Cell Biology- Chapter 12

Principles and Molecules of Transmembrane Support

Transport proteins grant selectivity to lipid bilayers

Movement of an atom/molecule across membranes is governed by it’s concentration and properties

. The hydrophobic layer of membranes stands as a barrier to the movement of some molecules

. Larger and more polar molecules have more difficult time crossing than smaller and non-polar molecules

• ions can’t cross at all

. an Imbalance in the concentrations of a solute on either side of the membrane (a gradient) is a driving force for the movement of the solute.

. Hierarchy:

• small nonpolar

• small uncharged polar

• larger uncharged polar

• ions

Channels and transporters allow charged and polar solutes across membranes

. Channels create continuous open paths through which solutes rapidly flow

• some channels have open/closed states

• Only allow solutes to move according to gradients

. Transporters have fixed amounts of solutes at a time via conformational changes, they are slower than channels.

• some transporters participate in facilitated diffusion, others use active transport.

. both are selective

Ion Movement is governed by both its gradient and membrane voltage

• unequal Distibution of ions results in a charge difference across plasma membrane

• membrane potential influences movement of ions

• ions have both a chemical and electrical gradient

Water diffuses across membranes through aquaporin channels

• for a time, it was thought that water could move freely through membranes

• while water can cross at a low level, more rapid transport needs water channels

• Aquaporins

• All water-moving proteins are passive channels. Osmosis is the movement of water according to solute concentrations.

• Cells manipulate solute concentrations on either membrane side.

Cells of difference organisms manage internal water pressure differently.

• solutes pumped into contractile vacuoles. Water follows solutes to fill vaculoes, and then vacuoles fuse to membrane

• Plant cell walls can withstand high pressure without bursting

• Animal cells can export solutes to decrease their cytosolic solute concentration and reduce osmotic inflow.

Each cellular membrane has it’s own set of transport proteins

Passive transporters alternate between different conformations independent of solute binding

• passive transporters continuously alternate between different conformations that position the solute binding site on either site of membrane

• This conformation is independent of solute binding, so a solute moved in one direction can be moved in the opposite direction just as easily

• The glucose transporters depicted is massive, if there is more glucose on the outside of cell, it will enter the binding site more frequently than glucose on the inside, which makes a net inward movement.

Pumps use energy to move solutes against their electrochemical gradient

• gradient driven: moves substances (like ions or molecules) across a membrane using a gradient

• ATP-driven: uses energy from the hydrolysis of ATP (adenosine triphosphate) to transport ions or molecules across a membrane against their concentration gradient.

• light-driven: uses light energy to transport ions or molecules across a membrane

Sodium-potassium pumps use ATP to maintain gradients of both ions

ATP powered conformational cycling of the Na+-K+ pump.

The sarcoplasmic reticulum Ca++ pump was the first to be crystallized

• SR is a calcium rich modified ER found in muscle cells

• rapid increases in cytosolic calcium concentration due to opening of Ca++ channels on the SR membrane

• contraction is terminated by movement of calcium back into the SR by ATP powered pumps

• phosphorylation powers

Transporters are defined by the number and direction of solutes

• symport

• antiport

• coupled transport by gradient-driven pumps

Na+ glucose symporters facilitate glucose uptake by cells of the intestines

• passive glucose transporters cannot allow glucose to accumulate in absorptive epithelial cells, but the cell still wants more glucose

• to maintain a higher concentration, glucose uptake is coupled to inflow of sodium, which follows its concentration gradient

• maintained by ATP powered sodium pump

• same gradient is used by cells to uptake other nutrients.

Different membrane-domain restricted glucose transporters in the gut

• active sodium-glucose symporter brings glucose from the intestinal lumen to absorptive cell

• tight junctions limit transporter diffusion and keep them on apical cell side

• glucose is also low in extracellular fluid, so passive glucose transporters on basolateral surface are sufficient to move glucose out of cells

• Sodium also moved out of the cell by sodium-potassium pump, it prevents sodium accumulation

• amino acids are moved from the gut using a similar method

Ion Channels and Membrane Potential

Plant and animal cells use different ionic gradients

• Animals: sodium pump preferred

• Plant: proton pump preferred

• but all cells can do both

Ion channels have water-based selectivity filters

• most channels are selective

• example: sodium channels will not let a potassium enter

• Ions enter a channel at the vestibule

• the selectivity filter is narrow, and interacts with ions based on size

• How can a large potassium channel exclude small sodium ions???

• position of polar amino acids in vestibule drive selectivity. Strips shells off molecules

Ion channels fluctuate between open and closed states

• based on protein conformation

• some rapidly alternate between open and closed, others are gated

Unequal ion distribution establishes membrane potential

• when there is an imbalance in positive and negative charges on either side of the membrane, the excess cations and anions will be attracted to each other at the membrane, but the rest of the fluid is neutral

• The small number of ions at the membrane means very few ions move to alter membrane potential

Nerst equation describes individual ion contribution to membrane potential

• relative to inner leaflet

• positive potential means more positive charges in cell

• negative potential means more negative charges in cell

What is the equilibrium potential of an ion? at what membrane potential will there be no net movement of an ion?

• at equilibrium, electrical forces and chemical forces are balanced.

