Questions: Passive Transport

Question 1

A red blood cell is placed in a solution with a much higher solute concentration than its cytoplasm (hypertonic). What will happen and why?

• A. The cell will swell as water moves in by osmosis down its concentration gradient
• B. The cell will shrink as water moves out by osmosis toward lower water potential outside
• C. Solutes will rush into the cell through the membrane by simple diffusion
• D. Nothing will happen because the cell membrane blocks all movement

Answer: (B)

In a hypertonic environment, the solute concentration is higher outside than inside the cell, which means water potential is lower outside. Water moves by osmosis from high water potential (inside) to low water potential (outside), causing the cell to lose water and shrink (crenate). This is a direct application of osmosis — water always moves toward higher solute concentration across a semipermeable membrane.

Question 2

Facilitated diffusion requires the cell to expend ATP because transport proteins need energy to move molecules across the membrane.

• T. True
• F. False

Answer: False

Facilitated diffusion is passive — no ATP is consumed. Transport proteins (channels or carriers) provide a pathway that lowers the energy barrier for crossing the membrane, but the driving force is still the concentration gradient. Molecules still move from high to low concentration spontaneously. ATP is only required for active transport, which moves substances *against* their gradient.

Question 3

Explain why oxygen (O₂) can cross the cell membrane by simple diffusion, but glucose requires a transport protein (facilitated diffusion).

Model answer: O₂ is a small, nonpolar molecule that dissolves readily in the lipid bilayer and passes directly through. Glucose is a large, polar molecule that cannot dissolve in the hydrophobic core of the membrane, so it requires a protein channel or carrier to provide a hydrophilic pathway.

The cell membrane's core is a nonpolar lipid bilayer. Small nonpolar molecules (O₂, CO₂, lipid-soluble vitamins) dissolve in this layer and cross freely. Large or polar molecules face a steep energy barrier because inserting them into the hydrophobic core is thermodynamically unfavorable. Membrane proteins solve this by creating hydrophilic tunnels or shuttling mechanisms — but crucially, these proteins do not add energy to the process, they just lower the barrier.