Leaves act as the powerhouses of plants. Within each cell, structures called chloroplasts busily manufacture sugar from sunlight, supplying the plant with energy. Different types of leaf cells have different structures, depending on their function.
Epidermis cells cover and protect the leaf. Guard cells control the flow of air in and out of the leaf, while mesophyll cells undertake the bulk of photosynthesis.
Epidermal cells contain few chloroplasts, the organelles responsible for photosynthesis. The transparency of epidermal cells allows light to pass through and into the mesophyll.
Two types of cells fill the mesophyll, the middle layer of the leaf that lies between the upper and lower epidermises. Palisade cells are elongated and oriented perpendicularly to the leaf's surface. Each palisade cell can contain dozens of chloroplasts to perform most of the leaf's photosynthesis. Cells of the spongy layer have irregular shapes with large air spaces in between. Although they also contain chloroplasts, they contain fewer than the palisade cells and they primarily provide storage for sugars and gases.
Two guard cells flank each pore opening into the spongy layer. The inner cell wall of a guard cell is thicker than the outer cell wall. When water fills the cell, this tough inner layer stretches into a crescent shape that opens the pore and allows gases in and out of the leaf.