International A Level Biology / Cellular respiration
Anaerobic respiration and what happens to lactate revision notes
Edexcel International A Level Biology (YBI11), chapter 20: Cellular respiration, specification point 7.2, 7.5. Free lesson.
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Anaerobic respiration lets glycolysis continue when oxygen is unavailable, but it produces only a small amount of ATP. In animals, lactate is made and later carried to the liver, where it is processed.
Why animals make lactate
Without oxygen, the electron transport chain stops. Reduced NAD cannot pass on its hydrogen, so NAD is not regenerated in that way. Glycolysis still needs NAD, so pyruvate takes hydrogen from reduced NAD. This reduces pyruvate to lactate and frees NAD for glycolysis to continue. Glycolysis turns glucose into two pyruvate and has a net gain of 2 ATP. Hydrogen is added to NAD during glycolysis, making reduced NAD. Making lactate does not produce ATP. Its role is to regenerate NAD.
How anaerobic respiration differs between animals and yeast
Without oxygen, the link reaction and Krebs cycle stop as well as the electron transport chain. Anaerobic respiration therefore uses glycolysis only, giving a net 2 ATP per glucose. Glucose is only partly broken down, so much of its energy remains in the products. In animals, hydrogen ends up in lactate. No carbon dioxide is made. In yeast and plants, the products are ethanol and carbon dioxide. In yeast, pyruvate loses carbon dioxide to form ethanal. Ethanal is reduced to ethanol, regenerating NAD. In aerobic respiration, hydrogen ends up in water and far more ATP is produced.
What happens to lactate after exercise
Lactate builds up in muscle during anaerobic respiration. It comes from lactic acid, whose hydrogen ions lower pH and affect enzymes. Lactate diffuses into the blood and is carried to the liver. There it is oxidised to pyruvate, with hydrogen transferred back to NAD. Some pyruvate is respired aerobically through the link reaction and Krebs cycle, producing carbon dioxide, water and ATP. The rest is made into glucose, using ATP. The glucose can be stored as glycogen or released into the blood. The extra oxygen used afterwards to deal with lactate is called the oxygen debt.
How to describe blood lactate data
During hard exercise, muscles make lactate faster than it is removed, so blood lactate rises. It may continue to rise briefly after exercise stops as lactate diffuses out of the muscles. It then falls as the liver processes it. In the example, hard running lasts 10 minutes. Lactate peaks at 12.5 mmol dm⁻³ at 12 minutes. By 60 minutes it is 1.5 mmol dm⁻³, close to the resting value of about 1 mmol dm⁻³. When explaining a graph, describe only the time period asked about.
Explain what happens to lactate
A complete answer follows lactate from the blood to the liver, then gives the two possible fates of pyruvate. Naming where the conversion happens is important. Lactate is carried in the blood to the liver and oxidised to pyruvate. Some pyruvate is respired aerobically to carbon dioxide and water, making ATP. The rest is converted to glucose, which can be stored as glycogen.