1. Cell Transport Mechanisms Passive transport (Diffusion) – Movement of a substance down a concentration gradient (area where there is more of a substance to an area where there is less of a substance) until equilibrium is reached – speed depends on the concentration difference, the size of the substance being moved, and the thermal energy of the solution – diffusion of a solute is ALWAYS toward the area of least solute concentration – passive transport in organisms occurs across a biological membrane Facilitated Transport - Diffusion aided by a channel protein or a carrier protein – necessary for the transport of polar molecules through the hydrophobic interior of the plasma membrane – still follows the rules for active transport and osmosis – most carriers and channels are ion or molecule specific – channels greatly speed up the entry of water and small solutes into the cell – ion channels may be gated channels requiring a stimulus to open

2. Osmosis Osmosis - diffusion of water across a differentially permeable (semi- permeable or selectively permeable) membrane – aided by the presence of aquaporins (channel proteins) – water diffuses to an area of lower solute concentration to an area of higher solute concentration – the side of the membrane with higher solute pressure is said to have greater osmotic pressure (negative - sucking) – the side with the lower pressure is said to have greater hydrostatic pressure (positive - pushing) – osmoregulation - management of water through the manipulation of tonicity (solute concentration) iostonic - equal solute in 2 adjoining compartments (osmotic & hydrostatic press = 0) hypertonic - having a greater solute concentration than the adjoining compartment (osmotic press > 0) – a cell in a hypertonic environment loses water and shrivels, turger (firmness) decreases hypotonic - having a lower solute concentration than the adjoining compartment (hydrostatic press > 0) – a cell in a hypotonic environment gains water and expands, turger (firmness) increases & plasmolysis (splitting of the plasma membrane) is possible

3. Active Transport Active Transport - diffusion against a concentration gradient requiring energy – movement is toward the area of lower solute concentration – requires a carrier protein – requires the use of ATP (adenosine triphosphate) for energy – used to maintain the ion-potential of the cell membrane sodium-potassium pump – creates an electrochemical gradient (cytoplasm is negative compared to the extracellular environment) » called membrane potential (-50 to -200mV) » called an electrogenic pump in animals a proton pump in plants, bacteria, & fungi

4. Cotransport Cotransport - transport by a molecule that has been actively transported across a membrane – 2nd transport is by facilitated diffusion – H + -sucrose pump in plants

5. Bulk Transport Exocytosis vs Endocytosis – Exocytosis - a membrane bound compartment (from golgi, lysosome, …) that fuses with the plasma membrane releasing the contents the extracellular environment – Endocytosis - the infolding of the plasma membrane around a molecule releasing a vesicle in the intracellular environment. 3 types: Phagocytosis - cellular eating Pinocytosis - Cellular drinking Receptor mediated endocytosis - used to bring in ligands (molecules bound to a receptor)