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Energy
The human body depended upon external sources for energy.
Plants use solar radiation to make glucose and other nutrients.

The human body takes glucose and other nutrients directly or indirectly from plants.
The body receives oxygen from the air.

The energy that was once derived by plants from solar radiation is released within human cells by the process of metabolic oxidation. This involves the combination of glucose and other nutrients with oxygen, releasing the stored energy.

ATP
The mitochondria of the cells use the energy, released from nutrients, to form Adenosine triphosphate (ATP) molecules from Adenosine diphosphate (ADP) molecules.

ADP is converted to ATP by the addition of a "part of a molecule" called a phosphate radical.

The binding of this phosphate radical requires a large quantity of energy, which can be released later when the phosphate radical is separated off.

Adenosine triphosphate provides energy for cellular processes such as:

• active transport of substances across membranes
• synthesis of chemical compounds for the body
• mechanical work (such as muscle contraction)

When an ATP molecule provides energy for such a process, it loses a phosphate radical and becomes ADP. Then, the cycle begins again as ADP is converted into ATP within the mitochondria.

Certain cells, such as muscle cells and nerve cells, require great amounts of energy. Such cells have well-developed mitochondria.