What is Photosynthesis?

Photosynthesis usually refers to the process by which green plants (including algae) absorb light energy, synthesize carbon dioxide and water into energy-rich organic matter, and release oxygen at the same time. ^[1] It mainly includes two phases of light reaction and dark reaction. ^{[2] It} involves important reaction steps such as light absorption, electron transfer, photosynthetic phosphorylation, carbon assimilation, etc., which can realize the energy conversion in nature and maintain the carbon-oxygen balance of the atmosphere Is of great significance. ^[1]

Green plants make use of the sun

The process of photosynthesis is a relatively complicated problem. On the surface, the overall reaction formula of photosynthesis seems to be a simple redox process, but essentially includes a series of photochemical steps and matter conversion issues. According to modern data, the entire photosynthesis can be roughly divided into the following three steps: the original reaction, including absorption, transfer and conversion of light energy; electron transfer and photosynthetic phosphorylation to form active chemical energy (ATP and NADPH); Carbon assimilation converts active chemical energy into stable chemical energy (fixes CO ₂ to form sugars). ^[6]

C3 Photosynthetic C3 plant

Plants that fix CO ₂ through the C ₃ pathway are called C ₃ plants, and the starch from their photosynthesis is stored in mesophyll cells because this is the site of the Calvin cycle. C ₃ plants belong to the type of high-light breathing plants, which have low photosynthetic rate, many types, wide distribution, and mostly grow in warm and humid conditions, such as most trees, plant food, tobacco, etc. ^[3]

C4 Photosynthetic C4 plants

Plants that fix CO ₂ through the C ₄ pathway are called C ₄ plants, and they are mainly those living in drought

Figure 4 Schematic diagram of C4 plant metabolism

Tropical plants. In this environment, if plants open their stomata for a long time to absorb carbon dioxide, water will be lost through transpiration too quickly. Therefore, plants can only open the stomata for a short time, and the amount of carbon dioxide intake is bound to be small. Plants must use this small amount of carbon dioxide for photosynthesis to synthesize substances needed for their own growth. ^[3]

The biological characteristics of C ₄ plants are very different from C ₃ plants. They have higher water use efficiency and nitrogen use efficiency than C ₃ plants. C ₄ plant mesophyll cells contain a unique enzyme, namely phosphoenolpyruvate carbon oxidase, which makes carbon dioxide assimilated by a three-carbon compound phosphoenolpyruvate to form a four-carbon compound oxaloacetate. It is also the origin of the dark reaction type name. The advantage of this type is that the efficiency of carbon dioxide fixation is much higher than that of C3, which is conducive to plant growth in arid environments. The starch of the C ₄ plant will be stored in the vascular bundle sheath cells, because the Calvin cycle of the C ₄ plant occurs here. There are few species of C ₄ plants and their distribution is limited. They are suitable for growing in high temperature and dry climate conditions. Most of the weeds belong to C ₄ plants. ^[3]

CAM Photosynthetic CAM plants

If C ₄ plants are staggered in space with carbon dioxide fixation and Calvin cycle, crassulacean acid metabolism (CAM) is staggered in time. The plants that use this approach are those with enlarged fleshy leaves, such as pineapples. These plants open their stomata at night and absorb carbon dioxide, which also fixes CO ₂ via the Hatch-Slack pathway. The stomata are closed in the morning to avoid excessive water loss. At the same time the Calvin cycle begins in mesophyll cells. The carbon dioxide fixation efficiency of these plants is also very high. ^[3]

Photosynthesis algae and bacteria

Eukaryotic algae, such as red algae, green algae, brown algae, etc., have chloroplasts like higher plants and can also perform oxygen-generating photosynthesis. Light is absorbed by chlorophyll, and many algae have different pigments in their chloroplasts, giving them different colors. ^[3]

Photosynthetic bacteria do not have chloroplasts, but proceed directly from the cells themselves. Cyanobacteria (or "cyanobacteria"), which belong to prokaryotes, also contain chlorophyll, which produces oxygen and photosynthesis in the same way as chloroplasts. In fact, it is widely believed that chloroplasts evolved from cyanobacteria. Other photosynthetic bacteria have a variety of pigments, called bacterial chlorophyll or chlorophyll, but do not oxidize water to generate oxygen, and use other substances (such as hydrogen sulfide, sulfur or hydrogen) as electron donors. Non-oxygen-producing photosynthetic bacteria include purple sulfur bacteria, purple non-sulfur bacteria, green sulfur bacteria, green non-sulfur bacteria, and solar bacillus. ^[3]

In June 2018, a new study published in the American Science magazine showed that cyanobacteria can use near-infrared light for photosynthesis, and its mechanism is different from that previously known. This discovery is expected to bring new ideas for finding alien life and improving crops. New research finds that the above cyanobacteria normally use "chlorophyll-a" for photosynthesis in the presence of visible light, but if they are in a dark environment and lack visible light, they will switch to using "chlorophyll-f" and use near Infrared light performs photosynthesis. ^[8]