What effect different solutions, with the same water potential, have on potato and cucumber strips (by mass).

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Jessica Rossi

What effect different solutions, with the same water potential, have on potato and cucumber strips (by mass).

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Introduction

Plants in the soil have their roots in a dilute solution of various mineral ions. Sometimes, the soil they are planted in gets flooded with sea water (which has an average of 0.3 sodium chloride concentration). In these cases the roots are observed to wilt and become flaccid. In my experiment, I will simulate this situation using potato and cucumber strips, placing them in different solutions (sodium chloride, glucose and sucrose) at different molar concentrations.

A substance dissolved in water is called a solute. A solvent is a liquid that is able to dissolve another substance, a solute, to form a solution.

Water potential is a measure of the ability of a solution to give out water. Water potential (ψ) can be expressed as the sum of the solute potential (ψs) and the pressure potential (ψp).          (ψ) = (ψs) + (ψp)

The water potential of pure water is zero and all other solutions have a negative water potential. The greater the solute concentration, the more negative is the value for water potential.

Background Information

The water content of plants depends on environmental conditions. In land plants, water plays a vital role in structural support and mineral transport, thus, the lack of water may lead to wilting or possibly death.

Water is mainly absorbed through the roots by Osmosis. Osmosis is the diffusion of water molecules from and area of high water potential to an area of low water potential through a partially permeable membrane. The roots are covered in specially adapted root hair cells, which increase the surface area, and have thin walls which facilitate the process of Osmosis. The evaporation of water through the leave’s stomata causes a transpiration stream, causing the water to be drawn up through the xylem vessels.

Whether the water enters by Osmosis will depend on the balance between external and internal solute and water potentials. If the solutions on each side of the partially permeable membrane have the same water potential, then there will be no net movement of water molecules across the membrane.

The concentration of solute particles is described as molarity. One mole of any substance is the mass of 6.02 x 1023 particles of any substance.

If a plant was exposed to a waterlogged environment, with the external solute concentration to the cell being hypotonic to the vacuole contents, the cell will not continue to take in water by osmosis forever. This is because the cellulose wall provides a rigid barrier to uncontrolled expansion. A cell, when it is full of water, is described as turgid, and cannot expand as the inward force of the starched wall balances the outward pressure on the cell contents. This wall pressure is called turgor pressure, and the internal outward force on the wall is called osmotic pressure. At the other extreme, if a cell was to be placed in a solution that is hypertonic to its contents, it will lose water by Osmosis. The cytoplasm, in this case, will stop exerting a pressure on the cellulose wall and the cell, and will lack support. At this stage, the cell is described as flaccid. Water loss can continue to such extent that the cytoplasm, and the attached membrane, contracts and detaches itself from the cell wall. A cell in this condition is said to be plasmolysed, which is an irreversible damage.  

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        In my experiment, I will vary the solutions and their molarity, as well as using two different kinds of plant tissue samples; potato and cucumber cores.

        The dependant variables of this experiment are the changes in length and mass of the potato and cucumber cores, which should occur as a result of changing the dependant variables. However, calculating the change in length could lead to inaccurate results, as the measurements are more likely to be calculated imprecisely. Calculating mass, however, using a laboratory weighing scale, will give more accurate and detailed results, providing that each vegetable core ...

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