Lactic acid is formed in the muscles during exercise or running. It is formed :
- A.to give extra energy from fat.
- B.to give extra oxygen from lungs.
- C.to give extra energy from glucose.
- D.to give extra energy from vitamins.
Correct answer
Explanation
The correct answer is C, to give extra energy from glucose. When oxygen runs short during hard exercise, muscle cells break glucose down anaerobically, and lactic acid is the by-product.
In glycolysis, which takes place in the cytoplasm, one molecule of glucose splits into two molecules of pyruvate and yields a little ATP. With enough oxygen, pyruvate enters the mitochondria and is fully oxidised to carbon dioxide and water. During a sprint the oxygen supply cannot keep up, so pyruvate is reduced to lactic acid, which regenerates NAD+ and lets glycolysis keep making ATP. Only 2 ATP are gained per glucose this way, against about 36-38 in aerobic respiration. Build-up of lactic acid is linked with muscle fatigue; the liver later turns it back into glucose.
A is wrong: fat is broken down only through aerobic respiration, not into lactic acid.
B is wrong: lactic acid forms because oxygen is lacking; it supplies no oxygen.
D is wrong: vitamins are not used as fuel for energy.
Exam tip: glucose → pyruvate → lactic acid in muscles (no oxygen, 2 ATP); glucose → ethanol + CO2 in yeast.
Practice Questions
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