Cellular respiration: aerobic respiration and energy transfer

Learning objectives

By the end of this lesson, you should be able to:

• define cellular respiration
• write the word equation for aerobic respiration
• explain why cells need energy
• describe where aerobic respiration occurs in eukaryotic cells
• distinguish respiration from breathing and photosynthesis

Meaning of cellular respiration

Cellular respiration is the series of enzyme-controlled chemical reactions by which cells release energy from food molecules. Glucose is a common respiratory substrate. The released energy is transferred to ATP (adenosine triphosphate), which supplies energy for cellular activities.

Cells use energy for muscle contraction, active transport, synthesis of proteins and other molecules, cell division, growth, transmission of nerve impulses and maintaining body temperature in mammals. Energy is not simply stored as heat; ATP provides a usable, transferable form of chemical energy.

Aerobic respiration

Aerobic respiration takes place in the presence of oxygen and releases a large amount of energy from glucose. Its overall word equation is:

glucose + oxygen → carbon dioxide + water + energy

The balanced chemical equation is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

The process begins with glycolysis in the cytoplasm, where glucose is broken down into pyruvate. When oxygen is available, further stages occur in mitochondria. The link reaction and Krebs cycle release carbon dioxide and transfer energy to electron carriers. The electron transport chain on the inner mitochondrial membrane uses oxygen as the final electron acceptor and supports the production of much of the ATP.

Mitochondria are therefore important sites of aerobic respiration. Cells with high energy demands, such as many muscle cells, generally contain numerous mitochondria.

ATP and energy transfer

ATP is a small molecule that can release energy when its terminal phosphate group is removed by hydrolysis, producing ADP and inorganic phosphate. Energy from respiration is used to regenerate ATP from ADP and phosphate. ATP is not a long-term energy store like fat or glycogen; it is continually made and used.

The energy released during respiration supports reactions that would not proceed readily on their own. For example, active transport uses ATP to move substances against concentration gradients. Muscle fibres use ATP during contraction and relaxation.

Respiration is not breathing

Breathing or ventilation is the physical movement of air into and out of lungs (or water over gills). Gas exchange is diffusion of oxygen and carbon dioxide across an exchange surface. Cellular respiration is a chemical process inside cells. Breathing supplies oxygen for aerobic respiration and removes carbon dioxide, but the two processes are not the same.

Respiration and photosynthesis

Photosynthesis stores light energy in organic molecules. In simplified form, carbon dioxide and water are used to produce glucose and oxygen in the presence of light and chlorophyll. Respiration releases energy from organic molecules. Green plants carry out both photosynthesis and respiration. Photosynthesis occurs only when suitable light is available, whereas respiration occurs continuously in living cells.

WASSCE examination focus

Write the aerobic respiration equation in the correct direction and include oxygen as a reactant. If asked where it occurs, state that glycolysis occurs in the cytoplasm and later aerobic stages occur in mitochondria. Avoid saying that respiration happens only in animals: plants and other organisms respire too.

Quick check

1. Define cellular respiration.
2. Write the word equation for aerobic respiration.
3. State three cellular activities that require energy.
4. Why is breathing not the same as respiration?

Answers: 1. Enzyme-controlled reactions in cells that release energy from food molecules and transfer it to ATP. 2. Glucose + oxygen → carbon dioxide + water + energy. 3. Active transport, muscle contraction, protein synthesis, cell division or nerve impulse transmission (any three). 4. Breathing is movement of air; respiration is a chemical process in cells.