With a schematic diagram explain briefly the overall process of respiration
The oxygen inhaled is used to burn/oxidize/break down the food (glucose). This reaction produces energy required for all activities. Water and carbon dioxide are by-products of this reaction. This process occurs inside the mitochondria and is called cellular respiration. It is exactly opposite to the process of photosynthesis. It can be represented as:
Thus, cellular respiration is the process by which energy is released from the breakdown of organic substances (glucose).
Aerobic respiration: It involves four main steps:
Step 1: The first step is called glycolysis. It occurs in the cytoplasm of a cell. It does not require oxygen. Here, the 6-carbon molecule glucose is partially broken down into a 3-carbon molecule, pyruvate or pyruvic acid. In this step, one glucose molecule produces 2 molecules of pyruvate.
Glucose 
2 Pyruvate
Step 2: The two pyruvic acid molecules are converted into acetyl CoA, which can easily enter the mitochondria, which is the site for further reactions.
Step 3: These acetyl CoA molecules enter the mitochondrial matrix and take part in the Krebs cycle. This occurs in the presence of O2.
Step 4: In this step, the energy released in all the above steps is converted to ATP by ATP synthase enzyme.
In aerobic respiration, a total of 36 ATP molecules are produced from one molecule of glucose.
Anaerobic respiration: It is a two step process.
Step 1: The first step, glycolysis, is the same as that of aerobic respiration.
Glucose 2 pyruvic acid
Step 2: Pyruvic acid is not transported to the mitochondria. It remains in the cytoplasm. It is then broken down into waste products that can be removed from the cell. This entire process occurs in the absence of oxygen.
Glucose Pyruvic acid 
Anaerobic respiration occurs in the roots of some species of water logged plants, parasitic worms, yeast, animal muscles, and microorganisms.
In some microorganisms such as yeasts, anaerobic respiration is called fermentation. In the process of fermentation, ethanol is formed as a by-product. This is represented in the above reaction.