Questions: Membrane Transport: All Mechanisms Integrated

Question 1

Glucose is transported from the intestinal lumen into epithelial cells against its concentration gradient by a protein that simultaneously allows Na⁺ to flow down its gradient. No ATP is consumed directly by this transporter. What type of transport is this?

• A. Primary active transport, because glucose is moving against its concentration gradient
• B. Secondary active transport, because it harvests energy from the pre-existing Na⁺ gradient (built by the Na⁺/K⁺-ATPase) to power the uphill glucose movement
• C. Facilitated diffusion, because a protein carrier is involved and no ATP is directly consumed by this transporter
• D. Simple diffusion, because the overall process does not require ATP at the point of glucose entry

Answer: (B)

The key distinction: secondary active transport does not use ATP directly but still moves a molecule against its gradient — it couples that uphill movement to the downhill flow of another ion (here Na⁺). Option C is the classic trap: 'no ATP consumed directly' sounds like facilitated diffusion, but facilitated diffusion only works downhill. The Na⁺/K⁺-ATPase elsewhere in the cell paid the energy cost by building the Na⁺ gradient; the glucose-sodium symporter spends that stored energy.

Question 2

Which transport mechanism is used when O₂ moves from the bloodstream across the plasma membrane into a metabolically active cell?

• A. Facilitated diffusion through a specific O₂ channel protein, since O₂ is too reactive to diffuse freely
• B. Simple diffusion directly through the lipid bilayer, because O₂ is small and nonpolar
• C. Primary active transport using an O₂-ATPase, since metabolic activity requires precise concentration control
• D. Endocytosis, since gases require vesicular packaging to cross the membrane safely

Answer: (B)

Small, nonpolar molecules like O₂, CO₂, and N₂ are the only substances that cross the lipid bilayer by simple diffusion — they dissolve into the hydrophobic core and pass through. No protein is needed and no energy is spent. This works because O₂ is moving down its concentration gradient (it is consumed by mitochondria, keeping intracellular concentrations low). The polarity and size rules determine mechanism: O₂ is small and nonpolar, so simple diffusion is the correct answer.

Question 3

Facilitated diffusion requires no energy input because the transported molecule is moving down its concentration or electrochemical gradient.

• T. True
• F. False

Answer: True

This is correct. Facilitated diffusion is passive — the channel or carrier protein lowers the activation energy for crossing the hydrophobic membrane interior, but the driving force is the preexisting concentration (or electrochemical) gradient. No ATP is consumed. The cell can only use this mechanism when the net movement is thermodynamically spontaneous, i.e., downhill. When a molecule must move uphill, the cell must use active transport (primary or secondary), which has an energy cost.

Question 4

Secondary active transport directly uses ATP hydrolysis to power the movement of molecules against their concentration gradient.

• T. True
• F. False

Answer: False

Secondary active transport does NOT directly use ATP. Instead, it couples the uphill movement of one molecule to the downhill flow of another (usually Na⁺ or H⁺), harvesting energy stored in a pre-existing ionic gradient. The gradient itself was built by primary active transport (e.g., the Na⁺/K⁺-ATPase, which does use ATP). This is a critical distinction: secondary active transport is indirectly powered by ATP, but no ATP is hydrolyzed at the secondary transporter itself.

Question 5

If the Na⁺/K⁺-ATPase in intestinal epithelial cells were completely inhibited, explain step by step how this would ultimately impair the secondary active transport of glucose across the apical membrane.

Model answer: The Na⁺/K⁺-ATPase normally pumps Na⁺ out of the cell, maintaining a low intracellular Na⁺ concentration and a strong inward Na⁺ electrochemical gradient. The glucose-sodium symporter on the apical membrane uses this gradient to pull glucose uphill into the cell. If the ATPase is inhibited, Na⁺ accumulates inside the cell. The Na⁺ gradient dissipates. Without the driving force of Na⁺ flowing inward, the symporter can no longer pull glucose against its gradient, and glucose uptake stops.

This question tests whether students understand that secondary active transport is ultimately powered by primary active transport upstream. The Na⁺/K⁺-ATPase is not at the same membrane as the glucose symporter, but its function is the prerequisite. This energy chain — ATP → Na⁺ gradient → glucose gradient — is a general principle. Many drugs and toxins (e.g., cardiac glycosides like digoxin) work by targeting the Na⁺/K⁺-ATPase, with widespread downstream effects on secondary transporters.