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Experiment 4B: Agar blocks

Aim: To explore the relationship between surface area to volume ration and rate of exchanging materials.

Hypothesis: (2x2x2) cm 3 solid agar block cut into 64 blocks will produce the highest concentration of ions when immersed in water, while the (2x2x2)cm 3 solid agar block produces the lowest concentration of ions when immersed in water.

__Reason__ Although the surface area of the (2x2x2) cm 3 solid agar block may be larger than surface area of one of the 64 small blocks cut from the (2x2x2) cm 3 solid agar block, however, diffusion is dependent on surface area: volume ratio. Volume increases with surface area, but surface area experiences a more significant increase than surface area. Hence the (2x2x2) cm 3 solid agar block will have a smaller surface area per volume ratio, making it more difficult to exchange materials with its surroundings. Therefore, the sodium and chlorine ions diffuse out of the (2x2x2) cm 3 agar block (largest in size) at the slowest rate, resulting in the lowest concentration of ions. The ions diffuse out of the (2x2x2) cm 3 agar block cut into 64 pieces (smallest in size) at the fastest rate, resulting in the highest concentration of ions.

Independent variable: The size of the agar block(s) immersed in water.

Dependent variable: The concentration of ions in the distilled water.

Constant variables: Volume of agar in each beaker, amount of time agar was immersed in water, amount of strength and speed used to stir the agar and distilled water mixture, amount of water in each beaker, the amount of disturbance the conductivity probe is subject to during stirring, the data logger used throughout the experiment (the same machine was used).

Apparatus/Materials:
 * Razor blade/scalpel
 * White tile (for cutting)
 * Ruler
 * Stirrer
 * Stop watch
 * 3 beakers
 * 3 pieces of (2x2x2) cm 3 solid agar
 * Data logger with conductivity probe
 * Tap water

Procedure: Results: Agar block - 1 piece (2x2x2) cm 3
 * 1) Take one piece of the agar block and place it on the white tile. Using a ruler and scalpel, measure out (2x2x2 )cm 3 and cut accordingly. Throw away the excess.
 * 2) Take the second piece of Take one piece of the agar block and place it on the white tile. Using a ruler and scalpel, measure out (2x2x2) cm 3 and cut accordingly. Throw away the excess. Divide the remaining block into 8 pieces so that each side is 1cm in length.
 * 3) Take the third piece of the agar block and place it on the white tile. Using a ruler and scalpel, measure out (2x2x2) cm 3 and cut accordingly. Throw away the excess. Divide the remaining block into 64 pieces so that each side is 0.5cm in length.
 * 4) Switch on the data logger and launch MultiLab CE by clicking the icon on the screen.
 * 5) Plug the conductivity probe into sensor port I/O-1 of the data logger.
 * 6) Click the “setup” icon and click the “samples” tab. Scroll down and choose the continuous option.
 * 7) Fill a beaker with 200ml of tap water and place the conductivity probe in the water.
 * 8) Starting with the largest single agar block, place the agar block in the beaker and click the “run” icon.
 * 9) Stir the water with the stirring rod continuously and gently while the data logger takes the reading every 10 seconds.
 * 10) Press the red “hand” button that appears in place of the “run” icon to stop the experiment after 2 minutes by using a stopwatch. Record the change in reading after 2 minutes.
 * 11) Take note of the graph and read the highest conductivity in the period of the 2 minutes.
 * 12) Repeat steps 7-11 for all of the different sized cubes and record the results in a table.
 * 13) Dispose of the contents of the beaker after the experiment. Do not throw the agar into the sink.
 * 14) Plug your thumbdrive into one of the ports on the side.
 * 15) Go to File > Export CSV file.
 * 16) The save as dialog box will open. Find the “Removable Disk” option by clicking on the folder icon with the green arrow.
 * 17) Enter the name of your CSV file using the pop-up keypad, then click “ok” at the top right hand corner of the box.
 * 18) The data logger will start to open MS excel program. In the pop-up import options window select “comma” as the separator and click “ok”.
 * 19) The excel document will open and you can view your results.
 * 20) Go to File > Save as to save a copy of the excel file in your “Removable disk”.


 * Time(s) || Conductivity I/O-1(mS) ||
 * 0 || 0.14 ||
 * 10 || 0.34 ||
 * 20 || 0.53 ||
 * 30 || 0.6 ||
 * 40 || 0.64 ||
 * 50 || 0.63 ||
 * 60 || 0.67 ||
 * 70 || 0.85 ||
 * 80 || 0.81 ||
 * 90 || 0.9 ||
 * 100 || 0.9 ||
 * 110 || 0.95 ||
 * 120 || 1.01 ||

Agar block - 8 pieces (8 x 1x1x1) cm 3


 * Time(s) || Conductivity I/O-1(mS) ||
 * 0 || 0.33 ||
 * 10 || 0.3 ||
 * 20 || 0.39 ||
 * 30 || 0.86 ||
 * 40 || 1.32 ||
 * 50 || 1.11 ||
 * 60 || 1.27 ||
 * 70 || 1.45 ||
 * 80 || 1.51 ||
 * 90 || 1.61 ||
 * 100 || 1.71 ||
 * 110 || 1.76 ||
 * 120 || 1.84 ||
 * 130 || 1.91 ||
 * 140 || 1.95 ||
 * 150 || 2.04 ||

