A-LevelBiologyCell structure9700/23 · May/June 2017 · Q215 Marks

Fig. 2.1 is a transmission electron micrograph of a cell from a leaf. [Figure 2.1]

(a)Use the scale bar to calculate the magnification of the image in Fig. 2.1. Write down the formula that you will use and show your working. formula magnification × ..............................................................[3]
(b)Name structure X and state one function of this structure. name function[2]
(c)Name two structures, visible in the cell in Fig. 2.1, that contain DNA. 1 2[2]
(d)Plant cells can be infected with viruses. Fig. 2.2 shows the key features of a plant virus. [Figure 2.2] Identify the structures labelled A and B in Fig. 2.2. A B[2]
(e)(i)Some plant cells produce a polypeptide called systemin. Describe the role of DNA in the production of systemin.[2]
(e)(ii)Systemin stimulates plant cells to produce enzyme inhibitors known as serpins. One of these serpins is a competitive inhibitor of some protease enzymes. It inhibits the protease enzymes found in herbivores, but does not inhibit the proteases in plants. Suggest how this serpin inhibits only the protease enzymes in herbivores but not those in plants.[3]
(e)(iii)The presence of competitive inhibitors, such as serpins, increases the Michaelis-Menten constant (Km) for the enzymes they inhibit. Explain why the Km value increases.[1]

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 2017). The mark or mark type is shown in brackets.

[3] one mark for the formula and two marks for the correct answer (magnification =) image/scale bar (length) ; A triangle / letters only actual/object (length) 3000 ;; 30 000 + 10 ; using 30mm as measured length A calculated values for measured lengths of 29 mm or 31 mm allow one mark if correct answer given with units allow one mark if incorrect answer and correct measurement and correct working correct measurement and formula but incorrect conversion measurement ± 2mm and correct working
[2] name (large permanent) vacuole ; function one from store of/ holds / AW, cell sap; R if contains organelles store of /holds / AW, water / ions / named ion(s) / minerals / salts / pigments / sugars / named sugars ; A nutrients I substances / molecules / food I storage unqualified pushes chloroplast to edge of cell ; gives, turgidity / turgor pressure / hydrostatic pressure / support / AW; A makes, firm / rigid A controls / maintains, turgidity I gives shape / strengthen / provides structure store of /holds, waste (products) ; A breakdown of (large) waste products I reactions occur in vacuole, unqualified allow function mark even if name of organelle left blank or incorrect
[max 2] nucleus; chloroplast(s); mitochondrion ; A mitochondria
[2] A nucleic acid / DNA / RNA ; A genetic material I strand R circular DNA B protein coat/capsid / capsomere / protein subunit ;
[2] gene coding for systemin is (a length of) DNA ; A idea that a length of DNA codes for systemin transcription/production of mRNA ;
[max 3] herbivore proteases and plant proteases have different (shaped) active sites ; serpin / competitive inhibitor, only complementary to (shape of) active site of herbivore proteases ; ora A can only fit serpin is specific to (active sites of) herbivore proteases ; AW proteases act on different, substrates / (parts of) proteins; A similar shape only to herbivore substrates AVP; e.g herbivore proteases cut at different amino acid sequences allow other acceptable suggestions e.g. suggestion that serpin physically separated from plant proteases ; detail, e.g. in vesicle / plant proteases in vacuole ; cannot bind to active site of plant proteases ; serpin released when plant parts ingested by herbivores ;
[1] affinity for substrate has decreased; needs a higher concentration of substrate to saturate active sites / AW ; A needs a higher concentration to reach, ½ Vmax/Vmax idea that inhibitor prevents substrate from binding (to active site) so more substrate required (to compete);