Biology Test 357

The light-independent reactions (dark or Blackman's reactions) of photosynthesis take place in stroma or matrix of chloroplasts. These reactions are enzymatic reactions which catalyse assimilation of CO₂ into carbohydrates.

In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The simplest example of arithmetic growth is explained by a root elongating at a constant rate.

When the length of the organ is plotted against time, a linear curve is obtained.

The living beings continuously interact with the environment.Living beings also show continuous progressive evolution to become more advanced.Example, in the continuous exposure to the antibiotics, bacteria develop antibiotic resistance and become more evolved.Non living things do not interact with the environment and they do not evolve.

The C₄ plants are adapted to dry tropical regions and have greater productivity of biomass. They have special type of leaf anatomy known as Kranz anatomy. In this type of anatomy the bundle sheath cells form several layers around the vascular bundles; they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.

Calvin, Benson and their colleagues in California, U.S.A. fed Chlorella and Scenedesmus with radioactive ¹⁴C in carbon dioxide. Radioactive carbon, ¹⁴C has a half life of 5568 years. Therefore, the path of CO₂ fixation can be easily traced with its help. Algal suspension, illuminated and carrying out photosynthesis with normal carbon dioxide, was supplied ¹⁴CO₂. The alga was killed at intervals in near boiling methanol. It immediately stopped photosynthesis activity due to denaturation of enzymes. Alcohol was evaporated and after crushing the alga, the product was made into paste. The paste was placed on paper chromatogram and the different compounds were separated by two dimensional chromatography. The radioactive compounds were identified by comparing their position on the chromatogram with standard chemicals. Calvin and co-workers found that after three seconds, radioactivity appeared in phosphoglyceric acid or PGA. Phosphoglyceric acid is, therefore, the first stable product of photosynthesis.

Z scheme involves both PSI and PSII to transfer electron excited by light starting from PSII uphill to the acceptor, down to the electron transport chain to PSI, which further comprise of excitation of electrons, transfer to another acceptor and finally downhill to NADP⁺ causing reduction of it to NADPH + H⁺. Stroma lamella contains PSI only.

In kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. Vascular bundles are surrounded by large sized bundle sheath cells which are arranged in a wreath-like manner in one to several layers. C₄ plants, both monocots and dicots, such as sugarcane, maize, sorghum have kranz anatomy in leaf.

The dark reactions of photosynthesis is purely enzymatic and slower than the primary photochemical reaction. It takes place in stroma portion of the chloroplast and is independent of light i.e., it can occur either in presence or in absence of light provided that assimilatory power is available.

C₄ plants are photosynthetically more efficient than C₃ plants because C4 plant contain two types of chloroplast i.e., bundle sheath chloroplast and mesophyll chloroplast. So such plants operate a dicarboxylic acid cycle in addition to Calvin cycle. CO₂ acceptor molecule (PEP) is present in large bundle sheath cell which has higher efficiency in picking up CO₂. Thus, photosynthesis continues even at low CO₂ concentration and the rate of photorespiration is also negligible.

In C₄ plants, C₄ cycle occurs in mesophyll cells and C₃ - cycle occurs in bundle sheath cells.