2023 IB DP Biology Practice Paper with Answers

During exercise, sympathetic stimulation is activated. This leads to: 1. An increase in cardiac output by increasing both heart rate and stroke volume (force of contraction). 2. Bronchodilation of the airways in the lungs to reduce resistance and increase ventilation. 3. Vasoconstriction of arterioles supplying non-essential organs (such as the kidneys and digestive tract) to redirect blood flow to active skeletal muscles (making choice A incorrect). Vasodilation occurs in active muscles and coronary arteries. The vagus nerve is part of the parasympathetic nervous system, which decreases heart rate, not increases it (making choice D incorrect).

Award 1 mark for the correct option (B). Reject A because sympathetic stimulation causes vasoconstriction, not vasodilation, of arterioles supplying the digestive tract and kidneys. Reject C because ventilation rate increases and coronary arteries dilate. Reject D because the vagus nerve is parasympathetic and slows heart activity.

Dehydration leads to an increase in blood osmolarity (solute concentration), which is detected by hypothalamic osmoreceptors. This stimulates the release of Antidiuretic Hormone (ADH) from the posterior pituitary gland. ADH travels in the blood to the kidneys, where it binds to receptors on the collecting duct cells. This triggers a signaling cascade resulting in the insertion of aquaporin water channels into the apical membranes, increasing water reabsorption back into the blood and producing concentrated urine.

Award 1 mark for the correct option (B). Reject A because dehydration increases ADH release, not decreases it. Reject C because decreased aldosterone would decrease sodium retention, and aldosterone is not the primary immediate response to high blood osmolarity. Reject D because increasing GFR would increase urine output, worsening dehydration.

Character displacement is the evolutionary divergence of traits in sympatric populations (coexisting in the same area) compared to allopatric populations (living in separate areas). When species A and B live together, competition for seeds of intermediate sizes drives natural selection for different beak sizes, allowing resource partitioning. When living alone, species A faces less interspecific competition and can utilize a broader range of seed sizes, resulting in a broader range of beak sizes.

Award 1 mark for the correct option (B). Reject A because competitive exclusion would lead to the elimination of one species from the habitat. Reject C because biomagnification refers to the concentration of toxins up a food chain. Reject D because primary succession is the colonization of previously lifeless areas.

In X-linked dominant inheritance: 1. An affected father has one X chromosome with the dominant disease allele (\(X^D Y\)). 2. He passes his Y chromosome to all of his sons (so they do not inherit the condition from him). 3. He passes his \(X^D\) chromosome to all of his daughters, so they must all inherit the dominant allele and exhibit the condition. 4. An affected mother can be heterozygous (\(X^D X^d\)) or homozygous (\(X^D X^D\)) and can pass the mutant X chromosome to either sons or daughters with equal probability.

Award 1 mark for the correct option (C). Reject A because autosomal recessive traits would not show a pattern where 100% of daughters of affected fathers are affected while 0% of sons are affected. Reject B because Y-linked traits are only passed from father to son. Reject D because if it were X-linked recessive, an affected father (\(X^d Y\)) would only pass the carrier state to daughters, not the fully affected condition (unless the mother also carried the allele).

Apoplastic phloem loading is an active transport mechanism. Proton pumps (H+-ATPases) in the plasma membrane of companion cells actively transport hydrogen ions (\(H^+\)) out of the cell into the cell wall spaces. This builds up a high concentration of protons outside the cell. The protons then diffuse back down their electrochemical gradient into the companion cell through a sucrose-proton symporter (cotransport protein), bringing sucrose against its concentration gradient with them. Sucrose then diffuses into the sieve tube element via plasmodesmata.

Award 1 mark for the correct option (B). Reject A because passive loading through plasmodesmata is symplastic and does not represent the active loading described. Reject C because animal cells use Na+/K+ pumps, but plants use H+ pumps for secondary active transport. Reject D because starch is insoluble and cannot be transported in phloem; sucrose must remain as a soluble sugar.

During early ventricular systole, the ventricles begin to contract, which rapidly increases the pressure of the blood inside them. This pressure causes the atrioventricular (AV) valves (tricuspid and bicuspid) to snap shut to prevent backflow. However, the pressure in the ventricles is not yet high enough to exceed the pressure in the aorta and pulmonary artery, so the semilunar valves remain closed. Because all four valves are closed, no blood can enter or leave the ventricles, resulting in 'isovolumetric' contraction.

Award 1 mark for the correct option (A). Reject B because during atrial systole, the AV valves are open. Reject C because during ventricular diastole, the ventricles are relaxing. Reject D because during isovolumetric relaxation, the ventricles are relaxing (not contracting) after the semilunar valves close but before the AV valves open.

Thyroxin (T4) is secreted by the thyroid gland in response to Thyroid Stimulating Hormone (TSH) from the anterior pituitary (which is itself stimulated by Thyrotropin-Releasing Hormone, TRH, from the hypothalamus). Thyroxin acts on almost all body cells to increase their basal metabolic rate, which also increases body temperature. High levels of thyroxin exert negative feedback on both the hypothalamus and pituitary gland to inhibit further release of TRH and TSH.

Award 1 mark for the correct option (B). Reject A because insulin and glucagon regulate blood glucose. Reject C because thyroxin increases body temperature and metabolic rate, and is not regulated by the adrenal medulla. Reject D because calcitonin (also from the thyroid) and PTH regulate calcium homeostasis, not thyroxin.

As a population reaches its carrying capacity (\(K\)), density-dependent limiting factors (like food shortages, disease, and predation) increase environmental resistance. In the logistic growth model, represented by the equation \(dN/dt = rN(K-N)/K\), as \(N\) approaches \(K\), the term \((K-N)/K\) approaches 0, which reduces the overall population growth rate (\(dN/dt\)) to zero, leading to a stable population size.

Award 1 mark for the correct option (B). Reject A because birth rate decreases and/or death rate increases as resources become scarce. Reject C because carrying capacity (\(K\)) is determined by environmental factors, not by population size itself. Reject D because the population transitions from exponential growth to logistic growth as it approaches carrying capacity.

During intense exercise, increased cell respiration releases more CO2, which dissolves in blood plasma to form carbonic acid, lowering blood pH. Chemoreceptors in the medulla oblongata, carotid bodies, and aortic bodies detect this decrease in pH and send nerve impulses to the control centres in the medulla oblongata. The medulla oblongata then increases the rate of ventilation and increases heart rate via sympathetic nerve stimulation of the sinoatrial (SA) node to rapidly deliver oxygen and remove carbon dioxide.

