STPM Biology - chapter 4(old syllabus) chapter 6 (new syllabus): Revision essay question 6

6. Describe the pathway taken by atmospheric carbon dioxide until it is used in the synthesis of hexone sugars in

-C₃ plants

-C₄ plants

Answer:

C₃ plants

C₃ plants, for example are tomato, wheat and legume. All of these plants only contain and take places in mesophyll cells. The carbon dioxide acceptor in these plants is ribulose biphosphate RuBP (5C). RuBP carboxylase is inefficient at low carbon dioxide concentration. Carbon dioxide from the atmosphere diffuses through the stomata into the intercellular spaces of the leaf. The carbon dioxide molecule combines with a five-carbon acceptor, ribulose bisphousphate (RuBP) to form an unstable six-carbon sugar. The process required enzyme RuBP carboxylase (Rubisco) to catalysed. The six-carbon compound splits into two molecules of glycerate 3-phosphete (GP) (3-carbon compound). The glycerate 3- phosphate combine with a phosphate group from ATP to become glycerate 1, 3-diphosphate. Glycerate 1, 3-diphosphate is converted into glyceraldehydes 3-phosphate by combines with hydrogen

atoms from reduced dinucleotide phosphate (NADPH+H⁺). Some of the glyceraldehydes 3-phosphate molecules are rearranged in a series of complex reactions to regenerate ribulose bisphosphate, this process requires ATP. The ribulose bisphosphate is combined with carbon dioxide from the atmosphere and the Calvin cycle is repeated.

C₄ plants

Examples of C₄ plants are maize, sorghum, sunflower, sugarcane and eleusine. There are growing in tropical and subtropical areas. The pathway of C₄ plants for carbon dioxide fixation is the C₄ pathway and also known as Hatch-Slack pathway. Moreover, C₄ pathway more efficient in carbon dioxide fixation than C₃ pathway. C₄ plant leaves have two layers of green cells that are filled with chloroplasts that is the mesophyll cell and inner vascular bundle cell. In mesophyll cells, the chloroplasts are few and small. When carbon dioxide in the atmosphere diffuses into the mesophyll cells, they combine with phosphenolpyruvate (PEP, 3C acceptor) to produce oxaloacetate (4C). This process required enzyme phosphoenol-pyruvate carboxylase which has higher affinity for carbon dioxide at low concentration. Oxaloacetate (4C) combines with hydrogen atoms from reduced dinucleotide phosphate (NADPH+H⁺) and is converted into malate acid (4C). After that, malate is shunted through plasmodesmata to bundle sheath cell. Plasmodesmata are the cell walls possess minute pores and can form living connections between neighboring cells. Malate is then oxidized into pyruvate by the removal of hydrogen and carbon dioxide. These in turn, increases the

carbon dioxide concentration in the stroma of the chloroplasts. The pyruvate produced is diffuses into mesophyll cells and reuse to regenerate phosphenolpyruvate produces more and more carbon dioxide. The carbon dioxide enters in the Calvin cycle and produces organic molecules like glucose and starch.

Assignment submitted by S.M., Kang, E.S., Ong, T.W., Tan, K.Y., Ho. X.H., Wong 2011/2012 (IBM)