Photosynthesis

Photosynthsis occurs only in the chlorophyll-containing cells of green plants, algae, and certain protists and bacteria. Overall , it is a process that converts light energy into chemical energy that is strored in the form of molecular bonds. Form the point of view of chemistry and energetics, it is the opposite of cellular respiration. Whereas cellular respiration is highly exergonic and releases energy, photosynthesis requires energy and is highly endergonic.

光合作用只发生在包含叶绿素的植物细胞，藻类，某些原生动物和细菌内。总的来说，它是一个将光能转化成化学能的过程，化学能以分子键的形式存储。从化学和能量学的观点来看，它与细胞呼吸作用相反。但是细胞呼吸作用高放能并释放能量，光合作用高吸能并需要能量。

Photosynthysis starts with CO2 and H2O as raw materials and proceeds through two sets of partial reactions. In the first set, called the light-dependendt reactions, water molecules are split (oxidized), O2 is released ,and ATP and NADPH are formed. These reactions must take place in the presence of light energy. In the second set , called light-independent reactions, CO2 is reduced (via the addition of H atoms) to carbohydrate. These chemical events rely on the electron carrier NADPH and ATP generated by the first set of reactions.

光合作用以CO2 和H2O作为原料启动，通过两个局部反应受益。第一阶段称为光反应阶段，水分子裂解（被氧化），氧气释放。第二阶段称为暗反应阶段，CO2（通过增加H原子）被还原成碳水化合物。这些化学反应依赖于电子载体NADPH和第一阶段产生的ATP。

Both sets of reactions take place in chloroplasts. Most of the enzymes and pigments for the light-dependent reactions are embedded in the thylakoid memberane fo chloroplasts. The dark reaction take place in the stroma.

这两个反应阶段都发生在叶绿体内。大多数光反应阶段的酶和色素包埋在叶绿体的类囊体内。暗反映阶段发生在基质中。

1.       How Light Energy Reaches Photosynthetic Cells

光能是怎样到达光合细胞的

The energy in light photon in the visible part of the spectrum can be captured by biological meoelcules to do constructive work. The pigment chlorophyll in plant cells absorbs photons within a particular absorption spectrum-a statement of the amount of light abosrbed by chlorophyll at different wave-lenghs. When light is absorbed it alters the arrangement of electrons in the absorbing molecule. The added energy of the photon boosts the energy condition of the molecule from a stable state to a less-stable excited state. During the light-dependent reactions of photosynthesis , as the absorbing molecule returns to the ground state, the “excess” excitation energy is transmitted to other molecules and stroed as chemical energy.

生物分子可捕获光谱中可见光光子中的能量，做建设性的工作。植物细胞中的叶绿素可以吸收在特殊的吸收光谱内的光子-叶绿素吸收不同的波长的光的总量的声明。当吸收光能后，就会改变吸收分子中电子的排列。光子吸收的能量激发了分子中的能量条件，使其从稳定态转入不稳定不兴奋态。在光合作用的光反应阶段，吸收分子返回到基态，过度兴奋作以化学能量的形式存储并将能量转移给其他分子。

All photosynthetic organisms contain various classes of chlorophyllls and one or more carotenoid (accessory) pigments that also contribute to photosynthsis. Groups of pigment molecules called antenna complexes are present on thylakoids. Light striking any one of the pigment molecules is funneled to special chorophyll a molecule, temed a reaction-center chlorophyll, which directly participates in photosynthsis. Most photosynthetic organisms possess two types of reaction-center chlorophylls, P680 aggregations are known respectively as photosystemＩ（P700）and photosystem II (P680)

所有的光合细胞器包含各种各样的叶绿素和一个或多个胡萝卜素

2.       The Light-Dependent Reaction: Converting Solar Energy into Chemical-Bond Energy

The photosystems of the light-dependent reactions are responsible for the packaging of light energy in the chemical compounds ATP and NADPH. This packaging takes place through a series of oxidation-reduction reactions set in motion when light strikes the P680 reaction center in photosystem II. In this initial event water molecules are cleaved, oxygen is released, and electrons are donated. These electrons are accepted first by plastiquinone and then by a series of carriers as they descend an delctron transprot chain. For each four electrons that pass down the chain, two ATPs are formed .The last acceptor in the chain is the P700 reaction center of photosystem I.At this point incoming photons boost the enzyme NADP+ is reduced to the NADPH. The ATP generated previously and the NADPH then take part in the light-independent reactions.

