BILD1: Membrane Transport

Which type of molecule can freely cross a biological membrane without a transporter?

Small, nonpolar molecules like O₂, CO₂, and lipids. Polar or charged molecules require channels or transporters.

Osmosis is the opposite of diffusion.

False — Osmosis is diffusion of water through a semipermeable membrane, driven by differences in solute concentration (water moves from low solute → high solute).

Distinguish between channels and pumps in membrane transport.

• Channels: Passive, allow movement down the concentration gradient, no ATP required.

• Pumps: Active, use ATP to move substances against their concentration gradient.

Which of the following statements about the Na⁺/K⁺-ATPase pump is true?
A) Moves Na⁺ into the cell
B) Moves K⁺ out of the cell
C) Requires ATP
D) Works only when Na⁺ levels are high

C — It uses ATP to move 3 Na⁺ out and 2 K⁺ in, maintaining electrochemical gradients. The pump is always active.

If Na⁺ channels open on the blood side of an epithelial cell, which direction will Na⁺ move? Does this require energy?

Na⁺ flows into the cell down its concentration gradient. No energy (ATP) is required — passive transport.

What happens to Cl⁻ and water movement when CFTR channels open at the airway side?

Cl⁻ moves out into the mucus (down gradient); water follows by osmosis, keeping mucus thin.

Predict what happens to the mucus in cystic fibrosis when CFTR channels malfunction.

Less Cl⁻ exits → less water follows → mucus becomes thick and sticky, trapping bacteria.

In cystic fibrosis, why do drugs that keep CFTR channels open longer help?

They allow more Cl⁻ and water to move into mucus, restoring hydration and clearing ability.

Red blood cells in a hypotonic solution will:

Gain water, swell, and possibly burst (lyse), because water moves into the cell where solute concentration is higher.

Red blood cells in a hypertonic solution will:

Lose water and shrink (crenate) as water exits toward the higher solute concentration outside.

“Solution C must be pulling water out of the cell using active transport.”
What’s wrong with this statement?

Water movement is always passive — osmosis, not active transport. The driving force is solute concentration differences.

Hospitals give patients isotonic IV fluids because...

They prevent water gain or loss from blood cells — isotonic means equal solute concentration on both sides of the membrane.

Why don’t plant cells burst in hypotonic solutions?

Their rigid cell walls resist expansion; water entry creates turgor pressure instead of lysis.

Draw and label a diagram showing movement of Na⁺, K⁺, and Cl⁻ across an epithelial cell in the lungs. Indicate where the Na⁺/K⁺ pump, Na⁺ channels, and CFTR are located.

• Na⁺/K⁺ pump on basolateral side: 3 Na⁺ out, 2 K⁺ in.

• Na⁺ channel on blood side → Na⁺ enters.

• CFTR channel on airway side → Cl⁻ exits.

• Water follows Cl⁻ into mucus layer.

Which of the following transport processes require ATP?
☐ Simple diffusion
☐ Facilitated diffusion
☐ Na⁺/K⁺ pump
☐ Endocytosis
☐ Osmosis

Na⁺/K⁺ pump
Endocytosis

All others are passive processes.

Which property determines whether a molecule can pass through the lipid bilayer by simple diffusion?

Polarity and size. Small, nonpolar molecules (O₂, CO₂, steroid hormones) can diffuse freely; large or charged molecules cannot.

What determines the direction of passive transport?

The concentration gradient — substances move from high to low concentration until equilibrium is reached.

Each substance’s movement across a membrane depends on its own concentration gradient.

True. Molecules diffuse independently of each other (except for coupled ions).

Substances only pass through membranes when a concentration gradient exists.

False. Random movement occurs both directions; a gradient only determines the net direction.

Define osmosis.

The diffusion of water across a semipermeable membrane from low solute (high water) to high solute (low water) concentration.

Osmosis is different from diffusion because it involves...

Movement of water, not solute — but it’s still diffusion (no energy required).

Which of the following are types of passive transport?
☐ Simple diffusion
☐ Facilitated diffusion
☐ Osmosis
☐ Active transport

Simple diffusion
Facilitated diffusion
Osmosis

Active transport requires ATP.

Distinguish between pumps and channels.

• Channels: Allow molecules to flow down gradient; passive.

• Pumps: Use ATP to move molecules against gradient; active.

Distinguish between pumps and transporters.

Both can move molecules against gradients, but:

• Pumps use ATP directly.

• Transporters use energy stored in another gradient (secondary active transport).

Pumps only “turn on” when concentration gradients are off balance.

False. Pumps work continuously to maintain gradients.

What is the function of the Na⁺/K⁺-ATPase pump?

Maintains electrochemical gradients:
3 Na⁺ pumped out, 2 K⁺ pumped in, using 1 ATP.

What would happen to membrane potential if Na⁺/K⁺ pumps stopped working?

Na⁺ would accumulate inside, K⁺ would leak out, leading to loss of electrochemical gradient and disrupted secondary transport.

How do cells use Na⁺ gradients to import glucose?

