A-LevelBiologyCell structure9700/23 · May/June 2020 · Q417 Marks

Saccharomyces cerevisiae is a unicellular fungus that is important in the brewing and baking industries. Fig. 4.1 is a diagram of a transmission electron micrograph of S. cerevisiae. [Figure 4.1]

(a)A student was asked to calculate the magnification of the image shown in Fig. 4.1. The student began by measuring the length of the scale bar in millimetres using a millimetre ruler. State what the student should do next to obtain the correct answer.[1]
(b)One function of the lipid droplets shown in Fig. 4.1 is to store triglycerides. The triglycerides in a lipid droplet are surrounded by a single layer (monolayer) of phospholipids. Suggest and explain why phospholipids, rather than triglycerides, are used for the outer monolayer of the lipid droplet.[2]
(c)The lysosomes and vacuole of S. cerevisiae contain acid hydrolases (hydrolytic enzymes) that function in an acid pH. Explain why lysosomes need hydrolases to carry out their function.[1]
(d)(i)A disaccharide, trehalose, is a reserve store of energy for S. cerevisiae when glycogen stores decrease. The monomer of glycogen and trehalose is a-glucose. Complete Fig. 4.2 to show the ring structure of one a-glucose molecule. [Figure 4.2][2]
(d)(ii)A student carried out tests on a solution of trehalose and correctly concluded that trehalose is a non-reducing sugar. Outline the procedure carried out by the student and state the results that were obtained.[3]
(e)(i)The hydrolysis of trehalose is catalysed by two different enzymes produced by S. cerevisiae, regulatory trehalase and non-regulatory trehalase. A study was carried out to compare regulatory trehalase and non-regulatory trehalase extracted from S. cerevisiae. The results of the study showed that: • regulatory trehalase had a higher Km value (Michaelis-Menten constant) than non-regulatory trehalase • the optimum pH of regulatory trehalase was pH 7.0–7.8 • the optimum pH of non-regulatory trehalase was pH 4.5–5.0. Explain what is meant by a higher Km value.[2]
(e)(ii)Regulatory trehalase is found only in the cytosol, the fluid part of the cytoplasm. Non-regulatory trehalase has been found on the external surface of the cell surface membrane and inside the cell. State the location inside the cell where non-regulatory trehalase is likely to be found and explain the reason for your answer.[2]
(e)(iii)Explain whether both types of trehalase, regulatory and non-regulatory, can be described as intracellular enzymes.[1]
(f)Saccharomyces boulardii is a strain of S. cerevisiae. It has been researched for its possible health benefits for some gut diseases. Researchers investigating trehalase extracted from S. boulardii concluded that only one type of trehalase was present in the extract. Fig. 4.3 shows the effect of pH on the activity of the trehalase extracted from S. boulardii. [Figure 4.3] With reference to Fig. 4.3 and to the two different types of trehalase enzyme produced by S. cerevisiae, state and explain what can be deduced about the type of trehalase present in S. boulardii.[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 (May/June 2020). The mark or mark type is shown in brackets.

[1] converted the measured length (in mm) to µm (and dropped the, µm / units)
or
multiplied the measurement by 1000;
[2] phosphate(-containing) heads are hydrophilic
or
triglycerides do not have a hydrophilic portion ;
lipid droplet is stored in / phosphate heads can interact with, cytosol / aqueous environment/watery environment;
[1] to, digest/break down, worn out organelles / waste metabolic products / toxic substances / AW ; A invading pathogens
[2] correct orientation of H and OH on, C1 and on C3 ;
OH added to carbon 6;
[3] positive result is coloured precipitate ; only allow if hydrolysis noted
A green / yellow / orange / brown / red, for colour
any two from
ref. to (heat with) Benedict's (reagent/solution) and, negative test / no colour change / remains blue, and test again with Benedict's;
boil with (hydrochloric) acid ; A hydrolyse with, acid / enzyme
AVP; e.g. use a fresh sample to hydrolyse
boil for 5-10 minutes with acid
cool before neutralising
neutralise (remaining acid) with alkali
test with universal indicator paper (to check for pH7)
compare with a control
allow one mark for heat with Benedict's and coloured precipitate
[2] any two from
(higher Km enzyme)
has a lower affinity for its substrate; A binds substrate less easily
needs a higher concentration of substrate to reach, Vmax / maximum activity / ½ Vmax ;
less likely to be saturated with substrate ;
variations in substrate have greater effect on rate of reaction;
[2] any two from
in the vacuole ; A in lysosome
qualified;
e.g. has, acidic / low pH, environment
contains, acid hydrolases / enzymes that require low pH
non-regulatory trehalase needs acidic conditions for optimum activity
cytosol has neutral pH so likely to be location of regulatory trehalase (and enzymes are in different locations)
if in cytosol then this would mean low pH and other enzymes, would (partially) denature / work below optimum
as acid conditions required, will be in area protected from rest of cell and vacuole has the tonoplast as barrier
A lysosomes are membrane bound
[1] yes because they both work, within the cell / inside the cell;
[3] any three from
likely to be regulatory trehalase / unlikely to be non-regulatory trehalase
nearer to optimum of regulatory trehalase (of S. cerevisiae); ora
pH 6.5/6.6 nearer to pH 7.0 (than to pH 5.0 of non-regulatory trehalase);
no / (very) low, activity at pH 4.5
or
greater activity at pH 7 than pH 4.5(–5.0);
alternative
suggestion that enzyme could be a different form of trehalase;
because has different optimum pH to both regulatory and non-regulatory ;
pH 6.5/6.6, rather than pH 7.0 or pH 5.0 ;
general
(so) likely to act in the cytosol with, neutral pH / pH7;
unlikely to be found in vacuole / lysosome, with low / acid, pH ;