14. RESPIRATION IN PLANTS
It is the partial oxidation (breakdown) of glucose to 2 molecules of pyruvic acid (C3H4O3) in the absence of O2.It occurs in cytoplasm of all living organisms.
Its scheme was given by Gustav Embden, Otto Meyerhof & J. Parnas. So it is also known as EMP pathway.
In anaerobes, it is the only process in respiration.
In plants, glucose is derived from sucrose (end product of photosynthesis) or from storage carbohydrates. Sucrose is converted to glucose & fructose by an enzyme, invertase. These 2 monosaccharides readily enter glycolytic pathway.
Glucose & fructose are phosphorylated to form glucose-6-phosphate by the enzyme hexokinase. It is then isomerised to produce fructose-6-phosphate. Subsequent steps of metabolism of glucose and fructose are same.
Steps of glycolysis:
It includes 10 steps under the control of different enzymes.
ATP is utilized at 2 steps:
- In the conversion of glucose into glucose 6-phosphate.
- In the conversion of fructose 6-phosphate to fructose 1, 6-diphosphate.
PGAL is oxidised and with inorganic phosphate get converted to 1, 3-bisphosphoglycerate (BPGA). During this, 2 redox-equivalents (2 H-atoms) are removed from PGAL and transferred to NAD+ forming NADH + H+.
BPGA becomes 3-phosphoglyceric acid (PGA) yielding energy. This energy is trapped by the formation of ATP.
ATP is also formed when PEP converts to pyruvic acid.
In glycolysis, 4 ATP molecules are directly synthesised from one glucose molecule.
Pyruvic acid (pyruvate) is the key product of glycolysis. Its metabolic fate depends on the cellular need.
In different cells, pyruvic acid is handled in 3 ways:
- Lactic acid fermentation.
- Alcoholic fermentation.
- Aerobic respiration (Krebs’ cycle).
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