to pass through the membrane, so their movement has to be facilitated. biological membranes contain transport
membrane proteins that facilitate the passage of selected ions and other polar molecules.
Membrane proteins do not set up a concentration gradient; a
concentration gradient must already be present for molecules to
diffuse, even if facilitated by the proteins. These membrane proteins
help to move substances across membranes without expenditure
of aTP energy. facilitated diffusion cannot cause net transport of
molecules, from a low to a high concentration which would require
input of energy.
facilitated diffusion is very specific, and allows a cell to select substances for uptake. It is sensitive to inhibitors,
which react with the protein side chains.
Two major types of transport membrane proteins are known viz, carrier proteins (also called carriers, transporters)
and channel proteins.
In transport mediated by carrier, the substance being
transported is initially bound to a specific site on the carrier
protein.
Carriers are highly selective for a particular substrate to
be transported. a conformational change in the protein is
required to transport individual molecules or ions hence
the rate of transport is slow.
Some carrier proteins allow transport, only if two types of molecules move together. This is called cotransport. When
a molecule moves across a membrane independent of other molecule, the process is called uniport.
Channels are transmembrane proteins that function as selective pores through which molecules or ions can diffuse
across the membrane. The size of pore and density of surface charges on its interior lining determine its transport specificity.
Channel transport is mainly limited to ions or water. Channels are not open all the time. They have structures called
gates that open and close the pore in response to external signals (voltage changes, hormone binding or light).
Porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing
molecules upto the size of small proteins to pass through.Ion channels allow passage of effective ions, e.g., K+
channels, during nerve conduction. Ion channels
may open in different voltage ranges or in response
to different signals which may include K+ or Ca2+
concentrations, pH, kinases, etc. This specificity
enables the transport of each ion to be fine-tuned
to the prevailing conditions.
Active transport
In active transport, the movable carrier proteins
are called pumps, and employ aTP energy
for transport across the membrane. It is uphill
transport, i.e., against concentration gradient and
is faster than passive transport. The rate of
active transport reaches the maximum, when all
the protein pumps are used in transport, which is
called saturation effect.