A-LevelBiologyBiological molecules9700/23 · May/June 2025 · Q212 Marks

Bees are insects that produce venom as a means of self-defence. Melittin is a polypeptide present in the venom of bees.

(a)(i)State the number of carbon atoms in the R group of lysine.[1]
(a)(ii)Complete the diagram of the structure of lysine in Fig. 2.1.[2]
(b)Descriptions of the structure of melittin are shown in Table 2.1. Complete Table 2.1 by writing the level of protein structure that applies to each description. Table 2.1 description | level of protein structure in some conditions, four melittin polypeptides can bind to each other | a melittin polypeptide consists of a sequence of 26 amino acids | alpha helices are formed at each end of a melittin polypeptide |[3]
(c)When a bee stings a person, venom enters the body. Melittin in the venom interacts with cell surface membranes of body cells, as shown in Fig. 2.2. [Figure 2.2] Outline how melittin affects the structure of a cell surface membrane, as shown in Fig. 2.2.[2]
(d)Cells that have been affected by melittin break down into cell fragments. These cell fragments are taken in by phagocytes for further breakdown. Describe the process by which phagocytes take in and break down these cell fragments.[4]

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 2025). The mark or mark type is shown in brackets.

[1] 4 / four ;
[2] COOH group bonded to the top carbon ;
[2] NH2 group bonded to the top carbon ; A with or without lines indicating bonds within functional groups ; R if atoms other than C and N are shown bonded to the carbon
[3] in some conditions, four melittin polypeptides can bind to each other: quaternary ;
[3] a melittin polypeptide consists of a sequence of 26 amino acids: primary ;
[3] alpha helices are formed at each end of a melittin polypeptide: secondary ;
[2] produces a, pore / gap / channel / opening / AW, (in cell surface membrane) ; A forms a channel protein
[2] ref. to phospholipids / phospholipid bilayer ; e.g. displaces / disrupts / separates, phospholipids
[4] any four from:
[4] 1 binding/attachment / joining / AW, of cell fragment to, (phagocyte cell surface) receptors / (phagocyte) membrane ;
[4] 2 endocytosis/phagocytosis
[4] 2 (cell surface) membrane, surrounds / AW, cell fragment ; or pseudopodia, surrounds / form round / AW, cell fragment ; or (phagocytic) cell, envelops / engulfs, cell fragment ;
[4] 3 membrane fusion / (phagocytic), vacuole pinches off / AW ; or phagocytic vacuole formed ; allow vacuole if phagocytosis stated ; A phagosome formed ; A vesicle for vacuole
[4] break down cell fragments
[4] 4 lysosome (containing enzymes), fuses / AW, with phagocytic vacuole ; A phagolysosome formed
[4] 5 fragment broken down by, hydrolytic / digestive, enzymes ; R lysosome digests
[4] 6 named enzyme and product or two named enzymes ;
[4] AVP; idea of opsonisation of/antibody binding to, cell fragment