A-LevelBiologyBiological molecules9700/23 · Oct/Nov 2021 · Q410 Marks

In thale cress, Arabidopsis thaliana, most of the stomata are on the lower surface of the leaves. Fig. 4.1 is a diagram of an open stoma and two guard cells. Some of the cellulose fibres in the cell wall of the guard cells are shown. [Figure 4.1]

(a)The cellulose fibres shown in Fig. 4.1 are composed of bundles of cellulose microfibrils. Explain how molecules of cellulose are arranged into a microfibril.[2]
(b)(i)The width of stomatal openings in A. thaliana is regulated by movement of ions. These ions move through channel proteins in the cell surface membranes of the guard cells. Draw a diagram to show part of a cell surface membrane with a channel protein. Label your diagram.[3]
(b)(ii)Explain why channel proteins are needed for the movement of ions into and out of cells.[2]
(c)The outward movement of ions from guard cells causes stomata to close. A variety of A. thaliana does not have channel proteins for the outward movement of ions in the cell surface membranes of the guard cells. Suggest and explain the effect of not having these channel proteins on transpiration.[3]

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 (Oct/Nov 2021). The mark or mark type is shown in brackets.

[2] any two from: cellulose molecules
1 straight (chains) / linear / unbranched ;
2 arranged in parallel ;
3 hydrogen bonds between (cellulose) molecules ;
4 any detail, e.g. between H of –OH groups and –O of –OH groups ; A between -OH groups
5 AVP ; e.g. cellulose molecules have, staggered / overlapping, ends I overlapping microfibrils
[3] any three from:
1 two layers of phospholipid ;
2 label for, phospholipids / phospholipid bilayer ;
3 channel protein across whole bilayer with hole through middle + label; ecf if a monolayer shown or no phospholipids drawn
4 any additional feature + label; e.g. hydrophilic heads / hydrophobic tails / cholesterol / other named protein (e.g. glycoprotein)
[2] any two from:
1 to allow facilitated diffusion ; R active transport
2 ions are, charged / hydrophilic ;
3 cannot pass through, hydrophobic core / phospholipid bilayer ; A ora – channel proteins provide a, hydrophilic / water-filled, pathway (through the membrane) / AW A channel proteins are lined by polar R groups A ions are repelled by, hydrophobic core / phospholipid bilayer
[3] any three from:
1 transpiration (rate), will not decrease to a low level / remains the same (if conditions remain the same) / remain the same at night/ increases if (named) condition(s) change / AW ;
2 water vapour continues to diffuse out (from intercellular air spaces);
3 (because) stomata, cannot close / remain open ;
4 guard cells do not lose turgidity;
5 because no outward flow of water (from guard cells);
6 by osmosis / down water potential gradient (from guard cells) ;
7 AVP; e.g. ref. to more likely to wilt rates of gaseous exchange remain the same