2021 Summer Paper 21-23

actual width = image width ÷ magnification ;
Accept:
A = I ÷ M
M = I ÷ A
I = A × M

working = width divided by 4275 ;
e.g.
16 000 ÷ 4275 = 3.7 (μm)
17 000 ÷ 4275 = 4.0 (μm)
18 000 ÷ 4275 = 4.2 (μm)
19 000 ÷ 4275 = 4.4 (μm)
;
Reject answer if given to more than 1 dp or whole number.

Table 1.1

Chloroplast / mitochondrion ;

nucleus ; A chloroplast / mitochondrion
R nucleolus

Any two from:

1 (section at) high resolution ;
A suggestion of a correct value of resolution for a TEM

2 any named structure visible in Fig. 1.1 that can, only be seen in a TEM / not be seen in a photomicrograph ;
e.g. internal structure of chloroplasts / thylakoid(s) / grana
e.g. internal structure of mitochondria / cristae

3 high magnification / higher magnification (magnification > 1000 / higher than with light microscope) ;
in context of higher than light microscope

4 (very) thin ;

5 2D / no surface contours / no surface features / AW ; A not 3D

any three from:

1 (diagram shows) hydrogen bond is a weak bond ;
2 each oxygen (atom) forms two hydrogen bonds / each hydrogen (atom) forms one hydrogen bond ;
3 (attraction) between oxygen (atom) of one water molecule and hydrogen (atom) of another (forms a hydrogen bond) ;
R cohesion / adhesion – if used for attraction

4 water is dipolar ;

5 detail ;
e.g. electrons not shared equally between oxygen and hydrogen
A oxygen is more electronegative (than hydrogen)
A ref. to (two) lone pair(s) (of electrons) on oxygen

A uneven distribution of, electrons / charge
e.g. oxygen has, small / slight, negative charge / δ^–, and, hydrogen has, small / slight, positive charge / δ⁺
only needs to state ‘small’ once

idea that (H-bonds) maintain (shape / structure, form of)
α-helices / β-pleated sheets ;

Accept: allows formation of, α-helices / β-pleated sheets.
Reject: if bonds are between R groups.

any two from:
1 idea that hydrogen bonds help to, stabilise, further folding of, amylase / polypeptide / protein ;

2 between, R groups with amine and carboxyl groups ;
Accept: between R groups with -NH and, -CO / -OH

3 idea that may be between amino acids far apart in primary structure ;

4 either helps to maintain / form / AW, globular shape / 3D shape / structure (of amylase / polypeptide / protein)
or
maintains / forms / AW, (specific) shape / structure, of, active site / binding site ;

any three from:
1 dissolves / ions / minerals / salts, and (named) polar molecules ;
Accept: ‘assimilates’ as polar I substances / nutrients.

2 transports, solute(s) / named solutes / dissolved substance, in, xylem / phloem / xylem and phloem ;

3 storage of, solutes / named solutes, in vacuoles ;

4 metabolic / chemical / cellular, reactions occur in water ;

5 dissolves, carbon dioxide / oxygen, with ref to, respiration / photosynthesis;

any three from:

1 differences in height show that concentrations of sucrose, 0, 0.4 and 0.8 (mol dm^–3) or ⩽ / less than, 0.8 / 0.9, water moves out of Visking tubing ;
Accept: one of 0, 0.4 or 0.8.
Reject: sucrose moving.

2 concentrations of sucrose 1.2, 1.6, 2.0 (mol dm^-3) or ⩾ / more than, 0.9 /1.2, water moves into Visking tubing ;
Accept: one of 1.2, 1.6 or 2.0.
Reject: sucrose moving.

3 ref. to net water movement ;
if water enters Visking tubing.
Accept reverse argument: for water leaving

4 external solution has higher water potential (than contents of Visking tubing) ;
Accept: high water potential to low water potential

5 water moves, down, water potential /  Ψ, gradient ;

mark whole question to a max of four marks

descriptions:
1 at concentrations, less than / ⩽, 0.04 mol dm^–3 all cells, burst / AW ;

2 at concentrations between 0.04 and 0.14 mol dm^–3 decreasing percentage of cells burst / AW ; A use of percentages

3 at concentrations, greater than / ⩾, 0.14 mol dm^–3 no cells, burst / AW ;

explanations to max 3
4 in low concentrations / ⩽ 0.04, of sodium chloride water moves into cells down water potential gradient / from high Ψ to low Ψ ;

