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Why must many food molecules be broken into smaller molecules before the body can use them?
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human nutrition
MyPlate, a revised set of dietary guidelines introduced by the U.S. Department of Agriculture (USDA) in 2011, divides the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, the size of each section representing the relative dietary proportions of each food group. The small blue circle shown at the upper right illustrates the inclusion and recommended proportion of dairy products in the diet.
Food molecules must be broken down into smaller units for several essential physiological reasons:
- Membrane Permeability: Large, complex molecules like proteins and sugar polymers are generally too large to pass through cell membranes. Digestion converts these into smaller, soluble building blocksāsuch as amino acids and monosaccharidesāthat can be absorbed into the body and transported to cells.123
- Species-Specific Utilization: While many living forms use the same small molecules, large complex molecules are often unique to a specific species. An organism cannot directly use the proteins of another plant or animal; it must first break them down into their constituent amino acids to then recombine them into its own characteristic proteins.2
- Enzymatic Efficiency: Digestive enzymes can only act on molecules they come into direct contact with. Breaking down bulk food into smaller pieces or liquid chyme exposes more surface area, allowing enzymes to work more efficiently to hydrolyze chemical bonds.12