Measuring the rate of photosynthesis
by measuring the volume of oxygen produced with
a gas syringe
- varying light intensity, temperature and CO2 concentration
Doc Brown's Biology exam study revision notes
There are various sections to work through.
of PHOTOSYNTHESIS notes
Measuring the rate of photosynthesis
method 1 measuring the volume of oxygen produced with
a gas syringe
Possible practical work you
may have encountered - methods of measuring the rate of photosynthesis
You can investigate the need for chlorophyll for photosynthesis with variegated
Taking thin slices of potato and apple and adding iodine to observe
under the microscope - test for starch, which gives a blue colour with
Investigating the effects of light, temperature and
carbon dioxide levels (using Canadian pondweed, Cabomba, algal balls or leaf discs
from brassicas) on the rate of photosynthesis.
You can use computer simulations to model
the rate of photosynthesis in different conditions
You can use sensors to
investigate the effect of carbon dioxide and light levels on the rate of
photosynthesis and the release of oxygen.
You may have done/seen
experiments on the rate of photosynthesis in which the volume of oxygen
formed is measured with a gas syringe connected to a flask of sodium
hydrogen carbonate solution (to supply the carbon dioxide) and
Canadian pondweed immersed in it.
All experimental methods depend on
measuring the rate of oxygen production as a measure of the rate of
The faster the oxygen production the
faster the photosynthesis.
It is assumed that the rate of oxygen
production is proportional to the rate of photosynthesis.
So, how can we measure the rate of
Next, methods of measuring the rate of photosynthesis
can use this gas syringe system to measure the effects of changing temperature,
light intensity and carbon dioxide level (via different concentrations of sodium hydrogencarbonate
Method 1. Gas syringe system
A lamp and thermostated water bath are
not shown in this diagram, but they are in the apparatus diagram for
There are several aquatic plants you can
use, the most popular seems to Canadian pondweed (elodea canadensis), but
this is regarded as an invasive species, so perhaps some other oxygenated
aquatic plant should be used!
In this 'set-up' you measure the rate of
photosynthesis by measuring the rate of oxygen production as the gas is
collected in the gas syringe.
From the graph of volume of oxygen
versus time you measure the initial gradient to calculate the rate of
production of oxygen as a measure of the rate of photosynthesis.
The graph should be reasonably linear at
first e.g. rate of photosynthesis in cm3/min.
You can use sodium
hydrogencarbonate (NaHCO3) as source of carbon dioxide and vary its
vary the carbon dioxide concentration. You can use from 0.1% to 5% of NaHCO3
ie 0.1g to 5g per 100 cm3 of water.
With increasing concentration you should
see an increase in the rate of oxygen bubbles (eg cm3/min), but
you must keep the temperature constant eg lab. temp. 20-25oC, and
the light intensity constant by keeping the lamp (not shown in the diagram)
the same distance from the flask.
The light from the laboratory itself will
contribute, but the total light should be constant.
You need to use the same quantity and
batch of pondweed (or other oxygenating aquatic plant).
You use the same volume of
water/sodium hydrogencarbonate solution.
Using the set-up described in the
diagram, at constant temperature, constant light intensity - by using same
lamp at the same distance from the flask, you can investigate the effect of
the concentration of carbonate/carbon dioxide on the rate of photosynthesis.
To vary temperature you need to
immerse the conical flask in a thermostated water bath (not shown
method 2. Part 10
diagram) of different, but carefully controlled
Varying the light intensity is quite
difficult, you need to position a lamp at different measured distances away
from the flask, but for
accurate results you must take a light meter reading by the flask in the
direction of the lamp - but you can still use the basic set-up of apparatus
described in method 1. above.
This simple experiment can readily show
in principle the effect of changing the three controlling factors of the
rate of photosynthesis.
Problems and errors with the
Ideally the experiments should be done in
the dark, with the lamp the only source of light, not very convenient in a
classroom situation but it is particularly important when varying the light
intensity - I don't see how you can get accurate results for light intensity
though using a light meter might just ok?
Do you swirl the flask so the NaHCO3
concentration remains reasonably constant?, but will the same leaf area be
exposed to the light in the direction of the lamp?
When varying the temperature it is not easy
to maintain a constant temperature - if it falls a little, you could use the
average temperature, not as accurate, but better than nothing! A
thermostated water bath would be ideal.
The above apparatus is typical of that
used in rate of reaction experiments in chemistry.
How can we measure the speed or rate of a chemical reaction?
See graphs in
photosynthesis Part 6.
You can use other experiment designs
to look more conveniently, and hopefully more accurately at the three
factors that influence the rate of photosynthesis.
Keywords, phrases and learning objectives for this part on
Be able to describe and analyse the results of an
investigation experiment to measure rate of photosynthesis by
measuring the volume of oxygen produced using a gas syringe
Be able describe the experimental method, apparatus and
observation data of a gas volume experiment of photosynthesis and
also describe how to get experiment results at different temperatures,
different light intensities and different carbonate concentrations
(= varying CO2 concentration)..
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