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Active Transport
CONCEPT 5.4: Active transport uses energy to move solutes
against their concentration gradients
• Facilitated diffusion speeds transport of a solute by
providing efficient passage through the membrane but
does not alter the direction of transport
• Some transport proteins, however, can move solutes
against their concentration gradients. These are used for
active transport.
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Figure 5.15: Passive vs. Active Transport
Passive transport
Diffusion
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Active transport
Facilitated
diffusion
The Need for Energy in Active Transport
• Active transport moves substances against their
concentration gradients
• Active transport requires energy, usually in the form of
ATP
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• Active transport allows cells to maintain concentration
gradients that differ from their surroundings
• The sodium-potassium pump is one type of active
transport system
• Na/K Pump used in muscle contraction and to send signals in
nerve cells
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EXTRACELLULAR
FLUID
[Na] high
[K] low
Figure 5.14: the sodium potassium
pump: a specific case of active
transport
1
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CYTOPLASM
[Na] low
[K] high
2
6
3
5
4
ADP
How Ion Pumps Maintain Membrane Potential
• All cells have voltages across their plasma membranes
• Voltage: electrical potential energy
• The cytoplasmic side of the membrane is negative; the outside
of the cell is positive
• Membrane potential is the voltage across a membrane
• Serves as an energy source that controls the movement of
substances across the membrane
• Voltage is created by differences in the distribution of
positive and negative ions across a membrane
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• Two combined forces, collectively called the
electrochemical gradient, drive the diffusion of ions
across a membrane
• A chemical force (the ion’s concentration gradient)
• An electrical force (the effect of the membrane potential on
the ion’s movement)
• An ion diffuses down its concentration gradient and its
electrochemical gradient. Active transport is necessary
to move a particle whose concentration gradient is
opposite of its electrochemical gradient.
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• An electrogenic pump is a transport protein that
generates voltage across a membrane by transporting ions
• The sodium-potassium pump is the major electrogenic
pump of animal cells
• The main electrogenic pump of plants, fungi, and bacteria
is a proton pump (transports H+)
• Electrogenic pumps help store energy that can be used for
cellular work
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Figure 5.16
EXTRACELLULAR
FLUID
Proton pump
CYTOPLASM
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Cotransport: Coupled Transport by a Membrane Protein
• Cotransport occurs when active transport of a solute
indirectly drives transport of other solutes
• Plant cells use the gradient of hydrogen ions generated by
proton pumps to drive active transport of nutrients into
the cell
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Figure 5.17
Proton pump
Sucrose-H
cotransporter
Diffusion of H
Sucrose
Sucrose
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