Investigate the water potential of celeriac.

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Adhal Mahmood

Aim

Investigate the water potential of celeriac.

        Apparatus

Celeriac – this is the specimen that we will be finding the water potential of.

1.00 mol dm-3 sucrose solution – this will be placed in the test tube in which the experiment will take place. We will dilute the solution to produce a range of concentrations.

10.0ml Graduated Pipettes (x2) – I had a choice of 5.0ml, 10.0ml and 20.0ml graduated pipettes. I chose this size pipette because it is time efficient and accurate at the same time.  It is an accurate piece of equipment because as opposed to standard pipettes, this graduated pipette has 0.1ml graduations to ensure the utmost accuracy. One pipette will be used for water and one will be used for sucrose solution.

250ml Beakers (x2) – these will be used to hold water and the 1.00 mol dm-3 sucrose solution in a safe environment.

Cork Borer – this will be used to cut the precise shape of the celeriac. This will also ensure that the surface area to volume ratio is constant throughout the experiment.

Razor – this will be used to cut the pieces of celeriac to a precise length.

Ruler – this will be used to measure the length of celeriac that will be used in the experiment.

Scales – they will be used to measure the mass of celeriac before and after the experiment with accuracy. Therefore, the mass increase/decrease can be calculated after the experiment. These scales give a digital measurement to 2 decimal places; therefore they are more accurate than conventional scales.

Paper towels (x5) – they will be used to absorb any excess solution from the celeriac when it is removed from the appropriate test tube.

Stopwatch – this will be used to time the experiment. It is accurate to 2 decimal places.

Test tubes (x5) – this is where the experiment will take place. The test tube will be used to provide a safe environment for the experiment to take place in.

Test tube rack – this will be used to hold the test tubes in a steady position when the experiment is carried out.

        Prediction

I predict that as the concentration of the sucrose solution increases, the mass of celeriac will decrease. I think this because, as the concentration of the sucrose solution increases, the water potential of the solution increases in negativity. So therefore as the molarity increases, the water molecules will travel by osmosis from the less negative celeriac to the more negative sucrose solution.

When a solute and solvent are mixed together, they produce a solution. For example, a sucrose solution is consists the solute, sucrose and the solvent, water. Osmosis can be defined as ‘the movement of water molecules from a region of high water potential to a region of lower negative water potential through a partially permeable membrane’ 2.

The water potential of pure water is 0kPa and the water potential of 1.00 mol dm-3 sucrose solution is -3512kPa. Also water potential is defined as ‘a measure of the ability of any object or substance to draw water into itself; an object (such as a cell wall) that has a negative water potential will draw water into itself from any other object that has a less negative water potential’3.

Water potential can be worked out using the following formula – water potential = osmotic potential (solute potential) + pressure potential. The addition of a solute will decrease water potential and the addition of pressure will decrease water potential. In our experiment, the pressure potential will be constant, so it will not affect the water potential. However, if a solute such as a reducing sugar is present, the water potential of a cell will increase in negativity.

Hypertonic Solutions

If in the above diagram, the red dots represent solute molecules and the black dots represent water molecules, the solution to the left of the membrane has a greater solute potential than the solution to the right of the membrane, and therefore has a more negative water potential. The solution on the left is said to be hypertonic compared to the one on the right. As osmosis states, the water molecules must move from a less negative water potential to a more negative water potential. Therefore water molecules will move from the right side of the membrane, to the left side of the membrane. If the right side of the membrane represents a cell and the left side is a hypertonic solution, then the cell would become flaccid and possibly collapse due to the large loss of water 4.

                                                                             

        Hypotonic solutions

If in the above diagram, the red dots represent solute molecules and the black dots represent water molecules, the solution to the right of the membrane has a greater solute potential than the solution to the left of the membrane, and therefore has a more negative water potential. The solution on the left is said to be hypotonic compared to the one on the right. As osmosis states, the water molecules must move from a less negative water potential to a more negative water potential. Therefore water molecules will move from the left side of the membrane, to the right side of the membrane. If the right side of the membrane represents a cell and the left side is a hypotonic solution, then the cell would become turgid and possibly burst due to the large intake of water 4.

Plasmolysis occurs when a cell loses a vast amount of water, usually in a hypertonic solution.. The cell is usually in a solution, such as concentrated sucrose solution. Water leaves the cell by osmosis due to a water potential gradient, and the protoplast steadily shrinks away from the cell wall. When the protoplast no longer touches the cell wall, the pressure potential is zero. The cell is now plasmolysed 5.

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When water potential outside of the cell is equal to the water potential inside of the cell, it is in a state of equilibrium.

From previous experiments, I know that carrots have more sucrose dissolved in them than celeriac and potatoes.

        Results for reducing sugar test

When I carried out the Benedict’s reagent test for reducing sugars, such as glucose, I found that when the carrot was heated in a water bath, the solution of Benedict’s reagent turned from the Copper II Sulphate blue, to a brick red precipitate. Whereas when the potato was heated in the water ...

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***** This is a thoroughly and carefully written up account of the investigation. The author has a clear understanding of water potential theory and uses A level biological terminology accurately and appropriately throughout. There is a high level of attention to detail.