Cell membrane (Transport and structure).pptx

cell membrane transport

1. Transport across membrane

2. Plasma membrane is selectively permeable Plasma membrane is selectively permeable: Water molecules move much more rapidly through a cell membrane than do dissolved ions or small polar organic solutes, which are essentially non-penetrating. Because of this difference in the permeability of water versus solutes, membranes are said to be semipermeable Plasma membrane having hydrophobic core forms a barrier that prevent the loss of polar solutes and ions from a cell By preventing the unimpeded movement of molecules and ions into and out of the cell, the plasma membrane maintains essential differences between the composition of the extracellular fluid and that of the cytosol For example the conc. of Na + in extracellular fluid is around 150 mM while around 15 mM inside cytosol (reverse is for K + conc.) H + conc. inside lysosome is about 100 fold higher than in cytosol

3. Relative permeability of a pure phospholipid bilayer If cellular membranes were pure phospholipid bilayers , they would be excellent chemical barriers, impermeable to virtually all ions, amino acids, sugars and other water-soluble molecules

4. Transport through membrane However, provisions for the movement of substances through membranes are there Basically two means of transport: Passive transport by diffusion and active transport The transport may be a thermodynamically favourable (downhill) or unfavourable (uphill) process. Uphill transport must be coupled with energy (from hydrolysis of ATP) or another thermodynamically favourable process

5. Types of Transport Downhill Transport Passive Transport TRANSPORT Uphill Transport Active Transport Membrane protein involved Facilitated Diffusion No membrane protein involved Simple Diffusion Transport coupled with ATP hydrolysis Primary Active Transport Transport coupled with existing ion gradient Secondary Active Transport Carrier protein mediated Channel protein mediated

6. Passive Transport Downhill transport Two types: Simple diffusion or facilitated diffusion Transportation of: Gases (such as O 2 and CO 2 ), hydrophobic molecules (such as benzene), small polar but uncharged molecules (such as H 2 O and ethanol), large polar but uncharged molecules (such as glucose, amino acids, nucleosides), ions (such as Na + , K + , Ca ++ )

7. Simple Diffusion Nonselective process of transport through lipid bilayer No transport proteins are required No metabolic energy is required The net flow of molecules is always down their concentration gradient Molecule simply dissolves in the phospholipid bilayer……>diffuses across it…….>then dissolves in the aqueous solution at the other side of the membrane…….>equilibrium attained The diffusion rate is proportional to its concentration gradient across the bilayer and to its hydrophobicity (measured by partition coefficient, K) and size Examples of passively transported molecules: Gases (such as O 2 and CO 2 ), non-polar molecules (such as benzene), and small polar but uncharged molecules (such as H 2 O and ethanol)

8. Hydrophobicity/Partition Coefficient A partition coefficient (P) is the ratio of concentrations  of a compound in a mixture of two immiscible solvents at  equilibrium . This ratio is therefore a comparison of the solubility of the solute in these two liquids. However the solvents can be gases such as air, liquids such as water or olive oil or complex mixtures such as blood or other tissues.

9. Facilitated Diffusion (Transport) It is a selective process , i.e., the membrane allows only selective molecules and ions to pass through it preventing other molecules from passing through the membrane . Utilized by molecules that are unable to freely cross the phospholipid bilayer  Transported molecule do not dissolve in the phospholipid bilayer The net flow of molecules is always down their concentration gradient (for uncharged molecules) or concentration and electrical gradient (for charged molecules) Channel proteins and carrier proteins are involved in the process Examples of molecules transported: Large polar molecules (such as glucose, amino acids, nucleosides), ions (such as Na + , K + , Ca ++ )

10. Channel proteins and Carrier proteins

11. Ion channels Integral lipoproteins which contain a pore via which uncharged polar molecules or ions may cross from one side of the membrane to the other Channel proteins are ion-selective. Some are always open and others may be gated to regulate the passage of ions in response to certain stimuli Channel proteins have a much faster rate of transport than carrier proteins Carrier Proteins Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second

