The flow of energy from one form of life to the next begins with plants. Through the process of photosynthesis, plants harness the Sun's energy and use it to transform carbon dioxide and water into the food molecules that sustain them. The energy stored in these molecules nourishes not only the plants that produce them, but also nearly every organism on Earth. Wolves, toucans, and fungi all depend on plants to play the role of primary producers. In an interesting evolutionary twist, several groups of plants have turned the tables on the animal world, and play both producer and consumer in their unique environments.

For plants to be productive, they require access to sufficient light, water, and nutrients. Light and water make photosynthesis possible. Nutrients such as nitrogen, phosphorous, and potassium are essential to a plant's ability to build many important molecules, including proteins, DNA, and the pigment chlorophyll that enables plants to absorb light energy. Although most plants obtain the nutrients they need from the soil, not all nutrients are readily available in every environment. For example, nitrogen is often a limiting factor in the productivity of plants. Without a sufficient quantity of usable nitrogen in the soil, plants typically fail to thrive and/or reproduce.

Despite the fact that nitrogen is one of the most plentiful elements on Earth—making up approximately 78 percent of our atmosphere—much of it is unusable to plants. Before plants can use nitrogen, it must first be "fixed," or converted, into compounds such as nitrate or ammonium. This conversion is most often carried out by specialized bacteria that live in the soil or in the roots of some plants, such as legumes.

Some soil types are particularly poor in nitrogen because they lack sufficient numbers of nitrogen-fixing bacteria. Wet soils, such as those found in bogs, are typically too acidic for bacteria to live in, so there is nothing to break organic molecules down into usable forms of nitrogen and other compounds. As a result, some of the plants that grow in these areas have evolved modified leaves and other specialized structures that enable them to obtain nitrogen from animals by capturing and digesting them.

Carnivorous plants use a variety of leaf modifications to capture and digest their prey. For example, pitcher plants capture and digest insects inside vessel-shaped leaves that contain rainwater and digestive enzymes; sundews trap insects on leaf hairs that are tipped with sticky secretions; and Venus flytraps, perhaps the best known of all carnivorous plants, capture insects with hinged leaves that close rapidly when an insect touches trigger hairs on the leaf surface. With such modifications, these plants have effectively solved the problem of nitrogen-poor soils, and can function as both producers and consumers.

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