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Chapter 1

Chapter 1, French

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University of Toronto St. George
Michelle French

Factors Affecting the Direction of Transport Passive Transport vs Active Transport When oxygen enters a cell/carbon dioxide leaves, molecules move across the plasma membrane spontaneously. + cells don't need to expend energy to make this happen + contrast: energy is expended when glucose molecules are transported into epithelial cells in the intestine active transport: transport of molecules across a membrane if it requires energy passive transport: does not require energy simple diffusion: movement of a molecule into or out of the cell by its own thermal motion + one form of passive transport, but certain forms of protein mediated transport are also passive + contrast: active transport is always mediated by transport proteins (pumps) transport of molecules + spontaneous (of passive) movement occurs from high energy to low energy; to move in the opposite opposition requires an input of energy + the energy of a solution depends on the solute concentration (and charge, if the solute is an ion) - energy increases as solute concentration increases solutes move massively from an area where they are in greater concentration to an area where they are in lesser concentration// down their concentration gradient - to move from low concentration to high concentration requires an input of energy Driving Forces Acting on Molecules: any difference in energy existing across a membrane acts as a driving force that push molecules in one direction or another. + direction is always from higher to lower energy (direction molecules would move) Driving forces can arise as a result of concentration differences/other factors that affect molecular energies Molecules influenced by three types of forces: chemical, electrical, and electrochemical driving forces. Chemical Driving Forces: concentration gradient exists across the membrane when a substance is present in different concentrations on either side of a membrane. + referred as a chemical driving force, the direction of which is always down the concentration gradient rate at which a substance is transported varies with the size of the concentration gradient and generally increases as the size of the gradient increases + magnitude of the chemical driving force increases as concentration increases Electrical Driving Forces: affects ions arise due to membrane potential, a difference in electrical potential/voltage that exists across the membranes of most cells Membrane Potential: + fluids in the body contain a wide variety of solutes (many of them are ions), possess electrical charge + cations (positive charge), others are anions (negative charge) + neutral when negative and positive charges cancel each other, giving a net (total) electrical charge of zero. + a separation of charge exist across the membrane + membrane potential creates an electrical driving force for the movement of ions - to determine the direction of the electrical driving force, need to know the valence (charge) of the ion. Membrane potential is negative + magnitude of the electrical driving force on an ion depends on the size of the membrane potential and the quantity of charge carried by the ion and it increases as either of these factors get larger. Electrochemical Driving Force: when ions are transported across membranes, two driving forces are influential: + 1. a chemical force reflecting the ions' tendency to move down their concentration gradient + 2. an electrical force reflecting the ions' tendency to be pushed in one direction or the other by the membrane potential electrochemical driving force:total force acting on the ions is the combinations of chemical and electrical driving forces equilibrium potential: a hypothetical value for the membrane potential at which the electrical driving force is equal and opposite to the chemical driving force, producing an electrochemical driving force of zero. + ion won't move spontaneously in either direction Rate of Transport: the rate at which a substance is transported across a membrane refers to the number of molecules that cross the membrane refers to the number of molecules that cross the membrane in a given time flux Passive Transport: molecules move across the membrane down their chemical or electrochemical gradients no energy required simple diffusion, facilitated diffusion, diffusion through ion channels diffusion: movement of molecules from one location to another simply as a result of their own thermal motion Factors that affect the rates of simple diffusion: magnitude of the driving force, membrane surface area, and the permeability of the membrane When two concentrations become equal, the net flux becomes zero. + diminishes as the size of the concentration gradient decreases + no longer change Larger the surface area, faster the molecules travel permeability of membrane depends on the lipid solubility of the diffusing substance, the size and shape of diffusing molecules, temperature, membrane thickness Facilitated Diffusion: Passive Transport Through Membrane Proteins mediated transport: substances that crosses membranes by the way of transport membranes carrier: a transmembrane protein that binds molecules on one side of a membrane and transports them to the other side by means of a conformational change, or a change in shape. + posses one or more binding sites that are usually specific for molecules of certain substances or classes of substances + to be transported by a carrier, a molecule must first enter a binding site. Then it undergoes a conformational change that exposes the binding site to the fluid on the other side of the membrane molecule is free to dissociate from the carrier and be released into the fluid. + two factors affect the binding of a solute to a carrier: the affinity of the binding site on the carrier and the concentration gradient of the solute across the membrane Factors affecting the rate of Facilitated Diffusion: 1. the transport rate of individual carriers 2. the number of carriers in the membrane 3. the magnitude of the concentration gradient of the transported substance Diffusion Through Channels: channel: a transmembrane protein that transports molecules via a passageway/pore that extends from one side of the membrane to the other + specific for certain substances/classes of substances types: water channels (aquaporins) and ion channels Diffusion of Water Through Aquaporins: - aquaporins:highly selective pores that permit water, but no solutes, to move across the membrane by diffusion Active Transport: requires energy if the direction of the net flux is down an electrochemical gradient, the transport is passive if the direction of the net flux is up an electrochemical gradient, the transport is active two basic forms of active transport primary and secondary active transport differ in the nature of the energy source expended primary active transport uses ATP or some other chemical energy source directly to transport substances. Secondary active transport: powered by a concentration gradient that was previously created by primary active transport pumps: transport proteins that carry out active transport, similar to carriers in many respects but possess an ability that carriers do not have pumps can harness energy to drive the transport of molecules in a preferred direction across a membrane. Steady state: no further changes in concentration to maintain this state requires energy) Primary Active Transport: membrane proteins that perform primary active transport function both as transport proteins and as enzymes + proteins harness energy from ATP by catalyzing ATP hydrolysis proteins referred as ATPases.
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