Understanding Eco-Chemical Cycles - AP Biology

Energy, in an ecological context, refers to its flow in an ecosystem. Energy is passed from trophic level to trophic level; however, each time this occurs about ten percent of this energy is lost to the environment or as digested food. Energy is thus not recycled in this context.

In contrast, water, carbon, oxygen, nitrogen, and hydrogen are passed from one organism to another as organic material. This material is digested and incorporated into the organism's own biology, or released back into the ecosystem for continued use by other organisms in the environment. Energy can be passed from one organism to another in the form of chemical bonds and is released in the form of heat, but cannot be used by other organisms once it is returned to the environment and does not follow an ecological cycle.

In Earth’s atmosphere, carbon can be found in two forms: carbon dioxide and methane. Both are responsible for the greenhouse effect as they absorb and retain heat.

Carbon leaves the atmosphere and enters the terrestrial biosphere through photosynthesis and dissolution. Thus, carbon enters bodies of water and plants, where it is incorporated into organisms as organic molecules. Then the different trophic levels will consume these organic molecules. Upon their death, the carbon will be recycled within the terrestrial biosphere.

Carbon leaves the atmospheric biosphere and enters the terrestrial biosphere through photosynthesis and dissolution into bodies of water. Recall that carbon dioxide is a reactant of photosynthesis, and it gets converted into glucose.

When carbon enters into bodies of water through precipitation, it reacts with water to form carbonic acid. Carbonic acid then is absorbed into rocks in the sedimentary biosphere. Carbonic acid is very soluble in water, and is the form that most of carbon is in when dissolved in water.

The burning of fossil fuels has increased the amount of carbon in the atmosphere, contributing to the greenhouse effect and global warming. Burning fossil fuels mainly involves combustion reactions in which the fuel is oxidized, producing carbon dioxide and water. While the other answer choices may add to the carbon dioxide levels of the atmosphere, the question asks for a direct cause.

In the terrestrial biosphere, carbon can be found in all living and dead organisms, stored in the soil, and in inorganic forms. Inorganic carbon refers to carbon in minerals and ores. It is often said that life on earth is carbon-based due to its versatility and ubiquity.

Plants and animals receive carbon through photosynthesis and consumption, respectively.

Carbon that is contained in the terrestrial biosphere leaves and returns to the atmospheric biosphere through animal, plant, and soil respiration. These terms refer to the process by which food molecules (ie. glucose) are burned and converted to more usable forms of energy. The byproducts are often carbon dioxide, water, and heat. In animal respiration, carbon dioxide is produced during the transition step between glycolysis and Krebs cycle, and twice in the Krebs cycle.

Carbon from terrestrial organisms can enter into the oceanic biosphere and bodies of water through dead tissue, dissolution, or in the form of shells as calcium carbonate.

Industrialization has increased the burning of fossil fuels, such as coal and oil. This releases carbon from the geosphere into the atmosphere, increasing the carbon in the atmosphere. Carbon dioxide and methane are big contributors to the greenhouse effect and global warming.

Modern human activities greatly impact the carbon cycle. Increase in land usage decreases the presence of natural ecosystems and the ability of organisms to remove carbon from the atmosphere. Pollution damages organisms, which reenters the terrestrial biosphere and increases erosion. Deforestation directly removes carbon rom ecosystems and decreases the amount of carbon absorbed from the atmospheric biosphere.

The carbon cycle includes three levels: the geosphere, the terrestrial biosphere, and the atmospheric biosphere. The geosphere refers to the solid part of the earth, the terrestrial biosphere refers to all the living organisms on the land, and the atmospheric biosphere refers to the living organisms in space between the land and the outer ends of the atmosphere. Carbon is present in all levels and travels throughout the levels through various processes including photosynthesis, dissolution, and decay.

Nitrogen makes up most of the Earth’s atmosphere; however, this reserve of nitrogen is limited for biological use. Nitrogen exists in diatomic forms in the atmosphere and the process of nitrogen fixation frees atoms for biological use. As a result, nitrogen is a limiting resource in many ecosystems.

Nitrogen is necessary for all life; therefore, it is present in the territorial biosphere in living organisms, humus, and decomposing matter.

Nitrogen fixation can be described as the process in the nitrogen cycle in which bacteria “fix” atmospheric nitrogen into ammonia and usable forms of organic nitrogen.

In the process of assimilation, organic nitrogen is incorporated into plants through the roots. The type of nitrogen incorporated may be nitrate ions, nitrite ions, or ammonium ions. Symbiotic bacteria may assist this process.

The processes of denitrification and nitrogen fixation both involve atmospheric nitrogen. Nitrogen fixation is the conversion of atmospheric nitrogen into ammonia and organic nitrogen by nitrogen fixing bacteria. In denitrification, denitrifying bacteria convert nitrates into nitrogen gas, which is released into the atmosphere.

Denitrification is the final process of the nitrogen cycle in which soil-dwelling denitrifying bacteria convert nitrates back into nitrogen gas. The nitrogen gas is released back into the atmosphere.