Osmosis in Animals and Plants

Section 1 of 6Osmosis and water potential (Extended)

Definition

• Definition 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.

Why use water potential?

• Why use water potential?
  • Talking about the 'concentration of water' is confusing, because we also call solutions concentrated when they contain a lot of solute.
• A dilute solution has a high water potential; a concentrated solution has a low water potential. Pure water has the highest water potential of all.

Direction of travel

• Direction of travel Water always moves down its water potential gradient, from dilute to concentrated, until the two sides are balanced.

Section 2 of 6Plant cells in dilute solution

1Higher water potential outside

• Higher water potential outside The cell is placed in distilled water or a very dilute solution.

2Water enters by osmosis

• Water enters by osmosis Water molecules move into the cell through the partially permeable membrane.

3Turgor pressure rises

• Turgor pressure rises The vacuole swells and pushes the cell membrane against the cell wall.

4Cell becomes turgid

• Cell becomes turgid The strong wall stops the cell bursting, and turgid cells help to support the plant.

Section 3 of 6Plant cells in concentrated solution

1Lower water potential outside

• Lower water potential outside The cell is placed in a concentrated solution.

2Water leaves by osmosis

• Water leaves by osmosis Water moves out of the cell and the contents shrink.

3Cell becomes flaccid

• Cell becomes flaccid Turgor pressure falls, so the plant loses support and wilts.

4Plasmolysis

• Plasmolysis If enough water is lost, the membrane pulls away from the cell wall.
  • Under a microscope the cell is described as plasmolysed.

Section 4 of 6Animal cells compared

Solution	Water movement	Result for an animal cell
Lower water potential than the cell (concentrated, e.g. strong sugar solution)	Water leaves the cell by osmosis	The cell shrinks and becomes crenated (shrivelled)
Higher water potential than the cell (e.g. distilled water)	Water enters the cell by osmosis	The cell swells and, with no cell wall to resist, may burst
Same water potential as the cell	No net movement	The cell stays the same size

Section 5 of 6Key terms

Term	Meaning
Water potential	A measure of the tendency of water to move; dilute solutions have a high water potential and concentrated solutions have a low one.
Turgid	Describes a plant cell that is firm and full of water, with the membrane pressing on the cell wall.
Flaccid	Describes a plant cell that has lost water and become soft because turgor pressure has fallen.
Plasmolysed	Describes a plant cell in which the cell membrane has pulled away from the cell wall.
Crenated	Describes an animal cell that has shrivelled after losing water by osmosis.
Turgor pressure	The pressure of the cell contents pushing outwards against the cell wall.

Section 6 of 6Exam technique

1

Saying water moves from 'high concentration to low concentration' without specifying water potential — write 'from higher to lower water potential'.

2

Saying an animal cell becomes turgid; only plant cells have a cell wall, so animal cells swell and burst instead.

3

Mixing up flaccid and plasmolysedflaccid means soft through loss of turgor, plasmolysed means the membrane has pulled away from the wall.

4

Forgetting to mention the partially permeable membrane in a definition of osmosis.

5

Describing a dilute solution as having a low water potential — it is the other way round.