Diffusion and Osmosis

Biology 1010

Fall 2009

DIFFUSION, OSMOSIS AND TRANSPORT

I. Membranes were covered in a previous section

II. Diffusion and Osmosis

A. Movement of substances is the result of internal energy which keeps them in constant motion, and concentration gradients

1. Concentration gradient = differences in the concentration or pressure of materials in one part of a contained area compared to another

2. Materials tend to move down concentration gradients, i.e. from regions of higher concentrations to regions of lower concentrations

3. Materials move down concentration gradients because under high concentrations molecules collide with each other more than they would if spread over a larger area

4. Diffusion = tendency of materials to move from areas of higher concentration to areas of lower concentrations

a. A solution contains a solute, usually a solid, and a solvent, usually a liquid

B. Many materials, particularly water enter and leave cells via simple diffusion due to a concentration gradient

1. Passive transport = the movement of a substance into or out of a cell due to a concentration gradient. It is passive because it spontaneously happens with no input of energy

a. However, the lipid bilayer is permeable to many small molecules but not larger or electrically charged molecules - therefore it is selectively permeable

2. Osmosis = movement of water through a selectively permeable membrane in response to a concentration or pressure gradient

a. Water moves from regions of higher concentration (of water molecules) to where it is present in lower concentrations

b. Osmotic pressure = hydrostatic pressure, on side of membrane with higher solute concentration, produced by water diffusing to that side of membrane

4. Tonicity = relative concentrations of solutes in two solutions; determines movement of water into or out of cells

a. Isotonic solution = solute concentration inside and outside cells is equal. Therefore no net movement of water into or out of cell

b. Hypotonic (low) solution = low in solutes (high in water molecules). Therefore water moves into cell

c. Hypertonic (high) solution = high in solutes (low in water molecules). Therefore water moves out of cell

C. Active transport

1. Plasma membrane impedes passage of most substances but many molecules enter or leave at rapid rates

2. Carrier proteins = membrane proteins that combine with and transport only one type of molecule; are believed to undergo a change in shape to move molecule across in active and facilitated transport

3. Active transport = transport of specific solutes across plasma membranes against the concentration gradient through use of cellular energy