cell membrane transport
CELL MEMBRANE TRANSPORT Factors Affecting the Direction of Transport PASSIVE TRANSPORT VS. ACTIVE TRANSPORT Active transport: transport across a membrane requiring energy; Passive transport: without the use of energy. Simple diffusion: passive transport, uses thermal motion to move molecules into or out of cells; Pumps: transport proteins used in active transport. Spontaneous (passive; down the concentration gradient) movement from high to low energy; the opposite requires an input of energy. Energy increases as solute concentration increases. DRIVING FORCES ACTING ON MOLECULES Any differences in energy existing across a membrane acts as a driving force that tends to push molecules in one direction or another (higher to lower energy). CHEMICAL DRIVING FORCES o When a substance is present in different concentrations on either side of a membrane, a concentration gradient exists. The rate at which concentration changes with distance. o Refer to concentration gradient as a chemical driving force, the direction is down the concentration gradient. Magnitude increases when concentration increases. ELECTRICAL DRIVING FORCES o Electrical driving forces in addition to chemical driving forces influence ions. o They arise due to the membrane potential, a difference in electrical potential or voltage that exists across the membrane. MEMBRANE POTENTIAL o In intra. or extra. fluid, cations and anions are unequal, therefore not electrically neutral. o Intra. fluids have more anions than cations (negative charge). Extra. fluids contain more cations than anions (positive charge). Separation of charge across the membrane. o The greater the difference in charge between the two sides of a membrane, the larger the membrane potential. o By convention sign of membrane potential is taken to be the sign of the net charge inside the cell relative to outside. HOW THE MEMBRANE POTENTIAL CREATES AN ELECTRICAL DRIVING FORCE THAT ACTS ON IONS o Electrical force has the potential to cause charged particles to move. FACTORS AFFECTING THE DIRECTION AND MAGNITUDE OF THE ELECTRICAL DRIVING FORCE o The separation of charge is a potential energy for current to flow across the membrane. o The direction of the electrical driving force on a positively charged ion is inward. On a negatively charged ion is outward. o The magnitude of the electrical driving force on an ion depends on the size of the membrane potential and the quantity of the charge carried by the ion. ELECTROCHEMICAL DRIVING FORCES o The direction of the electrochemical driving force acting on an ion depends on the net direction of the electrical chemical driving forces. o 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. Rate of Transport
The rate at which a substance is transported across a membrane refers to the number of molecules that cross the membrane in a given length of time (flux). The number of molecules crossing the membrane per unit time in just one direction is called the one-way flux (unidirectional flux). When concentrations do not change, the system is diffusional equilibrium.
CELL MEMBRANE TRANSPORT Passive Transport SIMPLE DIFFUSION: PASSIVE TRANSPORT THROUGH THE LIPID BILAYER THE BASIS FOR SIMPLE DIFFUSION o Diffusion: movement of molecules from one location to another by their thermal motion. o Thermal motion is called random thermal motion because individual molecules move in various directions because of collisions with other molecules. o Individual molecules move randomly, population of molecules move down its concentration gradient. THE MAGNITUDE OF THE DRIVING FORCE o In simple diffusion, the net flux of a substance is directly proportional to the size of the concentration gradient. MEMBRANE SURFACE AREA o The rate at which molecules are transported across a membrane varies in direct proportion to the membrane’s surface area. MEMBRANE PERMEABILITY o Depends on the nature of the transported substance and the various properties of the membrane that influence the ease with which molecules are able to penetrate it. o Lipid solubility of the diffusing substance: hydrophobic substances are the most lipid soluble; they enter the bilayer more readily. o The size and shape of diffusing molecules: larger molecules and those with more irregular shapes move through the bilayer more slowly. o Temperature: molecules move faster at higher temperatures; relatively constant. o Membrane thickness: a thicker membrane has a lower permeability. FACILITATED DIFFUSION: PASSIVE TRANSPORT THROUGH MEMBRANE PROTEINS Mediated transport: molecules passively transported through proteins; facilitated diffusion. CARRIERS IN FACILITATED DIFFUSION o Carrier: transmembrane protein that binds molecules on one side of a membrane and transport them to the other side by conformational change; have binding site(s) that are specific to molecules. o The net flux of facilitated diffusion depends on the frequency of solute binding to the carrier molecule (down the concentration gradient); net transport occurs from the side with greater frequency of binding. o Two factors that affect the binding of a solute to a carrier: affinity of binding site on the carrier, and concentration (electrochemical) gradient. Affinity is the same in intra. and extra fluid. Difference in binding is due to concentration gradient. FACTORS AFFECTING THE RATE OF FACILITATED DIFFUSION o The rate is determined by: 1. Transport rate of individual carriers 2. Number of carriers in the membrane 3. Magnitude of the concentration (electrochemical) gradient. o At a given moment in time, there is a fixed number of carriers, each with its binding site(s). o An increase in carriers will increase the likelihood that solute will bind to carrier and be transported. DIFFUSION THROUGH CHANNELS Channel: transmembrane protein that transports molecules via a passageway or pore that extends from one side of the membrane to the other. DIFFUSION OF WATER THROUGH AQUAPORINS o Aquaporins: highly selective pores that permit water, but no solutes, to move across the membrane by diffusion. DIFFUSION THROUGH ION CHANNELS o Some ion channels have pores to act as water-filled paths where ions diffuse. Have one or more binding sites where ions jump from one site to the other.
