Levels of Organization Image Map
Before we can answer why energy is important, we have to know what energy is.  Any ideas?

 Energy is defined as a  force that can do work.  Energy can exist in two main forms:

  • Potential energy is a stored form of energy. Chemical bonds are a form of potential energy.
  • Kinetic energy is a form of energy that is being released, and some of that energy is lost in the form of heat


Energy can exist in several forms, ranging from a stored form (potential energy) to a form that does something (kinetic energy) to a form that is wasted as heat. Molecules are held together by an energy force. Breaking the bonds between atoms releases that energy. Whether or not that bond energy does anything depends on whether it can be captured and "put to work."  The body's job is to break food down so that cells can break down some of the chemical bonds. Then that energy can be used to do things (see below), before any left- over energy is finally lost as heat.

Some released energy escapes as heat.

Released energy is like steam rising over a stove pot. It disappears. If energy is released all at once, much of it will escape as heat.  

But even heat energy has its uses. Ever notice how frogs or fish are sluggish on a cold day, but people can be just as active as always. People make use of their "escaped" heat energy to make chemical reactions run better and to make muscles contract faster and stronger, even on cold days. Cold-blooded animals can't do that. 

Tea Kettle Image

Why do living systems need energy?

  • to start and control chemical reactions
  • to transport chemicals inside and outside of cells
  • to make proteins change shape and, as a result, change their function. 

This kinetic energy has to be delivered in small and controlled steps, so that things happen in an orderly way. 

Energy containing nutrients are transported into cells, where the nutrients are converted into an energy form readily used by the cell.
Cell Image

Energy-bearing nutrients have to be brought to the cells by blood and other tissue fluids. These nutrients then pass into cells.  Glucose, a 6-carbon sugar molecule, is the common breakdown product of table sugar, flour, potatoes, and  other starches. Even proteins and fats can get broken down into glucoses. Once inside the cell, glucose can be picked up by mitochondria where the energy in glucose is released and trapped in a form that the cells can use. This process is called cellular respiration.

One More Thing!  Recent research has revealed that mitochondria control death of cells. When cells die, it seems that the cause arises because the membranes of mitochondria loose their voltage charge, and this in turn causes a release of certain proteins from mitochondria into the cytoplasm of the cell. These proteins trigger a series of chemical reactions that kill the cell.

Self Quiz

The organelle that breaks down glucose to release and trap its energy is:

A.  Endoplasmic reticulum
B.  Golgi apparatus
C. Mitochondria
D. Cell membrane


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