All organisms are made of small microscopic units called cells. A Cell is the smallest structural and functional unit of an organism. Each cell of an organism performs certain functions such as nutrition, excretion and reproduction. All living organisms respire to get energy from food. The air we breathe in is transported to all parts of the body and ultimately to each cell. Air helps in breaking down of food. This process of breakdown of food in cell with the release of energy is called cellular respiration. Cellular respiration takes place in the cell of all organisms. In the cells, the food glucose is broken down into carbon-dioxide and water, using oxygen. When the breakdown of glucose occurs with the use of oxygen it is called aerobic respiration. Food can also be broken down without using oxygen. This is called anaerobic respiration. Breakdown of food releases energy. Glucose reacts with oxygen and produces carbon dioxide + water energy.

The organisms such as yeast that can survive in the absence of air. they are called anaerobe. They get energy through anaerobic respiration. In the absence oxygen, glucose breaks down into alcohol and carbon dioxide, as given below.

Glucose without the use of oxygen produces-

Alcohol + carbon dioxide + energy.

Our muscle cells can also respire an aerobically but only for a short time. Then anaerobic respiration takes place in the new cell to fulfill the demand of energy.

Glucose in the absence of oxygen in muscle- lactic acid +energy.