light. They also reflect green, due to which plants have a characteristic color for them. In the tilaktoid membranes, chlorophyll molecules are organized into photosystems. For blue-green algae and plants, systems 1 and 2 are characteristic. 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 multistep. In particular, two groups of reactions are distinguished. They are the dark and light phase of photosynthesis. The latter takes place with the participation of the enzyme ATP, proteins, carrying electrons, and chlorophyll. The light phase of photosynthesis occurs in the membranes of the tilaktoid. Chlorophyll electrons are excited and leave the molecule. After that they fall on the outer surface of the tilaktoid membrane. It, in turn, is charged negatively. After oxidation, the chlorophyll molecules are restored. They select electrons from the water that is present in the intralakoid space. Thus, the light phase of photosynthesis proceeds in the membrane upon decay (photolysis): H2 O + Q light → H + + OH -
Hydroxyl ions are converted into reactive radicals, giving up their electrons:
OH - → • OH + e -
• OH radicals combine to form free oxygen and water:
At the same time, oxygen is removed into the surrounding (external) environment, and inside the tilaktoid is the accumulation of protons in a special "reservoir". As a result, where the light phase of photosynthesis proceeds, the thylactoid membrane due to H + on one side receives a positive charge. At the same time, due to electrons, it charges negatively.
Phosphorylation of ADP
Where there is a light phase of photosynthesis, there is a potential difference between the inner and outer surfaces of the membrane. When it reaches 200 mV, protons are propelled through the ATP synthetase channels. 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 recovery of a special carrier of nicotinamide adenine dinucleotide phosphate NADP + to NADP • H2:
2H + + 2e - + NADP → NADP • H2
The light phase of photosynthesis, thus, includes the photolysis of water. He, in turn, is accompanied by three major reactions:
- Synthesis of ATP.
- Formation of NADP • H2 .
- Formation of oxygen.
The light phase of photosynthesis is accompanied by the release of the latter into the atmosphere. NADP • H2 and ATP move to 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 in the stroma of the chloroplast. The reactions are represented as 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. As the acceptor of carbon dioxide acts Ribulozobiphosphate (five-carbon sugar) RIBF. The catalyst in the reaction is ribulose-bisphosphate-carboxylase (enzyme). As a result of carboxylation of RibF, a six-carbon, unstable compound is formed. It almost instantly decomposes into two molecules of PGA (phosphoglyceric acid). After this, a series of reactions takes place, where it passes through several intermediate products into glucose. They use the energy of NADP • H2 and ATP, which were converted when the light phase of photosynthesis was moving. The cycle of these reactions is called 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, glycerin, amino acids, nucleotides.
They represent a type of photosynthesis, in which three-carbon compounds are formed as the first product. It was he described above as the Calvin cycle. As characteristic features of C3-photosynthesis are:
- ReBF is an acceptor for carbon dioxide.
- The carboxylation reaction catalyzes the RibF-carboxylase.
- A six-carbon substance is formed, which subsequently decomposes into 2 FGCs.
Phosphoglyceric acid is reduced to TF (triose phosphate). Some of them are directed to the regeneration of ribulosebobosphate, and the rest is converted to glucose.
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 set for millet, sorghum, sugarcane, corn. These cultures began to be called C4-plants. In the following, 1966, Slack and Hatch (Australian scientists) found that they almost completely lack photorespiration. It was also found that such C4 plants are much more effective in absorbing carbon dioxide. As a result, the path of carbon transformation in such cultures began to be called Hatch-Slack.
The importance of photosynthesis is very high. Thanks to it from the atmosphere carbon dioxide is annually absorbed in huge volumes (billions tons). Instead, not less oxygen is released. 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 short-wave UV radiation. In the process of photosynthesis, the leaf absorbs only 1% of the total energy of light incident on it. Its productivity is within 1 g of organic compound per 1 sq. Km. m surface per hour.