Diffusion and Osmosis: How Substances Move In and Out of Cells — Answers
Answer sheet with model answers and success criteria for the 8th Grade cells & classification worksheet.
Success TutoringWorksheetsAnswer sheet8th Grade · Science · Cells & ClassificationDiffusion and Osmosis: How Substances Move In and Out of Cells — Answers
Part A: Diffusion
Particles in liquids and gases are always moving randomly. Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated, until they are spread evenly.
Cells rely on diffusion. Oxygen diffuses from the blood into cells, where it is used in respiration, and carbon dioxide diffuses out. Diffusion is faster when the temperature is higher, the concentration difference is larger and the distance is shorter.
Answer: higher
Particles spread out until they are evenly mixed.
- AUsing warmer water
- BUsing colder water
- CUsing a taller beaker
- DAdding the drop more slowly
Answer: Using warmer water
Particles move faster at higher temperatures, so diffusion is faster.
Answer: In: oxygen (or glucose). Out: carbon dioxide (or other wastes such as urea).
Answer: False
Diffusion is passive: it happens because of the random movement of particles, so no energy from the cell is needed.
Part B: Osmosis
The cell membrane is partially permeable: it lets small molecules such as water through but stops larger ones. Osmosis is the movement of water across a partially permeable membrane from a dilute solution (more water) to a concentrated solution (less water).
Plant cells that take in water become firm, or turgid, because the cell wall resists. Plant cells that lose water become soft, or flaccid. Animal cells have no cell wall, so they can burst if too much water enters.
- ASalt
- BSugar
- CWater
- DOxygen
Answer: Water
Osmosis is a special case of diffusion involving water only.
- AWater moves in, and the cell swells and may burst
- BWater moves out, and the cell shrinks
- CNothing, because water cannot cross the membrane
- DSalt moves into the cell
Answer: Water moves in, and the cell swells and may burst
The cell contents are more concentrated than pure water, so water moves in. Animal cells have no cell wall, so the cell may burst.
Answer: The strong cell wall stops the cell from expanding too far. The cell becomes firm (turgid) instead of bursting.
Answer: The salty water is more concentrated than the cell contents, so water leaves the cells by osmosis. The vacuoles shrink and the cells lose their firmness, so the leaf goes limp.
Part C: Potato strip experiment
Students cut three potato strips, each with a mass of 5.0 g. They placed one strip in each solution for 30 minutes, dried them gently and measured their mass again.
Results: pure water 5.5 g; medium salt solution 4.6 g; strong salt solution 4.2 g.
Percentage change = (final mass − starting mass) ÷ starting mass × 100
Answer: +10%
- Change = 5.5 − 5.0 = +0.5 g
- 0.5 ÷ 5.0 × 100 = +10% (a gain)
Answer: −8%
- Change = 4.6 − 5.0 = −0.4 g
- −0.4 ÷ 5.0 × 100 = −8% (a loss)
Answer: −16%
- Change = 4.2 − 5.0 = −0.8 g
- −0.8 ÷ 5.0 × 100 = −16% (a loss)
Answer: In pure water, water moved into the potato cells by osmosis, so the strip gained mass. In the salt solutions, water moved out of the cells by osmosis, and the stronger the solution, the more water was lost, so the greater the loss in mass.
Answer: It allows a fair comparison even if the strips do not all start with exactly the same mass.
Part D: Why are cells so small?
Answer: Surface area 6 cm², volume 1 cm³, ratio 6 : 1
- Surface area = 6 faces × (1 × 1) = 6 cm²
- Volume = 1 × 1 × 1 = 1 cm³
- Ratio = 6 : 1
Answer: Surface area 24 cm², volume 8 cm³, ratio 3 : 1
- Surface area = 6 × (2 × 2) = 24 cm²
- Volume = 2 × 2 × 2 = 8 cm³
- Ratio = 24 : 8 = 3 : 1
Model answer: As a cube gets bigger, its volume increases faster than its surface area, so its surface area to volume ratio falls (6 : 1 for the 1 cm cube, but only 3 : 1 for the 2 cm cube). A cell takes in oxygen and nutrients and removes wastes through its surface by diffusion. A small cell has a large surface area compared with its volume, so substances can enter and leave quickly enough, and the distance to the center is short. A giant cell would not have enough surface for its volume, and diffusion to its center would be too slow, so large organisms are made of many small cells.
Success criteria:
- States that the surface area to volume ratio decreases as size increases, using the calculated values
- Links surface area to the exchange of substances by diffusion
- Explains that diffusion over long distances is too slow
- Concludes why organisms are made of many small cells
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.