<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />
Skip Navigation

4.1: Energy for Life

Difficulty Level: At Grade Created by: CK-12
Turn In

Lesson Objectives

  • State why living things need energy.
  • Describe how autotrophs and heterotrophs obtain energy.
  • Compare and contrast glucose and ATP.
  • Outline how living things make and use food.


  • autotroph
  • cellular respiration
  • consumer
  • energy
  • food
  • glucose
  • heterotroph
  • photosynthesis
  • producer


All living things need energy, which is defined as the ability to do work. You can often see energy at work in living things—a bird flies through the air, a firefly glows in the dark, a dog wags its tail. These are obvious ways that living things use energy, but living things constantly use energy in less obvious ways as well.

Why Living Things Need Energy

Inside every cell of all living things, energy is needed to carry out life processes. Energy is required to break down and build up molecules and to transport molecules across plasma membranes. All life’s work needs energy. A lot of energy is also simply lost to the environment as heat. The story of life is a story of energy flow—its capture, its change of form, its use for work, and its loss as heat. Energy, unlike matter, cannot be recycled, so organisms require a constant input of energy. Life runs on chemical energy. Where do living organisms get this chemical energy?

How Organisms Get Energy: Autotrophs and Heterotrophs

The chemical energy that organisms need comes from food. Food consists of organic molecules that store energy in their chemical bonds. In terms of obtaining food for energy, there are two types of organisms: autotrophs and heterotrophs.


Autotrophs are organisms that make their own food. Most autotrophs use the energy in sunlight to make food in a process called photosynthesis. Only three types of organisms—plants, algae, and some bacteria—can make food through photosynthesis. Examples of each type of photosynthetic organism are shown in Figure below.

Photosynthetic autotrophs, which make food using the energy in sunlight, include (a) plants, (b) algae, and (c) certain bacteria.

Autotrophs are also called producers. They produce food not only for themselves but for all other living things as well (which are known as consumers). This is why autotrophs form the basis of food chains, such as the food chain shown in Figure below.

A food chain shows how energy and matter flow from producers to consumers. Matter is recycled, but energy must keep flowing into the system. Where does this energy come from? (Though the decomposers are shown as the final step in this food chain, these organisms decompose material from each step of the food chain. See the Ecology Concepts for additional information.)


Heterotrophs are living things that cannot make their own food. Instead, they get their food by consuming other organisms, which is why they are also called consumers. They may consume autotrophs or other heterotrophs. Heterotrophs include all animals and fungi and many single-celled organisms. In Figure above, all of the organisms are consumers except for the grass. What do you think would happen to consumers if all producers were to vanish from Earth?

Energy Molecules: Glucose and ATP

Organisms mainly use two types of molecules for chemical energy: glucose and ATP. Both molecules are used as fuels throughout the living world. Both molecules are also key players in the process of photosynthesis.


Glucose is a simple carbohydrate with the chemical formula C6H12O6. It stores chemical energy in a concentrated, stable form. In your body, glucose is the form of energy that is carried in your blood and taken up by each of your trillions of cells. Glucose is the end product of photosynthesis, and it is the nearly universal food for life.


ATP (adenosine triphosphate) is the energy-carrying molecule that cells use for energy. ATP is made during the first half of photosynthesis and then used for energy during the second half of photosynthesis, when glucose is made. It is also used for energy by cells for most other cellular processes. ATP releases energy when it gives up one of its three phosphate groups and changes to ADP (adenosine diphosphate [two phosphates]).

Why Organisms Need Both Glucose and ATP

Why do living things need glucose if ATP is the molecule that cells use for energy? Why don’t autotrophs just make ATP and be done with it? The answer is in the “packaging.” A molecule of glucose contains more chemical energy in a smaller “package” than a molecule of ATP. Glucose is also more stable than ATP. Therefore, glucose is better for storing and transporting energy. However, glucose is too powerful for cells to use. ATP, on the other hand, contains just the right amount of energy to power life processes within cells. For these reasons, both glucose and ATP are needed by living things.

