Photosynthesis is the most important process underlying the emergence and existence of the vast majority of organisms on Earth.
Photosynthesis is the process of forming organic compounds from carbon dioxide (CO2) and water (H2O) using light energy.
Chloroplasts in plant cells and folds of the cytoplasmic membrane of prokaryotes contain a green pigment – chlorophyll. Chlorophyll has a special chemical structure that allows it to capture light quanta. The chlorophyll molecule is capable of being excited by sunlight, donating its electrons and moving them to higher energy levels.
Example: This process can be compared to a ball tossed upwards. As it rises, the ball stores up potential energy; falling, he loses it. The electrons do not fall back, but are picked up by the electron carrier molecules NADP + (nicotinamide adenine dinucleotide phosphate). In this case, their energy is partially spent on the formation of ATP.
The process of photosynthesis includes two successive phases: light and dark.
The light phase is the stage at which the light energy absorbed by chlorophyll is converted into electrochemical energy in the electron transport chain. It is carried out in the light, in the membranes of gran thylakoids, with the participation of carrier proteins and ATP synthetase.
The light phase of plant photosynthesis includes non-cyclic phosphorylation and photolysis of water.
The following processes take place on the photosynthetic membranes of gran chloroplasts:
- excitation of chlorophyll electrons by light quanta and their transition to a higher energy level;
- restoration of electron acceptors – NADP + to NADP⋅H2;
- photolysis of water, which occurs with the participation of light quanta:
2H2O → 4H ++ 4e− + O2.
The results of light reactions are:
- photolysis of water with the formation of free oxygen;
- synthesis of ATP;
- restoration of NADP + to NADP⋅N.
In the reactions of the light phase of photosynthesis, energy is accumulated in NADPH and ATP, which is spent in the processes of the dark phase.
The synthesis of ATP from ADP at the expense of light energy is a very effective process: in the same time, chloroplasts produce 30 times (!) More ATP than in mitochondria.
During the light phase, energy-rich molecules and hydrogen ions are formed, which are necessary for the dark phase of photosynthesis. Further processes of photosynthesis can proceed without sunlight.
The reactions of the dark phase of photosynthesis proceed independently of light.
The dark phase is the process of converting CO2 into glucose using the energy stored in the ATP and NADPH molecules.
These reactions take place in the stroma of chloroplasts, where energy-rich substances come from thylakoids: NADPH and ATP, accumulated in the reactions of the light phase of photosynthesis.
The plant receives its carbon source (CO2) from the air through the stomata.
The transformation of carbon dioxide into glucose during the dark phase of photosynthesis is called the Calvin cycle after its discoverer.
The result of dark reactions is the conversion of carbon dioxide into glucose and then into starch.
In addition to glucose molecules, amino acids, nucleotides, and alcohols are formed in the stroma of chloroplasts.
The summary equations and partial reactions of photosynthesis are presented in the table.
The importance of photosynthesis
1. In the process of photosynthesis, free oxygen is formed, which is necessary for the respiration of organisms.
2. Photosynthesis ensures a constant level of CO2 and O2 in the atmosphere.
3. Photosynthesis provides the formation of organic matter, and therefore food for all living things.
4. In the upper layers of the Earth’s air envelope, ozone O3 is formed from oxygen, from which a protective ozone screen is formed, which protects organisms from the life-threatening effects of ultraviolet radiation.