Diffusion, Osmosis, Imbibition


Connect to directory of images related to the diffusion activity.

Students must be able to interpret specific examples of situations observed in lab and be able to answer questions demonstrating an understanding of the following concepts.


Heat: For diffusion to occur there must be molecular movement which is simply heat. The relationship between heat and motion can be illustrated by observing Brownian Motion which is the random motion of small particles suspended in water due to the uneven collisions of water molecules.

Diffusion: is the movement of a substances dissolved in a fluid from areas of higher concentration to areas of lower concentration. This was demonstrated by the movement of janus green dye and by potassium dichromate through getatine.

Differentially Permeable Membranes: allow some molecules to pass but not others. In lab, we used dialysis tubing to demonstrate this principle. Dialysis tubing is made of cellaphane, and the size of the pores in the cellaphane determines whether a given molecule can freely pass through.

Osmosis: is the diffusion of water across a differentially permeable membrane. There are three possible conditions in regards to a cell's water concentration relative to its environment:

1. Isotonic - the solute concentration is the same on either side of the cell membrane. This is the condition of the cells in the human body and in animal bodies in general. The fluids in our body (blood and plasma) are isotonic to the cells in our body. Healthy animal cells exist within an isotonic environment.

2. Hypotonic - the solute concentration is lower outside of the cell than inside. Under these circumstances a cell is at risk of bursting. If blood cells are placed in distilled water they will burst. Some protozoans have contractile vacuoles that expel excess water. Many protists, all the fungi and all plant cells regulate the inflow of water under these conditions by means of the cell wall. While the membrane is in contact with the cell wall the wall will provide a static force that equals the force generated by the tendency of water to move into the cell. This not only prevents the cell from bursting, but, in the case of herbaceous plants, also provides support like that provided by an inner tube in a tire. Healthy plant cells exist within a hypotonic environment.

3. Hypertonic - The solute concentration is greater outside the cell than inside. A cell under these conditions will tend to shrink. This often represents an abnormal, stressful condition (the cells of a wilting plant, dehydration due to drinking sea water etc.).

Turgor Pressure: Protoplasts of healthy plant cells are emmersed in a hypotonic medium. In this condition. water tends to move into the protoplast due to osmosis. However, the protoplast is bounded by a cell wall which limits its volume. This condition generates pressure sufficient to stop the net movement of water. This pressure is termed 'turgor pressure'. Turgot pressure is inmportant in the support of herbaceous plant tissues.

Diffusion and imbibition: Another physical phenomenon linked to diffusion is imbibition. Imbibition is the adhesion of water molecules to colloidal particles. For our purposes, it can be described as the diffusion of water into a dehydrated substrate having an affinity for water. Examples of imbibition include the expansion of seeds, doors and dry sponges when they absorb water. Imbibition is totally unrelated to osmosis.