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

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

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

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

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

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

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 Cell Membranes: Ion Channels

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

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

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

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

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

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

Crossing Membranes: Active Transport It is electrogenic Helps create the concentration & electrical gradients for the action potential 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

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

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

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

Resting Membrane Potential : The Big Picture 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) Where do the electrical and concentration gradients push K + ? Where do the electrical and concentration gradients push Na + ?

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”

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

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

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