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Chapter 4 Cell Membrane Transport Chapter 4 Cell Membrane Transport • Plasma Membrane Review o Functions . Separate ICF / ECF . Allow exchange of materials between ICF / ECF such as obtaining O2 and nutrients and getting rid of waste products • Property of a Membrane o _________ permeable or semipermeable. o Membrane permeability is affected by . The lipids ____________of the diffusing substance . The size and shape of the diffusing substance . _____________ . Membrane __________ o Permeability of a membrane . ___________: non-polar and small polar molecules such as O2, CO2, fatty acids, water are easily transported . Not permeable: large polar molecules and ions such as glucose, proteins and Na+ 4.1 Factors Affecting the Direction of Transport • Depending on the requirement of energy o Passive transport is spontaneous; ________ cell energy is required. The direction of electrochemical force is downhill movement. o Active transport requires the consumption of energy o Table 4.2 Characteristics of Transport Processes • Driving Forces Acting on Molecules: Chemical, Electrical and Electrochemical o Chemical Driving Force Alone (Figure 4.1) . Direction of chemical driving force: ___________the chemical gradient (∆C) . A molecule travels along its own ____ o Electrical Driving Force Alone (Figure 4.3) . Due to the uneven distribution of ion across the cell membrane, a cell has a difference in electrical potential or voltage called _________ _________ . Membrane potential = potential energy o Electrochemical Force . Electrochemical force= the combination of both chemical and electrical forces . At equilibrium potential (E), the electrochemical force=_______. o Electrochemical Force on Potassium Movement (Figure 4.5) + . When the Vm = Ek, electrochemical force= zero, _______net K movement. + . When Vm < Ek , the electrochemical force is outward, K moves__________. 1 ∣ ∣ ∣ ∣ + . When Vm > EK , , the electrochemical force is ________, K moves inward. 4.2 Rate of Transport∣ ∣ ∣ ∣ • Flux is the number of molecules traveling through the membrane per unit time per unit surface area (mole/s/area) • Net Flux is the overall movement before achieving equilibrium. The net flux in (Figure 4.6b) goes from side ___ to side ___. 4.3 Passive Transport • Simple diffusion • Facilitated diffusion • Diffusion through channels • Osmosis • Simple diffusion is the passive transport of molecules through the membrane without the help of transport protein. Examples: fatty acids, O2 and CO2. o Simple diffusion rate is directly proportional to lipid solubility (membrane permeability), the magnitude of the driving force, and membrane surface area. It is inversely proportional to membrane thickness, the size and shape of the diffusing molecule • Facilitated diffusion (Figure 4.10) is the passive transport through a carrier o Characteristics of a carrier . Has binding sites for specific particles . Binding occurs one side at a time . Random conformational changes . Examples: organic molecules use carriers o Factors Affecting the Rate of Facilitated Diffusion . Rate of transport of each carrier . Number of carriers in membrane Carriers demonstrate saturation Cells can regulate the transport rate by modify number of carrier in membrane . The magnitude of concentration (or electrochemical) gradient • Diffusion through Channels (Figure 4.12) o Characteristics of a channel . Functions like a passageway or pore . Substance specific . Accessible from only one side o Types of Channels . Aquaporin: 13 classes of water transporter . Ion Channels: Examples: inorganic ions (Na+, K+, Cl-, Ca 2+) Leak channels Gated channels o Factors Affecting the Rate of Transport through Ion Channels: Similar to facilitated diffusion 4.5 Osmosis: Passive transport of water across membranes (Figure 4.17) • Osmosis: Water moves from high concentration to low concentration across the plasma membrane. • Molarity & Molality o 1 molar solution (1.0M) = 1mole of solute per liter solution o 1 molal solution (1.0m) = 1 mole of solute per kg H2O . May be more than one liter solution 2 o Osmolarity (Osm) is total concentration of permeant and impermeant solute particles of a solution . Normal” osmolarity for most body