Leaf Biology: Photosynthesis, Structure, and Seasonal Cycles

The Leaf's Primary Function

Despite variations in appearance, all leaves manufacture sugars and carbohydrates through photosynthesis: the interaction of light energy with chlorophyll converts carbon dioxide and water into sugars and oxygen.

How Photosynthesis Works

Water drawn from roots meets carbon dioxide entering through stomata (minute pores primarily on leaf undersides). Stomata also release oxygen and excess water vapor (transpiration). They can close to prevent dehydration, and the epidermis (outer coat) may be hairy, thick, or waxy for additional protection.

Seasonal Patterns

Photosynthesis halts in deciduous plants during leafless dormancy and slows in evergreens during cold-weather dormancy. Thus, plants need far less water from late fall to early spring, making this the ideal time for transplanting.

Threats to Photosynthesis

Damage from pruning or insects reduces leaf area for nutrient production. Soot, grime, and dust block stomata and reduce sunlight. A smothering leaf mat on lawns halts chlorophyll production, yellowing grass.

Photosynthesis Summary

• Sunlight acts on chlorophyll to convert water and CO₂ to sugars and oxygen.
• Leaves absorb CO₂ and release oxygen.
• Vascular system transports water to leaves and circulates sugars throughout the plant.