Plant tissues: spongy mesophyll

The idea

Spongy mesophyll is the tissue below the palisade layer in a leaf. Its cells are rounded and loosely arranged, leaving many large air spaces between them. These air spaces connect to the stomata on the lower surface, so carbon dioxide can diffuse through the leaf to reach the photosynthetic cells, and oxygen can diffuse out. Spongy mesophyll cells contain some chloroplasts, so they also photosynthesise, but they receive less light than the palisade layer. In an exam, you may be asked to label spongy mesophyll on a leaf section, explain how its air spaces help gas exchange, or compare it with palisade mesophyll. The key idea is that loose packing and air spaces make diffusion efficient.

What earns the marks

• Spongy mesophyll cells are loosely arranged with large air spaces between them.
• The air spaces connect to the stomata, allowing carbon dioxide to diffuse into the leaf and oxygen to diffuse out.
• Spongy mesophyll cells contain chloroplasts and carry out some photosynthesis, although they receive less light than palisade cells.
• The large surface area of the air spaces speeds up diffusion of gases to and from the photosynthetic cells.
• Spongy mesophyll is found below the palisade mesophyll and above the lower epidermis in a typical leaf section.
• Its main role is gas exchange and supporting photosynthesis, not transport of water or sugars.

Common mistakes and how to put them right

• Saying spongy mesophyll has no chloroplasts.

  Put it right: It has fewer chloroplasts than palisade mesophyll but still photosynthesises.

• Confusing the air spaces with the stomata.

  Put it right: Stomata are pores in the epidermis, while air spaces are gaps between spongy mesophyll cells inside the leaf.

• Describing spongy mesophyll as tightly packed.

  Put it right: Its cells are loosely arranged, which is what creates the air spaces needed for diffusion.

Exam tips

• Use the words air spaces and diffusion when explaining the role of spongy mesophyll, and link the spaces to the stomata.
• When comparing tissues, give one clear difference, such as palisade cells are tightly packed near the top while spongy cells are loosely packed lower down.
• If asked to label a diagram, place the label in the lower half of the leaf section, below the palisade layer.