photosynthesis

[guāng hé zuò yòng]

The process of green plants absorbing light energy to synthesize carbon dioxide and water into organic matter and release oxygen at the same time

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synonym Plant photosynthesis (Photosynthesis) Generally refers to photosynthesis (the process in which green plants absorb light energy to synthesize carbon dioxide and water into organic matter and release oxygen at the same time)

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Photosynthesis, usually refers to Green plants (including algae ）Absorption Light energy ，把 carbon dioxide Energy rich synthesis with water organic compound , release at the same time oxygen Process.^[1] It mainly includes Photoreaction 、 Dark reaction Two stages,^[2] involve light absorption 、 electron transport 、 Photosynthetic phosphorylation 、 Carbon assimilation And other important reactions step , for the realization of natural energy conversion , atmospheric Carbon oxygen balance It is of great significance.^[1]

Chinese name: photosynthesis
Foreign name: Photosynthesis
Place of occurrence: Green plants (including algae ）Of chloroplast Photosynthetic bacteria^[3]

Reaction conditions: Photopigment molecular enzyme, carbon dioxide (or hydrogen sulfide)
Reaction product: Enrich energy organic compound^[1]
environmental effect: Regulate the atmosphere Carbon oxygen balance^[1]

definition

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Green plants utilize the sun Light energy , assimilating carbon dioxide (CO two ）And water (H two O) The process of producing organic matter and releasing oxygen is called photosynthesis. The organic substances produced by photosynthesis are mainly carbohydrate And release oxygen.^[4]

photosynthesis

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The earliest photosynthesis

Fig. 2 Bitter plants for photosynthesis

In 1990, a red alga fossil was found in the Canadian Arctic. This red alga is the first known species of sexual reproduction on earth, and is also considered to be the oldest ancestor of modern animals and plants. There was no consensus on the age of red algae fossils before, most of which believed that they lived about 1.2 billion years ago.

In order to determine the age of this red algal fossil, researchers went to Baffin Island, Canada to collect the black shale containing this red algal fossil and analyzed it with rhenium and osmium isotope dating method. It is believed that the red algal fossil has a history of 1.047 billion years.

On the basis of confirming the age of red algae fossils, the researchers used a mathematical model called "molecular clock" to calculate biological evolution events based on gene mutation rate. They concluded that about 1.25 billion years ago, eukaryotes began to evolve chlorophyll that can carry out photosynthesis.^[5]

Main research progress

Ancient Greek philosopher Aristotle It is believed that all the nutrients needed for plant growth come from the soil.

Belgian in 1642 Jan Baptist van Helmont (Jan Baptista van Helmont) conducted the "willow experiment", and only watered for five consecutive years. The willow weight increased by 75kg and the soil mass decreased by only 60g. From this, it can be inferred that the weight of plants mainly comes from water rather than soil. But he did not find that the substances in the air also participated in the formation of organic matter.^[10]

In 1771, an English priest and chemist Joseph Priestley (Joseph Priestley). He found that the candles in the airtight bell jar with plants would not go out, and the mice would not suffocate to death. So in 1776, Priestley proposed that plants could "purify" the air. But he could not repeat his experiment many times, which showed that plants could not always "purify" the air.^[1]

In 1773, the Dutch doctor Jan Ingenhousz carried out many experiments on the basis of Priestley's research, and found that the reason why Priestley's experiment could not be repeated many times was that he ignored the role of light, and plants could "purify" the air only under light. The above three scientists are pioneers of photosynthesis research. Priestley is generally the discoverer of photosynthesis, and 1771 is the year of discovery of photosynthesis.^[1]

In 1782, the Swiss Jeanne Senabier (Jean Senebier) discovered by chemical method: CO two It is necessary for photosynthesis, O two yes Photosynthetic products 。^[1]

In 1804, Nicolas Th é odore de Saussure, a Swiss, proved through quantitative experiments that the organic matter produced by plants and the total amount released are more than the CO consumed two More, which further confirmed that photosynthesis and water participate in the reaction.^[1]

In 1864, J. V. Sachs found that illuminated leaves would turn blue when exposed to iodine, which proved that photosynthesis formed carbohydrates（ starch ）。^[1]

At the end of the 19th century, it was proved that the raw material for photosynthesis was CO in the air two And H in soil two O， The energy is solar radiation, and the product is sugar and O two 。^[1]