Award [1] mark for identifying A as the correct option. Other options are incorrect because baroreceptors monitor blood pressure, epinephrine increases tidal volume, and exercise-induced carbon dioxide accumulation decreases rather than increases blood pH.

Drinking a large volume of water dilutes the blood plasma, lowering its osmolarity. Hypothalamic osmoreceptors detect this decrease and reduce their signaling. Consequently, the posterior pituitary gland releases less antidiuretic hormone (ADH). Lower ADH levels reduce the water permeability of the collecting ducts in the kidneys, meaning less water is reabsorbed back into the blood, and a large volume of dilute urine is excreted.

Award [1] mark for identifying B as the correct option. Option A is incorrect because water intake decreases plasma solute concentration. Option C is incorrect because aldosterone regulates sodium, and ADH controls water permeability. Option D is incorrect because ADH release would decrease.

Resource partitioning occurs when species living in the same habitat use different resources or use the same resource in different ways. By foraging in different parts of the same tree, the warblers minimize niche overlap and reduce direct interspecific competition, enabling multiple species to co-exist in the same habitat.

Award [1] mark for identifying C as the correct option. Reject A because both species continue to co-exist. Reject B because this is interaction between different species (interspecific), not within the same species. Reject D because succession refers to community changes over time, not spatial distribution.

Identify the recombinant phenotypes, which are the non-parental phenotypes: Purple stem, potato leaves (112) and Green stem, cut leaves (104). The total number of offspring is 388 + 396 + 112 + 104 = 1000. The sum of the recombinants is 112 + 104 = 216. The recombination frequency is (216 / 1000) * 100 = 21.6%.

Award [1] mark for identifying B as the correct option. Show working: Recombination Frequency = (Number of recombinant offspring / Total offspring) * 100 = (216 / 1000) * 100 = 21.6%.

Water columns in xylem are under high tension. The continuous flow depends on cohesion (hydrogen bonds between water molecules that hold them together, preventing the column from breaking) and adhesion (hydrogen bonds between water molecules and the hydrophilic cellulose components of the xylem cell wall, which helps support the column against gravity).

Award [1] mark for identifying B as the correct option. Reject A because density differences do not provide tensile strength. Reject C because xylem vessel elements are dead at maturity and cannot perform active transport. Reject D because water has a high specific heat capacity.

Archaea are prokaryotic organisms. Unlike Bacteria, their cell walls lack peptidoglycan, and their cell membranes are composed of ether-linked lipids (often branched hydrocarbons) rather than the ester-linked lipids found in Bacteria and Eukaryota.

Award [1] mark for identifying B as the correct option. Reject A and D because eukaryotes have membrane-bound organelles and ester-linked lipids. Reject C because bacteria have ester-linked lipids and peptidoglycan cell walls.

Glycolysis requires oxidized NAD+ to convert glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate, producing NADH. Under anaerobic conditions, there is no electron transport chain to oxidize NADH back to NAD+. Fermentation processes use pyruvate or acetaldehyde as an electron acceptor to oxidize NADH back to NAD+, enabling glycolysis (and thus ATP production) to continue.

Award [1] mark for identifying B as the correct option. Reject A because fermentation itself does not produce any additional ATP. Reject C because oxygen is not produced. Reject D because yeast cells produce ethanol, not lactic acid.

A competitive inhibitor competes with the substrate for binding to the active site. Because the inhibitor and substrate compete, increasing the substrate concentration to a very high level eventually overwhelms the inhibitor, allowing the reaction to reach the same maximum velocity (Vmax). However, because more substrate is needed to overcome the competition, the Michaelis constant (Km, which is the substrate concentration at 1/2 Vmax) increases.

Award [1] mark for identifying B as the correct option. Reject A because non-competitive inhibitors decrease Vmax and cannot be overcome by high substrate concentrations. Reject C because irreversible inhibitors reduce active enzyme concentration, lowering Vmax.

During exercise, increased cellular respiration elevates blood carbon dioxide levels, which lowers blood pH. This is detected by chemoreceptors in the carotid and aortic bodies, as well as the medulla oblongata. Nerve impulses are sent to the cardiovascular centre in the medulla oblongata, which increases sympathetic nerve impulses to the sinoatrial (SA) node to increase heart rate, while simultaneously stimulating the adrenal glands to release epinephrine into the blood, reinforcing and prolonging the effect.

Award 1 mark for the correct option (C). Award 0 marks for incorrect options. Reject option A because the parasympathetic nervous system decreases heart rate via acetylcholine. Reject option B because the motor cortex does not directly stimulate the SA node in response to baroreceptors. Reject option D because increased blood pressure would trigger negative feedback to decrease heart rate, not increase it.

The sight and smell of food triggers the cephalic phase of digestion, which is mediated entirely by the nervous system (vagus nerve). This directly stimulates gastric secretion (including hydrochloric acid from parietal cells) before food even enters the stomach. In contrast, the other options involve the action of hormones (gastrin, secretin, and cholecystokinin) circulating in the blood.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options.

Normally, TRH from the hypothalamus stimulates TSH from the pituitary, which stimulates thyroxine from the thyroid. Thyroxine exerts negative feedback on both. If the pituitary cannot secrete TSH, thyroxine levels will decrease significantly. Because thyroxine levels are very low, there is minimal negative feedback on the hypothalamus. Consequently, the hypothalamus will secrete high amounts of TRH in an attempt to stimulate the pituitary.

Award 1 mark for the correct option (A). Award 0 marks for incorrect options.

When osmoreceptors in the hypothalamus detect high blood solute concentration (dehydration), the posterior pituitary gland is stimulated to release ADH (antidiuretic hormone). ADH targets the cells of the collecting ducts in the kidney, causing the insertion of aquaporin water channels into their membranes, which increases water reabsorption.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options.

The stationary phase (Phase IV) is reached when the growth rate becomes zero, meaning the natality rate equals the mortality rate (in a closed culture with no migration). The primary limiting factor for a yeast population in a closed vessel is the depletion of the finite nutrient resources in the growth medium.

Award 1 mark for the correct option (C). Award 0 marks for incorrect options.

Gause's principle of competitive exclusion states that two species competing for the exact same limiting resource cannot coexist if other ecological factors remain constant. One species will always have a slight advantage, leading to the exclusion of the other.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options.

A test cross of a dihybrid (DdHh x ddhh) is expected to produce a 1:1:1:1 ratio if the genes assort independently. Here, the offspring show a high frequency of parental phenotypes (Tall, hairy and Dwarf, hairless) and a low frequency of recombinant phenotypes (Tall, hairless and Dwarf, hairy). This indicates gene linkage. The total number of offspring is 1000, and the number of recombinant offspring is 200 (104 + 96). The recombination frequency is (200 / 1000) * 100% = 20%.

