Part 1 of 4What Osmosis Is
Definition
- Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration across a selectively permeable membrane.
Water concentration
- A dilute solution has a high water concentration, while a concentrated solution has a lower water concentration.
Solute
- The dissolved substance, such as sugar or salt; it cannot pass freely through the membrane.
Passive
- Like diffusion, osmosis needs no energy from the cell.
Part 2 of 4Animal Cells
Animal Cells
Hypotonic solution
- The outside has a higher water concentration, so water enters the cell, which may swell and burst.
Hypertonic solution
- The outside has a lower water concentration, so water leaves and the cell shrinks and shrivels.
Isotonic solution
- Water concentration is equal inside and outside, so there is no net water movement and the cell stays the same.
No cell wall
- Animal cells have no wall to resist swelling, so they are especially vulnerable in dilute solutions.
| Solution outside | Water moves | Effect on animal cell |
|---|---|---|
| Hypotonic | Into the cell | Swells, may burst |
| Isotonic | No net movement | No change |
| Hypertonic | Out of the cell | Shrinks |
Part 3 of 4Plant Cells
1
In a hypotonic solution water enters the cell by osmosis and the vacuole swells, pushing the membrane against the wall.
2
The cellulose cell wall is strong and stops the cell bursting, so the cell becomes firm and turgid.
3
In an isotonic solution there is no net movement, and the cell becomes soft and limp, described as flaccid.
4
In a hypertonic solution water leaves, the cytoplasm shrinks and the membrane pulls away from the wall; the cell is plasmolysed.
Part 4 of 4Exam Checklist & Tips
1.2.4 – Key Points for the Exam
- Osmosis The movement of water across a selectively permeable membrane from higher to lower water concentration.
- Turgid Firm, because a plant cell is full of water pressing on the cell wall.
- Plasmolysed A plant cell in which the membrane has pulled away from the wall after losing water.
- Hypotonic A solution with a higher water concentration than the cell.
- Hypertonic A solution with a lower water concentration than the cell.
- Flaccid Soft and limp, as a plant cell with little water pressure.
Common Mistakes
- Saying water moves towards a higher concentration of water;
- it moves from higher to lower water concentration.
- Writing that salt or sugar moves in osmosis; only water is involved.
- Forgetting that plant cells do not burst because of the cell wall.
- Mixing up the terms hypotonic and hypertonic; check which solution has more water.