Lesson 7 of 12 / Nutrition and transport in flowering plants
Light, dark and the compensation point
Does a leaf take in or give out carbon dioxide?
I can explain net gas exchange in light and dark, the compensation point and hydrogencarbonate indicator results.
I can investigate and describe light, CO2 and temperature effects on photosynthesis.Syllabus
Syllabus K224 / K225, Topic 8(h). Investigate and describe light, CO2 and temperature effects on photosynthesis.
About 7 min
Make a guess. You are not marked.
Do plants respire during the day, or only at night?
Show the answer
All the time, day and night.
Respiration never stops. In bright light, photosynthesis takes in carbon dioxide faster than respiration releases it.
The key idea
A leaf respires all the time but photosynthesises only in light. Net gas exchange depends on which process is faster; at the compensation point they are equal.
Hydrogencarbonate indicator shows carbon dioxide concentration. It is red when in equilibrium with normal air, yellow when carbon dioxide rises and purple when carbon dioxide falls.
At the compensation point the rate of photosynthesis equals the rate of respiration. The carbon dioxide used equals the carbon dioxide released, so the indicator stays red.
Net carbon dioxide uptake by a leaf over 24 hours
Shaded: night. Above the dashed line the leaf takes in carbon dioxide; below it the leaf releases carbon dioxide. Illustrative curve, not measured data.
Explanation
A leaf respires all the time, day and night. Respiration gives out carbon dioxide and takes in oxygen. In light, the leaf also photosynthesises, which takes in carbon dioxide and gives out oxygen. What we measure from outside is the difference between the two.
In the dark, only respiration happens, so the leaf gives out carbon dioxide. In bright light, photosynthesis is faster than respiration, so overall the leaf takes in carbon dioxide and gives out oxygen.
The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration. At this point there is no overall gain or loss of carbon dioxide or oxygen. Both processes are still happening; they just balance.
Over 24 hours, the leaf gives out carbon dioxide at night. Soon after dawn it passes the compensation point and starts taking carbon dioxide in, most of all around midday. Near dusk it passes the compensation point again. A plant grows only if, over the whole day, it takes in more carbon by photosynthesis than it loses in respiration.
Hydrogencarbonate indicator shows these changes by colour. It is red in normal air, yellow when there is more carbon dioxide and purple when there is less. A leaf in a sealed tube in the dark turns it from red to yellow. In bright light it turns it from red to purple. At the compensation point it stays red. A tube with no leaf is the control and stays red.
Always give both colours: "the indicator changed from red to yellow". Writing only "turned yellow" may lose the mark.
Step by step
- 1
Name both processes
Respiration happens all the time; photosynthesis only in light.
- 2
Compare the rates
Photosynthesis faster: net uptake of carbon dioxide. Respiration faster: net release. Equal: compensation point.
- 3
Link to the indicator
Less carbon dioxide: red to purple. More carbon dioxide: red to yellow. No net change: stays red.
Worked example
Three tubes
Three sealed tubes contain hydrogencarbonate indicator and a leaf. Tube A is in bright light, tube B is in the dark and tube C is in dim light. After two hours A is purple, B is yellow and C is red. Explain the results.
One way to explain it
In A, photosynthesis is faster than respiration, so the leaf removes carbon dioxide and the indicator changes from red to purple. In B there is no photosynthesis; respiration releases carbon dioxide, so the indicator changes from red to yellow. In C the light intensity is at the compensation point: photosynthesis and respiration are equal, so there is no net change in carbon dioxide and the indicator stays red.
Why this answer works
- Give the colour change from and to.
- Compare the rates of the two processes for each tube.
- A control tube without a leaf would show that the leaf caused the changes.
Watch out for this
A student says: "Plants photosynthesise in the day and respire only at night." What is wrong with this?
Show the answer
Plants respire all the time. In daylight the carbon dioxide from respiration is used in photosynthesis, so the release is hidden by the faster uptake.
Check your understanding
A leaf in a sealed tube of hydrogencarbonate indicator is kept in dim light. After an hour the indicator is still red, the same as a control tube without a leaf. What is the best explanation?
Show the answer
The rate of photosynthesis equals the rate of respiration.
This is the compensation point: carbon dioxide used equals carbon dioxide released, so there is no net change.
Quick recall
Does a leaf take in or give out carbon dioxide?