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(a) Respiratory Quotient (RQ) is defined as the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed by an organism during respiration over a given period of time.

(b) The germinating seeds are placed in the experimental chamber of the respirometer with potassium hydroxide (KOH) solution kept beneath or in a separate tube to absorb any carbon dioxide produced during respiration. The apparatus is sealed, and a capillary tube containing a droplet of coloured liquid (manometer) is connected. As the seeds consume oxygen, the volume of gas decreases because the carbon dioxide produced is absorbed by the KOH. This reduces the pressure, causing the liquid droplet to move towards the chamber. The distance moved by the droplet in a set time is measured, and the volume of oxygen consumed is calculated using the cross-sectional area of the tube.

(c) (i) Net change in gas volume = volume of oxygen consumed - volume of carbon dioxide produced
\( 0.36\text{ cm}^3 = 1.80\text{ cm}^3 - \text{CO}_2 \text{ produced} \)
\( \text{CO}_2 \text{ produced} = 1.80\text{ cm}^3 - 0.36\text{ cm}^3 = 1.44\text{ cm}^3 \).

(c) (ii) \( \text{RQ} = \frac{\text{CO}_2 \text{ produced}}{\text{O}_2 \text{ consumed}} = \frac{1.44}{1.80} = 0.80 \).

(d) An RQ value of 0.80 indicates that the seeds are respiring protein as their primary respiratory substrate (proteins typically have an RQ of ~0.8). Alternatively, it may indicate a mixture of lipid (RQ ~0.7) and carbohydrate (RQ ~1.0) being respired simultaneously.

(a)
1 mark: Ratio of carbon dioxide produced to oxygen consumed.
1 mark: Volume/moles of gas measured over a given period of time.

(b) Max 3 marks:
1 mark: KOH absorbs carbon dioxide.
1 mark: As oxygen is consumed, the volume/pressure of gas in the chamber decreases (causing the dye/manometer droplet to move towards the chamber).
1 mark: Record distance moved by the liquid in a given unit of time.
1 mark: Control temperature using a water bath.

(c)(i)
1 mark: \( 1.44\text{ cm}^3 \) (allow 1.44 with or without units).

(c)(ii)
1 mark: Correct formula or substitution: \( \frac{1.44}{1.80} \) (or ecf based on c(i)).
1 mark: Correct calculation: 0.80 (accept 0.8).

(d) Max 2 marks:
1 mark: Identify protein as the primary substrate.
1 mark: Explain that protein has an RQ of approximately 0.8 (OR explain that it could represent a mixture of lipid (0.7) and carbohydrate (1.0)).

(a) Allopatric speciation begins with geographical isolation, where a physical barrier (such as a body of water or rising sea levels) splits the ancestral population of lizards into separate, isolated populations on different islands. This prevents gene flow between the populations. Each island presents different environmental conditions and selection pressures (e.g., different food sources, predators, or microclimates). Random mutations and genetic drift occur independently in each population. Over time, natural selection favours different alleles in each environment, altering allele frequencies. Accumulation of these genetic differences eventually leads to reproductive isolation, meaning that if the populations reunite, they can no longer interbreed to produce fertile offspring.

(b) The ancestral population of lizards would have possessed genetic variation affecting traits such as limb length, toe pad structure, or colour. In the tree canopy, selection pressures favour individuals with larger toe pads and sticky scales to grip leaves and prevent falling. On tree trunks, selection pressures favour individuals with longer legs to run fast and escape predators. Individuals with these advantageous variations are more likely to survive, reproduce, and pass on their beneficial alleles to their offspring. Over many generations, disruptive selection or directional selection in different sub-habitats leads to adaptation to distinct ecological niches, minimizing interspecific competition.

(c) Two pre-zygotic isolating mechanisms are:
1. Behavioral isolation: differences in courtship rituals, displays (such as dewlap extension), or mating signals.
2. Temporal isolation: breeding/mating at different times of day or different seasons.

(a) Max 4 marks:
1 mark: Geographical isolation / physical barrier prevents gene flow.
1 mark: Different selection pressures / environmental conditions on each island.
1 mark: Role of random mutations / genetic drift in generating distinct gene pools.
1 mark: Natural selection leads to change in allele frequencies.
1 mark: Reproductive isolation occurs (cannot interbreed to produce fertile offspring).

