Effect of surface area to volume ratio on the rate of diffusion of agar blocks PDF

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Effect of surface area to volume ratio on the rate of diffusion of agar blocks Introduction : Cells are the basic units of life. There are different types of cells varying in shape, size, structure and function. For example, the average diameter of white blood cells found within the human body measures about 12-17 µm and is around 3 times larger than a red blood cell. The egg cell is the largest cell in the human body whereas the sperm cells are the smallest and the nerve cells being the longest. Cells can be observed through microscopes. There are different methods and ways to measure the size of a specific cell. The 3 dimensional space occupied by a cell is known as its volume while the outermost part of the cell exposed to the environment is its surface area. In this experiment, the relationship between the volume and the surface area of a cell is investigated to determine which one of these measurements are more crucial in limiting its size. In this experiment the cells are represented by blocks of agar jelly. The rate of diffusion of sulphuric acid into the agar jelly is determined by measuring the time taken for the jelly to decolourise. Aim: To determine the casual relationship between the surface area-volume ratio of the block and the percentage of block diffused with acid in a given amount of time. Hypothesis: The more the surface area-volume ratio the more the rate of diffusion in the agar blocks. Materials and equipment : See practical activity handout entitled Investigating Cell Size Limitations. Procedure : See practical activity handout entitled Investigating Cell Size Limitations. Results : Bloc k

Dimension s of block

A

20 x 20 x20 mm 12 x 12 x 12 mm 20 x 30 x 3 mm 10 x 18 x 10 mm

B C D

0.3

Thickness of coloured section (mm) 14

Volume of coloured section Vr 2016

Volume of discoloure d section Vd 5984

% of block diffused with acid 74.80

1728

0.5

7

252

612

35.42

1500

1800

0.83

0

0

1800

100.00

920

1800

0.51

12

120

800

44.44

Surface area (mm2) SA 2400

Voloume (mm3) Vi

SA : V

8000

864

The volume of the discoloured section was calculated using the formula V d = Vi – Vr and the percentage of the block that has been diffused with acid was calculated by dividing the volume of

discoloured section, Vd by the initial volume, Vi and multiplying the answer by 100. Block C had the highest percentage of diffused acid (100%) followed by block A (74.8%), block D (44.4%) and block B with the least percentage of only 35.4%. Generally, from the results tabulated it can be seen that when the surface area to volume ratio of the blocks increased from block B (0.5) to block D(0.51) to block C(0.83) the percentage of block diffused with acid increased as well except for block A. Whereby, the surface area to volume ratio of block A is only 0.3 but 74.8 percent of the block was diffused with acid. This may be an outlier data. Analysis and discussion : 1. The amount of diffusion of acid over the given time of 5 minutes is represented by the volume of the discoloured portion of the agar blocks. From the results obtained, it can be stated that when the volume of the block increases the amount of acid diffused into the block in five minutes increases as well. For example, block A which has the highest volume of 8000mm3 has the highest amount of acid diffused in it (5984 mm 3). Whereas, block B which has the least volume of only 1728 mm 3 has the least amount of acid diffused into it (612 mm3) 2. The lesser the surface area of the block the lesser the amount of diffusion of acid in 5 minutes. Block A has the largest surface area (2400mm 3), followed by block C (1500mm 3), block D (920mm3 ) and block B with the least surface area (864mm 3). Block A also has the highest amount of diffusion of acid in 5 minutes (5984mm 3 ), followed by block C (1800mm3), block D (800mm 3) and block B with the least amount of acid diffused (612mm 3). From, this we can observe a decrease in the volume of acid diffused when the surface area of the block decreased. 3. The more the SA:V of the block the more the more the amount of diffusion of acid in 5 minutes. Block C and block D both have the same volume of 1800 mm 3 but than the amount of acid diffused in block C (1800mm2 ) is more than the amount of acid diffused in block D (800mm3). Block C was entirely diffused and decolourised by the sulphuric acid. This is because block C has higher SA:V than block D. Block C has a surface area volume ratio of 0.83 and block B, 0.51. 4. The hypothesis is supported. Block C had the highest percentage of diffused acid (100%) followed by block A (74.8%), block D (44.4%) and block B with the least percentage of only 35.4%. Generally, from the results tabulated it can be seen that when the surface area to volume ratio of the blocks increased from block B (0.5) to block D(0.51) to block C(0.83) the percentage of block diffused with acid increased as well except for block A. Whereby, the surface area to volume ratio of block A is only 0.3 but 74.8 percent of the block was diffused with acid. This may be an outlier data. 5. Cells tend to be very small in size so that they can have a higher surface area to volume ratio. If cells are large they will have lesser surface area exposed to the enivironment decreasing the surface area to volume ratio. Having higher surface area to volume ratio increases the rate of diffusion. This is important in cells to allow movement of substances quickly in and out of the cell. More molecules, nutrients, ions and oxygen can be taken in by the cell. 6. (A) Wear safety goggles at all times to avoid the hydrochloric acid from splashing on to our eyes.

(B) Be careful while using the scalpel to cut the agar blocks so that we do not injure ourselves or others. (C) Wear gloves while handling the hydrochloric acid as it is a corrosive substance. (D) Make sure to rinse of the acid immediately at the emergency shower station if it comes in contact to our body. 7. All agar blocks/cubes must be cut out from the same block based on the required dimensions so that the composition of materials making up each block does not vary. Repeat the experiment several times to obtain the average value. Use agar blocks of many different sizes and shape.

8. I would want to conduct a different experiment to investigate cell size limitation and compare and contrast the findings of both experiments. I would cut different sizes and shapes of potatoes and put them in an iodine solution for a given amount of time. Than, I’ll measure how much of the potato was penetrated by iodine. 9. Cell A would have the highest rate of diffusion as it has larger surface area exposed compared to all the other cells. This would increase it’s surface area volume ratio. Hence allowing for the highest rate of diffusion. 10. When an animal stretches out or curls itself up depending on the weather it is manipulating it’s surface area. When it is cold they curl themselves up, thus reducing surface area exposure which in turn decreases SA: V. Therefore, heat loss through radiation can be prevented and more heat can be stored within the body to keep themselves warm. Inversely, when they stretch out during a hot day this increases the surface area and consequently increase the SA:V. Therefore, there will be more heat released from the body to cool themselves down. Conclusion : The more the surface area volume ratio the more the rate of diffusion in the agar blocks. The hypothesis is accepted. This relates closely to why the cells are very small in size. Smaller size gives them more surface area volume ratio, increasing rate of diffusion thus providing a better medium for the movement of substances in and out of the cell. This is how the cells in our body discharge waste, take in water, oxygen and carbon dioxide.

References : 1) Monash College practical handout (2019) 2) Reviewmylife.co.uk. (2019). Surface Area / Volume Ratio Biology Experiment. [online] Available at: https://www.reviewmylife.co.uk/blog/2008/06/02/surface-area-volumeratio-biology-experiment/ [Accessed 9 Aug. 2019].

3) How to Handle Acids. (2019). Retrieved 9 August https://www.wikihow.com/Handle-Acids 4) Microbiology@Morningside. (2019). Retrieved http://morningsidemicro.wikidot.com/why-cells-are-so-small 5) Agar Cell Diffusion. (2019). Retrieved 9 August https://www.exploratorium.edu/snacks/agar-cell-diffusion

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