BMB 251

Final Exam

What is the key function of cellular membranes in eukaryotes?

to establish subcellular compartments with distinct aqueous environments (i.e. nucleus, ER, mitochondria)

How are subcellular compartments with distinct aqueous environments achieved in eukaryotic cellular membranes?

1.) selective transport of molecules into and out of these membrane-bound compartments, and 2.) restricting the movement of both small solutes and macromolecules from the compartments

True or false. Lipid bilayers are relatively impermeable to MOST compounds.

TRUE

True or false. Charged molecules (ions) are able to diffuse across membranes without "help."

FALSE. Charged molecules (ions) are NOT able to diffuse across membranes without help.

Can gases (i.e. O2, CO2) and hydrophobic molecules (steroids) diffuse freely across membranes?

YES

Small \_________ polar molecules (i.e. water, glycerol) can also \_______ across membranes, but much more slowly than gases and hydrophobic molecules.

uncharged, diffuse

What is a huge challenge for cells?

how to get the many water-soluble solutes, ions, nutrients/waste products, etc. across the hydrophobic bilayer

How do cells get the many water-soluble solutes, ions, nutrients/waste products, etc. across the hydrophobic bilayer?

by using a special class of transmembrane proteins known as membrane transport proteins

What do membrane transport proteins do?

form aqueous pores across membranes that prevent contact between the hydrophobic bilayer and the solutes that they transport

What are the two classes of membrane transport proteins?

1.) channel proteins, and 2.) transporter proteins

What are transporter proteins known as?

carriers

Transporter Proteins: On binding their substrates, ...

a conformational change occurs that results in depositing substrate on the opposite side of the membrane

True or false. the rate of transport by carriers is much SLOWER than that observed by channel proteins.

TRUE

Carriers display characteristics of \_________ (i.e. specificity for ligands, saturation, etc.).

enzymes

Do channel proteins bind to their substrates?

NO

What do channel proteins do instead of binding their substrates?

form aqueous (hydrophilic) pores to permit diffusion through the membrane

The rate of transport in channel proteins is thousands of times \_______ than observed with transporter proteins.

FASTER

What do cells and their membrane-bound compartments possess?

an electrochemical gradient

What are the charges of the electrochemical gradient on the inside and on the outside?

inside: negative, outside: positive

How is the electrochemical gradient negative on the inside and positive on the outside?

due to unequal distribution of both solutes and charged ions

True or false. Solutes and ions can only be transported along transmembrane electrochemical gradients.

FALSE. Solutes and ions can be transported either along OR against transmembrane electrochemical gradients.

What direction are solutes and ions transported in passive transport?

along or down the gradient

What direction are solutes and ions transported in active transport?

against or up the gradient

In passive transport, no \_______ is required for substate transport.

energy

Passive transport can be mediated by both \_________ and \_______ \________.

transporters, channel proteins

In active transport, what is used for transport of substrates?

energy!

Active transport can only be accomplished by what?

carriers/transporters

In active transport, transporters bind on \_____ side of the membrane, undergo \_________ change, and release solute on the \_______ side of the membrane.

one, conformational, opposite

What are the three sources of energy that active transport can use?

1.) ATP hydrolysis by ATP-driven pumps, 2.) coupled substrate transport, and 3.) light energy driven transport

What happens in coupled substrate transport?

unfavorable transport of one molecule is coupled to the favorable transport of a different molecule

Three classes of transporters are defined by both the \________ and the \_________ of solutes that are transported.

direction, number

What are the three classes of transporters defined by both direction and number of solutes that are transported?

1.) uniporters, and 2&3.) coupled transporters (symporters and antiporters)

What do uniporters do?

transport a single solute

What do coupled transporters do?

move more than one solute

The transport of one solute \_______ its electrochemical gradient provides the energy to move the other solute \________ its electrochemical gradient.

down, against

What are the two types of coupled transporters?

1.) symporters, and 2.) antiporters

What do symporters do?

engage in co-transport of two different molecules in the same direction across a membrane (i.e. Na-Glucose symporter of the GI tract)

What do antiporters do?

engage in co-transport of two different molecules in the opposite direction (i.e. Na-K ATPase or Na-K pump)

Many ion pumps (Na-K pump, Ca pump) are \__________.

ATPases

What is ion movement linked to?

ATP hydrolysis

The Na-K pump establishes concentration gradients on ions in which Na is \_____ inside the cell and \_______ outside, while K is \_______ inside the cell and \_____ outside the cell.

low, high; high, low

Why is the Na-K pump electrogenic?

because 3 Na+ ions are transported in exchange for 2 K+ ions

What are two important function of the Na-K pump?

1.) driving glucose transport across the intestinal epithelium, and 2.) stabilizing cell volumes

How does the Na-K pump stabilize cell volume (and prevent osmotic lysis)?

the higher concentration of solutes inside cells would ordinarily make water move into the cell by osmosis; since Na-K pump maintains a higher concentration of Na+ (and indirectly Cl) outside the cell, the higher concentration of NaCl outside the cell counteracts the flow of water into the cell

Inhibiting the Na-K pump with a toxin (ouabain) causes cells to \______ and \_______.

swell, burst

The ABC (ATP-Binding Cassette) family of transporters are involved in what?

transport of ligands

True or false. Each member of the ABC transports a specific ligand (or related class, usually small molecules instead of ions).

