Osmosis In Potato Cells

For this coursework we are going to carry out tests to see the effects on sugar concentration on osmosis. My test subjects will be pieces of potato.

Before I begin, I will briefly explain my own understanding of Osmosis. This is a form of diffusion which occurs only in water. In this water will move from a high concentration of water to a low concentration through a partially permeable membrane until the concentration gradient is equal. Once this is balanced there will be no further net movement of water.

This shows that the transfer of water is from a high to a low concentration of water. It also shows that the solute cannot change which side of the partially permeable membrane it is on, because it is too large to pass through the membrane.

This knowledge will allow me to carry out a series of tests to investigate the affect different levels of solute will have on the speed, and quantity of water moved during osmosis.

Firstly I must decide on a suitable number f variables. I am going to use a scale called molar. This is a measure of concentration. I am going to use the following measurements in my investigation:

• 0M (molar) пÑ" distilled water

• 0.2M

• 0.4M

• 0.6M

• 0.8M

This should be enough of a range to give me accurate results. Now I will predict the results I will find.

Prediction

I think that when the potato piece is in the distilled water it will increase in mass by the most. This is because I think the concentration of sugar will be less in the solution that inside the potato. This should cause water to move via osmosis into the potato, increasing its mass. I think that similar results will occur in 0.2 that but the increase in mass will be to a lesser extent. In 0.6M and 0.8M I think that the mass of the potato piece will decrease, with 0.8M having the greater decrease. This decrease will be caused by the fact that the concentration of sucrose in the potato piece will be lower than that of the solution. This should cause a net movement of water out of the potato, causing a decrease in mass.

I feel that 0.4M will be the closest to an even concentration gradient out of all the measurements. Therefore, I predict that the mass will change the least, or possibly not at all, in 0.4M.

Next I must decide on what I need to keep constant and what I must vary to keep the test fair. The only variable will be the amount of sugar, so this will be my constant variable.

The things I am going to have to keep the same are:

• Time which the pieces are in the solution

• Surface area of the pieces

• Same Potato (type) must be used, to keep sugar levels as constant as possible

• Temperature of the water and the environment

These must be kept constant in order to ensure that the only thing affecting the rate of osmosis is the sugar levels in the solution.

ALSO all pieces must be dried before the experiment and after for weighing so that there are accurate results.

To keep these constant and unvaried, we must keep all the test tubes in the same environment, and cover them so that no water evaporates.

The environment I will chose is an incubator, in which all the test tubes will be in the same environment and the test as accurate as possible. This will also make it easier to keep the other variables (Time, surface area, temperature) constant.

To measure the differences between the solutions, I am going to weigh the pieces before and after the experiment 3 TIMES each. To do this I will use a very sensitive set of electronic scales. From these measurements I will calculate an average difference and percentage change for each different value. Finding an average will help keep the results accurate. The percentage change in mass will allow us to compare the pieces in relative terms, i.e., they will all be in the same ratio (just like converting inches to cm to make understanding relations easier).

Once I have the results, I will place the results in the table in a table. I will then transfer them onto a percentage change (mass)/ sugar concentration graph. This will allow me to see the results clearly and concisely.

Once I have the graph plotted, I will be able to plot a line of best fit, which in turn will allow me to discover the concentration of the sugar in the potato. This will be indicated where the line crosses the Y axis.

This is an example of the table I will use to write my results on:

Concentration of sugar (/m) Initial mass (/g) Finishing mass (/g) Average mass change (/g) %change in mass

0

0.2

0.4

0.6

0.8

Healthy & Safety

During this experiment there are a few rules which must be abided by to ensure a safe environment in which the test can be carried out. These are:

 Clear desks

 Wearing goggles at ALL TIMES

 DO NOT misuse apparatus

 Be extra careful with sharp or dangerous apparatus (e.g. knife, scalpel)

List of apparatus

Cork borer

2 beakers

15 test tubes

Scales-electronic accurate to 2 D.P

Tile

Measuring cylinder

Knife/Scalpel

Solutions

Stop clock