Chapter 3
Cells: The Living Units
69
3
due to pore size and the charges of the amino acids lining the
channel.
Leakage channels
are always open and simply allow
ions or water to move according to concentration gradients.
Gated channels
are controlled (opened or closed) by chemical
or electrical signals.
Like carriers, many channels can be inhibited by cer-
tain molecules, show saturation, and tend to be specific.
Substances moving through them also follow the con-
centration gradient (always moving down the gradient).
When a substance crosses the membrane by simple dif-
fusion, the rate of diffusion is not controllable because
the lipid solubility of the membrane is not immediately
changeable. By contrast, the rate of facilitated diffusion
is
controllable because the permeability of the membrane
can be altered by regulating the activity or number of in-
dividual carriers or channels.
Oxygen, water, glucose, and various ions are vitally impor-
tant to cellular homeostasis. Teir passive transport by diffusion
(either simple or facilitated) represents a tremendous saving of
cellular energy. Indeed, if these substances had to be transported
actively, cell expenditures of A±P would increase exponentially!
Osmosis
Te diffusion of a solvent, such as water, through a
selectively permeable membrane is
osmosis
(oz-mo
9
sis;
osmos
5
pushing). Even though water is highly polar, it passes via os-
mosis through the lipid bilayer (Figure 3.7d). Tis is surprising
because you’d expect water to be repelled by the hydrophobic
lipid tails. One hypothesis is that random movements of the
membrane lipids open small gaps between their wiggling tails,
allowing water to slip and slide its way through the membrane
by moving from gap to gap.
passively even though they are unable to pass through the li-
pid bilayer. Instead they move through the membrane by a pas-
sive transport process called
facilitated diffusion
in which the
transported substance either (1) binds to protein carriers in the
membrane and is ferried across or (2) moves through water-
filled protein channels.
Carrier-mediated facilitated diffusion.
Carriers
are trans-
membrane integral proteins that are specific for transporting
certain polar molecules or classes of molecules, such as sug-
ars and amino acids, that are too large to pass through mem-
brane channels. Alterations in the shape of the carrier allow
it to first envelop and then release the transported substance,
shielding it en route from the nonpolar regions of the mem-
brane. Essentially, changes in the conformation of the carrier
protein move the binding site from one face of the mem-
brane to the other (Figure 3.7b and ±able 3.1).
Note that a substance transported by carrier-mediated
facilitated diffusion, such as glucose, moves down its con-
centration gradient, just as in simple diffusion. Glucose is
normally in higher concentrations in the blood than in the
cells, where it is rapidly used for A±P synthesis. So, glucose
transport within the body is
typically
unidirectional—into
the cells. However, carrier-mediated transport is limited by
the number of protein carriers present. For example, when
all the glucose carriers are “engaged,” they are said to be
saturated
, and glucose transport is occurring at its maxi-
mum rate.
Channel-mediated facilitated diffusion.
Channels
are trans-
membrane proteins that transport substances, usually ions or
water, through aqueous channels from one side of the mem-
brane to the other (Figure 3.7c and d). Channels are selective
Extracellular fluid
Lipid-
soluble
solutes
Cytoplasm
Lipid-insoluble solutes
(such as sugars or
amino acids)
Small lipid-
insoluble
solutes
Water
molecules
Lipid
bilayer
Aquaporin
(a) Simple diffusion
of
fat-soluble molecules
directly through the
phospholipid bilayer
(b) Carrier-mediated facilitated diffusion
via protein carrier specific for one
chemical; binding of substrate causes
transport protein to change shape
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(d) Osmosis
, diffusion of a
solvent such as water
through a specific
channel protein
(aquaporin) or through
the lipid bilayer
Figure 3.7
Diffusion through the plasma membrane.
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