Cambridge O Level Biology · Syllabus 5090 · Plant Nutrition
Chlorophyll
What is Chlorophyll?
The green pigment found inside chloroplasts. Chlorophyll absorbs light and transfers the light energy into chemical energy that is used to make carbohydrate from carbon dioxide and water. It is a condition for photosynthesis rather than a reactant, because it is not consumed by the reaction. A shortage of chlorophyll, from magnesium deficiency or in the non-green regions of a variegated leaf, reduces or prevents photosynthesis there.
This definition is part of the Plant Nutrition chapter in Cambridge O Level Biology.
Chlorophyll in context
Photosynthesis is the process by which plants make carbohydrates from raw materials using energy from light. It happens in chloroplasts, and the green pigment inside them, chlorophyll, absorbs light and transfers that light energy into chemical energy stored in carbohydrate. Two raw materials go in — carbon dioxide and water. Two products come out — glucose and oxygen. Light and chlorophyll are conditions, not reactants: light supplies the energy and chlorophyll transfers it, and neither appears on the reactant side of the equation. The glucose then goes four ways: it is respired, stored as starch, built into cellulose for cell walls, or converted to sucrose for transport in the phloem. How fast the whole thing runs is set by whichever necessary factor is in shortest supply — the limiting factor — and the leaf is built to keep light, carbon dioxide and water in good supply while letting oxygen out.
To test a leaf for starch: boil it in water for about a minute to kill the cells, stop reactions and make the leaf permeable; heat it in ethanol in a hot water bath to remove the chlorophyll; rinse it in warm water to soften the brittle, decolourised leaf; spread it flat on a white tile; and add iodine solution. Regions containing starch turn blue-black; regions without starch stay the orange-brown colour of the iodine solution itself.
Common mistakes with Chlorophyll
- “Chlorophyll is used up during photosynthesis.” Why it is wrong: a leaf that has photosynthesised all day is still green. Chlorophyll absorbs light, passes the energy on, and is ready to do it again. Say instead Chlorophyll is not consumed. It is a condition of the reaction and is written below the arrow.
- “Magnesium ions are used to make protein.” Why it is wrong: that is nitrate’s job. The two ions are constantly swapped in answers. Say instead Magnesium ions are needed to make chlorophyll. A shortage causes chlorosis — yellowing — and so a lower rate of photosynthesis.
- “Stomata absorb light for the plant.” Why it is wrong: a stoma is a hole. Light is absorbed by chlorophyll inside chloroplasts, and a pore contains neither. Say instead Stomata are pores that allow carbon dioxide to diffuse in and oxygen and water vapour to diffuse out. Light absorption is done by the chloroplasts in the mesophyll.
Questions students ask about Chlorophyll
Why is chlorophyll not written in the photosynthesis equation?
Because it is not used up. Chlorophyll absorbs light, transfers the energy into the reaction, and is unchanged afterwards — which is why a leaf that has photosynthesised all day is still green. Substances that are not consumed are written below the arrow as conditions, never as reactants or products.
What causes yellow leaves in a plant grown without magnesium?
Magnesium ions are needed to make chlorophyll. Without enough of them less chlorophyll is produced, so the leaves turn yellow — a condition called chlorosis. Because there is less chlorophyll, less light can be absorbed, so the rate of photosynthesis also falls.