Biology Essay

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Diffusion of Cell Size

Kayla Szabo
Loxton High School

Rate of Diffusion
Practical report
Diffusion is the movement of a substance from an area of high concentration to an area of low concentration. Cells need to utilise diffusion to exchange materials from outside of the cell to the inside of the cell for them to survive. Diffusion is a passive process, which means it doesn’t involve the need of energy.

For a cell to survive it relies greatly on its size or volume. The larger the cell, the less efficient the cell will be, as there will be relatively more surface area to exchange substances. A cells ability to exchange substances relies greatly on the fact that the surface area is sufficient for the volume of the cell. Hence the smaller the cell, the more surface area it has in comparison to its volume which means it’s more efficient in supplying the cell with the right amount of nutrients.

The theory of surface area to volume ratio and its effect on the rate of diffusion is the basis of the experiment to be conducted. Cell size will be modelled using agar cubes soaked in the indicator phenolphthalein. The cubes will be cut into various sizes of 1cm to 3cm, then will be immersed into the acid, therefor the rate of diffusion can be measured by the decolouration that take place as the acid diffuses into the agar cube.

Aim: To investigate how varying the surface area to volume ratio affects the rate of diffusion between three agar cubes.

Hypothesis: The greater the surface area to volume ratio of the phenolphthalein agar cubes, the greater the percentage of decolouration that takes place.

Independent variable: size of agar cubes
Dependent variable: volume (or %) of decolouration of agar cubes Factors held constant:
* Shape of agar cubes
* Type of concentration of acid (0.1M sulfuric acid)
* Amount of acid (150mL)
* Time in which cubes are immersed (10minutes)
* Temperature of the solution

* Phenolphthalein cubes
* Knife white tiles
* Ruler
* Paper towel
* Spoon
* 3 x 250mL beakers
* Dilute (0.1M) sulfuric acid
* 100mL measuring cylinder

1. Obtain an agar block that is sufficient for you to cut 3 cubes with following dimensions: 1cm³, 2cm³, and 3cm³. 2. Measure 150mL of the dilute sulfuric acid into each of the 250mL beakers 3. Place 1 cube into each beaker and begin timing. The acid in solution will diffuse into the agar and the rate of diffusion can be measured by the amount of decolouration occurring in the cube (phenolphthalein is an indicator that turns colourless in the presence of acid). Leave the cubes in the acid for 10 minutes. 4. Use the spoon to carefully remove each cube from the beaker and blot it dry by placing the cube on the paper towel. 5. Cut each cube in half and measure the dimensions of the coloured section remaining. 6. Collect the results from two other groups.

Table 1 – The percentage of decolouration of different sized phenolphthalein agar cubes immersed in 0.1m sulfuric acid for 10 minutes. | % Volume Diffused|
Cube Size (cmᶟ)| Surface Area (cm²)| Volume (cm³)| SA:V| Group 1| Group 2| Group 3| Group 4| Group 5| Average| 1.| 6| 1| 6| 87.50%| 40.00%| 49.00%| 68.30%| 66.00%| 62.16%| 2.| 24| 8| 3| 27.10%| 32.50%| 40.50%| 35.20%| 14.25%| 29.91%| 3.| 54| 27| 2| 9.67%| 27.04%| 31.10%| 13.00%| 9.67%| 18.10%|

Figure 1 – A relationship between the surface area to volume ratio and the percentage of diffusion of phenolphthalein agar cubes of varying size immersed in 0.1M sulfuric acid for 10 minutes.

Cube size

The results show that as the agar cube size increases, the percent of decolouration of the agar cubes decreases. In Figure 1, the average percent of decolouration has a dramatic drop from 62.16% to 18.10% as the dimensions of the...
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