The purpose of proteins in the diet of a mammal is to

Analyze the question: It asks about the main purpose of dietary protein in a mammal, so we need the primary role of protein as a nutrient.

Explain the concept: Proteins are made of amino acids. When a mammal eats protein, digestion breaks it down into amino acids, which the body uses mainly to:

• build new cells and tissues (growth)
• repair worn-out or damaged tissues (e.g. healing wounds)
• make enzymes, hormones and antibodies (secondary roles)

This is why proteins are called the body-building foods.

Evaluate the options:

• A. Promote growth and repair cells. Correct. This is the classic primary function of dietary protein.
• B. Break down molecules. Not the purpose of dietary protein. Breaking down molecules is the job of enzymes (which happen to be proteins the body makes), and the question asks about protein in the diet.
• C. Regulate the flow of chyme. Chyme movement is controlled by muscular action (peristalsis) and sphincters, such as the pyloric sphincter, not by dietary protein.
• D. Serve as co-factors or enzymes. Co-factors are usually minerals or vitamins (coenzymes), not proteins. Enzymes are proteins, but they are made by the body, so this is not the main purpose of protein in the diet.

Answer: Option A, promote growth and repair cells.

Exam tip: Remember the nutrient functions with this memory trick:

• Carbohydrates and fats supply energy.
• Proteins supply growth and repair.
• Vitamins and minerals support protection and regulation.

When a WASSCE question asks for the "purpose of protein in the diet," think "growth and repair" first.