Osmosis — Cambridge IGCSE Combined Science

Revision notes

4 short notes, in the order of the specification. Each one in short:

1. Water and osmosis

   Osmosis is the diffusion of water through a partially permeable membrane; water is also the solvent that carries dissolved substances in organisms.

2. Investigating osmosis

   Practical

   Osmosis can be investigated using dialysis tubing or potato cylinders in sugar solutions. A tubing bag of concentrated sucrose solution placed in water gains mass as water enters by osmosis. Potato cylinders gain mass in dilute solutions and lose mass in concentrated ones. Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100.

3. Water potential

   Supplement

   Osmosis is the net movement of water molecules from higher to lower water potential through a partially permeable membrane. Cambridge IGCSE Biology 3.2.

4. Turgor, flaccid cells and plasmolysis

   Supplement

   A turgid plant cell has gained water by osmosis, so its vacuole presses the cell contents against the cell wall; this pressure is turgor pressure. In a concentrated solution, water leaves and the cell becomes flaccid. If more water leaves, the cytoplasm shrinks and the cell membrane pulls away from the cell wall, which is plasmolysis.