If these conditions are met, the substance will diffuse from
the side of the membrane with the higher concentration to
the side with the lower concentration until equilibrium is
reached (see Figure 3.11).
What makes a biological membrane permeable to a
substance? The answer depends on both the nature of the
substance being transported and characteristics of pro-
teins in the membrane that move substances across it—
that is, transport proteins:
If the substance can dissolve in the lipids of the membrane,
it will pass directly through it (Figure 3.12a).
If the substance can be dissolved in water and is larger
than the space between the lipids, it must pass through
a transport protein within the membrane to get to the
Transport proteins can be channels or pores, which
are tunnels with an opening large enough to allow the
substance to pass through. The channel may have a
gate to control the movement of the substance (Figure
Transport proteins may be carriers. The substance
binds to the outside surface of the carrier, the carrier
changes shape, the substance is released on the inside,
and the carrier returns to its original shape so the
process can occur again (
Diffusion across a membrane with the aid of a transport
protein is called facilitated diffusion or facilitated
transport. Like diffusion, facilitated transport requires
the presence of a concentration gradient across the mem-
brane for any net movement of the substance to occur.
Diffusion and facilitated diffusion • Figure 3.12
There are three types of diffusion: simple diffusion (a), facilitated diffu-
sion through a channel (b), and diffusion facilitated by a carrier (c).
Lipid-soluble molecules diffuse across the
membrane down their concentration gradient.
Details of the K+ channel
b. Facilitated Diffusion Through
(e.g., K+ channel)
• K+ concentration is higher inside the cell than outside.
• K+ diffuse through the opening of the channel to the outside.
• The channel’s gate controls the movement of K+ ions through the channel.
Facilitated Diffusion Through
(e.g., glucose transport)
Glucose binds to the outside surface of the
The carrier undergoes a change in shape that
moves glucose through the membrane.
3. The carrier releases glucose inside the cell.
Once glucose is released, the carrier returns to
its original shape and the process repeats itself
as glucose continues to move down its gradient.
Cells Carry Out Many Processes 59