2023 Cambridge IAS-Level Biology (9700) 模擬試題連答案詳解

The Michaelis-Menten constant (\(K_m\)) is the substrate concentration at which the reaction rate is half of its maximum velocity (\(V_{\max}\)). A lower \(K_m\) value indicates a higher affinity of the enzyme for that substrate, because less substrate is needed to occupy half of the active sites. Since \(5.0 \times 10^{-5} \text{ mol dm}^{-3}\) (for Y) is smaller than \(2.0 \times 10^{-3} \text{ mol dm}^{-3}\) (for X), the enzyme has a higher affinity for substrate Y than for substrate X.

1 mark for identifying that a lower \(K_m\) corresponds to a higher affinity, making B the correct choice.

Eukaryotic plant cells contain chloroplasts and mitochondria, both of which contain circular DNA and 70S ribosomes. Prokaryotic cells also contain circular DNA and 70S ribosomes. Both cell types possess a cell surface membrane. Linear DNA and 80S ribosomes are characteristic of the eukaryotic cytoplasm, but are not found in prokaryotes.

1 mark for recognizing that circular DNA, 70S ribosomes, and a cell surface membrane are features shared by both prokaryotes and organelles within eukaryotic plant cells.

In metaphase (Cell 1), chromosomes align individually along the metaphase plate (equator). In anaphase (Cell 2), the centromeres divide and the sister chromatids are pulled apart toward opposite poles by the spindle fibres.

1 mark for identifying Cell 1 as metaphase and Cell 2 as anaphase.

Capillaries have walls that are extremely thin—consisting of only a single layer of endothelial cells (tunica intima) on a basement membrane. This minimizes diffusion distance. Arterioles have a tunica media containing smooth muscle and elastic fibres, making their walls much thicker. Capillaries do not have valves, and their lumen is very narrow (only wide enough for red blood cells to pass through in single file).

1 mark for identifying the single-layered endothelial wall as the unique functional capillary feature.

The mRNA codon is 5'-GUC-3'. The complementary DNA template strand must run antiparallel and have complementary bases, which is 3'-CAG-5'. The tRNA anticodon must also pair antiparallelly with the mRNA codon, meaning its 3' to 5' sequence is complementary to the mRNA's 5' to 3' sequence. Thus, the tRNA anticodon is 3'-CAG-5' (or 5'-GAC-3' if written 5' to 3'). Option B correctly lists 3'-CAG-5' for both.

1 mark for correctly matching both the tRNA anticodon (3'-CAG-5') and the DNA template triplet (3'-CAG-5') based on complementary base pairing and antiparallel directionality.

Xylem vessel elements are dead cells at maturity; they have no cytoplasm, no nucleus, and their walls are heavily thickened with waterproof lignin. Phloem sieve tube elements are living cells containing a thin layer of peripheral cytoplasm and a cell surface membrane, and their walls are made of cellulose (not lignified).

1 mark for identifying that lignified walls and the complete absence of cytoplasm are features of mature xylem but not mature phloem sieve tubes.

The trachea contains C-shaped rings of cartilage, goblet cells in the ciliated epithelium, and smooth muscle. Bronchioles lack cartilage and goblet cells, but still contain smooth muscle to control the diameter of the airway.

1 mark for identifying the correct distribution of these three tissue types in the trachea and bronchioles.

T-lymphocytes are responsible for cell-mediated immunity (destroying infected host cells or activating other immune cells), whereas B-lymphocytes differentiate into plasma cells that secrete antibodies, which is the humoral immune response. T-lymphocytes mature in the thymus (B-lymphocytes mature in the bone marrow), and both cell types have highly specific antigen receptors.

1 mark for identifying the difference in the type of immunity (cell-mediated vs humoral) between T-lymphocytes and B-lymphocytes.

Lysosomes are bounded by a single membrane and contain hydrolytic enzymes (such as proteases, nucleases, and lipases) used to break down biological molecules. They are present in animal cells. The nucleolus is not membrane-bound (no membrane). Centrioles are not membrane-bound. The rough endoplasmic reticulum is a single membrane-bound organelle.

Award 1 mark for selecting option A, which correctly identifies that a lysosome is single-membrane bound, contains hydrolytic enzymes, and is present in animal cells.

