Electrochemical Gradients & Signaling

Question 1

Define diffusion.

Answer 1

A spontaneous process in which a substance moves from a region of high concentration to a region of low concentration, eventually eliminating the concentration of difference between the two regions.

Question 2

How does free energy relate to the way in which an electrolyte diffuses across the plasma membrane?

Answer 2

The greater the difference in charge (the potential difference or voltage) between the two compartments, the greater the difference in free energy. Thus, the tendency of an electrolyte to diffuse between two compartments depends on two gradients: a chemical gradient, determined by the concentration difference of the substance in the two compartments, and the electric potential gradient determined by the difference in charge.

Question 3

What two conditions must be met in order for a substance to diffuse passively across a plasma membrane?

Answer 3

1. The substance must be present at higher concentration on one side of the membrane than the other
2. The membrane must be permeable to the substance.

Question 4

What two conditions can make a membrane permeable to a given solute?

Answer 4

1. The solute can pass directly through the lipid bilayer.
2. The solute can transverse an aqueous pore that spans the membrane.

Question 5

Define partition coefficient.

Answer 5

The ratio of a substance’s solubility in a nonpolar solvent to that in water. It describes the amount of energy needed to transport a hydrophilic molecule over the hydrophobic bilayer.

Question 6

Define osmosis.

Answer 6

The movement of water from a region of low solute to a region of high solute.

Question 7

What do the terms hypertonic and hypotonic mean?

Answer 7

The hypertonic compartment is the compartment of higher solute concentration in relation to the hypotonic compartment, which has a lower solute concentration.

Question 8

How do aquaporins keep hydrogen protons from entering the cell?

Answer 8

Each aquaporin subunit contains a central channel that is lined with hydrophobic amino acid residues that is highly specific for water molecules. Water molecules interact with the inner surface of the channel, reorienting the molecule so that it cannot maintain the hydrogen bonds that normally link it to the other water molecules.

Question 9

Define ion channels.

Answer 9

Openings in the membrane that are permeable to specific ions. Each is formed by integral membrane proteins that enclose an aqueous pore.

Question 10

Do ion channels require energy to run?

Answer 10

No; they rely on passive transport. Ions moving through these channels always move from a place of higher energy to a place of lower energy.

Question 11

What does the conformational state of a voltage-gated channel rely upon?

Answer 11

The conformation depends on the difference in ionic charge on the two sides of the membrane.

Question 12

What does the conformational state of a ligand-gated channel rely upon?

Answer 12

It depends on the binding of a specific molecule (the ligand), which is usually not the solute that passes through the channel.

Question 13

What does the conformational state of a mechano-gated channel rely upon?

Answer 13

It depends on mechanical forces (such as stretch tension) that are applied to the membrane.

Question 14

Why can’t Na+ ions penetrate a K+ ion pore in a KcsA channel?

Answer 14

Inside the pore are eight oxygen atoms with which a K+ ion temporarily coordinates with; it is a type of selectivity filter. However, while this selectivity filter is a perfect match for K+, an Na+ ion is too large. Therefore, it cannot interact optimally with the eight oxygen atoms necessary to stabilize it in the pore and overcome the higher energy barrier required to penetrate the pore.

Question 15

How is the gating of molecules in a KcsA channel accomplished?

Answer 15

Gating is accomplished by conformational changes of the cytoplasmic ends of the inner alpha helices. In the closed conformation, the helices are straight and cross over one another to form a “helix bundle” that seals the cytoplasmic face of the pore. The helices bend at a certain point where a glycine residue is located.

Question 16

Describe the two distinct domains of the six helices (S1-S6) of a eukaryotic Kv channel.

Answer 16

1. A pore domain which contains the selectivity filter that permits the selective passage of K+ ions.
2. A voltage-sensing domain consisting of helices S1-S4 that senses the voltage across the plasma membrane.

Question 17

What causes the S4 helix of a K+ channel to move, and why is it important?

