(a) Describe the process of spermatogenesis. Add a note on the role of Golgi bodies in the formation of acrosome. (15+5=20…

(a)(i) Process of Spermatogenesis

Spermatogenesis is the continuous process occurring within the seminiferous tubules of testes, wherein diploid spermatogonia transform into haploid spermatozoa. It proceeds through three major phases: the proliferative phase, meiotic phase, and spermiogenesis.

Type A spermatogonia undergo mitotic divisions to maintain the stem cell pool and yield Type B spermatogonia, which enlarge into primary spermatocytes (46, XY). Under the influence of follicle-stimulating hormone (FSH) acting on Sertoli cells and luteinizing hormone (LH) stimulating Leydig cells to secrete testosterone, primary spermatocytes undergo Meiosis I to form two haploid secondary spermatocytes (23, X or 23, Y). These rapidly enter Meiosis II, producing four non-motile, spherical spermatids.

During spermiogenesis, spermatids differentiate into spermatozoa without further division: the nucleus condenses as histones are replaced by protamines, the distal centriole forms the axoneme of the flagellum, mitochondria wrap spirally round the middle piece, and excess cytoplasm is shed as residual bodies that Sertoli cells phagocytose. Sertoli cells also form the blood–testis barrier and nourish the germ cells, and mature sperm are released into the lumen by spermiation. In humans one cycle from spermatogonium to sperm takes about 64–74 days.

(a)(ii) Role of Golgi Bodies in Acrosome Formation

The Golgi apparatus plays a central role in generating the acrosome, an apical cap acting as a specialized lysosome. Acrosome formation passes through the Golgi, cap, acrosome and maturation phases. Ultrastructurally, the Golgi cisternae produce multiple electron-dense proacrosomal vesicles that coalesce into a single large acrosomal vesicle containing an acrosomal granule. This vesicle adheres to the nuclear membrane at the anterior pole of the condensing nucleus, spreading over its anterior two-thirds to form the acrosomal cap. The remnant Golgi apparatus moves away as the Golgi remnant. The completed acrosome contains hydrolytic enzymes, notably hyaluronidase, corona penetrating enzyme, and acrosin, which are essential for penetrating the cumulus oophorus and zona pellucida during fertilization.

(b) Why Vitamins Are Called Coenzymes

Vitamins, specifically water-soluble B-complex vitamins, are designated as coenzymes because they serve as essential, non-protein organic precursors that bind with apoenzymes to form catalytically active holoenzymes. Unlike macronutrients, vitamins function catalytically in micro-quantities rather than stoichiometrically as substrates.

The biochemical justification rests on their obligate transformation into reactive prosthetic groups or co-substrates: Thiamine (Vitamin B1) is phosphorylated to Thiamine Pyrophosphate (TPP) for oxidative decarboxylation; Riboflavin (Vitamin B2) forms Flavin Mononucleotide (FMN) and Flavin Adenine Dinucleotide (FAD) for cellular redox reactions; Niacin (Vitamin B3) yields NAD+ and NADP+, serving as universal hydride-ion carriers; Pantothenic acid (Vitamin B5) forms the functional core of Coenzyme A for acyl transfers; Pyridoxine (Vitamin B6) converts to Pyridoxal Phosphate (PLP) for transamination and amino acid metabolism; Biotin (Vitamin B7) acts as a covalently bound coenzyme in carboxylation reactions; Folic acid (Vitamin B9) converts to Tetrahydrofolate (THF) for one-carbon unit transfer; and Cobalamin (Vitamin B12) forms Methylcobalamin and Adenosylcobalamin for isomerase and methyltransferase reactions. Strictly, the vitamin is the precursor and its derivative is the coenzyme, which is why deficiency of a vitamin shows up as failure of particular enzymes: TPP deficiency causes beriberi, NAD deficiency pellagra, and folate or B12 deficiency megaloblastic anaemia.

Unlike fat-soluble vitamins (A, D, E, K), which primarily function as nuclear receptor ligands, visual pigments or lipid-soluble antioxidants (although vitamin K is a true cofactor for γ-glutamyl carboxylase in clotting-factor synthesis, and vitamin C for prolyl and lysyl hydroxylases in collagen synthesis), water-soluble vitamins undergo limited tissue storage and function directly in intermediary metabolic enzymatic complexes.

(c)(i) Mechanism of Contraction in Skeletal Muscle

Skeletal muscle contraction operates via the sliding filament mechanism coupled with excitation-contraction coupling. At the neuromuscular junction, acetylcholine released from the motor nerve ending binds nicotinic receptors and produces an end-plate potential that triggers a muscle action potential, which propagates across the sarcolemma and down the transverse (T) tubules. This depolarizes dihydropyridine receptors, opening ryanodine receptors on the sarcoplasmic reticulum to release Ca²⁺ into the sarcoplasm.

Elevated Ca²⁺ binds to troponin C, producing a conformational shift in troponin I and troponin T that pulls tropomyosin away from actin active sites. Myosin heads, energized by hydrolyzed ATP (bound to ADP and Pi), bind to exposed actin to form cross-bridges. The release of Pi and ADP triggers the power stroke, tilting the myosin head by 45 degrees and pulling the thin actin filament toward the center of the sarcomere (H-zone narrowing). Binding of a new ATP molecule causes myosin detachment; ATP hydrolysis re-cocks the head for subsequent cycling until Ca²⁺ is actively pumped back by SERCA pumps. During contraction the A band keeps its length while the I band and H zone shorten, because the filaments slide rather than shorten; without ATP the cross-bridges cannot detach, which explains rigor mortis.

(c)(ii) Rheobase and Chronaxie

Neuromuscular excitability is evaluated using the strength-duration curve. Rheobase is defined as the minimum galvanic electrical stimulus intensity (current or voltage) of infinite duration required to elicit a threshold muscle contraction. Chronaxie is the minimum stimulus duration required to excite the tissue when using a current twice the strength of the rheobase. Chronaxie provides a precise index of excitability: tissues with shorter chronaxie values, such as myelinated nerves and fast skeletal muscle fibers, exhibit higher excitability.