Page 227 - Color Atlas of Biochemistry

218 Organelles
Transport processes
A. Permeability
Only small, uncharged molecules such as
gases, water, ammonia, glycerol, or urea are
able to pass through biological membranes by
free diffusion. With increasing size,evencom-
pounds of this type are no longer able to pass
through. Membranes are impermeable to glu-
cose and other sugars, for example.
The polarity of a molecule is also impor-
tant. Apolar substances, such as benzene,
ethanol, diethyl ether, and many narcotic
agents are able to enter biological membranes
easily. By contrast, membranes are imperme-
able to strongly polar compounds, particu-
larly those that are electrically charged. To
be able to take up or release molecules of
this type, cells have specialized channels and
transporters in their membranes (see below).
B. Passive and active transport
Free diffusion is the simplest form of mem-
brane transport. When it is supported by in-
tegral membrane proteins, it is known as fa-
cilitated diffusion (or facilitated transport). for example (see p. 350).
1. Channel proteins have a polar pore
through which ions and other hydrophilic
compounds can pass. For example, there are
channels that allow selected ions to pass (ion
channels; see p. 222) and porins that allow
molecules below a specific size to pass in a
more or less nonspecific fashion (see p. 212).
2. Transporters recognize and bind the
molecules to be transported and help them
to pass through the membrane as a result of a
conformational change. These proteins (per-
meases) are thus comparable with enzy-
mes—although with the difference that they
“catalyze” vectorial transport rather than an
enzymatic reaction. Like enzymes, they show
acertain affinity for each molecule trans-
ported (expressed as the dissociation
–1
constant, K d in mol L )and a maximum
transport capacity (V).
Free diffusion and transport processes fa-
cilitated by ion channels and transport pro-
teins always follow a concentration gradient—
i. e., the direction of transport is from the
site of higher concentration to the site of
lower concentration. In ions, the membrane
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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potential also plays a role; the processes are
summed up by the term “electrochemical
gradient” (see p. 126). These processes there-
fore involve passive transport, which runs
“downhill” on the slope of a gradient.
By contrast, active transport can also run
“uphill”—i. e., against a concentration or
charge gradient. It therefore requires an input
of energy, which is usually supplied by the
hydrolysis of ATP (see p. 124). The transporter
first binds its “cargo” on one side of the mem-
brane. ATP-dependent phosphorylation then
causes a conformation change that releases
the cargo on the other side of the membrane
(see p. 220). A non-spontaneous transport
process can also take place through coupling
to another active transport process (known as
secondary active transport; see p. 220).
Using the transport systems in the mem-
branes, cells regulate their volume, internal
pH value, and ionic environment. They con-
centrate metabolites that are important for
energy metabolism and biosynthesis, and ex-
clude toxic substances. Transport systems
also serve to establish ion gradients, which
are required for oxidative phosphorylation
and stimulation of muscle and nerve cells,
C. Transport processes
Another classification of transport processes
is based on the number of particles trans-
ported and the direction in which they
move. When a single molecule or ion passes
through the membrane with the help of a
channel or transporter, the process is de-
scribed as a uniport (example: the transport
of glucose into liver cells). Simultaneous
transport of two different particles can take
place either as a symport (example: the trans-
port of amino acids or glucose together with
Na ions into intestinal epithelial cells) or as
+
an antiport. Ions are often transported in an
antiport in exchange for another similarly
charged ion. This process is electroneutral
and therefore more energetically favorable
– –
(example: the exchange of HCO 3 for Cl at
the erythrocyte membrane).