Working With and Against Gradients

Working With and Against Gradients

• Polar molecules and ions cannot diffuse across a lipid bilayer
    * They require the help of transport proteins
• Many solutes cross the membrane by diffusion through a channel or tunnel inside transport proteins
    * As a solute molecule or ion enters the channel the protein’s shape changes
    * The channel closes behind the solute and opens in the front
    * When the solute leaves the channel, the protein goes back to its original shape
• Passive transport: a concentration gradient, electric gradient, or both drive diffusion of a substance across a cell membrane, through a channel inside a transport protein
    * Does not require energy input to assist directional movement (known as facilitated diffusion)
• Some passive transports are open channels and some are closed with molecular gates that close and open depending on the conditions
    * Example: Glucose transporters - when one end of the transporter shut the other opens and glucose can diffuse in
• The net movement of a solute through passive transporters tends to be toward the side of the membrane (where the solute is less concentrated)
    * This occurs because molecules and ions collide with transporters more with the membrane where it is more concentrated
• If nothing else is happening, passive transport will continue until the concentration of the membrane is equal on both sides
    * This equilibrium rarely happens in a living system
    * Example: Glucose diffuses through glucose transporters, but cells tend to use it up fast as they get it
      * A cell’s use of glucose helps maintain a gradient that favors the uptake of more glucose
    * Solute concentrations shift constantly in the cytoplasm and extracellular fluid
      * Maintaining concentration at a level necessary for the metabolic reaction to proceed often means moving the solute against its gradient to the side of a membrane where it is more concentrated (does not occur without energy input - ATP)
    * Active transport: a transport protein uses energy to pump a solute across a cell membrane, against its gradient
    * Only specific solutes can bind to the interior channel of an active transport
      * Energy changes the transporter’s shape
      * What allows the transporter to release the solute to the other side of the membrane
    * Example: Calcium pumps move calcium ions across muscle cell membranes
      * Allows calcium ions to escape from special locations
      * Calcium pumps can keep the concentrations of calcium in that compartment 1000 to 10000 times higher than it is in the muscle cell cytoplasm
    * Sodium-potassium pumps (cotransporter) - move two substances at the same time
      * Almost all body cells have these pumps that maintain gradients by pumping sodium and potassium ions in opposite directions across the membrane
      * Sodium Ions in the cytoplasm diffuse into the pump’s open and bind to its interior
        * Pumps change shape after receiving ATP
      * Potassium ions from the extracellular fluid diffuse into the channel and bind to the interior
        * Releases phosphate group and goes back to original shape
      * The channel opens to the cytoplasm, and the potassium ion is released there
    * Membranes of all cells have membrane pumps