I. Movement of substances across the cell membrane
1. Molecules randomly move and distribute along concentration gradient
2. Warm temperatures and small molecules diffuse quicker
3. Efficient only for short distances
4. Diffusion across plasma membranes
B. Osmosis– diffusion of water across cell membrane
1. Osmotic pressure– the force associated with the movement of water – or the tendency of water to move by osmosis, a “pulling” pressure. Osmotic pressure is measured in milliosmoles/liter (a measurement of dissolved solutes in a solution)
2. Movement of water across membrane
a. Low solubility in lipids
b. Water diffuses easily across aquaporins (transmembrane water channels)
3. Osmolarity and Tonicity
a. Osmolarity is the number of osmoles per liter of solution
b. Tonicity is the ability of a solution to affect the fluid volume and pressure in a cell
c. Isotonic solution
d. Hypotonic solution
e. Hypertonic solution
C. Transport systems –molecules cross the membrane using mechanisms mediated by membrane proteins called transporters or carriers.
1. Facilitated diffusion
a. Follows concentration gradient
b. Shows saturation & specificity
c. Diffusion of ions using small selective protein channels
- How permeable a membrane is to an ion depends on the density of channels for that ion.
- Important is the number of open channels.
- Ion channels open and close.
- The opening and closing of ion channels is called channel gating.
d. Voltage sensitive channels
e. Ligand sensitive channels
f. Mechanogated channels
2. Active transport
a. Against concentration gradient
b. Energy must be used
c. Shows specificity and saturation
d. Primary AT –uses ATP as source of energy
-For example, Sodium-Potassium Pump = Na/K ATPase
e. Secondary AT –uses ion gradients as source of energy (cotransport)
f. Symport –the carrier used to move molecules in same direction with cotransport
g. Antiport –the carrier used to move molecules in opposite directions