At the beginning of the 20th century, the molecular mechanism of photosynthesis made breakthrough progress. The milestone work was mainly: Wilstatter et al. (1915) won the Nobel Prize for purifying chlorophyll and clarifying its chemical structure.^[1]

Later, Blackman from Britain and O Warburg et al. conducted flash experiments with algae to prove that photosynthesis can be divided into light reaction and dark reaction without light.^[1]

In 1932, R. Emersen and W Arnold defined the "photosynthetic unit" through the flash test of chlorella suspension, that is, to release 1 molecule or assimilate 1 molecule of CO two The number of 2500 chlorophyll molecules required. Then 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 existing on the thylakoid membrane that can conduct complete photoreaction.^[1]

From 1940s to the end of 1950s, Melvin Calvin (Melvin Ellis Calvin), etc Carbon14 Research on photosynthetic carbon assimilation, and clarify CO two Biochemical pathway for conversion to organic matter. Calvin won the Nobel Prize in 1961. After that, CAM pathway (M. Thomas, 1960) and C4 pathway (M. D. Hatch and C. B. Slack, 1966) were successively determined.^[1]

In 1954, American scientist D 1. Arnon et al. found that ATP and NADPH will be produced in the system when inorganic phosphorus, ADP and NADP+are supplied to the system when illuminating chloroplasts. At the same time, as long as ATP and NADPH are supplied, even in the dark, chloroplasts can convert CO two Convert to sugar. Therefore, it is concluded that the essence of photoreaction is to produce "Assimilation force" (assimilatory power) to promote dark reaction, and the essence of dark reaction is to use "Assimilation force" The inorganic carbon (CO two ）Convert to organic carbon (CH two O）。^[1]

In 1957, Emerson observed that Chlorella added a little light of shorter wavelength (such as 650 nm light) when it was irradiated by far red light, and the quantum yield was higher than the sum of the two wavelengths of light alone. This phenomenon that short wavelength light is added to long wavelength red light to promote photosynthetic efficiency is called double light gain effect, or Emerson enhancement effect. It was later known that this is because photosynthesis requires the synergy of two photochemical reactions.^[1]

In 1960, Hill et al. put forward the concept of two photo system. The system that absorbs long wave light was called photosystem I (PS I), and the system that absorbs short wave light was called photosystem II (PS II), which promoted the research of biochemistry and molecular biology such as the separation and purification of PS I and PS.^[1]

In 1965, Robert Burns· Woodward Robert Burns Woodward won the Nobel Prize in Chemistry for his work on the synthesis of chlorophyll molecules.^[15]

In the early 1980s, P. Mitchell put forward the chemical permeation hypothesis. Jagendorf et al. conducted a phased study of photosynthetic phosphorylation with chloroplasts, proving that the high-energy state of photosynthetic phosphorylation is the transmembrane proton gradient in the chemical osmosis hypothesis. This not only enables people to understand the mechanism of energy conversion in photosynthesis, but also leads to the study of proton dynamic potential in connection with ion transport, dynamic changes of thylakoid structure and the regulation process of energy conversion reaction.^[1]

In the late 1980s, Johann Deisenhofer (Johann Deisenhofer) et al. measured the structure of the reaction center of photosynthetic bacteria, made outstanding progress in understanding the details of membrane protein complexes and the primary reaction of photosynthesis, and won the 1988 Nobel Prize in Chemistry.^[1]

In 1992, Rudolph A. Marcus (Rudolph A. Marcus) won the Nobel Prize for his research on the electron transfer theory of life system, including photosynthesis electron transfer.^[1]

At the end of the 1990s, great progress was made in the study of the dynamic structure and reaction mechanism of enzymes that catalyze photosynthetic phosphorylation and oxidative phosphorylation of respiration. John Walker (John E. Walker) and Paul Boyer Paul D. Boyer won the Nobel Prize in Chemistry in 1997.^[1]

On August 19, 2010, Dr. Min Chen and others from the University of Sydney in Western Australia Shark Bay The fifth kind of chlorophyll is found in "stromatolite"—— Chlorophyll f , a much redder absorption spectrum than other types of chlorophyll, extending to the near-infrared range, with the absorption peak mainly at 722nm.^[12]

Andrea Fantuzzi and A William Rutherford's research team found cyanobacteria（ Chroococcidiopsis thermalis ）In the case of visible light, it will normally use "chlorophyll a" for photosynthesis, but if it is in a dark environment and lacks visible light, it will use "chlorophyll f" for photosynthesis. This kind of photosynthesis can use lower energy infrared light for photosynthetic reaction, which exceeds the "red light limit" of photosynthesis, which represents the third type of photosynthesis widely existing in nature.^[13]