Collagen is a fibrous protein made of three polypeptide chains wound together to form a tight triple helix. Haemoglobin is a quaternary globular protein composed of four polypeptide chains (two alpha and two beta chains), each containing a prosthetic haem group with an iron ion. Collagen does not contain a prosthetic group and is insoluble, while haemoglobin is soluble.

Award 1 mark for identifying option A as the correct comparison between collagen and haemoglobin.

Cholesterol acts as a fluidity buffer in cell membranes. At low temperatures, it prevents the fatty acid tails of phospholipids from packing too closely together, which prevents the membrane from becoming too rigid. The phospholipid bilayer (not glycoprotein) acts as a barrier to polar molecules. Glycoproteins/glycolipids (not phospholipids) act as receptors. Carbohydrate chains of glycoproteins/glycolipids (not peripheral proteins) form hydrogen bonds with water to stabilise the membrane.

Award 1 mark for selecting option A, which correctly matches cholesterol with its role in regulating membrane fluidity.

A competitive inhibitor (X) competes with substrate molecules for the active site. At very high substrate concentrations, the substrate molecules outcompete the inhibitor, so the maximum rate of reaction (\(V_{\max}\)) is achieved. A non-competitive inhibitor (Y) binds to an allosteric site, altering the shape of the active site so that the substrate can no longer bind. This effectively reduces the concentration of active enzyme, meaning \(V_{\max}\) is decreased and cannot be restored by adding more substrate.

Award 1 mark for option A, which correctly identifies inhibitor X as a competitive inhibitor and inhibitor Y as a non-competitive inhibitor based on their effects on \(V_{\max}\).

Chromatin condensation and disappearance of the nucleolus occur during prophase (Stage 1). The separation of sister chromatids and their movement to opposite poles occurs during anaphase (Stage 2). The reformation of the nuclear envelopes occurs during telophase (Stage 3).

Award 1 mark for selecting option A, which correctly identifies the mitotic stages for all three stages described.

During transcription, complementary base pairing occurs between the DNA template strand and the forming mRNA molecule. Adenine (A) on the DNA template strand pairs with Uracil (U) on the RNA strand. Therefore, the percentage of uracil in the mRNA will be exactly equal to the percentage of adenine in the template strand, which is 18%.

Award 1 mark for option A, since uracil in mRNA is complementary to adenine in the DNA template strand (18%).

The trachea contains C-shaped rings of cartilage, smooth muscle, and elastic fibres in its walls. Bronchioles lack cartilage entirely but contain smooth muscle to control airway diameter and elastic fibres. Alveoli lack both cartilage and smooth muscle but contain abundant elastic fibres to allow for expansion and elastic recoil during ventilation.

Award 1 mark for option A, which correctly identifies the distribution of cartilage, smooth muscle, and elastic fibres.

Penicillin inhibits the enzyme transpeptidase, which is responsible for forming covalent cross-links between peptidoglycan chains in the bacterial cell wall. This inhibition prevents the cell wall from being reinforced, leading to osmotic lysis when water enters the cell.

Award 1 mark for option B, which correctly states that penicillin prevents the formation of cross-links between peptidoglycan chains.

Chloroplasts, mitochondria, and prokaryotic cells all contain 70S ribosomes and circular DNA as part of their evolutionary origin (endosymbiotic theory for the organelles). The nucleoplasm of eukaryotic cells contains linear chromosomes and does not contain ribosomes, which are assembled in the nucleolus as subunits and function as 80S ribosomes in the cytoplasm/RER. Thus, 1, 2, and 4 are correct.

1 mark for identifying that chloroplasts, mitochondria, and prokaryotic cytoplasm contain both 70S ribosomes and circular DNA, while eukaryotic nucleoplasm does not.

The Michaelis-Menten constant (\(K_m\)) is defined as the substrate concentration at which the rate of the enzyme-catalysed reaction is half of the maximum rate (\(V_{max}\)). Since the maximum rate is \(80\text{ }\mu\text{mol dm}^{-3}\text{ s}^{-1}\), half of this rate is \(40\text{ }\mu\text{mol dm}^{-3}\text{ s}^{-1}\). Therefore, the substrate concentration at this rate is equal to \(K_m\).

1 mark for identifying that half of the maximum rate of reaction corresponds to a substrate concentration equal to the Michaelis-Menten constant (\(K_m\)).

