The primary function of an elodea leaf is to make food for the plant using photosynthesis. Elodea is a water plant, and those same leaves provide a home for tiny invertebrate life forms. When the plant dies, the leaves decompose, providing food for these same creatures.
The dark green leaves are filled with chlorophyll, a green substance that is used in photosynthesis. The plant floats just below the surface, giving it access to plenty of sunshine. Unlike land plants that pull carbon dioxide from the air, elodea pulls it from the water. Elodea does anchor to the pond bottom so it does pull water and nutrients through its root system but also absorbs some through the leaves.
During photosynthesis, the chlorophyll takes the energy from the sun and converts the carbon dioxide and water to sugars and oxygen. The sugars are used for plant food, while the oxygen is released into the pond.
Since elodea survives year-round, it provides underwater habit for aquatic insects and fish year-round. The plants sometimes break apart and float independently because of the dense leaf structure. Under the right conditions, stem fragments take root on the pond bottom and produces new plants. The downside of this is plant overgrowth which must be controlled chemically or manually.