AST是什么

楼上说的不正确。首先低浓度和高浓度的CO2是相对的。其次C4植物并不是喜欢在低浓度CO2环境下生长，而是因为它在低浓度的CO2环境下有优势所以会得到选择。在高CO2环境下因为没有了光呼吸C3反而会节省一些资源，所以C3有一定优势。现在的大环境下是C4有优势。第三 温度 楼上说反了。楼主的补充是正确的，原因在于高温时植物会因为蒸腾作用大量失水。为了节水叶片会被动地合上气孔，但是这样会降低细胞内CO2浓度。低CO2浓度下光呼吸增加，所以C4有优势。

生理上的区别主要是C4植物光反应和暗反应分离在不同类型的细胞内，CO2首先被PEP抓住合成一个四碳的化合物，反应由PEP carboxylase进行。然后这个四碳化合物（应该先是OAA然后转化成苹果酸）再回到mesophyll cell里放出CO2提高叶绿体内CO2浓度。简单说就这样。

这里抄一大段维基的科普

C4类二氧化碳固定

C4类植物比C3类植物在二氧化碳固定方面更进一步。因为该类植物在二氧化碳固定的过程中，第一个可观察得到的产物是一个四碳化合物，人们就命名其为C4类植物，为的是跟C3类植物在名称上有所区别。单子叶植物玉米、中国芒、甘蔗和小米都属于C4类。

在15亿年前，随着光合作用的出现，氧气开始在地球的大气层积聚。二氧化碳固定过程中的关键酶二磷酸核酮糖羧化酶同时具有加氧酶的功能，它在一个重要的副反应里也催化了氧的固定。氧气可以与二氧化碳争夺二磷酸核酮糖羧化酶的活性部位。在原始大气里，氧气缺乏，在上面提到的副反应里面，二碳化合物积聚，碳循环受阻，同化作用在这种环境下并不能顺利进行。回收二碳化合物的过程对于植物来说也是费时耗力的。此过程需要耗氧，人们称之为光呼吸。

随着温度的升高，二磷酸核酮糖羧化酶与氧气的亲和力递增迅速，超过了对二氧化碳的递增速度，这对于生长在干旱热带地区的植物来说并不是好消息，它们需要另外的途径以固定二氧化碳。植物发展出"ATP驱动的 CO2泵"，从而创造出一种与原始大气相适应的内环境。

除了Rubisco-反应外，叶肉细胞还发展出PEP-羧化途径以固定二氧化碳。在这个过程里CO2会被磷酸烯醇式丙酮酸（缩写PEP）所固定，之后生成四碳化合物草酰乙酸（缩写 OA），这就是C4类植物名称的由来。 草酰乙酸转换为苹果酸或天门冬氨酸后进入维管束鞘，在苹果酸酶的作用下生成丙酮酸和CO2。在维管束鞘里CO2浓度高，卡尔文循环能高效的运行。

[编辑] 哈奇-斯莱克-循环

20世纪60年代，马沙·哈奇和罗杰·斯莱克阐明了这种发生在相邻两种类型细胞里的四碳双羧酸途径的反应，后世便以他们的名字命名该循环。循环开始于叶肉细胞，但那里缺少二磷酸核酮糖羧化酶，反应转到维管束鞘里面进行，在这里，就遵循C3类植物的科里循环途径发生反应。

并不是很好，推荐英文的。

C4 carbon fixation is one of three biochemical mechanisms, along with C3 and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon atoms present in the first product of carbon fixation in these plants, in contrast to the 3-carbon atom products in C3 plants.

C4 fixation is an elaboration of the more common C3 carbon fixation and is believed to have evolved more recently. C4 and CAM overcome the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in what is called photorespiration. This is achieved by using a more efficient enzyme to fix CO2 in mesophyll cells and shuttling this fixed carbon via malate or oxaloacetate to bundle-sheath cells. In these bundle-sheath cells, RuBisCO is isolated from atmospheric oxygen and saturated with the CO2 released by decarboxylation of the malate or oxaloacetate. These additional steps, however, require more energy in the form of ATP. Because of this extra energy requirement, C4 plants are able to more efficiently fix carbon in only certain conditions, with the more common C3 pathway being more efficient in other conditions.