• Can be calculated with Nernst equation

Potassium leak channels are a major driver of membrane resting potential

• leak channels are ion channels the open/close randomly

• Na+ leaks operate at around 5% the activity of K+ leak channels

• between these 2 ions, K+ has a stronger influence on resting membrane potential

• if potassium leak channels are added, potassium will leave

• exiting positively charged potassium makes inner leaflet more negative

• As the membrane potential becomes more negative, it becomes more difficult for potassium to leave- this creates potassium-driven resting potential.

• Potassium leak channels and Na-K pumps that move sodium out and potassium in maintain a resting potential of around -70mV

Patch clamp experiments measure the activity of indiviudal ion channels

• difficult to measure an individual ion channel’s function with whole cells

• patch clamping uses a fine pipette to detach a patch of membrane and a single ion channel

• relationship between stimulus strength and time spent open- more stimulus= longer open times

• in the absence of acetylcholine, the channel opens very rarely.

• Different ion channels are opened by different stimuli

Different ion channels are opened by different stimuli

• voltage gated

• ligand gated (extracellular)

• ligand gated (intracellular)

• mechanically gated

Hearing is based on mechanically-gated ion channels

• detected by sensory organs

• vibration causes basilar membrane to lift hair cells and press stereocilia against the tectorial membrane.

• Stereocilia displacement pulls mechanically gated ion channels open which leads to intracellular signaling that communicates with an associated auditory nerve filter.

• brain interprets signal as sound.

Some plants use a combination of mechanically- and voltage-gated channels to move

• regional control of turgor pressure

• some plants (like Venus flytraps) use sensory cells- insect presence triggers changes in electrical potential that leads to closure of leaves

• Mimosa has mechanically gated ion channels that produce and electrical signal upon touch that causes leaves to fold (shy plant)

Ion Channels and Nerve Cell Signaling

Neurons rapidly propagate signals

• the cell body contains the nucleus and most organelles

• dendrites are input zones for stimulatory or inhibitory signals

• Axons propagate electrical signals down the length of the neuron

• Nerve Terminals communicate chemically with target cells

The unique nervous system of a longfin inshore squid was essential to understanding neuron function

• sends signals to brain via giant axons

• in general, the thicker the axon, the faster the signal is read

• Application of electrical stimulation to the axon produced a characteristic transient change in membrane potential called an action potential

• action potentials are all or nothing changed in membrane electrical properties! - increasing intensity did not alter shape of action potential, would form as long as stimulation was sufficient.

Replacement of giant axon axoplasm with defined solutions identified key icons in action potential formation

• replacement of giant axon axoplasm with solutions of defined electrolyte concentrations revealed the key ions of action potentials

  • Action potentials could only be generated if the fluid matched the ion composition of the axoplasm particularly with respect to sodium and potassium – no other solutes or biochemical process appeared to be necessary

Extracellular sodium concentration effects action potential intensity

• altering solution produced different effects on giant axon membrane potentials

• altering sodium concentration changed membrane depolarization

Action potentials are initiated by sufficient membrane depolarization

• depolarization: reductions of charge difference between inner and outer plasma membrane leaflet

• entry of cations causes depolarization

• entry of anions cause hyperpolarization

• Threshold potentials is the level of depolarization required to initiate an action potential

• At threshold, voltage-gated sodium channels opens and accentuate the depolarization = action potential.

Threshold potential is the membrane voltage at which voltage-gated Na channels open

• at sub-threshold membrane potentials, voltage-gated Na channels are closed

• channel opens with sufficient depolarization

• influx of cations produces an action potential

• at the peak of action potential, a separate domain of the channel called the inactivation gate swings into the channel and blocks sodium flow

• During refractory period, the inactivation gate remains in place

• after repolarization, the gate resets, channel is ready to open again

Na channel conformation changes during an action potential cycle

Na channel inactivation gates keep action potential propagation in one direction

• the stronger the stimulus you provide the neuron, the more action potentials fire through this region per unit time (increases action potential frequency)

Action potential initiation- Na+ inflow through mechanically gated channels makes membrane potential more positive

• 1- stimulated channels in dendrites open - initiation

• 2- voltage gated Na+ channels (axon) closed - open at firing

• 3- voltage gated K+ channels closed - open at Termination

• 4- Total recovery to resting potential (channels closed)

Neurons communicate with target cells via synapses

• action potentials travel quickly down axons and stimulate signaling to associated target cells

• a synapse is the space between…

Action Potential arrival stimulates neurotransmitter release from the axon terminal by opening voltage-gated Ca++ channel at presynaptic nerve terminal

Neurotransmitter receptors on target cells govern response to stimulation at postsynaptic membrane

Skeletal acetylcholine receptors are ligand-gated sodium channels

There are multiple ion channels with divergent stimuli

• mainly ligand-gated sodium channels

• Cl- flows in, hyperpolarize, harder to reach threshold potential

Psychoactive drugs work by altering synaptic signaling

• Stimulant: Nicotine

• Sedative: Cannabis

A single neuron can receive inputs form many axon terminals

• whether or not this neuron fires is based on the sum of inhibitory and stimulatory signaling it’s receiving.

Optogenetics uses channel rhodopsin to study neuron function with light