AS & A Level Biology - 9700 Paper 2 2020 Winter Zone 2

Fig. 1.1 is a diagram drawn from a photomicrograph of a transverse section through part of a leaf.
The arrows in Fig. 1.1 show the movement of water through the cells of the leaf after it has left the xylem.


(a) Water from the xylem can enter cell A and then moves to cells B and C without crossing their cell walls.
The cell structures through which water passes from cell A to cell B are not visible in Fig. 1.1.
(i) Name the cell structures through which water passes from cell A to cell B without crossing their cell walls.
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(ii) Explain what causes water to move from cell B to cell C.
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(iii) Name the pathway taken by water between cell A and cell C.
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(b) Most of the water that arrives at the leaf passes to the external atmosphere.
With reference to Fig. 1.1, describe and explain the sequence of events occurring between point P and point Q.
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(c) The actual diameter of cell D in Fig. 1.1 along the length X–Y is 25μm.
Calculate the magnification of the image.
Write down the formula used to make your calculation. Show your working.
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formula
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answer = x ........................................................... [3]

[Total: 10]

(a) Susceptible strains of \textit{M. tuberculosis} will be killed using any one of the antibiotics listed in Table 2.1. However, combination treatment is preferred as it is one method that can be used to reduce the impact to society of antibiotic resistance.

With reference to Table 2.1, explain how combination treatment for TB can help to reduce the impact of antibiotic resistance compared to single antibiotic treatment.

[Table 2.1]

Rifampicin (R) - 6 months - enters bacterial cells and inhibits protein synthesis
Isoniazid (H) - 6 months - prevents the synthesis of cell wall components known as mycolic acids
Ethambutol (E) - first two months - prevents mycolic acids from being added to the cell wall
Pyrazinamide (Z) - first two months - prevents the synthesis of fatty acids

[4]

(b) (i) RNA polymerase catalyses the formation of messenger RNA (mRNA) from DNA.
State the term for this process.

[1]

(ii) During the formation of RNA, a number of events occur that involve the action of RNA polymerase.
Suggest ways in which rifampicin can affect the activity of RNA polymerase.

[3]

(c) RNA polymerase is composed of five different polypeptides. Gene \textit{rpoB} codes for one of these polypeptides known as the β-subunit.
One or more mutations in a specific region of \textit{rpoB} result in strains of \textit{M. tuberculosis} that are resistant to rifampicin. In these strains, mutations often occur in two DNA triplets in this region, in positions 526 and 531.
Table 2.2 summarises the results of an investigation into seven rifampicin-resistant strains, \textbf{A} to \textbf{G}, that have amino acid changes for positions 526 and 531.

Table 2.2 includes:
- the change in the \textbf{mRNA codon} for position 526 or position 531
- the amino acid change that has occurred as a result of the mutation
- the minimum concentration of rifampicin required to inhibit growth of the bacterial strain (MIC)
- the number of \textbf{other mutations} occurring within the specific region of \textit{rpoB}.

[Table 2.2]

| strain | codon involved | mRNA codon change | amino acid change | MIC / µg cm^-3 | other mutations in the specific region |
|--------|----------------|-------------------|------------------|----------------------------|------------------------------------|
| \textbf{A} | 526 | CAC → UAC | His → Tyr | ≤50 | 0 |
| \textbf{B} | 526 | CAC → AAC | His → Asn | ≥100 | 1 |
| \textbf{C} | 526 | CAC → CGC | His → Arg | ≈50–75 | 2 |
| \textbf{D} | 526 | CAC → CGC | His → Arg | >100 | 3 |
| \textbf{E} | 526 | CAC → CGC | His → Arg | ≈50 | 3 |
| \textbf{F} | 526 | CAC → UUC | His → .................. | >100 | 3 |
| 531 | UCG → UUG | Ser → Leu | | |
| \textbf{G} | 526 | CAC → UAC | His → .................. | >100 | 3 |
| 531 | UCG → UUC | Ser → Phe | | |

(i) Complete Table 2.2 to show the amino acid changes that have occurred in strains \textbf{F} and \textbf{G}.
[1]
(ii) With reference to Table 2.2, list the strains of \textit{M. tuberculosis} that show the greatest resistance to rifampicin.
[1]
(iii) Suggest reasons to explain why strains \textbf{C}, \textbf{D} and \textbf{E} show:
- resistance to rifampicin
- different levels of resistances to rifampicin.

