(a) Block to polyspermy (10 marks) (b) Fate map of frog embryo (10 marks) (c) Activation energy based mechanism of enzyme action…

(a) Block to polyspermy

Polyspermy, the entry of more than one sperm, is lethal in most animals because extra centrioles produce multipolar spindles and abnormal chromosome numbers. Eggs prevent it by two blocks.

• Fast block: within seconds of sperm fusion the egg membrane depolarises. In the sea urchin the potential rises from about −70 mV to about +20 mV through Na⁺ influx, and sperm cannot fuse with a positive membrane. In frogs (Xenopus) depolarisation comes from Cl⁻ efflux. The block is transient, and mammals have little or no electrical block.
• Slow block (cortical reaction): sperm entry releases Ca²⁺ from the endoplasmic reticulum via IP₃, and a wave of Ca²⁺ crosses the egg. It triggers exocytosis of cortical granules beneath the membrane. In the sea urchin, their proteases cut the links between vitelline envelope and membrane and release sperm-binding receptors, mucopolysaccharides draw in water to lift the envelope, and peroxidase cross-links it into a hard fertilisation envelope; hyalin forms a hyaline layer. In frogs the vitelline envelope is similarly converted.
• Zona reaction in mammals: the cortical granule protease ovastacin cleaves the zona glycoprotein ZP2, so the zona pellucida no longer binds sperm, and fused sperm-binding proteins (Juno) are shed from the egg membrane.

Together, the fast block gives immediate protection and the slow block a permanent barrier, ensuring monospermy.

(b) Fate map of frog embryo

A fate map shows what each region of an early embryo will normally become. For the frog it was prepared by W. Vogt (1929) by marking the blastula surface with vital dyes (Nile blue, neutral red) and following the stained cells; modern maps of Xenopus use injected lineage tracers.

• Animal hemisphere (presumptive ectoderm): the dorsal part becomes neural ectoderm (brain, spinal cord, neural crest), and the ventral and lateral parts become epidermis.
• Marginal zone (presumptive mesoderm): a belt around the equator. Its dorsal sector, at the future dorsal lip of the blastopore opposite the sperm entry point, forms the prechordal plate and notochord; dorsolateral regions form somites; lateral regions form lateral plate mesoderm (heart, kidney, body-wall lining); and the ventral sector forms blood islands. In Xenopus much of this mesoderm lies in the deep layer beneath the surface.
• Vegetal hemisphere (presumptive endoderm): forms the lining of the gut and its derivatives such as liver, pancreas and lungs.

During gastrulation the marginal zone and endoderm move inside through the blastopore, and the ectoderm spreads over the embryo by epiboly. The dorsal lip is the Spemann–Mangold organiser (1924): grafted to the ventral side, it induces a second body axis with a neural tube. Mesoderm itself is induced in the marginal zone by signals (Nodal-related proteins, VegT) from the vegetal cells, with the dorsal-vegetal Nieuwkoop centre specifying the organiser.

(c) Activation-energy mechanism of enzyme action

For a reaction S → P to occur, molecules must pass through a high-energy, unstable transition state; the free energy needed to reach it is the activation energy (ΔG‡ or Ea). The rate constant falls exponentially as ΔG‡ rises (k ∝ e^(−ΔG‡/RT)), so most biological reactions are very slow without catalysts.

Enzymes speed reactions by providing a pathway with a lower activation energy. In an energy profile, the enzyme-catalysed curve has a lower peak (through an ES complex and a stabilised transition state, EX‡), but the free energies of S and P, and so the overall ΔG and equilibrium, are unchanged; the enzyme speeds the forward and reverse reactions equally and is not consumed. Lowering ΔG‡ by about 5.9 kJ/mol raises the rate about tenfold at 37 °C, so enzymes achieve accelerations of 10⁶–10¹⁷ (catalase and carbonic anhydrase work close to the diffusion limit).