Answer 17

The S4 helix connects the voltage-sensing domain of the channel to the pore domain. S4 responds to membrane depolarization (when the change in potential becomes more positive), which initiates a series of conformational changes within the protein that opens the gate at the cytoplasmic end of the channel.

Question 18

How does inactivation of a K+ channel occur?

Answer 18

Inactivation is accomplished by movement of a small peptide that dangles from the cytoplasmic portion of the protein. When it moves up into the mouth of the pore, the passage of ions is blocked and the channel is inactivated. Later, the inactivation peptide is released and the gate to the channel is closed.

Question 19

Define facilitated transport.

Answer 19

Substances always diffuse from a place of high concentration to low concentration, but they do not always do this through the lipid bilayer or a channel. Sometimes the substance first binds to a membrane-spanning protein called a facilitative transporter. that facilitates the diffusion process.

Question 20

How is facilitated diffusion like an enzyme-catalyzed reaction?

Answer 20

Like enzymes, transporters exhibit saturation-type kinetics. Unlike ion channels, which can conduct millions of ions per second, most transporters can move only hundreds to thousands of solute molecules per second across the membrane. Facilitated diffusion is important in mediating the entry and exit of polar solutes, such as sugars and amino acids.

Question 21

How is a favorable gradient maintained for glucose diffusion?

Answer 21

The cell phosphorylates the sugar after it enters the cytoplasm, thus lowering the intracellular glucose concentration.

Question 22

Why must the affinity of a protein for sodium or potassium ions change during the activity of the sodium/potassium pump?

Answer 22

To begin, the pump must take K+ or Na+ ions from a place of low concentration, which requires high affinity to pull it away. Then, to be able to release the ions into an area of high concentration, the protein must lessen its affinity for the ion.

Question 23

How does a protein decrease its affinity for an ion after it has moved from low to high concentration in an ATPase pump?

Answer 23

This is achieved by ATP hydrolysis and the subsequent release of ADP, which induces a large conformational change within the protein molecule.

Question 24

How does secondary active transport work?

Answer 24

The establishment of concentration gradients provides a means by which free energy can be stored in a cell. The potential energy stored by these gradients can be used to transport other solutes.

Question 25

Describe how the movement of glucose across the apical plasma membrane illustrates the idea of cotransport and secondary active transport.

Answer 25

Polysaccharides are hydrolyzed in the intestine, and the glucose that results needs to be moved into the epithelial cells of the intestine. Sodium ion concentration is kept low within the the epithelial cells , so the tendency for those sodium ions to diffuse back across the apical plasma membrane is “tapped” by the epithelial cells to drive the cotransport of glucose molecules into the cell against a concentration gradient. The sodium ion and glucose are coupled together to pass through the membrane.

Question 26

Define terminal knob.

Answer 26

A terminal knob is the specialized site where impulses are transmitted from neuron to target cell.

Question 27

How do K+ ions affect the membrane potential?

Answer 27

Thinking in terms of concentration, K+ ions want to flow out of the cell into a place of lower concentration. However, when they do so through K+ leak channels, they leave behind a negative charge. This favors the retention of K+ ions within the cell. This conflict of interest between concentration and voltage results in an equilibrium of K+ ions.

Question 28

Define depolarization in the context of a membrane.

Answer 28

Depolarization refers to the increase in positivity in the membrane voltage, which causes a decrease in polarity between the two sides of the membrane: depolarization.

Question 29

Describe what happens when the threshold for a stimulus has been reached.

Answer 29

The threshold refers to the depolarization of the membrane up to a point around - 50 mV. The change in voltage causes the voltage-gated sodium channels to open. As a result, sodium ions diffuse freely into the cell. The increased permeability of the membrane to sodium ions and movement of positive charge into the cell causes the membrane to reverse briefly, becoming positive at about +40 mV.

Question 30

What is the relationship between the diameter of an axon and the speed with which an action potential can travel down it?

