Photosynthesis Quiz: 10 Questions with Answers

Answer all ten questions before you look at the answers, which are printed upside down at the foot of the infographic. Each question tests one part of the Photosynthesis series. If you get one wrong, read the explanation below and then revisit that part.

Plants take in carbon dioxide from the air through their stomata. Oxygen is a product of photosynthesis, not a reactant, although plants do use some oxygen for respiration. Carbon dioxide makes up only about 0.04% of the air, which is why it is often a limiting factor.

The answer is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Option B is the equation for aerobic respiration, the same reaction run backwards. Option C is not balanced, and option D has oxygen on the wrong side. Check by counting atoms: 6 carbon, 12 hydrogen and 18 oxygen on each side.

Most photosynthesis happens in the palisade mesophyll: tall, tightly packed cells just under the upper surface, full of chloroplasts and in the brightest light. The upper epidermis has no chloroplasts, and the phloem carries sugar rather than making it. The spongy mesophyll does photosynthesise, but its cells are looser and have fewer chloroplasts.

Guard cells open and close the stomata. Open stomata let carbon dioxide in and oxygen out; closing them saves water, but also cuts off the supply of carbon dioxide.

The oxygen comes from water, split in the chloroplasts using light energy. This was shown in 1941 with water labelled with a heavy oxygen isotope. It is one of the most common myths that the oxygen comes from carbon dioxide.

When more light no longer speeds up photosynthesis, light is no longer the limiting factor: another factor, such as carbon dioxide concentration or temperature, is now in short supply.

Light intensity is proportional to 1 ÷ distance². Doubling the distance from 10 cm to 20 cm gives 1 ÷ 2² = one quarter of the light, not a half. Going the other way, halving the distance from 20 cm to 10 cm makes the light four times as intense. This only changes the rate of photosynthesis while light is the limiting factor.

Plants respire all the time, day and night, in every living cell. In the light they also photosynthesise, usually faster than they respire.

Glucose is built into cellulose, the fibres that strengthen cell walls. Starch and oils are energy stores, and amino acids are used to build proteins. To make amino acids, a plant also needs nitrate ions from the soil, because glucose contains no nitrogen.

In 1779 Jan Ingenhousz showed that plants only restore air in the light, and only their green parts do it. Priestley had shown in 1771 that plants restore air, but not that light was needed.