On July 13, 2020, the Robertsa Croce research group of the Free University of Amsterdam showed that the insertion of chlorophyll f would slow down the overall energy capture in the two light systems, especially the efficiency of the light system II. However, although the energy output is low, it is still advantageous to insert red shifted chlorophyll f into the light system in an environment rich in far red light.^[14]

The research on photosynthesis in China has made great progress since the 1950s. For example, the Shanghai Institute of Plant Physiology, Chinese Academy of Sciences, has found and innovated in the research on energy conversion of photosynthesis, enzymology of photosynthetic carbon metabolism, and the primary reaction of photosynthesis and research on photosynthetic pigment protein complexes.

In May 2023, Wan Yinglang, a professor of the School of Tropical Crops of Hainan University, and his team found in the mangroves in Hainan Dongzhaigang National Nature Reserve and the coral reef in Yunlong Bay, Wenchang City White sided sidefoot manatee and Haitians tomentosa 2 newly recorded species. This is the first officially recorded mollusk in mainland China that can provide "food" by photosynthesis.^[11]

In February 2024, according to the official website of Okayama University in Japan, Okayama University Professor Shen Jianren and others successfully captured the moment when the catalyst in the protein responsible for plant photosynthesis absorbed water molecules.^[16]

Although the history of photosynthesis research is not long, through the efforts of many scientific researchers, remarkable progress has been made, which provides sufficient theoretical basis for guiding agricultural production.^[1]

significance

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Turn solar energy into chemical energy

Plants in assimilation inorganic carbide While transforming solar energy into chemical energy , stored in the formed Organic compound Medium. The solar energy assimilated by photosynthesis is about 10 times of the energy required by human beings every year. Chemical energy stored in organic matter, except for the plant itself and all Heterotroph In addition to its use, it is more important to provide energy sources for human nutrition and activities.^[4] Therefore, it can be said that photosynthesis provides the main energy today. Green plants are a huge energy conversion station.^[6]

Change inorganic matter into organic matter

The scale of organic matter production by plants through photosynthesis is very huge.^[6] It is estimated that plants can absorb CO every year two About 7 × 10 eleven Tons, and about 500 billion tons of organic compounds were synthesized.^[4]^[9] Of the carbon assimilated by autotrophic plants on the earth, 40% is assimilated by phytoplankton, and the remaining 60% is assimilated by terrestrial plants.^[6] The food, oil, fiber, wood, sugar, fruit, etc. needed by human beings all come from photosynthesis. Without photosynthesis, human beings would have no food and all kinds of daily necessities. In other words, there is no survival and development of human beings without photosynthesis.^[4]

Maintain carbon oxygen balance in the atmosphere

atmosphere The reason why the oxygen content of 21% can always be maintained mainly depends on photosynthesis (about oxygen release during photosynthesis). Photosynthesis, on the one hand, provides conditions for aerobic respiration; on the other hand, the accumulation of oxygen gradually forms ozone (O three ）Layer. ozone layer Capable of absorbing sunlight that is harmful to organisms Ultraviolet radiation 。 Although photosynthesis of plants can remove a large amount of CO from the atmosphere two , but CO in the atmosphere two The concentration of is still increasing, which is mainly caused by urbanization and industrialization.^[4]

Reaction stage

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Photoreaction stage

Fig. 3 Photosynthesis process diagram

Photoreaction The phase is characterized by that the electrons released by the oxidation of water molecules driven by light pass through mitochondrion breathing Electron transfer chain Such electronic transmission system is transmitted to NADP + , restore it to NADPH. Another result of electron transfer is stroma in proton To be pumped thylakoid In the cavity, the formed transmembrane proton gradient drives ADP Phosphorylation Generate ATP.^[2]

Reaction formula:

Dark reaction stage

Dark reaction The stage is to use light reaction to generate NADPH and ATP for carbon assimilation , reducing the gas carbon dioxide to sugar. Because this stage basically does not rely directly on light, but only on the provision of NADPH and ATP, it is called the dark reaction stage.^[2]

Reaction formula:

Total reaction formula:

； Among them,

Represents sugars.

