9.4Chapter 9: Plants and Photosynthesis

Evidence of photosynthesis: oxygen bubbles and pondweed — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 9 2 quizzes

📚 Alhussan Biology - Stage 9 Hub / Topical Revision

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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

Part 1 of 4Method

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.

Part 2 of 4Identifying the gas and results

Test for oxygen

Test for oxygen collect the gas in a test tube and insert a glowing splint, which relights.

Trend

Trend the number of bubbles falls as the lamp is moved further away, because light intensity falls.

Light intensity

Light intensity it is proportional to 1 ÷ distance², so a graph of rate against 1 ÷ distance² is often a straight line.

Levelling off

Levelling off at high light intensity the rate stops rising because another factor,
- such as carbon dioxide concentration or temperature, becomes limiting.

Part 3 of 4Variables and reliability

1Independent variable

Independent variable light intensity, changed by the distance from the lamp.

2Dependent variable

Dependent variable the number of bubbles per minute.

3Control variables

Control variables temperature, the same plant, carbon dioxide concentration and time.

4Errors

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.

5Better measurement

Better measurement collecting and measuring the volume of gas is more precise than counting.

6Control experiment

Control experiment a plant kept in darkness shows that bubbles come from photosynthesis and not from dissolved air.

7Net oxygen

Net oxygen plants also respire and use some oxygen, so the bubbles are the net oxygen released.

8Reliability

Reliability a wider range of measurements and more repeats improve it, and conclusions are limited by the quantity and reliability of the data.

Part 4 of 4Factors affecting the rate

Factors

Factors light intensity, carbon dioxide concentration and temperature all affect photosynthesis.

Limiting factor

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

Use in growing greenhouse growers add artificial light, extra carbon dioxide and heating to increase yields.

Key terms

Term	Meaning
Elodea	A water plant (Canadian pondweed) used to show oxygen release.
Rate of photosynthesis	How quickly photosynthesis is taking place.
Limiting factor	The factor in shortest supply that sets the rate of a process.
Glowing splint	A test that relights in oxygen.
Independent variable	The factor deliberately changed in an investigation.
Dependent variable	The factor measured in an investigation.