Cycling of Matter in Food Chains

The elements most commonly found in living things include carbon, oxygen, hydrogen, nitrogen, phosphorus, potassium, calcium, and sulfur. Atoms of these elements can be combined to produce a few different types of compounds that are found in nearly all living organisms. These compounds are called biomolecules, and they can be classified into four main types: carbohydrates, fats, proteins, and nucleic acids.

All forms of life on Earth are carbon-based. Each of the four types of biomolecules is made up of carbon bonded with atoms of other elements. All forms of life also require water to survive. Water is important both as a solvent for many of the body's solutes and as a key component in many metabolic processes. Water is the single most abundant compound in all living organisms. Around two-thirds of the weight of a cell is accounted for by water. Nitrogen and oxygen are also essential for life and found in significant quantities in the biomolecules that make up living organisms. Nitrogen is found in proteins and nucleic acids, such as DNA. Oxygen is a component of all four types of biomolecules.

The primary source of the elements used to form biomolecules is food. Food is the way that matter is transferred from one organism to another. Food web diagrams can be used to show the feeding relationships in an ecosystem

Food Web Diagrams

Food webs are commonly used to show the flow of energy through an ecosystem. However, they are useful tools for following the flow of matter. In food web diagrams, the arrows indicate the direction the energy and organic matter flow within an ecosystem.

In one of the feeding relationships shown in this food web, organic matter from the grass—in the form of the sugars, proteins, and water that make up the grass's leaves—becomes incorporated into the grasshopper, which is transferred to the frog and finally to the copperhead. When the copperhead dies, its matter will be broken down by decomposers into smaller molecules which can be released into the atmosphere or become part of the soil. Ultimately, the atoms in the released molecules will be used by other organisms to build body structures.

Flow of Matter through Ecosystems

Just as producers are the vectors by which energy enters an ecosystem, they also begin the cycle of matter. At the beginning of the cycle, producers perform photosynthesis by using sunlight to convert carbon dioxide gas from the atmosphere and water and nutrients absorbed from the soil into carbohydrates (sugars) and other types of biomolecules. The producers then incorporate these biomolecules into structures such as leaves, stems, and roots.

Consumers eat producers or other consumers and transform the matter they obtain into building materials for their own bodies. The matter needed for a consumer to grow larger comes from the organisms they consume.

Decomposers break down animal wastes and the remains of dead plants and animals. Through this process, matter from these materials is converted back to water and nutrients which are in the form of elements and small simple molecules. These nutrients are then returned to the soil and the atmosphere, where producers can use them to begin a new matter cycle.

Matter is Conserved in Closed Systems

Energy, in the form of sunlight, is constantly entering the Earth's atmosphere. This energy is converted by producers into chemical energy. As this energy moves from the producers through the food web, it changes forms many times, and some of the energy is lost to the atmosphere as heat at each step. Eventually, excess heat is radiated by the atmosphere out into space. Therefore, Earth is not a closed system in terms of energy; energy enters and exits the Earth "system." However, the Earth is essentially a closed system in terms of matter. With the exception of the occasional addition of matter by asteroids and meteoroids that enter the atmosphere and fall to Earth, and a small number of atmospheric molecules that drift off into space, no matter enters or exits the Earth system. The atoms of carbon, nitrogen, oxygen, and other elements on Earth today have been in the Earth system since the planet formed.

Ecosystems are not generally closed systems in terms of matter. Organisms in an ecosystem give off gases, water, and other wastes that may find their way into the atmosphere or other ecosystems. Likewise, when producers take in carbon dioxide and produce sugars with them, they are taking new matter into the ecosystem from the surroundings.

However, matter is conserved as it moves from one step in a food chain or web to another. For example, if a bird swallows a grasshopper, all of the matter that made up the grasshopper's body is inside the bird's body. Some of the matter will be used to make new building and energy-producing materials for the bird's body. The rest of the matter will be converted into waste materials that are eliminated from the bird's body and deposited back into the environment as urine, feces, and waste gases.