A higher (least negative, closest to zero) water potential in the external solution means water will move down the water potential gradient into the potato cells by osmosis, causing an increase in mass. Therefore, solution \(Q\) (mass change of \(+2.1\%\)) has the highest water potential. At solution \(R\) (\(0.0\%\) mass change), the water potential is equal to the potato tissue. Solutions \(S\) and \(P\) have lower water potentials than the potato tissue, causing water to leave the cells and resulting in a loss of mass, with \(P\) (mass change of \(-4.5\%\)) having the lowest (most negative) water potential. The correct order is \(Q \rightarrow R \rightarrow S \rightarrow P\).

1 mark for determining the correct order of water potentials from highest (least negative / mass gain) to lowest (most negative / mass loss).

During metaphase of mitosis, the DNA has replicated, so each of the 12 chromosomes consists of two sister chromatids. This gives \(12 \times 2 = 24\) chromatids. Each chromatid is a single linear double-stranded DNA molecule, which has 2 telomeres (one at each end). Therefore, 24 chromatids will have \(24 \times 2 = 48\) telomeres.

1 mark for calculating both the correct number of chromatids (24) and telomeres (48) in metaphase.

Mature xylem vessel elements are dead cells that are heavily reinforced with lignin in their cell walls to withstand tension. In contrast, mature phloem sieve tube elements are living cells with non-lignified cellulose walls. Cytoplasm, plasmodesmata, and sieve plates are characteristic of sieve tube elements but absent in xylem vessels.

1 mark for identifying that lignified cell walls are present in mature xylem but completely absent in phloem sieve tube elements.

In a healthy human, the trachea wall contains cartilage rings, goblet cells, and smooth muscle. Bronchioles lack cartilage and goblet cells (to prevent clogging of small airways) but retain smooth muscle to regulate airflow. Alveoli lack cartilage, goblet cells, and smooth muscle, consisting of simple squamous epithelium and elastic fibres to facilitate rapid gas exchange.

1 mark for identifying the correct distribution of cartilage, goblet cells, and smooth muscle in all three regions.

Penicillin is an antibiotic that inhibits the enzyme transpeptidase, which is responsible for forming cross-links in the peptidoglycan cell wall of bacteria. This weakens the cell wall, causing osmotic lysis of growing bacteria. Since viruses do not have a cell wall (nor do they have peptidoglycan), penicillin has no effect on them.

1 mark for identifying that penicillin's mechanism of action targets peptidoglycan cross-linking in bacterial cell walls, which viruses lack.

The total coding segment is \(1500 - 150 = 1350\) base pairs of DNA. When transcribed, this yields an mRNA molecule with \(1350\) coding nucleotides. Since each codon consists of 3 nucleotides (a triplet codon), the maximum number of codons is \(1350 / 3 = 450\). Thus, the maximum number of amino acids in the polypeptide chain is 450.

1 mark for calculating the correct number of amino acids by subtracting the introns (150 bp) from total base pairs (1500 bp) and dividing the remaining coding sequence (1350 bp) by 3.

In secretory cells like goblet cells, proteins (such as mucin) are synthesized on the ribosomes of the rough endoplasmic reticulum (RER). They are then transported in vesicles to the Golgi body, where they are modified (e.g., glycosylated to form glycoproteins). From the Golgi body, they are packaged into secretory vesicles which move towards the apical membrane and fuse with the cell surface membrane, releasing the proteins via exocytosis.

Award 1 mark for identifying the correct physiological pathway of a secreted protein (RER -> Golgi body -> secretory vesicles -> cell surface membrane).

The inhibitor is competitive because its effect can be overcome at high substrate concentrations, allowing the reaction to reach the original \(V_{\max}\). A competitive inhibitor decreases the affinity of the enzyme for its substrate, which increases the value of the Michaelis-Menten constant (\(K_{\m}\)) because a higher concentration of substrate is required to reach half of the \(V_{\max}\).

Award 1 mark for identifying that the inhibitor is competitive and that it increases the \(K_{\m}\) value.

During metaphase, there are 16 chromosomes aligned at the equator, each consisting of two sister chromatids joined at a centromere, giving a total of 32 chromatids. During anaphase, the centromeres split and the sister chromatids separate to opposite poles. Once separated, each chromatid is defined as an individual chromosome. Therefore, during anaphase, there are 32 chromosomes and 0 chromatids (as they are no longer paired).

