Chapter 12: Membrane Transport, Ion Channels, Membrane Potential, Nerve Cell Signaling

Q: What is the difference between transporters and channels?

A: Transporters can be active or passive; channels are always passive.

Q: What does passive transport mean?

A: Movement down a concentration or electrochemical gradient without energy input.

Q: What does active transport mean?

A: Movement against a gradient using energy (usually ATP or another gradient).

Q: What are the three types of active transporters?

A: ATP-driven pumps, coupled transporters (symport/antiport), and light-driven pumps (in some bacteria).

Q: What are uniport, symport, and antiport?

A: Uniport moves one molecule; symport moves two in the same direction; antiport moves two in opposite directions.

Q: What does the Na⁺/K⁺ pump do?

A: Uses ATP to move 3 Na⁺ out and 2 K⁺ in, maintaining resting potential.

Q: Why is the Na⁺/K⁺ pump essential?

A: It maintains ion gradients critical for nerve impulses and osmotic balance.

Q: What is the role of ATP in the Na⁺/K⁺ pump?

Q: What does the Ca²⁺ pump do?

A: Uses ATP to keep cytosolic calcium levels low by pumping Ca²⁺ into the ER or out of the cell.

Q: What is the glucose–Na⁺ symport?

A: A coupled transporter that uses Na⁺ gradient to bring glucose into the cell against its own gradient.

Q: What is the K⁺ leak channel and why is it important?

A: Allows K⁺ to leak out of the cell, generating the resting membrane potential.

Q: What creates the resting membrane potential?

A: K⁺ leaving through leak channels makes the inside slightly negative.

Q: What does the Nernst equation describe?

A: The balance point (equilibrium potential) where electrical and chemical gradients are equal and opposite.

Q: What happens during an action potential?

A: Na⁺ channels open → Na⁺ enters → depolarization → K⁺ channels open → K⁺ leaves → repolarization.

Q: What does “refractory period” mean?

A: The short time when Na⁺ channels are inactivated and another action potential cannot start.

Q: What ion triggers neurotransmitter release?

A: Calcium (Ca²⁺).

Q: Explain synaptic transmission in steps.

A: Action potential arrives → Ca²⁺ channels open → Ca²⁺ enters → vesicles fuse with membrane → neurotransmitter released → binds receptor on next neuron.

Q: What is acetylcholine?

A: A neurotransmitter that binds to ligand-gated Na⁺ channels to start a new action potential.

Q: What is the difference between voltage-gated and ligand-gated channels?

A: Voltage-gated respond to changes in membrane potential; ligand-gated open when a signaling molecule binds.

Q: What kind of pump do plants, fungi, and bacteria use instead of Na⁺/K⁺ pumps?

A: H⁺ (proton) pumps.

Q: What is an electrochemical gradient?

A: The combined effect of ion concentration difference and electrical charge difference across the membrane.

Q: What is the direction of Na⁺ movement in most cells?

A: Into the cell (down its gradient).

Q: What is the direction of K⁺ movement in most cells?

A: Out of the cell (down its gradient).

Q: What is depolarization?

A: When Na⁺ enters the cell, making the inside more positive.

Q: What is repolarization?

A: When K⁺ leaves the cell, restoring the negative potential.

Q: Why does the membrane potential not reach +60 mV even though Na⁺ enters?

A: Na⁺ channels inactivate and K⁺ channels open, balancing charge.

Q: How does the Na⁺/K⁺ pump restore gradients after an action potential?

A: It pumps Na⁺ out and K⁺ in to re-establish resting conditions.

Q: What are the three types of channel gating?

A: Voltage-gated, ligand-gated, and mechanically-gated.