5 cells increase in, volume / size / internal pressure ;

6 either
cell membranes are not strong enough to withstand increase in volume / pressure
or
red blood cells burst because they have no cell wall ;

7 between 0.04 and 0.14 (mol dm^–3) water potential gradient into cells, decreases / becomes less steep / AW ;

8 above 0.14 (mol dm^–3) / in high concentrations, water potential inside cells is the same or higher than the sodium chloride solution ;

9 at high concentration / ⩾ 0.14, water leaves cells / cells shrink / cells shrivel / cells show crenation ;

TIP: if a questions that comes with a graph says, describe, then step by step write down what the major trends are from start to end in a sequencial order. Explaining means say WHY each of the trends took place.

idea of shape needs to be in the answer for mp2 and mp4
Ignore: ‘substrate changes shape’

any three from:

1 active site is not (fully) complementary to substrate ;

2 active site, changes shape / moulds around, to fit the substrate ;
Accept: conformational change for shape change.

3 enzyme-substrate complex / ESC, forms ;

4 active site returns to original shape on release of product ;

5 Any other valid point:
e.g. change of shape (to give complementary fit) lowers activation energy / puts strain on bonds / AW
ref. to binding site / catalytic site (of active site)

Accept: peroxidase for enzyme.

any three from:

rate of reaction increases as substrate concentration increases to 0.33 – 0.35 mmol dm^-3
1 (up to 0.33 to 0.35 mmol dm^–3) some active sites are not occupied ;

2 more collisions between, enzyme / active site, and substrate molecules ;
Accept: more collisions lead to increase in formation of enzyme-substrate complexes.

rate of reaction remains constant above 0.33–0.35 mmol dm^–3
3 all active sites, occupied / saturated ;

4 correct reference to limiting factor for slope ; Accept: limits / limiting
e.g. at low substrate concentrations, substrate concentration limiting / enzyme concentration not limiting.

5 correct reference to limiting factor for plateau ; Accept: limits / limiting
e.g. at high substrate concentrations, enzyme concentration limiting / substrate concentration not limiting.

idea that determined V_max / maximum rate / ref. to 5.6 (μmol min^–1) ;
Accept: if V_max shown on graph

K_m is the substrate concentration at half the V_max ;

any three from:

in respiring tissue
1 (carbonic anhydrase) catalyses the reaction between water and carbon dioxide in, red (blood) cells / erythrocytes ;

2 to form carbonic acid, which dissociates to form HCO₃^– and H⁺ ;
Accept: from equation.
Accept: use of word ion(s).

3 HCO₃^–, moves / passes / diffuses (through membrane proteins from red blood cells) into plasma ;

4 (activity of enzyme) maintains (steep) concentration gradient for diffusion of carbon dioxide into red blood cells ;

5 idea that so helps to remove large quantities of carbon dioxide ;
in lungs

6 (carbonic anhydrase) converts HCO₃^– back into carbon dioxide so can be, excreted / removed / exhaled ;
if described earlier accept catalyses reverse reaction so carbon dioxide can be, excreted.

1 vector is female Anopheles (mosquito) ;

2 mosquito / (female) Anopheles, takes blood from infected person ;
Ignore: ‘bites’ alone

3 (vector / mosquito / Anopheles) inserts / AW, saliva / anticoagulant, with, pathogen / Plasmodium / parasite, into (blood of) uninfected person ;
Reject: if incorrect type of pathogen

B-lymphocytes / B-cells ;

fuse / combine / join / merge.
Reject: hybridise / bind / bond

hybridoma ;

any three from:
1 results are accurate (rely on monoclonal antibodies) ;
2 results are obtained quickly / diagnosis is quick ;
3 low cost / cheap ;
4 does not need highly trained medical professional / easy to read / easy to use / AW ;
5 does not need specialised equipment ;
6 easily, transported / distributed (in bulk) ;
7 can ensure appropriate treatment is given immediately malaria is confirmed ;
8 reduces unnecessary use of antimalarial drugs (just in case) ;
9 (a small area for S so) only a small sample of blood needed ;
10 Accept alternative wording: e.g. can identity different species of Plasmodium.
Ignore: ref. to line C and working correctly.

ignore: ref. to sickle cell anaemia

any five from:

habitat of Anopheles mosquito:
1 tropical / warm and humid / 
Alternative wording: climate / regions ;
Accept: in context of global warming (increases life span of Anopheles)

