A-LevelBiologyCell membranes and transport9700/22 · Feb/Mar 2020 · Q613 Marks

Fig. 6.1 is a transmission electron micrograph of a plant parenchyma cell. [Figure 6.1]

(a)The external environment of the parenchyma cell has a higher water potential than the internal environment of the cell. One function of parenchyma cells is to provide support to the plant. With reference to Fig. 6.1, suggest how parenchyma cells provide support to the plant.[2]
(b)(i)The image shown in Fig. 6.1 is at a higher magnification than can be obtained using a typical light microscope. Explain what is meant by the term magnification.[1]
(b)(ii)The actual diameter of the parenchyma cell in Fig. 6.1 along the line X-Y is 35 µm. Calculate the magnification of the image. magnification = x[2]
(c)(i)The cell sap in the vacuole of the cell shown in Fig. 6.1 has a pH of 5.0. The cytosol has a pH of 7.2. The tonoplast controls the passage of hydrogen ions from the cytosol into the vacuole. The low pH created by the entry of hydrogen ions is optimum for the action of acid hydrolase enzymes in the vacuole. Acid hydrolase enzymes are also found in lysosomes in animal cells. Suggest which transport mechanism is used to move hydrogen ions from the cytosol of the parenchyma cell into the vacuole. Explain your choice. transport mechanism explanation[3]
(c)(ii)Suggest how the structure of the tonoplast allows hydrogen ions to be transported into the vacuole, but does not allow the ions to leave the vacuole.[3]
(c)(iii)The acid hydrolases in the vacuole cannot function in neutral conditions (pH 7.0) or alkaline conditions. Explain the advantage to the plant cell of having acid hydrolases that cannot function in neutral, near neutral or alkaline conditions.[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 2020). The mark or mark type is shown in brackets.

[2] 6(a) any two from:
1 water moves into, cell / vacuole, by osmosis / down water potential gradient ;
2 (large) vacuole full of, water / sap ;
3 turgid / vacuole exerts outward pressure ;
4 hydrostatic (support);
[1] 6(b)(i) number of times an image is larger than, actual / real, size ;
A image size + actual size
R increase in size of specimen or object
[2] 6(b)(ii) (×) 2000 ; ;
if incorrect:
one mark for correct working
70 000 / 35
or
one mark for correct measurement and division by 35
e.g. 70 mm / 35
7 cm / 35
[3] 6(c)(i) transport mechanism:
active, transport / uptake;
explanation (max 2):
(vacuole) has higher concentration of hydrogen ions (than in cytosol);
hydrogen ions need to move against the (concentration) gradient ;
ATP / energy, needed ;
membrane protein needed ;
[3] 6(c)(ii) any three from:
charged particles / ions, cannot cross, hydrophobic core / region of fatty acid tails / AW ;
movement through specific membrane protein ;
membrane protein only allows one-way movement ;
A no membrane proteins to allow outward flow
AVP; e.g. suggestion that binding site is on the cytosol side
[2] 6(c)(iii) any two from:
acid hydrolases, break down / digest / hydrolyse;
ref. to leakage from vacuole ;
avoids, damage to / breakdown of, cell contents / organelles / molecules (in the cytosol) ;