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

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
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