C4 pathway

The C4 pathway was discovered by M. D. Hatch and C. R. Slack, in Australia, in 1966, so it is sometimes called the Hatch-Slack pathway.[1]

In C3 plants, the first step in the light-independent reactions of photosynthesis involves the fixation of CO2 by the enzyme RuBisCO into 3-phosphoglycerate. However, due to the dual carboxylase / oxygenase activity of RuBisCo, an amount of the substrate is oxidized rather than carboxylated, resulting in loss of substrate and consumption of energy, in what is known as photorespiration. In order to bypass the photorespiration pathway, C4 plants have developed a mechanism to efficiently deliver CO2 to the RuBisCO enzyme. They utilize their specific leaf anatomy where chloroplasts exist, not only in the mesophyll cells in the outer part of their leaves but in the bundle sheath cells as well. Instead of direct fixation to RuBisCO in the Calvin cycle, CO2 is incorporated into a 4-carbon organic acid, which has the ability to regenerate CO2 in the chloroplasts of the bundle sheath cells. Bundle sheath cells can then utilize this CO2 to generate carbohydrates by the conventional C3 pathway.

The first step in the pathway is the conversion of pyruvate to PEP by the enzyme pyruvate-phosphate dikinase (pyruvate, orthophosphate dikinase). This reaction requires inorganic phosphate and ATP plus pyruvate, producing phosphoenolpyruvate, AMP, and inorganic pyrophosphate (PPi). The next step is the fixation of CO2 into PEP by the enzyme PEP carboxylase. Both of these steps occur in the mesophyll cells:

pyruvate + Pi + ATP → PEP + AMP + PPi

PEP carboxylase + PEP + CO2 → oxaloacetate

PEP carboxylase has a lower Km for CO2 — and, hence, higher affinity — than RuBisCO. Furthermore, O2 is a very poor substrate for this enzyme. Thus, at relatively low concentrations of CO2, most CO2 will be fixed by this pathway.

The product is usually converted to malate, a simple organic compound, which is transported to the bundle-sheath cells surrounding a nearby vein. Here, it is decarboxylated to produce CO2 and pyruvate. The CO2 now enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell.

Since every CO2 molecule has to be fixed twice, first by 4-carbon organic acid and second by RuBisCO, the C4 pathway uses more energy than the C3 pathway. The C3 pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C4 pathway requires 30 molecules of ATP. This energy debt is more than paid for by avoiding losing more than half of photosynthetic carbon in photorespiration as occurs in some tropical plants,[citation needed] making it an adaptive mechanism for minimizing the loss.

There are several variants of this pathway:

1. The 4-carbon acid transported from mesophyll cells may be malate, as above, or aspartate

2. The 3-carbon acid transported back from bundle-sheath cells may be pyruvate, as above, or alanine

3. The enzyme that catalyses decarboxylation in bundle-sheath cells differs. In maize and sugarcane, the enzyme is NADP-malic enzymein millet, it is NAD-malic enzymeand, in Panicum maximum, it is PEP carboxykinase.

[edit] C4 leaf anatomy

The C4 plants possess a characteristic leaf anatomy. Their vascular bundles are surrounded by two rings of cells, the inner ring, called bundle sheath cells, contain starch-rich chloroplasts lacking grana, which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. This peculiar anatomy is called kranz anatomy, from the German word for wreath. The primary function of kranz anatomy is to provide a site in which CO2 can be concentrated around RuBisCO, thereby reducing photorespiration. In order to facilitate the maintenance of a significantly higher CO2 concentration in the bundle sheath compared to the mesophyll, the boundary layer of the kranz has a low conductance to CO2, a property that may be enhanced by the presence of suberin.[2]

Although most C4 plants exhibit kranz anatomy, there are many species that operate a limited C4 cycle without any distinct bundle sheath tissue. Suaeda aralocaspica, Bienertia cycloptera and Bienertia sinuspersici (all chenopods) are terrestrial plants that inhabit dry, salty depressions in the deserts of south-east Asia. These plants have been shown to operate single-cell C4 CO2-concentrating mechanisms, which are unique among the known C4 mechanisms.[3][4][5] Although the cytology of both species differs slightly, the basic principle is that fluid-filled vacuoles are employed to divide the cell into two separate areas. Carboxylation enzymes in the cytosol can, therefore, be kept separate from decarboxylase enzymes and rubisco in the chloroplasts, and a diffusive barrier can be established between the chloroplasts (which contain rubisco) and the cytosol. This enables a bundle-sheath-type area and a mesophyll-type area to be established within a single cell. Although this does allow a limited C3 cycle to operate, it is relatively inefficient, with much leakage of CO2 from around rubisco occurring. There is also evidence for the exhibiting of inducible C4 photosynthesis by non-kranz aquatic macrophyte Hydrilla verticillata under warm conditions, although the mechanism by which CO2 leakage from around rubisco is minimised is currently uncertain.[6]

[edit] The evolution and advantages of the C4 pathway

Further information: Evolutionary history of plants#Advances in metabolism

C4 plants have a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or CO2 limitation. When grown in the same environment, at 30°C, C3 grasses lose approximately 833 molecules of water per CO2 molecule that is fixed, whereas C4 grasses lose only 277 water molecules per CO2 molecule fixed. This increased water use efficiency of C4 grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.[7]

