State four elements that are needed by living organisms, other than carbon, hydrogen and oxygen, giving one role of each.

Outline how light energy is used and how organic molecules are made in photosynthesis.

Explain the significance of complementary base pairing for replication, transcription and translation.

a. nitrogen – structure of organic molecules/proteins/nucleotides;

b. sulfur – amino acid / protein structure;

c. phosphorus – nucleic acids / energy carriers;

d. calcium – bone structure / trigger exocytosis (e.g. neurotransmitters);

e. iron – prosthetic groups / hemoglobin;

f. sodium – membrane potential;

Accept other valid roles for those elements already listed.

Accept other valid examples of elements with their roles.

To award [4 max], responses need an appropriate role for each element stated.

a. chlorophyll is the (main) photosynthetic pigment;

b. absorbs (mainly) red and blue light;

c. green light is reflected;

d. light energy absorbed is converted into chemical energy;

e. ATP produced;

f. water split;

g. to form oxygen and hydrogen;

h. ATP and hydrogen used to fix carbon dioxide to make organic molecules;

a. A-T and C-G in DNA;

b. A-U and C-G in RNA;

c. complementary base pairing in replication ensures identical nucleotide sequence of new complementary strands;

d. semi-conservative replication;

e. transcription produces RNA sequence complementary to the DNA sequence (of the gene);

f. triplets of nucleotides on mRNA are codons;

g. translation converts mRNA sequence of information into a specific amino acid chain (polypeptide);

h. (each class of) tRNA carries a specific triplet of (three) bases called an anticodon;

i. anticodons bind to codons by complementary base pairing;

j. (each class of) tRNA with specific complementary anticodons carry specific amino acids;

k. sequence of mRNA codons translates into specific amino acid sequence;

l. enables conservation of information transfer from DNA to RNA to polypeptide;

In general, well answered by those candidates who attempted the question. The syllabus content appeared to be well understood.

Most candidates mentioned the conversion of light energy into chemical energy during photosynthesis. Many candidates also referred to events of photolysis for additional marks. A common omission was reference to the wavelengths absorbed or reflected by chlorophyll. Finally, candidates wrote about fixation of carbon dioxide to make organic molecules but neglected to mention that the process requires ATP and hydrogen.

Reasonable accounts were given, though more emphasis was placed on replication than the other processes. Weaker candidates failed to mention identical nucleotide sequences in replication. For most candidates, the importance of complementary base pairing needed more development in transcription and translation. The complementarity of codons and anti-codons was often missed. Many times valid ideas were left undeveloped but then continued elsewhere in the essay. Some essays were embellished by good quality illustrations. Occasionally, understanding of the three processes was very muddled with some candidates describing mRNA as going into the nucleus to copy DNA.