Light, dark and the compensation point

Explanation

A leaf respires all the time, day and night, releasing carbon dioxide and taking in oxygen. In light it also photosynthesises, taking in carbon dioxide and releasing oxygen. What you measure from the outside is net gas exchange: the difference between the two processes.

In the dark only respiration takes place, so the leaf gives out carbon dioxide and takes in oxygen. In bright light the rate of photosynthesis is greater than the rate of respiration, so the leaf takes in carbon dioxide and gives out oxygen overall.

The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration. At this light intensity there is no net exchange of carbon dioxide or oxygen. The leaf is still photosynthesising and respiring; the two simply balance.

Over 24 hours, net carbon dioxide uptake is negative at night, rises through zero at the compensation point soon after dawn, peaks around midday and falls back through zero at the compensation point near dusk. A plant gains mass only if, over the whole day, it fixes more carbon by photosynthesis than it releases in respiration.

Hydrogencarbonate indicator lets you see these changes. Its colour depends on the carbon dioxide concentration: red in normal air, yellow when carbon dioxide increases, and purple when carbon dioxide decreases. A leaf in a sealed tube in the dark turns the indicator from red to yellow. In bright light it turns the indicator from red to purple. At the compensation point the indicator stays red. A tube with no leaf is the control: it stays red, showing that the leaf caused the change.

When you describe an indicator result, give both colours: "the indicator changed from red to yellow". Mark schemes often want the starting and final colour, and "turned yellow" alone may lose the mark.