2023 IB DP Biology 模擬試題連答案詳解

Water is a polar molecule due to the electronegativity difference between oxygen and hydrogen. This polarity allows water molecules to form intermolecular hydrogen bonds. These hydrogen bonds require a substantial amount of thermal energy to overcome, leading to high specific heat capacity and boiling points. In contrast, methane is nonpolar and only exhibits weak dispersion forces between its molecules, which require very little thermal energy to disrupt.

Award [1] for the correct answer (A). Award [0] for incorrect options. Method: Candidates must identify the role of hydrogen bonding in water versus weak intermolecular forces in methane.

The osmolarity of the tissue corresponds to the concentration of the external solution where there is no net movement of water into or out of the cells (0% change in mass). This point of zero-net movement lies between 0.2 M (where water entered the tissue, causing a +5% mass increase) and 0.4 M (where water left the tissue, causing a -2% mass decrease).

Award [1] for the correct answer (B). Award [0] for other options. Method: Identify the point of zero net mass change, which falls between 0.2 M and 0.4 M.

Cyclins are regulatory proteins that control the progression of the cell cycle. They function by binding to specific enzymes called cyclin-dependent kinases (CDKs). Once activated by cyclin binding, the CDKs phosphorylate target proteins that trigger specific events in the cell cycle (e.g., DNA replication or spindle formation).

Award [1] for the correct answer (A). Award [0] for other options. Method: Correctly link cyclins with the activation of CDKs and the phosphorylation of target proteins.

Active loading of sucrose into phloem sieve tubes involves proton pumps (H+-ATPases) that actively transport hydrogen ions (protons, \(H^+\)) out of the companion cells into the surrounding cell wall space, creating a concentration gradient. Protons then diffuse back into the companion cells down their electrochemical gradient through a co-transporter protein, carrying sucrose with them against its concentration gradient.

Award [1] for the correct answer (A). Award [0] for other options. Method: Confirm the role of proton pumps establishing a gradient for the co-transport of sucrose.

During exercise, increased cellular respiration produces more carbon dioxide, which dissolves in blood to form carbonic acid, lowering blood pH. Chemoreceptors in the carotid arteries and aorta detect this drop in pH and send nerve impulses to the cardiovascular center in the medulla oblongata. The medulla oblongata then increases the frequency of impulses along the sympathetic nerve to the sinoatrial (SA) node, accelerating the heart rate.

Award [1] for the correct answer (A). Award [0] for other options. Method: Correctly trace the signal transduction from chemoreceptors detecting low pH/high carbon dioxide, signaling the medulla, and stimulating the SA node via sympathetic nerves.

Let \(X^N\) represent the normal allele and \(X^n\) represent the color-blind allele. The woman has normal vision but her father was color-blind (genotype \(X^n Y\)). She must have inherited the \(X^n\) allele from her father, making her a heterozygous carrier with genotype \(X^N X^n\). The man has normal vision (genotype \(X^N Y\)). A cross between them (\(X^N X^n \times X^N Y\)) yields the following possible offspring genotypes: \(X^N X^N\) (normal female), \(X^N X^n\) (normal carrier female), \(X^N Y\) (normal male), and \(X^n Y\) (color-blind male). The probability of producing a color-blind male among all offspring is 1 in 4, which is 25%.

Award [1] for the correct answer (B). Award [0] for other options. Method: Correctly determine the genotypes of parents and calculate the probability of obtaining a color-blind male offspring.

A missense mutation is a type of point mutation where a single nucleotide substitution changes the codon so that it codes for a different amino acid (in this case, changing glutamic acid to valine, which is the exact mutation causing sickle cell anemia). Silent mutations code for the same amino acid, nonsense mutations create a premature stop codon, and frameshift mutations result from nucleotide insertions or deletions.

Award [1] for the correct answer (C). Award [0] for other options. Method: Correctly identify that a change in a single amino acid represents a missense mutation.

In yeast, anaerobic respiration (alcohol fermentation) converts pyruvate into ethanol and carbon dioxide (\(CO_2\)). In human muscle cells, anaerobic respiration (lactic acid fermentation) converts pyruvate into lactate (lactic acid) only, without the release of carbon dioxide.

Award [1] for the correct answer (B). Award [0] for other options. Method: Verify the exact end products for yeast fermentation (ethanol + carbon dioxide) versus lactate fermentation in humans (lactate).

