Question

Explain the production of a sugar molecule through the process of photosynthesis in a CAM plant. Describe the starting materials of the reaction, where the reaction takes place and the different steps in the production of sugar. Explain how energy is captured, held, and transferred to ultimately be stored in a sugar molecule. Include the terms chlorophyll, P680, primary electron acceptor, NADPH, light reactions, ATP, Calvin cycle, CO2, O2, H2O, and crassulacean acid metabolism.

Answer 1

Photosynthesis in the process of formation of sugar in the presence of Sunlight and chlorophyll molecules by using atmospheric carbon dioxide. This process is atmospheric carbon dioxide which is an abiotic source of carbon into biotic source of carbon in the form of glucose in the biomass of plants.

It is divided into two stages. The first stage is light dependent reaction which occurs in the presence of sunlight in thylakoid membrane of chloroplast and the second stage is dark reaction of Calvin cycle or light independent reaction which take place in the stroma of cell. Dark reaction can take place in the absence of presence of light but is dependent upon the product of light reactions.

During light reaction, light is absorbed by photosystem 2 (P700) which causes photolysis of water and release of oxygen as gas and excitation of electrons released by the photolysis of water. This results in the entry of electron into electron transport chain taking place in thylakoid membrane. In electron transport chain, electrons are transported from one electron carrier to the other. All the electron Carriers are arranged in increasing order of their electrochemical potential. As the electrochemical potential increases, the affinity of an electron carrier for electron also increases. The electrons pass from PS2 to cytochrome b6f complex and than to PS1 (P680). Ultimately the electrons are accepted by NADP+, primary electron acceptor, to form NAPDH which is a high energy electron carrier. When electrons are transported from one electron carrier to another, protons are pumped from the lumen of thylakoid to the stroma of chloroplast. This results in an increase in the acidity of stroma and differential concentration of protons across thylakoid membrane. This leads to the formation of proton gradient or electrochemical gradient across the membrane. The electrochemical energy stored in Proton gradient is used by ATP synthase enzyme for the synthesis of ATP.

Hence light reaction begin with the photolysis of water and results in the evolution of Oxygen and formation of ATP and NADPH.

Both ATP and NADPH are higher energy careers. From thylakoid they enter stroma and participate in Calvin cycle. Calvin cycle begins with the fixation of atmospheric carbon dioxide to form triose phosphate. It uses both of these high energy carriers for the fixation of carbon dioxide into triose phosphate. The key enzyme for Calvin cycle is rubisco.

So, to summarise, the electrons donated by water were passed on to NADP+ which was the primary electron acceptor and then the electrons were passed onto triose phosphate and finally accepted by glucose which is the final electron acceptor.

When triose phosphate molecules are synthesized, they are transported into the cytoplasm of the cell so that they can be converted into glucose. Glucose is then used by the plant for the generation of ATP by cellular respiration in mitochondria.

Carbon dioxide during photosynthesis is taken by plants through stomata and oxygen is also released through stomata. Calvin cycle is a Universal cycle because it takes place in all the plants. It is also called as C3 cycle because the first stable product is three carbon compound phosphoglyceric acid. Some plants which live in extreme dry climatic conditions, open and close stomata in a different manner. These plants show a similar method of photosynthesis but they have scotoactive stomata, which opens during night for the fixation of atmospheric carbon dioxide. This is an important mechanism in order to prevent water loss through stomata during the daytime when the temperature is high. This is called as crassulacean acid metabolism and is mainly shown by the plants belonging to family crassulaceae.

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