Energy and Life: The Transformation of Energy in Living Organisms

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  • 0:05 Energy and Life
  • 1:25 Plants Transform Sun's Energy
  • 2:11 Organisms Use Sugar Energy
  • 4:45 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

While the sun is an excellent source of energy, not all forms of life can utilize the sun's energy directly. This lesson describes how plants transform the sun's energy into potential energy stored in sugar, how living organisms utilize energy in sugar to perform work, and how the relationship between photosynthesis and cellular respiration is necessary for life.

Energy and Life

Organisms use sugar as a source of energy to do work.
Sugar Is Energy

All living things require energy to do the work necessary for survival and reproduction. This is true for bacteria, plants, and animals. But what is energy? Energy is simply the ability to do work, where work is done when a force moves an object. Let's consider your own needs for a moment. You need energy to turn on and turn off your computer. You need energy to get out of bed in the morning. You even need energy to listen to this lesson and think about what it says. And, yes, you need energy to reproduce. So where does energy come from and how do we use it? On Earth, energy ultimately comes from the sun. Plants use the sun's energy to make sugar. Organisms, in turn, use sugar as a source of energy to do work.

In this lesson, we will explore how living organisms utilize energy. We will first consider how plants use energy from the sun to make sugar. Then we will explore how organisms use energy from the sugar to do work.

How Plants Transform Energy from the Sun

Plants use energy from sunlight to make sugar and oxygen from carbon dioxide and water. The process by which carbon dioxide and water are converted to sugar and oxygen using sunlight is referred to as photosynthesis. This is an endergonic reaction, meaning energy is required by the reaction. Specifically, energy is required to put the carbon dioxide and the water molecules together to form sugar. Sun provides the energy needed to drive photosynthesis, and some of the energy used to make the sugar is stored in the sugar molecule.

The sun provides the energy needed to drive photosynthesis.
Sun Drives Photosynthesis

How Organisms Use Energy from Sugar

Now that we know how plants synthesize sugar, let's explore how organisms use the sugar as a source of energy. In short, organisms break down the sugar to release its stored energy. The energy released from the breakdown of sugar is used by the cells to make another chemical that we call adenosine triphosphate, or simply abbreviated ATP. The synthesis of ATP by cells is referred to as cellular respiration. This is an exergonic reaction as energy is released as a result of the reaction. Energy is released when the sugar is broken down into smaller parts: carbon dioxide and water. As you can see on the screen, sugar and oxygen are the reactants, and carbon dioxide and water are the products of cellular respiration. Does that reaction look familiar? Well it should, because cellular respiration is simply the reverse of photosynthesis. Photosynthesis and cellular respiration are related as the products of one become the reactants for the other. In fact, cellular respiration and photosynthesis are dependent on one another.

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