Membrane Proteins Unit 1

• Phosphate groups


  Addition or removal can affect a protein
• Protein kinases


  Catalyse the transfer of a phosphate group to other proteins
• Protein phosphatases


  Catalyse the reverse reaction of removing a phosphate group
• Phosphorylation


  Brings about conformational changes, which can affect a protein's activity (some activated, some inhibited)
• Cellular proteins regulated by phosphorylation


  • Enzymes
  • Receptors
• Fluid Mosaic model


  A model that describes the structure of the cell membrane
• Components of the plasma membrane

  • Integral glycoprotein
  • Peripheral glycoprotein
  • Carbohydrate
  • Membrane channel
  • Peripheral protein
  • Integral protein
• Hydrophobic regions


  Allow strong hydrophobic interactions that hold integral membrane proteins within the phospholipid bilayer
• Transmembrane proteins


  Integral membrane proteins that span the entirety of the cell membrane
• Peripheral membrane proteins


  Have hydrophilic R groups on their surface and are bound to the surface of membranes, mainly by ionic and hydrogen bond interactions
• The phospholipid bilayer is a barrier to ions and most uncharged polar molecules
• Simple diffusion


  Small molecules like oxygen and carbon dioxide pass through the bilayer by simple diffusion
• Channel proteins


  Multi-subunit proteins with the subunits arranged to form water-filled pores that extend across the membrane
• Facilitated diffusion


  Passive transport of substances across the membrane through specific transmembrane proteins
• Different cell types have different channel and transporter proteins to perform specialised functions
• Highly selective


  Most channel proteins in animal and plant cells let very few types of molecules pass through by changing conformation
• Types of gated channels

  • Ligand-gated channels
  • Voltage-gated channels
• Transporter proteins


  Bind to the specific substance to be transported and undergo a conformational change to transfer the solute across the membrane
• Transporter proteins


  Alternate between two conformations so that the binding site for a solute is sequentially exposed on one side of the bilayer, then the other
• Active transport


  Uses pump proteins that transfer substances across the membrane against their concentration gradient
• Pump proteins


  Transporter proteins coupled to an energy source
• ATPases


  Enzymes that hydrolyse ATP to provide the energy for the conformational change required to move substances across the membrane
• Electrochemical gradient


  Combination of concentration gradient and electrical potential difference that determines the transport of a solute carrying a net charge
• Membrane potential


  Electrical potential difference created when there is a difference in electrical charge on the two sides of the membrane
• Sodium-potassium pump


  1. Uses energy from ATP hydrolysis to establish and maintain ion gradients

  2. Actively transports sodium ions out of the cell and potassium ions into the cell

  3. Has high affinity for sodium ions inside the cell

  4. Binding occurs and the pump is phosphorylated by ATP

  5. Conformation changes and affinity for sodium decreases, releasing them outside

  6. Potassium ions bind outside the cell

  7. Dephosphorylation occurs and conformation changes again

  8. Potassium ions taken into cell and affinity returns to start
• For each ATP hydrolysed, three sodium ions are transported out of the cell and two potassium ions are transported into the cell, establishing chemical and electrical gradients
• The sodium-potassium pump is found in most animal cells, accounting for a high proportion of the basal metabolic rate in many organisms
• Glucose transport in the intestine

  1. Sodium-potassium pump generates a sodium ion gradient across the plasma membrane

  2. Glucose transporter responsible for glucose symport transports sodium ions and glucose in the same direction

  3. Sodium ions enter the cell down their concentration gradient, simultaneously transporting glucose into the cell against its concentration gradient