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Ecosystems | Lesson 3 - Matter Cycles and Energy Flows in Ecosystems

Lesson 3: Matter Cycles and Energy Flows in Ecosystems

In Lesson 2 students identified a pattern of the biomass pyramid in a meadow ecosystem. In Lesson 3 they explain why that pattern exists by tracing matter and energy and connecting scales: (a) matter cycling and energy flow among carbon pools at the ecosystem scale, (b) growth, life functions, and death of organisms at the macroscopic scale, and (c) carbon-transforming processes (photosynthesis, biosynthesis, digestion, cellular respiration) at the atomic-molecular scale.

Guiding Question

How do carbon atoms and energy move through an ecosystem?

Activities in this Lesson

  • Activity 3.1: Carbon Pools (20 min)
  • Activity 3.2: Carbon Dice Game (30 min)
  • Activity 3.3: Tracing Carbon Through Ecosystems (30 min)
  • (Optional) Activity 3.4: What Happens to Soil Carbon? (30 min)
  • Activity 3.5: Tracing Energy Through an Ecosystem (30 min)
  • Activity 3.6: Explaining Patterns in Ecosystem (30 min)


  • Describe carbon cycling within ecosystems as movement of carbon atoms among carbon pools associated with:
    • Movement of materials: Eating, defecation, death
    • Carbon-transforming processes: combustion, photosynthesis, digestion, biosynthesis, cellular respiration
  • Explain changes in size of carbon pools in terms of fluxes into and out of carbon pools.
  • Identify energy transformations involved in carbon fluxes
  • Describe energy as flowing through ecosystems, from sunlight to chemical energy to heat that is radiated into space

NGSS Performance Expectations

Middle School

  • Matter and Energy in Organisms and Ecosystems. MS-LS2-1. Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
  • Interdependent Relationships in Ecosystems. MS-LS2-2. Construct an explanation that predicts patterns of interactions among organisms across multiple ecosystems.
  • Matter and Energy in Organisms and Ecosystems. MS-LS2-3. Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.
  • Matter and Energy in Organisms and Ecosystems. MS-LS2-4. Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.
  • Matter and Energy in Organisms and Ecosystems. MS-LS1-6. Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy in and out of organisms.
  • Earth’s Systems. MS-ESS2-1. Develop a model to describe the cycling of earth’s materials and the flow of energy that drives this process.
  • Human Impacts. ESS3-4. Construct an argument supported by evidence for how increases in human population and per-capita consumption of natural resources impact Earth’s systems.

High School

  • Chemical Reactions. HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • Interdependent Relationships in Ecosystems. HS-LS2-1. Use mathematical and or computational representations to support explanations of factors that affect carrying capacity of ecosystems and different scales.
  • Interdependent Relationships in Ecosystems. HS-LS2-2. Use mathematical representations to support and revise explanations based on evidence about factors affecting biodiversity and populations in ecosystems at different scales.
  • Matter and Energy in Organisms and Ecosystems. HS-LS2-4. Use mathematical representations to support claims for the cycling of matter and flow of energy among organisms in an ecosystem.
  • Matter and Energy in Organisms and Ecosystems. HS-LS2-5: Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.
  • Earth’s Systems. HS-ESS2-6. Develop a quantitative model to describe the cycling of carbon among the hydrosphere, atmosphere, geosphere, and biosphere.

Background Information

In Lesson 3 students develop complete explanations for what causes the organic matter pyramid, the key pattern in ecosystems they examined in Lesson 2. Using carbon pools as a context, students trace matter through representations that connect carbon-transforming processes at the atomic-molecular, organismal, and ecosystem scales, and they consider the differences between matter cycling and energy flow through ecosystems.

Activity 3.1 introduces students to the carbon pools: atmosphere, soil, producer, herbivore, and carnivore. They further simplify these into two groups: “organic matter” and “inorganic matter” pools to help distinguish between pools with CO2 and pools with C-C and C-H bonds. These pools become the context through which matter and energy move throughout the rest of the unit. This forms a foundation for answering the Four Questions for the Large Scale: The Carbon Pools Question, the Carbon Movement Question, and the Energy Question. It is important to note that additional pools exist in ecosystems, but this ecosystem is intentionally simplified to focus on the organic matter pyramid as a theme in the unit.

Activity 3.2, Carbon Dice Game, helps students think about how carbon is cycled through different organisms in an ecosystem. The Meadow Simulation showed that the pattern of the biomass pyramid emerges repeatedly in ecosystems, but did not provide evidence for what drives this pattern. Through the dice game students can see that the biomass pyramid is a natural consequence of the carbon-transforming processes that take place in all organisms: photosynthesis, digestion/biosynthesis, cellular respiration, being eaten, or death/defecation. Students play the role of individual carbon atoms, and rolls of the dice represent the likelihood of which process will happen to them inside an organism. Students can see the pattern in their visits to different carbon pools: They go through plants, soil carbon, and the atmosphere often, through herbivores less often, and through carnivores hardly ever.

In Activity 3.3 students explain the reason for the pattern of size of carbon pools that was observed in the Meadows Simulation and the Carbon Dice Game. When they followed individual carbon atoms through an ecosystem in the Carbon Dice Game, they observed that the carbon atoms visited some pools more often than others. In this Activity they consider the implications of this observation for the size of the pools.

In order to explain the biomass pyramid, students need to think about movement of carbon in semi-quantitative ways. Most of organic carbon created through photosynthesis is used for cellular respiration (energy needs) is used by organisms and returns to the atmosphere. Because most of the organic carbon in an organism is used for cellular respiration (and much of it is lost during death and defecation), very little organic carbon is available to be passed from one level in a food chain to another. Thus in a “steady state” ecosystem, carbon atoms are constantly in motion, but the relative size of the large-scale carbon pools stays about the same.

In Activity 3.4, which is optional, students are further introduced to the soil carbon pool.

In Activity 3.5 students connect the game with the Large-Scale Four Questions to describe how carbon cycles and energy flows in ecosystems:

  • Carbon cycles: carbon atoms move between pools via carbon transforming processes
  • Energy flows: in describing energy flow students must add the energy source for all energy in ecosystems: sunlight, which is transformed into chemical energy and ultimately heat. That heat energy is ultimately radiated into space in the form of infrared light (this is why the earth cools down at night). So while heat energy may move through the atmosphere (and some of it may temporarily be trapped in the atmosphere due to the greenhouse effect), eventually all of it will be lost from ecosystems, and radiate into outer space.
  • It is especially important for students to understand that while matter and energy move through an ecosystem in tandem when they are combined in organic matter, they follow different pathways at the beginning and end of food chains and food webs:
    • Producers get matter ONLY from carbon dioxide, water, and minerals and energy ONLY from sunlight.
    • When organisms use organic matter for cellular respiration, ALL the matter goes back into carbon dioxide, water, and minerals, while ALL the energy leaves the ecosystem as heat (which is ultimately radiated out into space). So matter cycles, energy flows through ecosystems.

In Activity 3.6 students use the Explanations Tool for Ecosystems to explain the reason for the key pattern in this unit: the biomass pyramid. Students use their knowledge of energy flow and matter cycling to explain why the producer pool has more organic matter than the herbivore pool, and why the herbivore pool has more organic matter than the carnivore pool.

Key carbon-transforming processes: photosynthesis, cellular respiration, combustion

Ecosystems Unit Map