光反应：将太阳能转化成化学键能

光反应阶段的光合系统负责组装化学复合物ATP和NADPH中的光能。当光到达光合系统II中的P680反应中心后，组装过程通过一系列氧化还原反应开始进行，在这个最初的事件中，水分子裂解，氧气释放，电子放出。Plastiquinone首先接受这些电子，然后通过一些载体，传递下去，形成电子传递链。链上每传递四个电子，就形成两个ATP。光合系统I的P700反应中心是电子链上的最后受体。此时，先前产生的光能和随后产生的NADPH参与光反应阶段。

The production of ATP from the transport of electrons excited by light energy down an electron transport chain is termed photophosphorylation.. The one-way flow of electrons through photosystem II and I is called noncyclic photophosphorylation; plants also derive additional ATP through cyclic photophosphorylation, in which some electrons are shunted back through the elecron transport chain between photosystems II and I.

光能激发电子形成电子传递链。电子传递链又形成了ATP的过程，称为光合磷酸化作用。通过光合作用I和II的单向电子流叫做非环行光合磷酸化作用；植物通过环式光合磷酸化获得额外的ATP，一些电子在光合系统Ⅰ和Ⅱ之间的电子传递链中回流。

3.       The Light-Independent Reactions: Building Carbohydrates

In the light-independent reactions of photosynthesis, which are driven by ATP and NADPH, CO2 is converted to carbohydrate. The reactions are also known as the Calvin-Benson cycle. Atmospheric CO2 is fixed as it reacts with ribulose biphosphate (RuBP), a reaction that is catalyzed by the enzyme ribulose biphosphate carboxylase . The reduction of CO2 to carbohydrate (fructose diphosphate) is completed via several more steps of the cycles. Finally, RuBP is regenerated so that the sycle may continue.

暗反应: 合成碳水化合物

在光合作用中由ATP和NADPH驱动的暗反应阶段，CO2转变成碳水化合物。这个反应被称为Calvin-Benson循环。空气中的CO2在与由二磷酸核酮糖Rubp羧化酶催化的反应中被固定。CO2再通过几次循环步骤被还原成碳水化合物（果糖二磷酸）。最后，Rubp再生，此循环继续进行。

4.       Oxygen: An Inhibitor of Photosynthesis

High levels of oxygen in plant cells can disrupt photosynthesis and can also cause photorespiration-an inefficient from of the dark reactions in which O2 is fixed rather than CO2 and no carbohydrate is produced.

氧气：光合作用的抑制剂

植物细胞中高度的氧气可以扰乱光合作用，并且引起光呼吸---一种效率低下暗反应。反应中O2而不是CO2被固定，且不产生碳水化合物。

5.       Reprieve from Photorespiration:　The C4 Pathway

Most plants are C3 plants; they experience decreased carbohydrate production under hot ,dry conditions as a result of the effects of photorespiratin. Among C4 plants, however, special leaf anatomy and a unique biochemical pathway enable the plant to thrvive in arid conditions. Thus C4 plants lessen photorespiration by carrying out photosynthesis only in cells that are insulated from high levels of CO2. They also possess a novel mechanism for carbon fixation.

降低光呼吸：C4途经。

大多数的植物都是C3植物，它们在炎热、干燥的条件下由于光呼吸作用的影响减少碳水化合物的合成。然而，在C4植物中，特殊的叶片解剖结构和独特的生化途经使得C4植物可以在干旱的条件下茁壮成长。因此，C4植物仅通过与高水平CO2隔绝的细胞中进行光合作用来降低光呼吸。C4植物还有着固定碳的新奇机制。