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URI BIO 244 Ch 3

CH 3: Plasma Membrane and Membrane Potential

Plasma membrane A protein studded lipid bilayer that encloses each cell, separating it from the extracellular fluid.
Phospholipids polar head containing a negatively charged phosphate group and two nonpolar fatty acid chain tails. Polar heads are hydrophyllic (water loving), and the nonpolar tails are hydrophobic (water fearing).
Lipid Bilayer A double layer of lipid molecules with the hydrophobic tail burying itself in the center and the hydrophyllic heads lined up on both sides in contact with water.
Cholesterol Contributes to both the fluidity and the stability of the membrane. These molecules are tucked between the phospholipid molecules where they prevent the fatty acid chains from packing together and crystallizing. This would reduce membrane fluidity.
Membrane proteins These are inserted within or attached to the lipid bilayer. They include integral proteins, transmembrane proteins and peripheral proteins.
Integral Proteins Embedded in the lipid bilayer, with most extending through the entire thickness of the membrane.
Transmembrane proteins a plasma-membrane protein that extends entirely across the membrane
Peripheral proteins A plasma-membrane protein that studs the surface instead of penetrating the membrane
Describe the fluid mosaic model of the cell membrane references the membrane fluidity and the ever-changing mosaic pattern of the proteins embedded in the lipid layer
Membrane carbohydrate Found only inside of cells. Their function is to help cells recognize like cells so they can form tissues.
Channels Small, water-filled passageways through the plasma membrane; formed by membrane proteins that span the membrane and provide highly selective passage for small water-soluble substances such as ions. Examples are calcium, sodium, and potassium channels.
Leaky Channels Passageways that always permit passage of their selected ion.
Gated Channels May be open or closed to their specific ion as a result of changes in channel shape in response to controlling mechanisms.
Voltage Gated channels Open or closed depending on voltage.
Carrier Transport of a substance across the plasma membrane facilitated by a carrier molecule
Receptors Sites that "recognize" and bind with specific extracellular chemical messengers in the cell's environment.
Membrane bound enzymes proteins located on either the inner or the outer cell surface that control specific chemical reactions
Cell adhesion molecules (CAMs) Proteins that protrude from the surface of the plasma membrane and form loops or other appendages that the cells use to grip one another and the surrounding connective tissue fibers
Extracellular matrix Serves as the biological glue.
Collagen Forms flexible but nonelastic fibers or sheets that provide tensile strength. It is the most abdundant protein in the body, making up nearly half of total body protein by weight.
Elastin Rubbery protein fiber most plentiful in tissues that must easily stretch and then recoil after the stretching force is removed. One area it is found is in the lungs.
Fibroconectin Promotes cell adhesion and holds cells in position. Reduced amounts of this protein have been found within certain types of cancerous tissue, possibly accounting for cancer cells' inability to adhere well to one another leading to metastisizing.
Desmosomes Act like spot rivets that anchor together to adjacent but nontouching cells. They consist of two components: plaques and cadherins.
Tight Junction An impermeable junction between two adjacent epithelial cells formed by the sealing together of the cells’ lateral edges near their luminal borders; prevents passage of substances between the cells
Gap Junction A communicating junction formed between adjacent cells by small connecting tunnels that permit passage of charge-carrying ions between the cells so that electrical activity in one cell is spread to the adjacent cell
Describe how different types of molecules and ions cross the cell membranes. Lipid soluable substances and small water soluble substances can permeate the plasma membrane unassisted. Active forces use energy to move particles across the membrane, but passive forces do not.
Diffusion Random collisions and intermingling of molecules as a result of their continuous thermally induced random motion. Also known as simple diffusion.
Solutions Homogenous mixtures containing a relatively large amount of one substance called the solvent, which is the dissolving medium. In the body, this is water.
Solutes Smaller amounts of one or more dissolved substances
Concentration The amount of solute dissolved in a specific amount of solution. The greater the concentration of solute molecules (ions), the greater the likelihood of collisions.
Concentration Gradient A difference in concentration of a particular substance between two adjacent areas. Also known as a chemical gradient.
Electrical gradient A difference in charge between two adjacent areas
Fick's Law of Diffusion Rate of net diffusion of a substance across a membrane is directly proportional to the substance’s concentration gradient, the lipid solubility of the substance, and the surface area of the membrane and inversely proportional to the substance’s molecular
Osmosis The net diffusion of water down its own concentration gradient through a selectively permeable membrane.
Hydrostatic pressue The pressure exerted by a standing, or stationary, fluid on an object.
Osmotic pressure A measure of tendency for osmotic flow of water into that solution because of its relative concentration of nonpenetrating solutes and water.
Electrochemical gradient The simultaneous existence of an electrical gradient and concentration (chemical) gradient for a particular ion
Tonicity The effect the solution has on cell volume, whether the cell remains the same size, swells, or shrinks, when the solution surrounds the cells.
Active Transport Active carrier-mediated transport involving transport of a substance against its concentration gradient across the plasma membrane. Requires ATP.
Carrier Mediated Transport Transport of a substance across the plasma membrane facilitated by a carrier molecule
Facilitated Diffusion Passive carrier-mediated transport involving transport of a substance down its concentration gradient across the plasma membrane. No ATP required.
Surface markers Allow cells to recognize like cells.
Enzymes Speed up reactions.
Describe the sodium/potassium pump Carrier that transports Na+ out of the cell, concentrating it in the ECF, and picks up K+ from the outside, concentrating it in the ICF. Requires ATP.
Membrane potential A separation of charges across the membrane; a slight excess of negative charges lined up along the inside of the plasma membrane and separated from a slight excess of positive charges on the outside
Describe the electrical properties of the membrane Opposite charges tend to attract each other and like charges tend to repel each other. Work must be performed (energy expended) to separate opposite charges after they have come together.
milivolt The unit used to measure potential when the membrane potential is low.
Resting membrane potential The membrane potential that exists when an excitable cell is not displaying an electrical signal
Describe the permeability of the membrane at rest - K+ leaky channels It is usually much easier for K+ to get through the membrane because the membrane typically has many more leak channels always open for passive K+ traffic. At resting potential the membrane is typically 25 -30 times more permeable to K+.
A- (anions) The plasma membrane is virtually impermeable to these large, negatively charged proteins. They are found only inside the cell.
Equilibrium potential for K+ The potential that would exist when no further net movement of K+ occurs.
excitable tissues nerves and muscles that produce electrical signals when excited
Polarization The state of having membrane potential
Depolarization A reduction in membrane potential from resting potential; movement of the potential from resting toward 0 mV
Repolarization Return of membrane potential to resting potential following a depolarization
Hyperpolarization An increase in membrane potential from resting potential; potential becomes even more negative than at resting potential



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