A-LevelBiologyCell structure9700/22 · May/June 2018 · Q115 Marks

Fig. 1.1 is a drawing of a photomicrograph of a spongy mesophyll cell from a leaf. [Figure 1.1]

(a)On Fig. 1.1, add a label line and the correct letter for each of the three cell structures listed. • nucleolus = N • tonoplast = T • chloroplast = C[3]
(b)The drawing in Fig. 1.1 is 2000 times larger than the actual size of the cell. Describe the steps you would follow to determine the actual diameter of the cell in micrometres (µm), at X–Y.[2]
(c)The drawing in Fig. 1.1 was made using the high power objective lens of a light microscope. Some of the structures in Fig. 1.1 confirm that the cell is eukaryotic. An electron micrograph of the same cell would reveal additional cell structures that are found in eukaryotes and not in prokaryotes. List two examples of these additional cell structures.[2]
(d)(i)State, in terms of water movement from the xylem to the spongy mesophyll cells in the leaf, what is meant by the apoplastic pathway.[2]
(d)(ii)Outline the properties of water that contribute to the apoplastic movement of water to the spongy mesophyll cells and to the movement of water into the intercellular air spaces.[4]
(d)(iii)Describe what happens to the water vapour in the intercellular air spaces during the day and explain why this happens.[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 (May/June 2018). The mark or mark type is shown in brackets.

[3] one mark for each label to correct structure on Fig. 1.1;;;
tonoplast line to anywhere on the vacuolar membrane
R if two or more different structures stated for a single label
[2] ref. to conversion of measured (X-Y) line to µm / AW;
e.g. line is measured in, mm and then multiplied by 1000 / cm and then multiplied by 10000
evidence that, 60 (±1 mm) is multiplied by 1000 / 6 cm is multiplied by 10000
measured diameter is 60000µm
(and) divide by, magnification / 2000;
[2] two from:
rough endoplasmic reticulum ; A rough ER/RER
smooth endoplasmic reticulum ; A smooth ER/SER
endoplasmic reticulum ; acceptable only if the other structure is not SER/RER
80S / larger, ribosomes ; A 25-30 nm range
plasmodesma(ta);
microtubules ; A microfilaments
A cytoskeleton
lysosome(s);
Golgi (body / apparatus / complex) ;
secretory / Golgi, vesicles ;
AVP;; e.g. chromatin
EM detail of chloroplast
EM detail of mitochondrion A mitochondrion
nuclear pore
nuclear envelope R nuclear membrane
[2] two from:
cell wall, pathway / route / AW ; A involves only cell walls R if only in roots
movement / AW, through intercellular spaces / spaces between cells ;
further detail; e.g. water does not, enter cells / cross membranes / cross cytoplasm / move through plasmodesmata
osmosis does not occur
membranes not involved
R osmosis once only
[4] four from:
hydrogen bonding (between water molecules) ;
water molecules are polar;
movement to spongy mesophyll cells
adhesion / attraction, to, cellulose / cellulose fibres / cell walls ;
this is in context of leaf cells but also allow for xylem
R cell walls of lignin
A hydrophilic parts of lignin
cohesion between water molecules / (water molecules are) cohesive;
idea that movement of water (molecules) towards, spongy / mesophyll, cells, pulls / AW, other water molecules ;
A transpiration pull / continuous column / unbroken column
I continuous stream
movement to intercellular air spaces
water molecules absorb heat (energy);
bonds break between water molecules ;
evaporation / water to water vapour ;
from spongy cell, walls / surfaces;
I latent heat of vaporisation
[2] two from:
transpiration occurs ; in context of via stomata or via cuticle
diffuses / moves / AW, to, (external) environment / atmosphere / outside / out of the leaf/ from leaf surface ;
R evaporation / osmosis
R from cell surface
via (open) stomata (for, gas exchange / carbon dioxide entry / photosynthesis) ;
accept Ψ for water potential
I solute potential / ref. to concentration
(movement of water vapour) down water potential gradient ;
A down water vapour potential gradient
A high to low water potential / to lower water potential / from higher water potential
R from high to low water potential gradient