4. CELLS AND TRANSPORT PROCESS
• Membranes are selectively permeable and control the passage of
specific molecules and ions from one side of the membrane to the
• Cells need a higher concentration of solutes within the cell than
found typically outside the cell in order for many reactions to occur at
• Solutes cross membranes by simple diffusion, facilitated diffusion,
and active transport.
• It was evidenced by the fact that E.coli has 20% of genes in the aspect
of transport process.
8. Simple diffusion-unassisted movement down the
• Diffusion always moves solutes toward equilibrium.
• Diffusion always proceeds from regions of higher concentration to lower
• Equilibrium is the lowest energy state.
• Osmosis is the diffusion of water across a differentially permeable membrane.
• Simple diffusion is limited to small, nonpolar molecules.
9. SIMPLE DIFFUSION LIMITED TO SMALL AND
NON POLAR MOLECULES
• Diffusion is always movement toward equilibrium (minimum free energy).
Solute size (cut-off size is approx. 200 amu)
• Generally, membranes are more permeable to smaller molecules than
• For example: Glucose is too large.
• Permeable to nonpolar, more impermeable to polar.
• Membranes are impermeable to ions.
• Small nonpolar molecules: oxygen, carbon dioxide and ethanol can easily
diffuse across membrane.
Water diffuses from where the solute
concentration is lower, across a
to where the solute concentration is
12. • Osmolarity: solute concentration on one side of a membrane relative
to that on the other side of the membrane; drives the osmotic
movement of water across the membrane.
• Hypertonic: a solution with a higher solute concentration than inside
• Isotonic: a solution with an equal solute concentration as that inside
• Hypotonic: a solution with a lower solute concentration than inside
14. FACILITATED DIFFUSION
• Facilitated diffusion does not require input of energy.
• Two main classes of proteins involved with facilitated diffusion: Carriers
• Carrier proteins and channel proteins facilitate transport by different
• Carrier proteins (transporters or permeases) bind one or more solute
molecules on one side of the membrane and undergo conformational
change to deliver solute to the other side of membrane.
• Channel proteins: form hydrophilic channels, often transport ions.
• Ion channels
15. Carrier Proteins/Permeases/Transporters
• Carrier proteins probably alternate between two conformational
states – alternating confirmation model.
• Carrier proteins are analogous to enzymes in their specificity and
• Some carriers are extremely specific.(eg- carrier protein facilitates
diffusion of glucose into erythrocytes).
• Carrier proteins transport either one or two solutes.
• Uniport: single solute (glucose transporter)
• Cotransport: two solutes (couple).
• Symport: both in the same direction.
• Antiport: solutes are transported in opposite directions.
17. The Glucose Transporter Family GLUT-1 - GLUT-5
• The initial event in the cellular metabolism of glucose is its transport across the cell membrane into the
cytoplasm of the cell. 65 – 90 mg/100ml concentration in the blood plasma.
• This step is performed by the GLUT permeases .
• Members of the family include:
19. The erythrocyte anion exchange protein: An
• The erythrocyte anion exchange protein is also called band 3 protein,(Band
3 protein is a typical polytropic membraneprotein and mediates the
exchange of the cellular HCO3- with CI- in plasma, which has been known
as the "Chloride Shift") and chloride-bicarbonate exchanger.
• Solute binding site of the anion exchange protein interacts with different
ions on opposite sides of the membrane.
• Necessary to prevent net charge imbalance (one negative ion in for one
negative ion out).
• The anion exchanger transport Cl- across the membrane by counter-
transport with HCO3- (called the chloride shift).
• Important in the transport of carbon dioxide in the body and for helping to
regulate pH .
20. Direction of oxygen, carbon dioxide, and
bicarbonate transport in erythrocytes
• In the lung, oxygen diffuses from the inhaled air to the cytoplasm of
• In the capillaries, oxygen is release by hemoglobin and diffuses from
the cytoplasm into the blood plasma.
• Carbon dioxide is often transported in the form of bicarbonate ion.
• Carbon dioxide converted to bicarbonate in the capillaries and
released into blood plasma.
• In the lungs, bicarbonate is imported into the cytoplasm and
converted to carbon dioxide.
22. Transporters in plasma membrane regulate
• Most cells have one or more Na+ -driven antiporters in their plasma
membrane that help to maintain the cytosolic pH at ~7.2.
• These transporters use energy stored in the Na+ gradient to pump
out excess H+.
• Two mechanism
• H+ directly transported out by Na+ - H+ exchanger.
• Neutralize H+ in the cytosol with HCO3-.
• Na+– driven Cl--HCO3- exchanger.
23. Channel proteins facilitate diffusion by forming
hydrophilic transmembrane channels
• Three kinds of channel proteins:
• Ion channels: Transmembrane proteins that allow rapid passage of specific
• Porins: Transmembrane proteins that allow rapid passage of various solutes
.Beta barrel transmembrane region creates water-filled pore at its center
• Aquaporins: Transmembrane channels that allow rapid passage of water
• Can facilitate transport at a rate of several billion water molecules per second
• Found in certain tissues such as the proximal tubules of the kidneys that reabsorb
water as part of urine formation.
24. Ion channel proteins
• Ion channel proteins form hydrophilic pores to transport inorganic ions
across the membrane
• They only participate in passive transport (facilitated diffusion) whereby molecules
are transported “downhill” of the concentration and membrane potential; i.e.
• Ion channels are selective and gated (open briefly then close)
• Stimuli that open gates include:
• Mechanical stress (mechanical-gated ion channels)
• Changes in voltage across the cell membrane (voltage-gated ion channels)
• Ligand binding (ligand-gated ion channels)
• One of the most important types of ligand-gated ion channels is the transmitter-gated ion
channels that bind neurotransmitters and mediate ion movement.
• Depending on the ion involved, it may have an excitatory or inhibitory effect.
27. PORINS – pores formed by transmembrane protein
and allow Mol. Wt .upto 600 to diffuse.(ion
29. Cystic Fibrosis
• Molecular basis of lung disease in patients who have cystic fibrosis is
• In healthy individuals, the main epithelial cells lining the airways display at
least two types of channels at the surface facing the air passage. One - the
CFTR channel - releases chloride into the passage; the other (blue) takes up
• This arrangement somehow enables mucus produced by other cells to
remain wet, thin and easy to remove from the airways and so the airways
• In patients with cystic fibrosis, absence or malfunction of the CFTR channel
prevents chloride movement and indirectly causes cells to take up extra