To find out how different concentrations of sucrose solution affect the incipient plasmolysis of different root vegetables until the isotonic point has been reached.

AS Biology Coursework

Introduction: - to find out how different concentrations of sucrose solution affect the incipient plasmolysis of different root vegetables until the isotonic point has been reached.

Prediction

Osmosis is defined as the net movement of water molecules from a region of high water potential to a region of low water potential across a partially permeable membrane down the concentration gradient. A partially permeable membrane allows some molecule to diffuse through but not all, if the molecules are small enough like water molecules then they can diffuse through easily but large solute molecules can’t pass through. The water molecules will diffuse both ways through the partially permeable membrane however the net movement will be to the side with a lower concentration of water molecules.

Water potential is the potential of water molecules to diffuse out of a solution, also water molecules are more likely to diffuse out of a solution containing a higher concentration of water molecules and they are less likely to diffuse out of a solution containing a lower concentration of water molecules. Pure water contains the highest water potential which is zero kilopascals 0(ψ) All solutions have a lower water potential then pure water hence there water potentials are always negative, the more solute molecules present the lower more negative the water potential.

The water potential inside cells can be calculated depending on two factors:

Water Potential (ψ) = Solute Potential (ψs) + Pressure Potential (ψp)

The solute potential is the amount of solute molecules in a solution, the more solute molecules present the more weak chemical bonds formed with the water molecules slow down their movement.

Pressure Potential is caused by the existence of cell membranes and cell walls in plants, these exert a strong inward pressure on the cell and this can effectively be squeezing water molecules out of the cell.

Solutions that have the same solute potential and water potential are isotonic which means Water Potential = Solute Potential. Therefore if they are separated by a partially permeable membrane there is no net movement of water molecules.

Cells are affected by the water potential of solution in which they are surrounded by, there are three different types of solutions, Isotonic, Hypotonic and Hypertonic.

When plant cells are placed into an isotonic solution the water molecules pass into and out of the cell in equal amounts because the water potential in the solution is equal to the solute potential.

If a plant cell was placed in a hypotonic solution which has a lower solute potential and a higher water potential then water molecules will enter the plant cell making the net movement of water molecules into the cell, this makes the vacuole swell up and pushes the cytoplasm against the cell wall and the plant cell becomes turgid.

The opposite to this occurs in a hypertonic solution which contains a higher solute potential and a lower water potential therefore net movement of water molecules will be out of the cell and into the hypertonic solution because there is a higher water potential in the cell then there is in the hypertonic solution causing the cytoplasm to shrink and the membrane pulls away from the cell wall and the cell becomes flaccid which is called plasmolysis.

Preliminary Work

Swede:

Potato:

Carrot:

Parsnip:

Looking at my results I can see that the potato had the greatest percentage mass decrease which was -40% which means theoretically it should have the lowest isotonic point also the parsnip showed the smallest mass decrease which was -7.69% therefore should have the highest isotonic point out of all the root vegetables. Furthermore the carrot has the second highest percentage mass decrease which was 18.75% meaning it should have the second lowest water potential, also the swede has the second smallest mass decrease which was -6.67%.

Depending on the different types of root vegetables used they all will have different water potentials and solute potentials, I can use the core borer to examine which root vegetable is the juiciest ie. Which one has the most water inside, also I could taste the root vegetable and the one that tastes the sweetest will have the most sugar inside therefore will have a higher solute potential and lower water potential.

I predict that the higher the concentration of sucrose used such as 0.6M and 0.7M, there will be a greater decrease in length and mass of the root vegetable and it will become more flaccid. This is caused because there is a greater solute potential and a lower water potential in the sucrose solution and a higher water potential in the root vegetable. Therefore the net movement of water molecules is from the root vegetable cell into the sucrose solution. Water molecules have a tendency to move from a region of high concentration due to osmosis to a region of low concentration. The overall net movement of water molecules is out of the cells of the root vegetable therefore the mass and length will decrease until equilibrium is reached, this occurs when the water potential inside the root vegetable is equal to the water potential in the sucrose solution and there is no net movement of water molecules if a partially permeable membrane is placed between the solution and the root vegetable.

If the solution has a lower solute potential such as 0.3M sucrose solution, I predict that the root vegetables will become turgid and therefore increase the greatest in length and mass because 0.3M sucrose solution has the greatest amount of water molecules and the least amount of solute molecules. This occurs when there is a higher water potential in the sucrose solution and a lower water potential inside the root vegetable cells therefore this creates a bigger concentration gradient as there is a wider distance so the molecules travel faster and hence there is an increase in the rate of osmosis. The net movement of water molecules are into the cell from a high water potential to a low water potential. As the water molecules enter the cell it becomes turgid which means the vacuole will swell up and push the cytoplasm to the end of the cell wall. The greatest hypotonic solution should exist at 0.3M. When the cell becomes turgid this will create a pressure potential which will cause the protoplasts in the cell to push against the cell wall which cause the mass and length to increase.

Also at 0.4M sucrose solution the root vegetables should show a increase in length and size but not as much as in the 0.3M because more solute molecules are present and hence less water molecules. A hypotonic solution should exist at 0.4M.

I also predict that the isotonic point in all the root vegetables will occur around 0.5M sucrose solution. All the root vegetables should not change in length or mass at this point therefore I predict that the isotonic point will be reached at 0.5M.

At 0.6M there will be a higher solute potential and lower water potential therefore the net movement of water molecules will be out of the root vegetable and into the hypertonic solution, therefore I predict the root vegetable will decrease in length and mass. A hypertonic solution should exist at 0.6M.