Define external boundaries with a selectively permeable membrane.
Comparmentalization of cell components and organelles (Eukaryotes).
Molecule transport in and out of cells
Organize reaction sequences (restrict degrees of freedom) 3D to 2D
Cell-cell communication and adhesion
Diffusion as a rate-limiting step: Membrane Organization
Molecules can diffuse in three dimensions. Membranes can increase rates of reaction by restricting one degree of freedom for molecules to diffuse, and further allow molecules to react together.
Variability in membranes is due to functional differences in different types of membranes.
Plasma membranes are enriched in cholesterol and contain no cardiolipin.
Myelin sheath has high lipid proportions (insulator)
Mitochondrial membranes - high proportion of protein (site of many enzymatic reactions)
Biological Membrane Relative Composition
Biological membranes are mostly proteins and polar lipids. They account for almost all the mass of biological membranes. Carbohydrates are included into membranes in structures like glycoproteins and glycolipids.
Membranes as External Boundaries of cells-
Membranes are self-sealing and selectively permeable to polar solutes
Membranes are thin, effectively providing 2-D space for components to one around in.
Lipid bi-layer, hydrophobic tails together, hydrophilic heads on the outside.
Hydrophobic interactions hold proteins in bilayer
Proteins can be transmembrane, or integral, and can be positioned anywhere in membrane.
Lateral movement of lipids and proteins is allowed, due to only non-covalent interactions between molecules.