Level 3 — ProductionExcretory System & Homeostasis

Excretory System & Homeostasis

⏱ 45 minutes60 marksprintable — key stays hidden on paper

Level 3: Production Paper (from-scratch derivations & explain-out-loud)

Time limit: 45 minutes Total marks: 60

Answer all questions. Diagrams should be fully labelled. Where a "trace" or "derivation" is asked, show every step in order — marks are awarded for the chain of reasoning, not just the endpoint.

Question 1 — Build the nephron from scratch (12 marks)

From memory, produce a labelled sequential diagram of a nephron and its blood supply. Then, in a table, state for each of the following the structure and its single primary function: (a) glomerulus, (b) Bowman's capsule, (c) proximal convoluted tubule, (d) loop of Henle, (e) distal convoluted tubule, (f) collecting duct. (6 marks diagram/labels, 6 marks function table)

Question 2 — Derive urine formation as a mass-balance chain (12 marks)

(a) Write the general word equation: $\text{Amount excreted} = \text{Amount filtered} - \text{Amount reabsorbed} + \text{Amount secreted}$ Explain what each term physically represents. (3)

(b) Given the following renal data for glucose, sodium and creatinine, calculate the amount excreted per minute for each and state, with reasoning, which substance's clearance approximately equals GFR. (6)

Substance	Filtered (mg/min)	Reabsorbed (mg/min)	Secreted (mg/min)
Glucose	125	125	0
Sodium	500	495	0
Creatinine	1.4	0	0.1

Question 3 — Osmoregulation & the ADH feedback loop (11 marks)

A person runs a marathon on a hot day without drinking.

(a) From scratch, construct the negative feedback loop that restores blood water balance. Your answer must name in correct order: the stimulus, the receptor, the coordinating centre, the effector hormone, the target and the response. (6)

(b) Explain at the molecular/cellular level how the hormone in (a) changes collecting duct permeability. (3)

Question 4 — Nitrogenous waste trade-offs (9 marks)

(a) Complete the comparison table from memory for the three nitrogenous wastes: (6)

Waste	Toxicity	Water needed to excrete	Typical animal group
Ammonia
Urea
Uric acid

(b) A desert reptile and a freshwater fish excrete different wastes. Explain, using the water-toxicity trade-off, why each form is adaptive for that animal. (3)

Question 5 — Thermoregulation as a control system (9 marks)

Explain-out-loud: describe the complete homeostatic response of a mammal to a cold environment.

(a) Name the receptor and control centre. (2) (b) Describe four distinct effector responses and how each conserves or generates heat. (4) (c) State whether each response is nervous or hormonal control, and explain why a negative feedback (rather than positive feedback) system is appropriate for body temperature. (3)

Question 6 — The liver in homeostasis (7 marks)

(a) Trace, as a step-by-step pathway, what happens to excess amino acids in the liver, ending in a nitrogenous product ready for excretion. Name the two processes involved. (4)

(b) State three other homeostatic functions of the liver besides the pathway in (a). (3)

End of paper

Answer keyMark scheme & solutions

Question 1 (12)

Diagram (6): Correct order Bowman's capsule → PCT → descending loop → ascending loop → DCT → collecting duct; afferent & efferent arteriole, glomerulus inside capsule, peritubular capillaries/vasa recta. (1 mark each correct labelled structure, max 6)

Function table (6, 1 each):

Glomerulus — capillary knot; ultrafiltration under high hydrostatic pressure.
Bowman's capsule — cup around glomerulus; collects filtrate.
PCT — folded, many mitochondria/microvilli; selective reabsorption of glucose, amino acids, most ions & water.
Loop of Henle — hairpin; sets up medullary salt gradient (counter-current) for water conservation.
DCT — fine-tuning of ions/pH; site of aldosterone action.
Collecting duct — carries urine to pelvis; water reabsorption under ADH control.