CELL MEMBRANE TRANSPORT Difference between a channel with binding sites and a carrier is that a channel’s binding sites are accessible from both sides of the membrane at the same time. FACTORS AFFECTING THE RATE OF TRANSPORT THROUGH ION CHANNELS o Rate of ion movement depends on the number of open channels and the transport rate. o For pores, ion movement acts as simple diffusion except that ions move down an electrochemical gradient o For channels with binding sites, transport is slower, and saturated (facilitated diffusion). o
Active Transport
Transport of a substance up an electrochemical gradient requires an input of energy because molecules are moving against the electrochemical force that is pushing them. If direction of net flux is down an electrochemical gradient, passive. If direction is up, active. Primary active transport: uses ATP or other chemical energy source directly to transport substances. Secondary active transport: powered by a concentration gradient or an electrochemical gradient that was generated by a primary active transport. Pumps: can harness energy to transport molecules in a preferred direction across a membrane. The affinity of pumps is greater when the binding site is exposed to one side of the membrane. Pumps can also get saturated.
PRIMARY ACTIVE TRANSPORT Most primary active transport proteins are enzymes, they harness energy from ATP by catalyzing ATP hydrolysis (ATPases). The Na+/K+ pump transports 3 Na+ outside of the cell and K+ ions inside. Both move up their electrochemical gradients. Each cycle 1 ATP is hydrolyzed to provide energy. Intracellular fluid is rich in K+ but poor in Na+ relative to the extracellular fluid. SECONDARY ACTIVE TRANSPORT A transport protein couples the flow of on substance to that of another. One substance moves passively down its electrochemical gradient, in the process releasing energy that is then used to drive the movement of the other substance up its electrochemical gradient. Cotransport: transport of two substances in the same direction. Countertransport: transport of two substances in opposite directions. FACTORS AFFECTING RATES OF ACTIVE TRANSPORT Two factors to determine the rate at which molecules are actively transported are: rate of transport by individual pumps, and the number of pumps. PUMPS AND LEAKS: HOW ACTIVE AND PASSIVE TRANSPORT MECHANISMS COEXIST IN CELLS The composition of intra. fluid remains fairly steady because substances are simultaneously transported passively across the membrane in the opposite direction but at the same rate. The net flux across the membrane (passive + active) is zero. Osmosis: Passive Transport of Water Across Membranes
Water flow across membranes is always passive, is unaffected by membrane potentials, and is always driven by its own concentration gradient. Osmosis: the flow of water across a membrane down its concentration gradient. Water molecules flow passively into the cell down their concentration gradient, which is also in an area of higher solute concentration. The presence of solutes inside the cell reduces the water concentration inside by taking up a certain amount of space.
CELL MEMBRANE TRANSPORT
In osmosis, water movement occurs from a low concentration of solute to a high concentration of solute.
