Cells Need to Exchange
Materials with
the Extracellular Fluid
Membrane Transport
• Take in nutrients
Chapter 6
–
–
–
–
O2
energy substrates
building materials
cofactors
• Need to maintain complexity inside the cell
• Selectively Permeable
– some materials can pass
readily, others cannot
• Must regulate type and amount of material
entering and leaving the cell
Membrane Permeability
– the smaller the particle, the more permeable
– small molecules (O2, CO2, H2O) can
– large molecules (protein, DNA) cannot
Membrane Transport
•
Requires:
1. Permeability of the membrane
2. A driving force
•
Passive Transport
–
–
• Lipid Solubility
– YES: non-polar molecules (O2, cholesterol),
– NO: charged atoms/molecules (Na+, Cl-, HCO3-), large
polar molecules (glucose)
– CO2
– Urea
Plasma Membrane
Cells Must Control Movements
of Materials
• Size
• Dispose of wastes
•
movement of particles along a gradient
does not require energy expenditure
Active Transport
–
–
movement of particles against a gradient
requires energy expenditure
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Some Important Terms
• Solution
– mixture of two(+) substances that is uniform at the molecular level
Passive Membrane Transport
• Simple Diffusion
– movement of particles along a concentration gradient
• Solute
– particles (molecules or ions) present in a solution
• Solvent
• Osmosis
– diffusion of water across a semi-permeable membrane
– phase (generally a liquid) in which particles are dissolved (H2O)
• Facilitated Diffusion
• Concentration
– amt. solute dissolved in a given volume of solution or solvent
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
Gas Diffusion in Cells
– movement of particles along a concentration gradient
through a carrier protein
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
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
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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
• 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
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
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
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Osmosis
• Net diffusion of water across a semi-permeable
membrane
– diffusion of the solvent, not the solute
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
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
• 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
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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
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
ACMT vs. Facilitated Diffusion
• Similarities
– Carrier Protein Mediated
– Exhibit Chemical Specificity
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: Na+/ K+ pump
• Pumps Na+ out and K+ in
– 3 Na+ out per 2 K+ in
• Generates concentration
gradients
• Generates electrical gradient
Types of Active CarrierMediated Transport
• Primary Active Transport
– hydrolysis (breakdown) of ATP directly required for
the function of the carrier
– e.g. Ca2+ pump, Na+/K+ pump
• Differences
– ACMT requires energy (ATP)
– Binding affinity of carrier changes in ACMT
• does not change for facilitated diffusion - gradient
determines net movement
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Types of Active CarrierMediated Transport
Vesicular 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)
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
1. exocytosis - movement of material out of the cell
•
hormones, neurotransmitters, etc.
2. endocytosis - movement of material into the cell
•
cellular debris, bacteria, etc.
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