Topics: Area, Prism, Volume Pages: 6 (1248 words) Published: March 24, 2014

Cell Shape and Diffusion-Practical Report (Re-Written)


Year 11 IB Biology

Title: Cell Shape and Diffusion
Diffusion is a type of passive transport which allows particles to move from a region of high concentration to a region of low concentration. In a living system, diffusion involves a cell membrane. Cells also need to excrete waste which also occurs at the cell membrane. Aim: The aim is to use model cells made from agar which contains phenolphthalein and sodium hydroxide (cut in different sizes) to investigate the effect of shape on the time taken for sulfuric acid to diffuse into the centre of the ‘cells’. This will be measured by the time it takes for the agar to become colourless. Research Question: How will changing the shape or size of the agar prisms affect the time taken for the sulfuric acid to diffuse into the centre? Hypothesis: If the surface area to volume ratio of the agar prism is large (eg: 1 x 1 cm), then the time taken for the sulfuric acid to diffuse it will be shorter. The smaller the size of the agar block, less time will be needed to diffuse the agar block compared to a larger agar block with a lower SA: V ratio. Variables: The independent variable was the shapes of the agar prisms and its sizes. The surface area to volume ratio (cm 2 : cm 3 ). The sizes and shapes used were: Dimensions of each agar block: Table 1

Dimensions (cm)
Cube 1
1 x 1 x 1cm
Cube 2
1.5 x 1.5 x 1.5cm
Cube 3
2 x 2 x 2cm
Rectangular prism
1.5 x 3 x 1.5 cm
The dependent variable was the time in minutes (mins) that it took for the agar prisms to become colourless. The quantitative data that was measured was the time in minutes, that it took the sulfuric acid to diffuse the agar prisms. This was measured with the use of a stopwatch. Controlled variables: Table 2

Controlled variable
Why it must be controlled
How it was controlled
Room temperature and temperature of solution
So that it does not affect the rate of reaction
Stayed in the same room with the same temperature, throughout the experiment Concentration of sulfuric acid (0.1 M)
So that it does not affect the rate of reaction
Used the same concentration of sulfuric acid throughout the experiment Same coverage of agar
So that it does not affect the rate of reaction
Solution just covering the agar blocks

1 x Stopwatch
2 x 250 mL beaker
2 x 50 mL beaker
Sulfuric acid (0.1 M) (enough to cover each agar block, approximately 150 mL for the 250 ml beakers and 35 mL for the 50 mL beakers) 1 x knife
1 pair of safety glasses (for each student in the group)
1 x tile cutting board
4 different agar blocks containing phenolphthalein and sodium hydroxide 1st block- 1 x 1 x 1 cm (1 cm3 )
2nd block- 1.5 x 1.5 x 1.5 cm (3.375 cm3 )
3rd block- 2 x 2 x 2 cm (8 cm3 )
4th block- 3 x 1.5 x 1.5 cm (6.75 cm3 )
Diagram of the set up of experiment: Figure 1

50 mL beaker 50 mL beaker
1 x 1 x 1 cm cube 1.5 x 1.5 x 1.5 cm cube

250 mL beaker 250 mL 2 x 2 x 2 cm cube 1.5 x 3 x 1.5 cm rectangular prism

1. Cut the agar shapes into the 4 required shapes.
2. Place the two smaller blocks in the 50 mL beakers (one in each) and the other two blocks, each in a 250 mL beaker. 3. Pour approximately 150 mL of Sulfuric acid into each of the 250 mL beakers. Then pour approximately 35 mL of Sulfuric acid into each of the 50 mL beakers. 4. Start timer and record the time taken for the first shape to lose colour entirely. 5. Do the same for the remaining shapes.

6. Repeat steps 2-6 two more times or by using data collected by two other groups of students who used the same independent variable. Risks and Precautions
Sulfuric acid is dangerous if it comes in contact with your eyes. To stay safe, wear safety glasses while dealing with the sulfuric acid. Use the knife sensibly. To...
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