Question

Name the three phases of respiration in human beings.

Answer 1

Hint: If energy is to be useful to the cell, it should be able to utilise it to synthesise other molecules that the cell requires. The strategy that the animal cell uses is to catabolise the glucose molecule in such a way that not all the liberated energy goes out as heat. The logic behind this is to oxidise glucose not in one step but in several small steps enabling some steps to be just large enough such that the energy released can be coupled ATP synthesis. This is done by the process of cellular respiration.

Complete answer:
Respiration includes three phases:
1.Glycolysis:
- It is a common step of both types of respiration. Glycolysis (Greek glycon - for sugar and lysis – for splitting) means splitting up of sugar.
- This scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is called the EMP pathway. In anaerobic organisms, it is the only process in respiration. Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms.
- Glycolysis is defined as the process of partial oxidation of glucose to form two molecules of pyruvic acid.
- Glycolysis does not require oxygen. In plants, the glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates. Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway. - In animals, starch is digested into glucose and that is used in the glycolysis.
- The first half of this pathway activates glucose (glucose activation phase). The second half extracts the energy (energy extraction phase).
2. Krebs’s cycle:
- Acetyl CoA enters a cyclic pathway called as Krebs cycle, Krebs cycle was named after the
scientist Hans Krebs who first elucidated it in flight muscles of pigeons.
- In these reactions hydrogen atoms are removed from the acetyl CoA and transferred to the coenzymes for further processing in the electron transport system.
- The cycle is also called Citric Acid Cycle (CAC, because of the formation of citric acid in the first step of this cycle) or TCA cycle (because many intermediate compounds formed in the cycle have three carboxyl groups).
- The reactions of the Krebs cycle require the presence of oxygen and are confined to the mitochondrial matrix.
- This cycle serves as a common oxidative pathway for carbohydrates, fats and proteins. All enzymes are soluble in the mitochondrial matrix, but succinate dehydrogenase (SDH) is found attached to the inner mitochondrial membrane.
3. Electron Transport Chain:
- Till now, we have seen that glucose has been broken down completely but neither $\mathop O\nolimits_2 $, has been directly involved nor has the large number of ATP molecules been produced.
- The following steps in the respiratory process are to release and utilize the energy stored in $\mathop {NADH + H}\nolimits^ + $and $\mathop {FADH}\nolimits_2 $.
- This is accomplished when they are oxidised through the electron transport system and the electrons are passed on to $\mathop O\nolimits_2 $ resulting in the formation of $\mathop H\nolimits_2 \mathop O\nolimits_{} $.
- The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS). This process occurs in the inner mitochondrial membrane.

Note:-
So these three pathways constitute aerobic respiration. It is the process by which organisms can carry out complete oxidation of glucose and extract the energy store to synthesise a larger number of ATP molecules needed for cellular metabolism