Organic compounds are used as fuels in cellular respiration to allow the…

The idea

Cellular respiration is the controlled release of energy from organic compounds such as glucose. Glucose is broken down in enzyme-controlled reactions; aerobic respiration uses oxygen and yields carbon dioxide and water, while anaerobic respiration in animals yields lactic acid. The energy transferred powers the other chemical reactions necessary for life, including protein synthesis, active transport and muscle contraction. Respiration happens continuously in all living cells, including plant cells, and is not the same as breathing.

What earns the marks

• Organic compounds such as glucose act as fuels because their bonds store chemical energy that can be released in controlled steps.
• Respiration transfers the released energy to power cellular processes, rather than creating energy.
• Anaerobic respiration releases less energy per glucose molecule and produces lactic acid in animal cells or ethanol and carbon dioxide in plant and yeast cells.
• The energy supplied by respiration drives other chemical reactions necessary for life, such as protein synthesis and active transport.
• Respiration occurs in all living cells continuously, including plant cells, and is distinct from breathing or gas exchange.

Common mistakes and how to put them right

• Writing that respiration is the same as breathing.

  Put it right: Breathing is ventilation that brings oxygen to the lungs, whereas respiration is the chemical release of energy inside cells.

• Stating that plants photosynthesise instead of respiring.

  Put it right: Plants respire continuously and also photosynthesise in light.

• Claiming that respiration makes energy.

  Put it right: Respiration transfers energy from glucose, and energy is conserved rather than created.

Exam tips

• Learn the word equation for aerobic respiration, and if you are taking Higher Tier, learn the balanced symbol equation too.
• When asked why a cell needs respiration, link the released energy to a named process such as active transport or protein synthesis rather than saying 'for energy' alone.
• Compare aerobic and anaerobic respiration in a table of reactants, products and relative energy release to prepare for compare questions.