The Fluid Mosaic Model, proposed by S.J. Singer and Garth Nicolson in 1972, is the currently accepted model of cell membrane structure. Key features: Phospholipid bilayer: amphipathic phospholipid molecules arranged with hydrophilic heads facing the aqueous environments (cytoplasm and extracellular) and hydrophobic fatty acid tails facing inward. Fluid: the bilayer is not rigid but fluid — phospholipids and proteins can move laterally (diffuse) within the plane of the membrane. Mosaic: protein molecules are embedded in the fluid bilayer like tiles in a mosaic. Integral (transmembrane) proteins: span the entire bilayer. Peripheral proteins: attached to the surface (inner or outer). Glycoproteins and glycolipids: carbohydrate chains attached to proteins/lipids on outer leaflet — cell recognition. Cholesterol: inserted between phospholipids — regulates membrane fluidity (prevents crystallisation at low T, prevents excess fluidity at high T).

Cell membrane composition varies by cell type and membrane location. General composition: Lipids (~40%): phospholipids (most abundant — phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin), cholesterol (in animal cells — ~20-25 mol% of membrane lipids), glycolipids (on outer leaflet). Proteins (~52%): integral (transmembrane) and peripheral proteins. Carbohydrates (~8%): as glycoproteins and glycolipids on outer leaflet — form the glycocalyx. Human RBC membrane: 52% protein, 40% lipid, 8% carbohydrate — as stated in question. Myelin sheath: 80% lipid, 20% protein (specialised for electrical insulation). Inner mitochondrial membrane: 75-80% protein (ETS complexes), 20-25% lipid. The protein content reflects membrane function — membranes with high metabolic activity have more protein.

The two leaflets of the plasma membrane have different lipid and protein compositions (asymmetry). Outer leaflet (exoplasmic): phosphatidylcholine (PC), sphingomyelin, glycolipids, glycoproteins. Negative charge on carbohydrate groups. Inner leaflet (cytoplasmic): phosphatidylethanolamine (PE), phosphatidylserine (PS — negatively charged), phosphatidylinositol (PI — signalling). PS on inner leaflet is important: normally inward, when cell undergoes apoptosis, PS flips to outer leaflet → signal for phagocytosis (eat-me signal). Membrane flippases (ATP-dependent aminophospholipid translocases) maintain asymmetry by actively moving PS and PE from outer to inner leaflet. Scramblases mix lipids between leaflets during apoptosis. Asymmetry is essential for: signalling (PS on inner leaflet activates kinases), endocytosis, apoptosis signalling.

Substances cross the membrane by different mechanisms. Simple diffusion: small non-polar molecules (O2, CO2, N2, alcohols, steroid hormones). Moves down concentration gradient. No energy, no protein needed. Facilitated diffusion: charged/polar molecules (glucose, amino acids, ions) diffuse through protein channels or carriers. Down concentration gradient. No energy (passive). Examples: GLUT1 (glucose transporter), aquaporins (water), ion channels (Na+, K+, Ca2+, Cl-). Active transport: moves solutes AGAINST concentration gradient. Requires ATP energy. Proteins: Na+/K+-ATPase (pumps 3 Na+ out, 2 K+ in per ATP — maintains resting membrane potential). Ca2+-ATPase, H+/K+-ATPase (stomach, creates acid). Primary active: directly uses ATP. Secondary active: uses electrochemical gradient created by primary active transport (cotransporters, antiporters). Endocytosis: phagocytosis (large particles), pinocytosis (fluids), receptor-mediated endocytosis (specific ligands). Exocytosis: secretion of proteins, neurotransmitters.

Mesosomes are infoldings of the plasma membrane found ONLY in prokaryotes (bacteria). They are NOT present in eukaryotes. Proposed functions in bacteria (some controversial): Site of DNA attachment and segregation during cell division. Increase membrane surface area. Site of respiratory enzymes (functional role analogous to mitochondrial cristae). Associated with cell wall synthesis. Controversy: many researchers believe mesosomes are artifacts of chemical fixation for electron microscopy rather than true in vivo structures. However, for NEET purposes: mesosomes are associated with PROKARYOTES only. Statement C in this question incorrectly states that mesosomes are extensions of plasma membrane in eukaryotic cells — hence C is WRONG. In eukaryotes, the inner mitochondrial membrane forms cristae (analogous functionally, but NOT called mesosomes).

Evolution of membrane models: Overton (1895): proposed cell membrane is made of lipids (based on permeability studies). Gorter and Grendel (1925): extracted lipids from RBCs and showed they covered exactly twice the RBC surface area → proposed lipid bilayer. Danielli and Davson (1935): proposed Protein-Lipid-Protein sandwich model — proteins coat both surfaces of lipid bilayer. Robertson (1959): Unit Membrane Model — all biological membranes have same basic structure (protein-lipid-protein sandwich), observed by electron microscopy. Singer and Nicolson (1972): Fluid Mosaic Model — replaced sandwich model. Proteins embedded IN bilayer, not just coating surface. Bilayer is fluid, not rigid. Proved by: freeze-fracture electron microscopy, FRAP (fluorescence recovery after photobleaching), lateral diffusion studies. Current refinements: lipid rafts (microdomains enriched in cholesterol and sphingolipids), pickets and fences model (cytoskeleton restricts lateral diffusion).

Cell wall: rigid structure outside plasma membrane. Found in: plants, fungi, most bacteria, many algae. Absent in: animals, protozoa. Plant cell wall: Primary wall: cellulose microfibrils embedded in matrix of hemicellulose, pectin, and proteins. Flexible, present in growing cells. Secondary wall: additional cellulose layers + lignin (in wood cells — xylem). Rigid, waterproof, provides mechanical strength. Middle lamella: pectin-rich layer between adjacent cells (holds cells together). Plasmodesmata: cytoplasmic connections through cell walls — allow cell-to-cell communication. Bacterial cell wall: Gram-positive: thick peptidoglycan layer (20-80 nm). Stains purple with Gram stain. Examples: Staphylococcus, Streptococcus. Gram-negative: thin peptidoglycan + outer membrane (lipopolysaccharide = LPS = endotoxin). Stains pink/red. Examples: E. coli, Salmonella. Peptidoglycan synthesis inhibited by penicillin → bacteria swell and burst.

Glycocalyx: layer of carbohydrate chains on the outer surface of the plasma membrane. Components: oligosaccharide chains attached to membrane glycoproteins and glycolipids. Carbohydrate chains extend outward from cell surface forming a coat. Functions: cell recognition (ABO blood group antigens are glycoproteins/glycolipids), cell adhesion (selectins bind carbohydrates for leukocyte rolling), protection from mechanical/chemical damage, lubrication (synovial joint fluid), immune function (pathogen recognition). Red blood cell glycocalyx: contains ABO blood group antigens (glycoproteins and glycolipids). Blood group O: only H antigen (no A or B modification). Blood group A: A antigen. Blood group B: B antigen. Blood group AB: both. Cancer cells: altered glycocalyx → changes in cell adhesion → contributes to metastasis. Selectins: lectins on endothelial cells that bind carbohydrates on leukocyte glycocalyx → initial rolling during inflammation.