BIOLOGY 3 Marks Question

The significance of step-wise release of energy in respiration are:

• It facilitates the utilization of the relatively higher proportion of the energy in ATP synthesis.
• The activities of enzymes for the different steps may be enhanced or inhibited by specific compounds. This provides a mean of controlling the rate of the pathway and the energy output according to the need of the cell.
• The same pathway may be utilized for forming intermediates used in the synthesis of other biomolecules like amino acids.

Aerobic respiration and Anaerobic respiration:

	Aerobic respiration		Anaerobic respiration
i.	It occurs in the presence of oxygen.	i.	It occurs in the absence of oxygen.
ii.	It involves the complete breakdown of respiratory material.	ii.	It involves the incomplete breakdown of gases.
iii.	The end products are carbon dioxide and water.	iii.	The end products are ethanol and carbon dioxide.
iv.	It involves the exchange of gases.	iv.	It does not involve the exchange of gases.

The following are the main steps in aerobic respiration:

1. Glycolysis: It is the breakdown of glucose to pyruvic acid and occurs inside the cytosol of cytoplasm.
2. Oxidative decarboxylation of pyruvic acid to acetyl coenzyme-A: It occurs inside the mitochondrial matrix.
3. Tricarboxylic acid cycle or Kreb's cycle: It occurs inside the mitochondrial matrix.
4. Electron transport system and oxidative phosphorylation which occur in the inner mitochondrial membrane involving ATP synthase complex.

Aerobic respiration occurs in mitochondria.

Aerobic respiration and fermentation:

	Aerobic respiration		Fermentation
1.	Oxygen is used for deriving energy.	1.	Occurs in the absence of oxygen.
2.	Occurs in the cytoplasm and mitochondria.	2.	Occurs in the cytoplasm.
3.	End products are carbon dioxide and water.	3.	End products are ethyl alcohol and carbon dioxide.
4.	Complete oxidation of the respiratory substrate takes place.	4.	Incomplete oxidation of the respiratory substrate takes place.
5.	About 36 ATP molecules are produced.	5.	Only 2 ATP molecules are produced.

Respiration is generally assumed to be a catabolic process because during respiration, various substrates are broken down for deriving energy. Carbohydrates are broken down to glucose before entering respiratory pathways. Fats get converted into fatty acids and glycerol whereas fatty acids get converted into acetyl CoA before entering the respiration. In a similar manner, proteins are converted into amino acids, which enter respiration after deamination. During synthesis of fatty acids, acetyl CoA is withdrawn from respiratory pathway. Also, in the synthesis of proteins, respiratory substrates get withdrawn. Thus, respiration is also involved in anabolism. Therefore, respiration can be termed as amphibolic pathway as it involves both anabolism and catabolism.

Respiration is generally assumed to be a catabolic process because during respiration, various substrates are broken down for deriving energy. Carbohydrates are broken down to glucose before entering respiratory pathways. Fats get converted into fatty acids and glycerol whereas fatty acids get converted into acetyl CoA before entering the respiration. In a similar manner, proteins are converted into amino acids, which enter respiration after deamination. During synthesis of fatty acids, acetyl CoA is withdrawn from respiratory pathway. Also, in the synthesis of proteins, respiratory substrates get withdrawn. Thus, respiration is also involved in anabolism. Therefore, respiration can be termed as amphibolic pathway as it involves both anabolism and catabolism.

3. Currency notes.

Cells have developed a wonderful mechanism of storing energy in the form of ATP. ATP provides energy as and when needed. If this mechanism were not developed, then it could have been problematic for the living beings. A living being would be in constant need of food to get the required energy. We can compare a situation in which the petrol tank of a car becomes empty. Once the tank becomes empty, there is no way to move the car further. One needs immediate refilling to start the car. We would have suffered the same fate. Because of the facility of energy being stored in the form of ATP, we need to take food at certain intervals and can also do away with food for some time if we don't get the food or don't get the time to eat.