fluids = 300 mOsm (milliosmoles) • Osmotic Pressure (π) is the force that would have to be exerted to stop osmosis. o It is proportional to solute concentration. Water moves from lower π (low opposing force) to higher π. • Tonicity (Table 4.3) o Reflects the concentration of impermeant solutes in ECF relative to its concentration in ICF o Describes the effect of a solution on water movement of RBC . When a red blood cell is placed in an isotonic solution, it remains the same shape. When a RBC is place in a hypertonic solution, it _________________. When a RBC is placed in a hypotonic solution. It _________________. How come a person should not drink ocean water? _____________________________________ • Water toxicity also called water poisoning or dilutional hyponatremia caused by over-hydration will make brain cells swell and cause electrolytes imbalance. o It is a potentially fatal disturbance. o http://abcnews.go.com/GMA/jury-rules-radio-station-jennifer-strange-water-drinking/story?id=8970712 4.4 Active Transport • Features: o Non-spontaneous, requires energy o Involves a pump (membrane protein) o The net flux movement is uphill and against electrochemical gradient. • Characteristics of a Pump. A pump o Is a type of membrane protein o Functions as a transporter and an enzyme o Can harness energy to transport the molecules to a preferred direction o Has specific binding sites o Demonstrates saturation • Types of Active Transport o Primary Active Transport . Pump is both a transporter and an enzyme. Since energy is usually obtained from ATP hydrolysis, most of them are called ATPases. For example sodium-potassium pump (Figure 4.14) also called the Na+/K+ ATPase). How many sodium ions are pumped out? ______; how many potassium ions are pumped in _____? o Secondary Active Transport (Figure 4.15) . Energy released from electrochemical gradient of another solute drives a pump . Cotransport (symport): The transport of two substances in the same direction. For example, diffusion of Na+ provides energy to actively transports glucose . Counter transport (antiport): The transport of two substances in opposite direction. For example, diffusion of Na+ provides energy to actively transport H+ • Active and Passive Transport Coexist in Cells (Figure 4.16) 3 4.6 Transport of Material via Membrane-Bound Compartments • Bulk transport of macromolecules o Consume energy o Uses membrane compartments o Endocytosis o Exocytosis • Endocytosis (Figure 4.21) o Molecules enter the cell through the formation of vesicles (endosomes) o Phagocytosis: Cell eating; cell uses amoeboid movement to engulf particles. Example WBC o Pinocytosis: Cell - drinking o Receptor-mediated transport (Figure 4.21) . Specific transport that requires the recognition of the molecule and receptor. Familial hypercholesterolemia, FH (type II) patients have defect membrane surface receptors that help to remove LDLs from circulation • Exocytosis (Figure 4.22) is the reverse of endocytosis. o Functions of Exocytosis . To add components to the plasma membrane. To replace the membrane that is lost during endocytosis . To secrete molecules 4.7 Epithelial Transport: Movement of Molecules across Two Membranes • Apical membrane is lumen facing • Basolateral membrane is blood facing • Tight junctions limit materials transport • Epithelium: Absorption of Sodium (Figure 4.24) + o Apical membrane: Na channel (passive) o Basolateral membrane . Na+ / K+ pump (active) . K+ Channel o Overall transport of sodium is passive or active? • Epithelium: Absorption of Glucose and Sodium Ions (Figure 4.24) o Apical membrane: Sodium-linked cotransport system o Basolateral membrane . Na+ / K+ pump . K+ channel . Glucose carrier o Overall transport of glucose is passive or active. • Cystic Fibrosis (Clinic Connections) o Defected chloride transporter in membrane causes the inability of water secretion o Thick respiratory mucus clogs airway and promotes bacteria growth • Epithelia water transport (Figure 2.25) is done by actively pump solutes to create an osmotic gradient, osmosis follows. • Transcytosis (Figure 4.26) is the transport of macromolecules across epithelial cells which involve both endocytosis and exocytosis 4 .
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