Chapter+3

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 than be used by the plant as food, in order to sustain the functions of life. Animals do not have the ability to produce our own food. Thus we must obtain our 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 Water Cycle

Water is found throughout the biosphere, and is probably the most important substance needed to sustain life forms. 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 life forms must perform in order to survive. Water carries nutrients to various parts of a life form, and carries waste away from the different parts of a life form. Because water is so important, the most abundant substance in any organism is water.



The water found within your body as you read this 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 water or //**hydrologic**// cycle.

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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 life forms 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 life forms become buried by sand. Instead of returning to the atmosphere, the carbon from these life forms 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.

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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 life forms also remove oxygen from the atmosphere, as they breath. This oxygen is used by these life forms 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 life forms 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 life forms. 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.