Award 1 mark for selecting the row that correctly identifies 16 chromosomes and 32 chromatids in metaphase, and 32 chromosomes and 0 chromatids in anaphase.

Transcription of the template DNA strand \(3'\text{- T A C G G G C A T A T T -}5'\) produces complementary mRNA in the \(5'\) to \(3'\) direction: \(5'\text{- A U G C C C G U A U A A -}3'\). During translation, the tRNA anticodons align with these mRNA codons. The anticodons (which contain uracil instead of thymine) will be complementary to the codons, giving: \(U A C\), \(G G G\), \(C A U\), \(A U U\).

Award 1 mark for identifying the correct complementary mRNA strand containing uracil (not thymine) and the corresponding tRNA anticodons.

The formula for net filtration pressure is: \(\text{Net Pressure} = (\text{Blood Hydrostatic Pressure} - \text{Tissue Hydrostatic Pressure}) - (\text{Blood Oncotic Pressure} - \text{Tissue Oncotic Pressure})\).

At the arterial end: \(\text{Net Pressure} = (4.6 - 1.2) - (3.3 - 1.0) = 3.4 - 2.3 = +1.1\text{ kPa}\) (pointing outwards, causing filtration).

At the venous end: \(\text{Net Pressure} = (2.3 - 1.2) - (3.3 - 1.0) = 1.1 - 2.3 = -1.2\text{ kPa}\) (pointing inwards, causing reabsorption).

Award 1 mark for showing correct calculation steps for both the arterial (+1.1 kPa) and venous (-1.2 kPa) net pressures.

A bronchus contains cartilage (in plates), smooth muscle, ciliated epithelium, and goblet cells. A terminal bronchiole does not have cartilage or goblet cells (to prevent mucus blockage in very narrow passages), but it still possesses smooth muscle and a ciliated epithelium (often simple cuboidal and ciliated) to control airway diameter and sweep up any remaining particles.

Award 1 mark for correctly matching the tissue composition of both structures (bronchus: cartilage, smooth muscle, ciliated epithelium, goblet cells present; terminal bronchiole: cartilage and goblet cells absent, smooth muscle and ciliated epithelium present).

Statement 1 is correct: T-cells migrate to and mature in the thymus, while B-cells mature in the bone marrow. Statement 2 is incorrect: B-lymphocytes differentiate into plasma cells that produce antibodies, whereas T-lymphocytes do not produce antibodies. Statement 3 is incorrect: both cell types possess specific surface receptors (T-cell receptors and B-cell receptors/antibodies). Statement 4 is correct: activated T-helper cells release cytokines that trigger B-cell clonal expansion, and B-cells differentiate into antibody-secreting plasma cells.

Award 1 mark for identifying statements 1 and 4 as the only correct options.

Penicillin is a beta-lactam antibiotic. It inhibits the enzyme transpeptidase (glycoprotein peptidase), which catalyzes the cross-linking of peptidoglycan chains in the bacterial cell wall. This prevents the synthesis of a strong, structural cell wall. When the bacterium attempts to grow, the wall is too weak to withstand internal turgor pressure, leading to osmotic lysis (bursting) of the cell.

Award 1 mark for identifying the correct mechanism of action of penicillin: inhibition of transpeptidase and prevention of peptidoglycan cross-linking.

Competitive inhibitors compete with the substrate for the active site. They increase \(K_m\) (reducing substrate affinity) but do not affect \(V_{\text{max}}\) because high substrate concentrations can overcome the inhibition. Non-competitive inhibitors bind to an allosteric site, reducing the maximum rate of reaction (\(V_{\text{max}}\)) by rendering some enzymes inactive, but do not affect the affinity of the unaffected enzymes for the substrate, so \(K_m\) remains unchanged. Therefore, X is a competitive inhibitor and Y is a non-competitive inhibitor.

1 mark for identifying that inhibitor X is competitive and binds to the active site, and inhibitor Y is non-competitive and binds to an allosteric site.

Proteins destined for insertion into the cell surface membrane are synthesized by ribosomes bound to the rough endoplasmic reticulum (RER). They are folded within the RER and then packaged into transport vesicles. These vesicles fuse with the Golgi body, where the proteins undergo post-translational modifications (e.g., glycosylation). Modified proteins are then packaged into secretory vesicles which move to and fuse with the cell surface membrane.

