Transport across cell membrane
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The document discusses mechanisms of transport across cell membranes, including passive diffusion, facilitated diffusion, and active transport. It highlights the roles of various channels, transporters, and endo/exocytosis in moving molecules, as well as the impact of gradients and energy expenditure on these processes. Specific examples, such as sodium-potassium ATPase and various types of ion channels, are provided to illustrate these transport mechanisms.
Transport across cell membrane
- 3. Transfer of material and information across cell membrane O Cross-membrane movement of small molecules O Diffusion(passive and facilitated) O Active transport O Cross membrane movement of large molecules O Endocytosis O Exocytosis
- 4. O Signal transmission across membrane O Cell surface receptors O Signal transduction O Signal internalization(coupled with endocytosis)
- 5. Intracellular contact and communication O Passive O Facilitated diffusion O Active transport
- 6. diffusion O Simple diffusion O Facilitated diffusion O Simple diffusion passive flow of a solute from higher to lower concentration. It is limited by 1. Thermal agitation 2. Conc. Gradient 3. solubility
- 7. Factors that affect net diffusion O Conc. gradient across the membrane O Electrical potential O Permeability co-efficient O Hydrostatic pressure gradient O temperature
- 8. O Substances move from a higher chemical concentration to a lower concentration i.e.in response to concentration gradient. O Rate of transport is directly proportional to concentration gradient. O Electrical gradient. O molecules will move towards opposite charges.in renal glomeruli the filtering membrane possesses negative charge and does not permit the negative charge proteins to pass through it.
- 9. Facilitated diffusion Passive transport of a fluid from higher to lower concentration mediated by specific transporter and ion channels. Transporters may be O Specific proteins Transport may be O Uniport O Co transport O symport
- 12. Cont…. Rate at which solutes enter cell by facilitated diffusion O Con.gradient across membrane O Amount of carrier available O Affinity of the solute O Rapidity of conformational changes
- 13. O Hormones regulate the facilitated diffusion by changing the number of transporters available. O Insulin increases glucose transport in fats and muscles by recruiting transporters from an intracellular reservoir.
- 14. Ion-channels O Membranes have special devices called ion channels. Ion channels are transmembrane proteins that allow the selective entry of various ions. O These channels are for quick transport of electrolytes . O These channels usually remain closed but in response to a stimulus they open.
- 15. Some properties of ion channels
- 16. Comparison of transporters and ion channels
- 17. Voltage gated channels O They open or close in response to a change in membrane potential. O Example is sodium channel, potassium channel etc. O These are seen in nerve cells and involved in conduction of nerve impulses.
- 18. Sodium channel O Sodium channel has four subunits. O Each subunit has alpha helical transmembrane domain. O A pore like structure is formed through which ion pass.
- 19. Structure of the ion channel
- 20. K-channel
- 21. Ligand gated channels O They are opened by binding of effectors. O The binding of a ligand to a receptor site on the channels results in opening or closing of channels. O The ligand may be an extracellular signaling molecule or an intracellular messenger. O Examples are acetylcholine receptor or calcium channels etc.
- 22. Ionophores O Microbes synthesize small organic molecules called inophores . O They shuttle for movement of ions across the membrane. O Hydrophilic centre. O Surrounded by peripheral hydrophobic region. O Gramicidin fold-up to form hollow channels. O B barrel by hemolysin O Direct access channel by diphtheria toxin
- 23. Valinomycin-a peptide inophore that binds potassium
- 24. Aquaporins O tetrameric trasmembrane proteins . O Ten distinct aquaporins have been identified . O Crystallography and other studies have revealed how these channels permit the passage of water. But ,they exclude passage of ions and other proteins. O In essence the pores are too narrow to permit the passage of ions and proteins.
- 25. Active transport O Transport of a solute across the membrane against a concentration gradient with the expenditure of energy.
- 26. Na + K+ ATPase O Extracellular fluid has higher conc. Of sodium ions. O Intracellular fluid has higher conc. Of potassium. O Sodium ,potassium pump is an antiport pump. O Energy is provided by ATP.
- 28. H+ K+ ATPase O They are also called proton pumps. O They exchange hydrogen for potassium. O They are present in endosomes, lysosomes, mitochondria and plasma membrane of certain cells.
- 29. Plasma membrane calcium pumps O They move calcium from cytosol into the extracellular fluid. By decreasing calcium conc. Within cytosol it prevents formation of relatively insoluble calcium phosphate which could hinder cytosolic function.
- 30. Endoplasmic reticulum calcium pumps O They move calcium from cytosol into the endoplasmic reticulum.
- 31. Secondary active transport O Molecules are transported into cells by active transport property of sodium potassium ATPase which keeps the cell interior deficient in sodium ions. O Plasma membrane is negatively charged on its interior. O Chemical and electrical gradient favour the movement of sodium ions into the cell interior.
- 32. O Special transporter proteins acting as symporter bring about by facilitated diffusion a simultaneous movement of certain solutes along with sodium ion into the cell interior. O Later the sodium ions are pumped out of the cell by sodium potassium ATPase to lower its concentration within cell to help further movement of sodium along with its associated solute into the cell .
- 33. O Thus the movement of solute into the cell depend upon the active transport property of sodium potassium ATPase. O For this reason this transport is secondary to the movement of sodium ions.
- 34. Renal tubules O Action of sodium potassium ATPase causes a low content of sodium into the cell interior. O Movement of sodium ions into cell interior along its chemical and concentration gradient in association with glucose molecule is brought about by symporter namely sodium dependent glucose transporter. O Dissociation of sodium and glucose from their symporter in cell membrane.
