Measuring the rate of photosynthesis
by timing movement of a bubbles of gas in a capillary tube
Doc Brown's Biology exam study revision notes
There are various sections to work through,
after 1 they can be read and studied in any order.
of biology notes on PHOTOSYNTHESIS
Measuring the rate of photosynthesis: experimental
method 2 - timing movement of a bubbles of gas in a capillary tube
can use this gas syringe system to measure the effects of changing temperature,
light intensity and carbon dioxide level (via a sodium hydrogencarbonate
At the end of method 2 the inverse square
low of light intensity is explained.
experimental methods described in methods 1. and 2. above for determining the rate of photosynthesis
in Canadian pondweed experiment.
The 'set-up' probably the best
system I can devise sitting at home in front of the computer screen!
In method 2 the pondweed tube could
be enclosed in a large beaker of water that
acts as a simple thermostated bath to keep the temperature constant -
ideally a thermostated water bath.
The tube of pondweed is immersed in
NaHCO3 solution is subjected to a lamp emitting bright white
light at a specific distance from tube of pondweed.
You can again use sodium
hydrogencarbonate (NaHCO3) as source of carbon dioxide and vary its
vary the carbon dioxide concentration.
(i) The oxygen bubbles are still channelled into a
capillary tube but the gases and liquids allowed to freely exit from the
capillary tube - no problem with liquid in the syringe which might
quite stiff anyway and difficult to measure an accurate volume.
(ii) A T junction in the tubing
allows the 'injection' of water into the gas stream to make bubbles
of gas visible.
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.
What can you measure and vary?
rate of photosynthesis by measuring the rate of oxygen gas production in the gas syringe
is more accurate but requires more time to get a set of readings to plot
Measuring the speed of the horizontal
movement of the gas bubbles is quite easy via the accurate linear scale and
You can use quite a long uniform
capillary tube to increase the sensitivity and hence accuracy of the
For each set of experimental conditions
get at least three reasonably consistent readings and compute an average for
the best accuracy.
The speed of bubbles in cm/s gives you a
relative measure of the rate of the overall reaction of photosynthesis to
With increasing concentration (of
NaHCO3) you should see an increase in the rate of oxygen bubbles,
but you must keep the temperature constant eg lab. temp. 20-25oC,
and the light intensity constant by keeping the lamp a fixed distance from
the flask. The light from the laboratory itself will contribute, but the
total light should be constant and you can use a light meter to ensure the
same light intensity.
To vary temperature you need to
immerse the boiling tube in water baths of different carefully controlled
and constant temperatures - ideally using a thermostated water bath.
You should be able to get enough results
eg 5o increments from 15oC to 50oC to show
maximum the maximum rate of photosynthesis expected to be around 35-40oC.
The concentration of NaHCO3
and the light intensity should be both kept constant.
Varying the light intensity is quite
difficult, you need to position a lamp at different measured distances away
from the pondweed tube.
You can calculate the relative
intensity using the inverse square law, see
light intensity section on this page.
accurate results you should take a light meter reading by the flask in the
direction of the lamp (see the discussion on the inverse square law further
down the page).
You must choose, and keep constant, both
the temperature and sodium hydrogencarbonate concentration of appropriate
values eg a 2% solution of NaHCO3 and 25oC.
Although I think this is an
improvement on method 2, its still quite difficult to get
I think a light meter is essential
for accurate results - changing the lamp distance is relevant to
changing the light intensity, BUT, intensity is NOT a simple function of
You need to use the same sample of
pondweed, but is it always the same leaf area towards the light?
The experimental runs should not take
too long as the NaHCO3/CO2 concentration is
falling all the time.
Graphs of experimental data and their
The relative intensity of the
from a fixed power is governed by an inverse square law.
When investigating the influence of light
intensity on the rate of photosynthesis you must appreciate the inverse
square law applied to light intensity for a fixed lamp power and light
As you move the lamp further away, the
light intensity falls, and so should the rate of photosynthesis.
BUT the light intensity is inversely
proportional to the distance between the light source and the
experiment tube squared.
From a specific light source ...
relative light intensity = 1 / d2
light intensity is in arbitrary
units, d = distance of the lamp from experiment.
The effect of the law can be demonstrated
by some simple calculations ...
|distance from lamp
||distance to the experiment
|1 / d
||reciprocal of distance
|1 / d2
||reciprocal of distance squared
1 / d2
||arbitrary units calculated
by the inverse square law equation
||distance from lamp to
as a fraction
||decreasing with the inverse
||this is what it would be if
intensity = 1 / d
relative rate of photosynthesis
rate of photosynthesis is proportional to light intensity
The inverse square law for relative light
intensity means that the relative brightness that the plant experiences
falls away quite dramatically as the lamp is move further and further from
the experiment tube.
Graphs of rate of photosynthesis
versus distance of the lamp from experiments such as method 3.
These graphs are plots of the theoretical
data used in the table above assuming a constant light source (a lamp!).
Graph (a) shows how rapidly the
light intensity decreases as you move the experiment tube/flask away from
the light source, shown by the equally rapid decline in the rate of
photosynthesis. This is a consequence of the inverse square law of light
intensity. You can show by experiment the rate of photosynthesis is
proportional to the light intensity where it is the limiting factor. The
graph also shows that the relationship between rate of photosynthesis and
lamp distance is not linear.
Graph (b) shows that the rate
photosynthesis is not proportional to reciprocal of the lamp distance, but
it is a more linear graph than (a).
Graph (c) shows (for ideal
results) that the rate of photosynthesis is proportional to the reciprocal
of the lamp distance squared (and the lamp light intensity is proportional
to 1 / d2). Therefore in graph (c) the horizontal axis could be
also labelled relative light intensity, a proportional linear relationship
with the rate of photosynthesis.
See graphs of factors
in photosynthesis Part 6.
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