To investigate the effect of cell sizes on diffusion by modelling the process of agar blocks and dilute Hydrochloric acid.

The same source of agar must be used in order to be a fair test. 3. Next fill three beakers with 50 ml of hydrochloric acid. The volume of hydrochloric acid cubes are immersed in must remain the same in order to keep it a fair test. 4. Place the 3 different size pink agar blocks into the beakers and start three separate stop clocks immediately. 5. Stop the stop clock for each beaker once all the dye has disappeared and the block has turned white, throughout the experiment try and measure the ending point similarly in each experiment in order to be fair and gain accurate results. 6. Record this information in a table. Results Size of cube cm3 Surface area cm2 Volume cm3 SA:Vol Rate of diffusion in seconds (s) 1 6 1 6 454.12 2 24 8 3 548.75 3 54 27 2 573.72 Conclusion It is clear to see that the larger the SA:Vol ratio, the higher the rate of diffusion. This is because if a cell gets too big, then the mechanical forces on the membrane will break it. Small cells have a large SA:Vol ratio, enabling them to have a good supply of oxygen and glucose and having a small volume means that the demand for oxygen is less, subsequently the cell can use more oxygen and glucose for respiration than bigger cells. As the cell increases in size, the surface area to volume ratio decreases to the point where the supply of oxygen and glucose becomes limited. The cell will die if it gets any bigger. Evaluation Overall, I believe that this experiment was completed reasonably well. The results supported the scientific explanation of the effect of cell size on diffusion. However, there are many errors in this experiment, the major one being measuring the end point. In this experiment the stop clocks were stopped when the pink block turned white however, this point could be measured differently depending on people’s perception of the colour white. Also, there is the disadvantage of human’s reaction time to stop the stop clock, which will inevitably add error to the results. A way to overcome the error of measuring the end point could be to leave the different sized agar blocks in the hydrochloric acid for the same amount of time, then cut them open and measure the distance the acid has diffused. This could gain more accurate results. Another human error is the accuracy of the size of the agar blocks, to overcome this either use a machine to cut them or buy them already cut into shape. Finally to improve this experiment repetitions would be necessary along with more sized cubes in order to gain more accurate and reliable results.  As previously mentioned instead of measuring the time taken for the pink agar blocks to fully turn white, to improve the experiment it would be more accurate to leave the agar blocks in the hydrochloric acid for the same amount of time, cut them open and measure using a ruler the distance the acid has diffused. Also, in this experiment 5 different sized cubes would be used and the experiment will be repeated three times in order to gain accurate and reliable results. Main apparatus Choice of equipment Justification Precision/Reliability Agar Blocks Nearest thing to living cells, solid jelly with matrix so diffusion can occur. Also allows colour change. Reliability Hydrochloric acid As has small molecules that can diffuse. Reliability Stop clock accuracy precision 5 Beakers To hold solution and agar blocks. Gives same surroundings to all cubes reliability Knife To cut agar precision ruler To measure cubes and diffusion precision Measuring cylinder To accurately measure hydrochloric acid precision tile To cut agar accurately on precision Method 1. Gather apparatus as stated above. 2. Cut 15 cubes of agar, three of 1 cm3, three of 2 cm3 , three of 3cm3 , three of 4cm3 and three of 5cm3 on the white tile using the ruler to measure them and the knife to cut them. This is the independent variable. All blocks must e cubes in order to be a fair test. 3. Measure out 250ml of hydrochloric acid pouring 50 ml into each beaker. The same volume of hydrochloric acid must be used in order to be a fair test. 4. Place the 5 different sized agar blocks into each beaker and start the stop clock. 5. Leave each cube in there for 450 seconds then remove. 6. Place the blocks on a white tile and cut open. 7. Using a ruler measure the distance that the acid has diffused and record this number. This is the dependent variable as it responds to the independent variable. 8. Repeat steps 2-7 two more times in order to gain three results for each sized cube. This will make the results more accurate and reliable. Prediction I predict that the larger the surface area to volume ratio then the larger the distance, in relationship to the size of the cube, the acid will diffuse. I believe this as small cubes have a larger SA:Vol, therefore they enable more hydrochloric acid to diffuse through them then larger cubes. This relates to cells as, small cells have a large SA:Vol ratio, enabling them to have a good supply of oxygen and glucose and having a small volume means that the demand for oxygen is less, subsequently the cell can use more oxygen and glucose for respiration than bigger cells. As the cell increases in size, the surface area to volume ratio decreases to the point where the supply of oxygen and glucose becomes limited. The cell will die if it gets any bigger. This is why in our bodies we are mainly built of loads of small cells.