2 in areas where, Anopheles (mosquito) / vector, occurs ;
3 Anopheles mosquitoes only live in humid conditions ;
4 warm temperatures for, development / growth, of, parasite / Plasmodium (in vector / mosquito) ;
5 warm temperature for, development / growth, of mosquito larvae ;
6 mosquitoes require bodies of, still / AW, water for breeding ;
Accept: ponds, puddles, lakes, swamps

7 mosquitoes require places where there is sufficient rainfall ;
8 low altitude ;

resistance:
9 insecticide / repellent, resistance of mosquitoes ;
10 drug resistance of parasite ;

prevention:
11 in countries / areas, where, prevention / control measures, are not implemented by, governments / health authorities ;
12 further detail e.g. any example of a, prevention / control measure that is not used or not implemented fully ;

immunity:
13 immunity to malaria in human population (limits distribution) ;

people:
14 migration of infected people from areas (with high rates of malaria) ;
15 high rates of HIV infection ;

A – rough endoplasmic reticulum ;
E – Golgi, body / apparatus / complex ;

transport from, (R)ER / ribosomes, to Golgi ; Accept: from A to E.

Accept: transport from Golgi to, cell surface membrane / phagosome.
Accept: separate, acid hydrolases / enzymes, from, rest of cell.

phagocytosis / endocytosis ;

any three from:
1 break down / digest / destroy, bacteria / pathogen(s) ;

2 break down / digest / destroy, (worn out / defective), organelles / named organelle (in animal cell) ;
Accept: autophagy

3 catalyse / speed up, hydrolysis ;

4 any two named substrates ;
e.g. (any named) polysaccharides / proteins / (phospho)lipids / (named) nucleic acids

5 idea that recycle / reuse, biological molecules within cell ;

6 (macrophage / phagocyte) cut up to present antigen ;

moves / pump(s), hydrogen ions / protons, into the lysosome against concentration gradient ;
active transport / uses ATP / energy from respiration / ref to conformational change of carrier ;
Reject: if a ref to facilitated diffusion.

(a) Fig. 1.1 is a photomicrograph of a region of eukaryotic tissue. Some of the cells are in stages of mitosis.

Fig. 1.1

(i) Identify which stage of mitosis is shown in cell E and in cell F in Fig. 1.1. [2]

(ii) Microtubules are present within the cells that are in stages of mitosis, but these are not visible in Fig. 1.1.
State the function of microtubules in mitosis. [1]

(iii) State, with a reason, whether Fig. 1.1 shows a region of animal or plant tissue. [1]

(b) Semi‑conservative replication of DNA occurs during interphase, before mitosis begins.
Write the correct term in the spaces provided to complete each of statements A to D.

A The DNA double helix unwinds and is separated into two template strands when
…………………………………………………………….. bonds holding the two strands together
are broken.

B One of the template strands of DNA is copied in fragments. The enzyme
…………………………………………………………….. is required to join the fragments together
to form a continuous strand of DNA.

C Complementary DNA nucleotides are added to the template strands, catalysed by the
enzyme …………………………………………………………….. .

D …………………………………………………………….. are regions of repeating nucleotide
sequences at the ends of chromosomes that allow the continued replication of DNA,
without the loss of genes. [4]

[Total: 8]

Sugars are transported within phloem sieve tubes from a source, such as a mature leaf, to a young leaf, which acts as a sink. The young leaf also needs water and dissolved mineral ions, which arrive at the leaf within xylem vessels.

(a) As the young leaf matures, the quantity of sugar taken up by the leaf decreases to zero, but the need for water increases.
Suggest and explain why the quantity of sugar taken up by the developing leaf decreases to zero over time, but the need for water increases. [3]

(b) The features listed in Table 2.1 are present in one or more of the three cell types:
• companion cell
• phloem sieve tube element
• xylem vessel element.

Complete Table 2.1 using a tick (✓) if the feature is present and a cross (x) if the feature is
absent.

Table 2.1

[4]
[Total: 7]

In mammals, some cell signalling molecules are steroid (lipid) hormones. These hormones are transported in the bloodstream to reach capillary networks.

At a capillary network, hormones pass out of the blood into tissue fluid.

(a) Fig. 3.1 is a diagram of a capillary network.

