Respiration Notes

11.1 Respiration

Every living cell requires energy to function. In humans, cells need energy for various processes, including:

Contracting muscles for movement.
Protein synthesis, where amino acids are linked to form long chains.
Cell division for tissue repair and growth.
Active transport of substances across cell membranes against their concentration gradients.
Growth, by building new cells which can then divide to form new cells.
Transmitting nerve impulses for rapid information transfer.
Producing heat to maintain a constant body temperature.

This energy originates from the food we consume. Digestion breaks down food into smaller molecules, which are absorbed from the small intestine into the bloodstream. The blood then transports these nutrients to all the cells in the body.

The primary nutrient utilized for energy production in cells is glucose, which is rich in chemical energy. To harness this energy, cells break down glucose molecules through a series of metabolic reactions known as respiration. Like all metabolic reactions, respiration is facilitated by enzymes.

Aerobic Respiration

Most of the time, cells extract energy from glucose by combining it with oxygen, a process called aerobic respiration.

This process occurs in a series of small steps, each controlled by enzymes, primarily within the mitochondria.

The overall reaction of aerobic respiration can be summarized by the following equation:

glucose + oxygen \rightarrow carbon dioxide + water

The balanced equation is:

C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O

Anaerobic Respiration

It is possible to release energy from glucose without oxygen, though this process is less efficient than aerobic respiration, yielding less energy per glucose molecule. This is called anaerobic respiration, which occurs in the cytoplasm of a cell, not in mitochondria.

Yeast, a single-celled fungus, commonly respires anaerobically, breaking down glucose into alcohol and carbon dioxide.

glucose \rightarrow alcohol + carbon dioxide

The equation is:

C6H{12}O6 \rightarrow 2C2H5OH + 2CO2

Plants can also respire anaerobically in a similar manner, but only for short durations.

Some cells in the human body, especially muscle cells, can respire anaerobically for a limited time. However, they produce lactic acid instead of alcohol, and no carbon dioxide is generated. This occurs during vigorous exercise when the lungs and heart cannot supply oxygen to the muscles quickly enough. The muscle cells can release some energy from glucose without oxygen to sustain themselves until oxygen becomes available again.

glucose \rightarrow lactic acid