Respiration is the process of complete oxidation of carbon compounds to C02 and H”0, using molecular oxygen as the final electron acceptor, and the energy released being conserved as the high energy terminal bonds of ATP.

This process is also known as aerobe respiration, since 0, is used as the final electron acceptor. Nearly all eukaryotic organisms including plants and animals respire aerobically. In the absence of 02, yeasts, some bacteria, and even green plants resort to incomplete oxidation of carbon compounds producing cthanol or lactic acid and much less amount of ATP than is produced during aerobic respiration.

This process is termed as fermentation. There are relatively a few other types of bacteria which respire in the absence of 0″, an inorganic compound being the terminal electron acceptor; this process is called anaerobic respiration. Fermentation is also often referred to as anaerobic respiration.

BOX -1


Comparison of some important aspccts of the processes of photosynthesis and respiration

Photosynthesis Respiration

1. Takes place only in chlorophyll-containing cells 1. Takes place in all plant and animal cells of plants

2. Takes place only in the presence of light 2. Takes place continuously both in light and in dark


3. The raw materials for this process are CO, and 3. The raw materials for this process are sugars and 0, H20 (Other carbohydrates as well as fats and proteins are used as raw materials when soluble sugars are depleted)

4. The products of this process are sugars, water and 4. The products of this process are C02 and H20 02 (Soluble sugars are then used to produce other carbohydrates and provide carbon skeleton for the production of fats and proteins)

5. Synthesizes food materials 5. Oxidizes food materials

6. Converts light energy to chemical energy (ATP) 6. Releases energy from C-C bonds in usable form (ATP) and stores the energy in C-C bonds


7. Results in an increase in dry mater 7. Results in a decrease in dry mater

Aerobic respiration is the most common type of respiration in nearly all eukaryotic organisms. From a chemical standpoint, the overall process of aerobic respiration may be stated as per the following general equation with glucose as the substrate:

It may be noted here that this equation is written as a reversal of the equation for photosynthesis. As in the case of photosynthesis, this equation is a mere representation of the raw materials, necessary conditions, and the products of the respiratory reaction.

It in no way accounts for the large number of intermediate chemical reactions that actually occur during cellular respiration. Direct oxidation of glucose by molecular oxygen, as shown in the equation above, would release a large quantity of energy all at once that would be sufficient to incinerate the cell structure. Instead, during aerobic respiration the cell releases the large amount of free energy from the oxidation of glucose in a series of step by step reactions in a controlled manner in order to conserve this energy in metabolically useful forms (for example, ATP).


These reactions can be grouped into three major phases such as glycolysis, citric acid cycle, and respiratory electron transport chain. The cellular site for glycolysis is the cytoplasm, whereas citric acid cycle and respiratory electron transport chain occur in the mitochondrion.

Out of the three distinct phases of respiration, glycolysis is common to both aerobic and anaerobic respiration. Th- product of glycolysis is pyruvate which is reduced to cthanol or lactic acid in anaerobic respiration but is completely oxidized to C02 and II20 through the citric acid cycle and electron transport chain in aerobic respiration.