Biological Membrane Overview

Biological Membrane

• Composed of cell structures (eukaryotic): cell membrane, nucleus, organelles, cytoplasm, mitochondria, and cytoskeleton.
• Cell membrane functions: separating the inside from the outside, maintaining concentration gradients, maintaining membrane potential, binding cells together, mediating and regulating cell transport, and detecting and transmitting electrical and chemical signals.
• Cell membrane theory evolution: Overton (1890s): lipid nature of biological membranes; Langmuir (1917): lipid monolayer (hydrophilic/hydrophobic surfaces); Gorter and Grendel (1925): lipid bilayer; Danielli and Davson (1935): lipid bilayer + protein sheets ("sandwich" model); Singer and Nicolson (1972): fluid-mosaic theory; Unwin and Henderson (1975): membrane proteins contain transmembrane segments; Recent Findings (2000): Lipid rafts.
• Cell membrane (7-10nm): fluid mosaic theory (Singer and Nicolson, 1972); membrane lipids (40%): bilayer, phospholipids (dominate the structure), cholesterol (provides stability); membrane proteins (52%): peripheral proteins (confined to the surface), integral proteins (span the membrane); and membrane carbohydrates (8%): glycoproteins, glycolipids.

Membrane Structure

• Phospholipids: amphipathic molecules (one water-soluble portion and one lipid-soluble portion) with hydrophilic head groups and hydrophobic hydrocarbon tails. Spontaneously aggregate into a bilayer in water, with phosphate head groups facing outward toward the aqueous environment inside and outside the cell.
• Cholesterol: stabilizes the membrane and changes its fluidity. Lipid rafts are aggregations of lipids and cholesterol.

Membrane Fluidity

• Decreased cholesterol decreases fluidity
• Decreased saturation (FA) increases fluidity
• Increased cholesterol increases saturation (FA) increases fluidity
• Decreased temperature decreases fluidity
• Increased temperature increases fluidity
• Length of fatty acids (FA) affects fluidity. Short-chain FA increases fluidity, and long-chain FA decreases fluidity.
• Degree of saturation of FA affects fluidity. Unsaturated FA increases fluidity, and saturated FA decreases fluidity.

Lipid Composition

• Phospholipids are the most abundant lipid component in cell membranes.
• Phosphoglycerides (glycerol-based) and sphingolipids (sphingosine-based) are two major classes.
• Sterols (e.g., cholesterol in animals, ergosterol in fungi) are present in some membranes and influence fluidity.
• Lipids are distributed unequally in two monolayers. Glycolipids are mostly on the outer layer.

Protein Composition

• Integral proteins: covalently attached to fatty acid chains in the hydrophobic interior and typically have hydrophobic transmembrane segments.
• Peripheral proteins: attached to exposed polar heads of membrane lipids or integral proteins by weak electrostatic forces and hydrogen bonds.

Lipid-Anchored Proteins

• GPI-anchored proteins: covalently attached to glycosylphosphatidylinositol (GPI).

Protein Movements

• Protein movement within the membrane may be lateral and rotational (more restricted than phospholipids).

Membrane Carbohydrates

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) by glycosylation.

Cell-Cell Adhesion

• Cells are connected through extracellular matrix (ECM) components (like collagen, elastin, and fibronectin).
• Specialized cell junctions include tight junctions, desmosomes, and gap junctions.