The intricate processes by which organisms harness, consume, digest, repackage, and use energy are essential to life. Yet, without the Sun, the origin of all of this energy, life would not exist.

Photosynthesis is the process by which plants (and a few types of single-celled organisms) use energy from the Sun to transform carbon dioxide and water into carbohydrates. Although only a small portion of solar energy that falls on Earth is stored in the chemical bonds of plant compounds, photosynthesis sustains—either directly or indirectly—nearly all life forms.

While sunlight provides the energy needed to drive photosynthesis, a molecule called chlorophyll in the leaves of plants makes the reaction possible. This green pigment is found in specialized structures called chloroplasts inside a plant's photosynthetic cells. Chloroplasts serve a vital function in the conversion of light energy into chemical energy. They have much in common with mitochondria, the so-called powerhouses of the cell.

Mitochondria are the cell structures that convert energy that has been stored as carbohydrates into a molecule called adenosine triphospate (ATP), a form of energy the cell can readily use. Plant and animal cells both rely on mitochondria to release the energy needed to support vital functions. However, much of the energy plants harness remains in their tissues. Animals have evolved to take advantage of the energy stored in plant tissue. The majority of animals, including humans, rely on the carbohydrates from plant roots, stems, leaves, seeds, and fruits as their primary source of energy.

To access the energy contained in plant or animal tissue, humans must first break down their food, both physically and chemically, into glucose and other molecules more easily absorbed by the blood. This process takes place in the stomach and the intestines. From there, glucose moves into the bloodstream and is carried to the liver and muscles, where it is stored.

Glucose is the raw material that drives cellular respiration. In a process that has more than five dozen steps, a single glucose molecule is converted into 38 molecules of ATP. Each of these molecules can be used, or "spent," easily and efficiently inside the cells to drive even the smallest chemical reactions. It is this efficiency, the ability to distribute units of energy as needed, that drives the cellular respiration process.


Source: Teachers' Domain, Where Do You Get Your Energy?, published August 9, 2007, retrieved on November 4, 2010,http://www.teachersdomain.org/resource/lsps07.sci.life.stru.cellenergy/