PY1.1-7 | General Physiology — Gate Quiz

Correct! The fluid mosaic model (Singer & Nicolson, 1972) describes a phospholipid bilayer in which proteins float and move laterally — giving the membrane both fluidity and structural diversity.

Key concept: The cell membrane is a dynamic phospholipid bilayer. Cholesterol modulates fluidity. Integral proteins span the bilayer; peripheral proteins attach to the surface. This structure underlies all membrane transport and signalling.

Incorrect. The fluid mosaic model describes a phospholipid bilayer with embedded proteins that can move laterally within the lipid matrix.

Correct! 0.3% NaCl is hypotonic (normal saline is 0.9% NaCl). Water moves into the RBC by osmosis, causing it to swell and eventually lyse (haemolysis).

Key concept: Tonicity determines the direction of osmotic water flow. Isotonic = 0.9% NaCl (308 mOsm/L). Hypotonic solutions cause cell swelling; hypertonic solutions cause crenation. RBCs begin haemolysing at ~0.45% NaCl.

Incorrect. 0.3% NaCl is hypotonic relative to normal saline (0.9%). Water enters the RBC by osmosis, causing swelling and lysis.

Correct! The resting membrane potential of most neurons is approximately -70 mV (inside negative relative to outside), maintained primarily by the Na⁺/K⁺-ATPase and K⁺ leak channels.

Key concept: Resting membrane potential (~-70 mV in neurons, ~-90 mV in skeletal muscle, ~-55 mV in cardiac pacemaker) is maintained by: (1) Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in), (2) high K⁺ permeability via leak channels, (3) intracellular anions.

Incorrect. The resting membrane potential of a neuron is approximately -70 mV. The negative sign indicates that the inside is negative relative to the outside.

Correct! The Na⁺/K⁺-ATPase exports 3 Na⁺ and imports 2 K⁺ per ATP hydrolysed. This electrogenic net export of one positive charge contributes ~-4 mV to the resting membrane potential.

Key concept: Na⁺/K⁺-ATPase (3 Na⁺ out : 2 K⁺ in per ATP) is electrogenic — net export of one positive charge. Inhibited by ouabain/cardiac glycosides. Maintains the Na⁺ gradient used for secondary active transport (e.g., glucose co-transport in the gut).

Incorrect. The Na⁺/K⁺-ATPase transports 3 Na⁺ OUT and 2 K⁺ IN per ATP. This asymmetric ratio makes the pump electrogenic.

Correct! Rapid depolarisation is caused by opening of voltage-gated Na⁺ channels, allowing Na⁺ to rush into the cell down its electrochemical gradient, driving the membrane potential towards +35 mV.

Key concept: Action potential phases — (1) Depolarisation: voltage-gated Na⁺ channels open → Na⁺ influx; (2) Repolarisation: Na⁺ channels inactivate, voltage-gated K⁺ channels open → K⁺ efflux; (3) Afterhyperpolarisation: K⁺ channels slow to close.

Incorrect. The rapid depolarisation (rising) phase of the action potential is due to massive Na⁺ influx through voltage-gated Na⁺ channels.

Correct! A partial agonist has intrinsic activity between 0 and 1. It activates the receptor but cannot produce the full maximum response (Emax) of the endogenous ligand, even at receptor saturation.

Key concept: Agonist classification by intrinsic activity (α): Full agonist (α=1), Partial agonist (0<α<1), Antagonist (α=0), Inverse agonist (α<0 — reduces baseline activity). Buprenorphine is a classic example of a partial agonist at opioid receptors.

Incorrect. A drug that produces a submaximal effect even when occupying all receptors is a partial agonist (intrinsic activity 0–1).

Correct! Gs proteins stimulate adenylyl cyclase, increasing cAMP, which activates Protein Kinase A (PKA). This pathway mediates effects of adrenaline (β-receptors), glucagon, TSH, FSH, LH, and others.

Key concept: G-protein subtypes — Gs (stimulates adenylyl cyclase → ↑cAMP), Gi (inhibits adenylyl cyclase → ↓cAMP), Gq (activates PLC → IP₃+DAG → Ca²⁺+PKC). Examples: β₁-adrenoceptor = Gs; M₂ muscarinic = Gi; α₁-adrenoceptor = Gq.

Incorrect. Gs-coupled receptors activate adenylyl cyclase → ↑cAMP → activate PKA. (Phospholipase C is activated by Gq; inhibition of adenylyl cyclase is via Gi.)

Correct! This is classic negative feedback homeostasis. The deviation (↑blood glucose) triggers a response (↑insulin → ↑glucose uptake) that opposes and corrects the deviation, restoring the set point.

Key concept: Negative feedback is the primary mechanism of homeostasis — it opposes deviations. Positive feedback amplifies deviations (e.g., parturition, blood clotting, LH surge). Feed-forward anticipates changes before they occur.

Incorrect. When a deviation from the set point triggers a corrective response that opposes the deviation, that is negative feedback — the backbone of homeostasis.

Correct! Total body water (TBW) is approximately 60% of body weight in adult males (42 L in a 70 kg man). It is slightly lower in females (~50–55%) due to higher adipose tissue content.

Key concept: TBW = 60% BW. Distribution: ICF = 40% BW (2/3 of TBW); ECF = 20% BW (1/3 of TBW). ECF = plasma (5%) + interstitial fluid (15%). Indicator dilution method: use deuterium oxide for TBW, inulin for ECF, Evans Blue for plasma.

Incorrect. Total body water is approximately 60% of body weight in adult males (about 42 L in a 70 kg man).

Correct! In nephrotic syndrome, massive proteinuria (>3.5 g/day) leads to hypoalbuminaemia, reducing plasma oncotic pressure. Water moves from capillaries into interstitium (Starling forces), causing oedema.

Key concept: Starling forces govern fluid exchange: Net filtration = (Pc - Pi) - σ(πc - πi). Oedema types: ↑Pc (cardiac failure), ↓πc (nephrotic/liver failure), ↑permeability (inflammation/sepsis), ↓lymphatics (filariasis). Nephrotic syndrome = proteinuria + hypoalbuminaemia + ↓πc.

Incorrect. Nephrotic syndrome causes oedema primarily via reduced plasma oncotic pressure due to urinary albumin loss (hypoalbuminaemia).