Water has a high specific heat capacity, meaning it requires a relatively large amount of heat energy to raise its temperature by one degree. This is due to the many hydrogen bonds that must be broken before water molecules can move faster. This property buffers organisms and aquatic ecosystems against extreme temperature fluctuations.

Award [1] for correct answer (B). [0] for incorrect options.

Moving sodium ions against their concentration gradient requires active transport, which relies directly on energy derived from ATP hydrolysis. Simple diffusion, facilitated diffusion, and osmosis (movement through aquaporins) are passive processes that do not require ATP.

Award [1] for correct answer (B). [0] for incorrect options.

The mitotic index is calculated as the number of cells undergoing mitosis divided by the total number of cells observed. Number of cells in mitosis = 12 (prophase) + 4 (metaphase) + 3 (anaphase) + 6 (telophase) = 25. Total number of cells = 25 + 175 (interphase) = 200. Mitotic index = \(25 / 200 = 0.125\) or \(12.5\%\).

Award [1] for correct calculation and matching option (A). [0] for incorrect calculations.

In human muscle cells, anaerobic respiration (lactic acid fermentation) converts pyruvate into lactate to regenerate NAD+, yielding a net of 2 ATP per glucose molecule. No carbon dioxide or ethanol is produced in this pathway (unlike in yeast fermentation).

Award [1] for correct answer (B). [0] for incorrect options.

A heterozygous mother with blood group A has genotype \(I^A i\). A heterozygous father with blood group B has genotype \(I^B i\). A Punnett square of this cross yields: 1/4 \(I^A I^B\) (blood group AB), 1/4 \(I^A i\) (blood group A), 1/4 \(I^B i\) (blood group B), and 1/4 \(ii\) (blood group O). Therefore, there is a 25% chance of the child having blood group O.

Award [1] for correct probability (B). [0] for incorrect options.

Increased cellular respiration during exercise releases carbon dioxide, which lowers blood pH. Chemoreceptors detect this decrease and send nerve impulses to the cardiovascular center in the medulla oblongata. The medulla oblongata then increases sympathetic nervous signals via the cardiac nerve to the sinoatrial (SA) node, increasing the heart rate.

Award [1] for correct choice (B). Other options are incorrect because the motor cortex is not the primary sensor, baroreceptors detect pressure (not oxygen), and the SA node does not directly detect blood pH.

A base substitution that changes an amino acid codon to a stop codon is a nonsense mutation. It causes translation to terminate prematurely, producing a truncated polypeptide that is usually non-functional. Missense mutations change one amino acid to another. Silent mutations do not change the amino acid sequence. Frameshift mutations are caused by insertions or deletions, not single base substitutions.

Award [1] for correct choice (B). [0] for other options.

Active transport of sucrose into the sieve tube at the source makes the phloem sap hypertonic (high solute concentration). Consequently, water enters the sieve tube from the xylem by osmosis. This accumulation of water increases the hydrostatic pressure inside the phloem at the source, driving the bulk flow (mass flow) of sap toward the sink where the pressure is lower.

Award [1] for correct explanation (B). [0] for options describing incorrect pressure systems (e.g., negative pressure in xylem vs. positive hydrostatic pressure in phloem).

Cholesterol is an amphipathic molecule found in mammalian cell membranes. It acts as a bidirectional regulator of membrane fluidity: at high temperatures, it stabilizes the membrane and raises its melting point, thereby decreasing fluidity; at low temperatures, it inserts between phospholipids to prevent them from clustering and stiffening, thereby maintaining/increasing fluidity.

Award 1 mark for correct choice B. Reject other options as they describe incorrect physical effects of cholesterol on membrane fluidity across different temperature ranges.

The mitotic index is calculated as the ratio of the number of cells undergoing mitosis (prophase + metaphase + anaphase + telophase) to the total number of cells observed. \(\text{Mitotic cells} = 12 + 4 + 2 + 6 = 24\). \(\text{Total cells} = 100\). \(\text{Mitotic index} = \frac{24}{100} = 0.24\).

Award 1 mark for the correct calculation and choice of B. No partial credit.

During exercise, the cardiovascular center in the medulla oblongata detects changes in blood pH and pressure, sending sympathetic signals via the cardiac nerve to release noradrenaline at the sinoatrial node to increase heart rate. Concurrently, the endocrine system releases epinephrine (adrenaline) from the adrenal glands into the blood, which acts on the sinoatrial node to sustain the increased heart rate.