Award 1 mark for the correct option (B). Award 0 marks for incorrect options.

In apoplastic phloem loading, protons (H+) are actively pumped out of the companion cells into the cell wall space using ATP. This builds an electrochemical gradient. Protons then diffuse back down their gradient into the companion cells through a co-transporter protein, which simultaneously transports sucrose against its concentration gradient into the companion cells.

Award 1 mark for the correct option (A). Award 0 marks for incorrect options.

A decrease in blood pressure is detected by baroreceptors (stretch receptors) in the carotid sinus and aortic arch. This information is sent to the cardiovascular control center in the medulla oblongata. In response, the medulla increases sympathetic nervous output and decreases parasympathetic output. The increased sympathetic stimulation acts on the sinoatrial (SA) node to increase heart rate, on the ventricles to increase contractility, and on smooth muscle in arterioles to cause vasoconstriction, thereby raising blood pressure back to normal levels.

Award 1 mark for the correct option (C). Correct option identifies increased sympathetic stimulation to the SA node and vasoconstriction of arterioles as the correct homeostatic reflex to a decrease in blood pressure. Incorrect options describe parasympathetic actions or sympathetic inhibition, which would further lower blood pressure.

The cephalic phase of digestion is initiated by the sight, smell, taste, or thought of food. These sensory stimuli are processed by the brain, which sends impulses along the vagus nerve (parasympathetic nervous system) to the stomach. This nervous pathway stimulates gastric glands to secrete gastric juice (containing hydrochloric acid and pepsinogen) and triggers the release of gastrin, preparing the stomach for incoming food.

Award 1 mark for the correct option (B). The cephalic phase is characterized by nervous system regulation (via the vagus nerve) triggered by sensory inputs, preceding the mechanical or chemical entry of food into the stomach.

An increase in blood solute concentration (high osmolarity) is detected by hypothalamic osmoreceptors. This triggers the posterior pituitary gland to release antidiuretic hormone (ADH) into the bloodstream. ADH travels to the kidneys, where it binds to receptors on the cells of the collecting ducts, increasing the insertion of aquaporins (water channels) into their apical membranes. This increases water reabsorption back into the blood, producing a lower volume of highly concentrated urine and restoring blood osmolarity to normal.

Award 1 mark for the correct option (B). The pathway correctly links high solute concentration with increased ADH release, increased aquaporin density, and increased water reabsorption in the collecting ducts.

In response to cold stress, the thermoregulatory center in the hypothalamus coordinates physiological changes to conserve and generate heat. This includes vasoconstriction of cutaneous (skin) arterioles to divert blood away from the skin surface (minimizing heat loss) and the stimulation of rapid skeletal muscle contractions (shivering) to generate metabolic heat.

Award 1 mark for the correct option (C). The correct response to cold involves vasoconstriction (to reduce heat loss) and shivering (to generate heat).

Density-dependent factors are those whose effects on the size or growth of the population vary with the population density. In a closed batch culture of yeast, as the population density increases, yeast cells consume glucose and produce ethanol as a byproduct of anaerobic respiration. The accumulation of ethanol to toxic concentrations is directly dependent on yeast density and acts as a limiting factor that stalls population growth, pushing it into the stationary phase.

Award 1 mark for the correct option (B). Ethanol accumulation is a density-dependent metabolic byproduct. Temperature changes and UV light are density-independent factors, while dilution does not limit growth as a result of density changes.

Recombination frequency is calculated as: \(\text{Recombination Frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100\). The parent genotypes are \(GgLl\) (heterozygous) and \(ggll\) (homozygous recessive test-cross partner). The parental-type offspring are Gray, long (415) and Black, vestigial (385), as these represent the linked alleles in their original configurations. The recombinant offspring are Gray, vestigial (105) and Black, long (95). Total offspring \(= 415 + 385 + 105 + 95 = 1000\). Total recombinants \(= 105 + 95 = 200\). Recombination frequency \(= \frac{200}{1000} \times 100 = 20\%\).

Award 1 mark for the correct option (B). Method of calculation requires identifying the recombinants, summing them, and dividing by the total population.

Active transport is used to load sucrose into the phloem sieve tubes at the source. This high concentration of solute decreases the water potential inside the sieve tubes, causing water to enter from the adjacent xylem by osmosis. This accumulation of water increases the hydrostatic pressure at the source. At the sink, sucrose is unloaded, water potential increases, water leaves the phloem, and hydrostatic pressure drops. Sap flows down this hydrostatic pressure gradient from source to sink.

Award 1 mark for the correct option (C). The driving force for mass flow in phloem is the hydrostatic pressure gradient established by active loading of solutes and the resulting osmotic water movement.

Water is pulled up the xylem under tension (negative pressure) due to transpiration. To prevent the vessels from collapsing inward under this substantial sucking force, xylem vessel walls are heavily reinforced with lignin. Lignin is a strong, woody polymer deposited in helical, annular, or reticulate patterns within the cellulose cell wall, providing high tensile strength and structural support.

Award 1 mark for the correct option (B). Lignified cell walls are the specific evolutionary adaptation that prevents structural collapse of xylem vessels under negative pressure.

Carbon dioxide dissolves in the blood to form carbonic acid, which lowers blood pH. Chemoreceptors in the medulla oblongata detect this decrease in pH. The medulla oblongata coordinates a response via the autonomic nervous system, sending nerve impulses to the sinoatrial (SA) node of the heart and the respiratory muscles to increase both heart rate and ventilation rate, thereby accelerating carbon dioxide removal.

Award 1 mark for identifying the correct regulatory center (medulla oblongata), the stimulus (decreased pH), and the coordinated responses (increased ventilation and heart rate). Correct option is A.

An increase in blood solute concentration (high osmolarity) is detected by hypothalamic osmoreceptors, triggering the release of ADH (antidiuretic hormone) from the posterior pituitary. ADH binds to receptors on the collecting duct cells, initiating a cascade that leads to the exocytosis of vesicles containing aquaporins. These aquaporins insert into the luminal membrane, increasing its permeability to water and allowing more water to be reabsorbed back into the bloodstream.

Award 1 mark for linking high blood solute concentration to increased ADH secretion, followed by the insertion of aquaporins into the luminal membrane via exocytosis to increase permeability. Correct option is B.

Bottom-up control occurs when the population size of trophic levels is determined by the availability of resources (like nutrients and primary producers) at the base of the food chain. Here, nutrient runoff increases phytoplankton, which subsequently boosts zooplankton and fish, showing bottom-up control. The shark population is limited by top-down human exploitation (fishing), not bottom-up resources.

