Chapter 04
Lecture Outline*
Movement Across Cell
Membranes
Eric P. Widmaier
Boston University
Hershel Raff
Medical College of Wisconsin
Kevin T. Strang
University of Wisconsin - Madison
*See PowerPoint Image Slides for all
figures and tables pre-inserted into
PowerPoint without notes.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Diffusion
• The movement of molecules from one location
to another as a result of their random thermal
motion.
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Diffusion Equilibrium
Fig. 4-1
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Diffusion Equilibrium
Fig. 4-2
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Magnitude and Direction of Diffusion
Fig. 4-3
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Diffusion Rate vs. Distance
• Diffusion times increase in proportion to the
square of the distance over which the
molecules diffuse.
• Diffusion is limited by distance. If a substance
has to diffuse a long distance it is very slow
and not an effective way to move solutes.
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Diffusion through Membranes
• The rates at which molecules diffuse across
membranes, as measured by their permeability
coefficients, are a thousand to a million times slower
than the diffusion rates of the same molecules
through a water layer of equal thickness.
• Membranes act as barriers that considerably slow the
diffusion of molecules across their surfaces.
• The major factor limiting diffusion across a
membrane is the hydrophobic interior of its lipid
bilayer.
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Diffusion through Membranes
Fig. 4-4
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Diffusion through Membranes
• Oxygen, carbon dioxide, fatty acids, and steroid
hormones are examples of nonpolar molecules that
diffuse rapidly through the lipid portions of
membranes.
• Remember that lipophilic (lipid-loving) substances
move through easily.
• Polar molecules and hydrophilic (water-loving) do
not diffuse readily through the membranes.
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Diffusion through Ion Channels
• Ions such as Na+, K+, Cl–, and Ca2+ all use specific protein channels
to diffuse into and out of cells.
• Channels are integral membrane proteins that span the lipid bilayer.
• A single protein may have a conformation that looks like a
doughnut, with the hole in the middle providing the channel for ion
movement.
• More often, several proteins aggregate, each forming a subunit of
the walls of a channel.
• Specificity is determined by pore size of the channel, charge, and
binding sites.
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Diffusion
through
Ion
Channels
Fig. 4-5
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Membrane Potential
• Membrane potential is a separation of
electrical charges that exists across plasma
membranes.
• The membrane potential provides an electrical
force that influences the movement of ions
across the membrane.
• Remember that like charges repel and
opposites attract.
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Fig. 4-6
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Regulation of Diffusion through Ion Channels
• Channels are regulated to control the
movement of ions into and out of a cell.
• Types of Gated channels are:
– Ligand gated
– Voltage gated
– Mechanically gated
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Mediated-Transport Systems
• Many molecules (like glucose) are either too
large and charged to get into the cell without
help.
• The protein transporters (also called carriers)
bring these molecules into and out of cells by
conformation changes.
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Mediated-Transport Systems
Fig. 4-8
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Transporters
• Transporters are specific for their ligand.
• Transporters do not move as many molecules
as channels do because of binding and
conformational shifts.
• Transporters can be saturated. This means that
there is a maximum flux of molecules that can
be reached.
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Facilitated diffusion
Fig. 4-9
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Active Transport
• Active transport uses energy to move molecules against the
concentration gradient.
• These transporters are often called “pumps”.
• These pumps can also be saturated and use two types of
energy sources:
(1) The direct use of ATP in primary active transport
(2) The use of an electrochemical gradient across a membrane to
drive the process in secondary active transport
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Active Transport
Fig. 4-10
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Na+/K+ ATPase
Fig. 4-11
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Primary Active-Transporters
• The Na+/K+-ATPase primary active transporter
is found in every cell and helps establish and
maintain the membrane potential of the cell.
• In addition to the Na+/K+-ATPase transporter,
the major primary active-transport proteins
found in most cells are:
(1) Ca2+-ATPase
(2) H+-ATPase
(3) H+/K+-ATPase
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Secondary Active Transport
• Secondary active transport is distinguished
from primary active transport by its use of an
electrochemical gradient across a plasma
membrane as its energy source.