1 mark for the correct chronological sequence of organelles and transport structures involved in membrane protein synthesis and routing.

In prophase, DNA has replicated, so each of the 24 chromosomes consists of 2 sister chromatids joined at a centromere. This gives 24 chromosomes, 48 chromatids, and 48 DNA molecules. During anaphase, the sister chromatids separate to become individual chromosomes, doubling the chromosome count in the single cell to 48. In telophase, before cytokinesis separates the cytoplasm, there are 48 individual chromosomes, 0 chromatids (since they are no longer paired at centromeres), and 48 DNA molecules within the cell boundaries.

1 mark for identifying the correct counts of chromosomes, chromatids, and DNA molecules in both stages.

Arteries must withstand and maintain high hydrostatic pressure, thus possessing a thick tunica media with abundant elastic fibers and smooth muscle. Veins carry blood under low pressure and have a much thinner tunica media, fewer elastic fibers, and valves to prevent backflow. Capillaries consist solely of a single layer of endothelial cells (tunica intima) to allow rapid diffusion, completely lacking elastic fibers, smooth muscle, and valves.

1 mark for correctly matching the histological features of arteries, veins, and capillaries.

The DNA template strand \(3'\text{-TACGCTAGGAAG-5'}\) is transcribed into mRNA by complementary base pairing, yielding the mRNA sequence: \(5'\text{-AUGCGAUCCUUC-3'}\). The mRNA codons are \(5'\text{-AUG-3'}\), \(5'\text{-CGA-3'}\), \(5'\text{-UCC-3'}\), and \(5'\text{-UUC-3'}\). The complementary tRNA anticodons bind antiparallel, so they are written in the \(3' \rightarrow 5'\) direction as: \(3'\text{-UAC-5'}\), \(3'\text{-GCU-5'}\), \(3'\text{-AGG-5'}\), and \(3'\text{-AAG-5'}\).

1 mark for deriving the correct complementary mRNA codons and identifying the correct antiparallel tRNA anticodon sequences.

T-helper cells express T-cell receptors that bind to specific antigens presented on antigen-presenting cells (APCs). Once activated, they secrete signaling molecules called cytokines (such as interleukins). Cytokines stimulate clonal expansion and differentiation of antigen-specific B-lymphocytes into antibody-secreting plasma cells, and they also activate macrophages to perform phagocytosis more efficiently. Option B is incorrect because plasma cells secrete antibodies. Option C is incorrect because phagocytes present antigens. Option D describes the role of cytotoxic (T-killer) lymphocytes.

1 mark for identifying the correct physiological role of cytokines released by activated T-helper lymphocytes.

Trachea and bronchi both possess cartilage to keep the airways open (C-shaped rings in trachea, irregular plates in bronchi). Alveoli lack cilia, goblet cells, and smooth muscle entirely, consisting of simple squamous epithelium. Bronchioles lack cartilage but still contain smooth muscle, elastic fibers, and ciliated epithelium (with goblet cells present in larger bronchioles, although fewer than in the bronchi). Thus, the specimen is a bronchiole.

1 mark for identifying the bronchiole based on the presence of ciliated epithelium, smooth muscle, and the absence of cartilage.

1. Find the distance of 10 stage micrometer divisions: \(10 \times 0.01\text{ mm} = 0.1\text{ mm} = 100\ \mu\text{m}\).
2. Calculate the value of 1 eyepiece graticule division (epu): \(100\ \mu\text{m} / 50 = 2\ \mu\text{m}\) per epu.
3. Calculate the actual size of the plant cell: \(30\text{ epu} \times 2\ \mu\text{m}/\text{epu} = 60\ \mu\text{m}\).

1 mark for correct calibration of the eyepiece graticule and calculation of the cell's actual diameter.

(a) A non-competitive inhibitor binds to an allosteric site of the enzyme, altering its tertiary structure and changing the shape of the active site so that the substrate is no longer complementary, preventing the formation of enzyme-substrate complexes. This differs from a competitive inhibitor, which binds directly to the active site and whose inhibitory effect can be overcome by high substrate concentrations.

(b) (i) Decrease in rate = 2.8 - 1.2 = 1.6 \(\mu\text{mol dm}^{-3}\text{ min}^{-1}\).
Percentage decrease = \(\frac{1.6}{2.8} \times 100 = 57.1\\%\).

