The pondweed and lamp practical, rate from gas volume or bubble counts, the inverse square law for light, and reading limiting factor graphs.
What you will learn
- Describe the pondweed and lamp investigation and its variables
- Calculate a rate from a volume and a time, and a mean of repeat counts
- Use light intensity ∝ 1 ÷ d² (Higher)
- Say which factor limits the rate at a point on a graph
Pondweed, a lamp and a ruler
The rate of photosynthesis can be followed by the oxygen a water plant gives off.
- Independent variable: light intensity, changed by moving the lamp (distance d).
- Dependent variable: oxygen made per minute: count bubbles, or collect the gas in a syringe (better: bubbles vary in size).
- Control variables: temperature (a tank of water absorbs heat from the lamp, or use an LED), carbon dioxide (add sodium hydrogencarbonate), the same piece of pondweed. Work in a darkened room and let the plant settle for a few minutes at each distance.
rate = volume of oxygen ÷ time
e.g. cm³ ÷ minutes = cm³/min
- A syringe collects 3.6 cm³ of oxygen in 4 minutes
- rate = 3.6 ÷ 4
- rate = 0.9 cm³/min
The rest of this lesson and its 11 practice questions are locked.
In this lesson
- Pondweed, a lamp and a ruler
- The inverse square law (Higher)
- Limiting factors
- Try it: Your first rate
11 practice questions, marked as you go; about 25 minutes in all.
Key terms
Limits, Limiting factor, Light, Carbon dioxide, Temperature
In this unit: Bioenergetics: photosynthesis and respiration
Photosynthesis and its rate, limiting factors and the pondweed practical, aerobic and anaerobic respiration, exercise and metabolism.