• Transporters that mediate secondary active
transport have two binding sites, one for an ion
(e.g., Na+)and another for the cotransported
molecule (e.g., Glucose).
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Secondary Active Transport
Fig. 4-13
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Secondary Active Transport
Remember that Cotransporters (symporters) move molecules in the same direction.
Countertransporters (antiporters) move molecules in opposite directions.
Fig. 4-14
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Membrane Transport Proteins
Fig. 4-15
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Osmosis
• The net diffusion of water across a membrane
• Facilitated by channel proteins called
aquaporins
• Aquaporin expression and insertion into the
membrane varies among cell types. These are
especially important in the kidney.
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Osmolarity
• The total solute concentration of a solution is known as its osmolarity.
• One osmol is equal to 1 mol of solute particles.
• So a 1 M solution of glucose has a concentration of 1 Osm (1 osmol per liter),
whereas a 1 M solution of sodium chloride contains 2 osmol of solute per liter of
solution.
• A liter of solution containing 1 mol of glucose and 1 mol of sodium chloride has
an osmolarity of 3 Osm.
• Although osmolarity refers to the concentration of solute particles, it also
determines the water concentration in the solution because the higher the
osmolarity, the lower the water concentration.
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Tonic solutions
• Isotonic, hypotonic, and hypertonic solutions:
– Isotonic solutions have the same concentration of
nonpenetrating solutes as normal extracellular fluid.
– Hypotonic solutions have a lower concentration of
nonpenetrating solutes as normal extracellular fluid.
– Hypertonic solutions have a higher concentration of
nonpenetrating solutes as normal extracellular fluid.
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Extracellular Osmolarity & Cell Volume
Fig. 4-19
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Endocytosis & Exocytosis
Fig. 4-20
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Endocytosis
• Movement of molecules into the cell via
vessicles.
• There are three general types of endocytosis
that may occur in a cell:
1. Fluid endocytosis (pinocytosis)
2. Phagocytosis
3. Receptor-mediated endocytosis
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Forms of Endocytosis
Fig. 4-21
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Exocytosis
• Movement of molecules out of the cell via
vessicles.
• Exocytosis performs several functions for cells:
1. Provides a way to replace portions of the plasma
membrane that endocytosis has removed
2. Adds new membrane components to the membrane
3. Provides a route by which membrane-impermeable
molecules (such as protein hormones) the cell
synthesizes can be secreted into the extracellular fluid
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Epithelial Transport
• Paracellular pathway: diffusion between
adjacent cells
• Transcellular pathway: movement into a cell,
through the cytosol, and exit across the
opposite membrane
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Epithelial Cell terms
• One surface of an epithelial cell generally faces
a hollow or fluid-filled chamber, and the
plasma membrane on this side is referred to as
the apical or luminal membrane.
• The plasma membrane on the opposite surface,
which is usually adjacent to a network of blood
vessels, is referred to as the basolateral
membrane (also known as the serosal
membrane).
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Paracellular Transport
• Diffusion through the paracellular pathway is limited by
the presence of tight junctions between adjacent cells.
The tight junctions form a seal around the apical end of
the epithelial cells.
• Although small ions and water can diffuse to some degree
through tight junctions, the amount of paracellular
diffusion is limited by the tightness of the junctional seal
and the relatively small area available for diffusion.
• The permeability of the paracellular pathway varies in
different types of epithelia, with some being very
permeable and others very tight.
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Transepithelial Transport of Na+
Fig. 4-22
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Transepithelial Transport of Organic Solutes
Fig. 4-23
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Transepithelial Osmosis
Fig. 4-24
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Clinical Case Study
• The patient in this study suffered from severe
hyponatremia.
• Instead of pure water, what should she have
consumed at her rest stops?
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Clinical Case Study
• The patient in this study suffered from severe
hyponatremia.
• Instead of pure water, what should she have
consumed at her rest stops?
• Gatorade, PowerAde, or any of the sports electrolyte
solutions that are on the market.
• This type of fluid loss is also why doctors tell you
that after severe vomiting or diarrhea you should use
Pedialyte for kids.
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