1. What is the aerobic respiration?

Glucose takes part in a chemical reaction with oxygen inside the cell. During this reaction, glucose is broken down to carbon dioxide and water, and energy is released. This process is called aerobic respiration.

2. Write short note on glycolysis.

Glycolysis is a purely anaerobic reaction. Glycolysis is a determined sequence of ten reactions involving ten intermediate compounds. They contain quite amounts of energy, glucose molecules are relatively uncreative. They must be activated before glycolysis can proceed. This is done by the addition of a phosphate. Each of these additions of a phosphate group is done by transferring a phosphate group to the sugar from ATP. The ATP is converted into ADP in the process; Glycolysis has used two ATP molecules. The six-carbon fructose biphosphate molecule is split into two three-carbon molecules, trios’ phosphate.

Each of these is then converted to glycerate 3 phosphates and then to pyruvate in a series of small steps. These steps release enough energy from the glycerate 3 phosphates molecules to make some ATP. Four molecules of ATP are made directly, there and then in the cytoplasm, using energy released as the ribose phosphate molecules are gradually changed to pyruvate. So although two molecules of ATP were put into the process at the beginning, four have been made at the end. The conversion of trios phosphate into glycerate 3 phosphates also releases hydrogen ions (H⁺) and electrons (e^-) which are transferred to the coenzyme Nicotinamide adenine dinucleotide. These hydrogen ions and high energy electrons are passed into mitochondrion where they can be used to produce up to 5 ATP molecules in oxidative phosphorylation.

3. Define the Krebs cycle.

The Krebs cycle takes place in the matrix of mitochondrion. If the oxygen is available, the pyruvate formed in glycolysis passes into a mitochondrion through the outer and inner mitochondrial membranes. Once in the matrix of the mitochondrion, pyruvate is converted to acetyl coenzyme. One hydrogen ion, two electrons and one carbon dioxide molecule are released during this process. The acetyl group of acetyl coenzyme A, which contains two carbon atoms, then combines with a four carbon compound called oxaloacetate. The resulting six- carbon compound, citrate, is gradually re converted to oxaloacetate. Many small steps are involved in this cyclic reaction. It is sometimes called the citric acid cycle, but sometimes the name Krebs cycle.

At two stages in the Krebs cycle, carbon dioxide is removed from the compounds involved. This process is called decarboxylation. This carbon dioxide, plus that which was produced when pyruvate was converted to acetyl coenzyme A, diffuses out of the cell, and eventually out of the organism. Other important products of the Krebs cycle are electrons and hydrogen ions which are both picked up by the oxidized coenzyme NAD, and some by oxidized FAD.

                    NAD⁺ + H⁺ + 2e^- → NADH

Oxidized NAD is reduced to NAD. These coenzymes can hold electrons which will then be fed into the electron transport chain to make ATP. When one glucose molecule is respired, two pyruvate are produced and they result in the production of six reduced NAD and two reduced FAD in the Krebs cycle. Two more reduced NAD molecules are produced in the conversion of pyruvate to acetyl coenzyme A. One step in the Krebs cycle makes ATP directly. Two ATP molecules are produced in this way per original glucose molecule.

4. Define oxidative phosphorylation.

An important result of respiration is the formation of ATP. ATP is made by the addition of inorganic phosphate, to ADP, a phosphorylation reaction. In respiration, this process requires oxygen and so is known as oxidative phosphorylation.

5. What is the importance of the Riboflavin or Vitamin B₂ in the reactions of Krebs cycle?

Riboflavin (Vitamin B₂) is used to make Flavin adenine dinucleotide (FAD). FAD is a coenzyme which acts as a hydrogen and electron acceptor in the Krebs cycle. It is a coenzyme because the dehydrogenate enzyme catalyzing the reaction which removes the hydrogens cannot do so unless FAD is available to accept them.

6. Write short note on the involvement of Niacin and Thiamine in the reactions of Krebs cycle.

Niacin or nicotinic acid is used to make NAD (Nicotinamide adenine di nucleotide). This, like FAD, is a coenzyme acting as a hydrogen and electron acceptor, which it does both in glycolysis and in the Krebs cycle. Both NAD and FAD act as coenzymes in other metabolic reactions besides those of respiration. Thiamine is converted in the body to thiamine pyrophosphate, TPP. This acts as a coenzyme in some reactions involving acetyl group transfer, and is needed for the reaction in which pyruvate is converted to acetyl coenzyme A.

7. Explain the mitochondrion structures matrix and crista.

The matrix is where the enzymes of the Krebs cycle are found. The Krebs cycle removes high energy electrons from the intermediates of the Krebs cycle and produces reduced NAD and FAD. The inner mitochondrial membrane is highly folded. This creates a large surface area to hold electron carriers and ATP synthesis molecule.

8. What is an ATP synthase?

This is the molecule that makes ATP from ADP and Pi using the energy from electrons obtained from reduced NAD and FAD. The molecule spans the inner membrane and has a channel in it which allows H⁺ ions to pass through.

9. Write short notes on the inner membrane of mitochondria.

The outer mitochondrial membrane is a barrier that stops large molecules from entering the mitochondrion and possibly interfering with the Krebs cycle and oxidative phosphorylation. However, it allows pyruvate needed for the Krebs cycle to cross freely into matrix. Inner membrane has electron carriers and ATP synthetase molecules embedded in it, the membrane is also a barrier which prevents many small molecules from the cytoplasm from entering the matrix and interfering with the Krebs cycle and oxidative phosphorylation. Pyruvate, however, is transported across it.

10. Define the two types of anaerobic respirations. Write their advantages to human beings.

1. Alcoholic fermentation :- Alcoholic fermentation is one form of the anerobic respiration. This is used by fungi and plants. Yeast converts pyruvate to ethanol. Carbon dioxide is removed from pyruvate to produce ethanal, the enzyme alcohol dehydrogenate convert the ethanal to ethanol. This step requires hydrogen which is taken from reduced NAD. This process has been used by humans for thousands of years. If yeast is provided with a supply of carbohydrate, it will carry out glycolysis and alcoholic fermentation. In bread making, we provide it with starch in flour, and make use of the carbon dioxide it releases to make the bread rise. In the making of alcoholic drinks, it is the ethanol which is required.

2. Lactic fermentation :- This is used by animals. If muscle is exercising hard, it may run out of oxygen and have to respire anaerobically for a short period. The pyruvate produced by glycolysis is converted to lactate. The enzyme responsible for this conversion is lactate dehydrogenase, and the process requires hydrogen from reduced NAD. It is the build-up of lactate, which forms lactic acid, in muscle which causes the pain. The lactate must be broken down, and for this to take place it is transported in blood to the liver. Here some is converted back into glucose. This process requires oxygen, which is why you go on breathing deeply even when your strenuous exercise is over, supplying extra oxygen to the liver, to help it to deal with the lactate produced because of a shortage of oxygen earlier on.