Chlorophyll

In the syllabus and the exam

Chlorophyll is tested as the pigment responsible for capturing light energy during the first stage of photosynthesis, and candidates must be able to distinguish it clearly from the chloroplast, the organelle that contains it; chlorophyll is the pigment, while the chloroplast is the whole structure housing chlorophyll along with the enzymes and membranes needed for the full process. A commonly tested practical link is the leaf destarching and testing procedure, where a leaf is first boiled in ethanol to remove chlorophyll and decolourise it, making any colour change with iodine solution easier to observe when testing for starch. Structured questions sometimes ask why chlorophyll appears green, expecting candidates to explain that it absorbs red and blue light strongly but reflects green light, which is why leaves appear green to the human eye. A frequent error is stating that chlorophyll itself performs photosynthesis; chlorophyll only absorbs and converts light energy as the first step, while the subsequent chemical reactions that fix carbon dioxide into glucose occur through separate stages within the chloroplast, not within the pigment molecule itself. Candidates should also know that a plant with insufficient chlorophyll, whether from disease, poor nutrition, or a genetic condition producing pale or variegated leaves, photosynthesises at a reduced rate, which is sometimes used as the basis for a structured question comparing green and variegated leaves under the same light conditions. This is also why the classic starch test on a leaf begins by softening it in hot water before decolourising it in ethanol, since the combined steps make the tissue permeable enough for iodine solution to reach any starch present once the chlorophyll has been removed.

Example

Explaining light absorption, a candidate writes: 'Chlorophyll, the green pigment found inside chloroplasts, absorbs light energy, mainly in the red and blue regions of the spectrum, and converts it into chemical energy for photosynthesis.'

Related terms

ChloroplastPhotosynthesisPalisade mesophyll

Source:SRC-DSKP-EN