• In cells, there is a much?lower energy yield?from respiration in?anaerobic conditions?than in aerobic conditions
• There can be different ways in which oxygen becomes unavailable
  • When?oxygen supply can't keep up with demand?in heavily respiring cells
    • But a short supply of ATP is still required eg.?vigorous exercise?requiring a lot of muscle contraction
  • In conditions where oxygen?cannot reach the organisms?eg. in waterlogged soil
• In anaerobic respiration,?glucose is only partially oxidised?meaning only?a small part of its chemical energy is released?and transferred to ATP
  • The only ATP-producing reaction that continues is the first stage of respiration (around 2 ATP molecules per molecule of glucose)
• As there is no oxygen,?none of the remaining reactions?(of aerobic respiration) can take place
  • This means that? around?36 ATP molecules are not produced?anaerobically?that would otherwise have been produced in the presence of oxygen
  • 2 ATP molecules is better than zero ATP molecules, so anaerobic respiration can give?a short discharge of energy?when oxygen runs out
• Different types of organisms produce?different products?when respiring anaerobically
  • Plants and yeasts?produce?ethanol?and?CO2
  • Animals?produce?lactate

Anaerobic respiration in yeast produces different products to anaerobic respiration in animals

• Bakers can make use of anaerobic cell respiration in yeasts to produce ethanol and carbon dioxide in baking
• Yeasts are?single-celled fungi?that live in areas where sugars are present eg. on fruit or on leaves
• They can respire?aerobically?or?anaerobically
• Flour?contains starch, and when mixed with?water?and?yeast?can form a bread dough
• The dough is?kneaded?to mix everything together
• The dough is then?left in a warm place?to encourage the yeast to respire
• Yeast cells?grow rapidly?in number?while oxygen is still present in the dough
  • The yeast?hydrolyses the starch?into maltose and glucose and respire the sugars,?aerobically at first
• The dough soon?becomes anaerobic?(all the oxygen within it is used up aerobically by the yeast)
• Anaerobic respiration takes over?and?CO2?bubbles?begin to form in the dough
• These bubbles allow the dough to?rise?(swell up)
• Baking the dough?kills the yeast?and the bubbles form the fluffy texture of the finished bread
• Ethanol, the other product of anaerobic respiration of yeast, is produced but?evaporates?during the final baking stage

The role of anaerobic respiration of yeast in breadmaking to cause bread dough to rise

• Animals have evolved a system to provide?a short burst of energy?in exceptional circumstances
  • Anaerobic respiration can help to?generate powerful muscle contractions?in the short term eg. to escape from a predator or to catch prey
• For humans, anaerobic respiration plays a role in?sport and exercise?more than in survival
• Glucose is metabolised to?lactate?when oxygen can't be supplied quickly enough to muscle cells
• Lactate?accumulates?in those cells and tissues
• Lactate is?toxic above a certain level?and can cause discomfort and even pain (cramp)
• This?limits how long an athlete can perform anaerobically for?eg. sprinters, who typically only race over 400 metres or less
• After lactate is produced it has to be?broken down aerobically
• The breakdown of lactate needs extra oxygen
  • This extra oxygen is referred to as an?oxygen debt
  • It explains why animals?breathe deeper and faster?for a period of time?after exercise