(ii) Compound X has a shape complementary to the active site of the enzyme, enabling it to compete directly with substrate molecules for binding. This reduces the frequency of successful collisions between the substrate and the active site, leading to fewer enzyme-substrate complexes forming per unit time.

(c) At temperatures above 70 °C, high kinetic energy causes weak bonds (such as hydrogen and ionic bonds) holding the tertiary structure of the protein together to break. The enzyme denatures, causing a permanent change in the shape of its active site so that the substrate can no longer bind and no enzyme-substrate complexes can form.

(a) Max 3 marks:
1. Non-competitive inhibitor binds to an allosteric site / site other than the active site;
2. Changes tertiary structure / 3D shape of enzyme;
3. Active site shape is altered / is no longer complementary, so substrate cannot bind / no enzyme-substrate complexes (ESCs) form;
4. Contrast: Competitive inhibitor binds to active site / its effect can be overcome by increasing substrate concentration (whereas non-competitive cannot);

(b)(i) Max 2 marks:
1. Correct calculation formula: \(\frac{2.8 - 1.2}{2.8} \times 100\) (or equivalent shown in working);
2. Correct final answer to one decimal place: 57.1% (Accept 57.1. Reject 57 or 57.14);

(b)(ii) Max 3 marks:
1. Compound X has a similar shape to the substrate / shape complementary to the active site;
2. Competes with the substrate to bind to the active site;
3. Reduces the frequency of successful collisions / fewer ESCs form per unit time;

(c) Max 2 marks:
1. High kinetic energy causes hydrogen / ionic / hydrophobic bonds to break;
2. Loss of tertiary structure / denaturation of enzyme;
3. Active site changes shape permanently, so substrate cannot bind / no ESCs form.

(a) Translation occurs on ribosomes bound to the rough endoplasmic reticulum (RER). The polypeptide enters the lumen of the RER, where it is folded into its tertiary structure. Transport vesicles bud off from the RER and travel to and fuse with the Golgi body. In the Golgi body, proteins are modified (such as by adding carbohydrate chains to form glycoproteins) and sorted. Secretory vesicles then bud off the trans face of the Golgi body, move along microtubules, and fuse with the cell surface membrane to release the protein by exocytosis.

(b) (i) Tonoplast
(ii) The permanent vacuole stores cell sap (water, inorganic ions, pigments, or organic solutes) and maintains cell turgor pressure / turgidity to support the plant structure.

(c) Microtubules are hollow, unbranched cylinders composed of polymerised alpha- and beta-tubulin dimers. They act as intracellular 'tracks' along which motor proteins (such as kinesin or dynein) attach to organelles and transport them across the cytoplasm using energy released from ATP hydrolysis.

(a) Max 4 marks:
1. Translation occurs on ribosomes bound to the rough endoplasmic reticulum (RER);
2. Protein enters the RER lumen for folding into tertiary structure;
3. Transport vesicles bud off RER and move to / fuse with Golgi body;
4. In Golgi body, protein is modified / processed (e.g. addition of carbohydrate chain to form glycoprotein) / packaged into secretory vesicles;
5. Secretory vesicles move to cell surface membrane and fuse with it to release protein via exocytosis;

(b)(i) 1 mark:
1. Tonoplast;

(b)(ii) Max 2 marks:
1. Storage of water / inorganic ions / organic solutes / waste products / pigments;
2. Maintaining turgor pressure / keeping cell turgid (for support);
3. Hydrolysis of macromolecules (contains lysosomal-like enzymes);

(c) Max 3 marks:
1. Hollow cylinders / tubes;
2. Made of tubulin / protofilaments (composed of \(\alpha\)- and \(\beta\)-tubulin dimers);
3. Act as intracellular tracks / scaffolding;
4. Motor proteins (kinesin/dynein) attach to organelle and 'walk' along microtubules using ATP hydrolysis;

(a) (i) The stage is anaphase. During anaphase, the centromeres divide and the spindle fibres shorten and contract. This pulls the sister chromatids (now individual chromosomes) apart, centromeres leading, towards opposite poles of the cell.
(ii) The centromere holds the two sister chromatids together until they are ready to separate in anaphase. It is also the site of attachment for spindle fibres (at the kinetochore), allowing proper alignment and equal distribution of chromosomes to each pole.

