most photosynthesis takes places in leaves, mesophyll cells and organelles known as chloroplast The first step in photosynthesis is the absorption of light energy to be used in the chloroplast.
ATP is a key ingredient in the final phase of photosynthesis, which occurs independent of light.
Light Independent Stage The light independent stage is sometimes called the dark stage, assuming that it takes place in the dark.
These two stages of photosynthesis are known as the (the photo part of photosynthesis) and the (the synthesis part) (FIGURE 10-4).
C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called . When the CO2 concentration in the drops below about 50 ppm, the catalyst that helps to fix carbon begins to fix oxygen instead. This is highly wasteful of the energy that has been collected from the light, and causes the rubisco to operate at perhaps a quarter of its maximal rate.
C4 plants almost never saturate with light and under hot, dry conditions much outperform . They use a two-stage process were CO2 is fixed in thin-walled mesophyll cells to form a 4-carbon intermediate, typically malate (malic acid). The reaction involves phosphoenol pyruvate () which fixes CO2 in a reaction catalyzed by PEP-carboxylate. It forms oxaloacetic acid (OAA) which is quickly converted to malic acid. The 4-carbon acid is actively pumped across the cell membrane into a thick-walled bundle sheath cell where it is split to CO2 and a 3-carbon compound.
A byproduct of this chemical reaction is oxygen, which is released through holes in the leaf called stomates.
The light-dependent reactions, or photoreduction, is the first stage of photosynthesis, the process by which plants capture and store energy from sunlight.
Respiration refers to the metabolism of oxygen and the release of carbon dioxide. In it is a positive term, a process vital to life. But photorespiration is an entirely negative term because it represents a severe loss to the process of using light energy in photosynthetic organisms to fix carbon for subsequent carbohydrate synthesis. By leading to the loss of up to half of the carbon that has been fixed at the expense of light energy, photorespiration undoes the work of photosynthesis.
The acidity was found to arise from the opening of their stomata at night to take in CO2 and fix it into malic acid for storage in the large vacuoles of their photosynthetic cells. It could drop the pH to 4 with a malic acid concentration up to 0.3M . Then in the heat of the day, the stomata close tightly to conserve water and the malic acid is decarboxylated to release the CO2 for fixing by the Calvin cycle. PEP is used for the initial short-term carbon fixation as in the , but the entire chain of reactions occurs in the same cell rather than handing off to a separate cell as with the C4 plants. In the CAM strategy, the processes are separated temporally, the initial CO2 fixation at night, and the malic acid to Calvin cycle part taking place during the day.
Thylakoid membranes, especially those of the grana, are the sites of the light reactions, whereas the Calvin cycle occurs in the stoma.
In this process, light energy is converted into chemical energy, in the form of the energy-carrying molecules ATP(Adenosine triphosphate) and NADPH(Nicotinamide adenine dinucleotide phosphate).
Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the fight reactions regenerate the NADPH and ATP spent in the reduction Of C02 to sugar.
The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or fight-independent reactions, because none of the steps requires light directly.
However, although light-independent reactions are, by convention, also called dark reactions, they are not independent of the need of light, for they are driven by ATP and NADPH, products of light.