OSMOLARITY Osmolarity: the total solute particle concentration of a solution. Isomotic: two solutions having the same osmolarity. Same solute concentration and same water concentration. Hyperosmotic: a solution whose osmolarity is higher than another. Water concentration is lower than solute concentration. Hypo-osmotic: a solution with lower osmolarity. Water concentration is higher and the solute concentration is lower. OSMOTIC PRESSURE Osmotic pressure: an indirect measure of its solute concentration and is expressed in ordinary units of pressure. As the total solute concentration (osmolarity) increases, the osmotic pressure increases. When water moves by osmosis from low solute to high solute concentration, water is flowing up an osmotic pressure gradient. Hen water moves up a gradient of osmotic pressure, it is merely moving down its own concentration gradient. TONICITY Tonicity: is a function of the concentration of nonpermeating solutes outside a cell relative to the concentration inside the cell, and it determines the behavior of a cell placed in the solution. Isotonic: when a solution does not alter cell volume (neither shrinks now swells). Hypertonic: a solution that causes cells to shrink. Hypotonic: a solution that causes cells to swell. A solution is isotonic is it contains impermeant solutes at a concentration of 300mOsm, the normal concentration of impermeant solutes in intracellular fluid. If the concentration of impermeant solutes is greater or less than 300mOsm, the solution will be hypertonic or hypotonic, respectively. A solution’s tonicity is not affected by the concentration of any permeant solutes that may or may not be present. The degree to which it swells or shrinks is determined by the initial concentrations of impermeant solutes in intra. and extra. fluid.
The rate at which a substance is transported across a membrane refers to the number of molecules that cross the membrane in a given length of time (flux). The number of molecules crossing the membrane per unit time in just one direction is called the one-way flux (unidirectional flux). When concentrations do not change, the system is diffusional equilibrium.
CELL MEMBRANE TRANSPORT Passive Transport SIMPLE DIFFUSION: PASSIVE TRANSPORT THROUGH THE LIPID BILAYER THE BASIS FOR SIMPLE DIFFUSION o Diffusion: movement of molecules from one location to another by their thermal motion. o Thermal motion is called random thermal motion because individual molecules move in various directions because of collisions with other molecules. o Individual molecules move randomly, population of molecules move down its concentration gradient. THE MAGNITUDE OF THE DRIVING FORCE o In simple diffusion, the net flux of a substance is directly proportional to the size of the concentration gradient. MEMBRANE SURFACE AREA o The rate at which molecules are transported across a membrane varies in direct proportion to the membrane’s surface area. MEMBRANE PERMEABILITY o Depends on the nature of the transported substance and the various properties of the membrane that influence the ease with which molecules are able to penetrate it. o Lipid solubility of the diffusing substance: hydrophobic substances are the most lipid soluble; they enter the bilayer more readily. o The size and shape of diffusing molecules: larger molecules and those with more irregular shapes move through the bilayer more slowly. o Temperature: molecules move faster at higher temperatures; relatively constant. o Membrane thickness: a thicker membrane has a lower permeability. FACILITATED DIFFUSION: PASSIVE TRANSPORT THROUGH MEMBRANE PROTEINS Mediated transport: molecules passively transported through proteins; facilitated diffusion. CARRIERS IN FACILITATED DIFFUSION o Carrier: transmembrane protein that binds molecules on one side of a membrane and transport them to the other side by conformational change; have binding site(s) that are specific to molecules. o The net flux of facilitated diffusion depends on the frequency of solute binding to the carrier molecule (down the concentration gradient); net transport occurs from the side with greater frequency of binding. o Two factors that affect the binding of a solute to a carrier: affinity of binding site on the carrier, and concentration (electrochemical) gradient. Affinity is the same in intra. and extra fluid. Difference in binding is due to concentration gradient. FACTORS AFFECTING THE RATE OF FACILITATED DIFFUSION o The rate is determined by: 1. Transport rate of individual carriers 2. Number of carriers in the membrane 3. Magnitude of the concentration (electrochemical) gradient. o At a given moment in time, there is a fixed number of carriers, each with its binding site(s). o An increase in carriers will increase the likelihood that solute will bind to carrier and be transported. DIFFUSION THROUGH CHANNELS Channel: transmembrane protein that transports molecules via a passageway or pore that extends from one side of the membrane to the other. DIFFUSION OF WATER THROUGH AQUAPORINS o Aquaporins: highly selective pores that permit water, but no solutes, to move across the membrane by diffusion. DIFFUSION THROUGH ION CHANNELS o Some ion channels have pores to act as water-filled paths where ions diffuse. Have one or more binding sites where ions jump from one site to the other.