TRUE

What family of transporters constitutes the largest class of transport proteins?

the ABC (ATP-Binding Cassette)

What are some examples of members of the ABC family of transporters?

phospholipid translocases, MDR Protein (multiple drug resistance in cancer cells), cystic fibrosis gated chloride channel

What are ion channels?

multi-pass transmembrane proteins with many alpha-helical transmembrane domains organized into a hydrophilic channel

What does the selectivity filter within an ion channel do?

specifies which ion can pass through the channel

True or false. Transport of ions through a channel is approx. 100,000 times quicker than the fastest transporter.

TRUE

Channels are limited to what kind of transport?

PASSIVE transport - they do NOT engage in active transport

Ion channels are \_____ (i.e. they are not permanently open so that ion flow is regulated).

gated

True or false. Gates cannot be opened or closed in response to a variety of stimuli.

FALSE. Gates CAN be opened or closed in response to a variety of stimuli.

What do voltage-gated channels do?

respond to a change in membrane potential (i.e. depolarization)

What do ligand-gated channels do?

respond to association between the channel and small molecules called ligands

What do mechanically-gated channels do?

respond to physical movement

What kind of contacts do ions make with amino-acids in the center of the channel?

transient contacts

True or false. All cell membranes possess a transmembrane potential.

TRUE

What does establishing a resting membrane potential in animal cells result from?

the coordinated action of a transporter AND ion channels

What is resting potential?

the membrane potential occurring when there is NO net flow of ions

What is the typical range of resting potentials?

-20mV to -200mV

What is the major contributor to resting potential?

K+ leak channels (but other channels account for the wide range of resting potentials)

What do K+ leak channels do?

allow K+ to diffuse out of cells (passive transport)

What is another name for a neuron?

nerve cell

Where is the nucleus located in neurons?

the cell body

What is the axon of a neuron?

a single long protrusion that sends signals away from the cell body (i.e. the output region of the neuron)

What are the dendrites of a neuron?

relatively short protrusions from the cell body that receive signals from the axons of other neurons (i.e. the input region of the neuron)

What do nerve impulses result from?

electrical disturbances in the plasma membrane that spread from one part of the cell to another

What is an electrical disturbance in the plasma membrane called?

an action potential

What does an action potential consist of?

a wave of membrane depolarization that moves down the axon

What initiates the action potential?

a threshold depolarization

When a neuron receives a signal, a \______ \_______________ of the membrane occurs.

modest depolarization

What happens if the depolarization reaches a threshold?

all the voltage-gated Na+ channels experiencing this threshold depolarization will OPEN simultaneously

What happens when all of the voltage-gated Na+ channels open simultaneously?

Na+ rushes INTO the cell and the membrane becomes depolarized

Voltage-gated channels are the \______ to generating a(n) \_______ \__________.

key, action potential

What is the direct result of the the voltage-gated Na+ channel opening briefly?

it automatically becomes inactive

How long does the voltage-gated Na+ channel remain inactive (and closed)?

until it is reset and the membrane returns to its resting potential

What has to happen in order for the voltage-gated Na+ channel to once again respond to a threshold potential?

it needs to be reset first

The return to the resting membrane potential relies, in part, on \_____________ of the voltage-gated Na+ channel.

inactivation

What does the repolarization of the membrane to resting membrane potential also depend upon?

the opening of voltage-gated K+ channels

Voltage-gated K+ channels open more \________ than the voltage-gated channels.

slowly

What does the lag between the voltage-gated Na+ channels and the voltage-gated K+ channels ensure?

that there is time for the influx of Na+ to depolarize the membrane BEFORE the efflux of K+ begins to repolarize the membrane

The transmission of a nerve impulse involves the propagation of what? In what direction and where must this occur?

an action potential, down the axon of a neuron

What is the channel cycle of the voltage-gated Na+ channels?

"closed, opened, and inactivated states"

What does the initiation of an action potential require?

an event that causes the membrane to depolarize to the threshold potential in the first place

Where does initiation of an action potential occur?

at the synapse

A neurotransmitter is released from a \_________ \_____ when a(n) \________ \_______ arrives at the end of the terminal branches of its axon.

presynaptic cell, action potential

What does a neurotransmitter diffuse across after it is released from a presynaptic cell?

the synaptic cleft

True or false. A neurotransmitter diffuses across the synaptic cleft and associates with a voltage-gated Na+ channel (aka transmitter-gated Na+ channel).

FALSE. A neurotransmitter diffuses across the synaptic cleft and associates with a LIGAND-gated Na+ channel (aka transmitter-gated Na+ channel).

What happens when a ligand-gated Na+ channel opens?

an influx of Na+ ions depolarize the membrane

What two events happen when sufficient ligand-gated Na+ channels open?

1.) the threshold membrane potential will be reached, and 2.) an action potential will ensue as a result of the opening of the voltage-gated Na+ channels

True or false. Synapses can be either excitatory or inhibitory.

TRUE

What do excitatory neurotransmitters result in?

depolarization of membrane due to Na+ influx

What do inhibitory neurotransmitters result in?

opne Cl- or K+ channels and hyperpolarization of the membrane, making it more difficult to depolarize

There are thousands of \______ \__________, both excitatory and inhibitory, that form synapses with individual neurons.

nerve terminals

Where and under what conditions is an action potential initiated?

at the axon hillock if the sum of excitatory and inhibitory signals depolarizes the axon hillock to the level of the threshold potential

True or false. Action potentials only involve transmission between neurons.

FALSE. Action potentials do NOT only involve transmission between neurons.

The neuromuscular junction is an example of what?

how an action potential from a neuron triggers a response in a different cell type