BIOL 2107 Chapter 5

Cellular Membranes are Fluid Mosaics of Lipids and Proteins

• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

• A phospholipid bilayer can exist as a stable boundary between two aqueous compartments

• Most membrane proteins are also amphipathic and reside in the bilayer with their hydrophilic portions protruding

• The fluid mosaic model states that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

• The movement of phospholipids is rapid; proteins move more slowly

• As temperatures cool, membranes switch from a fluid state to a solid state; the exception being if it is rich in phospholipids with unsaturated hydrocarbon tails

Membrane Proteins and Their Functions

• Integral proteins (transmembrane proteins) penetrate the hydrophobic interior of the lipid bilayer

• The hydrophobic regions of an integral protein consist of one or more stretches or nonpolar amino acids, often coiled into alpha helices

• Peripheral proteins are loosely bound to the surface of the membrane

• Major functions of membrane proteins: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to the cytoskeleton and ECM

• Cells recognize each other by binding to surface molecules, often containing carbs, on the extracellular surface of the plasma membrane

• Membrane carbs may be covalently bonded to lipids (forming glycolipids) or to proteins (forming glycoproteins)

• The asymmetrical arrangement of proteins, lipids, and associated carbs in the plasma membrane is determined as the membrane is built by the ER and Golgi

Membrane Selective Permeability

• Hydrophobic (nonpolar) molecules can dissolve in the lipid bilayer of the membrane and cross it easily

• Polar molecules do not cross the membrane easily

• Transport proteins allow passage of hydrophilic substances across the membrane

• Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel

• Aquaporins facilitate the passage of water

• Carrier proteins bind to molecules and change shape to shuttle molecules across the membrane

• A transport protein is specific for the substance it moves

Passive Transport

• Diffusion of a substance across a membrane with no energy investment

• Diffusion is the tendency for molecules to spread out evenly into the available space

• Substances diffuse down their concentration gradient, from where it is more concentrated to where it is less concentrated

• Osmosis is the diffusion of free water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration until the solute concentration is equal on both sides

Water Balance of Cells without Cell Walls

• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water

• In isotonic solutions, solute concentration is the same as inside the cell

• In hypertonic solutions, solute concentration is greater than that inside the cell

• In hypotonic solutions, solute concentration is less that that inside the cell

• Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments

Facilitated Diffusion

• Transport proteins speed the passive movement of specific molecules across the membrane by providing corridors that allow a specific molecule or ion to cross the membrane (aquaporins and ion/gated channels)

• Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

• The shape change may be triggered by binding and release of the transported molecule

• No net energy input is required

Active Transport

• Moves substances across membranes against their concentration gradients using energy, usually in the form of ATP

• Allows cells to maintain concentration gradients that differ from their surroundings

• Sodium-potassium pump is one type of active transport system