CELL MEMBRANE TRANSPORT Difference between a channel with binding sites and a carrier is that a channel’s binding sites are accessible from both sides of the membrane at the same time. FACTORS AFFECTING THE RATE OF TRANSPORT THROUGH ION CHANNELS o Rate of ion movement depends on the number of open channels and the transport rate. o For pores, ion movement acts as simple diffusion except that ions move down an electrochemical gradient o For channels with binding sites, transport is slower, and saturated (facilitated diffusion). o
Active Transport
Transport of a substance up an electrochemical gradient requires an input of energy because molecules are moving against the electrochemical force that is pushing them. If direction of net flux is down an electrochemical gradient, passive. If direction is up, active. Primary active transport: uses ATP or other chemical energy source directly to transport substances. Secondary active transport: powered by a concentration gradient or an electrochemical gradient that was generated by a primary active transport. Pumps: can harness energy to transport molecules in a preferred direction across a membrane. The affinity of pumps is greater when the binding site is exposed to one side of the membrane. Pumps can also get saturated.
PRIMARY ACTIVE TRANSPORT Most primary active transport proteins are enzymes, they harness energy from ATP by catalyzing ATP hydrolysis (ATPases). The Na+/K+ pump transports 3 Na+ outside of the cell and K+ ions inside. Both move up their electrochemical gradients. Each cycle 1 ATP is hydrolyzed to provide energy. Intracellular fluid is rich in K+ but poor in Na+ relative to the extracellular fluid. SECONDARY ACTIVE TRANSPORT A transport protein couples the flow of on substance to that of another. One substance moves passively down its electrochemical gradient, in the process releasing energy that is then used to drive the movement of the other substance up its electrochemical gradient. Cotransport: transport of two substances in the same direction. Countertransport: transport of two substances in opposite directions. FACTORS AFFECTING RATES OF ACTIVE TRANSPORT Two factors to determine the rate at which molecules are actively transported are: rate of transport by individual pumps, and the number of pumps. PUMPS AND LEAKS: HOW ACTIVE AND PASSIVE TRANSPORT MECHANISMS COEXIST IN CELLS The composition of intra. fluid remains fairly steady because substances are simultaneously transported passively across the membrane in the opposite direction but at the same rate. The net flux across the membrane (passive + active) is zero. Osmosis: Passive Transport of Water Across Membranes
Water flow across membranes is always passive, is unaffected by membrane potentials, and is always driven by its own concentration gradient. Osmosis: the flow of water across a membrane down its concentration gradient. Water molecules flow passively into the cell down their concentration gradient, which is also in an area of higher solute concentration. The presence of solutes inside the cell reduces the water concentration inside by taking up a certain amount of space.
CELL MEMBRANE TRANSPORT
In osmosis, water movement occurs from a low concentration of solute to a high concentration of solute.
OSMOLARITY Osmolarity: the total solute particle concentration of a solution. Isomotic: two solutions having the same osmolarity. Same solute concentration and same water concentration. Hyperosmotic: a solution whose osmolarity is higher than another. Water concentration is lower than solute concentration. Hypo-osmotic: a solution with lower osmolarity. Water concentration is higher and the solute concentration is lower. OSMOTIC PRESSURE Osmotic pressure: an indirect measure of its solute concentration and is expressed in ordinary units of pressure. As the total solute concentration (osmolarity) increases, the osmotic pressure increases. When water moves by osmosis from low solute to high solute concentration, water is flowing up an osmotic pressure gradient. Hen water moves up a gradient of osmotic pressure, it is merely moving down its own concentration gradient. TONICITY Tonicity: is a function of the concentration of nonpermeating solutes outside a cell relative to the concentration inside the cell, and it determines the behavior of a cell placed in the solution. Isotonic: when a solution does not alter cell volume (neither shrinks now swells). Hypertonic: a solution that causes cells to shrink. Hypotonic: a solution that causes cells to swell. A solution is isotonic is it contains impermeant solutes at a concentration of 300mOsm, the normal concentration of impermeant solutes in intracellular fluid. If the concentration of impermeant solutes is greater or less than 300mOsm, the solution will be hypertonic or hypotonic, respectively. A solution’s tonicity is not affected by the concentration of any permeant solutes that may or may not be present. The degree to which it swells or shrinks is determined by the initial concentrations of impermeant solutes in intra. and extra. fluid.
