• Purpose
• Objectives
• Reading Assignment
• Vocabulary
• 
    Commentary
  
  Section 1.1 Section 1.2 Section 1.3 Section 2.1 Section 2.2
  
• Study Questions
• Progress Evaluation

Section 2.2—Nutrition and Energy Flow

All energy for living organisms on earth can be traced back to the sun. Organisms that can use the sun's energy to make food in the process of photosynthesis are called producers or autotrophs (like green plants). All other organisms must get their food from producers directly or indirectly. These organisms are called consumers or heterotrophs. Heterotrophs can then be divided into different groups depending on what they use for their source of food. A consumer that eats plants is called a herbivore (first-order consumer). A consumer that eats a herbivore is called a carnivore (second-order consumer). A consumer that feeds on both plants and animals is called an omnivore. Scavengers and decomposers also play an important role as they help clean up and recycle materials in an ecosystem.

Matter and energy both move through an ecosystem, and ecologists have developed models to help explain the movement. The simplest model is called a food chain, which consists of two, three, or four links that show how food and energy are passed along. The food chain always begins with a producer that uses the sun's energy, and the chain will include herbivores, carnivores, and decomposers.

In this simple food chain, the grass generates energy it gathers from the sun through photosynthesis, which is then passed along to the grasshopper, the frog, the snake, and finally the hawk.

The total amount of energy being passed from one link to another always decreases with each step along the chain, because some of the energy is transferred to the atmosphere and isn't available for use by the organism at the next step. Each of the links in a food chain is called a trophic level. Food webs are more complex, because they show all the possible feeding relationships, but they give a more complete view of what is actually happening in an ecosystem.

Another model that is often used to give us more information about the movement of food and energy through an ecosystem is the pyramid. Figures 2.14, 2.15, and 2.16 on pages 52–53 of the textbook represent three different pyramids. In all of them, producers are on the bottom (or form the base). Each level above the base represents successive feeding levels (or trophic levels). In the pyramid of energy, it is easy to see the energy loss at each level. In the pyramid of numbers, the number at each level represents the number of organisms consumed by the organisms on the level above it. As you go up the pyramid, fewer and fewer organisms are present at each trophic level. The pyramid of biomass (total mass of living material at each trophic level) shows the amount of mass needed to support the level just above it. Energy is lost at each trophic level. According to the law of conservation of energy this energy doesn't disappear, but it is lost to the environment and is not available for organisms to do biological work.

Matter cycles through the biosphere (the same materials are used over and over again). These materials remain in constant amounts (the total number of carbon, nitrogen, hydrogen, oxygen, and other atoms that compose all things do not change). Thus some of the atoms found in your body today might have been in an ancient tree or long extinct dinosaur. Studying the carbon and nitrogen cycles on pages 55 and 56 of the textbook will show how this is possible. Other important materials such as water also go through a cycle. Although you will not be tested on the water cycle, it is detailed in the image below.

Earth's natural reservoirs (oceans, lakes, rivers, etc.) constantly lose water through evaporation, but it is replenished through precipitation and runoff. During the water cycle, also called the hydrologic cycle, water could be in the form of a solid, liquid, or gas.

Section Activities

The following questions are found in this section's textbook reading assignment. It is important that you have written down the answers to these questions in your science journal before checking the answers provided here. Correctly answering these questions will help you review the material that you have been reading in this section. You can view the answers to the section activities by clicking the "Show Answer" button next to the icons below, or you can visit the printer-friendly version of this lesson to view the answers. See the course overview to view a list and description of the different types of section activities.

Reading Check Question: Page 50
Compare food webs and food chains.
Hint: How are they alike? Which is simpler?

Picture Caption Question: Page 47, Figure 2.11
What are some of the autotrophs and heterotrophs in this photograph?

The trees, bushes, shrubs, and grasses are autotrophs, while all the animals in the picture are heterotrophs.

Picture Caption Question: Page 55, Figure 2.18
How is carbon released from the bodies of organisms?

Carbon can be released in several ways, but mostly through the release of carbon dioxide when exhaling. Decay and burning are two other methods by which carbon can be released (be sure to study the diagrams on the carbon, water, nitrogen, and phosphorus cycles).

Picture Caption Question: Page 57, Figure 2.20
What is the function of mountains in the phosphorus cycle?

The rocks making up the mountains contain phosphorus.

Problem-Solving Lab 2.2: Page 50

Solve the Problem: Beginning from the bottom box in the diagram that you drew, you should have filled in autotroph, first-order heterotroph, second-order heterotroph, third-order heterotroph. The labels for the brackets starting from the bottom should be producer, herbivore, carnivore.

Thinking Critically: The arrows represent energy being passed from one level to another. The boxes form a shape similar to a pyramid representing the fact that there is less matter and energy available at each level.