Light Limits
The RP6 practical: measure how light intensity changes the rate of photosynthesis — and make it a fair, accurate test.
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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, and how to measure the rate.
The apparatus 🪴
Put a piece of **pondweed** (Cabomba or Elodea) in water in a boiling tube, with **sodium hydrogencarbonate** added to supply plenty of **carbon dioxide**. Shine a **lamp** on it. As it photosynthesises, the pondweed releases **oxygen** — you collect or count it. Move the lamp to change the light intensity.
What comes off?
As the pondweed photosynthesises, which gas is given off and used to measure the rate?
- Oxygen
- Carbon dioxide
- Hydrogen
- Nitrogen
The method 📋
The rate is measured by **counting the oxygen bubbles** given off in a set time (say, per minute) — or, more accurately, by collecting the gas in a **syringe** and measuring its volume. You take a reading, move the lamp to a new distance, and repeat.
Order the method
An interactive activity.
Keep it fair 🎛️
To be sure it's the **light** changing the rate — not something else — control the other variables: • **Temperature** — the lamp heats the water, so put a **heat shield** (a beaker of water) between lamp and plant to absorb the heat. • **Carbon dioxide** — kept constant by the sodium hydrogencarbonate. • **The pondweed and timing** — use the same piece and the same counting time throughout.
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 accurate** — bubbles vary in size and are easy to miscount. Collecting the gas in a **syringe** and measuring its **volume** is more accurate. Either way, **repeat** each reading and take a **mean** to make the results more reliable.
Work out the rate
An interactive activity.
Read the result
As the lamp is moved **closer** to the pondweed, what happens to the rate of photosynthesis, and why?
- It increases — closer means higher light intensity
- It decreases — closer means less light
- It stays the same — distance has no effect
- It stops — the lamp is too close
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.