How the barrier is lowered:

• Transition-state binding: as Pauling proposed, the active site is complementary to the transition state rather than the substrate, and the binding energy released stabilises it (the basis of transition-state-analogue inhibitors).
• Induced fit: substrate binding changes the enzyme's shape (Koshland), bringing catalytic groups into position and straining substrate bonds.
• Proximity and orientation: reactants are held close together in the correct alignment.
• General acid–base catalysis, covalent catalysis (e.g., the serine of chymotrypsin) and metal-ion catalysis provide alternative lower-energy routes.

In Michaelis–Menten kinetics, v = Vmax[S]/(Km + [S]); lowering the barrier appears mainly as a high turnover number (kcat).

(d) Structure of the dipeptide unit of a protein

A dipeptide forms when the α-carboxyl group of one amino acid condenses with the α-amino group of another, releasing water and forming a peptide (amide) bond; for glycylglycine: H₃N⁺–CH₂–CO–NH–CH₂–COO⁻. In general the dipeptide has an N-terminus with a free amino group, two α-carbons each bearing an H atom and a side chain (R), and a C-terminus with a free carboxyl group.

• Partial double-bond character: the peptide C–N bond resonates between C–N and C=N forms (the nitrogen's lone pair is shared with the carbonyl), so its length is about 1.32 Å, between a single C–N (1.47 Å) and a double C=N (about 1.27 Å). Rotation about it is restricted.
• Planar peptide unit: the six atoms Cα–C(=O)–N(H)–Cα lie in one rigid plane; the peptide bond is polar and uncharged at physiological pH, and the C=O and N–H groups form the hydrogen bonds of secondary structure.
• Trans configuration: the ω angle is usually 180°, with the two α-carbons on opposite sides, which avoids steric clash; cis bonds occur mainly before proline (about 5–10% of X–Pro bonds).
• Freedom at the α-carbon: rotation is possible about N–Cα (φ) and Cα–C (ψ). Steric clashes limit the allowed φ/ψ pairs, which G.N. Ramachandran plotted (1963); the allowed regions correspond to α-helix and β-sheet. Glycine is most flexible and proline most restricted.

Thus the protein backbone is a chain of rigid planar peptide units linked at the α-carbons, which act as swivels.

(e) Transmission of the nerve impulse through a synapse

A synapse is a junction between a neuron and another neuron, muscle or gland cell. It may be electrical or chemical.

• Electrical synapses: pre- and postsynaptic membranes are joined by gap junctions (connexons), so current passes directly with almost no delay and usually in both directions; they synchronise activity, as in escape responses and some brain regions.
• Chemical synapses: the presynaptic terminal holds vesicles of neurotransmitter, separated from the postsynaptic membrane by a synaptic cleft of about 20–40 nm; transmission is one-way.

Steps at a chemical synapse (e.g., the cholinergic neuromuscular junction):

1. The action potential depolarises the presynaptic terminal and opens voltage-gated Ca²⁺ channels at the active zones.
2. Ca²⁺ entry binds synaptotagmin and triggers SNARE-mediated fusion of vesicles with the membrane, releasing acetylcholine by exocytosis in quantal packets.
3. The transmitter diffuses across the cleft and binds postsynaptic receptors. Ionotropic receptors (the nicotinic ACh receptor, glutamate AMPA receptors) open channels directly; metabotropic receptors (muscarinic ACh, most monoamine receptors) act more slowly through G-proteins and second messengers.
4. Opening of Na⁺/cation channels produces an excitatory postsynaptic potential (EPSP); opening of Cl⁻ or K⁺ channels (GABA, glycine) produces an inhibitory postsynaptic potential (IPSP). These summate in space and time, and if the axon hillock reaches threshold a new action potential is fired.
5. The transmitter is removed by enzymatic breakdown (acetylcholinesterase splits ACh into acetate and choline), reuptake into the terminal or glia (e.g., serotonin, noradrenaline, glutamate), or diffusion.

The synaptic delay of about 0.5 ms reflects Ca²⁺ entry and exocytosis. Chemical synapses allow one-way conduction, summation, facilitation, fatigue and plasticity (the basis of learning), and they are the targets of many drugs and toxins such as curare, botulinum toxin and organophosphate nerve agents.