Agar block - 64 pieces (64 x 0.5x0.5x0.5 )cm 3


 * Time(s) || Conductivity I/O-1(mS) ||
 * 0 || -0.02 ||
 * 10 || -0.02 ||
 * 20 || 0.47 ||
 * 30 || 0.96 ||
 * 40 || 2.32 ||
 * 50 || 2.96 ||
 * 60 || 2.95 ||
 * 70 || 4.08 ||
 * 80 || 4.95 ||
 * 90 || 3.11 ||
 * 100 || 3.13 ||
 * 110 || 3.15 ||
 * 120 || 3.48 ||
 * 130 || 3.56 ||

green = 64 pieces blue= 8 pieces purple = 1piece
 * graph unable to upload

Analysis: Salt is made up of sodium and chloride ions. When salt dissolves, the bonds between the ions weaken, and the ions become free to move. As the ions are small enough, they will diffuse out of the agar cube into the surrounding water. The conductivity sensor is capable of monitoring the total concentration of ions in a solution. Hence it is used in this experiment to measure the rate of diffusion.

The ideal result would be that as the size of the agar pieces decrease in size, the concentration of ions in water would be higher. This is because diffusion is dependent on surface area to volume ratio. Volume increases with surface area and surface area increases with volume, but surface area experiences a more significant increase than surface area. In other words, when the value of surface area increase, the value of volume increases even more. Given the (2x2x2 )cm 3 solid agar block has a smaller surface area per volume ratio, the rate of diffusion in/out of the agar would be slower, therefore resulting in a lower concentration of ions in the water. Diffusion occurs in/out of the (2x2x2 )cm 3 agar block cut into 8 pieces at a faster rate compared to the sole piece of agar block, given it has a bigger surface area per volume ratio compared to the sole piece of agar block. Although the concentration of ions would be higher, however it will be less than the concentration of ions in the 64 piece (2x2x2) cm 3 set up. The small agar pieces have the largest surface area per volume ratio among all the set ups, hence diffusion occurs more rapidly.

For my results, it is possible to observe the relationship from the line graph, and compare these trends between the 3 experimental setups. It can be observed that there are a number of outlier values. For instance, the readings taken by the conductivity probe for the 64 piece agar increase along a steep gradient, and made a sudden drop before increasing gradually. This could be due to a change in speed or strength of stirring. Although this was stated as a constant variable as it should rightfully be, however it is still subject to human error. Also the graphs did not start at the origin, 0mS, at 0 seconds. Perhaps this could be due to a glitch in the data logger, as shown below.

This data was taken when the conductivity probe was immersed in only water.


 * Time(s) |||| Conductivity I/O-1(mS) ||
 * 0 |||| 0.1 ||
 * 10 |||| 0.09 ||

It was not a systemic error, given that when observed from the line graph displayed in the results, the initial reading at 0s differs in each set up. These could be random errors.

For this experiment, there is no class data given only half the class has entered their values. However, class data would help one’s experiment, in the case where the experiment generated many outlier values. Hence the average taken from the class data would still enable one to arrive at a logical conclusion. Class data is also similar to repeating the experiment many times, hence increasing the accuracy of the results obtained.

Some precautions I took were to ensure that the agar cubes were not left immersed in the water for a while before the data logger was set to run. This was to ensure that the data logger would be able to accurately record the concentration of sodium and chloride ions in the water as time progressed. By starting late, the few seconds would have possibly created some outlier values in the experiment. Also, I made sure that the conductivity probe was not hit by the stirrer during the process of stirring. This would affect the ability of the conductivity probe to measure accurate values. Conclusion: The greater the surface area per unit volume of the agar block, the faster the rate of diffusion.

Further Discussion: The 1 piece agar setup is a similar representation of a complex living organism, which has a small exposed surface area per volume ratio. The 64 and 8 piece agar set ups are similar representations of smaller and simple organisms, which have a bigger exposed surface area per volume ration. Hence diffusion within a small sized organism is simple while diffusion within a big sized organism is more complex. Yes, the cell membrane becomes less efficient as the cell size increases, given there is a smaller surface are to volume ratio. __1 big piece of agar__ Total surface area = 24cm 2 Total volume = 8cm 3 Ratio = 24/8 = 3 (mS)
 * 1) How do you relate this to the shape of a simple to complex living organism?
 * 1) Once a cell grows to a certain size it becomes too large for the complete diffusion of needed substances throughout its cytoplasm. As a cell grows, is the surface area of the cell membrane as efficient relative to the volume of the cell?
 * 1) Examine the agar cells below and work out the surfaced area to volume ratio of each cell. Which is the most efficient and which is the least?

__8 pieces of agar__ Total surface area = 48cm 2 Total volume = 8cm 3 Ratio = 48/8 = 6 (mS)

__64 pieces of agar__ Total surface area = 64cm 2 Total volume = 8cm 3 Ratio = 64/8 = 8 (mS)

Hence we can see that the 64 piece agar block set up is the most efficient and the 1 big piece agar set up is the least efficient.