Difference between two reactions

Reaction stage	Phase I	Phase II
Essence of reaction	Light energy → chemical energy, release	Assimilate CO two Formation (CH two O) (Enzymatic reaction)
reaction time	Short, in microseconds	Slower
Reaction conditions	Requires pigments, light, ADP, and enzymes	No pigment and light required, multiple enzymes required
Reaction site	Carried out on the membrane of chloroplast cystic structure	In chloroplast matrix
Substance transformation (Photoreaction)	2H two O→4[H]+O two ↑ (Catalyzed by light and pigments in chloroplasts)	CO two +C five →2C three (Catalyzed by enzyme)
Substance transformation (Dark reaction)	ADP+Pi→ATP (Catalyzed by enzyme)	C three +[H]→（CH two O）+C five (Under ATP energy supply and enzyme catalysis)
Energy conversion	Chlorophyll converts light energy into electric energy and then into active chemical energy and stores it in ATP	The active chemical energy in ATP is converted into stable chemical energy in organic substances such as sugars

Reaction process

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The process of photosynthesis is a relatively complex problem. On the surface, the total reaction formula of photosynthesis seems to be a simple redox process, but it actually includes a series of photochemical steps and material transformation problems. According to modern data, the whole photosynthesis can be roughly divided into the following three steps: ① primary reaction, including absorption, transmission and conversion of light energy; ② Electron transfer and photosynthetic phosphorylation, forming active chemical energy (ATP and NADPH); ③ Carbon assimilation, transforming active chemical energy into stable chemical energy (fixed CO two , forming sugars).^[6] Before introducing the reaction process of photosynthesis, it is necessary to understand the photosynthetic pigments and photosystem involved in the process of photosynthesis.

Photosynthetic pigment and photosystem

1. Photosynthetic pigment

Chloroplast consists of bilayer membrane, thylakoid and matrix. Thylakoid is a flat capsule with a single membrane, which is parallel to the long axis of chloroplasts. The membrane contains photosynthetic pigments and electron transport chain components, and the conversion of light energy to chemical energy is carried out on the thylakoid. There are two kinds of pigments on the thylakoid membrane: chlorophyll and orange carotenoid. Generally, the ratio of chlorophyll and carotenoid is about 3:1, and the ratio of chlorophyll a (chl a) and chlorophyll b (chl b) is also about 3:1. According to the functional differentiation, the pigments on the thylakoid membrane of chloroplasts can be divided into two types: one is the central pigment, and a few special chlorophyll-a molecules belong to this class; The other is the concentrating pigment. Most pigments (including most chlorophyll a and all chlorophyll b carotene and lutein) belong to the concentrating pigment.^[7] Among them, the light trapping pigment has no photochemical activity, but only the function of collecting light energy. It gathers light energy like a funnel and transmits it to the pigment in the reaction center to complete the photochemical reaction.^[6]

2. Photosynthesis center

Photosynthetic center, also called reaction center,^[6] It is the most basic pigment protein structure for primary reaction. It includes at least one light energy conversion pigment molecule (P), one primary electron acceptor (A) and one primary electron donor (D), which can lead to charge separation, convert light energy into electric energy, and accumulate. The photosynthetic center can be regarded as the basic unit of light energy conversion.^[7]

3. Optical system

The photochemical reaction of photosynthesis is completed by two photosystems including photosynthetic pigments, namely photosystem I (PS I for short) and photosystem II (PS II for short). Each light system has a special pigment complex and other substances.^[7]

Primary reaction

The first act of photosynthesis is the primary reaction. It refers to the process of photosynthesis from the light excitation of chlorophyll molecule to the first photochemical reaction, including the pigment molecule pair Absorption, Transmission and Conversion of Radioenergy Process. Both photosystems (PS Ⅰ and PS Ⅱ) participated in the primary reaction.^[6]

When the visible light with a wavelength range of 400~700 nm irradiates green plants, the pigment molecules of the concentrating pigment system become excited after absorbing light quanta. Because the pigment molecules on the thylakoid lamella are closely arranged (10~50 nm), light quanta are rapidly transferred between pigment molecules by induced resonance. In addition, energy can be transferred not only between the same pigment molecules, but also between different pigment molecules. Therefore, the energy transfer efficiency is very high. In this way, the concentrating pigment absorbs and gathers a large amount of light energy just like a lens focuses the light beam to the focus, and quickly transfers it to the pigment molecules in the reaction center.^[6]

