These questions ask you to predict a net movement, compare diffusion rates, define osmosis, or work out what happens to an animal or plant cell in a solution. Begin every time by naming what moves and drawing an imaginary arrow between the two sides.
Choose the process, direction, and rate signal
For diffusion, track the particles named in the question: their net movement is down their concentration gradient, even though individual particles continue moving randomly in both directions. No direct cellular energy input is needed. A larger concentration difference across the same distance makes the gradient steeper. Higher temperature generally increases particle speed, a larger surface gives more routes at once, and a shorter path reduces travel distance. For example, the same gas diffuses faster across a thin broad surface than across a thick narrow one when other conditions match.
For osmosis, track water only and check for a partially permeable membrane. Water moves from higher water potential to lower water potential. If an impermeable solute makes one side more concentrated, the same direction can be described as from the more dilute side toward the more concentrated side. These are two valid descriptions of the same water movement, not competing rules. Osmosis is passive even when energy-driven pumps helped establish a gradient beforehand.
| Cell and surroundings | Reasoning route |
|---|---|
| Animal cell in sufficiently dilute surroundings | Water enters; without a wall the cell swells and may burst. |
| Animal cell in concentrated surroundings | Water leaves and the cell shrinks. |
| Plant cell in dilute surroundings | Water enters; the wall resists expansion and the cell becomes turgid. |
| Plant cell in sufficiently concentrated surroundings | Water leaves; the membrane can pull away from the wall, producing plasmolysis. |
Avoid these tempting routes
| Tempting route | How to reject it |
|---|---|
| Reverse diffusion or claim there is no net movement immediately | Random motion occurs both ways, but unequal concentrations create a net movement from the more crowded region toward the less crowded region. |
| Attach ATP use to diffusion or osmosis | Both processes are passive. Energy-powered pumping is a different mechanism. |
| Choose cooler conditions because particles move slowly | Slower particle motion generally slows diffusion; warming generally speeds it when other conditions stay constant. |
| Define osmosis as solute movement or omit the membrane | The moving substance is water, and a partially permeable membrane is part of the definition. |
| Swap animal and plant cell outcomes | Use the wall test. Animal cells lack a wall and can burst; plant walls resist expansion and allow turgor. |
| Say the plant wall disappears during water loss | In plasmolysis the membrane and living contents withdraw from the wall; the wall remains. |
Try it
Other conditions remain constant. Which changes make a concentration gradient steeper and generally increase diffusion rate? Select all that apply.
- Increase the concentration on the already higher side.
- Raise the lower concentration until both sides are equal.
- Lower the higher concentration toward the lower one.
- Decrease the concentration on the already lower side.
A model lists four geometry changes: increasing surface area, decreasing surface area, shortening diffusion distance, and lengthening diffusion distance. Enter the exact number of changes that speed diffusion.
A partially permeable membrane separates a dilute solution from a concentrated solution, and the solute cannot cross. What is the net direction of water movement?
- The water has no net movement under these conditions.
- The solute crosses toward the dilute side and pulls no water.
- The water moves from the concentrated side toward the dilute side.
- The water moves from the dilute side toward the concentrated side.