Award 1 mark for choice B. Reject options involving parasympathetic/vagus nerve stimulation (which decrease heart rate) or somatic motor neuron control of cardiac muscle.

While high specific heat capacity prevents rapid temperature fluctuations, the primary cooling effect of transpiration is achieved when water undergoes a phase change from liquid to gas. Because water has a high latent heat of vaporization, evaporating a small volume of water absorbs a large amount of heat energy from the surrounding leaf cells, effectively cooling the plant.

Award 1 mark for choice B. Reject choices emphasizing transport mechanisms (cohesion/adhesion) or non-evaporative thermal regulation (specific heat capacity).

The father of the man has blood group O (genotype \(ii\)), which means the man must carry one recessive allele and has the genotype \(I^A i\). The mother of the woman has blood group O (genotype \(ii\)), meaning the woman has the genotype \(I^B i\). Crossing these two genotypes: \(I^A i \times I^B i\) yields a offspring genotypic ratio of \(1\ I^A I^B\) (AB) : \(1\ I^A i\) (A) : \(1\ I^B i\) (B) : \(1\ ii\) (O). Therefore, there is a 25% (or 1 in 4) probability of the child having blood group AB.

Award 1 mark for choice B. Award 0 marks for incorrect percentages.

The sgRNA is engineered to contain a spacer sequence complementary to the target DNA sequence of interest. It binds to the Cas9 endonuclease, forming a ribonucleoprotein complex, and guides the enzyme to the specific target locus via complementary base-pairing, enabling site-specific double-strand cleavage.

Award 1 mark for choice B. Reject choices stating sgRNA does the catalytic cleavage (which is done by Cas9) or acts as a repair template.

As high-energy electrons from NADH and \(\text{FADH}_2\) are passed along the proteins of the electron transport chain, they release energy in a series of redox reactions. This released energy is used by electron carriers (proton pumps) to actively transport hydrogen ions (protons) from the mitochondrial matrix into the intermembrane space, establishing an electrochemical gradient.

Award 1 mark for choice B. Reject option A (ATP hydrolysis is not the source; ATP is synthesized instead), C (this describes the mechanism of ATP synthesis, not proton pumping), and D (decarboxylation happens in the matrix, link, or Krebs cycle, not directly powering the proton pumps).

During a primary immune response, activated B cells undergo clonal selection and differentiate into plasma cells (which secrete antibodies) and memory B cells. These memory cells persist long-term. Upon secondary exposure to the same antigen, the memory cells immediately recognize the antigen, rapidly proliferate, and differentiate into a large population of active plasma cells, resulting in a much faster and greater production of antibodies.

Award 1 mark for choice B. Reject phagocyte antibody secretion (phagocytes do not produce antibodies), direct T helper cell antibody synthesis (T cells do not produce antibodies), or pathogen weakening mechanisms.

During vigorous exercise, increased cellular respiration produces more carbon dioxide, which dissolves in the blood to form carbonic acid, thereby decreasing the blood pH. This decrease in pH (and increase in partial pressure of carbon dioxide) is detected by chemoreceptors located in the medulla oblongata (central) as well as in the carotid and aortic bodies (peripheral). In response, the respiratory center in the medulla sends nerve impulses to the diaphragm and intercostal muscles to increase both the rate and depth of ventilation, helping to expel the excess carbon dioxide and restore pH balance.

Award 1 mark for the correct answer (A). Award 0 marks for incorrect distractors: B is incorrect because blood pH decreases, not increases, and baroreceptors detect pressure, not pH; C is incorrect because stretch receptors detect lung inflation and ventilation rate increases rather than decreases; D is incorrect because blood pH decreases and ventilation rate increases.

Cyclins are regulatory proteins whose levels fluctuate throughout the cell cycle. They do not have enzymatic activity themselves, but instead bind to and activate specific Cyclin-Dependent Kinases (CDKs). Once activated by cyclin binding, these kinases phosphorylate key target proteins that trigger specific events in the cell cycle, such as DNA replication or spindle formation.

Award 1 mark for selecting the correct mechanism of cyclin action (B). Distractor A is incorrect because cyclins do not bind directly to ribosomes to alter protein translation. Distractor C is incorrect because cyclins are not transcription factors and do not bind directly to DNA. Distractor D is incorrect because cyclins themselves are not kinases; they activate CDKs which carry out phosphorylation.

