Research Question: How does the size of the cell affect its efficiency in exchanging substances through several ways, like diffusion? Aim: To plan and carry out an investigation to show the relationship between volume/Cm3, surface area, and diffusion using agar cubes measured in time/s; and to demonstrate, using diffusion, why the size of cells is limited, keeping the room temperature and pressure constant.
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Hypothesis: I expect to find that when the surface area to volume of a cell reaches a certain level, diffusion alone won’t be able to supply the cell materials needed, and as single-celled organisms take in and release substances by means of diffusion across the cell membrane, this process limits cell size. I predict that as the rate of diffusion will be too slow to deliver materials to the center of the cell in the larger cells, it could result in cell malfunction. Variables: The independent variables are the dimensions of surface area to volume ratio of the agar cube. (1cm,1. 5cm,2cm) The dependant variable, the variable I am observing/recording, is the time of diffusion (NaOH turns the agar cube pink as it disperses into it). The controlled variables will be keeping the concentrations of the Sodium Hydroxide constant, to use agar jelly from the same batch.
Also, there’s the room temperature which is kept constant and the room pressure. Materials: 250 ml beaker Measuring tape Dull knife Stirring rod Phenolphthalein agar cubes (2x2x2 cm, 1. 5cm,1cm) Hydrochloric acid (1m) Paper towel Ruler Procedure: 1- Obtain agar cubes and hydrochloric acid from your lab instructor. 2- Cut the cubes so that one has a length of 1. 00 ±0. 05 cm, another 1. 50 ±0. 05 cm, and the 3rd 2. 00 ±0. 05 cm. 3- Examine the cubes closely and then place them in the 250ml beaker. Pour in enough hydrochloric acid to cover them. (50ml) ±0. 1 ml 4-
Start the stopwatch and use the stirring rod to turn the cubes 10 times until the color of each agar cube completely changes. Be careful not to cut or scratch the surfaces of the
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cubes. Record the time taken for the color to change in (s).5- Pool your results with the rest of the class Data Collection and Processing: Quantitative Results: This table shows the quantitative data results for the time taken for the color of agar to change completely for each cube, with an uncertainty of ±0. 05 seconds. Volume Of Cubes /cm3 Time of diffusion/ trial 1/s Time of diffusion/trial2/s Time of diffusion trial 3/s (1)3 cm3 1149. 00 1065. 00 1620. 00 (1. 5)3 cm3 2640. 09 2874 . 00 2236. 00 (2)3 cm3 4342. 00 5244. 00 4811. 00
Sources of uncertainties: ± 0. 05 seconds from the stopwatch.± 5 mL from the beaker used for HCl Qualitative Results: The color of the Agar cube changed from dark purple into light yellow after the time period for each cube mentioned above passed. Data Processing: Calculations for each cube size: 1. (1)3 cm3 cube: Average time for the diffusion to complete = (1149. 00 + 1065. 00 + 1620. 00) / 3 = 1278. 00 seconds. SA:V Ratio = (1)3 : 1×6 = 1:6. 00 2. (1. 5)3 cm3 cube: Average time for the diffusion to complete = (2640. 09 + 2874. 00 + 2236. 00) / 3 = 2583. 36 seconds. SA:V Ratio = (1. 5)3 : 1.5×6 = 3. 375:9 = 1 : 2. 67 3. (2)3 cm3 cube: Average time for the diffusion to complete = ( 4342. 00 + 5244. 00 + 4811. 00) / 3 = 4799 seconds SA:V Ratio = (2)3 : 2×6 = 8:12 = 1 : 1. 50 This Table shows the average time, SA:V ratio for each of the three cubes. The graph below shows the relationship between the time taken for diffusion to occur completely and the cube size. Conclusion: The experiment proved my hypothesis correct that as the size of the Agar cube grows larger; the diffusion rate will be slower therefore taking up more time.
The cubes were put into hydrochloric acid and left by, in order to exchange materials through diffusion, as a sample of cells in real life. As we can see the cube with the largest surface area and volume has the smallest surface area to volume ratio. If the surface area to volume ratio gets too small, then the substances won’t be able to enter the cell fast enough to fuel the reactions, because the surface area controls the rate of the exchange of materials while as the volume grows larger it will need more materials to enter and exit, in a more quicker rate, therefore waste products will start to accumulate within the cell as they will be produced faster than they can be excreted. In addition, the cells will not be able to lose heat fast enough and so may overheat. This was concluded from the experiment, where as the Agar cube dimension were larger, the rate of diffusion was much smaller from the Agar cube with the smaller dimension, where the last had a larger V: SA ratio than the larger cube. Errors and Improvements: Error
Improvement Parallax error, resulting in a misreading in volume. This is caused by reading the volume readings from an angle, thus to solve this error, readings must be read at the human eye level, where the meniscus being measured at and the eye, are at the same horizontal level for accurate reading intakes. The measuring cylinders that were used for the HCl were not divided by the same rate of division. Use the same type of cylinders for all trials, like a volumetric cylinder to measure the amount of aqueous (HCl ) more accurately.
The sizes of the cubes were not exact as mentioned ( Ex: 1 cm3 is not exact ) and as a result the results obtained may not be very accurate. When cutting, a more precise instrument must be used. If no other instrument is found, a more sharper knife can be used accompanied with a more specific ruler. Injury of the surface of the cubes due to hard stirring by the rod. Stir at a slower rate for a longer time than stirring hard for a little time, in addition to that a thinner rod can be used for stirring.See More on Bacteriology