BIOLOGY · Stage 9 · Chapter 9: Plants and Photosynthesis
Biology 9.4 Evidence of photosynthesis: oxygen bubbles and pondweed — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions
5 min read 5 parts Lesson 4 of 9
Lesson summary
9.4 Evidence of photosynthesis: oxygen bubbles and pondweed — Alhussan International Academy — Biology (Stages 7–9) notes: Describe a pondweed experiment that shows photosynthesis produces oxygen and investigate how light intensity affects its rate.Equation — carbon dioxide + wat…
- Land plants
- Water plants
- Test for oxygen
- Levelling off
- Independent variable
- Control variables
Section 1 of 5The idea behind the experiment
- Equation
- carbon dioxide + water → glucose + oxygen, in light and with chlorophyll.
- Evidence of photosynthesis
- starch in leaves, and oxygen released.
- Land plants
- their oxygen diffuses into the air, so it is hard to use as evidence.
- Water plants
- aquatic plants such as Elodea release oxygen as bubbles from the leaves and cut stems.
- (Canadian pondweed)
- Collecting the gas shows it is oxygen.
- Counting the bubbles in a fixed time shows how fast photosynthesis is taking place.
- aquatic plants such as Elodea release oxygen as bubbles from the leaves and cut stems.
Section 2 of 5Method
- Place a piece of pondweed with a freshly cut stem in a tube, cut end up, and cover it with water.
- Add a little sodium hydrogencarbonate to the water as a source of carbon dioxide.
- Stand the tube in a beaker of water on a ruler, with the plant at a measured distance from a bright lamp.
- Use the beaker of water, with a thermometer, to absorb heat from the lamp so that temperature stays constant.
- Leave the plant for a few minutes to adjust to the light intensity.
- Count the bubbles released from the cut end in one minute, or collect the gas in an upturned measuring tube.
- Repeat at least once more and calculate a mean.
- Move the lamp, for example 2 cm further away each time, and repeat, keeping other factors the same.
Section 3 of 5Identifying the gas and results
- Test for oxygen
- collect the gas in a test tube and insert a glowing splint, which relights.
- Trend
- the number of bubbles falls as the lamp is moved further away, because light intensity falls.
- Light intensity
- it is proportional to 1 ÷ distance², so a graph of rate against 1 ÷ distance² is often a straight line.
- Levelling off
- at high light intensity the rate stops rising because another factor, such as carbon dioxide concentration or temperature, becomes limiting.
Section 4 of 5Variables and reliability
- Independent variable
- light intensity, changed by the distance from the lamp.
- Dependent variable
- the number of bubbles per minute.
- Control variables
- temperature, the same plant, carbon dioxide concentration and time.
- Errors
- miscounting fast bubbles, bubbles of different sizes, heat from the lamp and the cut end healing over so fewer bubbles escape.
- Cutting the stem again keeps it open.
- Giving the plant longer to adjust to each new light intensity makes results more reliable.
- Better measurement
- collecting and measuring the volume of gas is more precise than counting.
- Control experiment
- a plant kept in darkness shows that bubbles come from photosynthesis and not from dissolved air.
- Net oxygen
- plants also respire and use some oxygen, so the bubbles are the net oxygen released.
- Reliability
- a wider range of measurements and more repeats improve it, and conclusions are limited by the quantity and reliability of the data.
Section 5 of 5Factors affecting the rate
- Factors
- light intensity, carbon dioxide concentration and temperature all affect photosynthesis.
- Limiting factor = the factor in shortest supply, which sets the rate.
- Increasing the limiting factor raises the rate until another factor limits it.
- Use in growing
- greenhouse growers add artificial light, extra carbon dioxide and heating to increase yields.
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