(b) Max 4 marks:
1 mark: Genetic variation exists within the ancestral population (for traits like limbs/toe pads).
1 mark: Different selection pressures in different niches (canopy vs. trunk).
1 mark: Description of selective advantage (e.g., large toe pads in canopy for grip, long legs on trunk for speed).
1 mark: Differential survival and reproduction (survive, pass on beneficial alleles).
1 mark: Over time/generations, allele frequencies for these traits increase in respective populations, reducing competition.

(c) 2 marks:
1 mark per correct pre-zygotic mechanism listed (accept: behavioral isolation, temporal isolation, mechanical isolation; reject: post-zygotic mechanisms like hybrid inviability or hybrid sterility).

(a) Three key differences are:
1. Selection pressure: In natural selection, the environment (e.g. climate, predators, pathogens) exerts the selection pressure, whereas in artificial selection, humans select the desired traits.
2. Fitness: Natural selection increases the ecological fitness and adaptation of the organism to the wild, whereas artificial selection select traits that benefit humans, which may actually reduce the organism's fitness in the wild.
3. Rate of change: Natural selection is generally a slow process occurring over evolutionary timescales, whereas artificial selection produces changes much faster due to intense human-controlled breeding.

(b) Polyploidy involves the multiplication of chromosome sets. When a polyploid individual is formed, it cannot successfully breed with the parental diploid individuals because meiosis is disrupted in the offspring (homologous pairing cannot occur, resulting in sterile triploid or irregular gametes). This creates immediate post-zygotic reproductive isolation. However, many plants can self-fertilise or reproduce vegetatively, allowing the new polyploid to reproduce and establish a new, fertile population that is a distinct species from the parents.

(c) (i) A lack of genetic diversity means that all individuals in the crop population are genetically similar or identical (high homozygosity). If a new pathogen or environmental change occurs to which the crop has no resistance, every individual in the crop is susceptible. This means the entire crop population can be wiped out simultaneously.

(c) (ii) Wild relatives of crop plants have not undergone intensive selective breeding and thus possess a wider, more diverse gene pool. They often carry alleles for resistance to pests, diseases, or extreme weather conditions. Breeders can cross (hybridise) the commercial crop with these wild relatives to introduce these beneficial alleles back into the crop gene pool, and then perform backcrossing to maintain high crop yield.

(a) Max 3 marks:
1 mark: In natural selection, the environment selects, whereas in artificial, humans select.
1 mark: Natural selection increases fitness in the wild, whereas artificial selection selects for traits useful to humans (which may decrease wild fitness).
1 mark: Natural selection is a slower process, whereas artificial selection is faster.

(b) Max 3 marks:
1 mark: Polyploids have extra sets of chromosomes.
1 mark: Crossing a polyploid with a parental diploid produces sterile offspring (cannot form homologous pairs during meiosis).
1 mark: Leads to immediate reproductive isolation.
1 mark: Self-pollination or vegetative propagation allows the polyploid to reproduce and establish.

(c)(i) Max 2 marks:
1 mark: Crop individuals are genetically uniform / have high homozygosity / lack of variation.
1 mark: If one individual is susceptible to a new disease, all individuals are, leading to the potential loss of the entire crop.

(c)(ii) Max 2 marks:
1 mark: Wild relatives have a wider gene pool / wider range of alleles (including resistance genes).
1 mark: Cross-breeding/hybridising commercial crops with wild relatives introduces these resistance alleles into the crop.

(a) (i) Primers are short, single-stranded sequences of DNA that are complementary to the sequences at the start of the target DNA region. They bind (anneal) to the single-stranded template DNA, providing a double-stranded section that serves as a starting point for DNA polymerase to bind and begin synthesizing the new strand.

(a) (ii) Taq polymerase is a heat-stable (thermostable) DNA polymerase. It synthesizes the complementary DNA strands by adding free deoxynucleoside triphosphates (dNTPs) to the 3' ends of the primers. Because it is thermostable, it does not denature at the high temperatures used during the denaturation step, meaning it does not need to be replaced after every cycle.

(b) (i) \( 95\text{ }^\circ\text{C} \): Denaturation occurs. The hydrogen bonds holding the complementary base pairs of the double-stranded DNA template together are broken, separating the DNA into two single strands.

(b) (ii) \( 55\text{ }^\circ\text{C} \): Annealing occurs. This lower temperature allows the primers to bind via complementary base pairing to their target sites on the single-stranded DNA templates.

