IGCSE Biology 0610

3.2 Osmosis

The Role of Water as a Solvent in Organisms

  • Dissolved substances easily transported
  • Digested food molecules moved to cells
  • Toxic substances and excess salts dissolve in water and urinated out
  • 75% of cytoplasm and most chemical reactions in cells are in water

Osmosis

  • Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
  • Water diffuses through partially permeable membranes by osmosis
  • Water moves into and out of cells by osmosis through the cell membrane
Bonus! Identifying higher and lower water potential

Think of higher water potential as having a higher concentration of water molecules. Water itself would obviously have lots and lots of water molecules. But if you were to mix some salt or sugar into the water, the concentration of water molecules would become lesser and therefore have a lower water potential than just water. So the more solute is added to the water, the lower the water potential would be.

Investigating Osmosis

Dialysis Tubing:

Dialysis tubing filled with orange sugar solution is placed into a beaker of water. The levels of water and sugar solution are aligned. After 30 minutes, the level of sugar solution has moved up.

Why?

The water is a dilute solution with high water potential. The sugar solution is a concentrated solution with low water potential. Water molecules diffuse through the dialysis tubing, which is partially permeable, to the sugar solution through osmosis.

Osmosis in Plant Tissues:

Distilled water has higher water potential than cell sap of potato. Since there is a water potential gradient, water molecules diffuse into cells through osmosis.
↓
Cell swells as water pushes against cell walls creating turgor pressure
↓
Cell contains as much water it can hold
↓
Cell wall stops it from bursting
↓
Cell is turgid

Turgid cells give plants support
(potato becomes longer)
Water potential of sugar solution & cell sap of potato about the same.
↓
No water potential gradient, no diffusion of water in or out
↓
No change
Cell sap of potato has higher water potential than concentrated sugar solution. Since there is a water potential gradient, water molecules diffuse out of cells through osmosis.
↓
Sap vacuole shrinks
↓
Cells become flaccid
↓
Cytoplasm moves away away from cell wall
↓
Cells are plasmolysed

(potato becomes shorter)

Importance of Water Potential and Osmosis for Organisms

Plants

  • Plants need osmosis to maintain cells being turgid
  • Plant cells become flaccid if they lose too much water
  • Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall

Animals

  • If too much water diffuses out of animal cells, they shrink
  • If too much water diffuses into animal cells, they burst as there is not cell wall
Previous Topic

Table of Contents

Next Topic
Scroll to Top