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Transport system - Active transport, Passive transport

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The document explains the transport mechanisms across the plasma membrane, highlighting its semi-permeable nature and the categories of transport which include passive transport, osmosis, active transport, and vesicular transport. It details different types of passive transport such as simple and facilitated diffusion, and explains active transport mechanisms driven by ATP, including primary and secondary active transport. The document also discusses the roles of carrier proteins and pumps in maintaining ion gradients essential for cellular functions.

  • membrane transport
  • active transport
  • passive transport
  • diffusion
  • osmosis

By Dr. Ayesha Fatima

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  1. Slide 1. PLASMA MEMBRANE : TRANSPORT SYSTEM Presented by: Dr. Ayesha Fatima Assistant Professor
  2. Slide 2. Transport across Cell Membrane • Plasma membrane acts as a barrier to most, but not all molecules. • Plasma membrane is a semi-permeable barrier separating the inner cellular environment from the outer cellular environment. • Since the plasma membrane is made up of a lipid bilayer with proteins attached on the surface allowing substances to pass across it's membrane, possibility of transport across this membrane. • All lipid soluble substances can easily and freely diffuse in and out, e.g. O 2 and CO 2 . Water soluble substances like ions, glucose and macromolecules should find a special way of transport with the help of integral and trans-membrane proteins which act as binding sites, channels and gates to facilitate movement. • Transport across cell membrane is classified into four ways: 1. Passive Transport 2. Osmosis 3. Active Transport 4. Vesicular Transport.
  3. Slide 4. Passive Transport : Diffusion It is the net movement of a substance (liquid or gas) from an area of higher concentration to lower concentration without expenditure of energy is called diffusion. Diffusion can be further divided as follows: A. Simple diffusion B. Facilitated diffusion. A. Simple Diffusion: It is further classified into two categories: i. Diffusion of lipid soluble substance through lipid bilayer. ii. Diffusion of lipid insoluble substance through protein channels. i. Diffusion of Lipid Soluble Substance through the Lipid Bilayer: Substance like oxygen and carbon dioxide and alcohols are highly lipid soluble and dissolve in the layer easily and diffuse through the membrane. The rate of diffusion is determined by the solubility of the substance. For example, exchange of gases in the lungs. ii. Diffusion of Lipid Insoluble Substance through Protein Channels: This is possible through either selective permeability of protein channel or through gated channels
  4. Slide 6. Facilitated diffusion : Two types, Both differs on the type of protein used. 1. Channel mediated 2. Carrier mediated
  5. Slide 7. CHANNEL-MEDIATED FACILITATED DIFFUSION • In channel-mediated facilitated diffusion , a solute moves down its concentration gradient across the lipid bilayer through a membrane channel. • Membrane channels are ion channels, integral transmembrane proteins that allow passage of small, inorganic ions that are too hydrophilic to penetrate the nonpolar interior of the lipid bilayer. • Each ion can diffuse across the membrane only at certain sites. • In typical plasma membranes, the most numerous ion channels are selective for K+ ions or Cl− ions; fewer channels are available for Na+ ions or Ca2+ ions. • Diffusion of ions through channels is generally slower than free diffusion through the lipid bilayer because channels occupy a smaller fraction of the membrane’s total surface area than lipids. • A channel is said to be gated when part of the channel protein acts as a “plug” or “gate,” changing shape in one way to open the pore and in another way to close it. Gated channels randomly alternate between the open and closed positions; r egulated by chemical or electrical changes inside and outside the cell. • When the gates of a channel are open, ions diffuse into or out of cells, down their electrochemical gradients.
  6. Slide 8. CARRIER-MEDIATED FACILITATED DIFFUSION • In carrier-mediated facilitated diffusion , a carrier (also called a transporter ) moves a solute down its concentration gradient across the plasma membrane • The solute binds to a specific carrier on one side of the membrane and is released on the other side after the carrier undergoes a change in shape. • The solute binds more often to the carrier on the side of the membrane with a higher concentration of solute. • Once the concentration is the same on both sides of the membrane, solute molecules bind to the carrier on the cytosolic side and move out to the extracellular fluid as rapidly as they bind to the carrier on the extracellular side and move into the cytosol. • The number of carriers available in a plasma membrane places an upper limit, called the transport maximum, on the rate at which facilitated diffusion can occur. • Once all of the carriers are occupied, the transport maximum is reached, and a further increase in the concentration gradient does not increase the rate of facilitated diffusion, the process of carrier-mediated facilitated diffusion exhibits saturation . • Substances that move across the plasma membrane by carrier mediated facilitated diffusion include glucose, fructose, galactose, and some vitamins