(b) (iii) \( 72\text{ }^\circ\text{C} \): Elongation/Extension occurs. This is the optimum temperature for Taq polymerase, which actively synthesizes the new complementary DNA strands.

(c) DNA fragments are loaded into wells at one end of an agarose gel, and a voltage (electric current) is applied across the gel. DNA has a net negative charge due to the phosphate groups in its backbone, so it migrates towards the positive electrode (anode). The gel matrix acts as a molecular sieve; smaller/shorter DNA fragments can move more easily and quickly through the pores of the gel than larger/longer fragments. Consequently, DNA fragments are separated purely according to their molecular size (length).

(a)(i) 2 marks:
1 mark: Primers are short, single-stranded DNA sequences complementary to the start of target DNA.
1 mark: Provide a double-stranded starting point for DNA polymerase.

(a)(ii) 2 marks:
1 mark: Synthesizes new complementary DNA strands (by adding dNTPs/nucleotides).
1 mark: It is heat-stable / thermostable so it does not denature at high temperatures (e.g. \( 95\text{ }^\circ\text{C} \)).

(b) 3 marks:
1 mark: \( 95\text{ }^\circ\text{C} \): Denaturation / breaking of hydrogen bonds to separate double-stranded DNA into single strands.
1 mark: \( 55\text{ }^\circ\text{C} \): Annealing / binding of primers to complementary sequences on template strands.
1 mark: \( 72\text{ }^\circ\text{C} \): Extension / optimum temperature for Taq polymerase to synthesize new DNA.

(c) Max 3 marks:
1 mark: Electric current/voltage is applied; DNA is negatively charged and moves towards the anode/positive electrode.
1 mark: Gel acts as a sieve/has pores.
1 mark: Smaller/shorter fragments move faster/further through the gel than larger/longer fragments.

(a) Ultrafiltration occurs due to high hydrostatic pressure in the glomerulus, which is created because the afferent arteriole entering the glomerulus has a wider lumen than the efferent arteriole leaving it. The barrier between the blood in the glomerular capillaries and the lumen of the Bowman's capsule has three layers:
1. The capillary endothelium, which has tiny pores (fenestrations) that allow water and small solutes through but prevent blood cells from passing.
2. The basement membrane, which acts as a selective molecular filter, preventing large proteins (with a relative molecular mass > 69,000) from passing.
3. The podocytes (epithelial cells of the Bowman's capsule), which have foot-like projections (pedicels) wrapping around the capillaries, leaving filtration slits between them for the easy flow of filtrate.

(b) (i)
Volume of filtrate reabsorbed = GFR - rate of urine production
\( 125\text{ cm}^3\text{ min}^{-1} - 1.25\text{ cm}^3\text{ min}^{-1} = 123.75\text{ cm}^3\text{ min}^{-1} \)
Percentage reabsorbed = \( \frac{123.75}{125} \times 100 = 99.0\% \).

(b) (ii) The cells lining the PCT are highly specialized for selective reabsorption:
1. The luminal membrane has microvilli, which enormously increase the surface area available for the insertion of transport proteins.
2. The cells contain numerous mitochondria to produce ATP, which is required for active transport.
3. Sodium-potassium pumps (\( \text{Na}^+/\text{K}^+ \)-ATPase) on the basolateral membrane actively pump sodium ions out of the PCT cells and into the blood, establishing a concentration gradient of sodium ions between the filtrate and the cell cytoplasm.
4. Sodium-glucose cotransporter proteins (SGLT) in the luminal membrane allow sodium ions to diffuse down their concentration gradient into the cell, carrying glucose with them against its concentration gradient (secondary active transport).
5. Facilitated diffusion carrier proteins (GLUT) on the basolateral membrane allow glucose to diffuse out of the cell down its concentration gradient to enter the blood.

(a) Max 4 marks:
1 mark: High hydrostatic pressure in glomerulus due to afferent arteriole being wider than efferent arteriole.
1 mark: Capillary endothelial fenestrations/pores allow solutes/water through but block blood cells.
1 mark: Basement membrane acts as a molecular filter, preventing large proteins from entering the filtrate.
1 mark: Podocytes have foot-like projections (pedicels) forming filtration slits.

(b)(i) 2 marks:
1 mark: Correct substitution or calculation of reabsorbed volume: \( 125 - 1.25 = 123.75 \).
1 mark: Correct final answer: \( 99.0\% \) (accept 99).