[3]

Fig. 3.1 is a photomicrograph of a section through lung tissue.



Fig. 3.1

(a) State the feature visible in Fig. 3.1 that identifies the structure in the centre of the image as the bronchus and list other visible features that help to confirm this identification.

feature to identify the bronchus ..................................................................................................................
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other features .................................................................................................................................................
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[3]

(b) Identify the structure labelled J in Fig. 3.1.
State the evidence visible in Fig. 3.1 that supports your answer.

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[2]

(c) The ciliated epithelium labelled in Fig. 3.1 consists of goblet cells and ciliated epithelial cells.
Outline how goblet cells and cilia work together to maintain healthy lung tissue.

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[2]

[Total: 7]

In the immune system, a plasma cell develops from an activated B-lymphocyte. Mature plasma cells synthesise and secrete antibody molecules.
(a) Fig. 4.1 is a diagram of a transmission electron micrograph of a plasma cell.
[Image_Fig_4.1]
The plasma cell can be seen in greater detail using an electron microscope compared with using a light microscope.
(i) Describe the extra detail of the nucleus that can be seen using an electron microscope.
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[3]
(ii) Explain why cell structures, such as ribosomes and the rough and smooth endoplasmic reticulum, cannot be seen using a light microscope.
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[2]

(b) The transition from the activated B-lymphocyte to the fully mature plasma cell requires a number of mitotic cell cycles to occur. This process, which is known as clonal expansion, results in a large number of genetically identical plasma cells.
Fig. 4.2 describes events, A to F, that occur during the mitotic cell cycle of the B-lymphocyte.
[Image_Fig_4.2]
Table 4.1 lists the stages occurring during one cell cycle of the B-lymphocyte. These stages are not in the correct order.
[Table_4.1]
Complete Table 4.1 by writing the letter of the event described in Fig. 4.2 that correctly matches the stage of the cell cycle listed.
Leave a blank space if there is no matching description for the stage in the list. Use each letter once only.
One of the letters in Table 4.1 has already been added for you.
[5]

(c) Clonal expansion also results in the production of memory B-lymphocytes.
Explain the importance of clonal expansion and the production of memory B-lymphocytes in providing protection for a person against an infectious disease.
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[3]
(d) Myasthenia gravis is an example of a disease where the immune system fails to distinguish between self and non-self.
Explain what is meant by this statement.
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[2]

(a) Name reducing sugar X in Fig. 5.1.
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[1]

(b) In the absence of sucrose phosphorylase as a catalyst, the reaction shown in Fig. 5.1 would take too long to occur to allow the bacterial cell to function efficiently.
Explain why the reaction shown in Fig. 5.1 proceeds at a much faster rate in the presence of the enzyme.
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[2]

(c) An enzyme that catalyses a reaction of commercial interest needs to be investigated to see if it is suitable for use in industry.
For example:
• immobilised enzymes may be used as they have a longer shelf-life than the enzyme free in solution
• many industrial reactions are carried out at higher temperatures to minimise contamination of products by microorganisms.
Fig. 5.2 shows the results of an investigation to compare the activity of sucrose phosphorylase free in solution (free enzyme) with immobilised sucrose phosphorylase (immobilised enzyme) at different pHs.
Fig. 5.3 shows the activity of the free enzyme and immobilised enzyme at different temperatures.

[Images: Fig. 5.1, Fig. 5.2, Fig. 5.3]
With reference to the results shown in Fig. 5.2 and Fig. 5.3, discuss which sucrose phosphorylase enzyme, free or immobilised, is better for use in industrial reactions.
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[4]

(a) Fig. 6.1 shows an oxygen dissociation curve for adult human haemoglobin.

An increase in the partial pressure of carbon dioxide ($pCO_2$) in respiring tissue causes the Bohr effect.
(i) Sketch on Fig. 6.1 to show how the Bohr effect changes the oxygen dissociation curve. [1]
(ii) Explain how an increase in $pCO_2$ produces the Bohr effect and state the benefit of this effect for the tissue. [3]
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(b) Carbon dioxide ($CO_2$) is transported across the cell surface membrane of the red blood cell using a different mechanism to the transport of hydrogen carbonate ions ($HCO_3^-$).
Name the different mechanisms of transport used for $CO_2$ and for $HCO_3^-$ and explain why they are transported across the membrane by different mechanisms.
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explanation ....................................................................................................................................................
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