(a) Draw the molecular structure of ATP synthase and diagrammatically explain the chemiosmotic concept of ATP synthesis. 20 (b)…

(a) ATP synthase is a rotary molecular motor of the inner mitochondrial membrane, chloroplast thylakoid and bacterial plasma membrane. Schematic molecular structure:

Intermembrane side (high H⁺): H⁺ → a-subunit half-channels Membrane-embedded F₀: a, b₂, c-ring (c₈–c₁₂ in different species) Stalk: central γε rotor; peripheral stalk b₂δ/OSCP Matrix side F₁: α₃β₃ hexamer with catalytic β sites Matrix: ADP + Pi → ATP

F₀ conducts protons; F₁ is catalytic. The γε stalk connects them. The β subunits cycle through O (open), L (loose) and T (tight) conformations.

Chemiosmotic concept: Peter Mitchell proposed that electron transport chains I, III and IV pump H⁺ from the matrix to the intermembrane space. This creates a proton motive force. In standard form, Δp = Δψ − (2.303RT/F)ΔpH, where Δψ = ψ_out − ψ_in and ΔpH = pH_out − pH_in. In mitochondria, ψ_out > ψ_in, so Δψ is positive; H⁺ is pumped out, so pH_out < pH_in, making ΔpH negative, and the chemical term also favours H⁺ entry. R is in J mol⁻¹ K⁻¹, T is in K and F is in C mol⁻¹; Δψ and RT/F must be in the same units. Protons re-enter through F₀ down the gradient. Protonation/deprotonation of c-ring carboxylates and a-subunit half-channels rotate the c-ring. The γε rotor rotates inside α₃β₃, changing each β subunit O→L→T→O. In L, ADP and Pi bind; in T, ADP + Pi condense to ATP; in O, ATP is released. Typically about 3–4 H⁺ are used per ATP synthesised, depending on c-ring stoichiometry, plus about 1 H⁺ for ADP/ATP antiport. Validity: intact proton-impermeable inner membrane, functional ETC, O₂ as terminal acceptor, ADP + Pi, and Mg²⁺.

(b) Illustrate the following:

(i) In B-DNA, two antiparallel strands pair by Watson–Crick hydrogen bonds: A=T has two H bonds, and G≡C has three H bonds. Purine pairs with pyrimidine, keeping helix width ~2 nm.

5′—A—G—C—T—3′ = ≡ ≡ = 3′—T—C—G—A—5′

Thus A on one strand pairs only with T on the other; G only with C. G≡C has one extra H bond, increasing thermal stability. Bases stack hydrophobically; the phosphodiester backbone lies outside. Condition: right-handed B-DNA under normal ionic strength and pH.

(ii) Michaelis–Menten kinetics describes a single-substrate enzyme: E + S ⇌ ES → E + P. Let k₁, k₋₁ and k₂ be rate constants. At steady state, d[ES]/dt = 0: k₁[E][S] = (k₋₁ + k₂)[ES]. With [E]total = [E] + [ES], solving gives: [ES] = [E]total[S]/(Kₘ + [S]), where Kₘ = (k₋₁ + k₂)/k₁. Initial rate v₀ = k₂[ES], so: v₀ = Vₘₐₓ[S]/(Kₘ + [S]), with Vₘₐₓ = k₂[E]total.

Graph: v₀ versus [S] is a rectangular hyperbola. v₀ | __________ Vₘₐₓ | / | / | / |__/________________ [S] Kₘ at Vₘₐₓ/2

At low [S] (S << Kₘ), v₀ ≈ (Vₘₐₓ/Kₘ)[S], first-order. At high [S] (S >> Kₘ), v₀ ≈ Vₘₐₓ, zero-order. Kₘ is the substrate concentration at v₀ = Vₘₐₓ/2; unit mol L⁻¹. Vₘₐₓ has unit mol L⁻¹ s⁻¹ or µmol min⁻¹ mg⁻¹. Linear form: 1/v₀ = (Kₘ/Vₘₐₓ)(1/[S]) + 1/Vₘₐₓ. Conditions: [S] >> [E], initial velocity, steady state, no cooperativity, and negligible product.

(c) Steroid hormones are lipophilic cholesterol derivatives such as cortisol, aldosterone, testosterone, estradiol and progesterone. They cross the plasma membrane by simple diffusion. In target cells they bind intracellular receptors—cytoplasmic or nuclear. Unbound receptor is complexed with heat-shock proteins HSP90, HSP70, p23 and immunophilins, which mask the DNA-binding domain and nuclear localization signals.

Hormone binding causes a conformational change, dissociation of HSPs, receptor dimerisation and exposure of a zinc-finger DNA-binding domain. The hormone–receptor complex enters the nucleus and binds to hormone response elements (HREs) in promoter or enhancer regions of target genes. It recruits coactivators or corepressors and alters RNA polymerase II activity. This changes transcription, mRNA processing and protein synthesis. Effects are delayed (hours) but prolonged. Example: cortisol–glucocorticoid receptor activates PEPCK; aldosterone–mineralocorticoid receptor induces Na⁺/K⁺-ATPase; estrogen–estrogen receptor promotes cell proliferation. Some steroids also act rapidly at membrane receptors via second messengers. Termination occurs by metabolic inactivation and excretion. Validity: requires functional receptor, HRE, chromatin accessibility and cofactors.