Award 1 mark for correctly identifying that resource-driven population increases represent bottom-up control on zooplankton. Correct option is B.

The recombination frequency is calculated as (number of recombinants / total offspring) x 100. Total offspring = 420 + 80 = 500. Recombination frequency = (80 / 500) x 100 = 16%. Because the recombination frequency is less than 50%, the genes do not assort independently, meaning they are linked on the same chromosome.

Award 1 mark for the correct calculation of 16% recombination frequency and the logical deduction that this indicates gene linkage. Correct option is B.

During apoplastic phloem loading, proton pumps actively transport hydrogen ions (protons) out of the companion cells into the cell wall (apoplast) using ATP. This generates a high concentration of protons outside the cell. Protons then diffuse back down their electrochemical gradient into the companion cell through a co-transporter protein, which simultaneously carries sucrose against its concentration gradient.

Award 1 mark for recognizing that proton pumps actively transport hydrogen ions into the cell wall to create the concentration gradient driving sucrose co-transport. Correct option is C.

Increased carbon dioxide and lactic acid production lowers pH in actively respiring tissues. According to the Bohr effect, lower pH shifts the oxygen dissociation curve of hemoglobin to the right. This shift represents a decreased affinity of hemoglobin for oxygen at any given partial pressure, causing hemoglobin to release oxygen more readily to the tissues that need it.

Award 1 mark for identifying that decreased pH causes a rightward shift of the oxygen dissociation curve, reducing affinity and increasing oxygen release. Correct option is C.

To lower body temperature, the hypothalamus coordinates mechanisms to increase heat loss and decrease heat production. This includes: vasodilation of skin arterioles (bringing blood closer to the surface to radiate heat), activation of sweat glands (promoting evaporative cooling), and lowering general metabolic activity. Shivering and vasoconstriction are responses to cold.

Award 1 mark for selecting the correct physiological mechanisms of heat loss (vasodilation, sweating, decreased metabolism). Correct option is B.

Resource partitioning occurs when species divide a niche to avoid direct competition for resources. By feeding on different parts of the same tree (different ages of needles), the two beetle species minimize direct competition and can coexist within the same habitat.

Award 1 mark for identifying resource partitioning as the division of a resource to reduce competition and allow coexistence. Correct option is B.

Detailed explanation of the autonomic control of heart and ventilation rate via medullary chemoreceptors (a), the hormonal action of epinephrine via the bloodstream to target tissues (b), and systemic blood redistribution and stroke volume increase during exercise (c).

a) Award [1] for chemoreceptors in the medulla oblongata (or major arteries) detecting a drop in pH / rise in carbon dioxide concentration. Award [1] for sympathetic impulses sent via the cardiac accelerator nerve to the sinoatrial (SA) node to increase heart rate. Award [1] for impulses sent to the diaphragm / intercostal muscles to increase ventilation rate/depth.
b) Award [1] for the sympathetic nervous system stimulating the adrenal glands/medulla to release epinephrine. Award [1] for epinephrine traveling through the blood to target tissues (linking systems). Award [1] for explaining a specific effect: stimulating glycogenolysis in the liver / causing bronchodilation in the lungs / increasing cardiac stroke volume.
c) Award [1] for each correct cardiovascular change up to [2]: Vasodilation of arterioles supplying active skeletal muscles; Vasoconstriction of arterioles supplying non-essential organs (e.g., gut/kidneys); Increased stroke volume.

Comprehensive description of the loop of Henle countercurrent multiplier system (a), the cellular mechanism of ADH action on aquaporin insertion in the collecting duct membranes (b), and structural adaptations of nephrons to different environmental water availabilities (c).

a) Award [1] for stating that the descending limb is permeable to water but impermeable to solute (water leaves filtrate by osmosis). Award [1] for stating that the ascending limb actively transports sodium and chloride ions out into the medulla tissue fluid (and is impermeable to water). Award [1] for establishing a high solute concentration / hypertonic gradient in the medulla tissue.
b) Award [1] for high blood solute concentration stimulating ADH release from the posterior pituitary. Award [1] for ADH binding to receptors on the collecting duct cells, initiating a signaling cascade. Award [1] for the insertion of aquaporins into the cell membranes, making the collecting duct highly permeable to water.
c) Award [1] for desert mammals having longer loops of Henle / thicker medulla to allow greater water reabsorption. Award [1] for freshwater mammals having shorter loops of Henle / thinner medulla as water conservation is less critical.

Detailed analysis of the three phases of sigmoid growth in relation to birth and death rates (a), categorization and examples of limiting factors (b), and definition of carrying capacity with ecological consequences of overshoot (c).

a) Award [1] for exponential phase: birth rate far exceeds death rate, leading to rapid increase. Award [1] for transitional phase: birth rate begins to fall and/or death rate begins to rise, slowing growth. Award [1] for plateau phase: birth rate equals death rate, and growth rate becomes zero.
b) Award [1] for defining density-dependent factors as those whose effect depends on population density, with a valid example (e.g., competition for food, disease, predation). Award [1] for defining density-independent factors as those affecting the population regardless of its density, with a valid example (e.g., natural disasters, extreme temperatures). Award [1] for contrasting their mechanisms of action / impact relative to density.
c) Award [1] for defining carrying capacity (\(K\)) as the maximum population size that an environment can sustainably support over time. Award [1] for explaining that overshoot leads to overexploitation of resources, reducing the environment's future carrying capacity and causing a rapid population crash/starvation.

Step-by-step resolution of dihybrid test cross involving autosomal linkage: independent assortment ratio prediction (a), interpretation of deviation from expected ratio due to linkage and crossing-over (b), calculation of recombination frequency from experimental data (c), and determination of recombinant genotypes (d).

a) Award [1] for 1:1:1:1 (or equal proportions of the four phenotypes).
b) Award [1] for identifying that the genes are linked on the same chromosome. Award [1] for stating that the dominant alleles \(R\) and \(T\) (and recessive \(r\) and \(t\)) are located together on the parental chromosomes. Award [1] for identifying that the less frequent phenotypes (tall/yellow and dwarf/red) are recombinants. Award [1] for explaining that recombinants only arise from crossing over between non-sister chromatids during prophase I of meiosis.
c) Award [1] for showing correct working: \(\frac{21 + 19}{400} \times 100\%\) (or \(\frac{40}{400}\)). Award [1] for the correct final answer of 10%.
d) Award [1] for both correct genotypes: \(Ttrr\) and \(ttRr\) (accept equivalent representations).