The sodium-potassium pump (\\text{Na}^+/\\text{K}^+\\text{ ATPase}) uses energy from the hydrolysis of one ATP molecule to actively transport three sodium ions (\\text{Na}^+) out of the cell, against their concentration gradient, and two potassium ions (\\text{K}^+) into the cell, against their concentration gradient.

Award 1 mark for the correct direction of ion transport and stoichiometry associated with ATP hydrolysis (A). B is incorrect because the pump moves sodium out and potassium in, not vice versa. C is incorrect because energy is required (hydrolysis of ATP), not released. D is incorrect because only 1 ATP molecule is hydrolyzed per cycle, and sodium is transported out of the cell, not in.

The tension (negative pressure) is generated by the transpiration of water from the leaves, where evaporation from cell walls draws water out of xylem vessels. This pulling force is transmitted all the way down to the roots because water molecules are held together by cohesive forces, which are due to hydrogen bonding. This prevents the water column from breaking (cavitation) under high tension.

Award 1 mark for identifying evaporation/transpiration as the cause of tension and cohesion as the mechanism preventing the column from breaking (C). Distractor A is incorrect because root pressure is a positive pressure, not tension, and cannot drive water transport in tall trees. Distractor B is incorrect because sucrose translocation occurs in the phloem, not xylem. Distractor D is incorrect because phloem transport relies on positive hydrostatic pressure, and covalent bonds are not what holds the water column together.

The high specific heat capacity of water is due to the presence of extensive hydrogen bonding between polar water molecules. A large amount of thermal energy must be absorbed to break these hydrogen bonds before the kinetic energy (temperature) of the water molecules can increase. Biologically, this prevents rapid temperature fluctuations, providing a highly stable thermal environment for aquatic life.

Award 1 mark for the correct physical explanation of high specific heat capacity and its biological significance (A). Distractor B is incorrect because water molecules form hydrogen bonds with one another, not covalent bonds, and vaporization relates to latent heat of vaporization, not specific heat capacity. Distractor C describes solubility, not thermal properties. Distractor D is incorrect because ice is less dense than liquid water, which is why it floats, rather than sinking to freeze lakes from the bottom up.

An insertion of a single nucleotide alters the triplet reading frame of the genetic code from the point of mutation onward. This frameshift mutation changes all downstream codons, potentially resulting in an entirely different sequence of amino acids and/or a premature stop codon. In contrast, a single nucleotide substitution only affects one codon, which might change a single amino acid (missense) or have no effect on the amino acid sequence at all due to the degeneracy of the genetic code (silent).

Award 1 mark for identifying that insertion causes a frameshift changing multiple amino acids, whereas a substitution typically alters only one (B). Distractor A is incorrect because insertions are rarely silent and substitutions do not always lead to shorter proteins (only nonsense substitutions do). Distractor C is incorrect because both types of mutations are transcribed into mRNA and translated. Distractor D is incorrect because mutations in coding sequences do not prevent transcription initiation or cause the ribosome to skip the gene.

1. Following a meal, elevated blood glucose levels are sensed directly by pancreatic beta cells. 2. Beta cells secrete the hormone insulin into the bloodstream. 3. Insulin promotes glucose uptake in target tissues (skeletal muscle, adipose tissue) and glycogenesis in the liver. 4. Parasympathetic nervous system activity increases, augmenting insulin secretion. 5. During fasting, low blood glucose levels are sensed by pancreatic alpha cells, triggering glucagon secretion. 6. Glucagon acts on liver cells to stimulate glycogenolysis (breakdown of glycogen) and gluconeogenesis (synthesis of glucose). 7. The sympathetic nervous system is activated during fasting/stress, releasing adrenaline from the adrenal medulla to rapidly increase blood glucose levels.