The following assumptions are made during the calculation of net gain of ATP:

• There is a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS pathway following one after another.
• The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
• None of the intermediates in the pathway are utilised to synthesise any other compound.
• Only glucose is being respired – no other alternative substrates are entering in the pathway at any of the intermediary stages.

The ATP yielding reactions of glycolysis are:

1. Two molecules 1, 3-diphosphoglycerate reacts with ADP in presence of enzyme phosphoglycerate kinase to form two molecules of 3-phosphoglycerate and two ATP molecules.

1, 3-diphosphoglycerate + ADP → 3-phosphoglycerate +ATP.

2. The two molecules of phosphoenol pyruvate in presence of Pyruvate kinase get phosphorylated to produces two molecules of pyruvate and two ATP.

Phosphoenol pyruvate + ADP → Pyruvate +ATP.

Glycolysis and Fermentation:

	Glycolysis		Fermentation
i.	It is the first step of respiration which occurs without oxygen and is common to both aerobic and anaerobic modes of respiration.	i.	It is anaerobic respiration which does not require oxygen.
ii.	It produces pyruvic acid.	ii.	It produces different products such as ethanol and lactic acid.
iii.	It produces two molecules of NADH per glucose molecule.	iii.	It generally utilises NADH produced during glycolysis.
iv.	It forms 2 ATP molecules per glucose molecule.	iv.	It does not produce ATP.

Glycolysis and Citric acid cycle:

	Glycolysis		Citric acid cycle
i.	It occurs inside the cytoplasm.	i.	It operates inside the mitochondria.
ii.	It is a straight or linear pathway.	ii.	It is a cyclic pathway.
iii.	It is the first step of respiration in which glucose is broken down to the level of pyruvate.	iii.	It is the second step in respiration where an active acetyl group is broken down completely.
iv.	The net gain of energy is equal to 8 ATP.	iv.	The net gain of energy is equal to 24 molecules of ATP.

1. It is the process of breaking of the C-C bonds of complex compounds through oxidation within the cells, to release considerable amount of energy.
2. RQ for proteins is 0.9.
3. RQ for fats is 0.7.

A single step release of large amount of energy will be harmful to the cell as most of it will remain unutilized and would cause damage to cellular organisation. Step-wise release of small amount of energy is useful as:

1. It can be trapped and stored for later use.
2. There will be little wastage.
3. Process of energy release can be controlled as per requirement.
4. It produces a number of intermediates which are used for synthesis of several types of substances.

When formation of ATP happens by direct transfer of a phosphoryl (PO3) group to ADP from a phosphorylated reactive intermediate; this is called substrate level phosphorylation. During glycolysis, ATP is formed at two stages because of direct transfer of phosphoryl group to ADP. When BPGA (biphosphoglyceric acid) is converted into PGA (phosphoglyceric acid). When PEP (phosphoenolpyruvate) is converted into pyruvic acid.

Following are the reasons why plants can get along without respiration organs:

1. Every part of the plant takes care of its own gas exchange needs and there is very little transport of gases from one part of the plant to another.
2. Plants do not have great demands for gas exchange; the rate of respiration is far lower than that of animals.
3. The distance for which the gases diffuse, even in large, bulky plants is not great as living cells in a plant are located quite close to the surface of the plant.

During breakdown and synthesis of protein, respiratory intermediates form the link. Breaking down processes within the living organism is catabolism, and synthesis is anabolism. Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.

On oxidation 1gm of fat releases 9.45 Kcal of energy, 1gm of protein releases 5.65Kcal of energy, and 1gm of glucose releases 4.1Kcal of energy. 0.5g of protein + 0.5g of glucose release 2.825Kcal + 2.05K cal = 4.875 Kcal of energy. The arrangement of amount of energy released in ascending order is: 1gm of glucose < 0.5gm of protein + 0.5gm of glucose < 1gm of protein < 1gm of fat.