Detailed explanation of transpiration pull driven by cohesion-tension theory in xylem (a), methodology of using a potometer to estimate transpiration rate via water uptake, including controls and precautions (b), and adaptation of xerophytes to arid environments (c).

a) Award [1] for cohesion where hydrogen bonding between water molecules maintains a continuous water column. Award [1] for adhesion where water molecules adhere to the hydrophilic cellulose walls of xylem vessels, resisting gravity/tension. Award [1] for xylem vessels having a hollow lumen / being dead / lacking end walls, reducing resistance to flow. Award [1] for lignified cell walls providing structural support to prevent the xylem from collapsing under negative pressure/tension.
b) Award [1] for explaining that a potometer measures the rate of water uptake by tracking the movement of an air bubble in a capillary tube. Award [1] for an essential precaution: cutting the plant stem under water to prevent air bubbles from blocking xylem vessels / sealing all joints with petroleum jelly (Vaseline) to ensure the system is airtight. Award [1] for stating how the rate is calculated: distance moved by the bubble divided by time (or calculating volume of water using cylinder volume formula).
c) Award [1] for any of the following: Thick waxy cuticle to block cuticular transpiration; Rolled leaves to trap humid air around stomata, reducing concentration gradient; Sunken stomata to create a microclimate of high humidity; Reduced leaves/spines to decrease total surface area for transpiration.

**(a) Outline the principle of negative feedback, using the control of blood glucose concentration as an example. [4]**
- Negative feedback is a regulatory mechanism where a change in a physiological variable triggers responses that counteract the change, restoring the variable back to its normal set point.
- If blood glucose levels rise (e.g., after eating), beta cells in the pancreatic islets of Langerhans detect this change and secrete insulin into the bloodstream.
- Insulin stimulates body cells (especially muscle and liver cells) to increase glucose uptake and triggers glycogenesis (conversion of glucose into glycogen for storage), lowering blood glucose levels.
- If blood glucose levels drop (e.g., during exercise or fasting), alpha cells in the pancreatic islets detect this and secrete glucagon into the bloodstream.
- Glucagon stimulates liver cells to undergo glycogenolysis (breakdown of glycogen to glucose) and release glucose into the blood, raising blood glucose levels back to normal.

**(b) Explain the role of the kidney and antidiuretic hormone (ADH) in maintaining water balance (osmoregulation) when the body is dehydrated. [6]**
- Dehydration causes an increase in blood solute concentration (osmolarity) / a decrease in blood water potential.
- Osmoreceptors in the hypothalamus detect this change in blood osmolarity.
- The hypothalamus stimulates the posterior pituitary gland to release antidiuretic hormone (ADH) into the bloodstream.
- ADH travels to the kidneys, where it binds to specific receptors on the cells lining the distal convoluted tubules and collecting ducts.
- This binding activates a secondary messenger system that causes vesicles containing aquaporins (water channels) to fuse with the apical membrane of these cells.
- The insertion of aquaporins significantly increases the permeability of the collecting duct to water.
- As the filtrate passes through the collecting duct, water is reabsorbed by osmosis into the highly concentrated (hypertonic) interstitial fluid of the renal medulla and then back into the capillaries.
- As a result, a smaller volume of highly concentrated (hypertonic) urine is excreted, conserving water in the body and returning blood osmolarity to normal.

**(c) Describe the physiological mechanisms used by endothermic homeotherms to increase core body temperature when exposed to cold conditions. [5]**
- Thermoreceptors in the skin and the hypothalamus detect a decrease in external or core body temperature.
- **Vasoconstriction:** Sympathetic nerves stimulate arterioles supplying the skin capillaries to constrict, while shunt vessels dilate. This reduces blood flow to the skin surface, minimizing heat loss via radiation, conduction, and convection.
- **Shivering:** Rapid, involuntary cycles of skeletal muscle contraction and relaxation are initiated, converting metabolic energy into heat.
- **Piloerection:** Hair erector muscles contract, pulling hairs upright to trap a layer of warm, still air close to the skin, which acts as an insulator (though this is more effective in furry mammals than humans).
- **Metabolic rate increase:** Endocrine glands release hormones such as thyroxine (from the thyroid gland) and adrenaline (from the adrenal medulla), which increase cellular metabolism and generate heat.

**(a) Negative feedback and blood glucose [Max 4 marks]:**
- Award 1 mark for defining negative feedback as counteracting a deviation from a set point to restore homeostatic balance.
- Award 1 mark for noting that high blood glucose stimulates insulin secretion from pancreatic beta cells.
- Award 1 mark for noting that insulin promotes glucose uptake by cells/glycogenesis in the liver/muscle.
- Award 1 mark for noting that low blood glucose stimulates glucagon secretion from pancreatic alpha cells.
- Award 1 mark for noting that glucagon promotes glycogenolysis/release of glucose from the liver.

**(b) Osmoregulation and ADH [Max 6 marks]:**
- Award 1 mark for stating that dehydration increases blood osmolarity/solute concentration (or decreases water potential).
- Award 1 mark for identifying that osmoreceptors in the hypothalamus detect this change.
- Award 1 mark for stating that ADH is released by the posterior pituitary gland.
- Award 1 mark for noting that ADH increases the permeability of the collecting duct (and distal convoluted tubule) to water.
- Award 1 mark for describing the mechanism: ADH causes aquaporins (water channels) to be inserted into the membrane.
- Award 1 mark for explaining that water is reabsorbed by osmosis into the hypertonic medulla/blood.
- Award 1 mark for noting that this results in a small volume of highly concentrated/hypertonic urine.

**(c) Response to cold [Max 5 marks]:**
- Award 1 mark for stating that thermoreceptors in the skin/hypothalamus detect the temperature drop.
- Award 1 mark for vasoconstriction: constriction of skin arterioles reducing blood flow and heat loss at the surface.
- Award 1 mark for shivering: rapid muscle contractions generating metabolic heat.
- Award 1 mark for piloerection: contraction of hair erector muscles trapping an insulating layer of air.
- Award 1 mark for increased metabolic rate: release of thyroxine/adrenaline to raise cellular respiration.

**Quality of Communication [1 mark]:**
- Award 1 mark if the overall response is written in a clear, structured, and coherent manner using precise biological terms (e.g., islet of Langerhans, aquaporins, vasoconstriction, medulla, etc.).

