A-LevelBiologyEnzymes9700/23 · May/June 2022 · Q512 Marks
Arachidonic acid is a fatty acid that is a common component of phospholipids. Phospholipids can be used as a source of arachidonic acid when it is metabolised within cells in an enzyme-catalysed pathway known as the cyclooxygenase (COX) pathway. The final products of the COX pathway can be different in different cell types, causing a range of responses. In some cells, the products are involved in the inflammatory response, which is a response by the body to infection. In other cells, cell division is stimulated. Fig. 5.1 shows the first reaction in the COX pathway. This reaction is catalysed by an enzyme known as COX-2. [Figure 5.1]
- (a)The enzymes involved in the COX pathway are located in the membrane of rough endoplasmic reticulum. Suggest the advantages to the cell of enzyme pathways being located in cell membranes, rather than in the cytosol of the cell (fluid portion of cytoplasm).[2]
- (b)(i)Sketch on Fig. 5.2 the curve obtained if an anti-inflammatory drug, which is a competitive inhibitor, is present with arachidonic acid.[1]
- (b)(ii)Complete the statements to show whether the maximum rate of reaction (Vmax) and the Michaelis-Menten constant (Km) of COX-2 increases, decreases, or stays the same in the presence of a competitive inhibitor. In the presence of a competitive inhibitor: Vmax of COX-2 ..................................................................................... Km of COX-2 .....................................................................................[2]
- (c)(i)Using gene PTGS2 and enzyme COX-2 as examples, explain what is meant by a gene.[2]
- (c)(ii)Some mutations in PTGS2 lead to an increased rate of transcription. These mutations have been linked to an increased risk of certain types of cancer. Suggest why mutations in PTGS2 may increase the risk of cancer.[2]
- (d)Fig. 5.3 shows the molecular structure of arachidonic acid. Not all hydrogen atoms are shown. [Figure 5.3] With reference to Fig. 5.3, explain why increasing the proportion of phospholipids with arachidonic acid in a cell will increase the fluidity of the cell surface membrane of the cell.[3]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.
- [2] any two from: ref. to can organise / order, enzymes / events, in pathway ; consequence in terms of distance ; e.g. product of one reaction is close to next enzyme (where it acts as substrate) enzyme (more likely to be) closer to substrate distance between enzymes shorter I short distance / less distance to travel, unqualified (so) increases chance of successful collisions between substrate and enzymes ; increased rate of formation of final product or decrease time for ES complex to occur; idea that products can pass to either side of the membrane ; all reactions, localised / occur in the same location (within the cell); AVP; e.g. membrane provides a source of arachidonic acid I phospholipid
- [1] (similar shaped) curve to the right of main curve and reaches Vmax; A increasing to reach Vmax
- [2] stays the same ; increases ;
- [2] the gene / PTGS2, is a sequence of nucleotides, that forms part of a DNA molecule / AW / on chromosome 1; A bases codes for (the production of) a, polypeptide / enzyme / COX-2 ; A protein I codes for an amino acid max one if no example used
- [2] any two from: more, product / compounds, produced that stimulate mitosis; increased, DNA replication / cell division / mitosis; I ref. to cancer cells replicating idea that, may lead to / increases chance of / AW, (other) mutations ; mutations can result in oncogenes ; AVP; e.g. idea that proof checking capacity impaired / increased chance of errors
- [3] any three from: phospholipids may be, used in / added to, the (cell surface) membrane ; arachidonic acid, is unsaturated / polyunsaturated ; has, C=C / (carbon-carbon) double bonds; allow ecf if stated as saturated unsaturated fatty acid tails, have kinks / not linear / AW ; A double bonds produce, kinks / AW increased distance between, phospholipids / other fatty acid tails; A phospholipids cannot pack closely together less hydrophobic interactions between phospholipid (molecules) ;
Write your answer and get it marked
On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.
Sign up free →