Light Limits
The RP6 practical: measure how light intensity changes the rate of photosynthesis, and make it a fair, accurate test.
Get the method right under pressure
Free interactive practice on the steps that lose marks under exam pressure.
Start revising freeWhat you'll cover
Light Limits
You already know light intensity affects photosynthesis. **RP6** is where you *prove* it, and the marks come from designing a **fair, accurate** experiment. This module walks the practical: the kit, the method, the variables to control, how to measure the rate, and how to plot what you get.
The apparatus
Everything on the bench has one job. Learn what each part is *for*, because the exam asks.
What comes off?
As the pondweed photosynthesises, which gas is given off and used to measure the rate?
- Oxygen
- Carbon dioxide
- Hydrogen
- Nitrogen
Order the method
Put the steps of the RP6 method into the correct order.
- Set up the pondweed with the lamp at a measured distance
- Let it settle, then start timing
- Count the oxygen bubbles given off in one minute
- Record the count, then move the lamp to a new distance
- Repeat at each distance and take a mean
Keep it fair
To be sure it is the **light** changing the rate and not something else, every other variable has to be held still. Three things try to drift:
Control each one
- Temperature
- Carbon dioxide
- The pondweed
- Timing
- Use a heat shield so the lamp doesn't warm the water
- Kept constant by sodium hydrogencarbonate
- Use the same piece throughout for a fair test
- Count for the same length of time each reading
Why the heat shield?
Why is a beaker of water (a heat shield) placed between the lamp and the pondweed?
- It absorbs the lamp's heat, so temperature stays constant and only light intensity changes
- It makes the light brighter
- It magnifies the pondweed so bubbles are easier to see
- It adds more carbon dioxide to the water
Accurate and reliable
Counting bubbles is quick but less _____, because bubbles vary in _____ and are easy to miscount. Collecting the gas in a _____ and measuring its volume is better. Whichever you use, _____ each reading and take a mean, which makes the results more reliable.
Work out the rate
At one distance, the pondweed gives off 45 oxygen bubbles in 3 minutes. What is the rate, in bubbles per minute?
Plot your results
The class repeated RP6 at six light intensities and recorded the rate in bubbles per minute: 1 → 2, 2 → 4, 3 → 6, 4 → 8, 5 → 8, 6 → 8. The first three readings are already plotted. Plot the remaining three.
Why does it flatten?
Your graph climbs in a straight line, then goes flat from intensity 4 onwards. What does the flat part tell you?
- Light is no longer limiting. Another factor, such as carbon dioxide or temperature, now caps the rate
- The pondweed has run out of the oxygen it needs to release bubbles
- The light has become so bright that it is damaging the pondweed
- Photosynthesis has stopped, so no more bubbles are produced
Take the mean
Three repeats at the same distance gave 24, 26 and 25 bubbles per minute. What is the mean rate, in bubbles per minute?
In the exam
Practical nailed. Grade-9 habits for RP6: • Measure the rate by **counting O₂ bubbles** (quick) or **gas volume in a syringe** (more accurate); **repeat and mean**. • **Control** temperature (heat shield), CO₂ (hydrogencarbonate), the pondweed and the timing. Change only the light. • Closer lamp → higher light intensity → faster rate; use **light intensity ∝ 1/distance²** to quantify it. • When the plotted line **flattens**, light has stopped being the limiting factor. Name the factor that has taken over.