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

Picornaviruses are small viruses that are 30 nm in diameter. Picornaviruses are able to enter the cells of mammals and birds and can replicate within these cells.

Fig. 1.1 shows the entry of a picornavirus into its host cell.

[Image]

Fig. 1.1

(a) State the key features of a virus, such as picornavirus.
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(b) State, with reasons, whether a picornavirus can be seen using the light microscope.
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(c) With reference to Fig. 1.1, describe how the picornavirus enters the host cell.
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In a healthy mammalian heart, contraction of the four chambers is coordinated by the action of the sinoatrial node (SAN) and atrioventricular node (AVN).

(a) After the atria fill with blood, atrial systole (contraction) occurs.

State the events that occur to initiate and cause atrial systole.

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(b) State and explain how the structure of the heart allows the atria to contract before the ventricles.

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(c) Fig. 2.1 shows blood pressure changes that occur in the left ventricle, left atrium and aorta during one cardiac cycle.

E, F, G and H are the points at which a valve opens or closes as a result of blood pressure changes.



(i) For each of the points E, F, G and H on Fig. 2.1, name the valve concerned and state whether the valve opens or closes.

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(ii) Explain how Fig. 2.1 provides evidence that the wall of the left atrium has a different thickness to the wall of the left ventricle.

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The Bacillus Calmette-Guérin (BCG) vaccine is the only vaccine used to provide protection against the infectious bacterial disease tuberculosis (TB). Most countries of the world have a BCG vaccination programme.
(a) TB is most commonly transmitted from person to person by aerosol infection. The causative organism is present in airborne droplets.
Name the species of causative organism of TB commonly passed from person to person by aerosol infection.
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(b) In general, the countries that do not have a BCG vaccination programme are high-income countries that have a low number of cases of TB. In most of these countries, the vaccine is given only to babies and children at high risk of developing TB.
Suggest one reason why a child in a country with a low number of cases of the disease could be at a high risk of developing TB.
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(c) Countries are classified by the World Bank into one of four income groups.
Table 3.1 shows the estimated incidence of TB for 2012 to 2016 for these income groups.
The incidence represents the number of new cases of TB occurring per 100 000 people in one year. The new cases include the number of cases that have occurred again after a period of recovery (relapse TB).

Table 3.1
[Table_1]

Describe the patterns and trends shown in Table 3.1.
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(d) There is evidence that the BCG vaccine has also provided protection against the disease leprosy.
Leprosy is caused by a bacterium that is closely related to the bacteria that cause TB.
Suggest why the BCG vaccine can also provide protection against leprosy.
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(e) A baby can gain artificial active immunity to TB after having the BCG vaccine. A baby can also gain natural passive immunity to TB.
State the differences between artificial active immunity and natural passive immunity.
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Collagen is a major component of the cartilage found in some of the structures of the human gas exchange system. Cells that synthesise and secrete the components of cartilage are known as chondrocytes.

(a) Fig. 4.1 is a transmission electron micrograph of a chondrocyte.



With reference to Fig. 4.1, explain two features of the chondrocyte that show how the cell is adapted to its function.
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(b) (i) Describe the distribution of cartilage in the human gas exchange system.
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(ii) Outline the function of cartilage in the human gas exchange system.
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(c) Fig. 4.2 shows part of the primary structure of a collagen polypeptide.



(i) Name the type of covalent bond formed between the amino acids shown in Fig. 4.2.
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(ii) Fig. 4.3 shows the molecular structure of the amino acid glycine (gly).


With reference to Fig. 4.2 and Fig. 4.3 and the function of collagen, explain how the structure of a collagen polypeptide makes it suitable to form a collagen molecule.
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In most plants, sucrose is the main sugar that is transported from sources to sinks.

(a) In the source, sucrose is transferred from a mesophyll cell to a phloem sieve tube through a companion cell.

Describe \textbf{and} explain how the transfer of sucrose into a phloem sieve tube from a companion cell can lead to the transport of the sugar to a sink.
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(b) Fig. 5.1 shows part of one pathway that is used in plant cells to synthesise sucrose. The enzyme sucrose synthase catalyses the transfer of glucose from UDPG (uridine diphosphate glucose) to fructose.


Fig. 5.1
The structure of UDPG is shown in Fig. 5.2.


Fig. 5.2
(i) Name the type of bond formed when sucrose is synthesised.
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(ii) Explain why UDP can be described as a phosphorylated nucleotide.
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(iii) Sucrose synthase acts by using an induced fit mechanism rather than a lock and key mechanism.

With reference to sucrose synthase and the \textbf{synthesis} of sucrose, outline the difference between the induced fit mechanism and lock and key mechanism of enzyme action.
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(c) UDPG is used in some algae (photosynthetic protoctists) to synthesise a storage compound known as floridean starch.

The molecular structure of floridean starch has been described as an intermediate between amylopectin and glycogen, with little or no amylose.
Describe the molecular structure of floridean starch by completing the passage.

Floridean starch is a polysaccharide composed of .............................................. monomers.
The monomers are joined by .............................................. and .............................................. linkages,
to give a branching structure that is less highly branched than ...................................................... . [4]

The mitotic cell cycle in dividing cells is very carefully controlled.

(a) Complete Table 6.1 to show the correct order of stages in the mitotic cell cycle.

Some of the stages have been completed for you.

[Table]
Table 6.1
| stage of cell cycle |
|-----------------------------|
| G$_1$ phase |
| ................................... |
| ................................... |
| ................................... |
| ................................... |
| ................................... |
| telophase |
| cytokinesis |


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At various points during the mitotic cell cycle, checks are made. A cell goes through cell death (apoptosis) if errors occur that cannot be repaired. This makes sure that the daughter cells produced are genetically identical to each other and to the original cell.

Drugs have been developed that can inhibit the mitotic cell cycle and cause the cell to carry out apoptosis. These drugs are used in the treatment of cancer.

(b) Vincristine and 5-fluorouracil are chemical compounds that act as cell cycle inhibitors and can lead to apoptosis.

Vincristine binds to spindle microtubules and prevents the spindle from carrying out its function.
- 5-fluorouracil prevents the synthesis of thymine nucleotides.

Complete Table 6.2 to show which event in the cell cycle will occur when Vincristine or 5-fluorouracil are added to healthy dividing cells at the start of the interphase stage of the cell cycle.

Place a tick (✓) if the event will occur or a cross (✗) if the event will not occur.
All boxes in the table should be completed.

[Table]
Table 6.2
| event in cell cycle | S-phase completes | cell enters prophase of mitosis | chromosomes line up at spindle equator | sister chromatids move towards opposite poles |
|---------------------------------------|-------------------|-------------------------------|----------------------------------------|---------------------------------------------|
| compound | | | | |
| Vincristine | | | | |
| 5-fluorouracil | | | | |

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(c) Vincristine has been used in the treatment of certain types of leukaemia.

Fig. 6.1 is a photomicrograph of a blood smear of a person with one form of leukaemia, which affects lymphocytes.


Fig. 6.1
Describe how:

- the blood smear shown in Fig. 6.1 differs from a blood smear of a healthy person
- the lymphocytes in a person with leukaemia, such as those shown in Fig. 6.1, differ from those of a healthy person.

blood smear differences ...................................
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lymphocyte differences ...................................
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