Cellular Respiration - Meaning, Equation and its Steps - Practice Questions & MCQ

• All living organisms need energy for carrying out daily life activities, be it absorption, transport, movement, reproduction, or even breathing.
• All the energy required for ‘life’ processes is obtained by oxidation of some macromolecules that we call ‘food’. 
• Only green plants and cyanobacteria can prepare their own food; by the process of photosynthesis, they trap light energy and convert it into chemical energy that is stored in the bonds of carbohydrates like glucose, sucrose, and starch. 
• In green plants too, not all cells, tissues, and organs photosynthesize; only cells containing chloroplasts, which are most often located in the superficial layers, carry out photosynthesis. 
• Hence, even in green plants all other organs, tissues, and cells that are non-green need food for oxidation. 
• Hence, food has to be translocated to all non-green parts.
•  Animals are heterotrophic, i.e., they obtain food from plants directly (herbivores) or indirectly (carnivores).
• Saprophytes like fungi are dependent on dead and decaying matter.
• What is important to recognize is that ultimately all the food that is respired for life processes comes from photosynthesis.
• Cellular respiration is the mechanism of the breakdown of food materials within the cell to release energy and the trapping of this energy for the synthesis of ATP.
• The breakdown of complex molecules to yield energy takes place in the cytoplasm and in the mitochondria.
• The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to the release of a considerable amount of energy is called respiration.
• The compounds that are oxidized during this process are known as respiratory substrates. 
• Usually, carbohydrates are oxidized to release energy, but proteins, fats, and even organic acids can be used as respiratory substances in some plants under certain conditions.

Calorific values of respiratory substrates:

• 1g of Carbohydrates = 4 cal.
• 1g of proteins = 4 cal.
• 1g of fats = 9 cal.
• Vitamins and minerals = no calories