Describe the structure of protein, explain its functions, distinguish high and low biological value proteins, and identify good food sources

What this dot point is asking

The syllabus wants you to describe what protein is made of, explain why the body needs it, and judge the quality of different protein foods using biological value. The central idea is that proteins are built from amino acids, that some of those amino acids must come from food because the body cannot make them, and that this is what separates a high-quality protein source from a lower-quality one.

The answer

What protein is made of

Proteins are large molecules built from smaller units called amino acids, joined in long chains. There are about twenty amino acids. Some can be made by the body, but a number of them, the essential amino acids, cannot be made and must be supplied by the diet. When you eat protein, digestion breaks it back down into amino acids, which the body then reassembles into the proteins it needs.

Functions of protein

• Growth, repair and maintenance of body tissues such as muscle, skin, hair and organs. This is protein's main job, and it is why children, teenagers and pregnant women need more.
• Making enzymes, hormones and antibodies, which control body reactions, regulate processes, and fight infection.
• A secondary source of energy, used when carbohydrate and fat intake is too low. Each gram releases $17\ \text{kJ}$.

Biological value: high versus low

The quality of a protein depends on which amino acids it contains.

High biological value (HBV) proteins contain all the essential amino acids. They are mostly from animals: meat, fish, eggs, milk and cheese. Soya bean is the main plant source of HBV protein.

Low biological value (LBV) proteins are missing one or more essential amino acids. They are mostly from plants: cereals (rice, wheat, bread), pulses (beans, lentils, peas) and nuts.

Protein complementation

Different LBV foods are usually missing different amino acids. By combining two LBV foods, the amino acid missing from one is supplied by the other, so the meal as a whole provides all the essential amino acids. This is complementation, and it is how vegetarians and vegans meet their protein needs. Classic pairings are rice with dhal, baked beans on toast, and tofu with rice.

Deficiency and excess

Too little protein, especially in children, causes poor growth, muscle wasting, a weak immune system and, in severe cases, kwashiorkor. Excess protein is not stored as protein; it is broken down, and the energy part is stored as fat while the nitrogen part is excreted.

Examples in context

Example 1. Rice with dhal. A staple across South Asian and Singaporean meals, this pairs an LBV cereal with an LBV pulse. The lysine that rice lacks is supplied by the lentils, and the methionine the lentils lack is supplied by the rice, so the meal delivers complete protein without any meat.

Example 2. Steamed fish with rice. Here the fish is an HBV protein providing all essential amino acids on its own, while the rice mainly adds carbohydrate energy. This shows the simpler animal-protein route to a complete meal, useful for growing teenagers who need plenty of high-quality protein.

Try this

• Cue. Define an essential amino acid and explain why HBV proteins are described as high quality. An essential amino acid cannot be made by the body and must come from food; HBV proteins are high quality because they contain all of them.
• Cue. Name two LBV foods that would complement each other and say why. Choose a cereal and a pulse, for example rice and beans, because each supplies the amino acid the other lacks.
• Cue. Describe two effects of protein deficiency in a young child. Recall stunted or poor growth, muscle wasting, weak immunity, or kwashiorkor with a swollen abdomen.