Respiration – iGCSE Biology CAIE Revision

Fundamentals of Respiration

Respiration is a series of biochemical reactions that happens in all living cells to release energy from glucose.
Energy obtained during respiration is used for various life processes including movement, growth and reproduction.
Respiration can be aerobic (requiring oxygen) or anaerobic (occurring without oxygen).

Aerobic respiration is the process of breaking down glucose in the presence of oxygen to produce carbon dioxide and water.
The chemical equation for aerobic respiration is glucose + oxygen -> carbon dioxide + water + energy.
This type of respiration is the most efficient way to produce energy, as it releases the most energy per molecule of glucose.

Aerobic respiration occurs in four stages: glycolysis, link reaction, Krebs cycle, and the electron transport chain (also known as oxidative phosphorylation).
Glycolysis takes place in the cell’s cytoplasm and the rest of the stages occur in the mitochondria.
During glycolysis, glucose is converted into two molecules of pyruvate, producing a small amount of energy.
In the link reaction, pyruvate enters the mitochondria and is converted into Acetyl-coenzyme A.
During the Krebs cycle, Acetyl-CoA is broken down into carbon dioxide and hydrogen, and a small amount of energy is released.
In the electron transport chain, hydrogen is used to generate a larger amount of energy.

Anaerobic respiration takes place in the absence of sufficient oxygen supply, typically when an organism has used up the available oxygen faster than it can be supplied.
The chemical equation for anaerobic respiration in animals including humans is glucose -> lactic acid + energy.
In plants and yeast, anaerobic respiration is referred to as fermentation and glucose is broken down into ethanol and carbon dioxide.
Compared to aerobic respiration, anaerobic respiration is less efficient, releasing less energy per molecule of glucose.

Energy production: Respiration is crucial for all life processes as it provides the necessary energy to maintain cell functions.
Production of metabolic water: Aerobic respiration produces water which can be useful for organisms living in dry environments.
Maintaining temperature: The heat generated during respiration can be used to maintain body temperature in warm-blooded animals.