Photochemical reaction is the core link of photosynthesis, which can directly convert light energy into chemical energy. When the special chlorophyll a pair (P) is excited by light, it becomes the excited state P * , release electrons to the primary electron acceptor (A). Chlorophyll a is oxidized to be positively charged (P + ）And the receptor is reduced to a band negative charge Reductive state of (A - ）。 Oxidized chlorophyll (P + ）After the loss of electrons, electrons can be obtained from the secondary electron donor (D) to restore the reduced state of electrons. In this way, the original electron acceptor releases high-energy electrons into the electron transfer chain until the final electron acceptor. Similarly, the electron donor (D + ）It is also necessary to capture electrons from the previous donor, once until the final electron donor is water.^[6]

The above is the whole process of photoreaction. The photochemical reaction of the photoreaction center is as follows:^[6]

Electron transfer chain

All organisms capable of oxygen releasing photosynthesis have two photosystems: PS Ⅰ and PS Ⅱ. Optical system I (PS I) can be excited by light with a wavelength of 700 nm, also known as P700; Optical system II (PS Ⅱ) absorption peak is at 680 nm, also known as P680. PS Ⅰ and PS Ⅱ are connected by electron transfer chain and arranged in high order on the thylakoid membrane, undertaking the task of electron transfer and proton transfer.^[6]

PS Ⅱ is mainly composed of PS Ⅱ reaction center (PS Ⅱ, reaction center）、 Light harvesting complex II (LHC II) and oxygen releasing complex (OEC) are composed of subunits. Its function is to use light energy to oxidize water and reduce plastoquinone. These two reactions are carried out on both sides of the thylakoid membrane, that is, to oxidize water on one side of the cavity to release protons into the cavity, and to reduce plastoquinone on one side of the matrix, so that a proton gradient is established on both sides of the thylakoid membrane.^[6]

Cytochrome b six F complex (cytochrome b six f complex，Cyt b six f) It is a large and multi subunit protein with several cofactors, Cyt b six The f complex contains two Cyt b, one Cyt c (formerly called Cyt f), one Rieske Fe-S protein (RFe-S), and two quinone redox sites. Cyt b six The main physiological function of f is: two The electrons in (reduced proton quinone) are transferred to plastocyanin (PC), and hydrogen protons are released into the cavity of thylakoid at the same time. Due to PQH two The hydrogen protons of are derived from the chloroplast matrix, so this process will actually carry out transmembrane transport, build a transmembrane proton gradient, and become the driving force for ATP synthesis. Cytb six The electron transfer route of f complex is: PQH two Transfer electrons to Cytb six After f, electrons are transferred to plastocyanine and then to P700 of PS Ⅰ through quinone cycle.^[6]

The particle size of PS Ⅰ complex is small, with a diameter of 11 nm, and it only exists in the non stacking area of matrix lamella and matrix lamella. The PS Ⅰ core complex is composed of the reaction center pigment P700 electron receptor and the PS Ⅰ light harvesting complex (LHC Ⅰ). The function of PS Ⅰ is to transfer electrons from plastocyanine to ferredoxin. PS Ⅰ participates in the electron transfer route: LHC Ⅰ around the core complex absorbs light energy, transfers it to P700 through induced resonance, and then transfers electrons to the primary electron receptor A in sequence zero (Chla) Secondary electron acceptor A one (Probably leaf quinone, i.e. vit K one ）, and then through the iron sulfur center (Fe-S), and finally to ferredoxin (Fd).^[6]

1. Acyclic electronic transfer chain

The acyclic electron transfer chain starts from photosystem II, which will crack water, release oxygen, and produce ATP and NADPH. Acyclic Electron transfer chain The process is roughly as follows:

Photosystem II → primary acceptor → plastoquinone → cytochrome complex → plastocyanin → photosystem I → primary acceptor → ferredoxin → reductase^[6]

2. Cyclic electron transfer chain

The circulating electron transfer chain does not produce oxygen because the electron source is not cracked water. The electrons start from the optical system I and eventually produce ATP. The process of circulating the electron transfer chain is as follows:

Photosystem I → primary acceptor → iron redox protein → cytochrome complex → plasmid blue → photosystem I^[6]

Photosynthetic phosphorylation

Photosynthetic phosphorylation (photophosphorylation or photophosphorylation) refers to the process of synthesizing phosphoric acid with the energy driven by light and stored in the proton gradient across the thylakoid membrane in photosynthesis. Photosynthetic phosphorylation has two types: Acyclic photophosphorylation and Cyclic photophosphorylation 。^[6]

