Lab 15: Population Genetics - eScience Labs

Lab 7: Osmosis
There are three types of tonicity (Figure 1) —remember that it is a relative term used to compare one
solution to another:
A hypertonic solution contains a greater concentration of solutes unable to cross the membrane
compared to the solution on the other side of the membrane.
A hypotonic solution contains a lower concentration of solutes unable to cross the membrane
compared to the solution on the other side of the membrane.
An isotonic solution has an equal concentration of impermeable solutes on both sides of the
semi‐permeable membrane.
When osmosis takes place, water flows from hypotonic solutions to hypertonic solutions, until the solutions
become isotonic. In most biological systems, cells are hypertonic and extracellular water flows
into them. If placed in pure water, they will burst (lyse) as a result of the increased pressure on the
membrane from the additional water that diffused into the cell.
Osmotic pressure (the force required to prevent osmosis) is directly correlated with tonicity (higher
tonicity causes an increase in osmotic pressure). Some cells, such as plant cells, have specialized structures
that regulate osmotic pressure and prevent lysis.
Experiment 1: Direction and concentration gradients
In this experiment, we will investigate the effect of solute concentration on osmosis. A semi‐permeable
membrane (dialysis tubing) and sucrose will create an osmotic environment similar to that of a cell.
Using different concentrations of sucrose (which is unable to cross the membrane) will allow us to examine
the net movement of water across the membrane.
Materials
30% Sucrose solution
4 15cm Pieces dialysis tubing**
3 250mL Beakers
8 Rubber bands
10mL Graduated cylinder
Blue, red, yellow, green beads
Concepts to explore:
Water*
Watch*
*You must provide
**Cut to exact length
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