(a)(i) Describe composition of plasma membrane. (10 marks) (a)(ii) Differentiate between facilitated and passive diffusion across…

(a)(i) Plasma membrane composition. The plasma membrane is best described by the Singer–Nicolson fluid mosaic model. It is a phospholipid bilayer in which hydrophilic phosphate heads face the aqueous exterior and interior, while hydrophobic fatty-acid tails form the interior. Integral proteins span or are embedded in the bilayer and form channels, carriers, receptors and enzyme sites; peripheral proteins attach to the surface or to integral proteins. Cholesterol is interspersed in the lipid layer, modulating fluidity and permeability. Carbohydrate chains attached to lipids and proteins form the glycocalyx on the outer surface, aiding cell recognition and protection. The bilayer is selectively permeable: lipid-soluble molecules cross directly, while many polar molecules require protein pathways.

(a)(ii) Passive and facilitated diffusion. Passive or simple diffusion is movement of small, non-polar molecules down their concentration gradient without membrane protein, e.g. O2 and CO2. Its rate depends on gradient, size and membrane permeability. Facilitated diffusion is also passive and down-gradient, but requires specific membrane proteins: channel proteins form aqueous pores for ions or water, while carrier proteins undergo conformational change to move solutes. Examples include GLUT glucose transporters and aquaporins for water. Thus both are energy-independent, but facilitated diffusion is protein-mediated, selective and saturable because transporters are finite.

(b) Mutations, variation and selection. Mutations are permanent changes in DNA and are the primary source of new genetic variation. Gene mutations include point mutations, which are single-base substitutions (transitions or transversions), and frameshift mutations caused by insertions or deletions of bases not in multiples of three. Chromosomal mutations involve deletions, duplications, inversions or translocations; genome mutations involve changes in chromosome number, such as aneuploidy or polyploidy. Mutations arise spontaneously during replication or by DNA damage, and can be induced by radiation or chemical mutagens. Most are neutral or deleterious, but some create alleles that can be advantageous in a particular environment. In a large population, mutation changes allele frequencies, so it violates Hardy–Weinberg equilibrium, where p + q = 1 and p² + 2pq + q² = 1 only when mutation, selection, drift, migration and non-random mating are absent. Natural selection acts on phenotypic variation produced by mutation and recombination: individuals with advantageous alleles survive and reproduce more, increasing those alleles. Examples are antibiotic resistance in bacteria, where resistant mutants persist after drug use, and industrial melanism in Biston betularia, where dark moths increased in soot-covered areas because they were less visible to birds.

(c) Pedigrees. Pedigree symbols are squares for males, circles for females, shaded for affected, half-shaded for carriers, horizontal lines for marriage, vertical lines for offspring, and Roman numerals for generations. For autosomal recessive thalassaemia, common in parts of India, draw three generations: I-1 (half-shaded square) and I-2 (half-shaded circle) are carrier parents; their daughter II-1 (half-shaded circle) marries her carrier cousin II-2 (half-shaded square), the son of I-3 and I-4 (both half-shaded), where I-3 is a sibling of I-1. Their children include III-1 (shaded square) and III-2 (shaded circle), affected males and females, and III-3 (unshaded square), normal. This shows consanguinity, skipped generations, unaffected carrier parents and equal sex ratio. For X-linked recessive haemophilia, draw I-1 (unshaded square) and I-2 (half-shaded circle). Their children are II-1 (shaded square, affected son), II-2 (half-shaded circle, carrier daughter) and II-3 (unshaded square). II-2 marries an unrelated normal male, II-4 (unshaded square); their children are III-1 (shaded square, affected son), III-2 (half-shaded circle, carrier daughter) and III-3 (unshaded square). This shows criss-cross inheritance, more males affected, and no male-to-male transmission. Thus molecular mechanisms link to evolution: membrane proteins regulate selective transport, mutation supplies heritable variation, selection changes allele frequencies, and pedigrees reveal how such variants are transmitted.