Diffusion & Osmosis

Learning objectives Describe the process of diffusion Give an example of facilitated diffusion Outline the process of osmosis State the effects of hypertonic and hypotonic solutions on cells and tissues

Dissolving KMnO4 crystal Diffusion Particles in a liquid or gas spread out… … from regions of high concentration… … to regions of low concentration… …until the particles are evenly spread out. Dissolving KMnO4 crystal

The difference between the regions of high concentration and low concentration is called the concentration gradient The steeper the concentration gradient, the faster diffusion takes place High concentration gradient Fast rate of diffusion Low concentration gradient Slow rate of diffusion

Diffusion occurs because the particles in gases and liquids are moving.

Dissolving substances in water The molecules in liquid water are constantly moving When water molecules bump into particles of a soluble substance, they stick to them Free moving water molecules Sugar molecules in sugar lump

When the water molecules move away… … they carry particles of the solute with them

Adding a solute to water reduces the amount of free water molecules

Partially permeable membrane Partially-Permeable Membranes A partially-permeable membrane will allow certain molecules to pass through it, but not others. Partially permeable membrane Generally, small particles can pass through… …but large particles cannot

Free water molecules diffuse in this direction Partially-permeable membrane More free water molecules on this side of membrane Water-solute particle is too large to pass through membrane Free water molecules diffuse in this direction

Osmosis Osmosis is the diffusion of free water molecules… … from a region of high concentration of free water molecules… … to a region of low concentration of free water molecules… …across a partially-permeable membrane… …until they are evenly spread out.

Osmosis and Animal Cells Sometimes called “haemolysis” Sometimes called crenation

Alternative image

Osmosis and Plant Cells

Osmosis and Tissues Consider potato cylinders placed in different solutions . . . . 20mm long Hypertonic Isotonic Hypotonic 17mm long 23mm long

Osmosis and Tissues Consider potato disks placed in different solutions . . . . 1.0g Hypertonic Isotonic Hypotonic 1.2g 0.8g

Internal Assessment Using a range of sugar solutions (or salt) of differing concentration and therefore water potentials, it is possible to determine the concentration of the cells. Where there is neither a gain or loss in mass (or length) is considered to be the isotonic point. This corresponds to the concentration of the cells. If students collect data individually and then collate the class results, standard deviation values can be drawn. This is particularly appropriate in larger schools where results can be collated across groups provided the same procedure is followed. Needless to say, the value obtained is only true for that variety of potato grown in a certain place and stored in the same conditions for the same period of time. Further investigative work can be done to compare varieties or different vegetables. Which will have the higher cell concentration, a potato or a sweet potato?

Error bars are ± 1 standard deviation. Drawn using Graphpad Prism. A curve would be more appropriate than connecting the points with straight lines.

Water potential  Water potential is not mentioned in the IB Biology guide but you may encounter it in some of your school’s textbooks. Water potential refers to the capacity of a system to lose water. The term system is used rather than solution as in turgid plant cells the cell wall will exert a pressure that will increase the water potential of the cell. Using a standard curve drawn from published data, it is possible to find the water potential (in kPa) by using the concentration value at the isotonic point. This is covered in clear detain in the UK A-Level text: Taylor DJ, NPO Green, and GW Stout. Biological Science 1 and 2. 3rd ed. :Cambridge University Press, 1997. Print.