7.1 Overview of Membrane Transport

7.2 ATP-Powered Pumps and the Intracellular Ionic Environment

7.3 Nongated Ion Channels and the Resting Membrane Potential

7.4 Cotransport by Symporters and Antiporters

7.5 Movement of Water

7.6 Transepithelial Transport

7.7 Voltage-Gated Ion Channels and the Propagation of Action Potentials in Nerve Cells

7.8 Neurotransmitters and Receptor and Transport Proteins in Signal Transmission at Synapses

7.ÏI Overview of Membrane Transport

The phospholipid bilayer, the basic structural unit of biomembranes, is essentially impermeable to most water-soluble molecules, ions, and water itself. After describing the factors that influence the permeability of lipid membranes, we briefly compare the three major classes of membrane proteins that increase the permeability of biomembranes. We then examine operation of the simplest type of transport protein to illustrate basic features of protein-mediated transport. Finally, two common experimental systems used in studying the functional properties of transport proteins are described.

Few Molecules Cross Membranes by Passive Diffusion

Gases, such as O2 and CO2, and small, uncharged polar molecules, such as urea and ethanol, can readily move by passive (simple) diffusion across an artificial membrane composed of pure phospholipid or of phospholipid and cholesterol (Figure 7-1). Such molecules also can diffuse across cellular membranes without the aid of transport proteins. No metabolic energy is expended because movement is from a high to a low concentration of the molecule, down its chemical con-

Small uncharged polar molecules


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