Carbohydrates, lipids and proteins · Year 13
Food contains three main kinds of large molecule: carbohydrates, proteins and lipids. Each is built from smaller units, and enzymes in the gut break them back down so that they can be absorbed. Simple chemical tests show which of them a food contains.
Key facts
- Carbohydrates contain carbon, hydrogen and oxygen. Simple sugars such as glucose join to make large carbohydrates: starch (stored in plants), glycogen (stored in animals) and cellulose (plant cell walls).
- Proteins are chains of amino acids folded into particular shapes. They contain nitrogen as well as carbon, hydrogen and oxygen. Enzymes, antibodies and many hormones are proteins.
- Lipids (fats and oils) are made of glycerol and fatty acids: a triglyceride has one glycerol and three fatty acids. Lipids store energy and insulate the body.
- Food tests: iodine turns from orange-brown to blue-black with starch; Benedict's solution, heated in a water bath, turns from blue to green, yellow or brick-red with reducing sugars; Biuret reagent turns from blue to purple with protein; Sudan III stains lipids red.
- Carbohydrases such as amylase break carbohydrates into simple sugars; proteases break proteins into amino acids; lipases break lipids into glycerol and fatty acids.
- Joining two units releases a molecule of water (a condensation reaction); splitting the bond uses one (hydrolysis). A chain of n units has n − 1 bonds: glycosidic bonds in carbohydrates, peptide bonds in proteins and ester bonds in lipids.