GCSE Biology Revision "Required Practical 6: Photosynthesis"

Required Practical Setup

Materials Needed

• Boiling tube

• LED light source (preferred due to low heat output)

• Normal light bulb (requires additional setup with a beaker of water)

• Sodium hydrogen carbonate solution (provides carbon dioxide)

• Pond weed

Experimental Procedure

1. Positioning the Light Source

   • Place the boiling tube 10 cm away from the LED light.

   • If using a normal light bulb, insert a beaker of water between the light and boiling tube to absorb excess heat produced by bulb.

   • An LED light is used as these don't release very much heat; too much heat would change the temperature of the experiment.

2. Preparing the Boiling Tube

   • Fill the boiling tube with sodium hydrogen carbonate solution — Sodium hydrogen carbonate solution releases carbon dioxide, which is needed for photosynthesis

   • Place a piece of pond weed into boiling tube with the cut end facing upwards.

   • Allow the setup to acclimatize to the conditions of the boiling tube for five minutes.

3. Observing Oxygen Production

   • Observe for gas bubbles produced from the pond weed; this gas is oxygen generated by photosynthesis.

   • Start a stopwatch and count the number of bubbles produced in one minute.

   • Repeat this for two more trials and calculate the mean no. of bubbles per minute.

4. Changing Distances

   • Repeat the experiment at distances of 20 cm, 30 cm, and 40 cm:

     

Challenges and Solutions

Problems Identifie

• Bubble Counting Difficulty: no. of Bubbles can be produced too rapidly to count accurately.

• Bubble Size Variation: Bubbles are not always the same size: for example, a large bubble would count the same as a small bubble.

• Bubbles vary in size, making consistent counting challenging.

Solutions

• Measure the volume of oxygen produced instead of counting bubbles. Do this by: Placing pond weed under funnel and catching the bubbles in a measuring cylinder filled with water. Then use measuring cylinder to measure the volume of the oxygen gas produced

  

• Collect bubbles in a measuring cylinder filled with water for accurate volume measurement.

Data Interpretation for Higher-Tier Students

Graphing Results

• Plot the mean number of bubbles per minute or the volume of oxygen per minute against the distance from the light source.

• If we plot the mean number of bubbles per minute/volume of oxygen per minute against the distance from the lamp to the pond weed, then we get this graph:

  

Inverse Square Law

• Key Observation: if we double the distance, then the number of bubbles per minute falls by a factor of four (this is called inverse square law) (y=k/x, light intensity= 1/distance²)

  • Example: Moving from 10 cm to 20 cm reduces bubbles produced from the pond weed by four times.

  • Again, moving from 20 cm to 40 cm also results in a fourfold decrease.

• Explanation: This phenomenon occurs because light intensity decreases with the square of the distance, thus affecting the rate of photosynthesis.

• The reason for this is that if we double the distance, the light intensity falls by four times(x1/4), and because we need light for photosynthesis, that causes the number of oxygen bubbles to fall by four times.