You can tell these three apart with three questions: is energy from respiration used, is the particle water crossing a partially permeable membrane, and which way is the concentration gradient? Once you ask them in order, the choice stops being a guess.
The fix is a decision routine and a habit of naming the direction. This page supports movement across membranes, and the confusion often returns in plant transport and excretion and homeostasis.
What is actually going wrong?
There are usually three causes.
- Memorised definitions with no examples. The words are known but not connected to a case.
- Osmosis treated as a separate event. It is diffusion of water, with a membrane and a direction.
- Direction stated loosely. “Water moves into the cell” lacks the reason, because it does not say from where or why.
A routine for choosing
Ask these in order.
- Does the cell use energy from respiration? If particles move against the gradient, from low to high concentration, it is active transport.
- Otherwise, is it water crossing a partially permeable membrane? Then it is osmosis: water moves from the dilute solution to the more concentrated one.
- Otherwise, are particles simply moving down their concentration gradient? Then it is diffusion.
- State the direction and the gradient in your answer, using the numbers or the words in the question.
| Gradient | Energy from respiration | Particles | |
|---|---|---|---|
| Diffusion | Down (high to low) | No | Any |
| Osmosis | Water, dilute to concentrated | No | Water only |
| Active transport | Against (low to high) | Yes | Ions or molecules |
Worked example
A student places potato cylinders in two liquids for two hours and records the mass of one cylinder each time.
| Liquid | Start mass (g) | End mass (g) |
|---|---|---|
| Pure water | 5.0 | 5.6 |
| Sugar solution | 5.0 | 4.3 |
Pure water. Is energy used? No, a potato cylinder in a dish is not using respiration to move water. Is it water crossing a membrane? Yes. The cell contents are more concentrated than pure water, so water moves into the cells by osmosis. Change: +0.6 g, or 0.6 ÷ 5.0 × 100 = +12%.
Sugar solution. The solution outside is more concentrated than the cell contents, so water leaves the cells by osmosis. Change: −0.7 g, or −0.7 ÷ 5.0 × 100 = −14%.
A third case. A root hair cell absorbs nitrate ions, although the soil water has a lower concentration of nitrate than the cell. That is against the gradient, so the cell must use energy from respiration: active transport.
Independent check. The mass rose in the solution that was more dilute than the cell and fell in the solution that was more concentrated. The direction of mass change matches the direction of water movement.
The mistake to watch for
A common slip is to call every movement of water “diffusion” and lose the reason.
Question: Explain why the potato cylinder in sugar solution lost mass.
Mistaken answer: “Water diffused out because there was more water inside.”
The idea is close but vague. It does not name osmosis, the membrane or the concentrations.
The correction: “The sugar solution is more concentrated than the cell contents, so water moved out of the cells by osmosis through the partially permeable membrane. The cylinder lost water, so its mass fell.” The answer names the process, the gradient and the membrane.
Check yourself
1. Oxygen moves from the air in the alveolus into the blood. No energy is used. Which process, and why?
Show answer
Diffusion. Oxygen moves down its concentration gradient, from a higher concentration in the alveolus to a lower concentration in the blood, without energy from the cell.
2. A plant in very salty soil wilts. Explain, using the correct process.
Show answer
The soil solution is more concentrated than the solution in the root hair cells, so water leaves the cells by osmosis through the partially permeable membrane. The cells lose water, so the plant wilts.
3. Cells in the lining of the small intestine absorb glucose when the concentration of glucose in the gut is lower than in the cells. Which process, and what must the cells use?
Show answer
Active transport. Glucose moves against the concentration gradient, so the cells must use energy from respiration.
What can you do next?
Try drawing a three-column chart for each new case you meet, using the routine above. The inheritance model board and the probability tree and counting board show the same habit of stating assumptions before drawing a conclusion, in a different part of the course.
For more questions, use the original practice hub. When you know the routine but still hesitate on new cases, online one-to-one Biology tuition lets a teacher give you unfamiliar examples and listen to how you decide.