Write your answer in about 150 words for each of the following: (a) Highlight role of various hormones in the regulation of…

(a) Hormonal regulation of digestive secretions. The gastrointestinal tract is controlled by peptide hormones released in response to chyme composition. Gastrin, from antral G-cells, is stimulated by peptides and gastric distension; it acts on parietal cells to increase HCl and pepsinogen secretion. Secretin, from duodenal S-cells, is released by acid in the duodenum; it stimulates pancreatic duct cells to secrete a watery HCO3-rich juice that neutralises acid and also inhibits gastric acid secretion. Cholecystokinin, from I-cells, responds to fatty acids and amino acids; it promotes pancreatic acinar enzyme secretion, gallbladder contraction, and relaxation of the sphincter of Oddi, while slowing gastric emptying. Glucose-dependent insulinotropic peptide, from K-cells, stimulates insulin release and can modulate pancreatic bicarbonate and gastric acid secretion. Motilin, from M-cells, promotes the migrating motor complex and gastric/colonic motility, helping coordinate secretion and propulsion.

(b) Glycolytic ATP balance. In glycolysis, one glucose molecule is converted to two pyruvate molecules through an investment and a payoff phase. The investment phase consumes two ATP: hexokinase phosphorylates glucose to glucose-6-phosphate, and phosphofructokinase-1 phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate. The payoff phase generates four ATP by substrate-level phosphorylation: phosphoglycerate kinase transfers a phosphate from 1,3-bisphosphoglycerate to ADP, and pyruvate kinase transfers a phosphate from phosphoenolpyruvate to ADP, each occurring twice per glucose. Thus gross ATP production is four ATP, while net gain is two ATP per glucose. In addition, glyceraldehyde-3-phosphate dehydrogenase oxidises glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, reducing NAD+ to 2 NADH + 2 H+. These NADH molecules can yield additional ATP, commonly three or five ATP per glucose depending on the cytosolic shuttle used, but the direct glycolytic ATP balance remains two ATP net.

(c) Acrosome reaction. The acrosome reaction is the first species-specific step in mammalian fertilisation. After capacitation, the sperm head contacts the cumulus oophorus; hyaluronidase released from the acrosome digests the hyaluronic acid matrix of the cumulus oophorus, allowing the sperm to reach the zona pellucida. Binding of the sperm to zona pellucida glycoproteins, especially ZP3, triggers a rise in intracellular calcium and acrosomal exocytosis. The outer acrosomal membrane fuses with the sperm plasma membrane, releasing acrosomal enzymes such as acrosin and further hyaluronidase. Acrosin helps digest zona pellucida proteins and exposes ZP2, which supports the second binding step. The sperm then penetrates the zona pellucida, reaches the oolemma, and the sperm plasma membrane fuses with the oocyte plasma membrane. This fusion permits sperm entry, depolarises the oocyte for the fast block, and initiates cortical granule exocytosis, which modifies ZP2 for the slow block to polyspermy.

(d) Neurotransmitters and the neuromuscular junction. Neurotransmitters are chemical messengers that transmit signals across synapses. They include acetylcholine; catecholamines such as noradrenaline, adrenaline, and dopamine; biogenic amines such as serotonin; amino acids such as glutamate, gamma-aminobutyric acid, and glycine; gaseous messengers such as nitric oxide; purines such as ATP; and neuropeptides such as substance P, enkephalins, and endorphins. At the neuromuscular junction, the specific neurotransmitter is acetylcholine. An action potential in the motor neuron opens voltage-gated calcium channels at the presynaptic terminal, causing synaptic vesicles to release acetylcholine in quanta into the synaptic cleft. Acetylcholine binds nicotinic cholinergic receptors on the motor end plate, opening cation channels that allow sodium influx and potassium efflux. This produces an end-plate potential, which, if sufficient, triggers an action potential in the muscle fibre and contraction. The signal is terminated rapidly by acetylcholinesterase, which hydrolyses acetylcholine, preventing repetitive stimulation.

(e) Enzyme inhibition. Enzyme inhibition reduces catalytic activity by binding to the enzyme. Competitive inhibition occurs when an inhibitor resembles the substrate and binds reversibly to the active site. Because it competes with substrate for the same site, increasing substrate concentration can displace the inhibitor and restore the maximum rate. In Michaelis-Menten terms, Vmax remains unchanged, but Km increases, meaning a higher substrate concentration is needed to reach half-maximal velocity. This pattern is typical of many drug designs, where the inhibitor blocks the active site without altering the enzyme's maximum catalytic capacity. Non-competitive inhibition occurs when the inhibitor binds to an allosteric site, not the active site. It may bind to the free enzyme or enzyme-substrate complex, changing enzyme conformation so that catalysis is reduced. Since substrate binding is not directly blocked, Km remains unchanged, but Vmax decreases because a fraction of enzyme is rendered less active or inactive. Increasing substrate concentration cannot fully overcome non-competitive inhibition, because the inhibitor still reduces the effective amount of active enzyme.

Together, these highlighted mechanisms show how digestive, energetic, reproductive, neural, and enzymatic processes are regulated by precise molecular signals, ensuring coordinated physiological function.