You Must Know
• The selective permeability of membranes.
• The role of facilitated diffusion.
• The role of active transport, including the sodium potassium pump and cotransport, in cells.
CONCEPT 5.2: Membrane structure results in selective permeability
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Channel protein
(a) A channelprotein
SoluteCYTOPLASM
EXTRA-CELLULARFLUID
Facilitated Diffusion:Channel Proteins
Hydrophilic
Facilitated diffusion does not alter the direction of transport
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Facilitated Diffusion – Carrier Proteins
Carrier protein
(b) A carrier protein
Solute
CONCEPT 5.4: Active Transport
• Active transport moves substances against their concentration gradients.
• Active transport requires energy, usually in the form of ATP.
• Active transport allows cells to maintain concentration gradients that differ from their surroundings
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• Sodium-potassium pump transports 3 sodium ions out of the cell and two potassium ions into the cell.
Cell
[K] high
EXTRACELLULARFLUID
CYTOPLASM
[Na] low
[K] low[Na] high
1
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ADP2
3
5 4
6
Phosphorylation
How Ion Pumps Maintain Membrane Potential
• Membrane potential is the voltage across a 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.
1. A chemical force (the ion’s concentration gradient).
2. An electrical force (the effect of the membrane potential on the ion’s movement).
• An electrogenic pump is a transport protein that generates voltage across a membrane.
• The sodium-potassium pump is the major electrogenic pump of animal cells.
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Figure 5.16
EXTRACELLULARFLUID
CYTOPLASM
Proton pump
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The main electrogenic pump of plants, fungi, and bacteria is a proton pump.
Electrogenic pumps help store energy that can be used for cellular work.
Cotransport: Coupled Transport by a Membrane Protein
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Sucrose
Proton pump
Sucrose-H
cotransporterDiffusion of H
Sucrose
