REVIEW AND QUESTIONS ON CELLULAR RESPIRATION

We have already discussed cell respiration but here is a recap. Cellular respiration is the process by which the chemical energy trapped in food molecules is released and partially captured in the form of ATP. All the different classes of food substances, carbohydrates, fats, and proteins can all be used as fuels in cellular respiration. They have to be converted to glucose before they can be used in the chemical pathway. The three stages of cellular respiration are:

1. Glycolysis: This occurs in the cytosol. In glycolysis, the 6-carbon sugar, glucose, is broken down into two molecules of a 3-carbon molecule called pyruvate. This change is accompanied by a net gain of 2 ATP molecules and 2 NADH molecules.

2. The Krebs cycle: This occurs in the matrix of the mitochondria. This takes place in the presence of oxygen.It generates a pool of chemical energy which includes ATP, NADH, and FADH_2,  from the oxidation of pyruvate, the end product of glycolysis.

3. Oxidative phosphorylation: This occurs in the electron transport chain. The electron transport chain is located on the inner mitochondrial membrane and consists of a series of molecules, mostly proteins. The electron transport chain allows the release of large amount of chemical energy stored in reduced NAD⁺ (NADH) and reduced FAD (FADH₂). This energy is captured in the form of ATP (3 ATP per NADH and 2 ATP per FADH₂). 

In the absence of oxygen, fermentation occurs. Fermentation leads to lactic acid production.

• Each molecule of glucose can generate 36-38 molecules of ATP in aerobic respiration.
•  Only 2 ATP molecules are generated in respiration without oxygen (through glycolysis and fermentation).

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