Outline the role of condensation and hydrolysis in the relationship between amino acids and polypeptides.

The protein hemoglobin transports oxygen to cells. Describe the processes that occur in the mitochondria of cells when oxygen is present.

Sickle-cell anemia affects the ability of red blood cells to transport oxygen.Explain the consequence of the mutation causing sickle-cell anemia in relation to the processes of transcription and translation.

Award [3 max] for condensation reactions

Condensation reactions:
condensation is two molecules joining (by a covalent bond) with the loss of a water molecule;
example of condensation reaction;

formation of peptide bond between amino acids;
(covalent) bond between carboxyl end of one amino acid molecule and amino end of other;
many amino acids joined by condensation to form polypeptide;

Hydrolysis reactions:
hydrolysis is the addition of water to break a large molecule into smaller ones;
polypeptide broken down into amino acids/dipeptides by hydrolysis;        
Award any of the above points for a clearly drawn correctly annotated diagram.

pyruvate decarboxylated/CO₂ is removed and reduced NAD/NADH + H⁺ is formed (when entering mitochondrion);
(both needed)

2-C molecule/acetyl group reacts with (reduced) coenzyme A to form acetyl CoA;

acetyl CoA enters Krebs cycle;

2 CO₂ molecules removed (as waste);

energy/electron rich NADH + H⁺/FADH₂ formed;

for each turn of cycle/each pyruvate, 3 NADH + H⁺ and 1 FADH₂ formed;

1 ATP formed per pyruvate each turn (by substrate-level phosphorylation);

reduced NAD/NADH + H⁺ and FADH₂ enter electron transport chain/ETC;

oxidative phosphorylation uses energy released by ETC to synthesise ATP;

as electrons move along ETC, protons/H⁺ move into intermembrane space;

creates H⁺ gradient across the membrane;

ATP synthesized by flow of H⁺ back across membrane through ATP synthase;

ATP synthesized by chemiosmosis;

ETC reduces oxygen/oxygen is final hydrogen (and electron) acceptor forming water;
Award any of the above points for a clearly drawn correctly annotated diagram.
Accept reduced NAD and NAD⁺ + H⁺ as alternatives to each other.

caused by single base substitution (mutation);
mutation in gene coding for (one of) polypeptide chain in hemoglobin/HbA;
GAG (on sense strand of DNA) mutated to GTG;
when transcribed, RNA sequence/codon becomes GUG rather than GAG;
during translation, have one amino acid substituted for another;
causes glutamic acid/glutamate to be replaced by valine;
change alters folding of Hb protein/makes RBCs sickle-shaped (in low oxygen);
sickle shaped cells block capillaries/cause tissue damage and pain;
Award any of the above points for a clearly drawn correctly annotated diagram.
(Plus up to [2] for quality)

Most were able to gain some marks on hydrolysis and condensation. Very few diagrams/ structures were completely correct.

The processes inside the mitochondria were well known by the better prepared candidates, who were able to explain in detail. Several candidates just tried to draw a diagram of the Krebs cycle without any annotation, hoping that the examiners would find some marks. A well annotated diagram can achieve full marks, but it must be clear. Many risked losing the quality marks by describing glycolysis in great detail, thus giving the impression that they were simply writing down everything they knew instead of answering the question.

Most knew that Sickle Cell Anaemia is due to a mutation, but only the better ones were able to correctly state that it was a single base substitution. Few correctly described that the mutation was in (one of) the polypeptide chain of Haemoglobin(A), with many vague statements attributing it to erythrocyte instead. Nearly every candidate remembered that glutamic acid was replaced by valine. Unfortunately this was the only mark gained by many.