(a) Enlist the molecular models of cell membrane and explain the models given by S.J. Singer and G. Nicholson (1972), Green and…

(a) Molecular models of the cell membrane

Models in order: Overton (1895–1899), a lipid film from permeability of lipid-soluble substances; Gorter and Grendel (1925), a lipid bilayer from extracted red-cell lipids; Danielli and Davson (1935), a lipid bilayer sandwiched between protein layers; Robertson (1959), the unit membrane, a trilaminar, universal and static structure seen in electron micrographs; the lipoprotein subunit models (Benson, 1966); Singer and Nicolson (1972); Green and Capaldi (1974); and Racker (1976).

Fluid mosaic model (Singer and Nicolson, 1972). The membrane is a fluid lipid bilayer in which globular proteins float like icebergs in a sea. Integral proteins span or are embedded in the bilayer (hydrophobic regions inside, hydrophilic ends outside), and peripheral proteins sit on the surface. Lipids and proteins diffuse laterally (shown by cell-fusion and FRAP experiments), the two leaflets are asymmetric, and there is no continuous protein layer. It explains fluidity, transport, receptors and membrane asymmetry, and later work added lipid rafts and cytoskeletal corrals.

Green and Capaldi (1974). Often presented as the lipid–globular protein mosaic view: the membrane consists of globular proteins embedded in a lipid matrix, with the lipid bilayer not continuous but interrupted by protein, and with lipid-protein associations of the functional complexes. It also stresses the role of the protein in organising lipids.

Racker (1976). From reconstitution experiments with vesicles, Racker stressed that membrane function (for example ATP-driven pumps) depends on specific protein complexes and on the lipids associated with them; the membrane is seen as a mosaic of such functional proteolipid units, not a uniform sheet.

(b) Mendel's seven pairs of contrasting characters in pea, and reasons for his success

The seven pairs (dominant versus recessive): stem height, tall and dwarf; flower position, axial and terminal; pod shape, inflated and constricted; pod colour, green and yellow; seed shape, round and wrinkled; seed colour, yellow and green; flower colour, violet and white.

Reasons for success: he chose the garden pea, which is self-pollinating (pure lines), easy to cross by emasculation, short-lived, prolific and with many clear-cut contrasting traits; he tested the lines for true breeding over two years; he studied one or two characters at a time, at first; he worked with discrete, qualitative traits showing complete dominance; the genes for the traits he chose were on separate chromosomes or far apart (so they assorted independently); he kept accurate records of large numbers of plants in all generations, including reciprocal crosses and backcrosses; and he applied a quantitative, statistical analysis that gave the 3:1 and 9:3:3:1 ratios, from which he inferred unit factors.

(c) Theories of inheritance and evidence for cytoplasmic inheritance

Theories: preformation (a miniature individual pre-exists in the gamete); pangenesis (Hippocrates, Darwin 1868), in which gemmules from all body parts gather in the gametes; blending inheritance; inheritance of acquired characters (Lamarck); germplasm (Weismann, 1892), separating germ cells from body cells; particulate inheritance (Mendel, 1866); the chromosome theory (Sutton and Boveri, 1902–03) and Morgan's gene theory; and the modern molecular view of DNA.

Evidence for cytoplasmic inheritance (traits that follow the mother and not Mendelian ratios): Correns (1908) found that variegation in Mirabilis jalapa follows the female parent in reciprocal crosses, through plastids; chloroplast mutations in Chlamydomonas (streptomycin resistance) pass through the mt+ parent; "petite" mutants in yeast and poky in Neurospora show mitochondrial inheritance; cytoplasmic male sterility in maize (T-cytoplasm), sorghum and pearl millet is passed through the female and is used in hybrid seed production; kappa particles in Paramecium and sigma virus in Drosophila (CO₂ sensitivity) are cytoplasmic. Reciprocal crosses differ, and the trait persists after repeated backcrossing to the male nuclear genome.