Membrane Structure (mix)

• Cell membranes enclose the cell's contents, separating intracellular components from the extracellular environment
• Precise control of internal conditions (homeostasis) is enabled by this separation
• Two key properties of cell membranes promote homeostatic regulation

Selective Permeability

• Cell membranes are semi-permeable, meaning some materials cannot cross the membrane without assistance
• Cell membranes are selective, meaning they can regulate the passage of materials

Phospholipid Bilayer Composition

• Membranes consist of a phospholipid bilayer
• Each phospholipid has a polar phosphate head and two non-polar fatty acid tails
• Phosphate heads are hydrophilic (water-loving)
• Fatty acid tails are hydrophobic (water-hating)
• Phospholipids are amphipathic, meaning they are both hydrophilic and hydrophobic

Bilayer Formation and Structure

• Phospholipids spontaneously arrange into a bilayer
• Hydrophilic phosphate heads face outwards towards aqueous solutions (cytosolic and extracellular fluids)
• Hydrophobic fatty acid tails face inwards to avoid exposure to polar fluids
• Bilayer is held together by weak hydrophobic associations between fatty acid tails
• This arrangement allows for membrane fluidity and flexibility

Membrane Proteins

• Phospholipid bilayers are embedded with proteins that can be permanently or temporarily attached

Integral Proteins

• Transmembrane proteins that span the bilayer and are permanently attached to the membrane
• Cannot be easily dissociated without disrupting the bilayer (e.g., using detergents)
• Examples include ion channels, carrier proteins, and protein pumps

Peripheral Proteins

• Associate with one side of the membrane and are temporarily attached to the bilayer
• Can easily be dissociated using a polar solvent
• Examples include receptor complexes associated with signal transduction pathways (e.g., G proteins)

Functions of Membrane Proteins

• Serve a variety of key functions

Junctions

• Connect cells together to form tissues (tight junctions)

Enzymes

• Immobilize enzymes on membranes to localize specific reactions

Transport

• Facilitate the passage of materials across the bilayer (channel proteins)

Recognition

• Act as markers for cell identification (e.g., antigens)

Adhesion

• Serve as attachment points for the cytoskeleton or extracellular matrix

Transduction

• Act as receptors for signaling pathways (glycoproteins)

Glycosylation

• Phospholipids and membrane proteins can have carbohydrate chains attached via glycosylation
• Glycosylation of phospholipids produces glycolipids
• Glycosylation of membrane proteins produces glycoproteins
• Carbohydrate chains are on the extracellular side of the membrane, playing roles in cell adhesion and recognition

Cell Adhesion

• Surface carbohydrates (glycoproteins) can serve as attachment points for cells (e.g., sperm binding sites)

Cell Recognition

• Surface carbohydrates can act as recognition points between cells (ABO antigens are glycolipids)
• Glycoproteins and glycolipids help maintain the structural integrity of the extracellular matrix
• Carbohydrate chains can link extracellular molecules, helping create a cohesive network

Fluid-Mosaic Model

• Describes two key qualities of the plasma membrane

Fluidity

• Phospholipids can move position, making membranes amorphous (able to change size or shape)

Mosaicism

• The bilayer is embedded with proteins and carbohydrates, creating a mosaic of components
• This model was proposed by Singer and Nicolson in 1972 and is currently accepted