Lecture 5
Cell Membrane Transport
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Overcoming the Cell Barrier
 The cell membrane is a barrier, but:
 Nutrients must get in
 Products and wastes must get out
 Permeability determines what moves in and out of a
cell
 A membrane is:
 Impermeable if it lets nothing in or out
 Freely permeable if it lets anything pass
 Selectively permeable if it restricts movement
 Cell membranes are selectively permeable:
 Allow some materials to move freely
 Restrict other materials
PLAY
Membrane Transport: Fat- and Water-Soluble Molecules
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Restricted Materials
 Selective permeability restricts materials based on:
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Size
Electrical charge
Molecular shape
Lipid solubility
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Diffusion in Solutions
 All molecules are constantly in
motion
 Molecules in solution move
randomly
 Random motion causes mixing
 Concentration is the amount of
solute in a solvent
 Concentration gradient:
 More solute in one part of a solvent
than another
 Solutes move down a concentration
gradient:
 Molecules mix randomly
 Solute spreads through solvent
 Eliminates concentration gradient
PLAY
Membrane Transport: Diffusion
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Factors Affecting Diffusion Rates
 Distance the particle has to move
 Molecule size:
 Smaller is faster
 Temperature:
 More heat, faster motion
 Gradient size:
 The difference between high and low concentration
 Electrical forces:
 Opposites attract, like charges repel
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Osmosis
 Osmosis is the movement of water across the cell membrane
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
•
Osmotic Pressure
is the force of a
concentration
gradient of water
•
Equals the force
(hydrostatic
pressure) needed
to block osmosis
Diffusion vs. Osmosis
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Effects of Osmosis on Cells
Tonicity – how a solution’s osmolarity affects cell volume
 Isotonic – solutions with the same solute concentration as that of the
cytosol
 Hypertonic – solutions having greater solute concentration than that of
the cytosol; water leaves the cell causing crenation (shrinkage)
 Hypotonic – solutions having lesser solute concentration than that of
the cytosol; water enters the cell causing swelling and potential lysis
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Hydrostatic and Osmotic Pressure
 Hydrostatic pressure = water
pressure
 Filtration is the passage of
water and solutes through a
membrane by hydrostatic
pressure
 Pressure gradient pushes
solute-containing fluid from a
higher-pressure area to a
lower-pressure area
 Osmotic pressure can create
an important counter force
against hydrostatic pressure
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
KEY CONCEPT
 Concentration gradients tend to even out
 In the absence of a membrane, diffusion eliminates
concentration gradients
 When different solute concentrations exist on either side of
a selectively permeable membrane, osmosis moves water
through the membrane to equalize the concentration
gradients
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Transport Through Cell Membranes
 Transport through a cell membrane can be:
 Active (requiring energy and ATP)
 Passive (no energy required)
 3 categories of transport
 Diffusion (passive)
 Carrier-mediated transport (passive or active)
 Vesicular transport (active)
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Diffusion and the Cell Membrane
 Diffusion can be simple, channel, or carrier mediated
 Channel & carrier mediated diffusion is:
 Specific: to size, charge, & interaction with the channel
 Subject to saturation: making the channels rate limiting
PLAY
Membrane Transport: Facilitated Diffusion
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Active Transport
 Active transport proteins:

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Move substrates against concentration gradient
Require energy, such as ATP
Ion pumps move ions (Na+, K+, Ca+, Mg2+)
Na+-K+ Exchange Pump moves both of these ions at the
same time, each in the opposite direction (called antiport
or countertransport)
 Proton Pump uses photosynthesis or food energy to
create a proton concentration gradient that then is used
to manufacture ATP
PLAY
Active Transport
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Sodium-Potassium Exchange Pump
 Active transport, carrier mediated:
 1 ATP moves 3 Na+ out 2 K+ in
 This creates an electrical potential across the membrane
 Called the Transmembrane Potential
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Transmembrane Potential
 Voltage across a membrane
 Resting membrane potential – the
point where K+ potential is
balanced by the membrane
potential
 Ranges from –20 to –200 mV
 Results from Na+ and K+
concentration gradients across the
membrane
 Differential permeability of the plasma
membrane to Na+ and K+
 Steady state potential is
maintained by active transport of
ions
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Proton Pump (in Mitochondrial Membranes)
 Expends metabolic energy to pump protons
across membranes
PLAY
Proton Pump
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Types of Active Transport
Primary active transport:
hydrolysis of ATP
phosphorylates the transport
protein causing
conformational change
Secondary active transport:
use of an exchange pump
(such as the Na+-K+ pump)
indirectly to drive the
transport of other solutes
Symport system – two
substances move across
a membrane in the same
direction (also called
cotransport)
Antiport system – two
substances move across
a membrane in opposite
directions (also called
countertransport)
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Vesicular Transport
 Also called bulk transport
 Transport of large particles and macromolecules across plasma
membranes
Directional Descriptive Terms
 Exocytosis – moves substance from the cell interior to the extracellular
space
 Endocytosis – enables large particles and macromolecules to enter the
cell
 Receptor-mediated
 Pinocytosis
 Phagocytosis
Functional Descriptive Terms
 Transcytosis – moving substances into, across, and then out of a cell
 Vesicular trafficking – moving substances from one area in the cell to
another
 Phagocytosis – pseudopods engulf solids and bring them into the cell’s
interior
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Receptor-Mediated Endocytosis
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Receptors (glycoproteins called clathrin) bind target molecules (ligands)
Coated vesicle (endosome) carries ligands and receptors into the cell
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Pinocytosis
 Pinocytosis (cell drinking)
 Endosomes “drink” extracellular fluid
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Phagocytosis
 Phagocytosis (cell eating)
 pseudopodia (psuedo = false, podia = feet)
 engulf large objects in phagosomes
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Exocytosis
 Is the reverse of endocytosis
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen
Summary
 The 7 methods of transport
Biology 11 Human Biology, TTh 8;00-9:20
Dr. Telleen