1. Non cyclic photosynthetic phosphorylation

After the water at OEC is cracked, it is released into the capsule like cavity, and the electrons are transferred to PS Ⅱ. When the electrons are transferred in the photosynthetic electron transfer chain, they are transferred to the cavity along with the transfer of the outer side of the thylakoid, which forms the concentration difference across the membrane, causing the formation; At the same time, the electrons are transferred to PS Ⅰ, which further increases the energy level and makes it reduce to, in addition, release. In this process, electron transfer is an open pathway, so it is called non cyclic photophosphorylation. Acyclic photophosphorylation is carried out in grana lamella, which plays a major role in photophosphorylation.^[6]

2. Cyclic photosynthetic phosphorylation

After the electrons produced by PS Ⅰ are transferred by some transferors, the concentration difference of H inside and outside the cavity is formed, which only causes the formation of the electron, but does not put it away, and there is no reduction reaction: in this process, the electrons reduce the energy level after a series of transfers, and finally return to the original starting point through the plastid blue, that is, the electron transfer is a closed loop, Therefore, it is called cyclic photophosphorylation. Cyclic photosynthetic phosphorylation is carried out in the matrix lamella, which may play a role in supplementing the deficiency in higher plants.^[6]

Carbon assimilation

CO two Assimilation (CO two Assimilation) is an important aspect of photosynthesis. Carbon assimilation is a series of CO promoted by and two Assimilation process, CO two It becomes organic matter such as sugar. CO fixation by higher plants two There are three biochemical pathways: Calvin cycle, C four Pathway and Sedum acid metabolic pathway. Among them, the Calvin cycle is the most basic way. At the same time, only this way has the ability to synthesize starch and other products; The other two pathways are not common (especially the Sedum acid metabolic pathway), and can only fix and operate CO two It can not form starch and other products.^[6]

1. Calvin cycle - C3 pathway

The Calvin cycle is the basic pathway of carbon assimilation in photosynthesis of all plants, which can be roughly divided into three stages, namely carboxylation stage, reduction stage and renewal stage.^[6]

1) Carboxylation stage: CO two It must go through the carboxylation stage, be fixed into carboxylic acid, and then be reduced. Ribulose 1,5-diphosphate (RuBP) is CO two Under the action of ribulose 1,5-diphosphate carboxylase/oxygenase (Rubisco), and CO two Form intermediate product, which is then mixed with 1 molecule H two O reaction to form 2 molecules of glycerate - 3 - phosphoric acid (PGA), which is CO two Carboxylation stage.^[6]

2) Reduction stage: Glycerate - 3 - phosphate is phosphorylated by ATP, forms glycerate - 1,3 - diphosphate (DPGA) under the catalysis of glycerate - 3 - phosphate kinase, and then is NADPH+H under the action of glyceraldehyde - 3 - phosphate dehydrogenase + Reduction to form glyceraldehyde - 3 phosphoric acid (PGAld).^[6]

3) Renewal stage: the renewal stage is the process of PGAld forming RuBP after a series of transformations, which is also the regeneration stage of RuBP.^[6]

2. C4 approach

On the basis of previous studies, Hatch and Slack (1966) found that CO in sugarcane, corn, etc two The initial stable product of fixation is tetracarboxylic acid compounds (malic acid and aspartic acid), so it is called tetracarboxylic acid pathway (C four -Dicarboxylicacidpath), referred to as C four The pathway is also called Hatch Slack pathway. Plants with this carbon assimilation pathway are called C four Plants (C four plant）。C four The pathway includes four steps: carboxylation, transformation, decarboxylation and reduction, and regeneration.^[6]

1) Carboxylation: C four CO of pathway two The receptor is PEP (phosphoenolpyruvate) in the mesophyll cytoplasm. Under the catalysis of PEPC, HCO is immobilized three - （CO two Dissolved in water) to produce oxaloacetic acid (OAA).^[6]

2) Transformation: oxaloacetic acid in chloroplasts of mesophyll cells passes NADP - The malate dehydrogenase acts and is reduced to malic acid. However, in some varieties, oxaloacetic acid and glutamic acid in the cytoplasm form aspartic acid and ketoglutaric acid under the action of aspartate aminotransferase. The above malic acid, aspartic acid and other C four After acid formation, it is transferred to vascular bundle sheath cells.^[6]

3) Decarboxylation and reduction: The tetracarboxylic acid becomes pyruvate or alanine after decarboxylation in the vascular bundle sheath. CO released two It is reduced to sugars through the Calvin cycle.^[6]

