Cell Membranes

1. Cell Membranes • Animal cells have a cell membrane that separates them from the environment • Cell membranes are phospholipid bilayers with associated proteins • Cell membranes may allow some substances to pass from one side to the other

2. Cell Membranes: Phospholipid Bilayer • Phospholipid bilayers are made of phospholipids • Phosphate head is polar (= charged) • Fatty acid tails are nonpolar (= not charged)

3. Cell Membranes: Phospholipid Bilayer • Phospholipid molecules naturally align themselves with their fatty acid tails joining together to form the middle of the membrane • The polar heads face outwards towards body fluids (water), and form hydrogen bonds with water molecules

4. Cell Membranes: Membrane Components • Proteins and other molecules are bound to the cell membrane • Peripheral proteins are bound only to one side of the membrane • Integral proteins pass completely through the membrane • Integral proteins often form ion channels

5. Calcium Channel Potassium Channels Calcium Channel Cell Membranes: Integral & Peripheral Proteins • Strings of amino acids corkscrew through the membrane and fold up to form ion channels

6. Channel Units and Subunits • Get used to the many different ways to draw a cartoon of an ion channel

7. Cell Membranes: Ion Channels • In living cells, a flow of ions occurs through ion channels in the cell membrane • This creates a difference in electrical potential between the two sides of the membrane • Neurons are electrically excitable due to the voltage difference across the membrane

8. Membrane Channels: Ion Channels • Ion channels allow ions to pass from one side of the membrane to the other • Ion channels can have selectivity mechanisms, which allow them to let some ions pass through while excluding other ions • An ion channel that allows anions to cross, but excludes cations

9. Ions • Ions are charged particles in solution • Many ionic compounds exist as crystals when not in solution (e.g. table salt)

10. Ions • Ionic compounds dissociate in solution, and individual ions exist as charged particles • Because water carries both partial positive and partial negative charges, ions are usually surrounded by water molecules

11. Diffusion • Solutes, including ions, diffuse in solution, until they reach equilibrium

12. Crossing Cell Membranes • Passive Diffusion • Wanders downhill across the membrane • Passive Transport • Downhill on an electrical or chemical gradient • Carrier Mediated • Primary Active Transport • Uphill against the gradient • Requires ATP • Secondary Active Transport • Uphill against the gradient • Hitches a ride with an ion going downhill

13. Crossing Membranes: Passive Transport • Some membrane channels are always open • Some membrane channels change conformation when a solute binds, and this allows the solute to pass from one side of a membrane to the other

14. Crossing Membranes: Active Transport • The sodium/potassium pump (Na+/K+/ATPase) which moves 3 Na+ out as it moves 2 K+ in is an example of active transport • It burns an ATP for each exchange • It is electrogenic • Helps create the concentration & electrical gradients for the action potential

15. Concentration Gradients • Concentration of ions is different inside & outside the cell membrane • Extracellular fluid rich in Na+ and Cl- • Cytosol full of K+, organic phosphate & amino acids • The result is a concentration gradient • Created in part by the sodium/ potassium pump

16. Electrical Gradients • Negative ions line the inside of cell membrane & positive ions line the outside • Potential energy difference at rest is -70 mV • Cell is polarized • The result is an electrical gradient • Created in part by the sodium/ potassium pump

17. Resting Membrane Potential • The overall concentration of positive and negative ions in the axoplasm is roughly equal • Positive ions line up on the outside of the axolemma • Negative ions line up on the inside of the axolemma

18. Resting Membrane Potential : The Big Picture • Where do the electrical and concentration gradients push K+? • Where do the electrical and concentration gradients push Na+? • The inside of the membrane is lined mostly with K+ and negatively charged protein anions • The outside of the membrane is lined mostly with Na+ and Cl- • The inside of the membrane is slightly negative relative to the outside (-70mV)

19. Leakage Ion Channels • Leakage (nongated) channels are always open • Nerve cells have more K+ than Na+ leakage channels • As a result, membrane permeability to K+ is higher • This explains the resting membrane potential of -70mV in most nerve tissue • The resting membrane is basically a “K+ membrane”

20. Gated Ion Channels • Gated channels open and close in response to a stimulus • Results in neuron excitability, and a change in membrane potential • There are three types of gated channels • Voltage-gated channels respond to a direct change in the membrane potential • Ligand-gated channels respond to the binding of a chemical stimulus (e.g. a neurotransmitter) • Mechanically gated channels respond to mechanical vibration or pressure

21. Voltage Gated Ion Channels • Voltage-gated channels respond to a direct change in the membrane potential • In particular, many voltage gated channels open as a result of a depolarization of the membrane

22. Ligand Gated Ion Channels • Ligand gated ion channels are one of the three types of gated channels • Ligand-gated channels respond to a specific chemical stimulus • In particular, when a neurotransmitter binds to a ligand gated channel,it often opens or facilitates the opening of the ion channel