Palisade mesophyll

In the syllabus and the exam

Palisade mesophyll is tested as one of the clearest examples in the whole syllabus of structure being suited to function, so questions on it almost always ask candidates to link a named feature to a specific advantage rather than simply to recall the definition. Candidates are expected to state that palisade cells are long, column-shaped, and arranged closely together with few air spaces, positioned near the upper leaf surface where light intensity is greatest, and that they contain a high density of chloroplasts, all of which maximises the leaf's rate of photosynthesis. This tissue is regularly compared with the spongy mesophyll located beneath it, which has more air spaces to aid gas diffusion but fewer chloroplasts, and Paper 2 diagrams of a leaf cross-section frequently ask candidates to identify and distinguish the two layers. A common error is describing palisade mesophyll cells as loosely packed or as being found on the lower surface of the leaf; candidates who reverse the position of palisade and spongy mesophyll in a labelled diagram lose marks even when their explanation of chloroplast density is otherwise correct. Candidates should also note that chloroplasts inside palisade cells can move within the cytoplasm to align with the direction of incoming light, an adaptation that further increases the efficiency of light absorption and is occasionally referenced in higher-level structured questions about leaf efficiency. This is also why a leaf's upper surface generally looks a deeper, more uniform green than its lower surface when viewed under a hand lens, a visible clue candidates can use to identify which side of a leaf section is uppermost in an unlabelled diagram.

Example

Describing leaf adaptations, a candidate writes: 'The palisade mesophyll cells are packed tightly together just below the upper epidermis and contain many chloroplasts, allowing them to absorb the maximum amount of light for photosynthesis.'

Related terms

ChloroplastPhotosynthesisCuticle

Source:SRC-DSKP-EN