(a)(i) Write down the reactions that produce NADH during Krebs cycle. (10 marks) (a)(ii) Explain the role of proton gradient in…

NADH-Producing Reactions in the Krebs Cycle

During the complete oxidation of acetyl-CoA in the mitochondrial matrix, three specific dehydrogenation reactions reduce NAD⁺ to NADH + H⁺:

1. Oxidative Decarboxylation of Isocitrate: Isocitrate is converted to α-ketoglutarate by the rate-limiting enzyme isocitrate dehydrogenase. The reaction proceeds through an oxalosuccinate intermediate, releasing the first molecule of CO₂ and generating NADH: Isocitrate + NAD⁺ xrightarrowIsocitrate Dehydrogenase α-Ketoglutarate + CO₂ + NADH + H⁺

2. Oxidative Decarboxylation of α-Ketoglutarate: The multienzyme α-ketoglutarate dehydrogenase complex (requiring TPP, lipoate, FAD, NAD⁺, and CoA) converts α-ketoglutarate to succinyl-CoA, releasing the second CO₂ and reducing NAD⁺: α-Ketoglutarate + NAD⁺ + CoA-SH xrightarrowα-Ketoglutarate Dehydrogenase Complex Succinyl-CoA + CO₂ + NADH + H⁺

3. Oxidation of L-Malate: In the final step of the cycle, malate dehydrogenase catalyzes the reversible oxidation of L-malate to regenerate oxaloacetate, yielding the third NADH: L-Malate + NAD⁺ xrightarrowMalate Dehydrogenase Oxaloacetate + NADH + H⁺

Proton Gradient in Oxidative ATP Synthesis

According to Peter Mitchell’s chemiosmotic hypothesis, electron transfer along the mitochondrial electron transport chain (Complexes I, III, and IV) is coupled to the active pumping of protons (H⁺) from the matrix into the intermembrane space. This unidirectional extrusion creates an electrochemical proton gradient (proton-motive force, Δ p), comprising an electrical membrane potential (Δ ψ) and a chemical pH gradient (Δ pH).

The dissipation of this proton gradient drives ATP synthesis via the F_oF₁-ATP synthase complex. Protons flow down their electrochemical gradient through the membrane-embedded Fₒ proton channel (c-ring), inducing mechanical rotation of the central stalk (γ/ε subunits). This rotation drives conformational changes within the catalytic β-subunits of the matrix-facing F₁ head through the binding change mechanism (open, loose, and tight states), synthesizing ATP from ADP and inorganic phosphate (Pᵢ). The thermodynamic yield reflects a P/O ratio of approximately 2.5 ATP per NADH and 1.5 ATP per FADH₂.

Characteristics and Functions of Leukocytes

Lymphocytes: Morphologically characterized by a large, darkly staining spherical nucleus with a thin rim of agranular cytoplasm and a lifespan ranging from weeks to several years. T lymphocytes differentiate into CD4⁺ helper cells (orchestrating immune responses via cytokine secretion), CD8⁺ cytotoxic cells (inducing apoptosis in virally infected or neoplastic cells via perforin/granzyme), and regulatory T cells (immune tolerance). B lymphocytes mediate humoral immunity by differentiating into antibody-secreting plasma cells and memory B cells. Natural Killer (NK) cells provide innate cytotoxicity without prior antigen sensitization.

Monocytes: The largest circulating leukocytes (12–20 mum), possessing an indented or kidney-shaped nucleus, abundant agranular cytoplasm, and a circulating lifespan of 1 to 3 days. They migrate into tissues to differentiate into resident macrophages (e.g., Kupffer cells, alveolar macrophages) and dendritic cells. Their functions include receptor-mediated phagocytosis, secretion of pro-inflammatory cytokines, and processing and presentation of exogenous antigens on MHC Class II molecules to activate naive T cells.

Neutrophils: Polymorphonuclear granulocytes featuring 3 to 5 nuclear lobes and a short circulating lifespan (6–10 hours). Their cytoplasm contains primary azurophilic granules (myeloperoxidase, defensins) and secondary specific granules (lactoferrin, lysozyme). They act as first responders to bacterial infection via chemotaxis, phagocytosis, oxidative respiratory burst (generation of reactive oxygen species via NADPH oxidase), and NETosis—extruding neutrophil extracellular traps of chromatin and antimicrobial peptides to immobilize pathogens.

`` Leukocyte Comparison: Lymphocyte: [ Large Spherical Nucleus | Scant Cytoplasm ] -> Adaptive Immunity (T/B/NK) Monocyte: [ Kidney-Shaped Nucleus | Abundant Cytosol ] -> Tissue Macrophages & APCs Neutrophil: [ Multi-lobed Nucleus | Specific Granules] -> Phagocytosis, ROS, NETs ``

Development of the Mammalian Heart

Mammalian cardiogenesis begins at human embryonic day 18–19 (mouse E7.5) with the induction of lateral plate splanchnic mesoderm into the cardiogenic plate, comprising the primary heart field (left ventricle and atria) and secondary heart field (right ventricle and outflow tract).

`` Cardiogenic Field (E18/E7.5) Straight Heart Tube (E21/E8.5) [ Atrial / Ventricular Precursors ] [ Truncus -> Bulbus -> Ventricle -> Atrium -> Sinus ] │ │ ▼ ▼ Dextral Looping (E23-28) Four-Chambered Heart (E50/E14.5) [ D-loop folds Ventricle Ventrally, [ Septum Primum/Secundum, IV Septum, Atrium shifts Dorso-cranially ] Spiral Aorticopulmonary Septum ] ``

Heart Tube Formation and Segmentation: Paired endocardial tubes fuse along the midline by day 21 (mouse E8.5) to form a single continuous heart tube lined by cardiac jelly and myocardium. Craniocaudally, it differentiates into five primitive dilatations: truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus.

Cardiac Looping: Between days 23 and 28, the rapidly elongating tube undergoes dextral looping (D-looping), bending rightward and folding the primitive ventricle ventrocaudally and the primitive atrium and sinus venosus dorsocranially.

Septation and Valve Formation: Atrial septation proceeds via the downward growth of septum primum toward the fused endocardial cushions, leaving the temporary foramen primum, followed by foramen secundum formation and the adjacent growth of the rigid septum secundum, leaving the patent foramen ovale. Ventricular septation occurs through an upward-growing muscular interventricular septum meeting the membranous septum formed by endocardial cushions and conotruncal ridges. Simultaneously, the aorticopulmonary spiral septum divides the truncus arteriosus into the ascending aorta and pulmonary trunk, establishing the integrated, mature four-chambered mammalian circulation.