Award [1] for each correct point up to [6.8]:
- Insulin is secreted by beta cells of pancreas in response to high glucose [1]
- Insulin stimulates glucose uptake by body cells / glycogenesis in liver [1]
- Parasympathetic nervous system activity stimulates insulin secretion [1]
- Glucagon is secreted by alpha cells of pancreas in response to low glucose [1]
- Glucagon stimulates glycogenolysis / gluconeogenesis in the liver [1]
- Sympathetic nervous system stimulates adrenal glands to release adrenaline [1]
- Adrenaline stimulates glycogen breakdown to elevate glucose levels [1]

1. Increased muscle respiration produces \(CO_2\), which reacts with water to form carbonic acid, decreasing blood pH. 2. Chemoreceptors in the medulla, aorta, and carotid arteries detect the drop in pH / rise in \(pCO_2\). 3. Nerve impulses are sent to the respiratory center in the medulla oblongata, which stimulates the diaphragm and external intercostal muscles. 4. This increases ventilation rate and depth (tidal volume), accelerating \(CO_2\) excretion and \(O_2\) uptake. 5. The cardiovascular center in the medulla receives inputs and increases sympathetic stimulation via the cardiac nerve to the sinoatrial (SA) node. 6. This increases heart rate and force of contraction (stroke volume), increasing cardiac output. 7. Vasodilation in skeletal muscle arterioles and vasoconstriction in splanchnic/renal circulations direct blood flow to working muscles.

Award [1] for each correct point up to [6.8]:
- \(CO_2\) from respiration decreases blood pH [1]
- Chemoreceptors in medulla/carotid/aorta detect low pH/high \(CO_2\) [1]
- Respiratory center in medulla stimulates intercostals and diaphragm to increase ventilation [1]
- Sympathetic nervous system stimulates SA node to increase heart rate [1]
- Stroke volume increases, elevating overall cardiac output [1]
- Local vasodilation increases blood flow to active muscles [1]
- Vasoconstriction reduces blood flow to digestive system / kidneys [1]

1. Cyclins are regulatory proteins that fluctuate in concentration during different phases of the cell cycle. 2. Cyclins bind to and activate specific cyclin-dependent kinases (CDKs). 3. The activated cyclin-CDK complex phosphorylates target proteins. 4. This phosphorylation activates or deactivates target proteins, triggering progression to the next phase of the cell cycle. 5. Once a checkpoint is passed, the cyclin is rapidly destroyed by proteolysis, resetting the CDK to an inactive state. 6. Mutations in genes regulating these cyclins/CDKs (such as proto-oncogenes or tumor suppressor genes) can lead to constant CDK activation. 7. This causes rapid, unregulated cell division, leading to oncogenesis / tumor progression.

Award [1] for each correct point up to [6.8]:
- Cyclins fluctuate in concentration during the cell cycle [1]
- Cyclins bind to and activate specific CDKs [1]
- Active cyclin-CDK complexes phosphorylate target proteins [1]
- Phosphorylation triggers specific events/transitions in the cycle [1]
- Cyclins are degraded after use, inactivating the CDK [1]
- Failure of regulation can result from mutations in proto-oncogenes/tumor suppressors [1]
- Uncontrolled cell division leads to tumor/cancer formation [1]

1. Transpiration occurs as water evaporates from cell walls in the leaves and diffuses out through stomata. 2. This creates low pressure (tension) in the xylem of the leaf. 3. Water molecules are highly cohesive due to hydrogen bonding, allowing a continuous column of water to be pulled upward under tension. 4. Adhesion of water to cellulose/lignin in the xylem walls assists in holding the water column up against gravity. 5. Lignin in xylem vessels strengthens the walls, preventing collapse under low pressure. 6. Humidity refers to the concentration of water vapor in the external air. 7. High external humidity decreases the concentration gradient of water vapor, reducing the rate of diffusion out of stomata, thereby lowering the transpiration rate.

Award [1] for each correct point up to [6.8]:
- Transpiration creates tension / negative pressure in leaves [1]
- Cohesion is due to hydrogen bonding between water molecules, maintaining a continuous column [1]
- Adhesion is the attraction between water and xylem cell walls, preventing cavitation [1]
- Transpiration pull moves water from roots to leaves [1]
- Lignified walls prevent xylem vessels from collapsing [1]
- High humidity decreases the concentration gradient of water vapor [1]
- Decreased gradient leads to a lower rate of transpiration [1]

1. The sodium-potassium pump is a primary active transport mechanism that uses energy from ATP hydrolysis. 2. Three intracellular \(Na^+\) ions bind to the pump's active sites. 3. ATP phosphorylates the pump, inducing a conformational change that expels \(Na^+\) to the extracellular fluid. 4. Two extracellular \(K^+\) ions bind to the pump, causing dephosphorylation. 5. The pump reverts to its original conformation, releasing \(K^+\) into the cytoplasm. 6. This establishes a high concentration of \(Na^+\) outside the cell and low concentration inside. 7. The \(Na^+\) gradient is used by sodium-glucose symporters (SGLT) to co-transport glucose into epithelial cells against its concentration gradient, using the downhill movement of \(Na^+\) (secondary active transport).

