Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in carbohydrates. It is the primary source of organic matter for almost all heterotrophs and maintains the balance of oxygen and carbon dioxide in the atmosphere.

光合作用是绿色植物、藻类和某些细菌将光能转化为储存在碳水化合物中的化学能的过程。它是几乎所有异养生物有机物的主要来源，并维持着大气中氧气和二氧化碳的平衡。

1. The Overall Equation and Importance of Photosynthesis | 光合作用总方程与重要性

The overall net reaction of photosynthesis can be summarised as:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This endergonic reaction requires light energy absorbed by photosynthetic pigments such as chlorophyll. The oxygen released comes from the splitting of water molecules, not from carbon dioxide.

光合作用的总净反应可概括为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
这个吸能反应需要叶绿素等光合色素吸收的光能。释放的氧气来自水分子的裂解，而不是二氧化碳。

Photosynthesis sustains life by producing oxygen for aerobic respiration and organic compounds that form the basis of food webs. It also removes CO₂ from the atmosphere, playing a crucial role in regulating global climate and the carbon cycle.

光合作用通过产生供有氧呼吸使用的氧气和构成食物网基础的有机化合物来维持生命。它还能移除大气中的CO₂，在调节全球气候和碳循环中起着关键作用。

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2. Chloroplast Structure | 叶绿体的结构

Chloroplasts are the specialised organelles where photosynthesis takes place. They are bounded by a double membrane – an outer and an inner membrane – which encloses the aqueous stroma. The stroma contains enzymes for the Calvin cycle, starch grains and chloroplast DNA.

叶绿体是进行光合作用的特化细胞器。它们由外膜和内膜组成双层膜包围，内部是含水的基质。基质中含有卡尔文循环所需的酶、淀粉粒和叶绿体DNA。

Suspended within the stroma is an extensive system of flattened membrane sacs called thylakoids. Stacks of thylakoids form grana (singular: granum), which are interconnected by unstacked lamellae. The thylakoid membranes house the photosynthetic pigments, photosystems, electron carriers and ATP synthase complexes, providing a large surface area for the light‑dependent reactions.

悬浮在基质中的是由扁平膜囊组成的广泛系统，称为类囊体。类囊体堆叠形成基粒（单数：基粒），并由非堆叠的片层相互连接。类囊体膜上含有光合色素、光系统、电子载体和ATP合酶复合体，为光依赖反应提供了巨大的表面积。

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3. Overview: Light-dependent and Light-independent Stages | 光依赖与光不依赖阶段概述

Photosynthesis is divided into two main sets of reactions.
The light‑dependent stage occurs on the thylakoid membranes and uses light energy to generate ATP and reduced NADP (NADPH). Water is split to provide electrons and protons, and oxygen is released as a by‑product.
The light‑independent stage (Calvin cycle) takes place in the stroma. It uses the ATP and NADPH from the light‑dependent stage to fix CO₂ and synthesise triose phosphates, which can be converted into glucose and other organic molecules.

光合作用分为两套主要反应。
光依赖阶段发生在类囊体膜上，利用光能生成ATP和还原型NADP (NADPH)。水被裂解以提供电子和质子，氧气作为副产品释放。
光不依赖阶段（卡尔文循环）在基质中进行。它利用来自光依赖阶段的ATP和NADPH来固定CO₂并合成磷酸丙糖，磷酸丙糖可进一步转化为葡萄糖和其他有机分子。

The two stages are linked by the energy‑carrying molecules ATP and NADPH, which are often referred to as ‘assimilation power’.

两个阶段通过能量载体分子ATP和NADPH联系起来，这两者通常被称为“同化力”。

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4. Light-dependent Reactions: Non-cyclic Photophosphorylation | 光依赖反应：非循环光合磷酸化

In non‑cyclic photophosphorylation, both photosystem II (PSII) and photosystem I (PSI) are involved. Light energy absorbed by PSII excites electrons in the reaction centre chlorophyll P680. These high‑energy electrons are passed along a chain of electron carriers (plastoquinone, cytochrome b₆f complex, plastocyanin) to PSI, replacing electrons lost by P700.

在非循环光合磷酸化中，光系统II (PSII) 和光系统I (PSI) 都参与其中。PSII吸收光能激发反应中心叶绿素P680中的电子。这些高能电子沿电子传递链（质体醌、细胞色素b₆f复合物、质体蓝素）传递到PSI，以替换P700失去的电子。

The loss of electrons from PSII is compensated by the photolysis of water:
2H₂O → O₂ + 4H⁺ + 4e⁻
This reaction releases oxygen gas and provides the protons that contribute to the proton gradient across the thylakoid membrane.