(b)(ii) Max 4 marks:
1 mark: Microvilli on luminal membrane increase surface area for transport.
1 mark: Mitochondria produce ATP for active transport of sodium ions.
1 mark: Sodium-potassium pumps on basolateral membrane pump \( \text{Na}^+ \) out to create a concentration gradient.
1 mark: Co-transporter proteins (SGLT) on luminal membrane transport glucose into the cell along with \( \text{Na}^+ \).
1 mark: Facilitated diffusion carriers (GLUT) on basolateral membrane allow glucose to enter blood.

(a) Glucagon is secreted by the alpha (\( \alpha \)) cells, which are located in the Islets of Langerhans of the pancreas.

(b) The cell-signalling pathway operates as follows:
1. Glucagon acts as a first messenger and binds to a specific G-protein coupled receptor (GPCR) on the outer surface of the hepatocyte cell membrane.
2. This binding causes a conformational change that activates a G-protein on the inside of the cell membrane.
3. The activated G-protein moves along the membrane and activates the enzyme adenylyl cyclase.
4. Adenylyl cyclase converts ATP into cyclic AMP (cAMP), which acts as a second messenger.
5. cAMP binds to and activates protein kinase A (PKA).
6. Active PKA initiates a phosphorylation cascade, ultimately activating glycogen phosphorylase.
7. Glycogen phosphorylase catalyses the breakdown of glycogen into glucose (glycogenolysis), which then leaves the hepatocyte.

(c) (i) In response to glucagon, glycogen breakdown (glycogenolysis) is activated and glycogen synthesis (glycogenesis) is inhibited. In contrast, in response to insulin, glycogen synthesis (glycogenesis) is activated and glycogen breakdown is inhibited.

(c) (ii) Insulin increases the translocation of glucose transporter proteins to the cell surface membrane to increase glucose uptake, whereas glucagon does not increase the number of these transporters, as its primary role is to promote the export of glucose out of the hepatocytes via existing GLUT2 transporters down its concentration gradient.

(a) 2 marks:
1 mark: Alpha (\( \alpha \)) cells.
1 mark: Islets of Langerhans (of pancreas).

(b) Max 5 marks:
1 mark: Glucagon binds to a cell surface / G-protein coupled receptor.
1 mark: Activation of G-protein.
1 mark: G-protein activates adenylyl cyclase.
1 mark: Adenylyl cyclase converts ATP to cAMP (second messenger).
1 mark: cAMP activates protein kinase A.
1 mark: Activates phosphorylation cascade / glycogen phosphorylase.
1 mark: Leading to glycogenolysis (breakdown of glycogen to glucose).

(c)(i) 2 marks:
1 mark: Glucagon promotes glycogenolysis (glycogen breakdown).
1 mark: Insulin promotes glycogenesis (glycogen synthesis).

(c)(ii) 1 mark:
1 mark: Insulin increases glucose transporter proteins (GLUT) on the cell membrane, whereas glucagon does not (or glucagon causes glucose export via GLUT2).

(a) The sequence of events is as follows:
1. The arrival of an action potential depolarises the presynaptic membrane.
2. This depolarisation causes voltage-gated calcium channels to open, and calcium ions (\( \text{Ca}^{2+} \)) diffuse into the presynaptic neurone down their electrochemical gradient.
3. The influx of \( \text{Ca}^{2+} \) causes synaptic vesicles containing acetylcholine (ACh) to move towards and fuse with the presynaptic membrane.
4. ACh is released into the synaptic cleft by exocytosis.
5. ACh diffuses across the synaptic cleft and binds to specific receptor proteins on the postsynaptic membrane.
6. This binding causes ligand-gated sodium channels to open, allowing sodium ions (\( \text{Na}^+ \)) to diffuse into the postsynaptic neurone.
7. The influx of sodium ions depolarises the postsynaptic membrane, generating an excitatory postsynaptic potential (EPSP). If the depolarisation reaches the threshold potential, voltage-gated sodium channels open, and a new action potential is generated.

(b) (i) Acetylcholinesterase is an enzyme located in the synaptic cleft. It hydrolyses acetylcholine into choline and acetate. This stops the continuous binding of ACh to receptors, preventing the continuous generation of action potentials in the postsynaptic neurone. It also allows the choline to be reabsorbed and recycled in the presynaptic neurone.

(b) (ii) Calcium ions are responsible for translating electrical signals into chemical signals. When they enter the presynaptic knob through voltage-gated channels, they cause the cytoskeleton to move synaptic vesicles towards the presynaptic membrane, triggering their fusion and the release of neurotransmitter by exocytosis.

