Some solutes pass readily through the lipid bilayer of a cell membrane, whereas others pass through
much more slowly, or not at all.
Small nonpolar (hydrophobic) molecules, such as dissolved gases (O , CO , N 2 and2sma2l lipids, can pass
directly through the membrane. They do so by interacting directly with the hydrophobic interior of the
Very small polar molecules such as water and glycerol can pass directly through the membrane, but
much more slowly than small nonpolar molecules. The mechanism that permits small polar molecules to
cross the hydrophobic interior of the lipid bilayer is not completely understood, but it must involve the
molecules squeezing between the hydrophobic tails of the lipids that make up the bilayer.
Polar molecules such as glucose and sucrose have very limited permeability.
Large molecules such as proteins cannot pass through the lipid bilayer.
Ions and charged molecules of any size are essentially impermeable to the lipid bilayer because they are
much more soluble in water than in the interior of the membrane.
Carrier proteins and channels are both transport proteins involved in facilitated diffusion, the passive
transport of solutes across a membrane down their concentration or electrochemical gradient. As
integral membrane proteins, both carriers and channels protect polar or charged solutes from coming
into contact with the hydrophobic interior of the lipid bilayer. Furthermore, all transport proteins are
specific for the solutes they transport, owing to the specificity of the interactions between the solute
and the transport protein.
Channels are protein-lined pores across the membrane. A channel may be open at all times (non-gated),
or may be gated such that the channel opens and closes under specific conditions. Channels transport
inorganic ions or water.
In contrast, carrier proteins do not have a pore. Binding of the transported solute to the carrier protein
on one side of the membrane induces a conformational change in the protein that exposes the solute
binding site to the opposite side of the membrane, where the solute is released. Carriers transport small
polar solutes such as sugars and amino acids.
Sodium Potassium Pump
The concentration gradient of Na ions across the membrane (higher Na concentration outside)
facilitates the diffusion of Na into the cell. At the same time, the electrical gradient across the
membrane (excess positive charge outside) drives Na into the cell.
The concentration gradient of K ions across the membrane (higher K concentration inside) facilitates
the diffusion of K out of the cell. However, the electrical gradient across the membrane (excess positive
charge outside) impedes the diffusion of K o