Bio 314 Chapter 12

transport across cell membrane

what molecule can cross the lipid bilayer without the help of membrane proteins?

protein-free, artificial lipid bilayer (liposome)

why do cells need to transport molecules across their membranes?

• Maintain ion concentrations

• Uptake nutrients

• Release waste

• Produce ATP

what are the two main types of transport proteins?

1. Channels — create pores in membrane for passive diffusion

2. Transporters — bind to molecule and change conformation

Channels vs Transporters

Channels: less specific, faster, size and charge

Transporters: more specific, slower

what controls the opening and closing of a channel?

external stimuli or conditions within the cell

what types of molecules can cross a cell membrane through either channels or transporters?

inorganic ions and small, polar organic molecules

what are the two components of an electrochemical gradient?

concentration gradient and membrane potential

How does water move through osmosis?

DOWN its concentration gradient from LOW to HIGH concentration.

what happens to a cell when the concentration of water is greater inside the cell than outside of it?

the cell will swell and burst as water rushes into the cell

what does an aquaporin-based water system do?

purifies water

what happens in a transporter to mediate the transport of a solute?

conformational changes

what are the 3 main ways that pumps carry out transport?

1. Gradient-driven pump

2. ATP-driven pump

3. Light-driven pump

what kind of pump is the Na+ - K+ pump (sodium potassium pump)?

ATP-driven

what does the Na+ - K+ pump do?

it helps keep the cytosolic conentration of Na+ low and K+ high (more Na+ out and more K+ in.)

what is the Ca2+ pump (ATPase pump) driven by?

phosphorylation

Gradient-driven pumps can be…

Symporters: move 2 diff molecules in same direction across membrane

Antiporters: move 2 diff molecules in opposite directions across membrane

Uniport: move one molecule in one direction across membrane

what is the glucose-Na+ symporter?

a transporter that uses the electrochemical Na+ gradient to drive active import of glucose against its gradient. (glucose and Na+ need to travel together..down the slide)

diagram of the alternate states of glucose-Na+ symporter

pump transitions between states

What is the uptake of glucose in the gut driven by?

the Na+ gradient (glucose is ACTIVELY taken up and PASSIVELY released for tissues)

what do plant cells use in replacement of an Na+ - K+ (ATPase) pump?

H+ - ATPase (electrochemical gradient of H+ is used for symports to import various solutes)

Ion channels are ___ and ____

ion-selective, gated

what type of functional groups help the pore select the ion in ion channels?

carbonyl groups

what is the main contribution to a cell’s membrane potential?

the distribution of ions on either side of the cell

What do K+ leak channels do?

when they open, K+ ions tend to leave the cell to go to lower concentration, which generates a membrane potential

what is patch-clamp recording used for?

it is used to monitor ion channel activity through measuring the current through a single ion channel

what do ion channels in patch-clamp recording respond to?

the ion channels open in response to the neurotransmitter acetylcholine

what type of cells are patch-clamp recordings tested on?

muscle cells

what are the four different types of ion channels based on their response to different stimuli?

1. voltage gated—open based on voltage nearby

2. ligand-gated — extracellular or intracellular: respond to specific neurotransmitter binding to receptor

3. mechanically-gated — physical pressure (e.g. allow us to hear through positioning of stereocilia)

4. always open

what does a typical neuron consist of?

dendrites, cell body, axon

describe how neurons receive and transmit signals

• neuron receives signal

• signal changes membrane potential

• membrane potential travels down axon

• axon terminal relays signal to next cell

what is special about the squid’s nervous system?

it can respond rapidly to threats because its cytoplasm can be removed and replaced with an artificial solution of pure ion

how do we study nerve cell excitability in a squid?

we study an isolated axon with an electrode that we insert into the cytoplasm (axoplasm)

what type of ion channels are responsible for triggering an action potential?

voltage-gated ion channels (resting membrane potential is -60 mV, threshold potential is -40, which triggers action potential)

How do membrane potentials return to resting potential?

inactivation of Na+ channels and opening of K+ channels

how do neurons connect to their target cells?

through synapses

how are signals converted and transmitted across the synapse?

the electrical signal is converted to a chemical signal, which is then converted back to electrical signal in the postsynaptic cell

synapses can be either ___ or ____

excitatory, inhibitory

what makes a synapse excitatory?

influx of Na+ depolarizes membrane, which increases likelihood of firing an action potential

what makes a synapse inhibitory?

influx of Cl- keeps membrane more polarized and decreases likelihood of firing an action potential

what do psychoactive drugs do when exposed to ligand-gated ion channels?

they bind the ligand-gated ion channels

what are optogenetics?

biological technique that controls neuron activity through the use of light