It usually refers to the process in 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. It mainly includes two stages: light reaction and dark reaction, involving important reaction steps such as light absorption, electron transfer, photosynthetic phosphorylation, and carbon assimilation. It is of great significance to achieve energy conversion in nature and maintain the carbon-oxygen balance of the atmosphere.
The earliest photosynthesis Figure 2 Bitter plants that perform photosynthesis In 1990, a red algae fossil was discovered in the Canadian Arctic. This red algae is the first known sexually reproducing species on Earth and is also considered to be the oldest ancestor of modern plants and animals discovered. There has been no consensus on the age of the red algae fossils, and most people believe that they lived about 1.2 billion years ago. In order to determine the age of this red algae fossil, researchers went to Baffin Island in Canada to collect black shale containing this red algae fossil and analyzed it using rhenium-osmium isotope dating. It is believed that the red algae fossil is 1.047 billion years old. Based on the confirmation of the age of the red algae fossils, the researchers used a mathematical model called the “molecular clock” to calculate biological evolution events based on gene mutation rates. They concluded that about 1.25 billion years ago, eukaryotes began to evolve chlorophyll that can perform photosynthesis.
Main research progress Ancient Greek philosopher Aristotle believed that all the nutrients needed for plant growth came from the soil. In 1642, Belgian Jan Baptista van Helmont conducted a “willow experiment”. He only watered the willow for 5 consecutive years. The weight of the willow increased by 75kg, and the soil mass only decreased by 60g. He inferred that the weight of the plant did not come mainly from the soil but from the water. But he did not find that substances in the air also participated in the formation of organic matter. In 1771, British priest and chemist Joseph Priestley conducted a closed bell jar experiment. He found that the candle would not go out in the closed bell jar with plants, and the mouse would not suffocate to death. So in 1776, Priestley proposed that plants can “purify” the air. But he could not repeat his experiment many times, which shows that plants are not always able to “purify” the air. [1] In 1773, Dutch doctor Jan Ingenhousz conducted several experiments based on Priestley’s research and found that the reason why Priestley’s experiments could not be repeated many times was that he ignored the role of light. Plants can only “purify” the air under light. The above three scientists are the pioneers of photosynthesis research. Priestley is generally regarded as the discoverer of photosynthesis, and 1771 is set as the year of discovery of photosynthesis. In 1782, Swiss Jean Senebier used chemical methods to discover that CO2 is a necessary substance for photosynthesis and O2 is a product of photosynthesis. [1] In 1804, Swiss Nicolas-Théodore de Saussure proved through quantitative experiments that the total amount of organic matter produced and released by plants is greater than the CO2 consumed, and further confirmed that water is involved in the reaction of photosynthesis. [1] In 1864, J. V. Sachs found that illuminated leaves would turn blue when exposed to iodine, proving that photosynthesis forms carbohydrates (starch). [1] At the end of the 19th century, it was proved that the raw materials of photosynthesis are CO2 in the air and H2O in the soil, the energy source is solar radiation, and the products are sugar and O2. [1] At the beginning of the 20th century, the molecular mechanism of photosynthesis made a breakthrough. The milestone work was: Wilstatter et al. (1915) won the Nobel Prize for purifying chlorophyll and clarifying its chemical structure. [1] Subsequently, Blackman in the UK and O. Warburg in Germany used algae to conduct flash experiments to prove that photosynthesis can be divided into two stages: light reaction (light reaction) and dark reaction (dark reaction) that do not require light. [1] In 1932, R. Emersen and W. Arnold defined the “photosynthetic unit” by conducting a flash experiment on a chlorella suspension, that is, the number of 2500 chlorophyll molecules required to release or assimilate one molecule of CO2. Later in 1986, Hall et al. pointed out that the photosynthetic unit should be about 600 chlorophyll molecules (300×2) including two reaction centers and the photosynthetic electron transport chain connecting the two reaction centers. Most people agree with Hall’s view that the so-called “photosynthetic unit” refers to the smallest structural unit that exists on the thylakoid membrane and can carry out a complete light reaction. [1] From the 1940s to the late 1950s, Melvin Ellis Calvin and others used carbon 14 to study photosynthetic carbon assimilation and clarified the biochemical pathway for the conversion of CO2 into organic matter. Calvin won the Nobel Prize in 1961. Later, the CAM pathway (M. Thomas, 1960) and the C4 pathway (M. D. Hatch and C. B. Slack, 1966) were successively identified. In 1954, American scientist D. I. Arnon and others discovered that when inorganic phosphorus, ADP and NADP+ were supplied to the system when illuminating chloroplasts, ATP and NADPH would be produced in the system. At the same time, as long as ATP and NADPH are supplied, chloroplasts can convert CO2 into sugar even in the dark. Therefore, it is concluded that the essence of the light reaction is to produce “assimilatory power” to promote the dark reaction, and the essence of the dark reaction is to use “assimilatory power” to convert inorganic carbon (CO2) into organic carbon (CH2O). In 1957, Emerson observed that when Chlorella was irradiated with far-red light and a little shorter wavelength light (such as 650 nm light) was added, the quantum yield was higher than the sum of the two wavelengths of light alone. This phenomenon of promoting photosynthetic efficiency by adding shorter wavelength light in addition to long-wave red light is called the double