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Transcript
Transport Across Membranes
 Solutes Cross Membranes by
 Simple Diffusion,Facilitated Diffusion,
 and Active Transport
 The Movement of a Solute Across a
Membrane
 Is Determined by Its Concentration Gradient
 or Its Electrochemical Potential
Simple Diffusion
 Simple Diffusion: Unassisted
 Movement Down the Gradient
 Diffusion Always Moves Solutes
 Toward Equilibrium
 Osmosis Is the Diffusion ofWater Across
 a Selectively Permeable Membrane
 Simple Diffusion Is Limited
 to Small, Nonpolar Molecules
 The Rate of Simple Diffusion Is Directly
 Proportional to the Concentration Gradient
Facilitated Diffusion
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Protein-Mediated Movement
Down the Gradient
Carrier Proteins and Channel Proteins
Carrier Proteins Alternate Between Two
Conformational States
Carrier Proteins Are Analogous to Enzymes
in Their Specificity and Kinetics
Carrier Proteins Transport Either
One or Two Solutes
The Erythrocyte Glucose Transporter
and Anion Exchange Protein Are Examples
of Carrier Proteins
Facilitated Transport
 Channel Proteins Facilitate Diffusion
 by Forming Hydrophilic Transmembrane
 Channels
 Ion Channels:Transmembrane Proteins That
Allow
 Rapid Passage of Specific Ions
 Porins: Transmembrane Proteins That Allow
Rapid
 Passage of Various Solutes
Active Transport
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Active Transport: Protein-Mediated
Movement Up the Gradient
The Coupling of Active Transport to an Energy
Source May Be Direct or Indirect
Direct Active Transport Depends on
Four Types of Transport ATPases
P-type ATPases
V-type ATPases
F-type ATPases
ABC-type ATPases
Indirect Active Transport Is Driven
by Ion Gradients
Examples For Active Transport
 Direct Active Transport: The Na/K Pump
 Maintains Electrochemical Ion Gradients
 Indirect Active Transport: Sodium Symport
 Drives the Uptake of Glucose
 The Bacteriorhodopsin Proton Pump Uses
 Light Energy to Transport Protons
Summary
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Cells and Transport Processes
■ The selective transport of molecules and ions across membrane
barriers ensures that necessary substances are moved
into and out of cells and cell compartments at the appropriate
time and at useful rates.
■ Nonpolar molecules and small, polar molecules cross the
membrane by simple diffusion. Transport of all other solutes,
including ions and many molecules of biological relevance, is
mediated by specific transport proteins that provide passage
through an otherwise impermeable membrane.
■ Each such transport protein has at least one, and frequently
several, hydrophobic membrane-spanning sequences that
embed the protein within the membrane and often act as the
channel itself. Typically, a separate regulatory domain controls
channel opening and closing.
Summary
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Simple Diffusion: Unassisted Movement
Down the Gradient
■ Simple diffusion through biological membranes is limited to
small or nonpolar molecules such as , , and lipids. Water
molecules, although polar, are small enough to diffuse across
membranes in a manner that is not entirely understood.
■ Membranes are permeable to lipids, which can pass through the
nonpolar interior of the lipid bilayer. Membrane permeability
of most compounds is directly proportional to their partition
coefficient—their relative solubility in oil versus water.
■ The direction of diffusion of a solute across a membrane is
determined by its concentration gradient and always moves
toward equilibrium. The solute will diffuse down the gradient
from a region of high concentration to a region of low
concentration
Facilitated Diffusion: Protein-Mediated
Movement Down the Gradient
 Transport can either be downhill or uphill in relation to an
 uncharged solute’s concentration gradient. For an ion, we
 must consider its electrochemical potential—the combined
 effect of the ion’s concentration gradient and the charge
 gradient across the membrane.
 ■ Downhill transport of large, polar molecules and ions, called
 facilitated diffusion, must be mediated by carrier proteins and
 channel proteins because these molecules and ions cannot
 diffuse through the membrane directly.
Facilitated Transport
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Carrier proteins function by alternating between two conformational
states. Examples include the glucose transporter and
the anion exchange protein found in the plasma membrane of
the erythrocyte.
■ Transport of a single kind of molecule or ion is called uniport.
The coupled transport of two or more molecules or ions at a
time may involve movement of both solutes in the same direction
(symport) or in opposite directions (antiport).
■ Channel proteins facilitate diffusion by forming transmembrane
channels lined with hydrophilic amino acids. Three
important categories of channel proteins are ion channels
(used mainly for transport of , , , , , and
), porins (for various high-molecular-weight solutes),
and aquaporins (for water).
Active Transport: Protein-Mediated
Movement Up the Gradient
 Active transport—the uphill transport of large, polar
 molecules and ions—requires a protein transporter and an
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input of energy. It may be powered by ATP hydrolysis, the
electrochemical potential of an ion gradient, or light
energy.
Active transport powered by ATP hydrolysis utilizes four
major classes of transport proteins: P-type, V-type, F-type,
and ABC-type ATPases. One widely encountered example
is the ATP-powered pump (a P-type ATPase),
which maintains electrochemical potentials for sodium
and potassium ions across the plasma membrane of
animal cells.
Active Transport
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Active transport driven by an electrochemical potential
usually depends on a gradient of either sodium ions (animal
cells) or protons (plant, fungal, and many bacterial cells). For
example, the inward transport of nutrients across the plasma
membrane is often driven by the symport of sodium ions that
were pumped outward by the / pump. As they flow
back into the cell, they drive inward transport of sugars, amino
acids, and other organic molecules.
■ In Halobacterium, active transport is powered by light energy.
As photons of light are absorbed by bacteriorhodopsin,
protons are pumped across the cell membrane and out of the
cell. As the protons flow back into the cell, ATP is synthesized.