Facilitated diffusion is the movement of substances across a membrane along a concentration gradient, aided by transport proteins like channel and carrier proteins. Channel proteins form hydrophilic pores through the membrane that allow rapid diffusion of small molecules and ions. Carrier proteins bind molecules and change shape to transport them, moving more slowly than channels. Gated channel proteins selectively open and close pores in response to stimuli like voltage changes, regulating substance flow across the membrane. Factors like temperature, concentration gradient, distance, and molecule size affect the rate of facilitated diffusion.

1. FACILITATED DIFFUSION
 Facilitated diffusion is the transport of substances across a biological
membrane from an area of higher concentration to an area of lower
concentration with the help of a transport molecule.
 Since substances move along the direction of their concentration gradient,
chemical energy is not directly required.
 Substances soluble in lipids diffuse through the membrane faster.
 Substances that are hydrophilic (ions) and large molecules (glucose), find it
difficult to pass through the membrane; their movement has to be facilitated.
 a concentration gradient must already be present for molecules to diffuse
even if facilitated by the proteins.
 Examples are glucose and amino acid transport, gas transport, and ion
transport.
 The plasma membrane is the cellular structure that is responsible for the
selective movements of substances.
Factors Affecting FacilitatedDiffusion
 Brownian motion is the force behind the diffusion of fluids. The main factors
affecting the process of facilitated diffusion are:
 Temperature- As the temperature increases, the movement of the molecules
increases due to an increase in energy.
 Concentration-The movement of the molecules takes place from the region
of higher concentration to lower concentration.
 Diffusion Distance- The diffusion rate is faster through smaller distance than
through the larger distance. For e.g., gas diffuses much faster through a thin
wall than through a thick wall.
 Size of the molecules- The smaller molecules diffuse faster than the larger
molecules.
Channel proteins
 a type of transport protein, acts like a porein the membrane that lets water
molecules or small ions through quickly.
 forming a channel
 Based on structural models, channel diameters are thought to be no more
than 4 – 5 Å ,

2.  These channels have hydrophilic domains exposed to both the extracellular
and intracellular matrix.
 Additionally, they have a hydrophilic core that provides a hydrated opening
through the membrane layers.
 An open channel protein, a type of transport protein, acts like a pore
 . In any case, as long as the channel pore is open, solutes that can penetrate
the pore diffuse through it extremely rapid: about 10^8 ions per second
through each channel protein.
 Aquaporins are the channels proteins that transport water across the plasma
membrane at a very high rate.
 A gatedchannel protein , is a transport protein that opens a "gate,"
allowing a molecule to pass through the membrane?
 The stimulus may be chemical or electrical signals, temperature, or
mechanical force, depending on the type of gated channel.
Gated channel proteins are of:
Voltage gated channels....... voltage-gated channels open or close in responseto
changes in the membrane potential.
Patch clamp studies reveal that, for potassium, a given membrane has a variety of
different channels. These channels may open in different voltage ranges, or in
responseto different signals, which may include K+ or Ca2+ concentrations, pH,
protein kinases and phosphatases, and so on
Opened by some mechanical stress.
 Glucose molecules, Due to its polar nature and large size, glucose
molecules cannot traverse the lipid membrane of the cell by simple
diffusion. they are moved across the membrane through gated channels. the
entry of glucose molecules into the cells is affected by a large family of
structurally related transport proteins known as glucose transporters. Two
main types of glucose transporters have been identified, namely, sodium–
glucose linked transporters (SGLTs)and facilitated diffusion glucose
transporters (GLUTs)
Carrier proteins

3.  Carrier proteins, as the name suggests, carry molecules across the
membrane.
 These proteins bind to specific regions in the molecules causing
conformational changes and then move the bound molecule to the interior of
the cell depending on the concentration gradient.
 The mechanism of the conformational change is not entirely understood, but
it is assumed that the hydrogen bonds are affected, which results in a change
in the shape of the molecule.
 Carrier proteins are involved in active transport and passive
 Because a conformational change in the protein is required to transport
individual molecules or ions, the rate of transport by a carrier is many orders
of magnitude slower than through a channel. Typically, carriers may
transport 100 to 1000 ions or molecules per second, which is about 106
times slower than transport through a channel.
Feature of proteins
 It acts like enzyme
 It has properties like selective saturation and inhibition
 Channels are selective ...e.g., if the channel is for glucose molecule it allows
only the transfer of glucose this channels name is glut 4 these channels will
not allow fructose to pass through
 Inhibitor bind at protein side chain and that destroythe shape of the protein
transporters
 Saturation can be increased by increasing the number of channels, which is
not possible
Classifiedby direction of movement
 Uniport
 Cotransport systems transfer two solutes: Symport - moves solutes in
same direction (e.g., Na+-sugar transporters or Na+-amino acid transporters)
Antiport - moves two molecules in opposite directions (e.g., Na+ in and
Ca2+ out)
Reference:
plant physiology by Taiz and Zeiger
Ncert 11 text book

4. By:
Abeer Ali
60071