Membrane Transport
Membrane Diffusion The simplest kind of transport is the unassisted diffusion of solutes across membranes (see Figure 1a). The kinds of molecules that transit in this fashion are more soluble in oil than water and so readily dissolve in and then spontaneously traverse the nonpolar lipid core of the membrane bilayer. Among these diffusible lipid-soluble molecules are steroid hormones, many kinds of drugs, the oxygen that cells respire, and the carbon dioxide they expire. Motion of all kinds must be impelled by some form of energy. In the case of simple membrane diffusion, movement across the bilayer is a random walk driven by the kinetic (heat) energy provided by the collisions of the solute with surrounding molecules. This is the Brownian motion that agitates all molecules. Random diffusion causes the solute molecules to end up at equal concentrations on the two sides of the membrane no matter how great the initial difference (gradient) was. Solute transport by these means is thus said to be downhill.
lipid fat or waxlike molecule, insoluble in water bilayer composed of two layers steroid hormone group of hormones that includes estrogen, testosterone, and progesterone respire use oxygen to burn cellular fuel
Membrane Channels Most cellular solutes have a polar chemical structure and are therefore strongly attracted to water. Consequently, these water-soluble molecules tend not to enter the lipid core of the membrane readily; indeed, the bilayer is designed keep them from doing so. To transport these solutes across the barrier, membranes are equipped with a variety of special protein structures. The simplest way to convey water-soluble solutes across membranes is through channels: membrane-spanning proteins with central pores. By these means, selected solutes diffuse downhill across membranes, passing single file along a narrow column of water molecules in these pores. Driven by random Brownian diffusion, a solute will ultimately reach an equal concentration in the two aqueous compartments. Channels can discriminate among solutes. They use the diameter of their pores as a sieve and place critical amino acid side chains along the pore to give it the proper shape and chemical profile. As a result, different channels strongly prefer Na+ or K+ or Ca++ or H+ and pass cations much better than anions; another channel is specialized for Cl-. Some channel families conduct larger solutes into cells; for example, nutrients like amino acids or sugars. There are also elaborate channels that enable newly synthesized
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ATP
ADP +P
polar partially charged, and usually soluble in water
protein complex molecule made from amino acids; used in cells for structure, signaling, and controlling reactions
amino acid a building block of protein
Figure 1. Modes of membrane transport. a) Some solute molecules can diffuse unassisted through the lipid bilayer. b) Certain solute molecules can diffuse though the aqueous pore of a specific channel protein. c) Reciprocating transporters can convey selected solute molecules across the bilayer by means of a fluctuating change in their shape. d) The energy released by the breakdown of ATP molecules can be coupled by a transport protein to pumping specific solute molecules against their concentration gradient.
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