2.
• The phospholipid bilayer,(a bio-membrane) is normally impermeable to most water-
soluble molecules, ions, and water itself.
• O2 and CO2, and small, uncharged polar molecules,(ex: urea and ethanol),move by
passive (simple) diffusion across an artificial membrane composed of mainly
phospholipids and cholesterol.
• The hydrophobicity of a substance given by partition coefficient K, is the ratio of its
concentration within the hydrophobic region of the bilayer Cm to its concentration in the
aqueous solution Caq).
• The higher a substance’s partition coefficient, the more lipid-soluble it is.
• The hydrophobicity of a molecule is, therefore directly proportional to its rate of
diffusion across a pure phospholipid bilayer.
Introduction
3. • Long chained Fatty acids are more hydrophobic than short chained fatty acids, therefore will
diffuse more rapidly across a pure phospholipid bilayer.
• The movement of substances carrying a net charge, is influenced by both its concentration
gradient and the membrane potential, which as a whole is referred to as electrochemical
gradient.
• The electrochemical gradient, determines the energetically favourable direction for the
transport the charged molecule across the membrane.
4.
• Active Transport: ATP-powered pumps use the energy of ATP hydrolysis to
transport ions and molecules across a membrane against a concentration
gradient(Chemical/Electric).
Facilitated diffusion: involves transport of water or specific ions via channel
proteins and hydrophilic small molecules down their concentration or
electric potential gradients
Passive transport does not involve any chemical energy and relies on the
difference in concentrations and of its component proteins and lipids.
Protein-assisted transport reactions occur faster than that of passive
diffusion, as they are substrate-specific.
Transport
mechanisms
5.
Transporters/Carriers move a wide variety of ions and molecules across cell
membranes
Types of transporters:
• Uniporters are involved in the transport of a single type of molecule down its
concentration gradient by facilitated diffusion. Most mammalian cells use uniporters in
the transportation of glucose and amino acids cross the bi-lipid layer(plasma
membrane).
• Antiporters execute coupled transport of two different molecules or ions through a
membrane in opposite directions by a common carrier mechanism
• Symporters also perform coupled transport of two different molecules or ions through
a membrane in the same direction with the help of a common carrier mechanism
Transporters
6. Antiporters and symporters are termed as cotransporters, as they transport two
different solutes simultaneously. antiporters and symporters couple the
movement of ions against the concentration gradient by transporting one or
more types of ions down its concentration gradient.
Co-transporters obtain energy stored in the electrochemical gradient unlike ATP
pumps which use energy obtained from hydrolysis of ATP, this property of the co-
transporters is termed as secondary active transport.
Conformational changes are an essential property in the function of all transport
proteins, this refers to the alteration in the tertiary structure of a protein as a
result of binding of a ligand or of a substrate)
The ATP-powered pumps and transporters experience conformational change
exposing the binding site (or sites) to one side of the
membrane,(Exoplasmic/Endoplasmic), in one conformation and alternates the
exposed site during the second conformation.
7.
The rate of facilitated diffusion by uniporters through a pure phospholipid
bilayer is considerably higher than passive diffusion
Transport occurs only through a few limited number of uniporter molecules,
rather than molecules present throughout the phospholipid bilayer
Transport is specific, i.e uniporter transports a single group of closely
related molecules or only a single type of molecule.
Difference between Uniport Transport &
Passive Diffusion
8.
The most widely studied uniporter is the glucose transporter GLUT1 found
in the plasma membrane of erythrocytes.
GLUT1
9.
Like other uniporters, GLUT1 undergoes a conformational change.
During the conformation, the glucose-binding site faces outward(Exoplasmic
side); in the other, the binding site faces inward(cytoplasmic side).
The glucose binds to the outward-facing site
Triggering a conformational change in the uniporter that results in the binding
site facing inward toward the cytosol.
Glucose is then released into the cytosol of the cell.
In the end, the transporter undergoes an inverted conformation, thereby
reverting the uniport to face the original outward-facing binding site(towards
the exoplasm)
If the concentration of glucose is greater inside the cell, then the cycle will work
in reverse
Mechanism involved
in GLUT1