Award [1] for each correct point up to [6.8]:
- \(Na^+/K^+\) pump uses ATP to transport ions against their concentration gradients [1]
- Three \(Na^+\) ions bind inside and are pumped out [1]
- Phosphorylation by ATP causes conformational change [1]
- Two \(K^+\) ions bind outside and are pumped in [1]
- The pump maintains a steep concentration gradient of \(Na^+\) (high outside, low inside) [1]
- Secondary active transport (co-transport) uses the \(Na^+\) gradient to move glucose [1]
- Glucose is transported against its gradient as \(Na^+\) moves down its electrochemical gradient [1]

Part (a) Solution:
During exercise, active skeletal muscles undergo high rates of aerobic respiration, producing large amounts of carbon dioxide (\(\text{CO}_2\)). This gas diffuses into the blood and cerebrospinal fluid, where it reacts with water to form carbonic acid, which dissociates into hydrogen ions (\(\text{H}^+\)) and hydrogen carbonate, lowering the pH. Chemoreceptors in the medulla oblongata (as well as peripheral chemoreceptors in the carotid and aortic bodies) detect this decrease in pH. The respiratory center in the medulla oblongata responds by sending nerve impulses via the phrenic and intercostal nerves to the diaphragm and external intercostal muscles, increasing both the rate and depth of ventilation to expel \(\text{CO}_2\) and take in \(\text{O}_2\).

Part (b) Solution:
Heart rate is primarily controlled by the sinoatrial (SA) node, which acts as the pacemaker. During physical activity, chemoreceptors detect low pH/high \(\text{CO}_2\) levels and baroreceptors detect blood pressure changes. These receptors send sensory inputs to the cardiovascular center in the medulla oblongata. The medulla oblongata transmits sympathetic nerve impulses via the cardiac nerve to the SA node, releasing noradrenaline to increase the heart rate and force of contraction. Additionally, the hormone adrenaline (epinephrine) is released from the adrenal glands into the bloodstream, directly stimulating the SA node for sustained elevation of heart rate. After exercise, parasympathetic signals via the vagus nerve release acetylcholine to return the heart rate to resting levels.

Part (c) Solution:
To prevent overheating (hyperthermia), the hypothalamus acts as the body's thermostat, detecting increases in core blood temperature and receiving sensory inputs from thermoreceptors in the skin. The nervous system coordinates autonomic responses to dissipate heat. First, vasodilation occurs as arterioles in the skin dilate, directing more blood flow to the superficial capillary networks, allowing heat to be lost to the environment via radiation and convection. Second, the nervous system stimulates sweat glands to secrete sweat. Sweat, which is primarily water, evaporates from the skin surface, absorbing and carrying away body heat due to water's high latent heat of vaporization. Lastly, behavior is modified (e.g., seeking shade or reducing exercise intensity) as a coordinated homeostatic effort.

Part (a) Marking Points (Maximum 4 marks):
- CO2 produced by respiring muscle tissues dissolves in blood/cerebrospinal fluid to form carbonic acid, lowering pH. [1]
- Chemoreceptors in the medulla oblongata (or carotid/aorta) detect the decrease in pH / increase in CO2 concentration. [1]
- The respiratory center in the medulla oblongata coordinates the response. [1]
- Nerve impulses are sent via the phrenic and intercostal nerves to the diaphragm and external intercostal muscles. [1]
- This leads to an increase in the rate and depth of ventilation (breathing). [1]

Part (b) Marking Points (Maximum 5 marks):
- Sinoatrial (SA) node acts as the pacemaker to initiate heart contractions. [1]
- Medulla oblongata (cardiovascular center) receives sensory inputs regarding pH, CO2, or blood pressure. [1]
- Sympathetic nerve pathway (cardiac nerve) carries impulses from the medulla to the SA node. [1]
- Noradrenaline is released at the SA node, increasing heart rate. [1]
- Adrenaline (hormone) is released from the adrenal glands and directly increases SA node activity. [1]
- Parasympathetic system (vagus nerve) releases acetylcholine to slow heart rate down post-exercise. [1]

