Membrane Transport Chapter 6

Cells Need to Exchange Materials with the Extracellular Fluid Take in nutrients O2 energy substrates building materials cofactors Dispose of wastes CO2 Urea

Cells Must Control Movements of Materials Need to maintain complexity inside the cell Must regulate type and amount of material entering and leaving the cell

Plasma Membrane Fig. 3.2 Selectively Permeable some materials can pass readily, others cannot Fig. 3.2

Membrane Permeability Size the smaller the particle, the more permeable small molecules (O2, CO2, H2O) can large molecules (protein, DNA) cannot Lipid Solubility YES: non-polar molecules (O2, cholesterol), NO: charged atoms/molecules (Na+, Cl-, HCO3-), large polar molecules (glucose)

Membrane Transport Requires: Passive Transport Active Transport Permeability of the membrane A driving force Passive Transport movement of particles along a gradient does not require energy expenditure Active Transport movement of particles against a gradient requires energy expenditure

Some Important Terms Solution Solute Solvent Concentration mixture of two(+) substances that is uniform at the molecular level Solute particles (molecules or ions) present in a solution Solvent phase (generally a liquid) in which particles are dissolved (H2O) Concentration amt. solute dissolved in a given volume of solution or solvent

Passive Membrane Transport Simple Diffusion movement of particles along a concentration gradient Osmosis diffusion of water across a semi-permeable membrane Facilitated Diffusion movement of particles along a concentration gradient through a carrier protein

Diffusion Molecules and ions in a solution are in a constant state of motion Tend to diffuse - become evenly dispersed throughout the solution Diffusion = movement of particles in a solution due to random thermal motion Fig. 6.2

Diffusion and Concentration Solute particles diffuse from regions of high concentration to regions of low concentration “Down” a concentration gradient (high  low) Continues until equilibrium is reached Fig. 6.2

Gas Diffusion in Cells Fig. 6.3

Diffusion and Ions Ions = charged particles Like charges repel, opposites attract Differences in charge between two areas = electrical gradient Ions move along an electrical gradient until charges are balanced

Diffusion and Ions

Diffusion and Ions Membrane impermeable to (-) NOTE: Electrical equilibrium may require movement against the concentration gradient

Electrochemical Gradient Net movement of ions due to the combined effects of the electrical gradient and the concentration gradient Equilibrium may be achieved across a membrane at a point of unequal concentrations and charges

Diffusion and Membrane Transport Lipid bilayer determines what substances can readily pass through the membrane if bilayer is permeable, substance can diffuse through if bilayer is impermeable, no diffusion even if gradient exists

Diffusion and Membrane Transport Substances to which the membrane is impermeable must pass via alternate means Facilitated Diffusion - movement across the cell membrane through a carrier protein Channel Proteins - allow flow of ions across the cell membrane Both allow regulation of flow Fig. 6.14 Fig. 6.4

Factors Affecting Rate of Diffusion magnitude of the gradient  gradient,  rate permeability of the membrane to the substance  permeability,  rate temperature of the solution  temperature,  rate the surface area of the membrane through which diffusion is taking place  SA,  rate

Osmosis Net diffusion of water across a semi-permeable membrane diffusion of the solvent, not the solute Figs. 6.5, 6.6

Osmosis For osmosis to occur: the membrane must be permeable to water and impermeable to at least one of the solutes in the solution there must be a difference in solute concentration between the two sides of the membrane

Osmotic Pressure Osmosis results in changes in volume on either side of the membrane Changes in volume could be stopped by applying an equal and opposite force would effectively stop osmosis Figs. 6.6, 6.7

Osmotic Pressure Osmotic pressure = amount of pressure that would have to be exerted in order to prevent osmosis measure of how strongly a solution “draws water into itself”  [solute] ,  osmotic pressure of the solution

Facilitated Diffusion Many molecules large and/or polar molecules are needed for metabolism cannot pass through lipid bilayer Shuttled across membrane by carrier proteins Facilitated diffusion – carrier-mediated transport along the conc. gradient no energy expended by the cell Fig. 6.14

Properties of Carrier Proteins in Facilitated Diffusion Specificity – transport only one or a few different substances possess special bind sites Saturation – limited rate of transport at high concentrations no further increase in transport rate will accompany increases in the conc. gradient Reversible - direction of movement across membrane is influenced by solute concentration If [Solute]out > [Solute]in mvmt is from out in If [Solute]in > [Solute]out mvmt is from in  out If [Solute]out = [Solute]in net diffusion = 0 Fig. 6.13

Active Membrane Transport Requires energy expenditure by the cell (use of ATP) Active Carrier Mediated Transport - use membrane proteins to move materials against a gradient Vesicular Transport - move large amounts of material into and out of the cell

Active Carrier-Mediated Transport A carrier-mediated transport system that moves a substance against its EC gradient across a cell membrane requires ATP usage pumps substances from low to high concentrations

Example: Ca2+pump Fig. 6.16 Ca2+ binds to protein ATP breakdown causes protein to change shape AND affinity for Ca2+ Ca+ ejected on opposite side of the membrane Fig. 6.16

Example: Na+/ K+ pump Fig. 6.17 Pumps Na+ out and K+ in 3 Na+ out per 2 K+ in Generates concentration gradients Generates electrical gradient Fig. 6.17

ACMT vs. Facilitated Diffusion Similarities Carrier Protein Mediated Exhibit Chemical Specificity Differences ACMT requires energy (ATP) Binding affinity of carrier changes in ACMT does not change for facilitated diffusion - gradient determines net movement

Types of Active Carrier-Mediated Transport Primary Active Transport hydrolysis (breakdown) of ATP directly required for the function of the carrier e.g. Ca2+ pump, Na+/K+ pump Figs. 6.16 & 6.17

Types of Active Carrier-Mediated Transport Secondary Active Transport (Coupled Transport) energy needed for movement of a substance against gradient is provided by the movement of another substance along its gradient Example: Na+-glucose cotransport indirectly requires ATP via Na+/K+ pump (establishes gradient) Figs. 6.18 & 6.19

Vesicular Transport Fig. 6.20 Transport of vesicle contents across cell membranes “bulk transport” - move large amounts of material very large molecules can be moved this way Two types of movement exocytosis - movement of material out of the cell hormones, neurotransmitters, etc. endocytosis - movement of material into the cell cellular debris, bacteria, etc. Fig. 6.20