Transport across membranes passive transport

TRANSPORT OF SIGNALS THROUGH PASSIVE MODE.SIMPLE DIFFUSION AND FACILITATED DIFFUSION

1. Transport across membranes - Passive transport Presented by BALAJI .S CAU/CPGS/MBB/M18/05 PLANT MOLECULAR BIOLOGY AND BIOTECHNOLOGY

2. Cell membrane is an amphipathic molecule

3. Membrane is a lipid bilayer with proteins embedded in it

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 other. Cells need a higher concentration of solutes within the cell than found typically outside the cell in order for many reactions to occur at reasonable rates. 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.

5. RELATIVE PERMEABILITY OF LIPID BILAYER TO DIFFERENT BIOMOLECULES

6. MOLECULES MOVE ACROSS MEMBRANES BY PASSIVE TRANSPORT ACTIVE TRANSPORT

7. Simple diffusion-unassisted movement down the gradient Diffusion always moves solutes toward equilibrium. Diffusion always proceeds from regions of higher concentration to lower concentration. 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.

8. 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 larger ones. For example: Glucose is too large. Solute polarity Permeable to nonpolar, more impermeable to polar. Ion Permeability. Membranes are impermeable to ions. Small nonpolar molecules: oxygen, carbon dioxide and ethanol can easily diffuse across membrane.

9. OSMOSIS Water diffuses from where the solute concentration is lower, across a differentially-permeable membrane, to where the solute concentration is higher.

10. 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 the cell. Isotonic : a solution with an equal solute concentration as that inside the cell. Hypotonic : a solution with a lower solute concentration than inside the cell.

11. FACILITATED DIFFUSION Facilitated diffusion does not require input of energy. Two main classes of proteins involved with facilitated diffusion : Carriers and channels. Carrier proteins and channel proteins facilitate transport by different mechanisms. 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 Porins Aquaporins

12. 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 kinetics. 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.

13. 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 c oncentration in the blood plasma. This step is performed by the GLUT permeases . Members of the family include:

14. The erythrocyte anion exchange protein: An antiport carrier 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 HCO 3 - ( called the chloride shift). Important in the transport of carbon dioxide in the body and for helping to regulate pH .

15. Direction of oxygen, carbon dioxide, and bicarbonate transport in erythrocytes In the lung, oxygen diffuses from the inhaled air to the cytoplasm of the cell. 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.

16. Transporters in plasma membrane regulate cytosolic pH 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- - HCO 3 - exchanger.

17. Channel proteins facilitate diffusion by forming hydrophilic transmembrane channels Three kinds of channel proteins: Ion channels: Transmembrane proteins that allow rapid passage of specific ions Voltage-gated Ligand-gated Mechanosensitive 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.

18. 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. electrochemical gradient. 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.

19. PORINS – pores formed by transmembrane protein and allow Mol. Wt . upto 600 to diffuse.(ion channel)

20. Cystic Fibrosis Molecular basis of lung disease in patients who have cystic fibrosis is complex. 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 sodium. This arrangement somehow enables mucus produced by other cells to remain wet, thin and easy to remove from the airways and so the airways remain open . In patients with cystic fibrosis , absence or malfunction of the CFTR channel prevents chloride movement and indirectly causes cells to take up extra sodium.

21. Voltage gated ion channel