(11) Some simple experiments to investigate aspects of photosynthesis

Some demonstrations of involving photosynthesis

Demonstrating the presence of starch in plant leaves

Simple experiments on starch production in plants

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.

 

Observations and interpretation

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.

 

(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, so less carbon dioxide, system less acidic.

(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.

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.

 6H₂O  +  6CO₂  == light ==>  C₆H₁₂O₆  +  6O₂

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.

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Keywords, phrases and learning objectives for this part on photosynthesis

Be able to describe (basic apparatus and method) and interpret results from simple experiments to investigate aspects of photosynthesis e.g. testing leaves with starch test with iodine, with experiments to show the need for light and that plants also need carbon dioxide for photosynthesis.

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