Author: Biology Junction Team

Cells and Transport Teaching Resources · High school biology; AP extensions where noted.

Use these original questions after teaching diffusion, membrane permeability, and tonicity. They replace an obsolete interactive quiz with a readable assessment that needs no plugin. Students should explain a prediction before looking at the feedback.

Objectives and setup

Students will distinguish movement from net movement, identify missing information, and classify a transport mechanism. Materials: notebook or a printed copy of the questions. Allow a short independent response followed by discussion; this is a formative check, not a complete unit test.

Student questions

1. Compartments A and B contain the same gas at 40% and 20%, respectively, under otherwise comparable conditions. An open connection allows the gas to move. Describe molecular movement in both directions and the initial net direction.
2. A molecule has a higher concentration outside a cell. Is that enough information to conclude it will diffuse through the cell membrane? Explain what is missing.
3. An animal cell contains 0.20 M nonpenetrating solute and is placed in 0.35 M nonpenetrating solute. The membrane permits water to cross, and initial pressures are equal. Predict its initial water movement and volume change.
4. A membrane protein carries a substance downhill along its relevant gradient without energy coupling. Is the process necessarily active? Name a suitable mechanism.
5. A pump moves an ion against its electrochemical gradient using ATP. Explain what distinguishes this from a passive channel.
6. At equilibrium, a student draws no arrows across a permeable membrane. What should change in the drawing?

Answer explanations

1. Gas molecules move both A→B and B→A. The initial net movement is A→B; it is incorrect to say all molecules move one way.
2. Permeability is missing. A gradient alone does not make a membrane permeable. For ions, electrical conditions also influence the net direction.
3. Water initially moves net out and the cell tends to shrink. The external solution is hypertonic relative to this cell under the given assumptions.
4. No. This description fits facilitated diffusion. Proteins participate in both passive and active mechanisms.
5. The pump couples an energy input to uphill transport. An open passive channel permits downhill movement and does not maintain that uphill transfer by itself.
6. Draw arrows in both directions with equal overall rates. Zero net movement is not zero molecular motion.

Teaching notes

Ask students to name the moving substance and permeability assumption before grading the direction. If a student answers “salt water shrinks cells,” ask what the salt concentration is relative to the cell: the phrase alone is underspecified. Score one point for a supported prediction and one for reasoning on each question. Use patterns of error to select the next lesson rather than treat the score as a comprehensive achievement measure.

AP extension: Revisit Question 2 with an ion. Ask how an electrical gradient could reinforce or oppose the concentration gradient, and what measurements would resolve the prediction.

Continue the unit

• Revisit osmosis, diffusion, and tonicity
• Review mechanisms before retrying
• Move to the cumulative transport review

Sources and further reading

• OpenStax Biology 2e: passive transport
• OpenStax Biology 2e: active transport