**(a) Explain how the nervous and endocrine systems cooperate in the 'fight-or-flight' response when an animal perceives a threat. [4]**
- When a threat is perceived, sensory inputs are processed by the brain, activating the amygdala and then the hypothalamus.
- The hypothalamus sends nerve impulses along sympathetic fibers of the autonomic nervous system directly to the adrenal medulla.
- This nervous stimulation causes the adrenal medulla to rapidly secrete the hormones adrenaline (epinephrine) and noradrenaline into the blood.
- This represents a direct nervous-to-endocrine relay (neuroendocrine integration).
- Concurrently, the hypothalamus activates the slower HPA (hypothalamic-pituitary-adrenal) endocrine axis, releasing Corticotropin-Releasing Hormone (CRH), which stimulates the pituitary to release ACTH, which in turn prompts the adrenal cortex to release cortisol.
- Together, these nervous and endocrine signals prepare the body by increasing heart rate, dilating airways, mobilizing glucose (via glycogen breakdown), and prioritizing blood flow to skeletal muscles.

**(b) Describe how the nervous, cardiovascular, and respiratory systems are integrated to meet the increased metabolic demands of active skeletal muscles during strenuous exercise. [6]**
- During exercise, active skeletal muscles undergo high rates of cellular respiration, consuming oxygen (\(O_2\)) and generating carbon dioxide (\(CO_2\)) and hydrogen ions (\(H^+\) / lactic acid).
- The accumulation of \(CO_2\) and \(H^+\) lowers blood pH.
- Chemoreceptors in the medulla oblongata (central) and in the carotid and aortic bodies (peripheral) detect this decrease in pH and increase in \(CO_2\).
- The respiratory center in the medulla oblongata sends more frequent nerve impulses (via the phrenic and intercostal nerves) to the diaphragm and external intercostal muscles, increasing ventilation rate and depth. This maximizes gas exchange in the lungs.
- Simultaneously, the cardiovascular center in the medulla oblongata increases sympathetic nervous output to the sinoatrial (SA) node of the heart, while decreasing parasympathetic (vagus) output.
- This increases both heart rate and stroke volume, boosting cardiac output to deliver blood faster to the muscles.
- Local chemical changes (low \(O_2\), high \(CO_2\), heat) cause local vasodilation of arterioles feeding active muscles, while sympathetic signals cause vasoconstriction of arterioles in non-essential organs (such as the digestive tract), redistributing oxygenated blood where it is needed most.

**(c) Outline how the nervous and endocrine systems coordinate the secretion of digestive juices in the stomach. [5]**
- The secretion of gastric juice is coordinated in phases involving both nervous and endocrine mechanisms.
- **Cephalic Phase (Nervous):** The sight, smell, taste, or thought of food triggers impulses in the cerebral cortex and hypothalamus, which send signals down the vagus nerve (parasympathetic) to the stomach.
- This nervous pathway directly stimulates gastric glands in the stomach mucosa to secrete hydrochloric acid (HCl) and pepsinogen.
- The vagus nerve also stimulates endocrine G-cells in the stomach wall to secrete the hormone gastrin into the blood.
- **Gastric Phase (Nervous and Endocrine):** Once food enters the stomach, physical distension (stretching) is detected by stretch receptors in the stomach wall.
- This sends impulses via local nervous pathways (enteric nervous system) and the vagus nerve, further stimulating gastric secretions.
- Chemical components of food (e.g., peptides) also stimulate G-cells directly to secrete more gastrin.
- Gastrin circulates in the blood and binds to receptors on parietal cells, strongly stimulating sustained secretion of hydrochloric acid.
- When the stomach contents become highly acidic (pH drops below 1.5–2), a negative feedback mechanism inhibits gastrin secretion, reducing further acid production.

**(a) Fight-or-flight response [Max 4 marks]:**
- Award 1 mark for stating that the hypothalamus processes the perception of threat.
- Award 1 mark for noting that the sympathetic nervous system stimulates the adrenal medulla.
- Award 1 mark for stating that the adrenal medulla secretes adrenaline/noradrenaline into the bloodstream.
- Award 1 mark for outlining the HPA axis (hypothalamus -> pituitary -> adrenal cortex releasing cortisol) as a secondary endocrine pathway.
- Award 1 mark for mentioning the combined physiological effect (e.g., increased heart rate, glycogen breakdown/mobilization of glucose).

**(b) Exercise integration [Max 6 marks]:**
- Award 1 mark for stating that active muscle respiration increases \(CO_2\) and decreases blood pH.
- Award 1 mark for noting that central/peripheral chemoreceptors detect these chemical changes.
- Award 1 mark for describing the respiratory response: the medulla oblongata stimulates the respiratory muscles (diaphragm/intercostals) to increase rate and depth of breathing.
- Award 1 mark for describing the cardiovascular response: the cardiovascular center in the medulla increases sympathetic input to the SA node.
- Award 1 mark for explaining that this increases heart rate and cardiac output.
- Award 1 mark for explaining vasodilation in active muscles and vasoconstriction in non-essential organs (blood shunting).

**(c) Gastric secretion [Max 5 marks]:**
- Award 1 mark for describing the cephalic phase where sensory stimuli trigger vagus nerve (parasympathetic) signals.
- Award 1 mark for stating that the vagus nerve stimulates gastric glands to secrete gastric juice/HCl/pepsinogen.
- Award 1 mark for stating that the vagus nerve (or stomach stretch) stimulates G-cells to release the hormone gastrin.
- Award 1 mark for describing the gastric phase: distension of the stomach activates stretch receptors leading to local nervous reflexes.
- Award 1 mark for explaining that gastrin travels via blood to stimulate acid secretion by gastric glands.
- Award 1 mark for mentioning the negative feedback loop where high acidity/low pH inhibits gastrin secretion.

**Quality of Communication [1 mark]:**
- Award 1 mark if the overall response is written in a clear, structured, and coherent manner using precise biological terms (e.g., chemoreceptors, medulla oblongata, vagus nerve, gastrin, etc.).

(a) At a light intensity of 0 \(\text{\mu mol m}^{-2}\text{s}^{-1}\), no photosynthesis occurs. The net oxygen production rate is -2.1 \(\text{\mu mol O}_2\text{ dm}^{-3}\text{ min}^{-1}\). This value represents the rate of oxygen consumption due to cellular respiration, which is 2.1 \(\text{\mu mol O}_2\text{ dm}^{-3}\text{ min}^{-1}\).

(b) Gross photosynthesis is calculated as:
Gross Photosynthesis = Net Photosynthesis + Respiration
Gross Photosynthesis = 4.8 + 2.1 = 6.9 \(\text{\mu mol O}_2\text{ dm}^{-3}\text{ min}^{-1}\).

(c) Between 400 and 800 \(\text{\mu mol m}^{-2}\text{s}^{-1}\), the rate increases only marginally from 8.5 to 8.6, indicating that the photosynthetic machinery is operating at maximum capacity under these conditions. Light intensity is no longer the factor that limits the rate of photosynthesis. Other environmental factors, such as ambient temperature or carbon dioxide concentration, are now the limiting factors.

