Protein: Functions and Complementation

This lesson covers the functions of protein in the body and the important concept of protein complementation, as required by AQA GCSE Food Preparation and Nutrition specification 8585 (section 3.2.1). Understanding these concepts is particularly important for questions about vegetarian and vegan diets, where getting all essential amino acids requires careful food combining.

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Functions of Protein in the Body

Protein performs a wide range of vital functions. You need to know the main functions and be able to explain each one:

Function	Explanation
Growth	Protein is essential for the growth of new cells and tissues. This is particularly important in children, adolescents, and during pregnancy.
Repair	Damaged or worn-out cells are replaced using amino acids from protein. This includes wound healing and recovery from illness.
Maintenance	Body tissues are constantly being broken down and rebuilt. Protein maintains muscles, skin, hair, nails, and internal organs.
Secondary energy source	If carbohydrate and fat stores are depleted, protein can be broken down and used for energy (4 kcal per gram). This is not ideal, as it diverts protein from growth and repair.
Enzymes	Many enzymes (biological catalysts that speed up chemical reactions in the body) are made of protein. Digestive enzymes such as amylase, protease, and lipase are all proteins.
Hormones	Some hormones are made of protein, including insulin (regulates blood sugar) and growth hormone.
Antibodies	Antibodies are proteins produced by the immune system to fight infections caused by bacteria and viruses.
Structural components	Proteins such as collagen (found in skin, bones, tendons) and keratin (found in hair and nails) provide structural support.

Exam Tip: When asked to state the functions of protein, aim for at least three distinct functions with brief explanations. Simply writing "growth and repair" is only one point — separate them into "growth of new cells" and "repair of damaged tissues" for two marks.

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How Protein Is Used in the Body

When protein is eaten, the following process occurs:

1. Protein in food is chewed and mixed with saliva in the mouth
2. In the stomach, hydrochloric acid denatures the protein, and the enzyme pepsin begins to break it down
3. In the small intestine, the enzyme trypsin (from the pancreas) continues protein digestion
4. Proteins are broken down into individual amino acids
5. Amino acids are absorbed through the wall of the small intestine into the blood
6. The blood carries amino acids to cells throughout the body
7. Cells reassemble amino acids into the specific proteins the body needs

graph LR
    A["Protein in food"] --> B["Digestion breaks peptide bonds"]
    B --> C["Individual amino acids"]
    C --> D["Absorbed into blood"]
    D --> E["Transported to cells"]
    E --> F["Reassembled into body proteins"]
    F --> G["Growth, repair, enzymes, hormones, antibodies"]

    style A fill:#e67e22,color:#fff
    style C fill:#4a90d9,color:#fff
    style F fill:#27ae60,color:#fff

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Protein Complementation

Protein complementation is the practice of combining two or more LBV protein foods in the same meal so that the essential amino acids missing from one food are provided by the other. Together, the combination provides all essential amino acids, effectively creating a meal with the same amino acid profile as an HBV protein.

Why Is Complementation Important?

• People following vegetarian or vegan diets may rely heavily on LBV protein sources
• No single LBV food contains all essential amino acids
• By combining different LBV foods, all essential amino acids can be obtained
• The two foods do not need to be eaten at exactly the same time, but ideally within the same day

How Complementation Works

Different LBV protein sources are deficient in different amino acids:

Food Group	Typically Lacking	Rich In
Cereals (wheat, rice, oats)	Lysine	Methionine
Pulses (lentils, beans, chickpeas)	Methionine	Lysine

When cereals and pulses are combined, each provides the amino acid the other lacks. The result is a complete protein meal.

graph TD
    A["Protein Complementation"] --> B["Cereal"]
    A --> C["Pulse"]
    B --> D["Low in LYSINE"]
    B --> E["Rich in METHIONINE"]
    C --> F["Rich in LYSINE"]
    C --> G["Low in METHIONINE"]
    D -.->|"complemented by"| F
    G -.->|"complemented by"| E
    F --> H["All essential amino acids provided"]
    E --> H

    style A fill:#4a90d9,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#27ae60,color:#fff
    style H fill:#8e44ad,color:#fff

Examples of Protein Complementation