The energy transferred by respiration in cells is used by the organism for…

The idea

Respiration releases energy from glucose in every living cell, and that energy does not simply escape as heat: it drives the chemical work of the organism. Metabolism is the sum of all the chemical reactions in a cell or body, and many of these reactions build larger molecules from smaller ones, so they are synthesis reactions. Because these reactions would not happen quickly enough on their own, enzymes control them, lowering the activation energy needed. The reactions are continual because cells constantly need new molecules for growth, repair and replacement. For example, amino acids are joined into proteins at ribosomes, and glucose molecules are joined into starch or glycogen. Energy from respiration powers these bonds, so a cell with blocked respiration cannot keep synthesising molecules and will die.

What earns the marks

• Respiration transfers energy from glucose in cells; this energy is not used directly for movement only but also for chemical reactions.
• Metabolism is the sum of all the chemical reactions in a cell or the body, including both synthesis and breakdown reactions.
• Synthesis reactions build larger molecules from smaller ones, for example joining amino acids into proteins or glucose into starch and glycogen.
• The metabolic reactions are enzyme controlled, so each step has a specific enzyme with a complementary active site.
• The processes are continual because new molecules are needed throughout the organism's life for growth, repair and replacement.
• Energy released by respiration is used to drive synthesis, for example in forming bonds when amino acids polymerise into polypeptides.

Common mistakes and how to put them right

• Saying respiration makes energy.

  Put it right by stating that respiration transfers energy from glucose, since energy is not created.

• Confusing metabolism only with digestion or only with breakdown.

  Put it right by defining metabolism as all reactions, including synthesis of new molecules.

• Writing that enzymes provide the energy for synthesis.

  Put it right by explaining that enzymes lower activation energy and speed up reactions, while respiration supplies the energy.

Exam tips

• Link the statement to a named example, such as amino acids joining to form proteins, so the idea of synthesising new molecules is concrete.
• Use the phrase enzyme controlled when describing metabolic reactions, because this shows the role of enzymes in the process.
• If asked why metabolism is continual, refer to growth, repair and replacement of cells rather than saying it happens all the time without reason.