Detailed Explanation of the Mechanism and Products of Photosynthesis

Photosynthesis is not a simple process; it can be divided into two major steps: the light reaction and the dark reaction. The light reaction can only be triggered in the presence of light, while the dark reaction can occur in either light or darkness. The two steps proceed continuously, and under natural conditions, both the light reaction and the dark reaction proceed continuously in the presence of light. Under strong light, the photosynthetic rate is affected by temperature, and within a certain range, it accelerates as temperature increases; under weak light, temperature has no effect. Under strong light, the "light reaction" is rapid, and the entire process is limited by temperature and the "dark reaction"; under weak light, it is limited by the "photochemical reaction." This proves that some steps in photosynthesis require light while others do not, which can improve photosynthetic efficiency. The mechanism of photosynthesis can be divided into four interconnected stages: the primary reaction, photosynthetic electron transport and photophosphorylation, and carbon assimilation. The primary reaction is the starting point of photosynthesis, in which photosynthetic pigments absorb sunlight, completing within 1 millisecond, and is unrelated to temperature. It includes the absorption and transfer of light energy and the photochemical reaction. In the light reaction, water is decomposed by the light energy absorbed by chlorophyll, that is, light with the help of chlorophyll decomposes water into H+ and OH-, and OH- then forms H2O and releases O2. The next step of photosynthesis is how H+ reduces CO2. This reaction does not require light and can proceed through a series of enzymatic reactions, so it is called the dark reaction. Its main content is the fixation and reduction of CO2. The hydrogen atoms released in the light reaction are transferred to CO2 molecules through various compounds, and chlorophyll acts as a hydrogen carrier to accomplish this step. Before CO2 is reduced, it must first undergo fixation, being fixed into a certain compound before it can be reduced by hydrogen into organic matter. CO2 fixation includes the C3 pathway, the C4 pathway, and the 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 also include proteins, fats, and organic acids, though these substances account for a relatively small proportion of the direct products of photosynthesis. Most proteins, fats, and organic acids are re-synthesized from intermediate products of carbohydrate metabolism. These substances are further synthesized into various other organic compounds such as nucleic acids, auxins, vitamins, lignin, plant alkaloids, anthocyanins, etc. The quantitative proportions of various photosynthetic products are closely related to the age of the plant and leaves, the quality and quantity of light, and the nitrogen nutrition status. Young tender leaves produce more proteins, so they are softer and more drooping; mature leaves tend to form only carbohydrates. When light increases, more carbohydrates are produced, so the leaves are stiffer and more likely to "head up"; when light decreases, more proteins are formed. Under red light, the plant synthesizes more carbohydrates; under blue light, it synthesizes more proteins. When nitrogen nutrition increases, more proteins are produced; when nitrogen nutrition decreases, more carbohydrates are formed. Therefore, under conditions of high nitrogen and weak light, more proteins are formed in the leaves, which often causes the plant to grow excessively tall and soft, prone to lodging, losing ornamental value and even commercial value. Therefore, in thecultivationlight management of Chinese orchids, measures such as using less nitrogen, slightly stronger light, and more red light should be adopted. (Liu Zhongjian)