Hypothesis
We predicted that as the Surface Area / Volume Ratio increased, the penetration volume of HCL into the agar cubes would increase. This is because a small block has a large amount of surface area compared to its volume so the hydrochloric acid will have a large surface area to diffuse through. A larger block has a smaller amount of surface area in relation to its size so it should take longer for the hydrochloric acid to diffuse into the centre of the cube. The actual rate of the hydrochloric acid diffusing through the agar should be the same for all the blocks but when the surface area / volume ratio goes up, it will take less time for the hydrochloric acid to reach the centre of the cube.
Variables
Controlled Variables:
Temperature – thermostatically controlled room
Time – the length of time you leave the cubes in the HCL (6 minutes)
Volume of HCL + Concentration of HCL – there has to be enough HCL in the beaker so that the agar cube is completely covered each time (50cm3) and the concentration of the HCL is 0.1 M
Source of agar – each agar cube that you cut should be from the same block of bromothymol blue agar
Independent Variable: Surface area to volume ratio of the cubes
Dependent Variable: penetration volume
Materials/Apparatus
-
1 beaker (250cm3)
- Bromothymol Blue Agar Cubes of sides 1cm, 2cm and 3cm
- 0.1M Hydrochloric Acid (HCL)
- Stop Watch
- Knife
- Chopping Board
-
Ruler (+/- 0.5mm x 2)
- Paper Towel
Method
- A block of bromothymol blue agar should be cut into the following sizes using the chopping board, knife and ruler:
-
1cm x 1cm x 1cm
-
2cm x 2cm x 2cm
-
3cm x 3cm x 3cm
-
Fill the beaker with hydrochloric acid and put the 1cm3 agar cube into the beaker, making sure to completely immerse the cube in the acid.
- Start the stop watch as soon as you put the agar cube into the HCL.
- Write down any qualitative observations you see.
- After 6 minutes, take the agar cube out onto a paper towel and cut the cube in half.
- Using a ruler, measure the uncoloured and coloured areas of one side of the cube.
- Calculate the volume and record all your results.
-
Repeat these steps for the agar cubes sizes 8cm3 and 27cm3
Results
Table 1 The penetration percentage (%) of hydrochloric acid (HCL-0.1M) when cubes of bromothymol blue agar (side lengths 3cm, 2cm, 1cm) with increasing surface area to volume ratio’s (2:1, 3:1, 6:1) are placed into the acid for a period of 6 minutes.
Table 2 The penetration volume (cm3) of hydrochloric acid (HCL-0.1M) when cubes of bromothymol blue agar (side lengths 3cm, 2cm, 1cm) are placed into the acid for a period of 6 minutes.
Table 3 The mean penetration percentage (%) and standard deviation of hydrochloric acid (HCL-0.1M) when cubes of bromothymol blue agar (side lengths 3cm, 2cm, 1cm) with increasing surface area to volume ratio’s (2:1, 3:1, 6:1) are placed into the acid for a period of 6 minutes.
Qualitative data: table 7 – qualitative data from experiment
Conclusion
In all the cubes of agar the penetration of the hydrochloric acid from each side were the same but all the cubes had different percentages of the cube being penetrated by the HCL (un-coloured) because of their different sizes and surface areas. As the cubes got bigger it took longer for the hydrochloric acid to diffuse completely through the cube. It took longer to reach the centre of the cube even though the rate of diffusion was the same for all the cubes.
The results of our experiment supported our hypothesis because as the surface are to volume ratio increased from the largest cube of agar to the smallest cube of agar, the penetration percentage also increase, with the smallest cube having a penetration percentage of 100% compared to the largest cube having a penetration percentage of 81.8%.
As the volume of the cubes goes up the surface area to volume ratio goes down. The larger cubes have a smaller proportion of surface area than the smaller cubes. The smallest cube has a surface area to volume ratio of 6:1 and the largest cube only has a surface are to volume ratio of 2:1.This means that the hydrochloric acid is able to diffuse to the centre of the smallest cube much faster than the largest cube. The HCL penetration percentage of the smallest cube was 100% while the percentage penetration of the largest cube was only 81.8 %. A living cell would not survive if oxygen only penetrated 81.8% of the cell so this shows us that living cells need to be very small and have a large surface area to volume ration in order to survive and make sure all the essentials can diffuse quickly and easily all the way through the cell.
When the surface area to volume ratio goes down it takes longer for the hydrochloric acid to diffuse into the cube but if the ratio goes up then the hydrochloric acid diffuses more quickly into the cube. Some shapes have a larger surface area to volume ratio so the shape of the object can also have an effect on the penetration percentage of diffusion.
It is important that cells have a large surface area to volume ratio that they can get enough nutrients into the cell. They can increase their surface area by flattening and becoming longer or by having a rough surface with lots of folds of cell membrane.
Single celled organisms such as amoebas have a large surface area to volume ratio because they are small. They are able to get all the oxygen and nutrients they need by diffusion through the cell membrane. By increasing the surface area the penetration percentage of diffusion will increase.
Evaluation
One of the weaknesses of our experiment was that we only had 4 trials, which means our results could be unreliable. In the future, we should do at least 5 trials to increase reliability.
Also, we did not turn the cubes over throughout the duration of the 6 minutes and because our experiment was about diffusion and surface area, if one side of the cube is touching the bottom of the beaker the whole time that surface is not exposed to diffusion as equally as the other sides. In the future, we would systematically turn the cube throughout the 6 minutes to ensure each side had even exposure to diffusion.
When we were measuring and cutting the cubes, it was hard to be exact because the agar was quite flexible and so when you cut through it, it was hard to cut it straight and to get an exact cube. Also, when we were measuring the cube, not all the edges were straight on the cube and also we were using a regular 30cm ruler with and uncertainty of 1 mm, so it was hard to be exact with our measurements. In the future, we could use set molds to increase the uniformity of the cubes and increase the exactness and straightness of all the sides.
The original big block of agar we received was also not consistently blue all the way through to start with which indicated that the bromothymol blue wasn’t consistently distributed or maybe a reaction had occurred between the agar and the air. However, when we cut our 3 different sizes of agar cubes, we still had a lot of agar left over so in the future, we would try to choose an area of the agar which was more consistently blue (therefore indicating that the bromothymol blue is more evenly distributed through the agar) and use that area to create our different sized cubes.