4) Regeneration: C four C formed by acid decarboxylation three Acids (pyruvate or alanine) are transported back to mesophyll cells, and in chloroplasts, they are catalyzed by pyruvate phosphate double kinase (PPDK) and acted by ATP to produce CO two Receptor PEP is a reaction cycle.^[6]

3. Sedum acid metabolic pathway (CAM)

The leaves of Crassulaceae plants, such as Sedum Sedum and ground-based plants, have special CO two Fixed mode. The pores open at night and absorb CO two Under the action of PEP carboxylase, it combines with PEP to form OAA, which is further reduced to malic acid and accumulated in vacuoles. During the day, when the stoma is closed, malic acid in the vacuole will be transported to the cytosol - Oxidative decarboxylation and CO release under the action of malic acid enzyme two , participate in the Calvin cycle, form starch, etc. In addition, triose phosphate forms PEP through glycolysis, and then circulates further. Therefore, the content of organic acids in plants is very high at night, while the content of sugars decreases; In the daytime, on the contrary, organic acids decreased while sugar increased. This metabolic type of diurnal changes in acid synthesis of young roots was first found in Crassulaceae plants, so it is called Crassulic Acid Metabolism.^[6]

Photosynthetic rate

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Definition of photosynthetic rate

Photosynthetic rate usually refers to the amount of carbon dioxide absorbed or oxygen released per unit leaf area per unit time, and can also be expressed by the amount of dry matter accumulated per unit leaf area per unit time.^[6]

Internal influencing factors

1. Different parts

In a certain range, the more chlorophyll content, the stronger photosynthesis. Take a leaf as an example, the photosynthetic rate of the youngest leaf is low. With the growth of the leaf, the photosynthetic rate continues to strengthen, reaching the peak, and then the leaf senescence, the photosynthetic rate decreases.^[6]

2. Different growth periods

The photosynthetic rate of each crop is different in different growth periods. Generally, it is the strongest in the vegetative growth period, and it drops at the end of the growth period. Taking rice as an example, the photosynthetic rate was faster at the peak tillering stage and decreased when the rice panicle was close to maturity. However, from the perspective of population, the photosynthetic capacity of population not only depends on the photosynthetic rate per unit leaf area, but also is greatly affected by the total leaf area and population structure.^[6]

External influencing factors

1. Light

（1） Effect of light intensity on photosynthesis

Photosynthesis is a photobiochemical reaction, so the photosynthetic rate increases and decreases with the increase of light intensity. In the dark, photosynthesis stops and respiration continuously releases CO two ； With the increase of light, the photosynthetic rate gradually increased, and gradually approached the respiratory rate. Finally, the photosynthetic rate and the respiratory rate reached a dynamic equilibrium. CO absorbed by the same leaf during photosynthesis at the same time two CO emitted in the process of photorespiration and respiration two The light intensity at the same time is called light compensation point. When plants are at the light compensation point, the formation and consumption of organic matter are equal, and dry matter cannot be accumulated, but dry matter is also consumed at night. Therefore, from the perspective of the whole day, the minimum light intensity required by plants must be higher than the light compensation point, so that plants can grow normally.^[6]

（2） Effect of light quality on photosynthesis

Light quality also affects the photosynthetic efficiency of plants. Under natural conditions, plants will be more or less exposed to light of different wavelengths. For example, light intensity on cloudy days is not only weak, but also blue and green light components increase; The leaves of trees absorb more red and blue light, so the light under the crown is rich in green light, especially in the wooded forest.^[6]

2. Carbon dioxide

Carbon dioxide is the raw material of photosynthesis and has a great impact on the photosynthetic rate. It is mainly through stoma Entering the leaves, strengthening ventilation or trying to increase carbon dioxide can significantly improve the photosynthetic rate of crops three Plants are particularly obvious. In addition, plants are sensitive to CO two The utilization of CO is related to the light intensity. In the case of weak light, only a lower concentration of CO can be used two The photosynthetic rate is slow. With the increase of light intensity, plants can absorb and utilize higher concentrations of CO two , the photosynthetic rate was accelerated.^[6]