PSII的电子损失由水的光解补偿：
2H₂O → O₂ + 4H⁺ + 4e⁻
该反应释放氧气，并提供质子，有助于在类囊体膜两侧建立质子梯度。

Electrons from PSI are transferred to the protein ferredoxin and then to the enzyme NADP⁺ reductase, which reduces NADP⁺ to NADPH. As electrons flow through the carriers, protons are pumped into the thylakoid lumen, creating an electrochemical gradient that drives ATP synthase to produce ATP (chemiosmosis). The overall products are ATP, NADPH and O₂.

来自PSI的电子传递给铁氧还蛋白，再传递给NADP⁺还原酶，将NADP⁺还原为NADPH。电子在传递体中流动时，质子被泵入类囊体腔内，形成电化学梯度，驱动ATP合酶产生ATP（化学渗透）。总产物是ATP、NADPH和O₂。

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5. Cyclic Photophosphorylation and Photolysis of Water | 循环光合磷酸化与水的光解

In cyclic photophosphorylation, only PSI is used. Electrons excited from P700 pass through ferredoxin and return to the cytochrome b₆f complex instead of reducing NADP⁺. This cyclic flow of electrons pumps protons across the membrane, generating a proton gradient that drives ATP synthesis, but no NADPH or O₂ is produced.

在循环光合磷酸化中，仅使用PSI。从P700激发的电子经过铁氧还蛋白后，不还原NADP⁺，而是返回细胞色素b₆f复合物。电子循环流动将质子泵过膜，形成质子梯度驱动ATP合成，但不产生NADPH和O₂。

This pathway allows the chloroplast to produce extra ATP to meet the demands of the Calvin cycle and other metabolic processes, especially when NADPH accumulates. The photolysis of water, however, is a feature only of non‑cyclic electron flow and is essential for the continuous supply of electrons to PSII.

这一途径使叶绿体能够产生额外的ATP，以满足卡尔文循环和其他代谢过程的需求，特别是当NADPH积累时。但水的光解仅是非循环电子流的特征，对持续供应PSII的电子至关重要。

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6. The Calvin Cycle: Carbon Fixation and Reduction | 卡尔文循环：碳固定与还原

The Calvin cycle occurs in the stroma and can be divided into three phases: carbon fixation, reduction and regeneration of RuBP. The cycle begins with the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO) catalysing the reaction of CO₂ with ribulose‑1,5‑bisphosphate (RuBP), a 5‑carbon compound. The unstable 6‑carbon intermediate immediately splits to give two molecules of 3‑phosphoglycerate (PGA), a 3‑carbon acid.

卡尔文循环在基质中进行，可分为三个阶段：碳固定、还原和RuBP的再生。循环开始于核酮糖二磷酸羧化酶/加氧酶 (RuBisCO) 催化CO₂与1,5‑二磷酸核酮糖 (RuBP，一种5碳化合物) 的反应。不稳定的6碳中间体立即裂解为两分子的3‑磷酸甘油酸 (PGA，一种3碳的酸)。

In the reduction phase, each PGA is phosphorylated by ATP to form 1,3‑bisphosphoglycerate, which is then reduced by NADPH to glyceraldehyde‑3‑phosphate (G3P), also called triose phosphate. For every three CO₂ molecules fixed, six G3P molecules are produced; five are used to regenerate RuBP and one exits to form carbohydrates such as glucose or starch.

在还原阶段，每个PGA被ATP磷酸化形成1,3‑二磷酸甘油酸，随后被NADPH还原为甘油醛‑3‑磷酸 (G3P)，也称磷酸丙糖。每固定三分子CO₂，产生六分子G3P；其中五分子用于再生RuBP，一分子输出形成葡萄糖或淀粉等碳水化合物。

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7. Regeneration of RuBP and Fate of Triose Phosphate | RuBP再生与磷酸丙糖去向

The regeneration of RuBP from G3P requires a series of enzyme‑catalysed reactions that rearrange carbon skeletons and consume additional ATP. For the continuous operation of the cycle, five out of six triose phosphates are recycled, ensuring that RuBP is available to accept new CO₂ molecules.

从G3P再生RuBP需要一系列酶促反应，重新排列碳骨架并消耗额外的ATP。为保证循环的持续运行，每六分子磷酸丙糖中有五分子被循环利用，确保RuBP可用于接受新的CO₂分子。

The G3P that leaves the cycle serves as a precursor for various biomolecules. In the stroma, two triose phosphate molecules can condense to form hexose phosphates, ultimately leading to the synthesis of glucose, sucrose and starch. Triose phosphate is also used to produce amino acids, lipids, and nucleotides, linking photosynthesis to primary metabolism.

