Energy flows directionally through ecosystems, entering as sunlight (or inorgani
Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for chemoautotrophs) and leaving as heat during energy transformation between trophic levels. Rather than flowing through an ecosystem, the matter that makes up organisms is conserved and recycled. The six most common elements associated with organic molecules—carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur—take various chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath Earth’s surface. Geologic processes, such as weathering, erosion, water drainage, and the subduction of continental plates, all play a role in the cycling of elements on Earth. Because geology and chemistry have major roles in studying these processes, recycling inorganic matter between living organisms and their nonliving environment is called biogeochemical cycles.
Organisms use the six aforementioned elements in a variety of ways. Hydrogen and oxygen are found in water and organic molecules, both essential to life. Carbon is found in all organic molecules, whereas nitrogen is an important component of nucleic acids and proteins. Phosphorus is used to make nucleic acids and the phospholipids that comprise biological membranes. Lastly, sulfur is critical to the three-dimensional shape of proteins.
The cycling of these elements is interconnected. For example, water movement is critical for leaching sulfur and phosphorus into rivers, lakes, and oceans. Minerals cycle through the biosphere between the biotic and abiotic components and from one organism to another.
The Water Cycle
The hydrosphere is the area of Earth where water movement and storage occurs: as liquid water on the surface (rivers, lakes, oceans) and beneath the surface (groundwater) or ice (polar ice caps and glaciers), and as water vapor in the atmosphere. The human body is about 60 percent water, and human cells are more than 70 percent water. Of the stores of water on Earth, 97.5 percent is salt water (see Figure 1 below). Of the remaining water, more than 99 percent is groundwater or ice. Thus, less than one percent of freshwater is present in lakes and rivers. Many organisms depend on this small percentage, a lack of which can negatively affect ecosystems. Humans have developed technologies to increase water availability, such as digging wells to harvest groundwater, storing rainwater, and using desalination to obtain drinkable water from the ocean. Although this pursuit of drinkable water has been ongoing throughout human history, freshwater supply remains a major issue. The various processes that occur during the cycling of water are illustrated in Figure 2 below.
The processes include the following:
- evaporation and sublimation
- condensation and precipitation
- subsurface water flow
- surface runoff and snowmelt
- streamflow
The Sun’s energy drives the water cycle as it warms the oceans and other surface waters. This leads to evaporation (liquid water to water vap