(a) Award [1] mark for the correct rate and units:
- 2.1 \(\text{\mu mol O}_2\text{ dm}^{-3}\text{ min}^{-1}\) (accept -2.1).

(b) Award [2] marks in total:
- [1] mark for correct working shown: \(4.8 + 2.1\) or \(4.8 - (-2.1)\).
- [1] mark for correct final answer: 6.9 \(\text{\mu mol O}_2\text{ dm}^{-3}\text{ min}^{-1}\) (accept unit-free answer if unit was graded in part a, but reject if incorrect units are written here).

(c) Award [2] marks in total:
- [1] mark for stating that the rate of oxygen production plateaus/reaches a maximum/remains virtually constant.
- [1] mark for explaining that light intensity is no longer the limiting factor / another factor (such as carbon dioxide concentration or temperature) is now limiting.

(a) The isotonic concentration is where there is no net movement of water, which corresponds to 0% change in mass. Looking at the data, the mass increases by +5.1% at 0.2 \(\text{mol dm}^{-3}\) and decreases by -1.2% at 0.3 \(\text{mol dm}^{-3}\). By linear interpolation, 0% change occurs at approximately 0.28 \(\text{mol dm}^{-3}\) (accept any value in the range of 0.27 to 0.29 \(\text{mol dm}^{-3}\)).

(b) Pure water (0.0 \(\text{mol dm}^{-3}\)) has a higher water potential (less negative) than the cytoplasm of the potato cells. Consequently, water moves down the water potential gradient from the surrounding solution into the potato cells via osmosis. This entry of water increases the turgor and mass of the tissues.

(c) Before weighing, each potato cylinder must be gently blotted with a paper towel. This ensures that any excess liquid adhering to the external surface of the potato is removed, preventing it from artificially inflating the mass measurement.

(a) Award [2] marks in total:
- [1] mark for correct estimated concentration: 0.28 \(\text{mol dm}^{-3}\) (accept range 0.27 to 0.29 \(\text{mol dm}^{-3}\)).
- [1] mark for explaining that this is the point where there is no net water movement / 0% change in mass.

(b) Award [2] marks in total:
- [1] mark for stating that the surrounding solution (pure water) has a higher water potential than the potato cell cytoplasm.
- [1] mark for stating that water moves into the cells by osmosis, increasing their mass.

(c) Award [1] mark for any of the following standard procedures:
- Blotting the potato cylinders dry / removing surface water with a paper towel before weighing.
- Using potato cylinders cut to the same initial length/diameter (surface area to volume ratio).
- Ensuring the cylinders are completely submerged in the solution.

(a) The Lincoln Index formula is:
\(N = \frac{n_1 \times n_2}{nm}\)
Where:
- \(n_1\) = 120
- \(n_2\) = 150
- \(nm\) = 30

Calculation:
\(N = \frac{120 \times 150}{30} = 600\) beetles.

(b) For the estimate to be valid, several assumptions must hold true over the 48 hours:
1. The population is closed: there is no significant birth, death, immigration, or emigration.
2. The marked beetles mix randomly with the unmarked individuals before the second sample is taken.
3. The marking technique does not affect the survival rate of the beetles (e.g., does not make them more visible to predators or toxic to them) or alter their behavior.
4. Marks are permanent and do not rub off.

(c) Reliability can be improved by repeating the capture-mark-recapture process multiple times to obtain a mean value, or by capturing larger sample sizes in both the first and second events to minimize sampling error.

(a) Award [2] marks in total:
- [1] mark for showing the correct working: \(\frac{120 \times 150}{30}\).
- [1] mark for the correct final answer: 600 (beetles).

(b) Award [2] marks for any two of the following conditions outlined:
- No migration (no immigration or emigration) / closed population.
- No births or deaths during the study period.
- The mark does not affect survival / does not increase predation risk.
- The mark does not wear off or fade.
- Marked individuals disperse/mix randomly back into the population.
- All individuals have an equal chance of being caught in both samples.

(c) Award [1] mark for:
- Repeating the entire procedure and calculating a mean.
- Increasing the size of the samples captured (larger \(n_1\) and/or \(n_2\)).

1. **Chemoreceptor detection**: Chemoreceptors in the carotid and aortic bodies detect a drop in blood pH caused by increased \(CO_2\) concentration during exercise.
2. **Medulla oblongata integration**: Sensory impulses are sent to the cardiovascular control centre in the medulla oblongata.
3. **Sympathetic nervous system stimulation**: The cardiovascular centre sends impulses along sympathetic motor neurons (specifically, the accelerator nerve) to the sinoatrial (SA) node.
4. **Neurotransmitter release**: Noradrenaline (norepinephrine) is released at the SA node, increasing the rate of depolarization and thus the heart rate.
5. **Endocrine contribution**: Adrenaline (epinephrine) is released from the adrenal medulla into the bloodstream, acting directly on the SA node and cardiac muscle to increase both heart rate and stroke volume (force of contraction).

Award 1 mark for each of the following points, up to a maximum of 5 marks:
- Detection of low blood pH / high carbon dioxide levels by chemoreceptors (in aorta/carotid arteries). [1]
- Signals sent to the cardiovascular control centre in the medulla oblongata. [1]
- Sympathetic nerves (accelerator nerve) send impulses to the sinoatrial (SA) node. [1]
- Release of neurotransmitter (noradrenaline) increases the heart rate/frequency of SA node firing. [1]
- Adrenal glands secrete epinephrine/adrenaline into the bloodstream. [1]
- Epinephrine binds to receptors on the SA node/cardiac muscle to increase heart rate and force of contraction (stroke volume). [1]

1. **Detection**: Hypothalamus acts as a thermostat, detecting a fall in blood temperature and receiving sensory input from skin thermoreceptors.
2. **Vasoconstriction**: Sympathetic signals cause vasoconstriction of arterioles supplying the skin capillaries, diverting warm blood away from the body surface to reduce convective and radiative heat loss.
3. **Shivering**: Involuntary, rapid contractions of skeletal muscles (shivering) are triggered, converting chemical energy from ATP into thermal energy to generate heat.
4. **Metabolic increase**: The hypothalamus stimulates the release of thyroxine (via TSH) and adrenaline, which increase the basal metabolic rate, generating more heat.
5. **Behavioral adjustments**: The cerebral cortex is stimulated to drive behavioral actions (e.g., huddling, seeking shelter, or putting on additional clothing).

