These eight original questions follow the lessons in movement across membranes. They get harder as you go, and each answer shows the reasoning. Write your own answer first, then open the reply.
The questions
Question 1. State the direction of movement of particles in diffusion.
Answer
Particles move from a region of higher concentration to a region of lower concentration, down the concentration gradient.
Question 2. A plant cell with cell sap equal to 0.25 M sucrose is placed in 0.05 M sucrose. Predict what happens.
Answer
The outside solution is hypotonic to the cell. Water moves into the cell by osmosis, the vacuole expands and the cell becomes turgid. The cell wall stops it from bursting.
Question 3. The same cell is placed in 0.50 M sucrose. Predict what happens.
Answer
The outside solution is hypertonic. Water leaves the cell by osmosis, the cell membrane pulls away from the wall, and the cell becomes plasmolysed.
Question 4. Why does an animal cell burst in a strongly hypotonic solution when a plant cell does not?
Answer
An animal cell has no cell wall, so it keeps taking in water until the membrane breaks. A plant cell has a rigid wall that resists the expansion, so it becomes turgid instead.
Question 5. Cell X has 1 unit of a substance inside and 7 outside, and cell Y has 4 inside and 6 outside. Which takes in the substance faster by diffusion? Explain.
Answer
Cell X. The gradient for X is 7 − 1 = 6 units and for Y it is 6 − 4 = 2 units. A steeper gradient gives a greater net movement of particles per second.
Question 6. Root hair cells take in nitrate ions even when the soil has a lower concentration of nitrate than the cell. Name the process and explain.
Answer
The process is active transport. The ions move against the concentration gradient, from low to high concentration. Carrier proteins in the membrane use energy from respiration.
Question 7. Carrot cylinders gained 6.0% in mass in 0.0 M sucrose, gained 1.0% in 0.2 M, and lost 7.0% in 0.4 M. Estimate the isotonic concentration.
Answer
The change crosses zero between 0.2 M (+1.0) and 0.4 M (−7.0). The fraction is 1.0 ÷ 8.0 = 0.125, so the point is 0.2 + 0.125 × 0.2 = 0.225 M. At that concentration there is no net movement of water.
Question 8. Write a conclusion for the data in Question 7 and name two variables to keep constant.
Answer
As sucrose concentration rises from 0.0 M to 0.4 M, the percentage change in mass falls from +6.0% to −7.0%, because water moves into the cells in weak solutions and out of them in strong solutions by osmosis. Keep the size of the cylinders and the time in solution constant. The temperature and volume of solution should also stay the same.
If you got these wrong
Questions 2 to 4 belong to predicting what happens to cells in different solutions. Questions 1, 5 and 6 belong to explaining concentration gradients.
Questions 7 and 8 need interpreting membrane transport experiment data. Note each slip in the mistake log tool, then build a timed round with the timed practice session builder.
If the same idea keeps going missing, a teacher in SPM Biology one-to-one tuition can check your written answers sentence by sentence. You can start with the one-hour trial class (from RM50).