Part (c) Marking Points (Maximum 7 marks):
- Hypothalamus acts as the control center / thermostat for temperature regulation. [1]
- Thermoreceptors in the skin and hypothalamus detect a rise in core/blood temperature. [1]
- Nervous system triggers vasodilation of skin arterioles. [1]
- Vasodilation increases blood flow near the skin surface, increasing heat loss via radiation/convection. [1]
- Sweat glands are stimulated by the nervous system to secrete sweat onto the skin. [1]
- Evaporation of sweat cools the skin because water has a high latent heat of vaporization. [1]
- Heat energy from the blood is transferred to the water in sweat to break hydrogen bonds. [1]
- Negative feedback mechanisms return body temperature back to the normal set point. [1]

(a) The independent variable is the concentration of the NaCl solution.
(b) As the concentration of NaCl increases, the percentage change in mass decreases. There is a negative correlation/relationship, transitioning from an increase in mass (positive percentage change) at low concentrations to a decrease in mass (negative percentage change) at higher concentrations.
(c) The isotonic concentration is approximately 0.33 to 0.35 mol dm\(^{-3}\). This is determined by finding the NaCl concentration where there is 0% change in mass, which lies between the concentrations of 0.2 mol dm\(^{-3}\) (where mass increased by 5.0%) and 0.4 mol dm\(^{-3}\) (where mass decreased by 2.5%).

(a) 1 mark for: Concentration of NaCl solution. [1]

(b) Max 2 marks for:
- Percentage change in mass decreases as NaCl concentration increases. [1]
- Mass increases/is positive at low concentrations and decreases/is negative at high concentrations. [1]

(c) Max 2 marks for:
- Identifying that the isotonic point is where there is 0% change in mass (no net water movement). [1]
- Estimating the concentration to be between 0.2 and 0.4 mol dm\(^{-3}\) (accept values in range 0.30 to 0.35 mol dm\(^{-3}\)). [1]

(a) The KOH absorbs the carbon dioxide released by the germinating mung beans during aerobic respiration.
(b) As the seeds respire, they consume oxygen gas from the chamber. The carbon dioxide they produce is continuously absorbed by the KOH, reducing the overall volume and pressure of gas inside the chamber. The higher external atmospheric pressure then pushes the liquid droplet toward the lower pressure chamber.
(c) The rate of oxygen consumption will increase. This occurs because higher temperature increases the kinetic energy of both enzymes and substrates, leading to more frequent collisions and a higher overall rate of respiration.

(a) 1 mark for: To absorb carbon dioxide (CO2) produced by respiration. [1]

(b) Max 2 marks for:
- Germinating seeds consume oxygen gas from the chamber. [1]
- The CO2 produced is absorbed by KOH, reducing overall gas pressure/volume inside the chamber. [1]
- Atmospheric pressure pushes the liquid droplet inward. [1]

(c) Max 2 marks for:
- Rate of oxygen consumption increases. [1]
- Because higher temperature increases kinetic energy of enzymes/substrates, increasing the rate of metabolic reactions. [1]

(a) The temperature of the reaction mixture, the pH, or the concentration/volume of the yeast extract (catalase source) must be kept constant.
(b) As hydrogen peroxide concentration increases from 1% to 4%, the volume of oxygen increases rapidly. From 4% to 6%, the volume of oxygen plateaus or levels off, showing very little change.
(c) At 3%, the substrate concentration is limiting, meaning some enzyme active sites are unoccupied, so increasing concentration increases the rate. At 6%, the enzyme active sites are fully saturated with substrate, meaning the enzyme concentration is now the limiting factor, and further increases in substrate concentration do not increase the reaction rate.

(a) 1 mark for any of the following: temperature / pH / concentration of catalase / volume of catalase extract [1]. Do not accept "amount" of enzyme.

(b) Max 2 marks for:
- Volume of oxygen increases as substrate concentration increases. [1]
- Rate of increase is rapid at lower concentrations (1% to 4%) and levels off/plateaus at higher concentrations (5% to 6%). [1]

(c) Max 2 marks for:
- At 3%, substrate concentration is limiting / enzyme active sites are not all occupied. [1]
- At 6%, enzymes are saturated with substrate / all active sites occupied. [1]
- Enzyme concentration is the limiting factor at 6%. [1]

During intense exercise, increased cellular respiration elevates blood \(CO_2\) levels, which lowers blood pH. Chemoreceptors located in the carotid arteries, aorta, and medulla oblongata detect this decrease in pH and send nerve impulses to the cardiovascular center in the medulla oblongata. The medulla oblongata then transmits impulses via the sympathetic nerve to the sinoatrial (SA) node of the heart, releasing noradrenaline to increase the rate of depolarization, thereby increasing the heart rate.

