What Is the Primary Function of Plant Leaves?

Photosynthesis occurs in the leaves

Photosynthesis is the primary function of plant leaves, though it is not the only important one. The entire food web, which can be thought of as a transfer of carbon, starts with plants photosynthesizing and creating carbohydrates from sunlight, carbon dioxide and water. The entire carbon cycle begins with the formation of carbohydrates in the leaf. Also, the oxygen we breathe is a byproduct of photosynthesis, escaping into the atmosphere through pores on the leaf.

Photosynthesis

Through a series of biochemical reactions, plants use the energy from sunlight to split water molecules into hydrogen and oxygen atoms. The hydrogen atoms are the bound to atmospheric carbon dioxide, forming basic carbohydrates. The oxygen atoms split from water and enter the atmosphere, supporting life. Photosynthesis typically increases during warmer months because there is more sunlight energy available and there is not a chance of leaf structures, which contain a large amount of water, to freeze.

Herbivory

Plants are autotrophic organisms, meaning they can generate their own food. Animals, on the other had, are heterotrophic and have to actively seek and consume food for energy. However, leaves contain more than just basic carbohydrates. Plants take in nutrients from the soil, such as nitrogen and phosphorus, which are used to make amino acids and proteins. When an animal consumes a plant leaf, carbon, nitrogen, phosphorus and other elements are transferred to the animal. In this way, plants serve a key link in the food chain.

Soil Organic Matter

Many plants and trees lose their leaves as the weather begins to turn colder. Some plants complete their life cycle in one year and other plants lose all of their above-ground biomass each year; these plants contribute their stems and leaves to the pool of soil organic matter. This organic matter is consumed by microbial communities in the soil, much like larger animals consume the live leaves. As the soil organic matter is broken down and consumed by the microbes, other elements that were bound in the leaf are also released, such as nitrogen and phosphorus. These nutrients can be taken by microbes, washed away from the soil or reabsorbed by other live plants.

Carbon Dioxide

The atmospheric concentration of carbon dioxide is primarily controlled by photosynthesis, which removes carbon dioxide from the atmosphere, and respiration, which releases it back into the atmosphere. Respiration does not only refer to breathing in animals, it also refers to the microbial degradation of organic matter in the soil, of which carbon dioxide is a byproduct. Carbon dioxide enters the leaf through small pores on the leaf surface called stomata. Photosynthesis is strongly dependent upon temperature and season; thus atmospheric concentrations of carbon dioxide have been found to fluctuate in response to the changing demand of photosynthesis in the leaves.

Carbon Cycle

The cycling of carbon through plants, animals, soil and the atmosphere starts with photosynthesis in the plant leaf. Carbon is pulled from the atmosphere and incorporated into the plant biomass, where it becomes available to plants through herbivory and to soil microbial communities through decomposition. Through respiration, the carbon from the animals and microbes is then returned to the atmosphere where it becomes available once again to be incorporated into photosynthesis, starting the cycle all over again.

References

• The Woods Hole Research Center; Understanding the Global Carbon Cycle
• Estrella Mountain Community College: Photosynthesis