OPEN-ENDED / EXAM-STYLE

A substance reaches equilibrium across a membrane. Have its molecules stopped moving?

No. Molecules continue moving in both directions, but movement is equal in each direction, so there is no net diffusion.

A molecule's diffusion distance doubles. Predict what happens to the time required for diffusion.

Approximately 4× longer.

A membrane suddenly becomes more permeable to a substance while surface area and concentration gradient remain unchanged. Predict the effect on diffusion.

Diffusion rate increases because membrane permeability is one of the factors determining diffusion rate.

A Na⁺ channel opens. Does opening the channel automatically mean Na⁺ will enter the cell?

No.

The channel provides the pathway. The gradient determines the direction of net movement.

Why can simple diffusion continue increasing with solute concentration while facilitated diffusion eventually plateaus?

Facilitated diffusion depends on a finite number of carriers.

At high concentrations, all carriers become occupied → saturation → transport maximum.

Simple diffusion doesn't have this carrier limitation.

A cell doubles the number of carrier proteins for a substance. What happens to its transport maximum?

Transport maximum increases because more transporters are available.

Why is secondary active transport considered active if its transporter doesn't directly hydrolyze ATP?

It uses energy stored in an ion concentration gradient—usually Na⁺—to move another substance against its gradient.

The Na⁺ gradient was established using energy from primary active transport.

The Na⁺/K⁺ ATPase is inhibited. Predict what will eventually happen to Na⁺-glucose cotransport.

Na⁺-glucose transport will decrease.

Without the Na⁺/K⁺ pump, the Na⁺ gradient will eventually decrease. Secondary active transport therefore loses its energy source.

A 300 mOsm cell is placed into 400 mOsm nonpenetrating solute. Predict the result

ECF = hypertonic
→ water moves OUT
→ cell shrinks.

A 300 mOsm cell is placed into 200 mOsm nonpenetrating solute. Predict the result.

ECF = hypotonic
→ water moves IN
→ cell swells.

Explain how the Na⁺/K⁺ pump and Na⁺-glucose symporter work together even though they are on different sides of an epithelial cell.

The basolateral Na⁺/K⁺ pump uses ATP to keep intracellular Na⁺ low.

That creates the Na⁺ gradient.

Na⁺ then enters across the apical membrane down its gradient through the Na⁺-glucose symporter.

The energy of Na⁺ movement drives glucose into the epithelial cell.

Trace glucose from the intestinal/kidney lumen completely across an epithelial cell into the ECF. Identify the transport mechanism and energy source at every step.

Lumen

↓ Na⁺ + glucose symporter

Secondary active transport
Energy = Na⁺ gradient

↓

Epithelial cell

↓ glucose carrier

Facilitated diffusion
Energy = concentration gradient

↓

ECF

Meanwhile:

Na⁺/K⁺ ATPase on the basolateral membrane uses ATP to maintain the Na⁺ gradient that makes the entire process possible.