The challenges of size in plants
During transpiration plants move water from the roots to their leaves for photosynthesis in xylem vessels. Glucose made in photosynthesis is then moved to all cells in phloem vessels for respiration.
Plant tissues: epidermis, palisade mesophyll and spongy mesophyll
The structure of a leaf
Plant leaves are adapted for photosynthesis, and the exchange of gases required for the process. The structure of the tissues is related to their functions in the plant.
Absorbing light energy
The palisade mesophyll layer of the leaf is adapted to absorb light efficiently. The cells:
- are packed with many chloroplasts
- are column-shaped and arranged closely together
- towards the upper surface of the leaf
Gas exchange
Spongy mesophyll tissue is packed loosely for efficient gas exchange. The spongy mesophyll cells are covered by a thin layer of water. Gases dissolve in this water as they move into and out of the cells.
When the plant is photosynthesising during the day, these features allow carbon dioxide to diffuse into the spongy mesophyll cells, and oxygen to diffuse out of them.
To enter the leaf, gases diffuse through small pores called stomata. As the stomata open, water is lost by the process of transpiration. Closing the stomata helps to control water loss.