Osmosis

An investigation how sugar solutions of different concentrations affect osmosis.

Research

Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution to one of a higher concentration.

If a solution has a low concentration then the amount of (in the case of my investigation) sugar, the solute, will be low and the solution will be weak. If there is high amount of sugar then the solution will be concentrated (have a high concentration).

When two solutions are divided by a partially permeable membrane; water will move from the area where there is a high concentration of water to where there is low until both sides are equal. This is known as reaching equilibrium.

A cell is turgid when it swells to its maximum volume due to large water content. Opposite to this is plasmolysis (or flaccid) where there is not enough water in the cell and the cell membrane begins to contract.

Image courtesy of the BBC website.

Factors which affect osmosis:

* The difference in concentration affects the speed ay which osmosis takes place - the greater the difference in concentrations, the faster the water molecules will diffuse.

* External Pressure- osmosis is slowed down if there is external pressure acting against the flow of water molecules. This is due to the fact that will need more energy in order to overcome the force of pressure so they can pass through the partially permeable membrane to the other side.

* Size of solute molecules- if the solute molecules are large then they will block the partially permeable membrane. Water molecules will not pass through where the membrane is blocked; slowing osmosis down because fewer water molecules will be able to pass through.

* Temperature- an increase in temperature gives water molecules more kinetic energy which causes them to move faster meaning they also diffuse faster. However, the temperatures need to be controlled because high temperatures destroy living partially permeable membranes.

The aim of this investigation is to study how the concentration of sucrose solution affects the rate of osmosis in potato cells. The mass change of the cells will be observed when they are placed in solutions with different sucrose concentrations. The results will show how the difference in concentration on both sides of the permeable membrane.

I predict that the pieces of potato placed in sucrose solution with a concentration of 0-0.4 m/l will increase the weight of the potato and the cells will become turgid, most likely when the concentration is 0. This is because as it states in my research, the concentration of the sucrose determines how much water will transfer across the permeable membrane. When the potato pieces are placed in the sucrose solution with a concentration of 0.0-1.0, will decrease in weight as the water from the potato cells will transfer across the membrane into the solution where the concentration is higher then the cells. The cells in these potatoes, according to my research are likely to become flaccid. I also predict that there won't be a large change of weight in pieces placed in the solutions with concentrations of 0.4 and 0.6. This is because the difference in concentration in the water and potato will not be great.

I predict my results graph will look like:

Apparatus

5 Beakers

Potatoes

Scalpel

13mm Borer

Knife

Cutting tile

Paper towels

Stopwatch

Electronic balances

Ruler (mm)

150ml of each of the following concentrations of sucrose solution:

* 0.0m/l

* 0.2m/l

* 0.4m/l

* 0.6m/l

* 0.8m/l

* 1.0m/l

0.0m/l will be distilled water.

(m/l= moles/litre) (342g of sucrose in 1 litre)

*The concentration is the independent variable in this investigation as it changes so that osmosis can be measured.

Method

1. Cut 5 tubes of potato using a 13mm borer. Cut all tubes of potato to exactly 25mm. make sure that the tubes are of a similar surface area, discard any that were on oblique cuts as this affects the surface area and will interfere with the results obtained.

2. Weigh the pieces of potato and be careful not to mix them up with pieces of potato that will be placed in other beakers. Record these weights.

3. 2 pieces of potato are required for each concentration so an average mass can be calculated. This will ensure more accurate results.

4. Pour 150ml of 0m/l sucrose solution (distilled water) into one of the beakers.

5. Do the same for the full range of sucrose solution concentrations.

6. Put two pieces of potato into each beaker.

7. Leave the solution for twenty minutes.

8. Remove these pieces of potato and re-weigh them.

9. Record these masses in the table.

Safety

The main safety precaution in this investigation include being careful with the sharp instruments used to cut the potatoes. Safety goggles need not be worn as sucrose solution is neither damaging to the skin or eyes.

Variables

The dependent

variable for this investigation is the mass of the potato cells. The original mass will be recorded prior to the pieces of potato being placed in the solutions and after they have been removed from the solution after the set period of time. It is the dependent