CLEAN > Teaching Climate and Energy Science > Teaching Energy Science > 3. Energy in Biological Processes

Energy Principle 3. Biological processes depend on energy flow through the Earth system.

Jump down to: Teaching with this principle | Find activities

Teaching this principle is supported by six key concepts:

3.1 The Sun is the major source of energy for organisms and the ecosystems of which they are a part. Producers such as plants, algae, and cyanobacteria use the energy from sunlight to make organic matter from carbon dioxide and water. This establishes the beginning of energy flow through almost all food webs.

Department of Energy Videos

Energy Principle #3

3.2 Food is a biofuel used by organisms to acquire energy for internal living processes. Food is composed of molecules that serve as fuel and building material for all organisms as energy stored in the molecules is released and used. The breakdown of food molecules enables cells to store energy in new molecules that are used to carry out the many functions of the cell and thus the organism.

3.3 Energy available to do useful work decreases as it is transferred from organism to organism. The chemical elements that make up the molecules of living things are passed through food chains and are combined and recombined in different ways. At each level in a food chain, some energy is stored in newly made chemical structures, but most is dissipated into the environment. Continual input of energy, mostly from sunlight, keeps the process going.

3.4 Energy flows through food webs in one direction, from producers to consumers and decomposers. An organism that eats lower on a food chain is more energy efficient than one eating higher on a food chain. Eating producers is the lowest, and thus most energy efficient, level at which an animal can eat.

3.5 Ecosystems are affected by changes in the availability of energy and matter. The amount and kind of energy and matter available constrains the distribution and abundance of organisms in an ecosystem and the ability of the ecosystem to recycle materials.

3.6 Humans are part of Earth's ecosystems and influence energy flow through these systems. Humans are modifying the energy balance of Earth's ecosystems at an increasing rate. The changes happen, for example, as a result of changes in agricultural and food processing technology, consumer habits, and human population size.

What does this principle mean?

This principle addresses the flow of energy through life on Earth. The Sun is the primary source of energy for nearly all biological systems. Sunlight allows plants, algae and cyanobacteria to use photosynthesis to convert carbon dioxide and water into organic compounds like carbohydrates. This process is the fundamental source of organic material in the biosphere. There are a few exceptions to this, such as ecosystems living around hydrothermal vents on the ocean floor, which derive their energy from the chemical compounds such as methane and hydrogen sulfide.

This principle also illustrates the role of energy in food webs. Plants use energy from the Sun to create organic matter. Plants are then eaten by primary consumers who are in turn eaten by secondary consumers, and so on. In each step, the energy that was originally emitted by the Sun is consumed, but energy also dissipates with each step. Animals use up 90% of the energy contained in the foods they eat for their normal activities. This leaves just 10% of the original energy available for the next consumer. These concepts illustrate several important points, such as that the overall productivity of an ecosystem is controlled by the total energy available, and that the efficiency of the food chain decreases as you go upward. It is most efficient to eat at the bottom of the food chain; this has implications for humans as we strive to keep a growing human population adequately nourished.

Why is this principle important?

This principle is important in two ways. First, it illustrates how energy flows from the Sun to nearly all living things and describes how energy moves through a food web or ecosystem. Second, it points out a critical factor in the distribution of energy in foods. Eating producers (plants) is most energy efficient. This same basic principle of energy apply to our food system too; eating at the bottom of the food chain is still the most efficient way for humans to acquire energy for living. The continual input of energy, mostly from sunlight, sustains the process of life.

This principle also introduces the origin of organic matter that later can become fossil fuels and biofuels. These ideas are further explored in Energy Principle 4.

What makes this principle challenging to teach?

While many students can readily identify with the idea of the terrestrial food web, the marine food web may be less familiar to them. Students may be surprised to learn that about half the Earth's primary productivity of organic material comes from the oceans.

Strategies for teaching this principle

Compared to Energy Principles 1 and 2, this principle is more concrete and easier to visualize. We all have direct experiences with different types of foods. Many of these concepts, such as how sunlight drives photosynthesis and food webs, are commonly taught in middle school and high school curricula. Educators can take these opportunities to tie in an energy theme with these topics.

Related teaching materials

Teaching about Food with teaching materials for college students

Fisheries Unit from EarthLabs

In middle school and high school, food webs are commonly taught with concept maps, such as this example Oceanic Food Web. The video The Ocean's Green Machines further explores the marine food web by examining phytoplankton These organisms form the base of the marine food web, are the source of half of the oxygen on Earth, and are an important means to remove CO2 from the atmosphere. This video is suitable for a middle school or high school audience.

A quantitative approach can be used to examine the energy embodied in different types of foods. Here are some examples of activities that do this.
How Cheeseburgers Impact the Oceans asks students to calculate the carbon emissions of cheeseburger production (middle school).

How Much Energy is on my Plate? leads students through a sequence of activities that highlight the embodied energy that is necessary to produce various types of food (high school or introductory-level college).

The Lifestyle Project challenges students to dramatically lower their energy use, and adopting a vegetarian diet is one of the paths that students may elect to take. This project can be used with middle school through college audiences.
A hands-on way to teach these topics is from the point of view of a meal or a community garden. All of the concepts contained in this principle can be illustrated in a garden that produces food.

College students can delve deeper into the concepts of energy in biological systems. Possible pathways include:
  • examining methods of measuring primary productivity in different ecosystems,
  • mapping the distribution of primary productivity across the oceans and on land,
  • calculating the available energy in different trophic levels,
  • calculating the embodied energy in different foods,
  • considering the science, technology, economics or ethics of agriculture and livestock production,
  • examining various impacts to the energy balance of ecosystems, such as fires, disease, population dynamics, and changes in land use.

Find Teaching Activities

See all activities for teaching this principle

Search by grade level:

Find middle school activities for this principle
Find high school activities for this principle

Find lower-level college activities for this principle

Find upper-level college activities for this principle


Energy Economics in Ecosystems What powers life? In most ecosystems, sunlight is absorbed and converted into usable forms of energy via photosynthesis. These usable forms of energy are carbon-based.

Ocean Productivity The purpose of this website is to provide the science and broader communities with global, updated ocean productivity estimates.

Campus Farmers The site offers a wealth of information and links to resources about starting an on-campus farm, managing farm finances, and staying in business.