A-LevelBiologyCell membranes and transport9700/22 · Feb/Mar 2024 · Q113 Marks

Fig. 1.1 is a diagram representing part of the phospholipid bilayer of a cell surface membrane. [Figure 1.1] Identify the part of a phospholipid molecule, labelled A in Fig. 1.1, that forms bonds with the phosphate heads and with the fatty acid tails.

(a)(i)Fig. 1.1 is a diagram representing part of the phospholipid bilayer of a cell surface membrane. [Figure 1.1] Identify the part of a phospholipid molecule, labelled A in Fig. 1.1, that forms bonds with the phosphate heads and with the fatty acid tails.[1]
(a)(ii)Cholesterol is an important lipid component of many cell surface membranes. Fig. 1.2 shows the structure of a cholesterol molecule. [Figure 1.2] Using the information in Fig. 1.2, explain the orientation (positioning) of cholesterol molecules in the phospholipid bilayer, as shown in Fig. 1.1.[1]
(a)(iii)State one role of cholesterol in phospholipid bilayers.[1]
(b)(i)Explain why sodium ions cannot cross phospholipid bilayers by simple diffusion.[1]
(b)(ii)Ions and some molecules move across cell surface membranes by facilitated diffusion and active transport. Compare facilitated diffusion and active transport by stating one way in which they are similar and two ways in which facilitated diffusion is different from active transport.[3]
(c)(i)Prostaglandins are small lipids produced in many tissues of the body. One role of prostaglandins is to cause inflammation at the site of an injury or infection. Inflammation is the normal first response of the immune system to injury or infection. Cyclooxygenase (COX) is an enzyme that catalyses one of the steps in the reaction pathway for the formation of prostaglandins from phospholipids. The reaction pathway occurs in the smooth endoplasmic reticulum (SER) of cells. Part of the reaction pathway is shown in Fig. 1.3. [Figure 1.3] Suggest an advantage for this reaction pathway occurring in the smooth endoplasmic reticulum of a cell rather than in the cytoplasm.[1]
(c)(ii)Sometimes inflammation can have side-effects, such as pain. Aspirin is a drug that can be used to reduce these side-effects. Aspirin reduces the catalytic activity of the COX enzyme by modifying the R-group of one of the amino acids. Suggest how modifying the R-group of an amino acid in the COX enzyme can reduce the catalytic activity of the enzyme.[3]
(c)(iii)Prostaglandins are examples of cell-signalling molecules. Outline the process of cell signalling that leads to a response by the cells involved in inflammation.[2]

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 (Feb/Mar 2024). The mark or mark type is shown in brackets.

[1] (a)(i) glycerol ;
[1] (a)(ii) any one from:
hydroxyl / polar, group, interacts with, phosphate heads ;
A idea that both are, polar / hydrophilic
A hydroxyl group, faces aqueous environment / AW, as it is, polar / hydrophilic
non-polar part, in region of / AW, fatty acid tails / AW, as both are, non-polar / hydrophobic ;
A non-polar part is hydrophobic so, in centre of membrane / away from aqueous environment
[1] (a)(iii) any one from:
maintains / regulates, fluidity of membrane ;
A detail e.g. reduces fluidity at high temperature / increases fluidity at low temperature / AW
maintains / regulates, (mechanical) stability of membrane ; AW
prevents entry of, hydrophilic substances / polar substances / ions ;
without cholesterol membranes would easily rupture
flat ring (structure) interferes with the movement of fatty acid tails
reduces lateral movement of phospholipids
[1] (b)(i) sodium ions are
(positively charged), so repelled / AW, by the, hydrophobic tails / non-polar core / non-polar tails / AW;
[3] (b)(ii) similarity:
any one from
1 both occur through / involve, a, membrane / transport, protein ; A carrier protein if context is correct
2 (both can be) specific to the, molecule / ion (passing through) ; A correct ref. to binding site(s) on protein
3 (both involve) conformational change of (carrier protein)) ; R if incorrect context of channel protein
AVP; e.g. can transport substances, into and out of cell / in both directions ;
differences:
facilitated diffusion: substances transported down the concentration gradient ; passive A does not require, ATP / (metabolic) energy ; involves channel and carrier proteins
active transport: substances transported against a concentration gradient ; requires, ATP / (metabolic) energy ; does not involve channel proteins / only involves carrier proteins
If not given as a similarity allow as differences
idea that active transport always involves conformational change (facilitated diffusion only carrier protein)
idea that active transport always involves specific binding site(s) (facilitated diffusion only with carrier proteins)
[1] (c)(i) any one from:
SER (membrane-bound so) can provide, phospholipid / arachidonic acid / substrate ;
(prostaglandins are lipids and) SER is involved in lipid transport;
prostaglandins can be transported (from SER) or stored (in SER) ;
idea of compartmentalisation / separated from other reactions in the cytoplasm / provides optimum conditions for pathway / higher concentration of enzymes (of pathway);
[3] (c)(ii) any three from:
ref. to effect on, ionic bonding / hydrogen bonding / hydrophobic interactions, (with other R-groups);
changes the, shape / conformation, of the active site ;
active site no longer complementary to, arachidonic acid / substrate ;
ref. to effect of activation energy not being reduced ; e.g. ref to changed charges (so no electron transfer)
no longer provides hydrophobic regions for reaction to occur
enzyme-substrate complex, not formed / formed at reduced rate ;
[2] (c)(iii) any two from:
A ligands for prostaglandins if in correct context
1 prostaglandins are, secreted / released, by cells
or
prostaglandins are, transported / AW, to target cells ;
2 prostaglandins bind to receptors (on target cell surface membranes) ; R antigens
3 example of events triggered leading to a response ; e.g. activation of secondary messenger
enzyme, cascade / activation
phosphorylation events
signal transduction