Factors That Influence the Rate of Diffusion

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-When an animal cell is surrounded by a hypotonic solution (a solution which has more water molecules than the cell’s contents. Water will enter the cell by osmosis and the cell will swell and burst.

Osmosis does not occur in an isotonic solution (a solution which has the same amount of water molecules as the contents of the cell which it surrounds), but movement of water molecules, still occurs. The amount of water that enters the cell is the same amount that exits.

CELLULAR STRUCTURES INVOLVED IN OSMOSIS AND THEIR FUNCTIONS

The vacuole contains the cell sap which is a dilute solution containing many dissolved nutrients.

The cytosol of the cytoplasm is also a dilute solution containing many dissolved nutrients and gases.

The water molecule concentration of both these solutions (cytosol and vacuole) help determine the direction of osmosis.

The tonoplast (vacuolar membrane) and the cell membrane both provide the selectively permeable barrier for osmosis to occur.

The cell wall is strong, rigid and is made of cellulose, which prevents osmotic bursting of the plant cell when it fills up with water.

OSMOSIS IN PLANT CELLS

When a plant cell is surrounded by a hypotonic solution, water will enter the cell and the cell will become turgid.

Turgid cells

Water enters the cell by osmosis, the vacuole swells and pushes against the cell wall. The cell doesn’t burst because the cell wall protects it, because the cell wall is not elastic like the cell membrane so it can only stretch a little. The vacuole takes in the water and become full, the cell stretches a little and the water pressure inside the vacuole builds up until it pushes against the wall with enough force to prevent more water from entering. The cell then becomes bigger and has a round shape.

When a plant cell is surrounded by an isotonic solution, the cell remains unchanged.

If a plant cell is surrounded by a hypertonic solution, water will leave the cell by osmosis and the cell will become plasmolysed. Plasmolysis is the tearing away of the cell membrane from the cell wall as water leaves the vacuole and cytoplasm by osmosis.

FLACCID PLANT CELLS

This occurs when there is just enough water inside to keep the cell membrane against the cell wall, but the vacuole is not full enough to exert a pressure against the wall and keep out more water.