Plants are species in Kingdom Plantae. Although there are a wide variety of plant species -- over 250,000 in all -- they all share certain fundamental characteristics. Among the most important of these is photosynthesis.
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Animals cannot produce their own food, so all terrestrial food chains are based on plants. Plants convert solar to chemical energy in organic compounds like sugars; animals exploit this stored energy when they eat plants to find the fuel they need for their own growth. Without plants, there would be no food for any land animals, including humans.
Plants use carbon dioxide as an ingredient for photosynthesis and release oxygen as a byproduct. The oxygen you breathe comes from plants and other photosynthetic organisms. Plants also reduce the concentration of carbon dioxide in Earth's atmosphere. Since carbon dioxide is a greenhouse gas, this activity has important implications for our climate.
Much of the fuel we use for energy is essentially ancient solar energy converted to chemical energy by plants. Coal, for example, formed from the remains of plant matter compressed by sediment over many millions of years. Modern industrial society depends heavily on fossil fuels like coal.