movement of molecules across membranes

• phospholipid composition


  hydrophilic head and a hydrophobic tail
• cell membrane composition

  made up of a bi-layer of phospholipids with protein embedded within the layers
• two types of membrane protein?

  integral and peripheral
• integral proteins


  penetrate the hydrophobic interior and form strong hydrophobic interactions with regions of hydrophobic R groups.

  Some only extend partly through the membrane and some are transmembrane proteins.
• transmembrane proteins


  integral proteins which span the width of the membrane
• peripheral proteins


  not embedded in the membrane- instead they form ionic and hydrogen binds to the surface of the membrane.

  Have hydrophilic R groups on their surface.

  Many interact with the surface of integral membrane proteins.
• Ligand-gated channel protein


  a signal molecule binds to the channel protein and changes its conformation which opens or closes the gate.
• Voltage-gated channel protein


  a change in ion concentration across the cell membrane causes a change in channel protein and it opens or closes.
• channel protein


  an type of integral protein which functions by having a hydrophilic channel that certain molecules use as a tunnel through the membrane. MOst channels are highly selective.
• what is the action of aquaporin?

  facilitates the passage of water molecules
• what are gated channels?

  Protein channels in cell membranes that can be opened or closed to control the flow of ions or molecules.
• Transporter/ carrier proteins


  Another type of integral protein that holds onto its passenger and changes shape to shuttle it across the membrane. They alternate between two conformation so that the binding site is exposed to one side of the membrane and then the other.
• What can pass through the membrane directly?

  Oxygen

  Carbon dioxide

  Hydrophobic signalling molecules i.e Thyroxine and Steroids

  Other non-polar molecules
• What cannot pass through the membrane directly?

  Charged ions; Sodium ions or Hydrogen ions

  Hydrophilic amino acids

  Polar molecules

  water

  glucose
• Facilitated diffusion


  The passive transport of substances across the membrane through specific transmembrane proteins
• How do molecules move across the cell membrane

  To perform specialised functions, different cell types have different types of protein within their cell membrane to help the molecules move across.
• how are some transmembrane proteins able to control the concentration gradient across a membrane?


  By controlling the number of ions on each side
• What is the ratio of sodium to potassium in the sodium potassium pump?

  The sodium potassium pump move 3 sodium out of the cell for every 2 potassium it moves in.
• Describe the mechanism of glucose symport.

  Sodium potassium pump pumps sodium ions out of the cell. Sodium potassium pump generates a lower concentration of sodium ions within the cell. Glucose symporter transports sodium ions down its concentration gradient alongside glucose which is transported against its concentration gradient.
• Describe the effect of insulin binding to its receptor.

  Triggers the recruitment of GLUT4 glucose transporters to the cell membrane of fat and muscle cells.
• Sodium potassium pump


  1. Transporter protein has ion binding sites exposed to cytoplasm

  2. Ions attach to binding sites

  3. Pump hydrolyses ATP, phosphorylating the pump

  4. Conformational change exposes binding sites to outside of cell

  5. New conformation has low affinity for sodium, sodium ions released

  6. New conformation has increased affinity for potassium, 2 potassium ions bind

  7. Dephosphorylation triggers binding sites returning to exposed to cytoplasm

  8. Cytoplasm has low affinity for potassium, potassium ions released

  9. Returns to high affinity for sodium, sodium ions bind, cycle repeats
• electrochemical gradient


  for a solute carrying a net charge, the concentration gradient and the electrical potential difference combine to form the electrochemical gradient
• What is the action of the electrochemical gradient?

  to determine the transport of the solute
• describe a function of the sodium potassium pump.


  maintains osmotic balance in cells, allows the sodium ion gradient in glucose symport, generates a gradient in kidney tubules and allows for resting potential in nerves
• how is the sodium gradient created in the small intestine?

  by the sodium potassium pump driving the active transport of glucose
• specialisation


  different tissues in multicellular organsims have different types of channel and transporter proteins in their membrane to move different materials across their membranes.
• where are glucose symporters found?


  lining of the small intestine and kidney nephron