Cell Membrane Structure & Permeability Plasma Membrane – The plasma membrane that surrounds the cell is a fluid mosaic of phospholipids and proteins that separates the living cell from its nonliving surroundings. The membrane is selectively permeable and functions to separate the internal and external environments of the cell

Membrane Structures Phospholipid bilayer – Hydrophilic on the external and internal side – Hydrophobic in the center on the membrane Membrane proteins – float freely with their hydrophobic regions in the center of the molecules and their hydrophilic regions extending outwards – some directed or held rigid by internal cytoskeleton fibers but most float freely – The membrane proteins serve as cell surface markers, some serve as channel proteins, and some communicate chemical messages to the internal environment

Cholesterol – makes the membrane more rigid – lowers the temperature at which the membrane solidifies

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 – diffusion of a solvent is ALWAYS toward the area of highest solute concentration – Diffusion occurs until equilibrium is reached Equal solute concentration on both sides

Cell Transport Mechanisms 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 passive 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

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

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)

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