Question 2 (12)

(a) (3): Filtered = amount entering nephron via ultrafiltration (GFR × plasma conc.); Reabsorbed = returned to blood across tubule; Secreted = actively added from blood into tubule; Excreted = net amount leaving in urine. (1 each, 3)

(b) (6): Excreted = Filtered − Reabsorbed + Secreted.

Glucose: $125-125+0 = 0$ mg/min (1)
Sodium: $500-495+0 = 5$ mg/min (1)
Creatinine: $1.4-0+0.1 = 1.5$ mg/min (1)

Substance whose clearance ≈ GFR: creatinine (1), because it is freely filtered and essentially not reabsorbed (1); (small secretion makes it a close but slight over-estimate of GFR) (1).

(c) (3): Glucose is completely reabsorbed in the PCT by active transport/co-transport as filtered load is below the transport maximum (2); sodium is only partially reabsorbed (bulk reabsorbed but some remains to be excreted, regulated by aldosterone) so some appears in urine (1).

Question 3 (11)

(a) (6, 1 each): Stimulus: rise in blood solute concentration / fall in water potential. Receptor: osmoreceptors in hypothalamus. Centre: hypothalamus / posterior pituitary releases hormone. Hormone: ADH (vasopressin). Target: collecting duct (and DCT). Response: increased water reabsorption, restoring blood water potential.

(b) (3): ADH binds receptors on collecting-duct cells (1), triggering insertion of aquaporin water channels into the apical membrane (1), so more water is reabsorbed by osmosis into the hypertonic medulla (1).

(c) (2): Urine is more concentrated / higher osmolarity (1) and smaller in volume (1).

Question 4 (9)

(a) (6, ½ each cell → round to 6):

Waste	Toxicity	Water	Group
Ammonia	very high	very much	aquatic/freshwater fish
Urea	moderate	moderate	mammals/amphibians
Uric acid	low	very little	birds/reptiles/insects

(b) (3): Freshwater fish are surrounded by water and lose it by osmosis anyway, so excreting toxic ammonia with lots of water is cheap/adaptive (1.5); desert reptile must conserve water, so excreting low-toxicity uric acid as a paste needs almost no water — adaptive for dry habitat (1.5).

Question 5 (9)

(a) (2): Thermoreceptors in skin (peripheral) and hypothalamus (core) (1); control centre = hypothalamus (1).

(b) (4, 1 each, any four): Vasoconstriction of skin arterioles → less heat lost; shivering (muscle contraction) → generates heat; erection of hair (piloerection) → traps insulating air; increased metabolic rate (thyroxine/adrenaline) → generates heat; behavioural (huddling/curling) (1 each max 4).

(c) (3): Vasoconstriction, shivering, piloerection = nervous; metabolic rate increase = hormonal (thyroxine/adrenaline) (1). Negative feedback opposes/reverses the change to return temperature to set point (1); positive feedback would drive temperature further from the set point, which is dangerous/unstable (1).

Question 6 (7)

(a) (4): Excess amino acids cannot be stored (1) → deamination: amino group removed forming ammonia + keto acid (used in respiration) (1.5) → ornithine cycle converts toxic ammonia + CO₂ into less-toxic urea (1.5), transported in blood to kidney for excretion.

(b) (3, 1 each): Regulation of blood glucose (glycogenesis/glycogenolysis/gluconeogenesis); detoxification (e.g. alcohol, drugs, hormone breakdown); production of plasma proteins; storage of iron; heat generation. (any three)

[
  {"claim":"Sodium excreted = 500-495+0 = 5 mg/min","code":"result = (500-495+0)==5"},
  {"claim":"Glucose excreted = 125-125+0 = 0 mg/min","code":"result = (125-125+0)==0"},
  {"claim":"Creatinine excreted = 1.4-0+0.1 = 1.5 mg/min","code":"result = abs((Rational(14,10)-0+Rational(1,10)) - Rational(15,10))==0"}
]