离开循环的G3P作为多种生物分子的前体。在基质中，两分子磷酸丙糖可缩合形成己糖磷酸，最终合成葡萄糖、蔗糖和淀粉。磷酸丙糖还用于生成氨基酸、脂质和核苷酸，将光合作用与初级代谢联系起来。

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8. Limiting Factors: Light Intensity and CO₂ Concentration | 限制因素：光照强度与二氧化碳浓度

The rate of photosynthesis is affected by several environmental factors. When light intensity is low, the rate increases almost linearly with increasing light because the light‑dependent stage is limiting. At higher intensities, the rate plateaus as another factor, such as CO₂ concentration or temperature, becomes limiting.

光合作用速率受多种环境因素影响。光照强度低时，速率随光强增加几乎呈线性上升，因为光依赖阶段是限制因素。在较高强度下，速率趋于平稳，因为其他因素如CO₂浓度或温度成为限制因素。

Carbon dioxide concentration directly influences the rate of the Calvin cycle. As CO₂ concentration rises, more RuBP is carboxylated, and the rate of photosynthesis increases until the active sites of RuBisCO become saturated. Beyond this point, further increases in CO₂ have no effect unless other limiting factors are relieved.

二氧化碳浓度直接影响卡尔文循环的速率。CO₂浓度升高时，更多的RuBP被羧化，光合作用速率提高，直至RuBisCO的活性位点饱和。达到该点后，除非其他限制因素得到缓解，否则进一步提高CO₂浓度无效果。

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9. Limiting Factors: Temperature and the Law of Limiting Factors | 限制因素：温度与限制因子定律

Temperature influences enzyme‑catalysed reactions in photosynthesis, particularly those of the Calvin cycle. At low temperatures, the rate is slow because the kinetic energy of molecules is low. As temperature rises, the rate increases to an optimum; above this, enzymes such as RuBisCO begin to denature and the rate declines sharply.

温度影响光合作用中的酶促反应，尤其是卡尔文循环中的反应。低温下速率缓慢，因为分子动能低。随着温度升高，速率加快直至最适温度；超过此温度，RuBisCO等酶开始变性，速率急剧下降。

The law of limiting factors states that at any given moment, the rate of photosynthesis is determined by the factor present in the least favourable amount. Even if light and temperature are optimal, a shortage of CO₂ will limit the overall rate. This concept is essential for interpreting experimental data and for controlled-environment agriculture.

限制因子定律表明，在任何特定时刻，光合作用的速率由处于最不利量的因子决定。即使光照和温度处于最适状态，CO₂的短缺也会限制整体速率。这一概念对解释实验数据以及受控环境农业至关重要。

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10. Investigating the Rate of Photosynthesis | 探究光合作用速率

A common practical to measure the rate of photosynthesis uses an aquatic plant such as Elodea. The plant is placed in water containing sodium hydrogencarbonate as a CO₂ source, and the number of oxygen bubbles released per unit time is counted under varying light intensities, CO₂ concentrations or temperatures. The bubble rate can be calibrated to determine the volume of oxygen produced.

测量光合作用速率的常用实验使用伊乐藻等水生植物。将植物置于含碳酸氢钠作为CO₂源的水中，在不同光照强度、CO₂浓度或温度下，计数单位时间释放的氧气气泡数。气泡速率可经校准以确定产生的氧气体积。

Alternatively, changes in CO₂ concentration can be monitored using a hydrogencarbonate indicator or pH changes due to the uptake of CO₂. Modern sensor technologies allow direct measurement of O₂ evolution or CO₂ consumption, enabling more accurate and quantitative analysis of the limiting factors in photosynthesis.

或者，可用碳酸氢盐指示剂监测CO₂浓度的变化，或利用因CO₂吸收引起的pH变化。现代传感器技术可直接测量O₂释放或CO₂消耗，从而能够更准确、定量地分析光合作用的限制因素。

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11. C4 Photosynthesis: A Spatial Adaptation | C4光合作用：空间适应

In many plants adapted to hot, dry climates, photorespiration – a wasteful reaction in which RuBisCO fixes O₂ instead of CO₂ – reduces photosynthetic efficiency. C4 plants such as maize and sugarcane overcome this by spatially separating initial CO₂ fixation from the Calvin cycle. Their leaves exhibit Kranz anatomy, with two distinct photosynthetic cell types: mesophyll cells and bundle sheath cells.

在许多适应炎热干燥气候的植物中，光呼吸——RuBisCO固定O₂而非CO₂的浪费性反应——会降低光合效率。C4植物如玉米和甘蔗通过将初始CO₂固定与卡尔文循环在空间上分离来克服这一问题。它们的叶片呈现Kranz解剖结构，具有两种不同的光合细胞类型：叶肉细胞和维管束鞘细胞。

In mesophyll cells, CO₂ is first fixed into a 4‑carbon compound, oxaloacetate, by the enzyme PEP carboxylase using phosphoenolpyruvate (PEP). This enzyme has a very high affinity for CO₂ and does not fix O₂. The oxaloacetate is converted to malate, which is transported into bundle sheath cells, where decarb

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