What is osmosis, and what happens to cells placed in different solutions?
Define osmosis as the movement of water across a partially permeable membrane and explain its effects on animal and plant cells
A scaffolded answer to the N(A)-Level Biology outcome on osmosis. What osmosis is, the partially permeable membrane, and what happens to animal and plant cells in dilute and concentrated solutions, including turgid, flaccid and plasmolysed states.
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What this dot point is asking
This outcome wants a clear definition of osmosis, which is a special kind of diffusion that only involves water. You should be able to explain what a partially permeable membrane is, say which way water moves, and predict what happens to animal cells and plant cells placed in dilute or concentrated solutions. The key terms to use correctly are turgid, flaccid and plasmolysed for plant cells.
The answer
What osmosis is
Osmosis is the net movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution), across a partially permeable membrane. Like diffusion, it is passive and needs no energy. A dilute solution has lots of water and little solute; a concentrated solution has little water and lots of solute.
The partially permeable membrane
A partially permeable membrane has tiny holes that let small water molecules pass through but hold back larger solute molecules such as sugar. The cell membrane is partially permeable, so water can move in and out by osmosis while sugars and salts stay put. This is why osmosis only moves water.
Plant cells in different solutions
- In a dilute solution (more water outside than in the cell), water moves into the cell by osmosis. The vacuole swells and pushes the cytoplasm against the cell wall. The cell becomes firm, or turgid. The strong cell wall stops it bursting. Turgid cells keep a plant upright.
- In a concentrated solution (less water outside), water moves out of the cell. The vacuole and cytoplasm shrink, and the membrane pulls away from the cell wall. The cell is plasmolysed. A cell that has lost water but is not yet plasmolysed is flaccid, and the plant wilts.
Animal cells in different solutions
Animal cells have no cell wall, so they are easily damaged.
- In a dilute solution, water moves in and the cell swells and may burst (this is called bursting, or lysis), because there is no wall to resist the pressure.
- In a concentrated solution, water moves out and the cell shrinks and shrivels (it becomes crenated).
Examples in context
Example 1. Wilting plants on a hot day. When a plant loses water faster than its roots take it up, the cells lose water and become flaccid. Without the firmness of turgid cells, the leaves and stem droop, so the plant wilts. Watering it restores turgor and the plant stands up again.
Example 2. Why we feel thirsty after salty food. Eating very salty food raises the salt concentration of the blood, lowering its water concentration. Water then tends to move out of body cells by osmosis. The body senses this and makes you feel thirsty so you drink water to restore the balance.
Try this
Q1. Define osmosis. [2 marks]
- Cue. The net movement of water from a region of higher water concentration to a region of lower water concentration, across a partially permeable membrane.
Q2. State the term used to describe a plant cell that is firm and full of water. [1 mark]
- Cue. Turgid.
Q3. Explain why a red blood cell bursts in pure water but a plant cell does not. [2 marks]
- Cue. Both take in water by osmosis, but the plant cell has a strong cell wall that resists the pressure, while the animal cell has no wall to stop it bursting.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksA piece of potato is left in pure water for an hour. Using osmosis, explain what happens to the mass of the potato and why.Show worked answer →
The mass of the potato increases.
The cell sap inside the potato cells is a more concentrated solution (it contains sugars and salts) than the pure water outside. So the water outside has a higher concentration of water molecules than the cell contents. Water therefore moves into the potato cells by osmosis, across the partially permeable cell membranes, down the water concentration gradient. As water enters the cells, they gain mass, so the whole potato gains mass.
What markers reward: the direction of water movement (into the cells), the reason (the cell sap is more concentrated, or the outside has more water), the words osmosis and partially permeable membrane, and the conclusion that the mass increases.
Original4 marksDescribe and explain what happens to a plant cell and to an animal (red blood) cell when each is placed in a concentrated salt solution.Show worked answer →
In a concentrated salt solution, the solution outside has a lower concentration of water than the cell contents, so water leaves both cells by osmosis.
Plant cell: as water leaves, the cytoplasm and vacuole shrink and the cell membrane pulls away from the cell wall. The cell becomes plasmolysed. The cell wall stops the cell collapsing completely.
Animal (red blood) cell: as water leaves, the cell shrinks and shrivels (it becomes crenated). With no cell wall, it has no support and changes shape badly.
What markers reward: water leaving both cells by osmosis, the correct term plasmolysed for the plant cell and shrunken or crenated for the red blood cell, and the point that the plant cell wall prevents total collapse while the animal cell has none.
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