Respiration in Plants

Plantslack specialised system for gaseous exchange.Stomata and lenticelspresent in them allow gaseous exchange by the process of diffusion.

Cellular respiration is a stepwise process of oxidation of respiratory substrates within the cell to release energy and store it in the form of ATP.

The compounds that are required during this process are known as respiratory substrates.If they are completely oxidised, their respiratory quotient will be one.

There are two types of respiration: anaerobic and aerobic respiration.

For both the types of respiration, glycolysis is the common stepthat takes place in the cytoplasm.

Glycolysis is a ten stepprocess of partial oxidation of glucose in the presence of enzymes to yield pyruvate.
Steps involved in glycolysis are:
•    Phosphorylation of glucose to form glucose-6-phosphate
•    Isomerisationof glucose-6-phosphate to form fructose-6-phosphate
•    Phosphorylation of fructose-6-phosphate to form fructose-1,6-biphosphate
•    Breakdown of fructose-1, 6-biphosphate to form dihydroxyacetone phosphate and glyceraldehyde-3-phosphate (PGAL)
•    Dihydroxyacetone phosphate is isomerised to glyceraldehyde-3-phosphate
•    Phosphorylation and Dehydrogenation of PGAL to yield 1,3-Diphosphoglyceric acid (DPGA)
•    Dephosphorylation of DPGA to form 3-Phosphoglycerate
•    Isomerisation of 3-Phosphoglycerate to form 2-Phosphoglycerate
•    Dehydration of 2-Phosphoglycerate to form phosphoenolpyruvate (PEP)
•    Dephosphorylation of PEP to yield pyruvate and ATP

The pyruvate produced in glycolysis is transported by a pyruvate carrieracross the mitochondrial membranes.
Steps involved are:
•    Oxidative decarboxylation of pyruvate in the presence of pyruvate dehydrogenase into mitochondrial matrix.
•    Production of two molecules of NADH and formation of acetyl CoA.
•    Entry of acetyl CoA in TCA cycle.

If an organism respires aerobically, it adopts TCA cycle to utilise the product of glycolysis (pyruvate) and release energy.

TCA Cycle is also known as the Krebs cycle or the tricarboxylic acid cycle. It takes place in mitochondria.
Steps involved are: -
•    Condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid
•    Isomerisation of citrate to form isocitrate
•    Decarboxylation of isocitrate to from a-ketoglutaric acid and then succinyl-CoA
•    Conversion of succinyl-CoA to succinic acid and synthesis of a molecule of GTP
•    Conversion of GTP to GDP with the simultaneous synthesis of ATP from ADP
•    Dehydrogenation of succinate to fumarate
•    Hydration of fumarate to form malate
•    Dehydrogenation of malate to OAA by malate dehydrogenase

NADH and FADH2, releasedby TCA cycle, are transported to the electron transport chain during oxidative phosphorylation, where the high energy electrons drive the synthesis of ATP.
On the other hand, if an organism respires anaerobically, it undergoes fermentation to release energy.

Fermentation refers to the process of incomplete oxidation of glucose in the presence of enzymes. Based on the type of product released, fermentation is of two types; namely, alcoholic fermentationand lactic acid fermentation.
Yeast cell respires anaerobically by alcoholic fermentation, while a muscle cell respires anaerobically by lactic acidfermentation.

Aerobic respiration is much more efficient than anaerobic respiration because it releases 36 ATP molecules from one molecule of glucose, while anaerobic respiration releases 2 ATP molecules from one molecule of glucose.

Since the respiratory pathway is involved in both anabolism and catabolism, it is known asamphibolic pathway.

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