AS & A Level Biology - 9700 Paper 2 2023 Winter Zone 2

During interphase and mitosis of the cell cycle, the chromosomes within a cell go through a number of changes. Each chromosome is composed of DNA complexed with proteins.

(a) In interphase, individual chromosomes are too diffuse (long and thin) to be visible using a microscope. In this stage, the chromosomal material is known as chromatin.

State the term used to describe the proteins that are complexed with DNA and form part of chromatin.

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(b) When viewed through a microscope, a chromosome is most clearly visible during the metaphase stage of mitosis.

Complete Fig. 1.1 to produce a labelled diagram of the metaphase stage of mitosis in an animal cell with two chromosomes.



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(c) Outline the changes that occur to the structure and behaviour of a chromosome:
• from the start of the S phase to the end of interphase
• during prophase of mitosis.

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Keratin is the structural protein in feathers of birds. Keratin polypeptides are composed of a high proportion of cysteine amino acids, which have sulfur-containing R groups.

Keratin polypeptides form filaments. The two main types of keratin in feathers are $\alpha$-keratin, which consists of many $\alpha$-helices, and $\beta$-keratin, consisting of many $\beta$-pleated sheets.

(a) Keratin can be classified as $\alpha$-keratin or $\beta$-keratin based on a study of protein structure.
Suggest the level of protein structure used to classify a protein as $\alpha$-keratin or $\beta$-keratin.
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(b) Protease enzymes hydrolyse proteins. Many proteases are able to hydrolyse more than one type of protein.
Suggest why it is possible for a protease to act on different types of protein.
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(c) Proteases known as keratinases vary in the extent to which they can hydrolyse keratin.
Feathers are not easily degraded (broken down) because keratin is a very stable protein.
Suggest features of keratin structure that contribute to its stability.
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Keratinaes are used to degrade the large quantities of waste feathers from chickens and turkeys that are processed in the food industry. The products of feather degradation can be used in animal feed.
Scientists investigated whether three different keratinases, K12, A22 and P3, were suitable as industrial enzymes. These enzymes were extracted from three different soil bacteria.
The effects of temperature and pH on the activity of each keratinase were investigated.
The results are shown in Fig. 2.1 and Fig. 2.2.



(d) To degrade feather waste from industry, it is an advantage to use keratinases that show at least 60% relative activity in conditions where temperature and pH can vary widely.
Table 2.1 shows, for each keratinase, the working range of temperature and pH where at least 60% relative activity is obtained.
Use Fig. 2.1 and Fig. 2.2 to complete Table 2.1 and use the completed table to:
• name the keratinase that has the widest working range of temperature
• name the keratinase that has the widest working range of pH

Table 2.1
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(a) The hepatic artery branches from the main artery that transports blood from the heart. Name the main artery that transports blood to the hepatic artery. .......................................................................................................................................................... [1]

(b) Blood arriving at the liver enters specialised blood vessels known as sinusoids. Fig. 3.1 is a diagram of part of a sinusoid and surrounding hepatocytes. A second type of cell found in the liver, a Kupffer cell, is also shown. Kupffer cells are phagocytic cells of the immune system. (i) Suggest one advantage of having large gaps between the endothelial cells of the sinusoid, as shown in Fig. 3.1. ................................................................................................................................................................................................................................................................................ [1] (ii) In addition to removing bacteria present in the blood inside the sinusoid, Kupffer cells are also able to remove old or damaged red blood cells. Describe the mode of action of a Kupffer cell in removing and breaking down a damaged red blood cell. ................................................................................................................................................................................................................................................................................................................................................................................................................................................. [4]

(c) Fig. 3.2 is a transmission electron micrograph of part of a hepatocyte showing some cell structures. The peroxisome shown in Fig. 3.2 is a spherical organelle bound by a single membrane. It carries out a variety of enzyme-catalysed metabolic reactions, including detoxification. Some of these reactions require oxygen. (i) Describe the evidence visible in Fig. 3.2, apart from the presence of a peroxisome, that indicates some of the functions of a hepatocyte. Add labels to Fig. 3.2 to identify the location of any cell structures, if not already labelled, that are part of your evidence. ................................................................................................................................................................................................................................................................................................................................................................................................................................................. [3] (ii) The mitochondria in Fig. 3.2 are larger than the peroxisome. State one other difference, visible in Fig. 3.2, between a peroxisome and a mitochondrion. ........................................................................................................................................................................................................ [1] (iii) Some of the enzymes used within mitochondria can be synthesised by the organelle. Peroxisomes cannot synthesise any of the enzymes that they contain. Suggest why a mitochondrion can synthesise enzymes, but a peroxisome cannot synthesise enzymes. ................................................................................................................................................................................................................................................................................................................................................................................................................................................. [2] (iv) One of the enzymes present in peroxisomes is catalase. This enzyme catalyses the breakdown of hydrogen peroxide to harmless products. Suggest why it is useful to the cell for this reaction to take place within peroxisomes. ................................................................................................................................................................................................................................................................................................................................................................................................................................................. [2]

