Photosynthesis Starch Required Practicals

Explained

Testing a leaf for starch, and the three experiments built on it

Starch is stored where photosynthesis has happened, and iodine solution shows it. So one test, applied to leaves treated in different ways, answers three separate questions about what photosynthesis needs.

Testing a leaf

Boil the leaf in water for about a minute. This kills it and stops any further reactions, and it breaks down the cell membranes so that the iodine can get in later.

Heat it in ethanol in a water bath until the leaf is white. The ethanol dissolves the chlorophyll out, and the leaf has to lose its green colour or you will not be able to see the result.

Never heat ethanol over a flame. It is highly flammable, so the tube of ethanol stands in a beaker of hot water, with the Bunsen turned off before the ethanol goes anywhere near it. Safety questions on this practical are asking for exactly that.

Dip the leaf back in hot water to soften it, since ethanol leaves it brittle and it will crumble otherwise.

Spread it on a white tile and add iodine solution. Blue black means starch is present. Orange brown, the colour the iodine started as, means no starch.

Testing whether light is needed

Destarch the plant first by leaving it in the dark for at least 24 hours, so that any starch found afterwards must have been made during the experiment.

Cover part of one leaf with foil or a stencil, leave the plant in light for several hours, then test that leaf.

The uncovered part turns blue black. The covered part stays orange brown. The two parts are on the same leaf, so everything else about them is identical, which is what makes the comparison worth anything.

Testing whether carbon dioxide is needed

Destarch as before. Seal a plant in a flask or bag with soda lime, which absorbs carbon dioxide, and a second identical plant with no soda lime as the control.

The leaf with access to carbon dioxide turns blue black. The one without stays orange brown.

The control has to be there. Without it, a negative result could be blamed on the plant, the bag, the light, or anything else about being sealed in a flask.

Testing whether chlorophyll is needed

Use a variegated leaf, which has green parts and white parts on the same leaf, and record which parts were which before you start.

After destarching and a period in light, test the leaf. The parts that were green turn blue black; the parts that were white stay orange brown.

Drawing the leaf beforehand is the step people skip, and it is the one that makes the result readable, because the boiling and the ethanol remove the very colours you were relying on to tell the regions apart.

Answering about the negative result

Give the chain rather than the observation.

There was no light, so no photosynthesis took place, so no glucose and no starch were made, so the iodine stayed orange brown.

Four clauses, and the same shape works for the carbon dioxide and chlorophyll versions with one word changed. Saying only that the leaf stayed orange describes what you saw and explains nothing.