Answer 30

The greater the diameter of the axon, the less the resistance to current flow and the more rapidly an action potential can be conducted. The speed of conduction increases with the square root of the increase in diameter.

Question 31

Define saltatory conduction.

Answer 31

A saltatory conduction is the mechanism by which action potentials are generated at nodes of Ranvier and conducted from node to node.

Question 32

What is the synaptic cleft?

Answer 32

A space of about 20 to 50 nm between a neuron and its target cell.

Question 33

What are synaptic vesicles?

Answer 33

Synaptic vesicles are found in the terminal knobs of the branches of an axon. They serve as storage sites for neurotransmitters.

Question 34

Describe the sequence of events of synaptic transmission.

Answer 34

When an impulse reaches a terminal knob, its depolarization induces the opening of a number of voltage-gated Ca2+ channels, which diffuse into the terminal knob of the neuron and greatly increase the concentration of Ca2+. This causes the release of neurotransmitter molecules into the synaptic cleft. These neurotransmitter then diffuse across the gap and selectively bind to receptor molecules in the postsynaptic membrane.

Question 35

What two effects can a neurotransmitter molecule have on its target cell membrane?

Answer 35

1. The bound transmitter can trigger the opening of cation-selective channels in the membrane, leading to an influx of sodium ions and a less negative membrane potential. This depolarization excites the cell, making it more likely to generate its own action potential.
2. The transmitter can trigger the opening of anion-selective channels, leading to an influx of chloride ions and a more negative membrane potential. Hyperpolarization of the postsynaptic membrane makes it less likely the cell will generate an action potential.

Question 36

What is the resting electrical potential of a typical cell?

Answer 36

About 70 mV.

Question 37

State Ohm’s Law.

Answer 37

V = IR
Voltage = current x resistance.

Question 38

What do patch/clamp recordings measure, and how do they do so?

Answer 38

Patch/clamp recordings measure the movement of ions in ion channels. A cell is attached to an electrode, which clamps onto a tiny portion of its membrane. This causes covalent bonds to form between the glass of the electrode and the membrane, creating a tight seal against the current. This gives the researcher the ability to measure the current and number of ions that flow through voltage-gated channels.

Question 39

What two characteristics of the selectivity filter in a K+ channel made it designed specifically for K+ ions?

Answer 39

1. The size of the channel; it is stabilized specifically to fit potassium ions–no others.
2. The pore R groups. Negatively-charged O of carbonyl groups interacts directly with the + ions. This replaces the normal water of hydration, energetically favoring ion passage.

Question 40

In reference to K+ channels, what is the water of hydration, and why must it be removed?

Answer 40

The water of hydration refers to the sphere of water molecules that surround the potassium ion (because water is so polar). This must be stripped away in order to fit through the pore. The only energetically favorable way to strip away the oxygens of the water is to replace them with oxygens from the amino acid residues of the pore.

Question 41

Briefly describe the process of generating an action potential.

Answer 41

1. Voltage change reaches the threshold.
2. Fast voltage-gated Na+ channels open first; Na+ enters the cell; depolarization.
3. Na+ channels then inactivate in a time-dependent fashion; Na+ current stops.
4. Slow voltage-gated channels open second; K+ exits the cell; hyperpolarization.
5. The cycle repeats.

Question 42

What is the vesicle hypothesis?

Answer 42

Vesicles containing a chemical signal fuse with the plasma membrane to deliver quantal packets of neurotransmitter.

Question 43

How does a freeze slammer work and what does it aim to study?

Answer 43

A freeze slammer is a machine designed to “stop time” at the neuromuscular junction in order to study it. The nerve is electrically stimulated and then stomped down onto a platform to rapidly freeze the synapse.

Question 44

Briefly describe the way an action potential is transmitted at the synapse.

Answer 44

1. Action potential arrives; terminal depolarized.
2. Ca2+ influx through voltage-gated channels.
3. Ca2+ triggers vesicle fusion; secretion of transmitter into cleft.
4. Transmitter binds receptor; opens ligand-gated ion channels.