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Respiration Transfers Energy in Every Living Cell

  1. Respiration is the process that transfers energy from glucose in every living cell.
  2. It happens continuously, in animals, plants and microorganisms.
  3. It is an exothermic reaction, so it transfers energy to the surroundings.
  4. Organisms need this energy to build larger molecules, to move, and, in mammals and birds, to keep warm.

A diagram showing four key roles of ATP in cells: molecule synthesis (building DNA, RNA, and proteins), active transport (moving substances across membranes), cell movement (powering muscle contraction), and heat production (maintaining body temperature).

An overview of energy generation in metabolism, showing how chemical energy from food is converted into ATP and heat, with carbon dioxide and water produced as chemical waste.

An illustration of a bird, an endotherm, showing heat energy radiating from its body. It defines endotherms as organisms that maintain a constant internal body temperature through metabolic heat production.

  1. Respiration can be aerobic, using oxygen, or anaerobic, without oxygen.
Key Idea

Respiration is not the same as breathing: it is the chemical reaction inside cells that transfers energy from glucose.

Aerobic Respiration Uses Oxygen

Definition

Aerobic respiration

Respiration that uses oxygen to break glucose down completely, transferring a large amount of energy and releasing carbon dioxide and water.

  1. Aerobic respiration uses oxygen to break glucose down completely.
  2. It transfers a large amount of energy and takes place mainly in the mitochondria.
  3. It produces carbon dioxide and water as waste products.

glucose + oxygen → carbon dioxide + water

C6H12O6+6O2→6CO2+6H2OC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2OC6​H12​O6​+6O2​→6CO2​+6H2​O

A chemical equation for aerobic cellular respiration showing glucose and oxygen reacting to produce carbon dioxide, water, and energy in the form of ATP. The diagram includes molecular structures and chemical formulas.

Note

You should recognise the symbols: glucose is C₆H₁₂O₆, oxygen is O₂, carbon dioxide is CO₂ and water is H₂O.

Anaerobic Respiration in Muscles

Definition

Anaerobic respiration

Respiration without oxygen that breaks glucose down incompletely, transferring much less energy than aerobic respiration.

  1. When muscles cannot get enough oxygen, they respire anaerobically, without oxygen.
  2. The glucose is broken down incompletely, so much less energy is transferred than in aerobic respiration.
  3. The only product is lactic acid.

glucose → lactic acid

Common Mistake

Anaerobic respiration transfers much less energy than aerobic respiration because the glucose is only partly broken down.

Anaerobic Respiration in Plants and Yeast

Definition

Fermentation

Anaerobic respiration in yeast, which produces ethanol and carbon dioxide and is used to make bread and alcoholic drinks.

  1. Plant cells and yeast can also respire without oxygen.
  2. In plants and yeast, anaerobic respiration produces ethanol and carbon dioxide.

glucose → ethanol + carbon dioxide

  1. Anaerobic respiration in yeast is called fermentation.

A diagram comparing aerobic and anaerobic respiration. It shows glucose breaking down into pyruvate, then following different paths depending on oxygen availability to produce ATP and waste products like CO2, water, lactic acid, or ethanol.

  1. Fermentation is economically important, because it is used to make bread and alcoholic drinks.
Example

In bread-making, the carbon dioxide from yeast fermentation makes the dough rise.

Comparing Aerobic and Anaerobic Respiration

  1. Oxygen:
    1. Aerobic respiration needs oxygen, while anaerobic respiration does not.
  2. Breakdown of glucose:
    1. Aerobic respiration breaks glucose down completely, while anaerobic respiration breaks it down only partly.
  3. Energy transferred:
    1. Aerobic respiration transfers much more energy than anaerobic respiration.
  4. Products:
    1. Aerobic respiration makes carbon dioxide and water, while anaerobic respiration makes lactic acid in animals, or ethanol and carbon dioxide in plants and yeast.

A diagram comparing aerobic and anaerobic respiration. Both start with glycolysis in the cytoplasm. Aerobic respiration continues in the mitochondria, using oxygen to produce CO2, water, and a high ATP yield. Anaerobic respiration occurs in the cytoplasm, producing lactate and a low ATP yield.

Self review
  • Give the word equation and the symbol equation for aerobic respiration.
  • What is the product of anaerobic respiration in muscles?
  • Write the word equation for anaerobic respiration in yeast.
  • Why does anaerobic respiration transfer less energy than aerobic respiration?
  • Give three things organisms need the energy from respiration for.
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4.2.1 Aerobic and anaerobic respiration Revision Guide

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