Biological processes depend on energy flow through the Earth's system.

Energy from the Sun fuels life on Earth

The continual input of energy, mostly from sunlight, sustains the process of life. 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 chemical compounds such as methane and hydrogen sulfide. In either case, the overall productivity of an ecosystem is controlled by the total energy available.

Energy flows through all life on Earth

Food webs show how energy moves throughout a system. 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 that 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. The efficiency of the food chain decreases as you go upward. (Learn more about the transfer of energy in ecosystems.)

This points out a critical factor in the distribution of energy in human foods too. Eating producers (plants) at the bottom of the food chain is the most efficient way for humans to acquire energy for living. This has implications for humans as we strive to keep a growing human population adequately nourished.

These ideas also introduce the origin of organic matter that later can become fossil fuels. The source of energy in fossil fuels is sunlight, which fueled photosynthesis. Oil and natural gas come from photosynthetic plankton which has been preserved in sediments on the ocean floor, heated, and chemically altered into hydrocarbons. Coal comes from plants that have been buried in sediment, compacted, and preserved. These ideas are further explored in Energy Principle 4. (Learn more about where oil comes from.)

Helping students understand these ideas

While many students can readily relate to 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.

Other topics that tie into this theme are:

• 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 on the energy balance of ecosystems, such as fires, disease, population dynamics, and changes in land use.

Bringing these ideas into your classroom

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.

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 Much Energy is on My Plate? leads students through a sequence of learning steps that highlight the embedded energy that is necessary to produce various types of food. 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.

Teaching materials from the CLEAN collection

Middle school

• To Boldly Go... is a web-based activity that tackles the broad reasons for undertaking ocean exploration - studying the interconnected issues of climate change, ocean health, energy, and human health. Students examine the types of technology ocean scientists use to collect important data.
• 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.
• Inland Fish and Warming Waters is an activity that relates water temperature to fishery health within inland freshwater watersheds.
• World of Change: Amazon Deforestation is a series of NASA satellite images taken over 10 years, 2000-2010, showing the extent of deforestation in western Brazil.

High school

• Stressed Out! is an activity where students research various topics about ocean health, such as overfishing, habitat destruction, invasive species, climate change, pollution, and ocean acidification. An optional extension activity has instructions to create an aquatic biosphere in a bottle and then manipulate variables.
• Food webs are commonly taught with concept maps, such as with the Oceanic Food Web. The concept map-like connections on this visualization encourages students to link the abiotic and biotic interactions within the oceanic food web. This is also suitable for introductory college students.
• Fermentation in a Bag and Bioprospecting for Cellulose-degrading Microbes are two hands-on activities that explore the production of cellulosic ethanol.

College

• How Much Energy is on My Plate? leads students through a sequence of learning steps that highlight the embedded energy that is necessary to produce various types of food. Students start by thinking through the components of a basic meal and are later asked to review the necessary energy to produce different types of protein. This activity may be paired with The Lifestyle Project.
• The Effects of El Niño/La Niña on Phytoplankton and Fish video illustrates the effects of climatic cycles on phytoplankton populations. Phytoplankton forms the base of the food web and supply half of all the oxygen that we breathe.