MAVO Year 4 · Plants · Section 9.1

Leaves

Leaf structure

A leaf contains epidermal tissue, photosynthetic leaf tissue and vascular bundles. The epidermis is a thin outer layer of closely packed cells, usually covered by a waxy cuticle. Together they help protect against water loss and pathogens.

Stomata in the epidermis allow gas exchange. Much of the internal leaf tissue contains cells with chloroplasts, where photosynthesis takes place. Vascular bundles carry water and dissolved substances and branch through the leaf as veins.

Photosynthesis

During photosynthesis, light energy is used to produce glucose and oxygen from carbon dioxide and water. It requires chlorophyll-containing structures, suitable light and appropriate conditions, including temperature.

Carbon dioxide + water + light energy → glucose + oxygen

Water arrives from the roots through the xylem. Carbon dioxide enters mainly through stomata, and oxygen can leave through them. Glucose is used as a fuel or as a raw material for other substances.

Support from water

Water entering a plant cell's vacuole produces turgor pressure against the cell wall. This helps keep cells and soft plant tissues firm. When too much water is lost, turgor falls and leaves or stems may wilt. If the cells have not been damaged, taking up water can restore firmness.

How stomata work

Each stoma is bordered by two guard cells. When they take up water and become turgid, their shape changes so the pore opens. When they lose water, the pore narrows or closes. This helps balance carbon dioxide uptake against water loss. Many plants close their stomata at night or during drought. Closing restricts carbon dioxide entry, reducing photosynthesis; it does not stop every cellular process.