  7. Slide 9. OSMOSIS • Osmosis is a type of diffusion in which there is net movement of a solvent through a selectively permeable membrane. It is a passive process • In osmosis, water moves through a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration . Or from an area of higher water concentration to lower water concentration. • During osmosis, water molecules pass through a plasma membrane in two ways: 1. by moving between neighboring phospholipid molecules in the lipid bilayer via simple diffusion, 2. by moving through aquaporins or AQPs , integral membrane proteins that function as water channels. • Different types of AQPs have been found in different cells and tissues throughout the body. AQPs are responsible for the production of cerebrospinal fluid, aqueous humor, tears, sweat, saliva, and the concentration of urine.
  8. Slide 10. • Osmosis occurs only when a membrane is permeable to water but is not permeable to certain solutes. • Diffusion of water across a membrane generates a pressure called osmotic pressure. If the pressure in the compartment into which water is flowing is raised to the equivalent of the osmotic pressure, movement of water will stop. This pressure is often called hydrostatic ('water-stopping') pressure • The amount of pressure needed to restore the startingcondition equals the osmotic pressure. Hydrostatic pressure is the pressure exerted by a fluid at rest due to gravity
  9. Slide 11. • The osmotic pressure of the cytosol is the same as the osmotic pressure of the interstitial fluid outside cells. Because the osmotic pressure on both sides of the plasma membrane is the same, cell volume remains relatively constant. • When body cells are placed in a solution having a different osmotic pressure than cytosol, however, the shape and volume of the cells change. • As water moves by osmosis into or out of the cells, their volume increases or decreases • A solution’s tonicity is a measure of the solution’s ability to change the volume of cells by altering their water content.
  10. Slide 12. • Isotonic: The solutions being compared have equal concentration of solutes. • When the cell is placed in isotonic solution, the water diffuses in and out of the cell in same rate. • Ex. RBC placed in 0.9% NaCl solution, RBC maintain its cell shape and volume. • Hypertonic: The solution with the higher concentration of solutes. • When cell is placed in hypertonic solution, water diffuses out of the cell causing the cell to shrink. Such shrinkage is called as Crenation. • Ex. RBC placed in 2% NaCl solution, RBC Shrink • Hypotonic: The solution with the lower concentration of solutes. • When cell is placed in hypotonic solution, the water diffuses into the cell causing it to swell and possible explode. Explosion/rupture by this process called as lysis. • Ex. RBC placed in 0.45% NaCl solution, RBC Swell and rupture called as hemolysis.
  11. Slide 14. Active Transport • Some polar or charged solutes that must enter or leave body cells cannot cross the plasma membrane through any form of passive transport because they would need to move “uphill,” against their concentration gradients. Such solutes may be able to cross the membrane by a process called active transport . • Active transport is an active process because energy is required for carrier proteins to move solutes across the membrane against a concentration gradient. • Two sources of cellular energy can be used to drive active transport: (1) Energy obtained from hydrolysis of ATP is the source in primary active transport; (2) energy stored in an ionic concentration gradient is the source in secondary active transport. • Active transport processes transport maximum and saturation. • Solutes actively transported across the plasma membrane include several ions, such as Na+, K+, H+, Ca2+, I− (iodide ions), and Cl−; amino acids; and monosaccharides
  12. Slide 15. • Small substances constantly pass through plasma membranes. • Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. • Active transport mechanisms depend on a cell’s metabolism for energy. • Two mechanisms exist for the transport of small-molecular weight material and small molecules. • Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. • Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP.