Fig. 3.1
(i) Describe the differences between the blood arriving at the arterial end of the capillary
network and the tissue fluid surrounding the body cells. [3]

(ii) Not all the tissue fluid passes back into the blood capillaries to enter the bloodstream.
Some of the tissue fluid drains into blind‑ended vessels, such as vessel X shown in
Fig. 3.1.
Name the fluid that is formed in vessel X. [1]

Hormone S is a steroid hormone involved in cell signalling.
Fig. 3.2 shows the sequence of events that occurs when hormone S enters a target cell.

Fig. 3.2
(b) Explain why hormone S, shown in Fig. 3.2, does not need to pass through a transport protein to enter the cytoplasm of the target cell. [2]

(c) The target cell can respond to other cell signalling molecules in addition to hormone S. The cell has receptors in the cell surface membrane, in the cytoplasm and in the nucleus.
Explain why hormone S binds only with receptor R in the cytoplasm and not with the other receptors shown in Fig. 3.2. [1]

(d) The hormone‑receptor complex shown in Fig. 3.2 enters the nucleus and binds to DNA. This switches on a gene coding for a polypeptide that is synthesised in the cytoplasm.
(i) Name the structure through which the hormone‑receptor complex enters the nucleus. [1]

(ii) Name the processes occurring at B and C. [2]

(iii) Name structure G. [1]

(e) Cell signalling by hormone S results in the production of a functioning globular protein
molecule composed of three identical polypeptide chains.
After the synthesis of these polypeptides, changes need to occur to form the functioning
globular protein molecule. Outline the changes that need to occur to form the functioning globular protein molecule. [4]

[Total: 15]

A person who is exposed to tobacco smoke is at greater risk of lung cancer and chronic obstructive pulmonary disease (COPD).
Many people with COPD have both chronic bronchitis and emphysema. These diseases cause changes in the gas exchange system. For example, changes occur in the total lung surface area to volume ratio (SA:V).

(a) Tar in tobacco smoke has a number of effects on the cells lining the gas exchange system.
State the main effects of tar on the cells lining the gas exchange system that are related to lung cancer and to chronic bronchitis. [3]

A student investigated the effect of SA:V on diffusion.
Agar was prepared with Universal Indicator solution and sodium hydroxide solution. The agar was coloured blue.
Three cubes, A, B, and C, were cut from a solid block of blue agar. Each cube was a different size.
Universal Indicator solution changes to a red colour in the presence of acid.
The student prepared Table 4.1 to show the sizes and SA:V of each cube.

Table 4.1

Complete Table 4.1 by:
• writing the correct units for volume
• calculating the total surface area, and volume, of cube C. [2]

(c) Cubes A, B and C were placed in a small beaker. At time 0 seconds, dilute hydrochloric acid was added to the beaker to cover the cubes.
The student timed how long it took for each cube to change colour completely.
Complete Fig. 4.1 to show the results that were obtained.

cube ……………………. → cube ……………………. → cube ……………………. [1]
Fig. 4.1

(d) Some people with emphysema may be offered lung volume reduction surgery (LVRS), in
which diseased lung tissue is surgically removed.
One expected outcome of the surgery is an improvement in total lung surface area to volume ratio (total lung SA:V).
Suggest why there is an improvement in total lung SA:V after the surgery has been carried
out. [2]

(e) In humans, blood that becomes oxygenated in the lungs reaches body tissues without coming into contact with blood that is deoxygenated.
Explain how the blood that becomes oxygenated in the lungs is kept separate from blood that is deoxygenated. [2]

[Total: 10]

Fig. 5.1 is a transmission electron micrograph showing parts of two plant cells. The function of the middle lamella is cell‑to‑cell adhesion. The middle lamella is composed of a polysaccharide known as pectin.
Pectin interacts with the polysaccharides cellulose and hemicellulose in the cell walls of the plant cells so that the cell walls are held close together, as shown in Fig. 5.1.

Fig. 5.1

(a) Cell structure X in Fig. 5.1 is a cytoplasmic channel with strands of cytoplasm passing through the cell walls of the two cells.
Name cell structure X and state one function of this cell structure. [2]

(b) Researchers have discovered that pectin is synthesised within the Golgi body. Golgi vesicles containing pectin are moved to the cell surface membrane for release.
(i) Suggest why researchers would not have investigated ribosomes as being the possible
location for the synthesis of pectin. [1]

(ii) Name the mechanism that is used to transport pectin out of the cell. [1]

Juices that are extracted commercially from fruits can be made less cloudy by the breakdown of the cell wall using the enzymes cellulase, pectinase and xylanase:
• cellulase hydrolyses cellulose
• pectinase hydrolyses pectin
• xylanase hydrolyses hemicellulose.

