(a) Describe the structure of mitochondrion. Why is it known as powerhouse of the cell? (20 marks) (b) Explain the mechanism of…

(a) Structure of the mitochondrion and why it is the powerhouse of the cell

Structure: the mitochondrion is a rod-shaped or spherical organelle, 0·5 to 1 μm wide and 1 to 10 μm long, enclosed by two membranes. Outer membrane: smooth, with porin channels that make it permeable to small molecules. Intermembrane space: a narrow space between the membranes, in which protons accumulate. Inner membrane: highly selective, and folded into cristae that increase its surface area. It bears the electron transport chain and elementary particles (F₀-F₁ ATP synthase), each with a head, stalk and base. Matrix: a dense fluid containing the enzymes of the Krebs cycle and fatty acid oxidation, a circular DNA, 70S ribosomes and tRNA, so that mitochondria make some of their own proteins and divide by fission. Why it is the powerhouse: Step 1: Pyruvate and fatty acids are oxidised in the matrix, and the Krebs cycle gives NADH and FADH₂. Step 2: These pass electrons to the electron transport chain in the inner membrane. As electrons flow to oxygen, protons are pumped from the matrix into the intermembrane space. Step 3: This creates a proton gradient, which is the chemiosmotic mechanism of Mitchell. Protons return through ATP synthase and drive the formation of ATP from ADP and phosphate (oxidative phosphorylation). Step 4: One glucose yields about 30 to 32 ATP in total, most of it formed in the mitochondrion, so the organelle supplies most of the cell's ATP. Mitochondria are most numerous in cells with a high energy demand, such as muscle and sperm tail.

(b) Mechanism of sex determination in humans

Humans have 22 pairs of autosomes and one pair of sex chromosomes, XX in females and XY in males. Sex is decided at fertilisation by the sperm: an X-bearing sperm gives a girl and a Y-bearing sperm gives a boy. The egg always carries an X, so the male is the heterogametic sex. Role of the Y chromosome: the SRY gene (sex-determining region Y) makes the testis-determining factor, which turns the indifferent gonad of the embryo into testis at about the seventh week. Without a Y chromosome the gonad becomes an ovary. Hormones after the gonad forms: Sertoli cells secrete anti-Müllerian hormone, which causes the Müllerian ducts to degenerate. Leydig cells secrete testosterone, which keeps the Wolffian ducts and forms male internal and external genitalia, and a derivative, dihydrotestosterone, forms the external genitalia. In the absence of these hormones, the Müllerian ducts form the uterus and oviducts. Evidence from chromosome disorders: Klinefelter (XXY) is male and Turner (XO) is female, which shows that the Y chromosome and not the number of X decides maleness. Dosage compensation: one X chromosome of each female cell is inactivated as a Barr body.

(c) Means by which gene frequencies change in a population

By the Hardy-Weinberg principle, allele frequencies stay constant under random mating in a large population, with no mutation, migration or selection. Changes in gene frequency come from failure of these conditions. Mutation: it alters alleles and produces new ones, but its rate is low, so its direct effect is small. It supplies the raw material for other forces. Gene flow (migration): movement of individuals or gametes between populations adds or removes alleles and makes populations more alike. Genetic drift: random change in the frequency of alleles in small populations, which can lead to fixation or loss. Its forms are the founder effect and the bottleneck effect. Natural selection: alleles that raise survival and reproduction become more common. Directional, stabilising and disruptive selection are the main modes. Example: the sickle-cell allele is kept at high frequency in malaria regions because heterozygotes resist malaria. Non-random mating: inbreeding raises homozygosity and changes genotype frequencies but not allele frequencies.