B2.1 Membranes and Membrane Transport

Lipid Bilayer

Phospholipids and other amphipathic lipids naturally form continuous sheet-like bilayers in water.

Fluid mosaic model

A model describing the structure of cell membranes as a mosaic of various components.

Selectively permeable

Allows substances meeting certain criteria to pass through unaided.

Hydrophobic core

Blocks large polar molecules and ions.

Passive Movement

Movement of substances without energy input.

Active Movement

Movement of substances that requires energy input.

Simple diffusion

The passive movement of a substance from a high concentration to a low concentration.

Facilitated diffusion

The passive movement of molecules from high to low concentration through a channel protein.

Osmosis

The passive, net movement of water molecules from a region of low solute (high H2O) concentration to a region of high solute concentration (low H2O).

Active transport

The movement of molecules against the concentration gradient via protein pumps using energy (ATP).

Bulk transport

Bulk movement of molecules into or out of the cell using energy.

Endocytosis

Bulk transport into the cell.

Exocytosis

Bulk transport out of the cell.

Cholesterol in membranes

Acts as a modulator of membrane fluidity.

Saturated fatty acids

Have no double bonds and straight tails.

Unsaturated fatty acids

Have one or more double bonds leading to bends in the molecule.

Gated ion channels

Channels that open in response to specific stimuli.

Ligand gated channels

Open when ligands bind with receptors on a protein.

Voltage gated channels

Opened by a change in charges of the membrane.

Sodium potassium pump

A transport protein that moves sodium out of and potassium into the cell.

Sodium glucose co-transporter

A protein that facilitates the transport of sodium and glucose into cells.

Cholesterol at high temps

Cholesterol restrains movement of the fatty acids

Stabilizes the membrane

Reduced permeability to small molecules

Cholesterol at low temps

Prevents stiffening of the membrane

Prevents tight packing of fatty acids

Maintains fluidity