Facilitated diffusion

• Diffusion works because the movement of molecules is random, so the odds of one highly concentrated molecule breaking away is likely because of how many there are, but because there are fewer of the molecule in less concentrated areas, the odds of one moving to a highly concentrated area are lower
• Diffusion is the movement of particles from areas of high concentration to low concentration
• The stronger the concentration gradient (difference between concentrations of two spaces) the faster diffusion will happen
• Facilitated diffusion


  When molecules diffuse across the plasma membrane with the help of membrane proteins, like channels or carriers
• Facilitated diffusion lets charged particles passively diffuse across the cell membrane without having to deal with its hydrophobic lipid core
• Channel proteins


  Membrane proteins that span the cellular membrane and form hydrophilic tunnels for polar molecules to pass through in order to avoid the hydrophobic part of the membrane. They are generally highly selective and each one will only let certain similar molecules through
• Aquaporins


  Channel proteins that let water pass in and out of a cell very quickly
• Some channel cells are open all the time, and some are gated, meaning they open and close in response to particular signals. This is especially important in the diffusion of ions for the nervous system, because that is how they transmit electrical charges
• Carrier proteins


  Change shape once one of the molecules they select for binds to their receptors to pull them in and release them on the other side
• Channel proteins typically transport much faster than carrier proteins because they are just tunnels that things flow through, instead of having to change shape and reset just to get one thing through
• Typical channel proteins let through tens of millions of molecules per second, while typical carrier proteins let through thousands per second
• Membrane potential


  The difference in electrical potential between the interior and exterior of a cell
• The membrane potential of the average cell is -40 to -80 milivolts
• The inside of a cell is typically more negative than its surroundings
• The inside of a cell tends to have a higher concentration of K+ ions than its surroundings. Because of this ratio, the K+ will be drawn out along its electric gradient (K+ goes outside to where its concentration is lower). HOW-EVERRR: K+ is also drawn into the cell by the membrane potential (negative charge of the cell), as the positive ions are drawn to the negative charge. In the end, K+'s movement is a balance between the two
• Electrochemical gradient


  The combination of concentration gradient and voltage that affects an ion's movements
• Primary active transport


  Directly uses sources of chemical energy (like ATP) to move molecules across a membrane against their gradient
• Secondary active transport or cotransport


  Uses an electrochemical gradient generated by active transport as an energy source to move molecules against their gradient, and so does not directly use a chemical energy source like ATP
• Electrogenic pump


  Cell membrane pumps that are involved in the creation and maintenance of voltages across a membrane (N+ and K+, or H+ in plants)
• Sodium-potassium pump


  Moves N+ out of cells and K+ into them using primary active transport
• The sodium-potassium pump gets rid of 3 N+ and brings in 2 K+
• Negative MP isn’t created just by pumping out more + ions than come in
• membrane potential

  
• Equilibrium potential


  The electrical potential difference that exactly balances the concentration gradient for an ion
• The Na-K pump helps build up so much K+ that it starts to diffuse out despite the outside + charge, until the relative -- charge of the cell gets too great, so K+ gets attracted back in until it reaches equilibrium
• Secondary active transport


  Uses the electrochemical gradients set up in primary active transport to move other ions against their own gradients
• Cotransporter


  Carrier protein that couples the moving on one ion with their gradient (downhill transport) and one ion against their gradient (uphill transport)
• Cotransporters use the energy of the downhill ion's gradient to drag in another ion against its gradient
• Symporter


  Secondary active transport protein that transports both ions in the same direction (e.g. both into or out of the cell)
• Antiporter


  Secondary active transport protein that transports the ions in opposite directions (e.g. one into the cell with its gradient and one out of the cell against its gradient or vice versa)