RESPIRATION & ENERGY TRANSFER
- Maintenance of life requires a continuous supply of energy.
- Respiration fulfills the continuous need for energy.
- Respiration is a catabolic process wherein complex organic substrate is oxidized to simple components to generate biological energy.
- It is cellular respiration that does not involve oxygen at all.
- It is also called "Fermentation".
- It is completed through steps like glycolysis and conversion of glycolytic product to any suitable product like lactic acid, ethanol, etc.
A.) GLYCOLYSIS:
- Occurs: Cytoplasm of a cell.
- It involves the breakdown of a glucose molecule into two pyruvic acid molecules.
- This is a common step in anaerobic as well as aerobic respiration.
- It is completed in two phases as preparatory phase and the pay-off phase.
- The overall process of glycolysis is completed in ten steps.
1.) Preparatory phase:
- The first five steps constitute the preparatory phase through which glucose is phosphorylated twice at the cost of two ATP molecules and a fructose 1,6-biphosphate is formed.
- This molecule is split to form: 1.) a molecule of glyceraldehyde 3-phosphate & 2.) a molecule of dihydroxyacetone phosphate.
- Both of these molecules are 3-carbon carbohydrates (trioses) and are isomers of each other.
- Dihydroxyacetone phosphate is isomerized to the second molecule of glyceraldehyde-3-phosphate.
- Thus, two molecules of glyceraldehyde-3-phosphate are formed, and here, the preparatory phase of glycolysis ends.
2.) Pay-off phase:
- Both the molecules of glyceraldehyde-3-phosphate are converted to two molecules of 1, 3-biphosphoglycerate by oxidation and phosphorylation.
- Phosphorylation is brought about with the help of inorganic phosphate (Pi) and not ATP.
- Both molecules of 1, 3-biphosphoglycerate are converted into two molecules of pyruvic acid through series of reactions accompanied by the release of energy.
- This released energy is used to produce ATP (4 molecules) by substrate-level phosphorylation.
- 2 ATP/glucose is the net outcome.
- Energy is also converted by the formation of 2-NADH molecules.
B.) Lactic Acid Fermentation (In Muscle):
- In muscles, the NADH+H ion produced during glycolysis is reoxidized to NAD+ by donating one proton and two electrons to pyruvic acid which yields lactic acid.
- In this reaction pyruvate is converted into a 3-carbon molecule called lactic acid.
- No production of carbon dioxide (CO2).
- The only benefit is serves is that it allows glycolysis to continue with the small gain of ATP generated.
- Skeletal muscles usually derive their energy by anaerobic respiration.
- After vigorous exercise lactic acid accumulates, leading to muscle fatigue.
- During rest, however, the lactic acid is reconvereted to pyruvic acid and is channeled back into the aerobic respiration pathway.
C.) Alcoholic Fermentation (In Yeast):
- In yeast, the pyruvate is decarboxylated to acetaldehyde.
- The acetaldehyde is then reduced by NADH+H ion to ethanol.
- Carbon dioxide (CO2) is also produced in this process.
- Accumulation of ethanol by fermentation in a culture of yeast may stop further multiplication and lead to the death of cells.
- In the presence of oxygen (O2) however, yeast can respire aerobically.
- Examples of food produced are alcoholic drinks, bread, cakes, etc.