A explanation of ATP as biological energy is found at http://www.youtube.com/watch?v=YQfWiDlFEcA.

Making and Using Food

The flow of energy through living organisms begins with photosynthesis. This process stores energy from sunlight in the chemical bonds of glucose. By breaking the chemical bonds in glucose, cells release the stored energy and make the ATP they need. The process in which glucose is broken down and ATP is made is called cellular respiration.

Photosynthesis and cellular respiration are like two sides of the same coin. This is apparent from Figure below. The products of one process are the reactants of the other. Together, the two processes store and release energy in living organisms. The two processes also work together to recycle oxygen in Earth’s atmosphere.

This diagram compares and contrasts photosynthesis and cellular respiration. It also shows how the two processes are related.


Photosynthesis is often considered to be the single most important life process on Earth. It changes light energy into chemical energy and also releases oxygen. Without photosynthesis, there would be no oxygen in the atmosphere. Photosynthesis involves many chemical reactions, but they can be summed up in a single chemical equation:

6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2.

Photosynthetic autotrophs capture light energy from the sun and absorb carbon dioxide and water from their environment. Using the light energy, they combine the reactants to produce glucose and oxygen, which is a waste product. They store the glucose, usually as starch, and they release the oxygen into the atmosphere.

Cellular Respiration

Cellular respiration actually “burns” glucose for energy. However, it doesn’t produce light or intense heat as some other types of burning do. This is because it releases the energy in glucose slowly, in many small steps. It uses the energy that is released to form molecules of ATP. Cellular respiration involves many chemical reactions, which can be summed up with this chemical equation:

C6H12O6 + 6O2 → 6CO2 + 6H2O + Chemical Energy (in ATP)

Cellular respiration occurs in the cells of all living things. It takes place in the cells of both autotrophs and heterotrophs. All of them burn glucose to form ATP.

Lesson Summary

  • Living things need energy to carry out all life processes. They get energy from food.
  • Autotrophs make their own food. Heterotrophs get food by eating other living things.
  • Glucose and ATP are used for energy by nearly all living things. Glucose is used to store and transport energy, and ATP is used to power life processes inside cells.
  • Many autotrophs make food through the process of photosynthesis, in which light energy from the sun is changed to chemical energy that is stored in glucose. All organisms use cellular respiration to break down glucose, release its energy, and make ATP.

Lesson Review Questions


1. Define energy, and state where living things get the energy they need.

2. What is an autotroph? Give an example.

3. How does photosynthesis change energy?

4. How do heterotrophs obtain food?

Apply Concepts

5. ATP and glucose are both molecules that organisms use for energy. They are like the tank of a tanker truck that delivers gas to a gas station and the gas tank that holds the fuel for a car. Which molecule is like the tank of the delivery truck, and which is like the gas tank of the car? Explain your answer.

Think Critically

6. Compare and contrast photosynthesis and cellular respiration. Why are the processes like two sides of the same coin?

7. Explain why living things need both glucose and ATP.

8. Explain how living things recycle oxygen in Earth’s atmosphere.

Points to Consider

Living things must have chemical energy from food to power life processes. Most of the chemical energy in food comes ultimately from the energy in sunlight.

  • Do you know how the energy in sunlight is captured by plants and other photosynthetic autotrophs?
  • How do you think light energy changes to chemical energy during the process of photosynthesis?
  • Some producers live in places that do not receive sunlight. How do you think they make food?

    Notes/Highlights Having trouble? Report an issue.

    Color Highlighted Text Notes
    Please to create your own Highlights / Notes
    Show More

    Image Attributions

    Show Hide Details
    Save or share your relevant files like activites, homework and worksheet.
    To add resources, you must be the owner of the section. Click Customize to make your own copy.
    Please wait...
    Please wait...
    Image Detail
    Sizes: Medium | Original
    Add Note
    Please to create your own Highlights / Notes