A-LevelBiologyCell membranes and transport9700/21 · May/June 2019 · Q29 Marks

Linoleic acid is an unsaturated fatty acid that is found in some triglycerides and some phospholipids. Phospholipids are components of cell membranes. [Figure 2.1] shows a molecule of linoleic acid.

(a)The composition of cell membranes of plants changes in response to changes in temperature. At the start of the cold season there is an increase in the proportion of phospholipids with unsaturated fatty acids in the chickpea, Cicer arietinum. Chickpea plants that do not make this change do not survive. Suggest how the increase in the proportion of phospholipids with unsaturated fatty acids helps plants, such as chickpea, survive decreases in temperature.[2]
(b)(i)State why triglycerides and phospholipids cannot be described as polymers.[1]
(b)(ii)State two differences in the structure of triglycerides and phospholipids.[2]
(c)Platelets metabolise linoleic acid to produce a molecule known as thromboxane. Thromboxane is released by platelets when blood loss occurs. Thromboxane acts on smooth muscle cells in the walls of arteries. This causes arteries to constrict, which reduces blood flow. Explain why the constriction of arteries following blood loss is an example of cell signalling.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

any two from:
('kinks') prevents close packing of, phospholipids / membrane components, (at low temperature);
keeps / maintains, fluidity; A increases A ora - prevents becoming too rigid
idea of preventing damage to membranes by freezing;
maintains movement of (named) substances across membranes ; A any named example of movement across membrane R increases
AVP; e.g. maintains movement of proteins within membrane
idea that triglycerides and phospholipids are not composed of, monomers / repeating (sub-)units ;
any two from:
phospholipids have two fatty acids (residuess / tails) not three ; A hydrocarbon chains / aliphatic chains A one less fatty acid (residue / tail) A ora
two ester bonds rather than three ;
a phosphate (group / head) ; A ora R 'phosphate not glycerol'
AVP; e.g. may have a (named) additional group, such as choline / AW e.g. triglycerides do not have nitrogen / phospholipids may have nitrogen
any four from:
1 thromboxane is a (cell) signalling molecule ; A 'thromboxane acts as a signal' I 'messenger' / hormone
2 released into / circulates in / AW, blood / plasma ;
3 (smooth) muscle, cell / tissue, is target;
4 thromboxane binds to receptors ;
5 ref. to thromboxane is complementary to receptor ;
6 (specific) response is smooth muscle (cell) contraction; R smooth muscle constricts
7 AVP; e.g. detail of change, such as activating G proteins / secondary messenger / enzyme cascade / chain of reactions / AW

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