C4 carbon fixation has evolved on up to 40 independent occasions in different families of plants, making it a prime example of convergent evolution.[8] C4 plants arose around 25 to 32 million years ago[8] during the Oligocene (precisely when is difficult to determine) and did not become ecologically significant until around 6 to 7 million years ago, in the Miocene Period.[8] C4 metabolism originated when grasses migrated from the shady forest undercanopy to more open environments,[9] where the high sunlight gave it an advantage over the C3 pathway.[10] Drought was not necessary for its innovationrather, the increased resistance to water stress was a by-product of the pathway and allowed C4 plants to more readily colonise arid environments.[10]

Today, C4 plants represent about 5% of Earth's plant biomass and 1% of its known plant species.[11] Despite this scarcity, they account for about 30% of terrestrial carbon fixation.[8] Increasing the proportion of C4 plants on earth could assist biosequestration of CO2 and represent an important climate change avoidance strategy. Present-day C4 plants are concentrated in the tropics (below latitudes of 45°) where the high air temperature contributes to higher possible levels of oxygenase activity by rubisco, which increases rates of photorespiration in C3 plants.

[edit] Plants that use C4 carbon fixation

About 7600 species of plants use C4 carbon fixation, which represents about 3% of all terrestrial species of plants. All these 7600 species are angiosperms. C4 carbon fixation is less common in dicots than in monocots, with only 4.5% of dicots using the C4 pathway, compared to 40% of monocots. Despite this, only three families of monocots utilise C4 carbon fixation compared to 15 dicot families. Of the monocot clades containing C4 plants, the grass (Poaceae) species use the C4 photosynthetic pathway most. Forty-six percent of grasses are C4 and together account for 61% of C4 species. These include the food crops maize, sugar cane, millet, and sorghum.[7][12] Of the dicot clades containing C4 species, the order, Caryophyllales contains the most species. Of the families in the Caryophyllales, the Chenopodiaceae use C4 carbon fixation the most, with 550 out of 1400 species using it. About 250 of the 1000 species of the related Amaranthaceae also use C4.[7][13]

Members of the sedge family Cyperaceae, and numerous families of Eudicots, including the daisies Asteraceae, cabbages Brassicaceae, and spurges Euphorbiaceae also use C4.

楼主要是要科研资料 来百度就是偷懒了。。。可以上google scholar或者scirus查。推荐Rowan Sage的论文，做的很不错。而且也做草坪草。 他老人家今天下午还来我们实验室了。

根据氨基酸合成时碳架来源不同，氨基酸可分为以下五族。

1、谷氨酸族氨基酸。的包括：谷氨酸(Glu)、谷氨酰胺(Gln)、脯(Pro)、 羟脯(Hyp)、精(Arg)

共同碳架：TCA中的α-酮戊二酸。

2、天冬氨酸族氨基酸。

包括:天冬氨酸(Asp)、天冬酰胺(Asn)、赖(Lys)、苏(Thr)、甲硫(Met)、异亮(Ile)

共同碳架:TCA中的草酰乙酸

3、丙酮酸族氨基酸。包括:丙(Ala)、缬(Val)、亮(Leu)

共同碳架:EMP中的丙酮酸

4、丝氨酸族氨基酸。包括:丝(Ser)、甘(Gly)、半胱(Cys)。

该族氨基酸碳架来源比较复杂，你自己查一下。

5、组氨酸和芳香族氨基酸。

包括:组AA(His)、色AA(Trp)、酪AA(Tyr)、苯丙AA(Phe)

。

芳香族氨基酸碳架:4-磷酸-赤藓糖(PPP)和PEP(EMP)

根据作用的氨基酸和酮酸不同，可将转氨酶分为数十种，其中以谷氨酸丙酮酸转氨酶（简称谷丙转氨酶――GPT，ALT）和谷氨酸草酰乙酸转氨酶（简称谷草转氨酶――GOT，AST）最为主要。在肝内，GPT主要分布于细胞浆水溶性部分，GOT则分布于细胞浆水溶部分和线粒体中。

在肝脏等脏器组织损伤或坏死时，细胞内酶释入血流，引起血清酶活力升高。由于整个肝脏内转氨酶活力比血清内该酶总活力高1000～1000倍，只要有1％肝细胞坏死便可使血清酶活力升高1倍（假设所有释出的酶均保持活性）。因此，在除外肝外脏器病变的情况下（如急性心肌梗塞、心肌炎和肌病），血清内转氨酶升高，在一定程度上即反映了肝细胞损害和坏死的程度。又由于肝内GPT活性超过体内其他任何脏器内的该酶活性，故测定GPT比GOT对反映肝脏损害更具特异性。

