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This lesson covers the sources and types of protein as required by AQA GCSE Food Preparation and Nutrition specification 8585 (section 3.2.1). You will learn the difference between high biological value (HBV) and low biological value (LBV) proteins, identify the main food sources of each, and understand the role of protein alternatives in the diet. This knowledge is essential for both the written exam and the food investigation coursework.
Protein is a macronutrient made up of long chains of amino acids joined together by peptide bonds. When we eat protein, the digestive system breaks it down into individual amino acids, which are then absorbed into the blood and reassembled into the specific proteins the body needs.
There are approximately 20 amino acids used by the human body. Of these:
The quality of a protein food depends on how many essential amino acids it contains.
Exam Tip: The term "essential" in biology means "must come from the diet" — it does NOT mean "more important." Non-essential amino acids are just as important for the body; the difference is simply that the body can manufacture them itself.
A protein is classified as HBV if it contains all of the essential amino acids in sufficient quantities.
Most animal-based protein foods are HBV:
| Food Source | Key Details |
|---|---|
| Meat (beef, lamb, pork, chicken) | Rich in protein; red meat also provides iron and B vitamins |
| Fish (cod, salmon, tuna, mackerel) | Oily fish also provides omega-3 fatty acids |
| Eggs | Contain all essential amino acids; often considered the "reference protein" |
| Milk | Contains casein and whey proteins; also provides calcium |
| Cheese | Concentrated source of protein and calcium; high in saturated fat |
| Yoghurt | Fermented dairy product; contains protein and probiotics |
A small number of plant foods are also HBV:
| Food Source | Key Details |
|---|---|
| Soya beans | The most significant plant-based HBV protein; used to make tofu, soya milk, TVP |
| Quinoa | A seed (often treated as a grain) containing all essential amino acids |
Exam Tip: Many students assume all plant proteins are LBV. Remember that soya and quinoa are HBV — this is a commonly tested exception. If asked to give a plant-based HBV protein, soya is the safest answer.
A protein is classified as LBV if it is missing one or more essential amino acids (or contains them in insufficient quantities). The missing amino acid is called the limiting amino acid.
| Food Source | Limiting Amino Acid(s) | Key Details |
|---|---|---|
| Cereals (wheat, rice, oats, maize) | Usually lysine | Staple foods worldwide; also provide carbohydrate |
| Pulses (lentils, chickpeas, kidney beans, baked beans) | Usually methionine | Good source of fibre as well as protein |
| Nuts (almonds, cashews, walnuts, peanuts) | Various | Also provide unsaturated fats and vitamin E |
| Seeds (sunflower, pumpkin, sesame, chia) | Various | Also provide essential fatty acids |
| Peas | Usually methionine | Widely available and affordable |
| Mycoprotein (Quorn — before fortification) | Naturally LBV, but commercially fortified | Derived from the fungus Fusarium venenatum |
Exam Tip: A question may ask you to explain the difference between HBV and LBV protein. Your answer should state: HBV contains all essential amino acids; LBV is missing one or more essential amino acids. Always give a named food example of each.
graph TD
A["Protein"] --> B["HBV Protein"]
A --> C["LBV Protein"]
B --> B1["Contains ALL essential amino acids"]
C --> C1["Missing one or more essential amino acids"]
B --> B2["Animal sources: meat, fish, eggs, dairy"]
B --> B3["Plant sources: soya, quinoa"]
C --> C2["Cereals: wheat, rice, oats"]
C --> C3["Pulses: lentils, chickpeas, beans"]
C --> C4["Nuts and seeds"]
style A fill:#4a90d9,color:#fff
style B fill:#27ae60,color:#fff
style C fill:#e67e22,color:#fff
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