Investigating Water Potential Of Potatoes.

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Investigating Water Potential Of Potatoes.

AIM

To investigate the effect of various sugar solutions (which have different concentrations of sugar solutions. on a potato. We will be investigating the length of the potato after a certain amount of time. Thus, enabling us to investigate the water potential of a potato.

INTRODUCTION

The fact that organisms rely on diffusion for the fulfilment of many of their needs has had a profound effect on their structure. Consider, for example, the way gas exchange relates to the size of an organism. The organism’s oxygen requirements (its needs) are proportional to its volume, i.e. the bulk of respiring tissue, which it contains. Its exchanges however, are proportional to the surface area over which diffusion of oxygen can take place. In an organism, the effective surface area must be sufficient to fulfil the needs of the respiring tissue (pg. 48 Biology a Functional Approach MBV Roberts).

This simple fact also occurs in Osmosis (movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane). Although the plasma membrane of a cell is fully permeable to respiratory gases, it is by no means permeable to all substances. The porous nature of the membrane means that only those molecules that are small enough will diffuse through it swiftly.

The plasma membrane is thus selectively permeable (or semi- permeable), permitting the passage of some substances but not of others. (Pg. 49, Biology a Functional Approach, MBV Roberts).

To appreciate the significance of this we must examine the properties of selectively permeable membranes. Consider the situation, where the mouth of a thistle funnel is covered with a selectively permeable membrane such as a cellophane or pig’s bladder. The funnel is then filled with a strong sugar solution and immersed in a beaker of pure water. What happens? Quite quickly, the level of solution in the tube starts to rise. Analysis of this solution shows that it gradually becomes more dilute, indicating that water is passing into it from the surrounding beaker.

In seeking for an explanation of this, it is necessary to appreciate that the membrane is permeable to the water molecules but impermeable to the much larger sugar molecules. With this in mind, consider the situation on either side of the membrane. In the beaker, there is nothing but water molecules and in the funnel, there are water molecules and sugar molecules. The presence of sugar molecules means that the concentration of water molecules in the funnel is less than that in the beaker. In other words, there is a net movement of molecules across a selectively permeable membrane, this is known as Osmosis. (Pg. 49, Biology a Functional Approach, MBV Roberts).

Osmosis is a special case of diffusion: it involves the passage of water molecules from a region of high water potential (the beaker) to a region of lower water potential (funnel).

It is the presence of sugar molecules in the funnel, which makes the concentration of water molecules lower on that side of the membrane. As water molecules continue to move from beaker to funnel a hydrostatic pressure develops in the latter. The head of pressure, which can be developed by a solution when permitted to take up water by osmosis, is known as osmotic pressure.

Osmosis will occur whenever solvent is separated from solute by a selectively permeable membrane. The membrane does not have to be completely impermeable to the solute molecules. Many naturally occurring membranes allow the passage of solute as well as solvent, though not to the same extents. All that is required for Osmosis to occur is that the solvent molecules move more rapidly than the solute molecules.

Plainly, the osmotic pressure a solution is capable of developing will depend on the concentration of its solute molecules because it is this which determines the concentration of the water molecules. Reverting back to the thistle funnel and beaker example, a much greater osmotic pressure will be developed if the sugar solution is saturated than if it is relatively dilute. It also follows that Osmosis will be reduced if solute molecules are present in the beaker aswell as in the funnel. If the concentration of solute is the same on both sides of the membrane, water molecules will move equally in both directions. If the solute concentration is greater in the beaker than in the funnel, there will be a net movement of water in the opposite direction –from funnel to beaker. (Pg. 49, Biology a Functional Approach, MBV Roberts).

Diagram 1(Pg. 49, Biology a Functional Approach, MBV Roberts).

Selectively permeable membrane

High Water Potential

Low concentration of solute molecules

High concentration of water molecules.

Low osmotic pressure

Low Water Potential

High concentration of solute molecules

Low concentration of water molecules.

High osmotic pressure

NET MOVEMENT OF WATER MOLECULES

After looking at diagram 1 and the previous information given, you will appreciate that there will be net movement of water molecules from the left of the selectively permeable membrane to the right. The reason, in terms of thermodynamics, is that the potential energy of the water molecules on the left is greater than the potential energy of the water molecules on the right. The potential energy of the water molecules is called the water potential. Water will diffuse from a region of high water potential to a region of lower water potential. The steeper the water potential gradient the greater will be the tendency for water to diffuse in this direction. (Mastering biology, OFG Kilgour, Pg. 89).

Having done research, I have found a number of things that will effect osmosis, and so are considered as variables, I have already mentioned a few in the research. Firstly, a higher temperature provides more energy to evaporate water and without water, osmosis cannot occur. By varying the temperature, the rate of osmosis may differ.

To continue, the size of the plant can also effect osmosis, this is because if the plant is big then there will be more cells, hence a greater are  for osmosis to occur.

However, this can also be dependent upon the concentration of the cells. If the concentration in the cell is higher than outside then osmosis will occur by removing water. However, if the concentration inside the cell is not too much more than out side the cell then osmosis will occur at a slower rate.

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Concentration is the variable we will be studying this will be done by using sugar as our concentration variable. We will be using potatoes of known masses and lengths, and placing it in a solution of known volume and known solute concentration. However each time the solute concentration will change.

PREDICTION

In my opinion, after researching, I think when the potato is placed in

distilled water without a concentration, the water potential of solute outside the cell will be less than that inside the cell, so the solution will be hypotonic. As always when the concentration/water ...

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