The mechanism and products of photosynthesis

Photosynthesis is not a simple process, it can be divided into two steps: light reaction and dark reaction. The light reaction can only be excited in the light, and the dark reaction can occur in the light or the dark. The two steps are carried out continuously, and under natural conditions, both light reaction and dark reaction are carried out continuously in the presence of light.

The photosynthetic rate is affected by temperature in strong light, and accelerates with the increase of temperature in a certain range, but has no effect in weak light. In strong light, the "light reaction" is rapid, and the whole process is limited by temperature and "dark reaction"; in low light, it is limited by "photochemical reaction". It is proved that some steps in photosynthesis need light and some steps do not need light, which can improve the photosynthetic efficiency.

The mechanism of photosynthesis can be divided into four interrelated links: primary reaction, photosynthetic electron transport, photophosphorylation and carbon assimilation.

The primary reaction is the starting point of photosynthesis, which is the absorption of sunlight by photosynthetic pigment, which is completed in 1 millisecond and has nothing to do with temperature. It includes the absorption, transfer and photochemical reaction of light energy.

In the light reaction, water is decomposed by the light energy absorbed by chlorophyll, that is, with the help of chlorophyll, water is decomposed into H + and OH-,OH- to form H2O and O2 is released.

The next step in photosynthesis is how to reduce CO2 by H +. This reaction can carry out a series of enzymatic reactions without light, so it is called dark reaction. Its main content is that CO2 is fixed and restored. The hydrogen atom decomposed in the light reaction is transferred to the CO2 molecule by different compounds, and chlorophyll is used as a hydrogen carrier to achieve this step. CO2 has to be fixed before it is reduced, and it must be fixed into a compound before it can be reduced to organic matter by hydrogen. CO2 is fixed by C3 pathway, C4 pathway and CAM pathway.

The direct products of photosynthesis are carbohydrates, including monosaccharides, disaccharides and polysaccharides, among which sucrose and starch are the most common. The direct products of photosynthesis are proteins, fats and organic acids, but these substances account for a small proportion of the direct products of photosynthesis, and most proteins, fats and organic acids are re-synthesized through intermediates of carbohydrate metabolism. These substances re-synthesize a variety of other organic compounds such as nucleic acid, auxin, vitamins, lignin, plant alkaloids, anthocyanins and so on.

The proportion of various products of photosynthesis in quantity is closely related to the age of plants and leaves, the quality and quantity of light and nitrogen nutrition. The young leaves produce more protein, so they are more saggy, and the growing leaves tend to form only carbohydrates; when the light increases, they produce more carbohydrates, so the leaves are stiff and easy to "knot", and the light is weakened. More proteins are formed; under red light, plants synthesize more carbohydrates. Under blue light, more protein was synthesized, nitrogen nutrition increased, more protein was produced, nitrogen nutrition decreased, and more carbohydrates were formed. Therefore, in the case of high nitrogen and weak light, more proteins are formed in the leaves, which often results in overgrowth and softness of plants, easy lodging, loss of ornamental value and even commercial value. Therefore, in the light management of national orchid cultivation, measures such as less nitrogen, slightly strong light and more red light should be adopted. (Liu Zhongjian)