Agar Lab- IB Biology
The effect of changing surface area on the rate at which HCl decolorizes each agar block Research Question: What is the effect of different sized colored agar cubes and their surface are: volume ratio on the rate of reaction at which they absorb HCl? There should be a direct correlation between the size of an agar cube, which represents a cell, and the rate at which they can exchange substances. This correlation should be: the bigger the size of the agar cube, the longer the period of time taken for the substance exchange to occur. Aim- the aim of this experiment is investigating how different surface areas affect the rate at which HCl completely decolorizes different sized agar blocks. The agar is colored with phenolphthalein indicator. The different sized agar blocks will be obtained by starting out with a 20mmX10mmX10mm agar block and cutting it up into half, then repeating the process until five different sized agar blocks are obtained. Each of the five agar blocks is then inserted into a test tube containing HCl at the same time as each other. The time taken for each block to turn colorless will then be measured with digital stopwatches in order to establish a relation between surface area and the rate of the reaction. The center of each agar block has to become completely colorless in order to be considered completely decolorized. Hypothesis- the smaller the Surface Area:Volume ratio, the slower the rate of reaction, therefore the longer the time taken for the cube of agar to be penetrated by the acid and for the phenolphthalein indicator to turn colorless. As the size of a cell increases, its volume and surface area increase with it, however they increase at a rate that limits the ratio between them, making it gradually smaller. This is because when the cell is smaller, it has a smaller volume but a greater surface area, therefore it can exchange more and at a faster rate. This would therefore mean that the smallest sized agar cube would be the one to get its phenolphthalein decolorized by the HCl the fastest. The following quotation supports the principles mentioned: “The important point is that the surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume.” Blamire, J. (2001).
Collected and Interpreted Data Raw (Unprocessed) Data: to show the correlation between cell size, Surface Area: Volume ratio and the rate at which substance exchange takes place
The dependent variable in this set of data is the time. Processed Data (including calculations): to show the principles behind geometrical values and proportions
The independent variable in this set of data is the Surface Area: Volume ratio, which is determined by the size of the agar cube, representing a cell. Data interpretation: Qualitative data to show the correlation between the results
Time taken for agar cubes to get decolorized by HCl (s)
This graph shows that as the cube decreases in size, so does the amount of time taken by the Hydrochloric acid to decolorize the phenolphthalein indicator. There is positive correlation between the time measurements and the size measurements. The shape of the graph is downwards, to the right, which means that a decrease is occurring. The line of best fit is passing through approximately 3 results, which is the majority of the overall quantity.
Time taken for decoloration (s) (average)
Conclusion and Discussion: From the data collected, it can be observed as the surface area and volume values decrease, so does the amount of time taken for the cube to be penetrated to its core by the HCl and get its phenolphthalein indicator completely decolorized. The cube that has the smallest surface area and volume can be labeled as the smallest cube the other trials: cube labeled as number 5, with a surface area of 100 mm^2 and a volume of 62.5 mm^3. According to the hypothesis, the bigger the size of a cube, the smaller its surface area: volume ratio. This statement is proven by the results of the largest cube of agar: volume of 100 mm^3 and surface area of 600 mm^2, where the surface area: volume ration is the smallest out of
all of the results: 0.6: 1. However, cube five had an opposite relation between its values, resulting in a bigger ratio: 1.6: 1. As a result of this bigger value in the ratio, it was recorded that the duration of the time taken for the acid to immerse the agar cube was the smallest, at only 14s, proving to be the shortest period of time out of the different trials. This supports the diffusion theory, which states that the larger the surface area, the faster the rate of a reaction. The final time values were increasingly larger as the ratio between surface area and volume decreased, this is because volume increases at a faster rate than surface area in a cell, so the cell ends up being too big in size and not exchanging sufficient substances for its significant size. Considering the direct relation between rate of metabolism and the amount of reactions performed in the cytoplasm, volume is deemed important when it comes to fuelling the cell and performing life processes. However, in order for metabolism to occur, substance exchange must occur across a permeable membrane, which is when surface area plays a very important part. Therefore, a cell cannot function without having a considerable volume and a considerable surface area, since they both provide space for life processes to occur. Consequentially, the ratio between them is very important as it establishes the way in which the cell will function. This claim is verified by a trustworthy, scholarly source: “The surface area to volume ration of a cell is therefore very important. If the ratio is too small then substances will not enter the cell as quickly as they are required and waste products will accumulate because they are produced more rapidly than they can be excreted.” Allott, A., & Mindorff, D. (2014). Evaluation: Source of error The depth of the colored agar poured in the petri dish was smaller than 10mm tall.
The concentration of the Hydrochloric acid possibly
Evaluation of Procedure
The students received a set of provisions at the start of the experiment and they were instructed to use them to the best of their abilities in order to establish a relation between the size of 5 individual agar cubes and the time taken by them to turn colorless in an HCl solution. Every group out of the five who performed the experiment received significantly different results for each of the agar cubes
In order to be able to obtain better results, the agar would have to be prepared by the students themselves, it would have to be carefully poured until reaching the desired height measurement. A fresh sample of Hydrochloric acid could be used, and shared by all the participants of the
changed over time.
The use of one test tube alone, as opposed to one per agar cube.
Imprecise cutting of agar cubes.
Difficulty knowing when the complete change from pink to colorless was
after using two different samples of Hydrochloric acid to decolorize them. Two different samples were used, out of which a test tube was filled by using a pipette. One of the latter steps of the experiment was to start the time watch as agar cubes were inserted into the test tube containing the Hydrochloric acid. Inserting each individual cube would take time on its own, so this would affect the timings on the stopwatch, as some of the cubes will have been inserted after the start of the stopwatch. Individual agar cubes would get a higher exposure to the Hydrochloric acid than other agar cubes, therefore impacting the time results and the rate of reaction. During the cutting of the agar procedure, the agar was segmented into cuboid shapes that would represent cells. The representations of cells would have to be of certain measurements in order to prove variation. These different measurements were achieved by measuring a portion of the agar with a ruler and making incisions with a scalpel, by free hand, which resulted in uneven edges and measurements that sometimes went off. The color of the phenolphthalein got very weak after a certain period of time, up to the point where it was difficult to discern whether there was any color left. At many
class, so that the concentration is known and it cannot affect the results significantly. An improvement would be to do the timing and inserting into an individual tube for each agar cube: the cube would be inserted and the stopwatch started, no other distraction would be ongoing. Since there were 5 agar cubes, 5 test tubes should be provided at the start of the experiment.
An improvement would be to use more professional utensils, such as a more accurate ruler, a specific cutter that would ensure that the edges being cut would come out straight and the size would meet the requirements.
An improvement would be to intensify the color of the agar so that even after most of the color is gone, the cores of the cubes are still
made by naked eye.
moments throughout the experiment, the color of the pink in the agar cubes got so dim that my partner and I were unsure of the nature of the cube at that given period of time. This may have affected the results due to lack of accurate visibility and certainty of correctness that the seen is true.
visible to the naked eye, without strain. This would improve the accuracy of the results, as there would be a higher certainty of knowing the exact timing when the shift from pink to colorless was complete.
Bibliography: Blamire, J. (2001). BIOdotEDU. Retrieved September 7, 2015, from http://www.brooklyn.cuny.edu/bc/ahp/LAD/C5/C5_ProbSize.html Allott, A., & Mindorff, D. (2014). 1.1 Introduction to cells. In Biology: course companion (2014 ed., p.9). Oxford.