3. Temperature

The carbon reaction in the process of photosynthesis is a chemical reaction catalyzed by enzymes, and temperature directly affects the activity of enzymes. Therefore, temperature has a great impact on photosynthesis. Except for a few examples, general plants can normally carry out photosynthesis at 10~35 ℃, of which 25~30 ℃ is the most suitable. When the temperature is above 35 ℃, the photosynthesis starts to decline, and when the temperature is 40~50 ℃, it stops completely. At low temperature, the enzymatic reaction decreases, which limits the progress of photosynthesis. At high temperature, on the one hand, high temperature destroys the structure of chloroplast and cytoplasm, and inactivates chloroplast enzymes; On the other hand, when dark respiration and photorespiration are strengthened, the photosynthetic rate decreases.^[6]

4. Mineral elements

Mineral elements directly or indirectly affect photosynthesis. Nitrogen, magnesium, iron and manganese are essential mineral elements for chlorophyll biosynthesis; Copper, iron, sulfur and chlorine are involved in the process of photosynthetic electron transfer and water splitting; Potassium, phosphorus, etc. are involved in carbohydrate metabolism, and their lack will affect the transformation and transportation of sugars, which will indirectly affect photosynthesis. At the same time, phosphorus is also involved in the transformation and energy transfer of photosynthetic intermediates, so it has a great impact on photosynthesis.^[6]

5. Moisture

Water is one of the raw materials for photosynthesis, and the water required for photosynthesis is only a small part (less than 1%) of the water absorbed by plants. Therefore, water shortage mainly indirectly affects the decline of photosynthetic rate. Specifically, water shortage will close the leaf pores, affecting CO two Entering the leaf; Water shortage will strengthen the hydrolysis of starch in leaves, accumulate sugars and slow the output of photosynthetic products, which will reduce the photosynthetic rate.^[6]

Photosynthetic organisms

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C3 plants

Via C three CO fixation by way two Is called C three For plants, the starch they get from photosynthesis will be stored in the mesophyll cells, because this is the place of the Calvin cycle. C three Like plants belong to the type of high light respiration plants, with low photosynthetic rate. They have many kinds and wide distribution, and most of them grow in warm and wet conditions, such as most trees, plant food, tobacco, etc.^[3]

C4 plants

Figure 4 Schematic Diagram of C4 Plant Metabolism

Via C four CO fixation by way two Is called C four Plants, which are mainly those living in arid tropical areas. In this environment, if plants open their stomas for a long time to absorb carbon dioxide, water will be lost too quickly through transpiration. Therefore, plants can only open their stomas for a short time, so their intake of carbon dioxide is bound to be low. Plants must use this small amount of carbon dioxide for photosynthesis and synthesis of substances needed for their own growth.^[3]

C four Biological Characteristics of Phytophytes and C three There are great differences between C and C three Plants have higher water use efficiency and nitrogen use efficiency. At C four Plant mesophyll cells contain a unique enzyme, namely phosphoenolpyruvate carbon oxidase, which makes carbon dioxide first assimilated by a three carbon compound phosphoenolpyruvate to form a four carbon compound oxaloacetic acid, which is the origin of the name of this dark reaction type. The advantage of this type is that carbon dioxide fixation efficiency is higher than C three It is much higher, which is conducive to the growth of plants in arid environment. C four Plant starch will be stored in vascular bundle sheath cells, because C four The Calvin cycle of plants takes place here. C four There are few species of Class A plants, their distribution is limited, and they are suitable for growing in high temperature and dry climate conditions. Most weeds belong to C four Similar plants.^[3]

CAM plants

If you say C four If plants stagger carbon dioxide fixation and Calvin cycle in space, then Sedum acid metabolism (Crushed acid metabolism, CAM) is to stagger the two in time. The plants that use this way are those with swollen fleshy leaves, such as pineapple. These plants open their stomas at night to absorb carbon dioxide, and also convert CO through Hatch Slake pathway two Fixed. In the morning, the air hole is closed to avoid too fast 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]

Algae and bacteria

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

Photosynthetic bacteria do not have chloroplasts, but are carried out directly by cells themselves. Blue algae (or "cyanobacteria") belonging to prokaryotes also contain chlorophyll, which, like chloroplasts, produces oxygen for photosynthesis. In fact, it is generally believed that chloroplasts evolved from cyanobacteria. Other photosynthetic bacteria have a variety of pigments, called bacterial chlorophyll or bacteriocyanin, 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 heliobacter.^[3]

June 2018, USA《 science 》A new study published in the journal shows 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 extraterrestrial life and improving crops. The new research found that the above cyanobacteria can be used normally under the condition of visible light "Chlorophyll - a" Photosynthesis, but if you are in a dark environment and lack of visible light, it will be used "Chlorophyll - f" , use near-infrared light for photosynthesis.^[8]

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