The Earth is continuously bombarded with energy from the Sun and radiates most of that energy as heat back into space, making Earth's biosphere an open system in terms of energy. In contrast, the Earth's biosphere is primarily a closed system in terms of water, minerals, and nutrients. There are finite amounts of these substances, which are recycled through the living and geological components of the biosphere, called a biogeochemical cycle. Carbon, a key element of organic life on Earth, cycles through the biosphere in this way. A tree like all plants, algae, and cyanobacteria is an autotroph, an organism that produces its own food from inorganic molecules to facilitate growth. Through photosynthesis, photosynthetic autotrophes use energy from the Sun to combine carbon dioxide from the atmosphere and water from the soil to form carbohydrates such as glucose. Oxygen is released as a byproduct of photosynthesis. This is where we will start to track the passage of carbon through the carbon cycle. When sunlight strikes a leaf, the energy in the sunlight is captured and used to convert six molecules of carbon dioxide gas from the atmosphere into the six carbon sugar glucose, which can be used as a source of energy for cellular reactions. Glucose can then be stored as starch to be used later for energy, or used to make other molecules such as cellulose that form the cell wall of plant cells and contribute to the biomass of the plant. Cellulose is one of the substances aiding in the rigid nature of many plant structures, like the leaves, branches, stems, and trunk. When the log or other portion of a tree is burned, the stored glucose is converted by oxidation back to carbon dioxide gas, which is released into the atmosphere. In a similar way, the burning of fossil fuels, which contain carbon that was sequestered in prehistoric decomposing plant and animal tissues and buried for millions of years, releases carbon dioxide into the atmosphere. Animals are not autotrophes, but heterotrophes. Because heterotrophes depend on the carbon cycle to obtain energy and molecules necessary for growth, they need to consume other organisms. They do this by eating plants, or by eating other animals that eat plants. The glucose a deer requires from eating plants contains carbon that the plant converted from carbon dioxide gas during photosynthesis. The process of converting glucose to energy, a process called oxidative respiration, uses oxygen to break down glucose molecules in cells, ultimately producing ATP, a denocene trifosate, the energy of most cellular reactions. Carbon dioxide generated as a waste product of the metabolic oxidation of glucose is released into the atmosphere. Through the same process of oxidative respiration, plants also release carbon dioxide into the atmosphere. This occurs predominantly at night when they are not using carbon dioxide for photosynthesis. In addition to emitting carbon dioxide through oxidative respiration, animals release some carbon as solid waste. These solid waste products are broken down in the soil by decomposers, sap robes and detritivores. Organisms such as bacteria and fungi that degrade the waste to remove recyclable nutrients that they use for energy and growth. In the process of breaking down animal feces, decaying plants and dead animals, decomposers also release carbon dioxide gas into the atmosphere. Carbon cycles in a similar way through aquatic ecosystems. Carbon is present in water as dissolved carbon dioxide and by carbonate ions. Both are sources of carbon for photosynthesis and are taken up by algae and aquatic plants. Carbon dioxide is released into the water through oxidative respiration in these organisms and in other aquatic organisms such as fish and in decomposition. Carbon dioxide is also exchanged between the water and the atmosphere. The constant absorption of carbon dioxide from the atmosphere by photosynthetic organisms and its conversion to carbohydrates by photosynthesis is key to sequestering carbon in the biotic environment. The breakdown of glucose by respiration or burning converts the carbon back to carbon dioxide, releasing it back into the atmosphere. These two processes contribute to the carbon cycle. Because there is a finite amount of carbon in the Earth's biosphere, nutrient cycling such as this is essential to life on Earth. When viewed as a biogeochemical cycle, one can see how the carbon cycle is a balance of carbon stores. In the atmosphere, in the bodies of living organisms and buried in the ground as fossil fuels. Maintaining this cycle in the closed system of the Earth's biosphere allows organisms to live on Earth and participate as key players in the carbon cycle.