Energy Production through Cellular Respiration

What cellular respiration is

Content standard 7.1 introduces respiration as a catabolic process in which cells break down a respiratory substrate, most commonly glucose, to release the chemical energy stored in it. This energy is captured and stored in molecules of ATP, which the cell then uses to power almost every life process, including muscle contraction, active transport, nerve impulses and biosynthesis.

Why every cell needs respiration

Respiration is not something only some cells do, it occurs continuously in every living cell, because none of a cell's activities can happen without a supply of usable energy. Without respiration, a cell could not maintain its structures, transport substances against a gradient, or carry out the chemical reactions needed to stay alive.

Aerobic versus anaerobic respiration: the overview

How it is examined

Questions at this standard typically ask you to state the general word equation for respiration, name the usual respiratory substrate, or explain in general terms why cells need a continuous supply of ATP. You may also be asked to state the basic difference between aerobic and anaerobic respiration in terms of oxygen requirement and amount of energy released, before more detailed questions on each type appear elsewhere in the paper.

ATP: the cell's energy currency

ATP (adenosine triphosphate) is the molecule that actually powers a cell's activities; it is often described as the universal energy currency because almost every energy-requiring process in the cell, muscle contraction, active transport, nerve impulse transmission, and biosynthesis, is paid for directly by ATP rather than by the respiratory substrate itself. When ATP is broken down to ADP and inorganic phosphate, the energy released can be used immediately by an enzyme or a cell structure; when energy becomes available again during respiration, ADP and phosphate are rejoined to reform ATP.

This ATP–ADP cycle allows a cell to keep only a small working supply of ATP rather than a large store, because ATP is regenerated as fast as it is used, in the same way a currency is exchanged over and over rather than spent once.

Respiration is not the same as breathing

A common source of confusion at this level is treating 'respiration' and 'breathing' as the same word. Breathing (ventilation) is a physical, mechanical process, the movement of air into and out of the lungs, and it happens only in organisms with lungs or similar gas-exchange organs.

Respiration, in contrast, is a biochemical process that takes place inside every living cell, whether or not that organism breathes air at all; a single-celled organism or a plant cell respires continuously without ever breathing. Breathing supplies the oxygen that aerobic respiration needs and removes the carbon dioxide it produces, but breathing itself releases no energy, it is respiration, occurring inside the mitochondria and cytoplasm of cells, that actually breaks down glucose and produces ATP.

Common mistakes to avoid

Worked exam-style question

Question. A resting muscle cell and a muscle cell undergoing vigorous exercise both respire continuously. (a) Name the respiratory substrate most commonly used by the muscle cell.

(b) State what happens to the rate of respiration when the muscle cell exercises vigorously, and explain why. (c) Name the molecule that directly supplies the energy for the muscle to contract.

Model answer. (a) The respiratory substrate is glucose. (b) The rate of respiration increases during vigorous exercise, because the contracting muscle cells use ATP much faster, so more ATP must be regenerated by respiration to keep pace with the higher energy demand.

(c) The molecule that directly supplies the energy is ATP, which is broken down to ADP and inorganic phosphate to release usable energy for contraction.

Practice question

Try it. A student claims that a plant respires only at night and photosynthesises only in the day. Explain whether this claim is correct.

Exam tip

Key terms

• Cellular respiration, the controlled breakdown of a respiratory substrate to release energy as ATP.
• ATP, the molecule that stores and supplies energy for cell activities.
• Aerobic respiration, respiration that requires oxygen and releases a large amount of energy.
• Anaerobic respiration, respiration without oxygen that releases only a small amount of energy.
• Fermentation, anaerobic respiration in yeast or muscle cells that produces ethanol or lactic acid.

Source:SRC-DSKP-EN

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