(c) Fig. 5.2 is a graph showing the effect of cellulose concentration on the activity of cellulase, which is used in making fruit juice less cloudy.

Fig. 5.2

Describe and explain the curve shown in Fig. 5.2. [3]

(d) Ultrasound is one possible method that can be used to destroy microorganisms that
contaminate fruit juices. Ultrasound is the term given to sound waves that are out of the range of human hearing.
An investigation was carried out into the effect of ultrasound on the activity of cellulase,
pectinase and xylanase used in fruit juice manufacture.

For each enzyme, the effect of ultrasound was compared with no ultrasound on the:
• maximum rate of reaction (Vmax)
• Michaelis‑Menten constant (Km)
• catalytic efficiency (Vmax/Km)

Table 5.1 summarises the results. A higher Vmax/Km indicates a higher catalytic efficiency.
Table 5.1

(i) In terms of changes in the interaction between enzyme and substrate when ultrasound
is used, suggest explanations for the lower Km for pectinase and the higher Vmax for
xylanase, as shown in Table 5.1. [3]

(ii) Explain whether the data shown in Table 5.1 supports the recommendation that
ultrasound can be used in the manufacture of fruit juices. [1]

[Total: 11]

The diseases myasthenia gravis (MG) and HIV/AIDS both involve disorders of the immune system.
(a) The cause of MG involves a response by B‑lymphocytes.
Explain why MG is called a disorder of the immune system. [2]

(b) Studies have indicated that T‑lymphocytes are involved in stimulating the B‑lymphocyte
response that causes MG.
Research has been carried out on a vaccine that will provide a person with active immunity against these T‑lymphocytes and B‑lymphocytes.
Suggest and explain how this vaccine will provide a person with active immunity against the T‑lymphocytes and B‑lymphocytes responsible for causing MG. [3]

(c) Many people who are living with HIV (infected with HIV) develop tuberculosis (TB). If a person does not have any symptoms of TB, one preventive measure is to prescribe antibiotics. This reduces the overall number of cases of TB and deaths from TB.
State the disadvantage of prescribing antibiotics as a preventive measure against TB. [1]

(d) Fig. 6.1 is a summary of some of the statistics published by UNAIDS (Joint United Nations Programme on HIV and AIDS) about HIV and HIV/AIDS for the year 2017.
The figures shown in Fig. 6.1 for 2017 are estimated.

Fig. 6.1

One other statistic published by UNAIDS indicated that, in 2017, only 75% of the estimated
36.9 million people living with HIV knew that they had been infected with the virus.
With reference to the information in Fig. 6.1, discuss the importance of this statistic. [3]

[Total: 9]

(a) Fig. 5.1 shows four types of cell that can be seen in a prepared slide of blood taken from a mammal.

Identify the four types of cell shown in Fig. 5.1.
Write the name of the cell type on the answer line provided by each cell in Fig. 5.1. [4]

allow one mark if monocyte and neutrophil both labelled as phagocytes and no other cell is labelled phagocyte.
allow phagocyte for either the monocyte or neutrophil if the other cell correct.

(b) During systole and diastole of the cardiac cycle, changes in blood pressure occur in the four chambers of the heart. Fig. 5.2 shows changes in the blood pressure in the left side of the heart and the aorta during one cardiac cycle.

With reference to the blood pressure changes shown in Fig. 5.2:
• state the maximum blood pressure reached in the left ventricle ………. kPa
• state the time at which the bicuspid (left atrioventricular) valve closes. ……. s [2]

(c) Chronic obstructive pulmonary disease (COPD) can cause a condition known as pulmonary hypertension. This involves an increase in systolic blood pressure in the right ventricle and in the pulmonary arteries.
Fig. 5.3 is a summary of some of the events that can result from COPD.

Figure 5.3

(i) Chronic alveolar hypoxia describes a condition where the partial pressure of oxygen in the gas exchange regions of the lungs is always lower than normal. Explain how COPD leads to chronic alveolar hypoxia. [3]

(ii) Explain how the loss of alveolar capillaries affects the functioning of the lungs in a person with COPD. [2]