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