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

• Selectively Permeable– some materials can pass

readily, others cannot

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:1. Permeability of the membrane2. 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

– 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

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

Gas Diffusion in Cells

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

NOTE: Electrical equilibrium may require movement against the concentration gradient

Membrane impermeable to (-)

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

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

Osmosis

• For osmosis to occur:1. the membrane must be permeable to water and

impermeable to at least one of the solutes in the solution

2. 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

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

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

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

• Ca2+ binds to protein

• ATP breakdown causes protein to change shape AND affinity for Ca2+

• Ca+ ejected on opposite side of the membrane

Example: Na+/ K+ pump

• Pumps Na+ out and K+ in– 3 Na+ out per 2 K+ in

• Generates concentration gradients

• Generates electrical gradient

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

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)

Vesicular Transport• 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 movement1. exocytosis - movement of material out of the

cell• hormones, neurotransmitters, etc.

2. endocytosis - movement of material into the cell• cellular debris, bacteria, etc.