light gain effect, or the Emerson enhancement effect. Later, it was learned that this is because photosynthesis requires the synergistic action of two photochemical reactions. [1] In 1960, Hill et al. proposed the concept of two photosystems, calling the system that absorbs long-wavelength light photosystem I (PS I) and the system that absorbs short-wavelength light photosystem II (PS II), which promoted the separation and purification of PS I and PS and other biochemical and molecular biological research. In 1965, Robert Burns Woodward won the Nobel Prize in Chemistry for his work on the total synthesis of chlorophyll molecules. [15] In the early 1980s, P. Mitchell proposed the chemiosmotic hypothesis. Jagendorf et al. conducted a phased study on photosynthetic phosphorylation using chloroplasts and proved that the high-energy state of photosynthetic phosphorylation is the transmembrane proton gradient in the chemiosmotic hypothesis. This not only enabled people to understand the energy conversion mechanism in photosynthesis, but also led to the study of the connection between proton motive force and ion movement, dynamic changes in thylakoid structure and the regulation process of energy conversion reactions. In the late 1980s, Johann Deisenhofer and others determined the structure of the reaction center of photosynthetic bacteria, making outstanding progress in understanding the details of membrane protein complexes and the study of primary photosynthetic reactions, and won the 1988 Nobel Prize in Chemistry. In 1992, Rudolph A. Marcus won the Nobel Prize for his research on the electron transfer theory of living systems, including photosynthetic electron transfer. [1] In the late 1990s, major progress was made in the study of the dynamic structure and reaction mechanism of enzymes catalyzing photosynthetic phosphorylation in photosynthesis and oxidative phosphorylation in respiration. John E. Walker and Paul D. Boyer won the 1997 Nobel Prize in Chemistry. [1] On August 19, 2010, Dr. Min Chen and others from the University of Sydney, Australia, discovered a fifth type of chlorophyll, chlorophyll f, in strmoatolite in Shark Bay, Western Australia. This type of chlorophyll has an absorption spectrum that is much redder than other types of chlorophyll, extending into the near-infrared range, with an absorption peak at 722nm. [12] On June 15, 2018, Andrea Fantuzzi and A. William Rutherford from Imperial College London discovered that cyanobacteria (Chroococcidiopsis thermalis) will normally use “chlorophyll a” for photosynthesis in the presence of visible light, but if it is in a dark environment and lacks visible light, it will switch to using “chlorophyll f” and use near-red light for photosynthesis. This type of photosynthesis can use lower energy infrared light for photosynthesis reactions, surpassing the “red light limit” of photosynthesis, which represents the third type of photosynthesis that is widely present in nature. On July 13, 2020, the research team of Roberta Croce of the Free University of Amsterdam showed that the insertion of chlorophyll f slowed down the overall energy capture of the two photosystems, especially greatly reducing the efficiency of photosystem II. However, despite the lower energy output, it is still advantageous to insert red-shifted chlorophyll f into the photosystem in an environment rich in far-red light. China’s photosynthesis research has made great progress since the 1950s. For example, the Shanghai Institute of Plant Physiology of the Chinese Academy of Sciences has made discoveries and innovations in the enzymatic research of photosynthetic energy conversion and photosynthetic carbon metabolism, and the Institute of Botany of the Chinese Academy of Sciences has made discoveries and innovations in the primary reactions of photosynthesis and the research of photosynthetic pigment protein complexes. In May 2023, Professor Wan Yinglang of the College of Tropical Crops of Hainan University and his team discovered two new species of white-edged side-foot sea slug and hairy sea slug in the mangroves of Dongzhaigang National Nature Reserve in Hainan and the coral reefs of Yunlong Bay in Wenchang City. This is the first officially recorded mollusk in mainland China that can rely on photosynthesis to provide “food”. In February 2024, according to the official website of Okayama University in Japan, Professor Jianren Shen and others from Okayama University in Japan successfully captured the moment when the catalyst in the protein responsible for plant photosynthesis absorbed water molecules. [Although the history of photosynthesis research is not long, through the hard work of many scientific researchers, remarkable progress has been made, providing a sufficient theoretical basis for guiding agricultural production.
Significance
Converting solar energy into chemical energy Plants convert solar energy into chemical energy while assimilating inorganic carbon compounds and store it in the organic compounds formed. The solar energy assimilated by photosynthesis each year is about 10 times the energy required by humans. The chemical energy stored in organic matter, in addition to being used by the plants themselves and all heterotrophic organisms, is more importantly a source of energy for human nutrition and activities. ] Therefore, it can be said that photosynthesis provides the main energy source today. Green plants are a giant energy conversion station. Converting inorganic matter into organic matter The scale of plants producing organic matter through photosynthesis is very large. It is estimated that plants can absorb about 7×1011 tons of CO2 and synthesize about 500 billion tons of organic matter each year. [40% of the carbon assimilated by autotrophic plants on the earth is assimilated by phytoplankton, and the remaining 60% is assimilated by terrestrial plants. The food, oil, fiber, wood, sugar, fruit, etc. needed by humans all come from photosynthesis. Without photosynthesis, humans would have no food and various daily necessities. In other words, without photosynthesis, there would be no human survival and development.