Award 1 mark for each of the following points, up to a maximum of 5 marks:
- Hypothalamus acts as the control centre / receives input from skin thermoreceptors / monitors blood temperature. [1]
- Vasoconstriction of arterioles supplying skin capillaries reduces heat loss by radiation/conduction. [1]
- Shivering / involuntary muscle contraction is stimulated to generate heat through respiration. [1]
- Increase in metabolic rate/cell respiration mediated by hormones (thyroxine/adrenaline). [1]
- Piloerection (erector pili muscles contract) traps a layer of insulating air. [1]
- Behavioral responses (e.g., putting on clothes/moving to warm areas) are triggered via cerebral cortex stimulation. [1]

1. **Bottom-up definition**: Population sizes are limited by the availability of resources (e.g., nutrients, water, light) at the base of the food web.
2. **Bottom-up example**: In aquatic ecosystems, nitrogen or phosphorus runoff causes algal blooms, which increases zooplankton populations, and subsequently higher trophic levels.
3. **Top-down definition**: Populations at lower trophic levels are regulated by predation, herbivory, or disease from higher trophic levels.
4. **Top-down example**: In a kelp forest ecosystem, sea otters (top predators) control sea urchin populations, preventing them from overgrazing and destroying the kelp forest.
5. **Key difference**: Bottom-up affects trophic levels upwards through energy flow, whereas top-down control exerts pressure downwards from consumers to producers.

Award 1 mark for each of the following points, up to a maximum of 5 marks:
- Bottom-up control is driven by the availability of resources/nutrients/primary producers. [1]
- Example of bottom-up control: e.g., nutrient enrichment (nitrogen/phosphorus) increasing phytoplankton and subsequently fish abundance, OR water/nutrients limiting plant biomass in grasslands. [1]
- Top-down control is driven by predation/herbivory from higher trophic levels. [1]
- Example of top-down control: e.g., wolves controlling deer populations which allows vegetation to regenerate, OR sea otters preying on sea urchins to preserve kelp forests. [1]
- Clear statement of contrast (e.g., bottom-up regulates from the base of the food chain upwards, whereas top-down regulates from the apex downwards). [1]

1. **Null Hypothesis**: State the null hypothesis (\(H_0\)) that the two genes assort independently (i.e., they are not linked).
2. **Expected Ratio**: For a test cross of a dihybrid (e.g., \(AaBb \times aabb\)), the expected Mendelian ratio of phenotypes is \(1:1:1:1\) (or \(9:3:3:1\) for an \(F_2\) self-cross).
3. **Calculation**: Calculate expected values (\(E\)) based on this ratio, then use the formula \(\chi^2 = \sum \frac{(O - E)^2}{E}\) where \(O\) represents observed offspring counts.
4. **Degrees of Freedom**: Determine the degrees of freedom (\(df = \text{number of classes} - 1\)), which is 3 for a dihybrid test cross.
5. **Comparison and Conclusion**: Compare the calculated \(\chi^2\) value to the critical value at a significance level of \(p = 0.05\). If the calculated value exceeds the critical value, reject the null hypothesis, demonstrating that the genes are linked (located on the same chromosome).

Award 1 mark for each of the following points, up to a maximum of 5 marks:
- State the null hypothesis (\(H_0\)): there is no link between the genes / they assort independently. [1]
- Calculate the expected phenotypic ratio: 1:1:1:1 for a test cross (or 9:3:3:1 for selfing of double heterozygotes). [1]
- Use of the chi-squared formula: \(\chi^2 = \sum \frac{(O-E)^2}{E}\) (with terms clearly defined). [1]
- Identify correct degrees of freedom: \(df = 3\) for four phenotypic classes. [1]
- Comparison with critical value: if calculated \(\chi^2 >\) critical value (at \(p = 0.05\)), reject the null hypothesis / conclude genes are linked. [1]

1. **Active Loading**: Sucrose is actively loaded into the companion cells and sieve tube elements at the source (e.g., leaves) against its concentration gradient using energy from ATP (specifically via a proton gradient and \(H^+\)/sucrose cotransporters).
2. **Osmosis at Source**: The high concentration of solute inside the sieve tube decreases the solute potential (and thus water potential), causing water to flow from the adjacent xylem into the phloem by osmosis.
3. **Hydrostatic Pressure Generation**: The entry of water into the non-elastic sieve tube walls increases hydrostatic pressure at the source end.
4. **Unloading and Osmosis at Sink**: At the sink (e.g., roots, fruits), sucrose is unloaded (actively or passively), which increases water potential in the phloem, causing water to exit back into the xylem.
5. **Mass Flow**: This creates a hydrostatic pressure gradient between the source (high pressure) and sink (low pressure), driving the bulk/mass flow of phloem sap along the sieve tube.

Award 1 mark for each of the following points, up to a maximum of 5 marks:
- Active transport / active loading of sucrose/solutes into companion cells/sieve tubes at the source. [1]
- Proton pumps create an \(H^+\) gradient to co-transport sucrose into phloem. [1]
- High solute concentration lowers water potential, causing water to enter phloem from xylem by osmosis. [1]
- Influx of water increases hydrostatic pressure at the source. [1]
- At the sink, sucrose unloading increases water potential, causing water to leave the phloem (lowering hydrostatic pressure). [1]
- A pressure gradient is established, causing mass flow/bulk flow of organic solutes from source to sink. [1]

1. **Renin release**: A drop in blood pressure or blood volume is detected by juxtaglomerular cells in the kidneys, which secrete the enzyme renin into the bloodstream.
2. **Angiotensin I formation**: Renin acts enzymatically on the circulating plasma protein angiotensinogen (produced by the liver) to convert it into angiotensin I.
3. **Angiotensin II conversion**: Angiotensin-converting enzyme (ACE) in the lungs converts angiotensin I to angiotensin II.
4. **Vasoconstriction**: Angiotensin II is a potent vasoconstrictor, causing systemic arterioles to constrict, which directly increases total peripheral resistance and blood pressure.
5. **Aldosterone action**: Angiotensin II also stimulates the adrenal cortex to secrete aldosterone, which increases \(Na^+\) reabsorption (and subsequent water reabsorption by osmosis) in the collecting ducts and distal tubules of the kidney, raising blood volume and pressure.

Award 1 mark for each of the following points, up to a maximum of 5 marks:
- Low blood pressure/volume stimulates the kidneys to secrete renin. [1]
- Renin converts angiotensinogen into angiotensin I. [1]
- Angiotensin-converting enzyme (ACE) converts angiotensin I into angiotensin II (in the lungs). [1]
- Angiotensin II causes vasoconstriction of arterioles, directly raising blood pressure. [1]
- Angiotensin II stimulates the adrenal cortex to secrete aldosterone. [1]
- Aldosterone increases sodium (and water) reabsorption in the distal tubules/collecting ducts, increasing blood volume. [1]