3.1.5B1. Characteristics of Living Organisms and Cells

Osmosis in Plants — Combined Science 0653 - Biology Revision Notes & Topical Questions

5 min read 7 parts Lesson 14 of 14 Unit quiz

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Part 1 of 5Osmosis and water potential

Osmosis

Osmosis Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
A dilute solution has a high water potential, while a concentrated solution has a low water potential.

Plant cell structure

Plant cell structure A plant cell has a partially permeable cell membrane inside a strong, freely permeable cellulose cell wall.
The wall can resist expansion, which is why plant cells behave differently from animal cells in water.

1Higher water potential outside

Higher water potential outside Water has a higher water potential outside the cell than in the cytoplasm and vacuole.

2Water enters by osmosis

Water enters by osmosis Water moves in through the membrane, and the vacuole and cytoplasm swell.

3Membrane pushes on wall

Membrane pushes on wall The swelling cell presses its membrane against the cell wall, increasing the turgor pressure.

4Cell becomes turgid

Cell becomes turgid The wall stops the cell bursting, and the firm cell helps to support the plant.

1Lower water potential outside

Lower water potential outside The outside solution has a lower water potential than the cell contents.

2Water leaves by osmosis

Water leaves by osmosis Water moves out of the vacuole and cytoplasm through the membrane.

3Turgor is lost

Turgor is lost The cell becomes soft and limp, which is described as flaccid.

4Plasmolysis

Plasmolysis With enough water loss the cytoplasm shrinks and the membrane pulls away from the cell wall;
- the cell is then plasmolysed.

State	Surrounding solution	Water movement	Appearance
Turgid	Dilute, with water potential higher than the cell	Net movement in	Firm, with the membrane pressed to the wall
Flaccid	Same as the cell, or somewhat more concentrated	Little net movement, or slight loss	Soft and limp
Plasmolysed	Very concentrated, with lower water potential than the cell	Net movement out	Membrane pulled away from the wall

Support

Support Turgid cells press against each other and make the stems and leaves of non-woody plants firm and upright.

Wilting

Wilting If a plant loses more water than it takes up, its cells become flaccid and the plant wilts and droops.
Watering allows water to enter again by osmosis and the plant recovers.

Over-fertilising

Over-fertilising Too much fertiliser makes the soil water more concentrated than the root cells, so water leaves the roots and the plant can wilt.

Term	Meaning
Osmosis	The net movement of water molecules from a higher to a lower water potential through a partially permeable membrane.
Water potential	A measure of the tendency of water to move; pure water has the highest water potential.
Turgid	Firm and swollen with water, with the membrane pressed against the cell wall.
Flaccid	Soft and limp because the cell has lost water and turgor.
Plasmolysis	The shrinking of the cytoplasm away from the cell wall when a cell loses a lot of water.
Turgor pressure	The pressure of the cell contents pushing against the cell wall.

Saying water moves from a 'high concentration of water' without using water potential. Use higher to lower water potential.

Writing that plant cells burst in pure water. The cell wall prevents this, so they become turgid.

Confusing flaccid with plasmolysedin plasmolysis the membrane has actually pulled away from the wall.

Describing osmosis as the movement of solute. It is the movement of water.

Forgetting 'partially permeable membrane' in the definition of osmosis.

Last updated: 20 October 2026