An Investigation Into Factors Affecting The Rate Of Osmosis

Plan

Aim

To find out if osmosis occurs in a potato, and to observe how different concentrations of a solution affect the rate of osmosis in the potato.

What Is Osmosis?

Before actually planning the experiment, I will do some research to find out about osmosis, and matters related to it, so that I can make predictions and work out a way to make this investigation fair and safe. Planning ahead would assist me in finding out how to do what, when to do it etc. This should lead me to good results at the end of the experiment.

Osmosis is the movement of water molecules from a solution of high water concentration to an area of low water concentration, through a semi permeable membrane. Below is a diagram illustrating the process of osmosis:

Water potential is the tendency of water to move from an area of higher concentration to one of lower concentration. This is a main factor that affects the rate of osmosis, as the higher the concentration of the water, the more likely it is to move to an area of low water concentration.

In this case, the tiny holes in the membrane of the potatoes will allow the water molecules to pass through in and out of the solution and the potato, depending on the concentration gradient of the two substances. So in this case, when the water concentration is lower in the potato, the water will go inside the tissue of the potato, and the potato will gain weight. If there is very little different in the two water concentrations, there shouldn’t be such a big change in weight. And if there is a higher concentration of water in the potato, the water will go out of the potato. The potato cells will have decreased in length, volume and mass due to a process called plasmolysis.

The cell is surrounded by water so it absorbs the water making the cells firm and turgid thus increasing the size and weight of the overall potato piece. However the cell does not rupture because the walls are very strong and thick. The cell is surrounded by a weak solution so there is not much change in the weight of the cell because the concentration is equal so there is no movement of water particles.

The cell is surrounded by a high concentration of salt solution so there is a lower concentration of water molecules outside the cell ...

This is a preview of the whole essay

The cell is surrounded by a high concentration of salt solution so there is a lower concentration of water molecules outside the cell so through osmosis the water molecules move from the cell into the surrounding liquid until equilibrium is reached. This leaves the cell flaccid due to the process of plasmolysis.

The solute concentration of a potato is 0.27m therefore I would expect the highest changes in mass to occur in higher concentrations than 0.27m rather than closer to it.

Factors Affecting Rate Of Osmosis

There are several factors that affect the rate of osmosis, namely:

Temperature

Cells move quicker at higher temperatures, therefore the higher the temperature the higher the rate of osmosis.

Difference In Water Potential (Key Variable)

The higher the water potential, the more likely it is for osmosis to occur due to the increased tendency for the molecules to move from an area of higher concentration to one of lower concentration.

Pressure

Areas of different pressure have different water potentials.

Surface Area

The larger the surface area, the more water can be absorbed into the cells, therefore altering the rate of osmosis.

Hypothesis

The rate of osmosis would increase when the difference in water potential is high.

Method

Apparatus:

Clock – To assist in timing our experiment
Cutting tile – To cut the potato on
Knife – To cut the potato
Petri Dish – To store the potato in the solution.
Different Molar Solutions (o-1m at 0.2m intervals) – To test the rate of osmosis.
Potatoes – To test the rate of osmosis
Tissue paper – To dry the potatoes/scales after the osmosis takes place.
Weighing scales – To weigh the potatoes at regular intervals
Cork borer – To cut out potato cylinders

First we will cut out 5 pieces of potatoes from the same potato using a borer so that we have all the potato strips at the same length and same mass. We are using potatoes because it is a cheap and efficient way of conducting the investigation as they have large amounts of homogenous tissue and this is perfect for osmosis. The cell surface membrane of potato cells acts as the semi-permeable membrane for osmosis to take place.

Then we will cut the potato with more accuracy using a magnifying glass and a ruler making sure that all the potatoes are as equal as possible in length. There will be 5 test tubes, 3 pieces of potato in each test tube. After we have cut all of the potato pieces, we will weigh them individually. They will be put with in the test tube and we will try to make the mass as even as possible as well.

After we have done this we will wipe each potato and put them in there test tube and then leave the potatoes in there for 30 minutes. Every 5 minutes we will weigh the potatoes and record any mass change then after 30 minutes we will take out the potatoes from the test tube. We will then wipe the potatoes again so there is no liquid on them and then weigh them to see the difference in mass. I think that this is an accurate way of proving whether concentration affects the rate of osmosis in a potato because this way we can see whether a potato gains or loses mass and at what rate due to different concentrations.

After the experiment I shall use my table of results to calculate the percentage mass change of each potato. I am using percentage mass change as it is an accurate way of calculating the rate of osmosis rather than using units such as grams etc, this would only be specific to the rate of osmosis in potatoes. Percentage mass change is a collective method of analysing the results. Below is a diagram illustrating the method and the apparatus used:

Fair Testing

Fair testing is essential in all investigations. If this experiment is not a fair test, we will be obtaining incorrect results, which could lead us to the wrong conclusions as they are not providing a true representation of the data at hand.

First of all, and most importantly, we will have to get the measurements and the weights of the solutions and the potatoes as exact, and as accurate as possible. We will try and get the measurements of the potatoes as accurate as possible for every single potato, evenly cutting the potato pieces, and making a record of the length to the nearest millimeter. And we will be using a very sensitive balance so that we can get the best readings possible. We also have to control all of the other factors that affect the rate of osmosis (previously stated).

Another important step in making this a fair test is to make sure that the potato is fully covered by the solution. This is because the potato should fully submerge, by having total contact with the solution.

When using the scales, we will make sure that the scale is reading zero before we put the potato pieces on it. This is so that we don’t get a false reading, with the weight of our potato with the reading it had before. And after the experiment, we will weigh the potato pieces that should be dried as much as possible, and weigh it the same way, taking the reading to the nearest 2 decimal places.

Carrying out the experiment in a constant temperature for the duration of the experiment is important. And to avoid temperature change, which might affect the different sets of results, we will take the temperature of the room/solution into account.

Getting and experimenting with the exact measurements of molars and water is vital to this task. If the volume of one solution in a test tube is higher or lower than another, it will affect the pattern of results later on.

Small things such as a unclean petri dish, and a slightly cracked measuring cylinder could still affect the results, and therefore we will take these into account as well.

Safety

Safety is an important aspect in every experiment, even if the experiment seems to be very harmless, and that is why we take this into consideration, no matter what.

We will be using a very sharp knife, which could injure someone if it is not handled properly. And we will also be careful that the solutions don’t get into our bodies internally, just in case, because we are not fully aware of the damage it could do to us. But other than that, there weren’t any other matters to be cautious of.