(a) Max 6 marks:
1 mark: Action potential depolarises presynaptic membrane, opening voltage-gated calcium channels.
1 mark: Calcium ions diffuse into presynaptic knob.
1 mark: Calcium ions trigger fusion of synaptic vesicles with presynaptic membrane.
1 mark: Acetylcholine (ACh) released into cleft via exocytosis.
1 mark: ACh diffuses across the cleft.
1 mark: ACh binds to receptor proteins on postsynaptic membrane.
1 mark: Ligand-gated sodium channels open and sodium ions enter postsynaptic neurone.
1 mark: Postsynaptic membrane depolarises (and if threshold is reached, action potential is generated).

(b)(i) 2 marks:
1 mark: Hydrolyses/breaks down acetylcholine (into choline and acetate).
1 mark: Prevents continuous stimulation of the postsynaptic membrane / allows recycling.

(b)(ii) 2 marks:
1 mark: Influx into presynaptic knob triggers vesicle movement.
1 mark: Causes vesicle fusion with presynaptic membrane / exocytosis of neurotransmitter.

(a) Thylakoid membranes.

(b) Non-cyclic photophosphorylation proceeds as follows:
1. Light energy is absorbed by accessory pigments in photosystem II (PSII) and passed to the primary pigment reaction centre (chlorophyll a, P680).
2. This energy excites electrons, which are emitted from PSII (photoactivation).
3. The excited electrons are captured by an electron acceptor and passed down an electron transport chain (ETC) of membrane-bound carriers.
4. As electrons pass along the ETC, they lose energy, which is used to pump protons into the thylakoid lumen, generating a proton gradient used to synthesize ATP via chemiosmosis.
5. The electrons reach photosystem I (PSI), which has also been photoactivated by light, losing its own excited electrons.
6. The electrons from the ETC replace those lost by PSI.
7. The excited electrons emitted from PSI are passed to an electron acceptor and then to NADP reductase, where they are used, along with protons, to reduce NADP to reduced NADP (NADPH).

(c) Water undergoes photolysis (\( 2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}_2 \)) catalysed by an enzyme in PSII:
1. The electrons produced replace the electrons lost by PSII, allowing non-cyclic photophosphorylation to continue.
2. The protons (\( \text{H}^+ \)) contribute to the proton gradient across the thylakoid membrane and are used in the stroma to reduce NADP.

(d) The Calvin cycle requires ATP and reduced NADP in a ratio of 3 ATP to 2 NADPH. Non-cyclic photophosphorylation produces ATP and reduced NADP in roughly equal quantities, which does not meet the high ATP demand. Cyclic photophosphorylation produces additional ATP (and no reduced NADP) to make up this deficit, ensuring there is a sufficient supply of ATP to drive the regeneration of RuBP from GP.

(a) 1 mark:
1 mark: Thylakoid membrane (accept grana / granum).

(b) Max 5 marks:
1 mark: Light absorption excites electrons in reaction centre of PSII.
1 mark: Excited electrons are emitted (photoactivation) and passed to an electron acceptor / carrier chain.
1 mark: Energy from electrons passing down the ETC is used to pump protons / synthesize ATP.
1 mark: PSI is also photoactivated by light / loses electrons.
1 mark: Electrons from PSII ETC replace the electrons lost by PSI.
1 mark: Electrons from PSI are used to reduce NADP (forming reduced NADP / NADPH).

(c) 2 marks:
1 mark: Photolysis of water produces electrons to replace those lost by PSII.
1 mark: Produces protons (\( \text{H}^+ \)) for the proton gradient / for reducing NADP (accept oxygen produced as waste).

(d) 2 marks:
1 mark: Calvin cycle requires more ATP than NADPH (ratio of 3:2).
1 mark: Cyclic photophosphorylation generates extra ATP without producing reduced NADP (to meet this deficit).

(a) Genomic DNA contains introns (non-coding sequences), whereas cDNA is synthesized from mRNA and therefore lacks introns. Sheep cells may not process human genomic introns correctly, or using cDNA ensures the mature, functional AAT protein is directly translated from the transcript without errors in splicing.

(b) (i) The promoter is the site where RNA polymerase binds to initiate transcription of the target gene (AAT), ensuring that the gene is expressed.
(ii) It ensures that the AAT protein is only synthesized and secreted into the milk of the mammary glands, preventing its systemic expression in other tissues which could harm the sheep's normal physiological functions.

