The Time Taken for Diffusion and Cell Dimension

Aim:

To investigate the relationship between the time taken for diffusion and cell dimension.

Procedure:

1. Place the gelatin block on a tile or Petri dish and use a scalpel or razor blade to cut it in half, producing two cubes of 10mm sides

2. Keep on of these cubes intact and cut the other in half

3. Repeat this cutting operation until you have 4 more cubes.

4. Fill a test-tube to within 10ml of the top with dilute HCl

5. Note the time: starting with the largest block drop all the blocks into the acid in the test-tube and close it securely with a rubber bung or cork.

6. Tilt the tube to spread the gelatin blocks along its length. Hold the tube horizontally and rotate it so that you can see each block clearly and from all sides. Try not to warm the tube too much with your hands or the gelatin may dissolve.

7. Note the time taken for the acid to penetrate to the center of the block as indicated by the disappearance of the orange color.

Data Collection:

This is a table of results showing the time for hydrochloric acid to penetrate a gelatin block with the variance of size which includes surface area, volume and surface area to volume ratio.

Uncertainty: 1% error (50ml measuring cylinder)

Cube Number

1

2

3

4

5

Cube Dimensions (mm)

10x10x10

10x10x5

10x5x5

5x5x5

5x5x2.5

Time for acid to penetrate (s)

709

359

187

95

46

Cube Surface Area (cm2)

6

4

2.5

1.5

1

Cube Volume (cm3)

1

0.5

0.25

0.125

0.0625

Cube SA/Vol ( cm2 /cm3) ratio

6

8

10

12

16

Conclusion:

We can see for the table that as the block of gelatin goes from 5*5*2.5mm to 10*10*10mm the volume is increasing at a faster rate than the surface area. That is why we see that the surface area to volume ratio decreases as size increases. As the gelatin block grows it needs to carry out more and more reactions with the hydrochloric acid in order to turn fully pink.

We can deduce the following information from the graphs:

* As surface area increases so does the time for the acid to penetrate.

* As volume increases so does the time for the acid to penetrate.

* From the graph we can generalize that if the gelatin block has a smaller surface area to volume ratio then it will take more time for the acid to penetrate.

Since the R2 value of all the graphs is around 1 (0.9631, 0.9976, 0.9972) this suggests

that the regression lines have a closer correlation to the points on the graph. From this we can deduce that though there were many possible sources of error in the lab they were not too severe.

Evaluation:

An error in the lab might have been that I was holding the test tube for long time and I was holding the test tube to the sunlight in order make a distinction between the orange and the red. Both holding the test tube with my warm hands and holding the test tube to the sunlight could have increased the temperature of the Hydrochloric acid and thus increasing the rate of penetration.

The results could have been affected due to the following reasons of human error. One place where human error takes place is when cutting the gelatin into blocks as we can never be sure if the sizes we have cut are exactly the sizes that are wanted. There are such problems as cutting in a straight line as well as measuring and handling a jelly-like substance. Another human error would be making the distinction between when the orange gelatin has become fully pink. By doing the experiment I can honestly say that it is very difficult to decide when the orange has become pink as it is a gradual change and not an immediate one. The results are based on what people perceive the color to be. There is also human error in stopping the stop watch. When you decide that the color has fully changed and you want to stop the stopwatch it takes at least a few seconds for your brain to process that and for your finger to hit the stop button. This is inevitable.

There is also error in measuring the water and the hydrochloric acid. We used a 50ml measuring cylinder accurate to �0.5ml and we measured 10ml of water and 40ml of hydrochloric acid. Therefore the error would be 1% percent. And we haven’t taken into account human error in reading the measuring cylinder.

As you can see this lab is full of errors which are virtually unavoidable. To improve this lab I would make sure to hold the test tube with tongs instead of my hands and instead of using sunlight to help me distinguish the colors I would used a piece of black or white paper. This would help keep the temperature of the hydrochloric acid constant.

I would also try to find of if possible create square cutters (like cookie cutters) in the correct dimensions so that u can simply stamp out the blocks. This would be much more accurate than cutting them free-hand.

To help distinguish the colors better I would use a more diluted hydrochloric acid. This would lower the rate of reaction as well as make the change in color slower and therefore my reading would be more accurate.

Discussion

We can infer from our results that the relationship between the rate of penetration of acid into gelatin and the size of the block is that the time for the acid to penetrate increase respectively with size.

To relate this lab to biology it would be useful to think of the gelatin block as a single-celled creature. Then we can put our results and conclusions into more scientific terms. When a cell grows, the volume increases at a faster rate than the surface area. Thus, as cells grow, the surface area to volume ratio decreases. A cell needs surface area in order to carry out metabolic functions (chemical reactions need a surface), and as a cell grows it needs to carry out increasingly more reactions. Therefore, since a cell must maintain a certain surface area to volume ratio, its size is limited.

Cells make up organisms so if we think of this lab on a larger scale we can deduce more information. Organisms have to exchange substances like food, waste, gases and heat with their surroundings. These substances diffuse between the organism and the surroundings. The rate at which a substance diffuses is given by Fick’s Law:

Rate of Diffusion a surface area x concentration difference

distance

The organism’s surface area that is in contact with the surroundings determines the rate of exchange of substances. The volume of the organism determines the requirement for materials. Thus the ability to meet requirements depends on the surface area to volume ratio. As organisms get bigger their volume and surface are both get bigger, however volume increases more rapidly than surface area and we can also see this in our results.

There is a problem with size however. When the organisms increase in size it becomes harder for them to exchange materials with their surroundings. Thus, as mentioned earlier, this problem sets a limit on the maximum size for a single cell of about 100mm. If it is any bigger than this the materials just can’t diffuse fast enough to support the reactions need to sustain life.