Outline what is meant by the term essential amino acid.

Phenylalanine is converted to tyrosine by the enzyme phenylalanine hydroxylase.

(i) Deduce the reason that tyrosine is considered to be a conditionally essential amino acid.

(ii) When infants with the condition phenylketonuria (PKU) are left untreated, they have a build-up of phenylalanine in the blood and high levels of phenylalanine in the urine. State the cause of this condition.

Evaluate human milk as an overall source of essential amino acids.

Outline the control of milk secretion by oxytocin and prolactin.

The diagram represents a lobule from a mammary gland. The mammary gland is an example of an exocrine gland.

Identify two features of an exocrine gland visible in the diagram.

«Essential amino acids» must be obtained from the diet

They cannot be synthesized by the body
OR
They cannot be synthesized from other amino acids

(i) Tyrosine can only be synthesized when phenylalanine is in the diet (Accept inverse: if phenylalanine is not in the diet the person will not
be able to synthesize tyrosine).

(ii) Recessive inherited «genetic» condition
OR
They lack «the enzyme» phenylalanine hydroxylase
OR
Mutated form of PAH gene

Contain all of the essential amino acids

Human milk has higher levels of 5 «out of 9» essential amino acids than cow milk
OR
Human milk has the same or more of essential amino acids except histidine and lysine compared to cow milk

Human milk is the same or higher in all essential amino acids except histidine compared with hen egg
OR
Human milk has higher levels of 4 «out of 9» essential amino acids compared with hen egg

Prolactin

Produced by the anterior pituitary

Stimulates mammary glands to grow

Stimulates the production of milk

Oxytocin

Produced in neurosecretory cells «in hypothalamus»
OR
stored/secreted by posterior pituitary

Suckling/nursing stimulates oxytocin release
OR
example of positive feedback

«Contractions» cause ejection/release of milk from mammary glands

Award [2 max] if response only mentions one of the hormones.

Presence of ducts

Presence acini/acinar/secretory cells

Ducts leads to a body surface/cavity/gut

Most were able to score 2 marks by outlining what is meant by an essential amino acid.

Most candidates showed a reasonable knowledge of PKU although many struggled to differentiate between what was required in (i) and (ii). Most were able to state the cause of PKU for part (ii) but fewer could deduce why tyrosine is considered to be a conditionally essential amino acid in (i). Many wrote the same answer in both sections.

There were many comments on this part of the question on G2 forms, some indicating that this was not on the syllabus. Content knowledge was not needed to answer the question as the data provided could be used to score full marks. This section was discriminating. Many candidates struggled to ‘evaluate’ human milk as a source of amino acids and did not use the data provided. Instead many wrote general statements about human milk, often talking about immunity. 

Many were awarded 2 marks but good, clear answers were not common. Candidates sometimes confused oxytocin and prolactin and most seemed unsure where these hormones came from.

Teacher comments on the G2 form indicated that some thought this question outside the scope of the syllabus although exocrine glands are mentioned in D2. While the question was ostensibly about mammary glands, the diagram was generic and could have been any exocrine gland seen with a light microspore. Most candidates received one mark for identifying the ducts visible in this general diagram and better candidates mentioned the secretory cells or acini.