CO₂ & Photosynthesis…

This Twig Prep page has a good video describing an experiment to determine if light and carbon dioxide are necessary for photosynthesis. I thought I would describe an experiment with CO₂ as the only independent variable, similar to one I saw in an exam paper several years ago. If nothing else it will remind you about a few of the key points in setting up a fair experiment and how to test a leaf for starch.

Investigation: Is CO₂ necessary for photosynthesis?

A de-starched plant that has been stored in the dark for 24 hours before the experiment is used.

The leaf in flask A is the control with which the leaf in flask B can be compared. The only difference between the two flasks is that flask B contains some solid lithium hydroxide which will remove any CO₂. The hypothesis is that since the leaf in flask B has no access to carbon dioxide it will be unable to produce starch. The plant is well lit and left for several hours.

p.s. – you could use calcium oxide, sodium hydroxide or potassium hydroxide to remove the carbon dioxide from flask B. I used lithium hydroxide because I read that it was used in the Apollo space program to remove carbon dioxide from the air the astronauts were breathing.

When enough time has passed, the leaves in flask A and B are cut off and placed into boiling water. This serves two purposes…

1. it kills the leaf, “fixing” any starch so that it cannot be broken down for respiration
2. the cell structure is softened up so that it is easier to remove the chlorophyll

The two leaves are then placed into a container with ethanol which can dissolve the chlorophyll out of the leaf. This makes the leaf paler in colour so that the starch test will show up more clearly. The leaves can either be left overnight for the chlorophyll to be removed slowly or the ethanol can be heated to speed the process up. A water bath is used to heat the ethanol because ethanol vapour is highly flammable.

We should now have two white leaves which can be placed into cold water with plenty of iodine added. The iodine will slowly diffuse into the leaf and turn blue/black as it reacts with any starch present. We expect leaf A to turn blue/black but leaf B not to change.

Leaf A had water from the roots of the plant, light through the glass of the flask and carbon dioxide from the air inside the flask. Leaf B had identical conditions except that all the carbon dioxide had been removed.

Another idea I sometimes see in exam papers is the use of leaf discs, which are cut out of living leaves (usually spinach). They are made to sink in water and then left to photosynthesise. As bubbles of oxygen stick to them they slowly rise to the surface of the water. The rate at which five discs (for example) rise to the surface is used as a measure for the rate of photosynthesis. Conditions can be changed and the experiment repeated. Because the discs are very small lots of them can be cut from a single leaf so it is possible to set up quite sophisticated sets of conditions for testing.

The following video is for a college-level course but it shows the process of making the discs, the fiddly business of extracting the air from the spongy mesophyll making them sink and then a nice sequence of them rising to the surface.

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Three other posts that go nicely with this one (if you have not already read them) are…

Photosynthesis

Leaves (part one)

Leaves (part two)

Questions…

1. How does storing a plant in the dark for 24 hours cause it to become de-starched?
2. What two substances are formed when carbon dioxide reacts with lithium hydroxide?
3. What type of reaction is carbon dioxide reacting with lithium hydroxide?
4. In the video, Mr Anderson uses water with baking soda added. What is the chemical name for baking soda?
5. What function does the baking soda perform in the experiment?
6. In photosynthesis, only the initial splitting of water to make hydrogen atoms (and releasing oxygen as a waste product) actually requires light. What is the light-independent stage of the process, that uses CO₂, usually called?