For hundreds of millions of years, the chemicals and elements found on Earth have remained relatively constant, or in other words, they have changed very little. The amount of one element or chemical in the Earth’s surface is practically the same as it was many millions of years ago. This consistency is one of the things that makes life on Earth possible.
This balance is maintained via complex interactions or cycles between the Earth and its organisms, or lifeforms. The elements of the Earth are taken into lifeforms, used to sustain the organism, and later released in the form of waste, or through decomposition upon the death of the organism.
Most of these cycles only take a few months or a few years to complete. However, some can take many millions of years.
The Energy Cycle
Each day as the Sun rises, our little blue world is showered with heat and light. This energy from the Sun is very important to all life on Earth. Without it, life could not exist.
While most of the heat and light that reaches the Earth is either reflected or radiated back into space, some of it is captured by plant life through the process of photosynthesis.
Photosynthesis takes place when plants use sunlight in order to produce sugars. These sugars can then be used by the plant as food in order to sustain the functions of life.
Animals and humans do not have the ability to produce their own food. Thus, they must obtain their energy by eating plants, or by eating other animals that have eaten plants.
At some point as plants or animals die, or release waste into the environment, the heat from the Sun is released back into the environment.
The Hydrologic (Water) Cycle
Water is found throughout the biosphere, and is probably the most important substance needed to sustain lifeforms. Humans can survive for many weeks without the energy obtained by eating food, however, we would only last a few days without water.
Water is used to carry out the many important and complex chemical reactions that all lifeforms must perform in order to survive. Water carries nutrients to various parts of a lifeform and carries waste away from the different parts of a lifeform. Because water is so important, the most abundant substance in any organism is water.
The water found within your body as you read this article has been used and re-used by organisms throughout the history of Earth. It is quite possible that there is water found in your body that was once inside of a Tyrannosaurus Rex, or inside of Julius Caesar.
The process of water moving through the environment is referred to as the hydrologic or water cycle.
The Carbon Cycle
Carbon is an important element to living things. As we learned earlier, the most abundant substance in organisms is water. The second most abundant substance is carbon. Much of the solid portions of lifeforms is made up of great amounts of carbon.
How do living things obtain carbon?
Carbon is extracted from the atmosphere by plants through the process known as photosynthesis. This carbon is combined with other elements in complex ways to form organic molecules important to life.
This carbon is later transferred to animals who consume, or eat plants. When plants and animals die, much of their carbon is returned to the atmosphere as the organisms decompose.
Every so often, a plant or animal does not decompose right away. Their bodies are trapped in locations where decomposition can simply not take place. This is most common at the bottom of oceans and seas where the lifeforms become buried by sand.
Instead of returning to the atmosphere, the carbon from these lifeforms is trapped within the Earth. Over millions of years, more and more of the carbon on Earth has been trapped in this manner. Today, almost 99% of all the carbon on Earth has been locked up deep within the Earth.
As rocks weather, this carbon is slowly released back into the atmosphere, creating a balance. For the past several hundred million years, the amount of carbon being locked up in the Earth and the amount being released by weathering rocks was almost perfectly balanced.
This important balance has been altered significantly in the past century as humans have begun using fossil fuels to produce energy. By burning the Earth’s store of carbon, mankind is able to create the energy needed to operate our communities. However, we must be careful as we do so. By releasing more carbon into the atmosphere than is being locked up, we risk causing damage to the delicate carbon cycle.
The Oxygen Cycle
Oxygen exists in our modern atmosphere in great amounts. Approximately 21% of the atmosphere is comprised of oxygen. This was not always the case, however.
Billions of years ago, it is believed that there was virtually no oxygen found in the atmosphere at all. What caused an increase in oxygen levels?
Most of the oxygen now found in our atmosphere was released by plants as a bi-product of photosynthesis. Over millions of years, as plants around the globe released oxygen, the levels continued to rise, until they reached a balance around 1 billion years ago.
For the last billion years, the amount of oxygen has remained relatively constant. At the same time that plants continue adding oxygen to the atmosphere, it is also being removed by various processes.
Oxygen is highly reactive. As the oxygen in our atmosphere interacts with other substances, it often bonds to them, becoming trapped. Many lifeforms also remove oxygen from the atmosphere as they breathe. This oxygen is used by these lifeforms to carry out the functions of life.
The Nitrogen Cycle
Nitrogen is the most abundant element in our planet’s atmosphere. Approximately 78% of the atmosphere is comprised of this important element.
Nitrogen is used by lifeforms to carry out many of the functions of life. This element is especially important to plant life. Yet, nitrogen in its gaseous form is almost entirely unusable to lifeforms. It must first be converted or ‘fixed’ into a more usable form. The process of converting nitrogen is called fixation.
There are specialized bacteria whose function it is to fix nitrogen, converting it, so that it can be used by plants. There are still other bacteria who do the reverse. That is, they return nitrogen to is gaseous form.
After nitrogen is fixed, it can be absorbed and used by plants, and subsequently by animals.
The process of nitrogen being fixed, used by plants and animals, and later returned to the atmosphere is referred to as the nitrogen cycle.