(b) (i) A chromosome is a single, double-stranded molecule of DNA (or a structure containing two identical sister chromatids joined at a centromere). A chromatid is one of the two identical copies of DNA that make up a duplicated chromosome prior to division.
(ii) DNA replication occurs during the S (synthesis) phase of interphase. It is essential because it duplicates the genetic material so that each daughter cell receives an identical, complete copy of the genome, maintaining genetic stability and ensuring no genes are lost.

(a)(i) Max 3 marks:
1. Anaphase;
2. Centromeres split / divide;
3. Spindle fibres contract / shorten;
4. Chromatids pulled / separated to opposite poles (centromeres first / V-shape);

(a)(ii) Max 2 marks:
1. Holds sister chromatids together (until anaphase);
2. Attachment site for spindle fibres / microtubules (via kinetochore);
3. Ensures equal segregation / correct alignment of chromosomes;

(b)(i) Max 2 marks:
1. Chromosome: refers to the complete unit of condensed chromatin; can exist as a single chromatid (G1 phase) or two sister chromatids (after S phase);
2. Chromatid: is one of the two identical strands of a replicated chromosome joined by a centromere;

(b)(ii) Max 3 marks:
1. S phase / synthesis phase (of interphase);
2. Duplicates DNA / genetic material exactly;
3. Ensures both daughter cells receive a full / diploid set of chromosomes / genetically identical copy;
4. Prevents loss of genetic information / maintains genetic stability;

(a) (i) A double circulatory system means that blood passes through the heart twice for each complete circuit of the body, involving two separate circuits: the pulmonary circulation (to the lungs) and the systemic circulation (to the body tissues).
(ii) Advantages include: maintaining a higher blood pressure in the systemic circulation to deliver oxygen and nutrients rapidly to metabolizing tissues, and keeping a lower blood pressure in the pulmonary circulation to protect delicate lung capillaries and allow slower flow for optimal gas exchange.

(b)
| Feature | Artery | Vein | Capillary |
| :--- | :---: | :---: | :---: |
| Has a wall containing elastic fibres | \(\checkmark\) | \(\checkmark\) | \(\times\) |
| Has a wall containing smooth muscle | \(\checkmark\) | \(\checkmark\) | \(\times\) |
| Has a lumen that is about 7–8 \(\mu\text{m}\) wide | \(\times\) | \(\times\) | \(\checkmark\) |

(c) At the arteriole end, the hydrostatic pressure of the blood is higher than the osmotic (oncotic) pressure. This net outward filtration force drives water and small dissolved solutes out of the capillary through small gaps in the endothelium into the tissue spaces, forming tissue fluid. At the venule end, the hydrostatic pressure falls significantly due to friction and fluid loss, making it lower than the osmotic pressure (which remains high due to large plasma proteins remaining in the blood). This creates a net inward force, causing water to return to the capillary by osmosis down a water potential gradient.

(a)(i) Max 2 marks:
1. Blood passes through the heart twice for each complete circuit;
2. Separate pulmonary (heart to lungs and back) and systemic (heart to body and back) circulations;

(a)(ii) Max 2 marks:
1. Allows high pressure in systemic circulation / rapid flow of blood to body tissues;
2. Low pressure in pulmonary circulation protects lung tissues / capillaries;
3. Oxygenated and deoxygenated blood are kept completely separate;

(b) 3 marks:
- All 9 entries correct = 3 marks
- 7 or 8 entries correct = 2 marks
- 5 or 6 entries correct = 1 mark
- Less than 5 correct = 0 marks

*Correct table entries:*
- Elastic fibres: Artery (\(\checkmark\)), Vein (\(\checkmark\)), Capillary (\(\times\))
- Smooth muscle: Artery (\(\checkmark\)), Vein (\(\checkmark\)), Capillary (\(\times\))
- Lumen 7-8 \(\mu\text{m}\): Artery (\(\times\)), Vein (\(\times\)), Capillary (\(\checkmark\))

(c) Max 3 marks:
1. At arteriole end, blood hydrostatic pressure exceeds solute / oncotic pressure;
2. Water and small solutes forced out of capillaries (through endothelial pores) into tissue spaces;
3. At venule end, hydrostatic pressure is lower than oncotic pressure (due to plasma proteins);
4. Water moves back into capillary by osmosis down water potential gradient;

(a) (i) RNA polymerase binds to the promoter region of DNA, unwinding the double helix to expose the template strand. It then moves along the template strand, lining up free RNA nucleotides by complementary base pairing (A to U, T to A, C to G). RNA polymerase catalyzes the formation of phosphodiester bonds to join adjacent RNA nucleotides together, forming a primary mRNA transcript.
(ii) DNA is double-stranded, contains deoxyribose sugar, and contains thymine, whereas mRNA is single-stranded, contains ribose sugar, and contains uracil.