  13. Slide 16. Carrier Proteins for Active Transport • An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three types of these proteins or transporters . • A Uniporter carries one specific ion or molecule. • A Symporter carries two different ions or molecules, both in the same direction. • An Antiporter also carries two different ions or molecules, but in different directions. • All of these transporters can also transport small, uncharged organic molecules like glucose. • Some examples of pumps for active transport are Na + -K + ATPase , which carries sodium and potassium ions, and H + -K + ATPase , which carries hydrogen and potassium ions. • Both of these are antiporter carrier proteins. • Two other carrier proteins (both are pumps) are Ca 2+ ATPase and H + ATPase, which carry only calcium and only hydrogen ions, respectively.
  14. Slide 17. A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions.
  15. Slide 18. PRIMARY ACTIVE TRANSPORT • In primary active transport, the energy is derived directly from the breakdown of ATP. Ion-pumps may be of two types: (i) Electro neutral pumps (ii) Electro genic pumps . 1. Electro neutral pumps are those which are associated with transport of ions with no net movement of charge across the membrane. • For example H + /K + -ATPase of some animal cells, pumps out one H + for each K + taken in with no net movement of charge. Therefore, it is an electro neutral pump. 2. Electro genic pumps on the other hand, transport ions involving net movement of charge across the membrane. • One of the most important pumps is the sodium-potassium pump (Na + -K + ATPase), which maintains the electrochemical gradient in living cells. • The sodium-potassium pump moves K + into the cell while moving Na + out at the same time, at a ratio of three Na + for every two K + ions moved in.
  16. Slide 19. Electrogenic pumps:- Na + /K + – ATPase • The most prevalent primary active transport mechanism expels sodium ions (Na+) from cells and brings potassium ions (K+) in. Because of the specific ions it moves, this carrier is called the sodium–potassium pump . Because a part of the sodium– potassium pump acts as an ATPase, an enzyme that hydrolyzes ATP, another name for this pump is Na + –K + ATPase . • These sodium– potassium pumps maintain a low concentration of Na+ in the cytosol, by pumping these ions into the extracellular fluid against the Na+ concentration gradient. • At the same time, the pumps move K+ into cells against the K+ concentration gradient. Because K+ and Na+ slowly leak back across the plasma membrane down their electrochemical gradients—through passive transport or secondary active transport—the sodium–potassium pumps must work nonstop to maintain a low concentration of Na+ and a high concentration of K+ in the cytosol.
  17. Slide 20. 1. Three Na+ in the cytosol bind to the pump protein. 2. Binding of Na+ triggers the hydrolysis of ATP into ADP, a reaction that also attaches a phosphate group P to the pump protein. This chemical reaction changes the shape of the pump protein, expelling the three Na+ into the extracellular fluid. Now the shape of the pump protein favors binding of two K+ in the extracellular fluid to the pump protein. 3. The binding of K+ triggers release of the phosphate group from the pump protein. This reaction again causes the shape of the pump protein to change. 4. As the pump protein reverts to its original shape, it releases K+ into the cytosol. At this point, the pump is again ready to bind three Na+, and the cycle repeats.
  18. Slide 22. Secondary active transport • In secondary active transport , the energy stored in a Na+ or H+ concentration gradient is used to drive other substances across the membrane against their own concentration gradients. • Because a Na+ or H+ gradient is established by primary active transport, secondary active transport indirectly uses energy obtained from the hydrolysis of ATP. • The sodium–potassium pump maintains concentration gradient of Na+ across the plasma membrane. As a result, the sodium ions have stored energy. • some of the stored energy converted to kinetic energy and used to transport other substances against their concentration gradients. • In secondary active transport, a carrier protein simultaneously binds to Na+ and another substance and then changes its shape so that both substances cross the membrane at the same time. • If these transporters move two substances in the same direction they are called symporters ; antiporters , by contrast, move two substances in opposite directions across the membrane
  19. Slide 23. • Plasma membranes contain several antiporters and symporters that are powered by the Na+ gradient. • For instance, the concentration of calcium ions (Ca2+) is low in the cytosol because Na+–Ca2+ antiporters eject calcium ions. • Likewise, Na+–H+ antiporters help regulate the cytosol’s pH by expelling excess H+. • By contrast, dietary glucose and amino acids are absorbed into cells that line the small intestine by Na+–glucose and Na+–amino acid symporters . • In each case, sodium ions are moving down their concentration gradient while the other solutes move “uphill,” against their concentration gradients.
  20. Slide 24. THANK YOU THANK YOU

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