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

The evolution of biological understanding spans from the structural elucidation of the cell membrane to the mechanistic decoding of heredity. While membrane biology transitioned from static lipid layers to dynamic protein-lipid interactions, genetics moved from classical Mendelian ratios to the recognition of cytoplasmic factors.

Molecular Models of the Cell Membrane The structural conception of the membrane evolved through several key models. Overton (1899) proposed that membranes are permeable to lipophilic substances, suggesting a lipid nature. Langmuir (1917) demonstrated that lipids form monolayers at the air-water interface. Gorter and Grendel (1925) deduced a lipid bilayer structure based on surface area calculations. The Danielli-Davson (1935) "sandwich" model proposed a lipid bilayer coated with protein layers. Robertson’s (1958) unit membrane model described a static trilaminar structure.

S.J. Singer and G. Nicholson (1972) introduced the Fluid Mosaic Model, which revolutionized membrane biology. They proposed that the lipid bilayer is fluid, allowing lateral movement of lipids and proteins. Proteins are not in a continuous layer but are embedded as a mosaic; integral proteins span the bilayer, while peripheral proteins attach to the surface. This model explains membrane asymmetry and the dynamic nature of transport.

Green and Capaldi (1974) refined this with the Lipid-Globular Protein Mosaic Model. Unlike the continuous protein layer of Danielli-Davson, they posited that globular proteins are embedded within a continuous lipid bilayer. These proteins are arranged in a mosaic pattern, interacting with lipids to facilitate specific functions, yet the lipid continuity is maintained.

Racker (1976) proposed the Subunit Model (or Complex Mosaic). Based on reconstitution experiments, Racker suggested that the membrane is not a continuous bilayer but consists of repeating lipoprotein subunits. Each subunit has a protein core surrounded by a lipid shell. These subunits pack together to form the membrane, creating a discontinuous lipid layer. This model emphasizes specific lipid-protein associations essential for transport functions.

**Mendel’s Experiments in *Pisum sativum*** Gregor Mendel studied seven contrasting character pairs in garden peas to establish the laws of inheritance:

1. Stem Height: Tall (dominant) vs. Dwarf (recessive).
2. Flower Position: Axial (dominant) vs. Terminal (recessive).
3. Pod Shape: Inflated (dominant) vs. Constricted (recessive).
4. Pod Color: Green (dominant) vs. Yellow (recessive).
5. Seed Shape: Round (dominant) vs. Wrinkled (recessive).
6. Seed Color: Yellow (dominant) vs. Green (recessive).
7. Flower Color: Violet (dominant) vs. White (recessive).

Mendel’s success was due to several strategic choices. He selected Pisum sativum for its true-breeding lines and distinct, discrete traits. He performed controlled self-pollination and cross-pollination to ensure genetic purity. By studying one character at a time (monohybrid crosses), he avoided the complexity of multiple variables. Furthermore, he applied statistical analysis to large sample sizes, allowing him to identify consistent 3:1 and 9:3:3:1 ratios, which revealed the particulate nature of heredity.

Theories of Inheritance and Cytoplasmic Inheritance Early theories included Preformationism, which suggested organisms pre-exist in gametes, and Pangenesis (Darwin), which proposed "gemmules" from body parts mix in gametes. The Germ Plasm Theory (Weismann) distinguished hereditary germ cells from somatic cells, rejecting acquired inheritance. The Chromosomal Theory (Sutton-Boveri) linked chromosomes to Mendelian factors. Epigenesis describes the gradual development of an organism from an undifferentiated egg. The Modern Synthetic Theory integrates Mendelian genetics with Darwinian evolution, recognizing mutation and selection.

Cytoplasmic Inheritance Cytoplasmic inheritance, or maternal inheritance, occurs when traits are encoded by DNA in mitochondria or chloroplasts, which are transmitted primarily through the egg. Evidence includes:

• Plastid Inheritance: In Mirabilis jalapa (4-o’clock plant), leaf variegation is maternally inherited. Crosses between variegated and green plants yield offspring with the phenotype of the female parent, as chloroplasts are passed via the cytoplasm of the egg.
• Mitochondrial Inheritance: In yeast, "petite" mutants (defective respiration) are maternally inherited. In Drosophila, sensitivity to CO2 is linked to sigma virus in mitochondria.
• Male Sterility: In maize and Indian sorghum, cytoplasmic male sterility (CMS) is maternally inherited. This is crucial for hybrid seed production in Indian agriculture, where CMS lines are used to produce high-yielding hybrid varieties without manual emasculation.

In conclusion, the progression from static membrane models to the fluid mosaic and subunit models highlights the dynamic nature of cellular boundaries. Similarly, the discovery of cytoplasmic inheritance expanded the nuclear-centric Mendelian framework, providing practical tools like CMS for crop improvement.