Energy and Biogeochemical Cycles
Energy is crucial to the survival of living organisms, but they also need water, minerals and other
compounds. 95 % of the body of a living organism is made up of the following four elements: carbon,
hydrogen, oxygen, and nitrogen.
Energy moves through an ecosystem in a one-way path. Energy enters an ecosystem in the form of
sunlight and exits the ecosystem in the form of heat. This energy cannot be recycled.
Matter, however, is recycled within and between ecosystems. Elements and compounds are recycled in
biogeochemical cycles.
A biogeochemical cycle is a process in which elements, chemical compounds, and other forms of matter
are passed from one organism to another and from one part of the biosphere to another.
The four processes that account for the movement of water molecules through an ecosystem:
1. Evaporation
2. Precipitation
3. Transpiration
4. Condensation
In the process of evaporation, water molecules are added to the atmosphere in the form of water vapor.
Heat causes water molecules to evaporate from bodies of water and from the bodies of living
organisms.
Transpiration is the loss of water from the leaves of plants.
The path that water travels as it enters a plant to the point that it leaves a plant is as follows:
1. Water enters plants through the roots, and travels to the leaves of the plant.
2. The leaves contain the pores called stomata that open to take in the carbon dioxide that is needed
for photosynthesis.
3. When the stomata are open, molecules if water evaporate from the leaf and enter the atmosphere.
Condensation is the phase change that occurs when water vapor is converted to a liquid.
The water vapor in the atmosphere condenses to form the clouds.
Precipitation (rainfall) returns water to the earth from the clouds.
It is important that carbon is recycled in the ecosystem because all of the carbohydrates, lipids, proteins,
and nucleic acids that compose all living things contain atoms of carbon.
The role of photosynthesis in the carbon cycle is that it removes carbon from the atmosphere in the
form of carbon dioxide. The carbon is used to build molecules of glucose, and other organic
compounds.
All living organisms convert glucose to ATP, giving off carbon dioxide in the process of cellular
respiration. The role of respiration in the carbon cycle is that it releases carbon dioxide back into the
atmosphere.
Other processes that release carbon dioxide into the atmosphere include geochemical processes such as
erosion and volcanic activity. These processes release carbon dioxide to the atmosphere and oceans.
Decomposition releases carbon dioxide. Human activities, such as mining, cutting and burning forests,
and burning fossil fuels, release carbon dioxide into the atmosphere.
Much carbon is contained in the atmosphere in the form of carbon dioxide. The ocean also serves as a
storage area for carbon. Many marine organisms combine carbon, calcium and oxygen to form calcium
carbonate. This calcium carbonate accumulates in marine sediments and in the bones and shells of
many marine organisms.
The dramatic increase in carbon dioxide levels in the atmosphere over the last forty years include the
burning of fossil fuels and deforestation.
The greenhouse effect is when carbon dioxide, water vapor and other gases trap the heat from the sun
in our atmosphere. This warms the Earth and insulates it from the deep cold of space. Life on Earth
depends on the “greenhouse effect.”
The increased levels of carbon dioxide increase the greenhouse effect. Today, too much heat is being
trapped by the atmosphere and this has led to the current period of global warming.
All organisms must have nitrogen in order to build proteins and nucleic acids.
Nitrogen gas makes up about 78% of the atmosphere. It would appear that nitrogen is readily available
for use in manufacturing proteins and amino acids. However, most organisms do not have the ability to
make use of atmospheric nitrogen. Living organisms depend upon soil dwelling, nitrogen fixing bacteria.
Nitrogen fixation is the process of converting nitrogen gas to nitrates.
A special group of soil-dwelling bacteria, known as nitrogen-fixing bacteria, are able to transform
atmospheric nitrogen into a form that other living cells can use. These bacteria take nitrogen from the
air and convert it into nitrates (NO3-)
The nitrates are absorbed by the roots of plants. Plants use nitrates to build proteins and nucleic acids.
The nitrogen is then passed up the food chain.
The five processes that are essential to the nitrogen cycle:
1. Nitrogen fixation
2. Ammonification
3. Denitrification
4. Assimilation
Nitrogen fixation is the process of converting atmospheric nitrogen into ammonia and nitrates.
Ammonification is the production of ammonia by bacteria during the decay of nitrogen containing
organic matter. Many animals excrete and eliminate nitrogen in urine and in feces. Soil bacteria
convert these wastes into ammonia (NH3). In addition, these bacteria convert the nitrogen compounds
in dead plants and animals to ammonia.
Nitrification is the production of nitrates from ammonia. Some of the ammonia in the soil is converted
by several kinds of bacteria to nitrates, NO3 -. These nitrates are absorbed from the soil by plants.
Denitrification is the conversion of nitrate to nitrogen gas. The nitrogen gas is released into the
atmosphere.
Ammonia and nitrates are picked up by plants. Plants use ammonia and nitrates to build proteins and
nucleic acids. When animals eat the plants, they use the nitrogen to build their own proteins and
nucleic acids. Nitrogen assimilation is the absorption and incorporation of nitrogen into plant and
animal compounds.
The nitrogen-fixing bacteria are able to take atmospheric nitrogen and convert it to ammonia.
Ammonifying bacteria are decomposers. They break down dead, organic matter and convert it to
ammonia.
Nitrifying bacteria convert ammonia to nitrates.
Denitrifying bacteria consume nitrates and release elemental nitrogen back into the atmosphere.
All living things must have a constant supply of phosphorous because it essential in all living organisms
because it is needed to build molecules of ATP and the nucleotides that compose of DNA and RNA.
Phosphorous is the key element in cell membranes and calcium phosphate is found in bones.
Phosphates are usually present in rocks and soil as calcium phosphate. Calcium phosphate dissolves in
water to form inorganic phosphate ions. As phosphates are released from soil and rocks, it washes into
streams and rivers, eventually making its way to the oceans where it is used by marine organisms.
A limiting nutrient is a single nutrient (such as nitrogen or phosphorous) that either is scarce or cycles
very slowly, limiting the growth of the organisms in the ecosystem.