(b) (i) A tRNA molecule has a specific three-base anticodon sequence that is complementary to an mRNA codon. It also features a specific amino acid attachment site at its 3' end to carry a corresponding amino acid. The single-stranded RNA folds into a stable cloverleaf shape held by internal hydrogen bonding, which is recognized by specific enzymes.
(ii) The complementary anticodon sequence is 3'-UAC-5' (accept UAC). The amino acid carried by this tRNA is methionine.

(a)(i) Max 3 marks:
1. Binds to promoter (on DNA) and unwinds DNA double helix / breaks hydrogen bonds;
2. Moves along template strand (in 3' to 5' direction);
3. Aligns free RNA nucleotides by complementary base pairing;
4. Catalyzes formation of phosphodiester bonds between RNA nucleotides;

(a)(ii) Max 2 marks (any two from):
1. DNA is double-stranded / double helix, whereas mRNA is single-stranded;
2. DNA contains deoxyribose sugar, whereas mRNA contains ribose sugar;
3. DNA contains thymine (T), whereas mRNA contains uracil (U);
4. DNA is long / has hydrogen bonds between strands, whereas mRNA is relatively short / linear;

(b)(i) Max 3 marks:
1. Has a specific anticodon loop (three bases) complementary to mRNA codon;
2. Has an amino acid binding site (at the 3' end) to carry a specific amino acid;
3. Cloverleaf/folded shape held by hydrogen bonds between bases, providing stability;
4. Has specific shapes/sites recognized by aminoacyl-tRNA synthetase enzymes;

(b)(ii) 2 marks:
1. Anticodon: UAC (accept 3'-UAC-5' or CAU if 5' to 3' specified, but standard is UAC / 3'-UAC-5') [1];
2. Amino acid: Methionine (accept Met) [1];

(a) (i) Xylem tissue transports water and dissolved mineral ions unidirectionally from the roots to the leaves. Phloem tissue transports organic solutes / assimilates (such as sucrose and amino acids) bidirectionally from sources (where they are produced) to sinks (where they are used or stored).
(ii) Xylem vessels are made of dead cells with no cytoplasm or organelles, leaving a completely hollow lumen that offers very low resistance to water flow. Their end walls are lost, creating continuous tubes for an uninterrupted column of water. The walls are thickened with lignin, which provides mechanical strength to prevent the vessel collapsing under the high tension created by transpiration pull. Pits in the lignified walls allow the lateral movement of water between adjacent vessels.

(b) (i) Companion cells have many mitochondria to generate ATP via aerobic respiration to power the active loading of sucrose. They contain many ribosomes and rough endoplasmic reticulum to synthesize the transport proteins, such as proton pumps and co-transporter proteins, required for transport. They also contain a prominent nucleus to coordinate metabolic activities for both themselves and the sieve tube elements.
(ii) Plasmodesmata (singular: plasmodesma).

(a)(i) Max 2 marks:
1. Xylem: transports water and mineral ions upwards / from roots to leaves / unidirectionally;
2. Phloem: transports assimilates / sucrose / organic solutes from source to sink / bidirectionally;

(a)(ii) Max 4 marks:
1. Lignified walls resist negative pressure / tension, preventing collapse;
2. Lignin provides waterproofing, preventing water loss from the vessel;
3. Cells are dead / contain no cytoplasm / no organelles, offering minimal resistance to flow;
4. End walls are absent / perforated, allowing a continuous column of water;
5. Pits in walls allow lateral movement of water (to bypass blocks);

(b)(i) Max 3 marks:
1. Large number of mitochondria to provide ATP for active transport / loading of sucrose;
2. Presence of proton-pumping proteins / sucrose co-transporter proteins in the membrane;
3. Numerous ribosomes / rough ER to produce transport proteins;
4. Nucleus is present to direct protein synthesis and metabolic processes for the anucleate sieve tube element;

(b)(ii) 1 mark:
1. Plasmodesmata (accept plasmodesma);