Light and Gas Exchange

Explained

Watching a plant change the air around it

Hydrogencarbonate indicator changes colour according to how much carbon dioxide is dissolved in it, which makes it a way of seeing a gas you cannot see.

Purple means less carbon dioxide than ordinary air. Red or orange means the same as ordinary air. Yellow means more than ordinary air.

Learn those three in that order, because the whole practical is read off them.

The method

Set up several boiling tubes, each containing a fixed volume of hydrogencarbonate indicator that has been equilibrated with the air so it starts red.

Put a piece of pondweed in each tube except the control, and seal every tube with a bung so no gas can enter or leave. The bung matters: without it, carbon dioxide from the room would exchange with the indicator and the colour would tell you nothing about the plant.

Place the tubes in different light conditions: bright light, dim light, and complete darkness, wrapped in foil. Leave them for an hour or two and record the final colours.

The results, and why

In bright light the indicator turns purple. Photosynthesis is faster than respiration, so the plant removes more carbon dioxide than it produces, and the level falls below that of ordinary air.

In darkness it turns yellow. Only respiration is happening, so carbon dioxide is added and the level rises above that of ordinary air.

In dim light it may stay red or orange. Photosynthesis and respiration are proceeding at similar rates, so there is little or no net change. That is the compensation point, and this tube is the one worth understanding rather than memorising.

The control tube, with no plant, should stay red throughout. If it changes, something other than the plant caused it, and the results from the other tubes cannot be trusted.

Why the control is the point

Without it, a colour change proves nothing. Warmth from the lamp, a leaky bung or a contaminated tube could each shift the indicator, and you would have no way of telling.

The control differs from the test tubes in exactly one way, the absence of the plant, so any difference between them must be caused by the plant. Saying that, rather than simply naming the control, is what the mark is for.

Making it fair

Use the same volume of indicator, the same mass or length of pondweed, the same species, the same time and the same temperature in every tube.

Temperature needs thought, because a lamp warms the tube it is nearest. Placing a beaker of water between the lamp and the tubes absorbs the infrared and keeps the temperature constant, so light intensity is the only variable that changes.

Handle the tubes by the top rather than the middle. Body heat from your hand, and the carbon dioxide you breathe out near an open tube, are both real sources of error in this experiment.