Award [1] for each of the following up to [5 max]:
- Increased respiration during exercise produces more \(CO_2\), which reacts with water to form carbonic acid, lowering blood pH / increasing \(H^+\) concentration.
- Chemoreceptors in the medulla oblongata, carotid arteries, and aorta detect this drop in pH.
- Nerve impulses are sent from the chemoreceptors to the cardiovascular center in the medulla oblongata.
- The cardiovascular center sends sympathetic nerve impulses (via the accelerator nerve) to the sinoatrial (SA) node.
- Noradrenaline (norepinephrine) is released at the SA node, increasing the rate of cardiac pacemaker potential / heart rate.

Under anaerobic conditions, oxygen is unavailable to act as the terminal electron acceptor in the electron transport chain, stopping the link reaction and Krebs cycle. To maintain glycolysis (the only source of ATP), NADH must be oxidized back to \(NAD^+\). Pyruvate is reduced to lactate, which oxidizes NADH to \(NAD^+\), replenishing the pool needed for glycolysis. After exercise, lactate is transported via the blood to the liver, where it is converted back into pyruvate or glucose using oxygen during recovery (repaying the oxygen debt).

Award [1] for each of the following up to [5 max]:
- Anaerobic glycolysis produces 2 net ATP, 2 pyruvate, and reduces \(NAD^+\) to NADH.
- In the absence of oxygen, the link reaction, Krebs cycle, and oxidative phosphorylation cannot occur.
- Glycolysis requires a continuous supply of \(NAD^+\) to continue generating ATP.
- Pyruvate is reduced to lactate by accepting electrons/hydrogen from NADH, regenerating \(NAD^+\).
- Lactate is transported by blood to the liver, where it is oxidized back to pyruvate/glucose (via gluconeogenesis) during recovery (repaying oxygen debt).

The sodium-potassium pump actively transports three sodium ions (\(Na^+\)) out of the cell and two potassium ions (\(K^+\)) into the cell against their concentration gradients using energy from ATP hydrolysis. ATP binds to the pump, phosphorylating it and causing a conformational change that releases the three \(Na^+\) ions into the extracellular fluid. Two extracellular \(K^+\) ions then bind, triggering dephosphorylation and returning the pump to its original shape, which releases the \(K^+\) ions intracellularly. Because three positive charges are pumped out for every two pumped in, this unequal movement of charge creates a net negative potential inside the cell, establishing the resting potential.

Award [1] for each of the following up to [5 max]:
- The pump binds three intracellular sodium (\(Na^+\)) ions.
- ATP is hydrolyzed and the pump is phosphorylated, which induces a conformational change.
- Three \(Na^+\) ions are released to the extracellular fluid.
- Two extracellular potassium (\(K^+\)) ions bind to the pump, triggering dephosphorylation.
- The pump returns to its original conformation, releasing two \(K^+\) ions inside the cell.
- Since more positive charges (3 \(Na^+\)) are moved out than are moved in (2 \(K^+\)), this creates a net negative charge inside the cell (electrogenic role).

An animal, such as a mouse, is injected with the target antigen to stimulate an immune response and the clonal selection of B cells. Plasma B cells producing the specific antibody are harvested from the animal's spleen. These B cells are then fused with tumor/myeloma cells to form hybridoma cells, which are both immortal and antibody-producing. The fusion is facilitated by a fusing agent like polyethylene glycol (PEG) or electrofusion. The resulting hybridoma cells are cultured in selective HAT medium, screened to isolate the specific clones producing the desired antibody, and then cultured to yield large quantities of monoclonal antibodies.

Award [1] for each of the following up to [5 max]:
- Antigen is injected into an animal (e.g., mouse) to stimulate an immune response / clonal selection of B cells.
- Active/plasma B cells producing the specific antibody are harvested from the animal's spleen.
- Harvested B cells are fused with tumor/myeloma cells to form hybridoma cells.
- Fusion is facilitated using a fusing agent (such as polyethylene glycol / PEG or electrofusion).
- Hybridoma cells are cultured in selective medium (such as HAT medium) and screened to identify the specific clone.
- Selected hybridomas are cloned/cultured to harvest large, pure quantities of monoclonal antibodies.