Respiration (Edexcel GCSE Biology: Combined Science)

Cellular respiration

• Cellular respiration is an exothermic reaction which is continuously occurring in living cells
• The chemical process of cellular respiration releases energy either in the presence of oxygen (aerobic respiration), or in the absence of oxygen (anaerobic respiration)
• The energy transferred supplies all the energy needed for metabolic processes to occur within cells and organisms as a whole
• Organisms need energy for:
  • Chemical reactions to build larger molecules from smaller molecules
  • Muscle contraction to allow movement
  • Keeping warm (to maintain a constant temperature suitable for enzyme activity)

Uses of the energy released from respiration

Aerobic Respiration

• Aerobic respiration requires oxygen
  • It is defined as the chemical reaction in cells that uses oxygen to break down nutrient molecules to release energy

• Aerobic respiration is the complete breakdown of glucose to release a relatively large amount of energy for use in cell processes and reactions
• Carbon dioxide and water are produced as waste products as well as releasing useful cellular energy

Word equation for aerobic respiration

• This equation can also be shown as a balanced symbol equation
  • One molecule of glucose combines with six molecules of oxygen to produce six molecules of carbon dioxide and six molecules of water

The balanced symbol equation for aerobic respiration

• Anaerobic respiration does not require oxygen
  • It is defined as the chemical reaction in cells that breaks down nutrient molecules to release energy without using oxygen

• It involves the incomplete breakdown of glucose and so releases a relatively small amount of energy for use in cell processes
• Different breakdown products are formed depending on the type of organism that the anaerobic respiration is taking place in
• You need to know the equations for anaerobic respiration in animals and plants (or fungi)

Anaerobic respiration in animals

• Anaerobic respiration mainly takes place in muscle cells during vigorous exercise
• When we exercise at high intensities, our muscles have a higher demand for energy
• Our bodies can only deliver so much oxygen to our muscle cells for aerobic respiration
• When oxygen runs out, glucose is broken down without it, producing lactic acid instead
• Glucose has not been fully broken down meaning there is still energy stored within the bonds of lactic acid molecules
• Anaerobic respiration releases less energy than aerobic respiration

Word equation for anaerobic respiration in animals

• This equation can also be shown as a balanced chemical equation
  • One molecule of glucose is broken down into two molecules of lactic acid

The balanced chemical equation for anaerobic respiration in animals

Lactic acid and oxygen debt

• Lactic acid builds up in muscle cells and lowers the pH of the muscle tissue (making the conditions more acidic)
  • Acidic conditions can denature the enzymes in cells

• Lactic acid will eventually be broken down using oxygen to produce carbon dioxide and water as waste products
• The amount of oxygen required to break down the lactic acid that has built up is referred to as the 'oxygen debt'
• The process of breaking down the lactic acid is known as ‘repaying the oxygen debt’

Anaerobic respiration in plants and fungi

• Plants and yeast can respire without oxygen as well, breaking down glucose in the absence of oxygen to produce ethanol and carbon dioxide
• Anaerobic respiration in yeast cells is called fermentation
• Fermentation is economically important in the manufacture of bread (where the carbon dioxide produced helps the dough to rise) and in brewing (where the ethanol produced makes beer)

Word equation for anaerobic respiration in plants and fungi

• This equation can also be shown as a balanced chemical equation
  • One molecule of glucose is broken down into two molecules of alcohol and two molecules of carbon dioxide

Balanced equation for anaerobic respiration in plants and yeast

• You need to be able to compare the processes of aerobic and anaerobic respiration with regard to the need for oxygen, the products and the relative amounts of energy transferred

Comparing Aerobic & Anaerobic Respiration Table