CBSE Class 10 Science 2019 · Paper 31/3/1 · Question 10

First step (common to both): In the cytoplasm, glucose, a six-carbon molecule, is broken down into pyruvate, a three-carbon molecule, and a little energy is released. (i) In the absence of oxygen (as in yeast, during fermentation), pyruvate is converted into ethanol and carbon dioxide, and energy is released. This is anaerobic respiration: $\text{Glucose}\stackrel{\text{in cytoplasm}}{\to }\text{Pyruvate}+\text{energy}\stackrel{\text{absence of oxygen (in yeast)}}{\to }\text{Ethanol}+{\text{CO}}_2+\text{energy}$ (ii) When there is a shortage of oxygen (as in our muscle cells during vigorous activity), pyruvate is converted into lactic acid, a three-carbon molecule, and energy is released. The build-up of lactic acid in the muscles causes cramps: $\text{Glucose}\stackrel{\text{in cytoplasm}}{\to }\text{Pyruvate}+\text{energy}\stackrel{\text{lack of oxygen (in muscle cells)}}{\to }\text{Lactic acid}+\text{energy}$ In both cases much less energy is released than in aerobic respiration, because glucose is not broken down completely.