3.1.2 Agar Blocks Practical

Aim of the Practical:

• To observe how the size and SA
  ratio of agar blocks affect the rate of diffusion of a substance.

Materials:

• Agar blocks containing phenolphthalein indicator.
• Dilute hydrochloric acid (HCl).
• A ruler or scalpel.
• Beakers.
• Stopwatch.
• Tweezers or forceps.

Method:

1. Prepare Agar Blocks:

• Cut agar blocks of different sizes (e.g., 1 cm³, 2 cm³, 3 cm³).
• These blocks contain phenolphthalein, which changes colour when neutralised by an acid.

2. Immerse Blocks in Acid:

• Place each block into a beaker containing dilute HCl.
• Start a stopwatch immediately.

3. Observe and Measure:

• As the acid diffuses into the blocks, it neutralises the phenolphthalein, causing the colour to fade.
• Measure the time taken for the colour to completely disappear from each block.

4. Record Results:

• Note the time taken for diffusion to complete and calculate the rate of diffusion for each block.

Variables:

1. Independent Variable:

• The size of the agar block (affecting SA
  ratio).

2. Dependent Variable:

• The time taken for the colour to disappear.

3. Controlled Variables:

• Volume and concentration of HCl.
• Temperature and time of immersion.
• Same concentration of phenolphthalein in all blocks.

Results and Observations:

1. Smaller Blocks (Higher SA
   Ratio):

• Colour fades faster, indicating a quicker rate of diffusion.
• The high SA
  ratio allows more acid to diffuse relative to the block's volume.

2. Larger Blocks (Lower SA
   Ratio):

• Colour fades slower, indicating a slower rate of diffusion.
• The low SA
  ratio reduces the efficiency of diffusion.

Conclusion:

• The rate of diffusion is faster in smaller agar blocks with a higher surface area to volume ratio.
• Larger blocks with a lower SA
  ratio take longer for the substance to diffuse completely.

Applications:

• This practical demonstrates why small organisms or cells rely on diffusion for substance exchange.
• Explains why larger organisms require specialised exchange surfaces and transport systems to meet their metabolic demands.

Key Learning Points:

1. SA
   Ratio:

• As the size of the object increases, the SA
  ratio decreases, making diffusion less efficient.

2. Diffusion Distance:

• Larger blocks have a longer distance for substances to diffuse, slowing the rate.

3. Importance in Biology:

• Cells are small to maintain a high SA
  ratio, ensuring efficient exchange of gases and nutrients.