CBSE Class 10 Answered

The proteins, lipids, and polysaccharides that make up most of the food we eat must be broken down into smaller molecules before our cells can use them—either as a source of energy or as building blocks for other molecules.

Stage 1 in the enzymatic breakdown of food molecules is therefore digestion, which occurs either in our intestine outside cells, or in a specialized organelle within cells, the lysosome. In either case, the large polymeric molecules in food are broken down during digestion into their monomer subunits—proteins into amino acids, polysaccharides into sugars, and fats into fatty acids and glycerol—through the action of enzymes. After digestion, the small organic molecules derived from food enter the cytosol of the cell, where their gradual oxidation begins. Oxidation occurs in two further stages of cellular catabolism: stage 2 (glycolysis) starts in the cytosol and ends in the major energy-converting organelle, the mitochondrion; stage 3 (Krebs cycle) is entirely confined to the mitochondrion.