Cellular Transport

Cell membranes

Lipid bilayer - two layers of Phospholipids

innner and outer surfaces - phosphate based - Hydrophilic

interior of membrane -lipid and cholesterol based – Hydrophobic; water soluble ions and molecules cannot enter this region

Glycocalyx - carbohydrate complexes on outer surface of cell membrane

serve as receptors that permit extracellular substances to control activity of cells

provides a genetically determined fingerprint that is recognized by the immune system as 'self' rather than 'foreign'

Integral proteins - proteins embedded in cell membrane

permit passageways through the cell membrane

Channels - passageways in the integral proteins

Gated Channels - channels that can open or close to control the passage of ions and molecules

Membrane permeability: Passive processes

Concentration gradient - difference between the high and low concentrations

Diffusion - movement of substances from an area of high concentration to an area of low concentration due to the random activity of molecules

Diffusion across cell membranes

lipid soluble substances can diffuse through the cell membrane

water soluble substances can pass through channels if;

Osmosis - diffusion of water across a cell membrane

Dialysis - diffusion of solutes across a cell membrane

Filtration - movement of water across a cell membrane due to hydrostatic pressure

Osmolality - concentration of dissolved substances (solutes)

determines the concentration gradient of the water

Isotonic - same osmolality as the cellular cytoplasm

Hypotonic - lower osmolality than the cytoplasm

Hypertonic - higher osmolality than the cytoplasm

Permeability - extent that substances can enter or leave a cell

Impermeable - nothing can cross the membrane

Freely Permeable - any substance can cross the membrane

Selectively permeable - only certain substances can cross

Membrane permeability: Active processes

Active transport - movement of substances across a cell membrane using a Carrier Protein and added energy

occurs regardless of the concentration gradient

Ion pumps - active transport of a single ion

Exchange pumps - active transport of one ion in one direction and another ion in the opposite direction

Sodium-potassium exchange pump - an exchange pump where three sodium ions are exchanged wtih two potassium ions

Transmembrane potential

extracellular fluid contains large numbers of sodium and chloride ions

intracellular fluid contains large numbers of potassium ions and negatively charged proteins

transmembrane potential - difference in charge between the inside and the outside of a cell (about -70 mV), due to


David G. Ward, Ph.D.  Last modified by wardd 23 May, 2006