Chapter 5Facilitated Diffusion and Active

Transport

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