Via secondary active transport — glucose moves into the cell against its gradient by coupling to Na⁺ movement down its gradient (symport).

In a healthy lung epithelial cell, what does the CFTR channel do?

Moves Cl⁻ ions out to the mucus layer; water follows by osmosis to keep mucus thin.

In CF patients, CFTR is nonfunctional. Predict:

Cl⁻ concentration in mucus

• Water content

• Mucus thickness

↓ Cl⁻ secretion → ↓ water → thick, sticky mucus → poor clearance of bacteria.

How does a CF drug that keeps CFTR channels open longer help patients?

Allows more Cl⁻ and water into mucus → rehydrates mucus and improves clearance.

Define “hypotonic” solution.

Lower solute concentration than the cell → water enters cell → cell swells.

Define “hypertonic” solution.

Higher solute concentration than the cell → water exits cell → cell shrinks.

Define “isotonic” solution.

Equal solute concentration inside and outside → no net water movement.

Red blood cells placed in a hypotonic solution will…

Swell and burst (lyse) as water enters the cell.

Red blood cells placed in a hypertonic solution will…

Shrink (crenate) as water leaves the cell.

Why must IV fluids be isotonic?

To prevent red blood cells from swelling or shrinking, which would impair function or cause hemolysis.

Plant cells in a hypotonic solution do not burst because...

Their cell wall provides structural support; water entry creates turgor pressure.

Explain what happens to a plant cell in a hypertonic solution.

Water leaves → plasma membrane pulls away from cell wall → plasmolysis.

Describe how water moves through membranes.

Via osmosis, primarily through aquaporins, which are channel proteins for water.

Which of the following describes secondary active transport?

Uses energy from one molecule moving down its gradient to move another against its gradient (e.g., Na⁺/glucose symporter).

If a cell’s ATP production stops, which transport processes would immediately cease?

Active and secondary active transport (e.g., Na⁺/K⁺ pump, Na⁺-dependent glucose uptake). Passive diffusion continues.

Draw a cell membrane showing:

• Phospholipid bilayer (hydrophobic tails, hydrophilic heads)

• Channel protein

• Carrier protein

• Pump
  Label which require ATP.

• Check diagram:

• Channels: passive

• Carriers: passive

• Pumps: active, ATP → ADP + Pi

Draw water movement across a semipermeable membrane where side A has 5% salt and side B has 10% salt.

Water moves from A → B (toward higher solute/lower water concentration).

Predict what happens if a cell in isotonic solution is moved into pure water.

Environment becomes hypotonic → water enters cell → it swells/bursts (if animal cell).

Predict what happens to osmotic balance if extracellular Na⁺ levels rise.

Water moves out of cells (hypertonic environment), causing cells to shrink.

Predict what happens if the Na⁺/K⁺ pump is inhibited by a toxin.

Na⁺ builds up inside, K⁺ leaks out → gradient collapses → secondary active transport fails → cell swells due to osmotic imbalance.

A student says: “Active transport always moves ions into the cell.”

Incorrect. Active transport can move substances into or out depending on the pump’s direction; the key is that it moves against the gradient and uses ATP.

What would happen if both sides of a membrane had equal water and solute concentrations?

Dynamic equilibrium — molecules continue moving but with no net flow.

What type of transport moves bulk materials like proteins or waste?

Vesicular transport (endocytosis and exocytosis), both requiring ATP.

Exocytosis involves which sequence of steps?

Vesicle from Golgi → moves to plasma membrane → fuses → releases contents outside cell. ATP and cytoskeletal elements are required.

Endocytosis involves…

Plasma membrane invaginates → engulfs external material → forms vesicle inside cell.

Phagocytosis vs. Pinocytosis — what’s the difference?

• Phagocytosis: Cell “eating” large particles/bacteria.

• Pinocytosis: Cell “drinking” small fluid droplets.

Which are true of facilitated diffusion?
☐ Uses ATP
☐ Uses transport proteins
☐ Moves down gradient
☐ Moves up gradient
☐ Saturable (can reach max rate)

Uses transport proteins
Moves down gradient
Saturable — rate limited by number of carriers

No ATP is used.

A cell is placed in a solution that causes no net water movement. Which of the following must be true?
A) The cell’s cytoplasm is hypertonic.
B) The external solution is isotonic.
C) Water concentration is higher outside.

B — Isotonic solutions produce no net water flow.

In cystic fibrosis, how does reduced water transport affect bacterial infection risk?

Thick mucus traps bacteria and impairs cilia movement, increasing infection susceptibility.

Draw and label the structure of a phospholipid and show how they arrange in a membrane.

Heads = hydrophilic (face water)

tails = hydrophobic (face inward); form bilayer barrier to polar substances.

Describe the relationship between diffusion rate and concentration difference.

Greater concentration difference → faster diffusion (until equilibrium).

If glucose levels are equal inside and outside the cell, but Na⁺ gradient is maintained, will glucose transport continue via Na⁺/glucose symporter?

Yes — as long as Na⁺ moves down its gradient, it can drive glucose uptake (secondary active transport).