- 35. O Three sodium ions are moved into the extracellular fluid in exchange for two potassium by sodium potassium ATPase. Low sodium content of cell is maintained. O This allows further movement of more sodium ions into the cell from tubular lumen along with glucose. O The glucose molecule that has moved into the cell is transported across the outer cell membrane into the extracellular fluid by facilitated diffusion aided by glucose transporter-2.
- 38. Ion channels O Ion channels are made up of transmembrane subunits that come together to form a central pore through which ions pass.
- 39. Na+ channels O The opening of these channels causes Na+ influx. O Generation of action potential. O Quinidine O It blocks the sodium channel preventing rapid rise in action potential in cardiac muscle cells. O It is used to treat cardiac arrythmias.
- 40. O Amiloride O Blocks Na+ channel present in apical membrane of cells lining renal tubules. O There will be excretion of more Na+ in urine. O There will be excretion of more water in the urine. O Lowering of B.P. O Used to treat hypertension
- 41. K+-channel O These channels on opening bring K+ efflux producing repolarization of excitable tissues; neuron and muscle cells. O Minoxidil O By opening k+ channels causes relaxation of smooth muscles of arterioles leading to their dilation. This decrease in peripheral vascular resistance lowers the arterial B.P.
- 42. Chloride channel O Entry of chloride in cell interior will produce hyper polarization of cell membrane. O Gamma aminobutyric acid acts as an inhibitory neurotransmitter by opening of chloride channels. O This action of GABA is much increased by drugs of benzodiazepam family.
- 43. Calcium channel blockers O The cardiac muscles and most type of smooth muscle are dependent on transmembrane calcium influx for normal tone and contractile response. O In treatment of certain heart diseases and hypertension ,a reduction in contractility of cardiac muscle fiber and relaxation of smooth muscle in arteriolar wall is aimed at. O Calcium channel blocker drugs are used in treating angina pectoris and hypertension.
- 44. Transport of macromolecule cells transport certain macromolecules across the plasma membrane by following mechanism O Endocytosis O Exocytosis
- 45. O Segment of plasma membrane invaginates enclosing small volume of extracellular fluid and its contents. O Fusion of endocytotic vesicle and primary lysosomes . O Formation of secondary lysosome. O Macromolecules contents are digested to yield O Amino acids O Simple sugars O nucleotide
- 46. O Endocytosis requires energy ,usually from hydrolysis of ATP. O Phagocytosis occurs in specialized cells such as Macrophages and granulocytes.
- 47. Types of endocytosis O Phagocytosis O Pinocytosis. O Pinocytosis is again of following types O Fluid-phase pinocytosis O Absorptive pinocytosis
- 48. endocytosis O Phagocytosis occurs in specialized cells such as macrophages and granulocytes.
- 49. pinocytosis O Cell drinking O Leads to cellular uptake of fluid and fluid contents.
- 50. It is a non-selective process in which the uptake of solute by formation of small vesicle is proportionate to its conc. In extracellular fluid. The formation of these vacuoles is extremely active process. It is receptor mediated endocytosis. It is responsible for uptake of macromolecules for which there are binding sites on plasma membrane.
- 51. Fibroblast ,for example internalize their plasma membrane at about 1/3 rate of macrophages .this process occurs more rapidly than membranes are made. The surface area and volume of cell do not change much, so membranes must be replaced by exocytose. High affinity receptors permit the selective conc. Of ligand from the medium. It will minimize the uptake of fluid or soluble unbound macro molecules. It will markedly increase the rate at which specific molecules enter the cell. Vesicles Coated pits It may be protein clatherin in the filamentous material .
- 52. Fluid phase pinocytosis O It is a non-selective process in which the uptake of solute in the form of small vesicles is proportionate to its concentration in the extracellular fluid. O Formation of these vesicles is extremely active process. O Fibroblasts internalize their membrane at about one third rate of macrophages . O This process occurs rapidly than membranes are made.
- 53. O The surface area and volume of cell shouldn't change.so,membranes must be replaced by exocytosis.
- 54. Absorptive pinocytosis O It is also called receptor mediated pinocytosis. O It is responsible for the uptake of specific macromolecules for which there are binding sites on the plasma membrane. O Vesicles derived are formed by invagination of that are coated on the cytoplasmic side with a filamentous material and named as coated pits. O In many situations the protein clathrin is the filamentous material. O It has three limbed structure.
- 55. O PIP2 plays important role in vesicle assembly. O Protein dynamin is necessary for pinching off clathrin coated vesicle
- 56. Exocytosis O Exocytosis releases certain macromolecules from the cell. O Signal is a hormone O Local and transient changes in calcium concentration. O Calcium triggers exocytosis. O Molecules releasd have three facts. O They are membrane proteins and associated with cell surface. O They can become part of extracellular matrix ,collagen and Gag,s. O They can enter extracellular fluid and signal other cells.
- 58. O Low density lipoprotein molecule and its receptor are internalized by means of coated pits containing LDL receptor.
- 60. Exocytosis O Most cells release macromolecules by exocytosis. O This process is involved in membrane remodeling. O Components synthesized in ER and Golgi are carried in vesicles that fuse with plasma membrane. O The signal for exocytosis is often a hormone. O It brings a local change in calcium concentration. O Calcium triggers exocytosis. O Molecules released by exocytosis has three facts. O They are membrane proteins and remain associated with the cell surface.
- 61. O They can become part of extracellular matrix. O They can enter extracellular fluid and signal other cells. O Insulin ,parathyroid and catecholamine are all packaged in granules processed within the cell.