Investigation Into Osmosis Potato Cylinders Essay Research

Investigation Into Osmosis, Potato Cylinders Essay, Research Paper

Introduction

Knowing that osmosis (a diffusion of water) will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and knowing that when this happens to cells they will either become turgid if water flows into them, or plasmolysed if water flows out of them, and thus change their volume, we want to test the hypothesis that:

If the concentration of a solution into which a cylinder of potato is placed is greater than a certain level the cylinder will contract, and if the concentration is less than that level it will expand.

We have studied turgidity and plasmolysis in a textbook (Key Science-Biology, pages 143-144) and in a preliminary experiment, where we first added 2% sucrose solution to rhubarb epidermal cells, and saw them become plasmolysed, and then added water, and saw them become turgid. However, we did not use different solution concentrations, and did not measure the amount of contraction or expansion that took place. From our results in the main experiment, we should be able to work out not only the amount of contraction or expansion caused by each strength of solution, but also the concentration of the sap inside the cells.

Apparatus

For the experiment we will require:

Either cylinders of potato with a diameter of 6.5mm and a height of 5mm, or a potato, a borer with a diameter of 6.5mm and a scalpel. (To allow us to make our own).

Solutions of varying strengths (of sucrose and NaCl), or a solution of a known strength and distilled water. (To allow us to make our own).

Pins (To ensure that cylinders remain separate while in the solutions.)

Test-tubes

Callipers (To measure cylinder height and diameter.)

Diagram One of the test-tubes during the experiment. Three potato discs on a pin, not touching.

Method

We take a cylinder of potato, with a diameter of 6.5mm, from the potato, and cut it into separate cylinders each with a height of 5mm. We then thread at least three of the cylinders, to make the experiment fair (in case one of the cylinders is abnormal or damaged), on to a pin, keeping them apart from each other. We then make up solutions of either sucrose or sodium chloride, either by % strength or by molarity, and place 4 millilitres of each strength into a separate test-tube. We used a range of % sucrose solutions, going from distilled water (0%) to 2% (which we knew from earlier experiments would plasmolyse the cells), and a range of sodium chloride solutions from distilled water (0) to 0.4 molar (which would again be enough to plasmolyse the cells). We then place each of the sets of three cylinders on a pin into each of the different solutions, making sure that the cylinders are covered by the solution, and leave all of the test-tubes close to each other for 24 hours.

We assume that this means that the pressure and temperature in each case is the same, as these are factors which could affect osmosis, and we know that the volume, size and surface area of each cylinder is the same, and as they are all from the same potato, the only variable that we are altering is the concentration of the solution. Although ideally the experiment would be repeated several times, we were not able to do this as we did not have sufficient time.

After 24 hours we remove the cylinders from solution and, with callipers, which are more accurate than a ruler and would cover the likely range of sizes (from 4mm to 7mm), measure the new diameter and height of the cylinders. The results, in table and graph form are recorded below in the Results section.

Results

ConcentrationCylinder Diameter/mmCylinder Height/mmVolume/mm3 (2dp)Ave. Cylinder Volume/mm3

Pre-immersion6.56.56.5555165.92165.92165.92165.92

Sodium Chloride solution

0.0 Molar6.86.665.56.45.2199.74218.96147.03188.58

0.1 Molar66.56.84.44.94.9124.41162.6177.95154.99

0.2 Molar5.65.95.754.54.5123.15123.03114.83120.34

0.3 Molar66.15.94.94.94.5138.54143.2123.03134.92

0.4 Molar5.9655.65.45153.1152.6898.17134.65

Sucrose Solution

0%6.876.85.75.55.3207.01211.66192.48203.72

0.25%5.56555.55118.79155.5198.17124.16

0.50%55.255.56.65107.99140.1798.17115.44

1%5.554.95.95.15140.17100.1494.29111.53

2%4.44.64.44.85.24.472.9986.4266.975.44 Concentration of SolutionAverage % Change in Volume From Original

NaCl solution

0.0 Molar13.66

0.1 Molar-6.59

0.2 Molar-27.47

0.3 Molar-18.68

0.4 Molar-18.84 Concentration of SolutionAverage % Change in Volume from Original

Sucrose Solution

0%22.78

0.25%-25.17

0.50%-30.42

1%-32.78

2%-54.53

Analysis

The results show that, in accordance with our hypothesis, the cylinders will expand when external solute concentration is low (high water concentration), and contract in strong solutions (low water concentration). This is due to osmosis, where water passes from weak solutions to strong solutions across a semi-permeable membrane, such as a cell membrane. The graphs of % change against solution strength show that the results tend to form a curve, crossing

Внимание, отключите Adblock

Вы посетили наш сайт со включенным блокировщиком рекламы!
Ссылка для скачивания станет доступной сразу после отключения Adblock!

Скачать