Living things are capable of searching out, capturing, storing, converting and using energy. Energy is needed to maintain the high levels of organization found in cells and multicellular organisms.
Vital chemical reactions that build macromolecules, construct organelles, and transport material across membranes are not possible without an input of energy from some external source. Without a steady supply of energy, all life would come rapidly to a halt.
Energy is defined as the capactiy to do work or produce heat.
Work is directed motion brought about when a force is applied to an object to move it through a distance.
Heat is rapid, disorganized or random motion.
Broadly speaking there are two types of energy: potential energy or stored energy, such as that found in a tank of gasoline or a bowl of cereal, and kinetic energy, the energy released when an object is set in motion.
All living things are capable of converting potential energy into kinetic energy and doing work. The movement of muscle, for example, is possibly only if potential energy (in the form of "food") is converted into the kinetic energy of muscle cell shape change and contration.
Life is also capable of a second type of energy conversion, in which energy is taken from its environment (either as food or sunlight) and then used to construct molecules, macromolecules, cells and multicellular organisms.
As far as we know there is no other phenomenon on earth (or elsewhere) capable of using energy in this way - only life. Everywhere else in the universe large, complex structures (such as stars) are breaking down and going to simpler forms of matter and lower forms of organized energy.
Only life and living things seem capable of starting with simpler substances (such as amino acids) and using them to make larger and more complex macromolecules (such as proteins). To do this, living things must use energy from other sources, convert this energy into forms they can manipulate, and then re-use it to build and synthesize all the higher levels of organization and structure.
Utilization of energy, and the ability to use energy to construct higher orders of complexity and structure, is a vital sign of life.