Plant tissues: palisade mesophyll

The idea

Palisade mesophyll is the main photosynthetic tissue of a leaf. It sits just below the upper epidermis and is packed with chloroplasts, so it absorbs most of the light that reaches the leaf. Its cells are tall, narrow and arranged in a regular layer, which fits many cells into a small area near the light. Air spaces between the cells let carbon dioxide reach them and oxygen leave. In an exam, you may be asked to identify palisade cells on a leaf section diagram, relate their features to photosynthesis, or explain why a leaf grown in shade develops thinner palisade tissue. The key idea is structure related to function: many chloroplasts and a position near the upper surface maximise light absorption for photosynthesis.

What earns the marks

• Palisade mesophyll cells contain many chloroplasts, which absorb light energy for photosynthesis.
• The cells form a regular layer near the upper surface of the leaf, so light reaches them before it is absorbed by other tissues.
• Their tall, narrow shape packs many photosynthetic cells into a thin layer, increasing the surface area available for light absorption.
• Air spaces around palisade cells allow carbon dioxide to diffuse in and oxygen to diffuse out.
• Palisade mesophyll is adapted for photosynthesis, not for support or transport, so it is not the same as xylem or phloem.
• A leaf section diagram should show palisade mesophyll as a distinct layer below the upper epidermis.

Common mistakes and how to put them right

• Confusing palisade mesophyll with spongy mesophyll.

  Put it right: Palisade cells are regular, tightly packed and near the upper surface, while spongy cells are rounded with large air spaces lower down.

• Thinking palisade cells carry out respiration only.

  Put it right: They respire, but their main role is photosynthesis because of their many chloroplasts.

• Stating that palisade cells have no air spaces at all.

  Put it right: Small air spaces remain between them so gases can diffuse to and from the cells.

Exam tips

• When describing a leaf section, name the tissue and give its position, for example palisade mesophyll lies just below the upper epidermis.
• Link each feature to a benefit: many chloroplasts absorb more light, and a position near the top means light is not blocked by other tissues.
• Use the phrase structure related to function to organise your answer, and avoid writing about transport when the question asks about photosynthesis.