12. Difference between channel proteins and carrier proteins Ion channels Carrier proteins Form open pores. Do not bind with solute molecules to be transported. Bind specific molecules to be transported. Carrier proteins u ndergo conformational changes upon binding Either always open or gated channels ( ie ., channels opened or closed by gates) Solute molecules bind to one side and released from the other side of the protein Transport is highly selective. Only molecule of appropriate size and charge is transported Polar uncharged molecules (sugars, amino acids, nucleosides) are transported Rapid transport (million ions per second) Slow transport (1000 times slower) Lipoproteins Glycoproteins Aquaporin , H + , Cl - , Na + , K + , Ca 2+ ion channels Glucose transporter

13. Facilitated diffusion: Aquaporins Channels A type of channel protein Rapid passage of water Osmosis: Osmosis is the net movement of water molecules across a selectively permeable membrane from an area with a high concentration of water molecules (low concentration of solute molecules) to an area of low concentration of water molecules ( high concentration of solute molecules ) Transport up the stem, transpiration in leaves, water balance in brain, excretion of sweat Aquaporins are impermeable to ions

14. Facilitated diffusion: Ion Channel Ion channels are membrane proteins that allow ions to travel into or out of a cell Leakage ion channel: The simplest type of ion channels are with more or less constant permeability because they are always open . The types of leakage channels with the greatest significance in neurons are potassium (K + ) and chloride channels . Potassium channels set the negative membrane potential (resting potential; -60mV) of neuron 3 types of gated ion channels . V oltage-sensitive , ligand-gated, or mechanically-gated in nature. Voltage-gated ion channels: Open and close in response to changes in membrane potential (voltage) across the membrane . Charge difference across membrane is triggered by changing ion concentration . Each type has particular ion selectivity for  (Na + ),  ( K + ),  ( Ca 2 + ),  (Cl − ) and a particular voltage dependence . Voltage-gated sodium ion channels contribute to the “spike” of a neuron’s action potential. Ligand-gated ion channels: Open when a chemical ligand such as a neurotransmitter binds to the protein . This type of ion channels function as neurotransmitter receptors occurring at postsynaptic sites and the chemical ligand that gates them is released by the presynaptic axon terminal. Mechanically-gated ion channels: O pen in response to physical deformation of the receptor , as in sensory receptors of touch and pressure

15. Ion Channels

16. Potassium Ion Channels The potassium ion channel is formed from the association of four identical subunits, one of which is shown S4 is voltage sensor Loop formed by S5 and S6 form the selectivity filter Selective for potassium ion, restrict sodium ion

17. Ligand-gated Ion Channels

18. Facilitated diffusion: Glucose Transporter A type of carrier protein 5% of total membrane proteins in human RBC  While the exact three-dimensional structure is not known, the binding of glucose probably causes a conformational change that makes the binding site face the interior of the cell. When glucose is released into the cell, the transporter returns to its original conformation Simulate enzyme-substrate reaction Net inward transport of glucose in RBC Outward excretion of synthesized glucose from liver cells (Reverse transport)

19. Active transport Active transport involves the movement of materials against a concentration gradient and requires an expenditure of energy Among the substances actively transported are ions (for example Na +, K+, H+, Ca2+, and Cl-), amino acids, and simple sugars. Although these substances can also be moved into cells by passive processes, their movement by active processes can go against the concentration gradient, allowing a cell to accumulate needed materials . Like facilitated diffusion, active transport depends on transporter proteins in the plasma membrane Ion pumps are not ion channels, but are critical membrane proteins that carry out active transport by using cellular energy (ATP) to “pump” the ions against their concentration gradient. This energy may be harnessed by one of two means : Primary active transport: The direct hydrolysis of ATP S econdary active transport: By coupling with the transport of another molecule moving along its electrochemical gradient

20. The coupled transport of two distinct molecules is called co-transport (the movement of a single molecule is called uniport ) If the two molecules are transported in the same direction it is called symport If the two molecules are transported in opposite directions it is called antiport The sodium-potassium pump is an example of an antiporter as sodium and potassium are pumped in opposite directions This is primary active transport as both molecules are pumped against their gradient and require ATP hydrolysis Glucose uptake in the kidneys is an example of symport as its movement is coupled to the parallel transport of sodium This is secondary active transport as the sodium is moving passively down an electrochemical gradient