Required Practical: Measuring the rate of photosynthesis

Method for effect of light intensity

1. Place a boiling tube 10cm from an LED light source

2. Fill boiling tube with sodium hydrogen carbonate solution(releases CO2)

3. Put a piece of pondweed into the boiling tube with the cut end at the top

4. Leave for 5 min to acclimatise to conditions

5. Start a stopwatch and count number of bubbles of oxygen in 1 min

6. Repeat 2 more times to calculate mean

7. Repeat step 1-6, increasing distance from light source by 10cm until 1m

Why do we use an LED light source?

• do not release as much heat as normal white light

• so temperature is controlled

What to do if using white light source?

place a beaker of water in between

What to use instead of counting bubbles?

1. place pondweed in beaker with funnel and measuring cylinder on top

2. use a delivering tube connected to a gas syringe

How to compare results?

measure rate in bubbles per min or cm3/min

How to change to investigate effect of CO2?

vary volume of sodium hydrogen carbonate

How to change to investigate effect of temp?

place beaker in water bath at different temperatures

Inverse square law

light intensity is proportional to the square of the distance

light intensity 1/d²

if d x ½

light intensity x4

if d x 4

light intensity x1/16