Cell Transport Mechanisms

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)