- To calculate surface-are-to-volume ratio (SA:V)
- To show the effect of surface-area-to-volume ratio on the diffusion rate of hydrochloric acid
- We used sharp razors to cut the agar into the correct sizes, so therefore make sure to cut away from yourself and keep them in an obvious place so you don’t touch them by accident
- Hydrochloric acid is an irritant so eye protection must be worn in case of exposure
- Rough cubes of agar around 20mm3
- Single edged razor blade
- Five boiling tubes, one for each size
- Pen to label test tubes
- 1M hydrochloric acid
- 5cm3 syringes to measure out HCl
- White tile
- Ruler or caliper to measure the cubes
- Stopwatch
Dependant – Time taken for the hydrochloric acid to diffuse through the cube
Independent – Size of the cube (mm) from 2 to 10
Control – Volume of Hydrochloric acid
Control – Concentration of Hydrochloric acid
Control – Shape of agar
Prediction
In most biological processes, the larger the surface area the more diffusion can occur. If the volume was larger than logically it would take longer for the hydrochloric acid diffuse throughout the cube. The larger the surface area to volume ratio the quicker the full diffusion of the hydrochloric acid into the cube.
Method
1- Cut the agar into five different-sized cubes (from 2mm to 10mm) using a sharp razor and calipers on a white tile
2- Add 5cm3 of 1M of hydrochloric acid to each of the boiling tubes and label the boiling tubes with the size of the agar cube
3- Manipulate the cubes into the boiling tube and start the stopwatch on contact of acid and agar. Make sure to not touch the cube as liquid make diffuse in or out of the agar on touch
4- Stop the stopwatch when the cube goes a light orange from a dark brown / orange in the cube
5- Record the results and plot a graph
Diagram
As according to my initial hypothesis, as the surface area: volume ratio decreased the time for the hydrochloric acid to diffuse completely through the cube. This is seen by comparing the 1:3 SA:Vol Ratio to the 0.6:1 SA : Vol Ratio. We can see the time for the largest SA : Vol Ratio was 32 seconds compare to the 0.6:1 ratio which took 446 seconds for complete diffusion. This is because although there is a larger surface area there is a larger volume to which the hydrochloric acid has to diffuse throughout. Therefore the volume counteracts the surface area increase so the higher the SA : Vol Ratio the quicker the diffusion throughout the cube
What additional procedures could you carry out to make these results more reliable and more accurate?
To increase reliability you could repeat the experiment multiple times or until you get concordant results. Then take an average by adding the results together and divide by the number of results you have. To increase accuracy we would need to refine how we cut the cubes as they are done by hand and agar is slippery so there is a large chance of error. To improve the method you could use a cheese wire to cut straight along an edge.
Explain the effect of surface-area-to-volume ratio on the rate of diffusion and how this is important in living organisms using your graph and your scientific knowledge
A decrease in surface-area-to-volume ratio inversely correlates to time take for full diffusion. This can be seen in the graph by the sloping, positive gradient. This is important in living organisms as a larger animal would have a smaller SA:V ratio and will be unable to obtain anough nutrients and oxygen by diffusion alone. For example a human has lungs which it draws oxygen into, it then diffuses into the blood stream but this is effective as they are able to draw enough oxygen into their lungs. An amoeba, a single celled protozoa gets its nutrients from diffusion alone and has no transport system.
What are the limitations of this experiment compared with living organisms?
It is difficult to cut the agar into exact measured cubes and you cannot guarantee that it will be an exact cube. We also judged when the hydrochloric acid had fully diffused through the cube by eye. This is quite unreliable as it is hard to define when it is finished.