Lecture13 Chapter 12

Chapter 12Membrane Transport

EssentialCell Biology

Third Edition

Figure 12-1 Essential Cell Biology (© Garland Science 2010)

Table 12-1 Essential Cell Biology (© Garland Science 2010)

(A) A transporter undergoes a series of conformational changes to transfer small water-soluble molecules across the lipid bilayer. (B) A channel, in contrast, forms a hydrophilic pore across the bilayer through which specific inorganic ions or in some cases other small molecules can diffuse. As would be expected, channels transfer molecules at a much greater rate than transporters. Ion channels can exist in either an open or a closed conformation, and they transport only in the open conformation, which is shown here. Channel opening and closing is usually controlled by an external stimulus or by conditions within the cell.

Some small uncharged molecules can move down their concentration gradient across the lipid bilayer by simple diffusion. But most solutes require the assistance of a channel or transporter. As indicated, movement of molecules in the same direction as their concentration gradient--passive transport-- occurs spontaneously, whereas transport against a concentration gradient--active transport--requires an input of energy. Only transporters can carry out active transport, but both transporters and channels can carry out passive transport.

Each cell membrane has its own characteristic set of transporters.

The actively transported molecule is shown in yellow, and the energy source is shown in red.

Some transporters carry a single solute across the membrane (uniports); others couple the uphill transport of one solute across to the downhill transport of another.

The glucose-Na+ symport protein uses the electrochemical Na+ gradient to drive the import of glucose.

An electrochemical gradient has two components.

This transporter uses the energy of ATP hydrolysis to pump Na+ out of the cell and K+ in, both against their electrochemical gradients

The diffusion of water is known as osmosis.

The animal cell keeps the intracellular solute concentration low by pumping out ions (A). The plant cell's tough wall prevents swelling (B). The protozoan avoids swelling by periodically ejecting the water that moves into the cell

Figure 12-19a,b Essential Cell Biology (© Garland Science 2010)

In animal cells, an electrochemical gradient of Na+, generated by the Na+-K+ pump (Na+-K+ ATPase), is often used to drive the active transport of solutes across the plasma membrane (A). An electrochemical gradient of H+, usually set up by an H+ ATPase, is often used for this purpose in plant cells (B), as well as in bacteria and fungi (not shown). The lysosomes in animal cells and the vacuoles in plant and fungal cells contain an H+ ATPase in their membrane that pumps in H+, helping to keep the internal environment of these organelles acidic. (C) An electron micrograph shows the vacuole in plant cells in a young tobacco leaf. (C, courtesy of J. Burgess.)

Figure 12-19c Essential Cell Biology (© Garland Science 2010)

Figure 12-23a,b Essential Cell Biology (© Garland Science 2010)

Membrane potential Action potential

Activation potential

Action potential

Lecture13 Chapter 12

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