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Monday, 30 January 2017

Respiration in Plants : Tricarboxylic Acid Cycle/Krebs Cycle

Respiration is an energy releasing enzymatically controlled catabolic process which involves a stepwise oxidative breakdown of food substance inside living cells.

  • Living organism require energy for all activities like absorption, movement, reproduction or even breathing. Energy required is obtained from oxidation of food during respiration.
  • Cellular respiration is the mechanism of breaking down of food materials within the cell to release energy for synthesis of ATP.
  • Breaking down of complex molecules takes place to produce energy in cytoplasm and in the mitochondria.
  • Breaking down of C-C bond of complex compounds through oxidation within the cells leading to release of considerable amount of energy is called respiration. The compounds that get oxidized is called respiratory substrates.
  • Energy released during oxidation is not used directly but utilized in synthesis of ATP, which is broken down when energy is required. Therefore, ATP is called energy currency of cells.
  • The process of respiration requires oxygen. In plants oxygen is taken in by stomata, lenticels and root hairs.
  • Plants can get along without respiratory organs because
  • a. Each plant part takes care of its own gas-exchange needs.
  • b. Plants do not present great demands for gas exchange.
  • c. Distance that gases must diffuse in large plant is not great.
  • d. During photosynthesis O2 is released in leaves and diffuse to other part of leaves.
  • During process of respiration oxygen is utilized and carbon dioxide and water is released along with energy molecules in form of ATP.
  • Respiratory Quotient is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time. RQ is equal to one for carbohydrate and less than one for protein and peptones.                        

Aerobic Respiration is an enzymatically controlled release of energy in a stepwise catabolic process of complete oxidation of organic food into carbon dioxide and water with oxygen acting as terminal oxidant.

  • The scheme of glycolysis is given by Gustav Embden, Otto Meyerhof, and J. Parnas. It is also called as EMP pathway.
  • Glycolysis is the partial oxidation of glucose or similar hexose sugar into two molecules of pyruvic acid through a series of enzyme mediated reaction releasing some ATP and NADH2. It occurs in cytoplasm.
  • In plans glucose is derived from sucrose or from storage carbohydrates. Sucrose is converted into glucose and fructose by enzyme invertase.
  • Glycolysis starts with phosphorylation of glucose in presence of enzyme hexokinase to form Glucose-6-phosphate. One molecules of ATP is used in this process.
  • In next steps Glucose-6-phosphate is converted into fructose-6-phosphate, catalyzed by enzyme phosphohexose isomerase.
  • Fructose-6-phosphate uses another molecules of ATP to form Fructose-1-6 biphospahte in presence of enzyme phosphfructokinase.
  • In glycolysis two molecules of ATP are consumed during double phosphorylation of glucose to fructose 1,6 biphosphate. Two molecules of NADPH2 are formed at the time of oxidation of glyceraldehyde 3-phosphate to 1,3 biphosphoglycerate. Each NADH is equivalent to 3ATP, so that net gain in glycolysis is 8 ATP.
  • Pyruvic acid is the key product of glycolysis, further breakdown of pyruvic acid depends upon the need of the cell.
  • In animal cells, like muscles during exercise, when oxygen is insufficient for aerobic respiration, pyruvic acid is reduced to Lactic acid by enzyme lactate dehydrogenase due reduction by NADH2
  • In fermentation by yeast, pyruvic acid is converted to ethanol and CO2. The enzyme involved is pyruvic acid decarboxylase and alcohol dehydrogenase catalyze this reaction.

  • In both lactic acid fermentation and alcohol fermentation very less amount of energy is released.
  • Yeasts poison themselves to death if concentration of alcohol reaches above 13%.
  • Final product of glycolysis, pyruvate is transported from the cytoplasm into mitochondria for further breakdown.
  • Oxidation of Pyruvate to Acetyl-CoA is done to produce CO2 and NADH. The reaction catalyzed by pyruvic dehydrogenase requires the participation of several Coenzymes including NAD+ .

  • The Acetyl CoA enters a cyclic pathway called TCA cycle or Kreb’s cycle.
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