Chapter 7 - Transport of Ions and Small Molecules Across Cell Membranes

T domains

The ________, each built of six membrane- spanning helices, form the pathway through which the transported substance (substrate) crosses the membrane and determine the substrate specificity of each ABC protein.

Pure phospholipid bilayers

________ are essentially impermeable to water, but most cellular membranes contain water- channel proteins that facilitate the rapid movement of water in and out of cells.

ox helices

Each subunit contains six membrane- spanning ________ that form a central pore through which water moves.

CO2

The ________ diffuses out of the cell, and the OH- ions combine with intracellular protons, forming water.

passive diffusion

The first and rate- limiting step in transport by ________ is the movement of a molecule from the aqueous solution into the hydrophobic interior of the phospholipid bilayer, which resembles oil in its chemical properties.

multiple excitatory

A single neuron can be affected simultaneously by signals received at ________ and inhibitory synapses.

Sensitive electric measurements

________ suggest that the opening of a voltage- gated Na+ or K+ channel is accompanied by the movement of 10 to 12 protein- bound positive charges from the cytosolic to the axoplasmic surface of the membrane.

concentration gradient

Uniporters** transport a single type of molecule down its ________ via facilitated diffusion.

phospholipid bilayer

The** ________**, the basic structural unit of biomembranes, is essentially impermeable to most water- soluble molecules, ions, and water itself.

Transition

________ to the E2 conformation also generates two high- affinity K+ sites accessible to the ectoplasmic face.

free energy

To calculate the ________ change △G corresponding to the transport of an ion across a membrane, consider the independent contributions from each of the forces to the** electrochemical gradient**

amino acids

Glucose and ________** cross the plasma membrane into most mammalian cells with the aid of uniporters.

none

Some proteins are synthesized in dendrites, but ________ are made in** axons or axon terminals**

electrochemical gradient

The proton ________ across the plant vacuole membrane is used in much the same way as the Na+ ________ across the animal- cell plasma membrane.

Myelination

# ________ Increases the Velocity of Impulse Conduction.

relative diffusion rate

The ________ of any substance across a pure phospholipid bilayer is proportional to its concentration gradient across the layer and to its hydrophobicity and size.