Comparing Osmosis in Plants & Animals (Cambridge (CIE) AS Biology): Revision Note

Cell membrane

• Present

• Composed of a phospholipid bilayer

• Selectively permeable

Cell wall

• Present

• Composed of cellulose

• Fully permeable

• Absent

Osmosis of water into and out of cell

• Can occur

Cell placed in solution with a lower water potential

• Water leaves through the selectively permeable cell surface membrane by osmosis

• Volume of the cell decreases

• Protoplast shrinks / pulls away from the cell wall

• Cell is plasmolysed

• Water leaves the cell through the selectively permeable cell surface membrane by osmosis

• Volume of cell decreases

• Cell shrinks / shrivels up

Cell placed in solution with a higher water potential

• Water enters the cell through the selectively permeable cell surface membrane by osmosis

• Volume of cell increases

• Protoplast expands / pushes against the cell wall

• The cell wall withstands the increased pressure created

• The pressure increases until the cell is rigid and firm (turgid)

• The cell is fully inflated with water and no more can enter

• Water enters the cell through the selectively permeable cell surface membrane by osmosis

• Volume of cell increases

• There is no cell wall to resist the increased pressure that is created

• The cell membrane is eventually overstretched and the cell bursts