(c) (i) Primers are short, single-stranded DNA sequences that are complementary to the 3' ends of the target DNA strand. They provide a starting point (free 3'-OH group) for Taq DNA polymerase to bind and begin synthesizing the new strand.
(ii) The temperature is lowered to allow hydrogen bonds to form between the primers and the complementary sequences on the single-stranded DNA templates (annealing).

(d) Two factors that determine the distance travelled by DNA fragments are: 1. The size/length of the DNA fragment (smaller fragments move faster and further through the pores of the gel matrix). 2. The concentration or density of the agarose gel (higher concentration slows down fragment movement).

(a) [Max 2 marks]
1. cDNA lacks introns / only contains exons (whereas genomic DNA has introns); [1]
2. Sheep cells may lack the specific splicing factors for human genomic introns / ensures functional mature mRNA is translated / prevents incorrect splicing; [1]

(b)(i) [Max 2 marks]
1. Binding site for RNA polymerase; [1]
2. Allows transcription of the target gene / controls gene expression; [1]

(b)(ii) [Max 1 mark]
1. Restricts expression to mammary gland cells / ensures protein is secreted into milk / prevents systemic expression that could harm the transgenic animal; [1]

(c)(i) [Max 2 marks]
1. Primers are complementary to the sequence at the end of the target gene; [1]
2. Provide double-stranded section / free 3'-OH group for Taq DNA polymerase to bind / start synthesis; [1]

(c)(ii) [Max 1 mark]
1. Allows primers to anneal / form hydrogen bonds with the single-stranded DNA template; [1]

(d) [Max 2 marks]
1. Net charge of DNA / size / length / molecular weight of the DNA fragment; [1]
2. Concentration of agarose gel / voltage applied / run time; [1]

(a) Under normal conditions, ADH binds to specific V2 receptors on the cell surface membrane of collecting duct cells. This binding activates a G-protein, which in turn stimulates the enzyme adenylyl cyclase. Adenylyl cyclase converts ATP to cyclic AMP (cAMP), acting as a second messenger. cAMP activates a cascade of protein kinase enzymes. This intracellular signal causes vesicles containing aquaporins (water channel proteins) to move towards and fuse with the luminal (apical) membrane of the cell, increasing its permeability to water.

(b) Exposure to nephrotoxin X prevents ADH from binding to the V2 receptors. Consequently, adenylyl cyclase is not activated, cAMP is not produced, and vesicles containing aquaporins do not fuse with the luminal membrane. The luminal membrane of the collecting duct remains impermeable to water. Therefore, water cannot be reabsorbed by osmosis down the water potential gradient into the tissue fluid and blood. This results in the production of a very large volume of dilute (hypotonic) urine.

(c) A decrease in blood water potential is detected by osmoreceptor cells located in the hypothalamus. When blood water potential falls, water moves out of these osmoreceptor cells by osmosis, causing them to shrink. This shrinkage depolarizes the osmoreceptor cells, which stimulates neurosecretory cells. Nerve impulses (action potentials) travel down the axons of the neurosecretory cells into the posterior pituitary gland, where ADH stored in vesicles is released into the blood by exocytosis.

(a) [Max 4 marks]
1. ADH binds to receptors on the cell surface membrane of collecting duct cells; [1]
2. Activates G-protein which activates adenylyl cyclase; [1]
3. Adenylyl cyclase converts ATP to cyclic AMP (cAMP); [1]
4. cAMP acts as second messenger to activate protein kinases; [1]
5. Vesicles containing aquaporins move towards and fuse with the luminal / apical membrane; [1]

(b) [Max 3 marks]
1. Nephrotoxin X prevents ADH binding, so no cAMP / cascade is triggered; [1]
2. Aquaporins are not inserted into the luminal membrane (membrane remains impermeable to water); [1]
3. No / less water is reabsorbed by osmosis from the lumen into the medulla / blood; [1]
4. Large volume of dilute urine produced; [1]

(c) [Max 3 marks]
1. Osmoreceptors in the hypothalamus detect the decrease in blood water potential; [1]
2. Water leaves osmoreceptor cells by osmosis and they shrink; [1]
3. Shrinkage stimulates / depolarizes neurosecretory cells; [1]
4. Action potentials travel down axons to posterior pituitary gland; [1]
5. ADH released from posterior pituitary by exocytosis into the blood; [1]