OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
Respiration: (a) the need for cellular respiration
Cellular respiration is the controlled release of energy from organic molecules inside cells, and the need for it runs through every living process.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 1 of 13
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Cellular respiration is the controlled release of energy from organic molecules inside cells, and the need for it runs through every living process. Cells must synthesise ATP because ATP is the immediate, universal energy currency: it transfers energy in small, usable amounts to reactions such as active transport, protein synthesis, DNA replication and muscle contraction. Respiration also supplies the reduced coenzymes and carbon skeletons that feed biosynthesis, and it maintains the concentration gradients and metabolic conditions that keep cells alive. Without a continuous supply of ATP, pumps fail, gradients collapse and metabolism stops. A useful method is to link each example of cell work to the ATP it consumes, then explain why diffusion alone cannot power that work.
What earns the marks
- Cellular respiration releases energy from organic molecules such as glucose in a series of controlled enzyme-catalysed reactions rather than a single explosive step.
- ATP is the immediate energy currency; hydrolysis of ATP to ADP and inorganic phosphate releases energy in small, usable amounts for cell work.
- Energy from respiration drives active transport, including ion pumps that maintain concentration gradients across membranes.
- Respiration supports anabolic reactions such as protein synthesis, DNA replication and the synthesis of other cell molecules.
- Respiration supplies reduced coenzymes and metabolic intermediates that link energy release to biosynthesis.
- Without continuous ATP supply, membrane transport, metabolism and cell maintenance fail, so respiration is essential for survival.
Common mistakes and how to put them right
Treating respiration as the same as breathing or gas exchange.
Put it right: Respiration is a series of intracellular biochemical reactions releasing energy, while breathing is ventilation of the lungs.
Assuming energy is released in one step with a large temperature rise.
Put it right: Many small enzyme-controlled steps release energy gradually so it can be captured in ATP.
Saying ATP stores large amounts of energy long term.
Put it right: ATP is a short-term, immediately usable energy carrier, and cells must keep regenerating it through respiration.
Exam tips
- For each example of cell work you give, name the specific process and state that ATP hydrolysis provides its energy.
- Use the phrase 'controlled release of energy in small steps' to show understanding of why respiration is not a single combustion reaction.
- When asked why respiration is needed, link your answer to a named cell activity rather than saying only 'for energy'.
Try the lesson
The start of the Respiration: (a) the need for cellular respiration lesson, made from checked teaching notes for this specification point.
Have a go first
Which molecule is the immediate energy currency that respiration supplies for cell activities?
Listen
Cellular respiration is the controlled release of energy from organic molecules inside cells, and the need for it runs through every living process. Cells must synthesise ATP because ATP is the immediate, universal energy currency: it transfers energy in small, usable amounts to reactions such as active transport, protein synthesis, DNA replication and muscle contraction. Respiration also supplies the reduced coenzymes and carbon skeletons that feed biosynthesis, and it maintains the concentration gradients and metabolic conditions that keep cells alive. Without a continuous supply of ATP, pumps fail, gradients collapse and metabolism stops. A useful method is to link each example of cell work to the ATP it consumes, then explain why diffusion alone cannot power that work. A strong answer earns marks for these points. Cellular respiration releases energy from organic molecules such as glucose in a series of controlled enzyme-catalysed reactions rather than a single explosive step. ATP is the immediate energy currency; hydrolysis of ATP to ADP and inorganic phosphate releases energy in small, usable amounts for cell work. Energy from respiration drives active transport, including ion pumps that maintain concentration gradients across membranes. Respiration supports anabolic reactions such as protein synthesis, DNA replication and the synthesis of other cell molecules. A common slip: Treating respiration as the same as breathing or gas exchange. Instead, respiration is a series of intracellular biochemical reactions releasing energy, while breathing is ventilation of the lungs. Exam tip: For each example of cell work you give, name the specific process and state that ATP hydrolysis provides its energy.
Key words
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ATP as the universal energy currency
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Controlled stepwise release of energy
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Active transport and maintenance of gradients
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Anabolic reactions and biosynthesis
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See every definition
- ATP as the universal energy currency
- ATP is the immediate, universal energy currency, transferring energy in small, usable amounts to reactions such as active transport, protein synthesis, DNA replication and muscle contraction.
- Controlled stepwise release of energy
- Energy is released from organic molecules such as glucose in a series of controlled enzyme-catalysed reactions rather than a single explosive step.
- Active transport and maintenance of gradients
- Energy from respiration drives active transport, including ion pumps that maintain concentration gradients across membranes.
- Anabolic reactions and biosynthesis
- Respiration supports anabolic reactions such as protein synthesis, DNA replication and the synthesis of other cell molecules.
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