What is the net gas exchange between a plant and its surroundings at the compensation point?

There is no net exchange, because all the carbon dioxide released by respiration is used in photosynthesis and all the oxygen used in respiration is supplied by it.

What is meant by the compensation period?

The compensation period is the time taken after dawn for a plant to reach the point at which photosynthesis has made up for the respiration carried out overnight.

Which has the lower compensation point, a shade plant or a sun plant, and why does that matter?

A shade plant has the lower compensation point, which allows it to make a net gain in biomass at the low light intensities found on a woodland floor.

On a graph of net carbon dioxide uptake against light intensity, what does the point where the line crosses the x-axis represent?

It is the compensation point, the light intensity at which net carbon dioxide uptake is zero.

OCR A-Level Biology B H422

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Photosynthesis, Food Production and Management of the Environment

The compensation point and crop production

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the compensation point and compensation period, shade and sun plants, reading net carbon dioxide uptake graphs, crop spacing and glasshouse lighting, and using hydrogencarbonate indicator, with controls and a water bath, to compare compensation points.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Define the compensation point.

Key term

What happens to a plant's biomass if it spends most of the day below its compensation point?

Key term

Why do growers space crop plants carefully in a field?

Questions from this subtopic

Each one with the model answer underneath.

1
Define the compensation point.
Model answer
The compensation point is the light intensity at which the rate of photosynthesis in a plant exactly equals its rate of respiration.
2
What is the net gas exchange between a plant and its surroundings at the compensation point?
Model answer
There is no net exchange, because all the carbon dioxide released by respiration is used in photosynthesis and all the oxygen used in respiration is supplied by it.
3
What is meant by the compensation period?
Model answer
The compensation period is the time taken after dawn for a plant to reach the point at which photosynthesis has made up for the respiration carried out overnight.
4
What happens to a plant's biomass if it spends most of the day below its compensation point?
Model answer
Respiration exceeds photosynthesis, so the plant breaks down more organic material than it makes and its biomass falls.
5
Which has the lower compensation point, a shade plant or a sun plant, and why does that matter?
Model answer
A shade plant has the lower compensation point, which allows it to make a net gain in biomass at the low light intensities found on a woodland floor.
6
On a graph of net carbon dioxide uptake against light intensity, what does the point where the line crosses the x-axis represent?
Model answer
It is the compensation point, the light intensity at which net carbon dioxide uptake is zero.

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