Experiment 1. To test for starch in leaves
Some of the glucose made by photosynthesis is
converted into starch. an energy food store.
A leaf, held with tongs/tweezers is 'dunked' into
boiling water - to stop all its chemical reactions.
The
leaf is placed in a boiling tube of alcohol and gently heated in an
electric water bath - this dissolves the green chlorophyll and turns the
leaf a very pale colour - no longer green!
Take care, ethanol is highly flammable - bunsen
burners not recommended!
The almost white leaf is rinsed with cold water and laid
out on a filter paper.
With a teat pipette, spot a few
drops of iodine solution onto the leaf.
If starch
is present , a blue-black colour will appear - the simple
standard
food test for starch molecules.
Preparation of plants and set-up
for experiments 2. and 3.
You can do simple experiments with a small plants in
plant pots, if necessary keep them enclosed in a bell jar.
You have to 'de-starch' the plants by leaving them in the dark for
at least 48 hours.
The plant will use up its starch
energy store to keep itself alive!
You can use this
set-up to do a couple of simple experiments, and, finally using the
starch test described above, to show what is required for
photosynthesis, or indeed, if photosynthesis was taking place.
Experiment 2. To show that light is needed for photosynthesis
From your stock of 'de-starched' plants, you keep
one in the dark and one into bright sunlight or artificial light.
After 24 hours you test a sample leaf from each plant for the
presence of starch.
The leaf from the plant in the
dark should not give the blue-black colour with iodine
solution - photosynthesis had not taken place.
The
leaf from the plant in the light should have produced starch from
photosynthesis, and after testing should give a blue-black colour
with iodine solution showing the presence of starch.
This shows light is need for photosynthesis.
The plants should
not be enclosed in the bell jar, so that
each plant has access to carbon dioxide in the atmosphere.
Ideally, the plants should be identical and kept at the same
temperature for a fair test.
Experiment 3. To show that carbon dioxide is needed
From your stock of 'de-starched' plants, two plants
are left out in daylight or artificial light.
BUT,
one of the plants is put in a bell jar with a small petri
dish of soda lime.
(a) This isolates one of the plants from the
surrounding 'normal' atmosphere.
(b) Soda lime
absorbs and chemically reacts with carbon dioxide to give a
solid product - thus removing carbon dioxide from the atmosphere
around the plant.
The plants are then left out for 24 hours.
After 24 hours you test a sample leaf from each plant for the
presence of starch.
The leaf from the plant left out
in the laboratory (not in the bell jar) with access to the
atmosphere should have produced starch from photosynthesis - after
testing should give a blue-black colour with iodine solution showing
the presence of starch.
However, the leaf from the
plant in the bell jar should not give the blue-black colour
with iodine solution - showing photosynthesis to form starch had not
taken place despite having access to light.
This
shows carbon dioxide is need for photosynthesis.
Ideally, the plants should be identical and kept at the same
temperature for a fair test.
Experiment 4.
To show that carbon dioxide is involved in the gas exchange of
photosynthesis
This is a simple photosynthesis gas exchange experiment
using hydrogencarbonate indicator and plant leaves.
Three test tubes are set up as in the diagram and
described below.
All three test tubes are exposed to the same
intensity of bright light - sunlight or lamp.
Remember
that carbon dioxide is a weakly acidic gas when dissolved in water.
1. Control
A test tube is set up just containing a few cm3
of the orange hydrogencarbonate indicator.
This acts as a control and shouldn't change from its
original orange colour, since there are no plant leaves in it
and it is sealed to the atmosphere, there should be just the normal
background level of carbon dioxide above the indicator.
If the indicator solution becomes more acidic,
it becomes yellow.
If the indicator solution becomes less acidic,
it becomes red.
Now remember, carbon dioxide gas is acidic
when dissolved in water.
Observations and interpretation
2. Leaves exposed to bright light
When exposed to light the leaves can photosynthesise and absorb carbon dioxide.
6H2O
+ 6CO2 ====> C6H12O6 + 6O2
Therefore carbon dioxide will be absorbed by the
plant and the reduction of carbon dioxide means conditions are less acidic.
So the hydrogencarbonate indicator changes to red
- solution less acidic.
In bright light the rate of photosynthesis will be
greater than the rate of respiration.
(Oxygen will replace the carbon dioxide, but that is
not detected in this experiment, but you could set up a system to
collect it and test the gas with a glowing splint - which should be
reignited.)
3. Leaves shaded from light
If little light can reach the surface of the leaves,
then photosynthesis cannot take place efficiently.
In order for the leaves (plant) to survive they must
be a switch from less photosynthesis to more respiration.
C6H12O6 + 6O2 ====>
6H2O
+ 6CO2
The respiring plant leaves give out carbon dioxide
which makes the condition more acidic.
Therefore the hydrogencarbonate indicator turns
yellow - solution more acidic.
In shade the rate of photosynthesis will be less
than the rate of respiration.
Experiment 5.
Simple demonstration of the effect of light on the rate of photosynthesis
You can use this simple investigation experiment to help
you design more sophisticated and more accurate quantitative experiments
described in
method 1. gas syringe system and
method 2.
moving gas bubble system.
You set up a beaker filled with water or sodium hydrogen
carbonate solution.
In the beaker you place an oxygenating plant like a
pondweed inside an inverted filter funnel.
You fill a test tube with water and invert it, still
filled with water, and place it over the exit from the filter funnel.
When you shine bright light (sunlight or lamp) on the
system, you should see bubbles of oxygen gas rising and collecting in
the inverted test tube.
6H2O
+ 6CO2 ==
light ==> C6H12O6 + 6O2
If you collect enough gas, it should ignite a glowing
splint - a simple chemical test for oxygen.
You can play around with a lamp distance to increase or
decrease the light intensity and note any difference in the rate of
bubble formation.
You should find adding sodium hydrogencarbonate speeds
up photosynthesis because it supplies more carbon dioxide - there is
only small amount dissolved in tap/deionised water.
Again, you could compare water with sodium
hydrogencarbonate solution at the same light intensity.
BUT, this set-up is no good for looking at temperature.
In fact the whole experiment isn't very accurate at all.
The bubbles tend to form randomly, no way of accurately
measuring gas volume or the rate at which the gas is evolved, no
thermostated bath to control and vary temperature.
Hence the need for
method 1.
gas syringe system and
method 2.
moving gas bubble system.