磷酸烯醇式丙酮酸英文缩写PEP 结构简式是 CH2=C(OH)-CO-O-PO3H2 ，结构看ATP就好了，含有高能磷酸键。（3 —磷酸甘油酸经2 —磷酸甘油酸转化为磷酸烯醇式丙酮酸（PEP），它的磷酸键吸收了自由能而变成高能磷酸键） 是一个P接一个羰基和两个羟基丙酮酸烯醇式丙酮酸 PEP是糖酵解中重要中间产物,在光反应阶段产生(主要化学式为:NADP*+2e-+H*→NADPH),为暗反应阶段提供能量与相应的酶(PEP缩合酶),也是C4植物中将CO2固定的化合物。只在C4植物中存在,是一种特殊的C3，C3(PEP)+CO2=C4。

糖解作用

在糖解作用中，此分子是2-磷酸甘油酸在烯醇化酶（enolase）的催化下生成，是一个高能磷酸分子。接下来磷酸烯醇丙酮酸将会进入糖解作用的第10个，也是最后一个步骤中。 在糖解作用的最后步骤里，磷酸烯醇丙酮酸将会经由丙酮酸激酶（Pyruvate kinase）的催化，使原本接在氧原子上的磷酸根转移到ADP上，进而生成ATP以及丙酮酸。这个反应会放出大量的能量，是一个难逆的反应，其标准自由能变化是31.4 kJ/mol（在pH=7、浓度55.5M的水中）。此外，这个反应也需要钾离子与镁离子（或其他二价阳离子）的参与。

糖质新生

由于糖解作用的最后步骤是个难逆反应，因此在糖质新生的过程中，需要一个替代途径，才能将丙酮酸还原成磷酸烯醇丙酮酸。首先丙酮酸必需要先在丙酮酸羧化酶（Pyruvate carboxylase）的催化之下，消耗ATP分子并转变成草酰乙酸（Oxaloacetate）。 之后草醋酸又会经由磷酸烯醇丙酮酸羧基激酶（Phosphoenolpyruvate carboxykinase）的催化，生成磷酸烯醇丙酮酸，在这个反应中，会消耗掉GTP，并生成GDP与二氧化碳。 与前后反应不同的是，以上的两个反应是在粒线体中进行。而且除了直接转变之外，草醋酸还可以利用另一个需要更多步骤的途径，来生成磷酸烯醇丙酮酸。此外，从磷酸烯醇丙酮酸的生成直到果糖-1,6-双磷酸产生为止，中间的糖质新生过程皆是糖解作用的逆反应。

碳水化合物代谢过程：

由于食物在口腔停留时间短暂，以致口腔唾液淀粉酶对碳水化合物的消化作用不大。胃液不含任何能水解碳水化合物的酶，其所含的胃酸对碳水化合物只可能有微小或极局限的水解，故碳水化合物在胃中几乎完全没有消化。碳水化合物的消化主要在小肠中进行。极少部分非淀粉多糖可在结肠内通过发酵消化。

肠腔中的主要水解酶是来自胰液的α-淀粉酶，称胰淀粉酶，可使淀粉变成麦芽糖、麦芽三糖、异麦芽糖、α-临界糊精及少量葡萄糖等。淀粉在口腔及肠腔中消化后的上述各种中间产物，可以在小肠黏膜上皮细胞表面进一步彻底消化，最后消化成大量的葡萄糖及少量的果糖及半乳糖。小肠内不被消化的碳水化合物到达结肠后，被结肠菌群分解，产生氢气、甲烷、二氧化碳和短链脂肪酸等，这一系列过程称为发酵。发酵也是消化的一种方式。所产生的气体经体循环转运，再经呼气和直肠排出体外，其他产物如短链脂肪酸被肠壁吸收并被机体代谢。

根据作用的氨基酸和酮酸不同，可将转氨酶分为数十种，其中以谷氨酸丙酮酸转氨酶（简称谷丙转氨酶――GPT，ALT）和谷氨酸草酰乙酸转氨酶（简称谷草转氨酶――GOT，AST）最为主要。在肝内，GPT主要分布于细胞浆水溶性部分，GOT则分布于细胞浆水溶部分和线粒体中。

在肝脏等脏器组织损伤或坏死时，细胞内酶释入血流，引起血清酶活力升高。由于整个肝脏内转氨酶活力比血清内该酶总活力高1000～1000倍，只要有1％肝细胞坏死便可使血清酶活力升高1倍（假设所有释出的酶均保持活性）。因此，在除外肝外脏器病变的情况下（如急性心肌梗塞、心肌炎和肌病），血清内转氨酶升高，在一定程度上即反映了肝细胞损害和坏死的程度。又由于肝内GPT活性超过体内其他任何脏器内的该酶活性，故测定GPT比GOT对反映肝脏损害更具特异性