Photosynthesis. Energy storage. С4-фотосинтез

you are: Photosynthesis. Energy storage

For a number of years was considered that as the first acceptor of carbon at all plants serves RuBP and that the first stable products of photosynthesis are three-carbon connections. Representation it has undergone to revision after it was found out that in experiences with 14СО2 some plants, for example corn, a sugar cane and related it tropical cereals, behave unusually: the label joins at them in four-carbon organic acids (shchavelevo-acetic, apple and asparaginovuju) much faster, than in FGK. Accordingly such plants began to name С4-растениями.

Leaves of many C4-plants on the anatomy are distinct from leaves of Sz-plants. Each vascular bunch is surrounded at them by a layer large parenhimnyh cages, or a so-called facing of a vascular bunch which is in turn surrounded by a layer of smaller cages mezofilla.

The Similar structure has received the name ' krants-anatomy '. Hloroplasty in both these types of cages morfologicheski are various: they contain very big starched grains in cages of a facing of a vascular bunch and are often deprived gran whereas in cages mezofilla in them are well expressed grany, but starch collects very little.

If in experiment to divide these two types of cages and to measure in them activity of enzymes rather accurate distinctions will be found out. In cages mezofilla above activity fosfoenolpiruvat (FEP)-karboksilazy, katalizirujushchej joining СО2 to FEP with formation of shchavelevo-acetic acid, and a facing cage is surpassed by cages mezofilla in activity RuBp-karboksilazy and other enzymes which are taking part in a cycle of Calvin - Bensona. There is an impression that both these groups of cages in plants kooperativno carry out transformation СО2 in geksozu and the subsequent transformation geksozy in starch.

The First of these reactions proceeds in cages mezofilla. Here FEP attaches arriving of atmosphere СО2 therefore acid which at one plants turns to apple acid is formed shchavelevouksusnaja, and at others - in asparaginovuju. According to one of hypotheses, apple;

Or asparaginovaja acid diffundiruet then from cages mezofilla in facing cages here again dekarboksiliruetsja with formation СО2 and three-carbon connection. The last again diffundiruet in mezofill where there is regeneration FEP then the cycle karboksilirovanija repeats with participation of the new molecule СО2 which has arrived from atmosphere. Simultaneously СО2, liberated in facing cages, enters Calvin's cycle - Bensona, i.e. Reacts with RuBP that leads to formation FGK and other intermediate products peculiar to Sz-plants, and eventually - to geksozofosfatam. The general scheme of these transformations is presented on fig. 4.20. Clearly in such a manner that even at Sm-plants carbon mastering basically occurs with the assistance of RuBp-karboksilazy; the final reaction leading to transformation geksozy in starch, proceeds at them in cages of a facing of vascular bunches.

We already noticed that С4-растения carry out photosynthesis more effectively, rather than С3-растения, partly because the photo breath is expressed at them poorly and, means, they for nothing do not spend already fixed carbon. Such behaviour С4-растений, probably, is connected with features of their anatomy (with ' krants-anatomy '). Photobreath includes formation glikolata and its subsequent disintegration in the presence of oxygen.

In С4-растениях final fixing СО2 in Calvin's cycle - Bensona proceeds in cages of a facing of the vascular bunch, densely pressed to the next cages. It is considered that in such compact fabric, without mezhkletnikov, it is not enough oxygen and what exactly this lowest level of oxygen limits photobreath. Clearly also that at absence mezhkletnikov round facing cages access СО2 to hloroplastam so if these cages functioned separately is complicated also, they could not carry out photosynthesis effectively. Probably that С4-клетки friable mezofilla play a role of collectors СО2 and concentrate it by means of FeP-karboksilazy in the form of organic Sts-acids. Enzyme FeP-karboksilaza is really capable to carry out this problem very effectively; it can fix СО2, transforming it into organic acids, at its much lower concentration, than it does RuBp-karboksilaza. The organic acids formed by such way are transported to hlorrplastam, localised in facing cages. Here СО2 it is liberated in high concentration and at rather low maintenance of oxygen thanks to what hloroplasty these cages can fix rather effectively СО2 in the form of Sugars through Calvin's cycle - Bensona. From this point of view С4-фикеация it is represented something like the pump delivering СО2 for the Sz-way. To it it is possible to add that position of cages of a facing creates conditions for program of end-products of photosynthesis (in particular, sucrose) is direct in sitovidnye tubes floemy on which these products can then be transported in other parts of a plant.

However, probably that all this explanation though also quite logical, nevertheless it is incorrect from the biological point of view. As it was found out recently, even in not differentiated cultures of fabrics С4-растений low level of photobreath remains. Meanwhile such cultures are grown up from one only cages mezofilla and, naturally, cannot possess krants-anatomy.

Considering this circumstance, perhaps, it is necessary to consider that the offered explanation is fair in chemical, but not in a structural part.

Thanks to the ability to especially effective recycling of SO2 С4-растения can fix it at lower concentration, than it do С3-растения. If, for example, to place on one plant of that and other group under a glass cap, i.e. In space with limited stock СО2 the Sz-plant will be lost from an exhaustion to which it will finish С4-растение. There will be it because all СО2, allocated with the Sz-plant in the course of photobreath, will be immediately grasped С4-растением, and the Sz-plant ' dodyshitsja ', at last, before full exhaustion of all resources. In tab. 4.2 the comparative is resulted! The characteristic of these two groups of plants.

The Reasons of occurrence in the course of evolution С4-пути are not quite clear. Among С4-растений there are many tropical cereals. As with rise in temperature photobreath sharply amplifies, ability to limit it in these conditions, undoubtedly, it is possible to consider as advantage. However, until then while function of photobreath to us is not known, it is difficult to judge why С4-путь has arisen and why it is available for one plants and is absent at others. In tropical areas and deserts S3- and С4-растения exist side by side with each other and neither that, nor other type does not find out any prevalence.

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