The alveoli of the lungs are the main gas exchange surface in humans. (a) Explain how blood flow through the alveolar capillaries helps to maintain steep diffusion gradients for gas exchange. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... [2]

(b) Ventilation of the lungs is the process of inhalation and exhalation. Ventilation helps to maintain steep diffusion gradients. Explain the role of elastic fibres in the alveolar wall during ventilation. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... [2]

(c) Some cells in the alveolar wall are specialised to secrete surfactant to prevent collapse of the alveoli at the end of exhalation. In these cells, surfactant is stored in membrane-bound organelles known as lamellar bodies. Surfactant is a mixture of lipids, mainly phospholipids, and some proteins. A protein known as ATP-binding cassette transporter A3 (ABCA3) is needed to move surfactant phospholipids into lamellar bodies from the surrounding cytosol (fluid part of cytoplasm). Suggest and explain the features of protein ABCA3 that make it suited to its function. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... [3]

(d) The gene ABCA3 codes for protein ABCA3. The gene is 80 kb (80 000 base pairs) long and is composed of introns and exons. Protein ABCA3 is composed of 1704 amino acids. (i) Fig. 4.1 shows the flow of genetic information in the production of ABCA3. gene ABCA3 → X ............................. ↓ primary transcript RNA ↓ messenger RNA (mRNA) → Y ............................. ↓ polypeptide ABCA3 ↓ protein ABCA3 Fig. 4.1 Complete Fig. 4.1 to name the processes occurring at X and Y. [2] (ii) A triplet of bases codes for one amino acid. This fact only partly explains how the activity of gene ABCA3, which is 80 kb long, can result in the protein ABCA3, which is only 1704 amino acids long. Suggest other reasons to explain the difference in the number of base pairs in gene ABCA3 compared with the number of amino acids in protein ABCA3. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... [3]

Tuberculosis (TB) is an infectious disease caused by a bacterium. In the majority of people, only the lungs are affected.

In most cases, the transmission of TB from an infected person to an uninfected person involves Mycobacterium tuberculosis. A different species of bacterium is involved in the transmission of TB from cattle, such as dairy cows, to humans.

(a) Name the species of bacterium causing the transmission of TB from dairy cows to humans.
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(b) In some areas, cattle cannot be regularly tested or treated for TB. In these areas, milk and dairy products from infected dairy cattle may enter the human food chain.
Outline a control measure that can be taken to protect people that consume milk and dairy products from these infected cattle.
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(c) Cattle are not usually affected by M. tuberculosis, but the pathogen can cause disease in other animals. A few cases of transmission of TB from people to animals have been reported.
Explain the most likely mode of transmission of TB from an infected person to an animal.
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(d) In most people, the response of the immune system to the infection of lung tissue by M. tuberculosis can prevent the spread of the bacterium to other organs of the body. The bacterium is contained in the lungs in a dormant state. This is known as latent TB.
Outline the treatment that is used to kill M. tuberculosis in latent TB infections.
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(e) M. tuberculosis can spread in the blood and lymph to other organs in the body.
In very rare cases, a disease known as mycotic aneurysm can be caused by infection of the arterial wall, particularly in elastic arteries. The damage caused by the pathogen can lead to a rupture (bursting) of the artery.
With reference to the structure of the wall of elastic arteries, suggest how damage caused by M. tuberculosis infection can lead to the rupture of the artery.
You may draw a diagram if you wish.
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Water that is absorbed from the soil solution by the roots of a plant enters xylem vessels and is transported to the leaves and buds.

Fig. 6.1 shows four important requirements for the efficient transport of water from the roots to the leaves of a plant.



(a) Name the specialised cells that are arranged end to end to form xylem vessels.
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(b) Explain how tension is created in the xylem vessels.
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(c) Fig. 6.1 highlights how the structure of xylem vessels must be related to their function. This means that during the development of xylem vessels changes need to occur to the cells forming the vessels.

(i) The walls of the cells forming the xylem vessel walls become lignified during development. Explain how this feature is important for the efficient transport of water.
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(ii) During the development of xylem vessels, the end walls of the cells forming the vessels break down. This contributes to minimising resistance to the flow of water. Describe one other main change that needs to occur to these cells so that their structure becomes suited to their function.
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