3 Membrane Proteins

  • Created by: danielle
  • Created on: 21-01-19 01:01
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  • 3 Membrane Proteins
    • a) Movement of molecules across membranes
      • molecules need to pass across the membrane
      • TRANSMEMBRANE PROTEIN controls the CONCENTRATION OF IONS & other molecules
      • To perform specialised functions, different cell types and different cell compartments have different channel and transporter proteins
        • a. Channel Proteins- allow molecules to move across membrane WITHOUT ENERGY- Passive Transport
          • Water can pass across the plasma membrane by diffusing through the phospholipid bilayer or through water channels which are called AQUAPORINS.
            • The direction of water movement is dependent upon the osmotic gradient
        • b. Gated channel Proteins
          • Some channels are GATED & change CONFORMATION to ALLOW/PREVENT diffusion
            • Gated channels respond to a stimulus which causes them to open or close. The stimulus may be chemical (ligand-gated) or electrical (voltage-gated).
              • b. Ligand gated channels are controlled by signal molecules [DIAGRAM]
              • c. Voltage gated channels are controlled by changes in ion concentration [DIAGRAM]
            • d. Transporter Protein
              • Change CONFORMATION to transport molecules across a membrane. They work by one of two mechanisms.
                • 2) Facilitated- is a PASSIVE process. It is not passive transport as the channel proteins in the facilitates transport involves a conformational change in a protein. Glucose moves acccorss the membrane by FACILITATED TRANSPORT
                • 1) ACTIVE- requires energy, brings about a conformational change from HYDROLYSIS of ATP e.g. NA/K-ATPase[DIAGRAM]
    • b) Signal transduction
      • Some proteins in the plasma membrane act as receptors. Receptor proteins convert an extracellular chemical signal to a speci?c intracellular response through a signal transduction pathway
        • Signal transduction may result in the activation of an enzyme or G-PROTEIN. An activated enzyme will bring about a specific chemical reaction within the cell.
          • G-proteins are involved in transmitting signals from outside the cell into the cell. G-proteins can be 'switched on' or activated by binding GTP; they can also be 'switched off' or deactivated by binding GDP.
      • Signal Transduction may  a change in the uptake or secretion of a molecule
        • For example, upon binding insulin, a cell will increase its glucose uptake. Signal transduction may also result in rearrangement of the cytoskeleton
          • ad may result in the activation of protein s that regulate gene transcription as shown in [DIAGRAM]
    • c) Ion Transport Pumps
      • a) The transporter protein has high af?nity for sodium ions inside the cell therefore binding occurs.b) Phosphorylation by ATP causes the conformation of the protein to change.c) The af?nity for ions changes resulting in sodium being released outside of the cell.d) Potassium ions from outside the cell bind to the sodium-potassium pump.e) Dephosphorylation occurs which causes the conformation of the protein to change.f) Potassium ions are taken into the cell and the af?nity returns to the start.
    • d) Ion Channels & Nerve Transmission
      • all cells have an ELECTRICAL POTENTIAL IFF. (V) across their membrane.
        • caused by VARYING CONCENTRATIONS of CHARGED MOLECULE; Membrane Potential
          • The membrane potential of a NEURON that is not transmitting signals is called the RESTING POTENTIAL
    • e) Nerve Transmission
      • Nerve transmission is a wave of depolarisation of the resting potential of a neuron.
        • This can be stimulated when an appropriate signal molecule, such as a neurotransmitter, triggers the opening of ligand-gated ion channels.
          • This then triggers depolarisation ->resting potential goes UP if the change is large enough, triggers ACTION potential.
            • LOOK AT DIAGRAM


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