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Leucine availability and chicken growth
Protein is an important part of a chicken's diet. Proteins contain 20 different types of amino acid. A chicken can make many of the 20 amino acids from other substances in the diet. Essential amino acids are amino acids the chicken cannot make, so they must be included in the diet. Maize seeds contain protein but the proportion of some essential amino acids is low. Scientists have produced a type of maize called Quality Protein Maize (QPM). Table 2 compares the proportions of seven essential amino acids in normal maize seeds and in QPM seeds.
| Amino acid | Normal maize | Quality protein maize (QPM) |
|---|---|---|
| Leucine | 122.2 | 88.1 |
| Lysine | 28.9 | 41.9 |
| Methionine | 19.9 | 18.1 |
| Phenylalanine | 49.4 | 40.9 |
| Threonine | 34.5 | 36.5 |
| Tryptophan | 7.3 | 16.3 |
| Valine | 45.9 | 51.2 |
Table 2 shows that 1 kg of QPM contains less leucine than 1 kg of normal maize. Suggest why a diet containing less leucine does not slow down the growth of chickens.
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Marking points
- 1 Explains that chickens need only a low amount of leucine for growth.
- 2 Explains that chicken proteins contain only a low proportion of leucine.
Full-mark answer
Chickens require only a small amount of leucine for growth, so the lower amount in QPM is still sufficient.
Why this answer loses marks
Leucine is not needed for growth.
The mark scheme rejects this: leucine is needed, but the amount required may be low.