SPEAK OUR LANGUAGE

• CELL – Core Experience Learning Lab
• SDR – Scientist Data Record

ASK WHY

Great scientists question the world around them. We encourage our LabLearner students to do the same. In anticipation of this, we explain the importance of learning the concepts in the Ask Why section within the CELL. Our hope is that these explanations help students understand why science matters.

BRANCH OUT

Each Investigation introduces students to a different branch of science or STEM (Science, Technology, Engineering, Mathematics) career that utilizes the scientific concepts of the CELL. These real-world connections will help students see the relevance of what they are learning. STEM connections are also integrated into each Performance Assessment.

GET FOCUSED

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:

Investigation 1:

• How does energy move through an ecosystem? The energy at each level of an ecosystem is contained in the biomass of organisms at that level. Therefore energy is transferred from one level to another as consumers in one level eat the biomass from the level below.

• What affects the amount of energy in an ecosystem? The amount of energy in an ecosystem is affected by the amount of energy that can be captured by plants through photosynthesis.

Investigation 2:

• What affects the efficiency of energy transfer within an ecosystem? The efficiency of energy transfer is affected by the use of energy by an organism. All organisms must use some energy to live, and therefore cannot store all the energy that they consume. Therefore, the transfer of energy from one level to another is not 100% efficient.

Investigation 3:

• What is the relationship between the energy and biomass of producers and the levels that an ecosystem can support? The amount of energy that is stored by producers determines the amount of biomass they can form. The amount of biomass and energy at the producer level determines the amount of biomass and energy that can be produced in each of the upper levels of the ecosystem. As the amount of biomass and energy increases in upper levels, the greater the number of levels an ecosystem can support.

• What affects the amount of energy in an ecosystem? The amount of energy in an ecosystem is determined by the amount of biomass in the ecosystem.

Note: These are succinct responses to the Focus Questions and are placed here for easy reference. Fully developed responses to the Focus Questions can be found on each PostLab page.

Note: Some questions may be revisited as the CELL progresses. As students acquire additional knowledge, their responses should reflect this.

LEARN THE LabLearner LINGO

The following list includes Key Terms that the teacher should introduce, as appropriate, within the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Backgrounds. These words are not Key Terms and are strictly emphasized for exposure at this time.

• Ecosystem: all the organisms in a given area and the abiotic factors with which they interact.
• Producers: organisms that are autotrophs, which means they make their own food.
• Consumers: organisms that are heterotrophs, which means they must obtain food by consuming producers or other consumers.
• Detritivore: an organism that obtains nutrition from detritus. Detritivores are scavengers, bacteria, or fungi.
• Detritus: dead plant and animal material.
• Trophic level: a group of organisms in an ecosystem identified by their food source.
• Biomass: the dry weight of the organisms within an ecosystem. Biomass = live weight – water content of the organism. Also known as dry matter.
• Law of Conservation of Energy: a law that states that energy is neither created nor destroyed, it simply changes form.
• Law of Conservation of Matter: a law that states that matter is neither created nor destroyed, it simply changes form.

BE PREPARED

An overview of the materials for each lab is placed here for easy reference. Specific teacher preparation for the labs is placed at the beginning of each Lab page.

EXTEND YOUR THINKING

The following information is included so that teachers have additional background knowledge pertaining to the concepts introduced in the CELL. Teachers may choose to use this information enrich students during instruction, but this is optional and not necessary for the intended students’ learning outcomes.

An ecosystem is composed of all the organisms that live in a given area as well as the abiotic factors with which those organisms interact. Abiotic factors are defined as those factors within the ecosystem which are not alive. For example, a pond ecosystem is made up of the animals, plants, and microorganisms living in or around the pond as well as the water and land areas making up the pond itself. In an ecosystem, the organisms interact not only with the abiotic factors but with each other.

How is an ecosystem identified? An ecosystem can be identified by its geographic location (i.e., a tropical rainforest), but its boundaries cannot be clearly assigned. For example, a suburban neighborhood could represent an ecosystem. A block within that neighborhood cannot be isolated and referred to as an ecosystem by itself. On the other hand, a subset of the entire neighborhood may be defined as an ecosystem. For example, all the areas of the neighborhood that receive full sun in the morning every day could be classified as an ecosystem. Organisms may inhabit specific regions of an ecosystem during specific parts of their day. In the full Sun ecosystem, for example, some animals and insects may move into this area and inhabit it for the duration of the sunlight, then move elsewhere as the sunlight wanes. Because these organisms move into and out of this specific area of the larger neighborhood ecosystem with regularity, they have created a smaller ecosystem within the larger one that exists during a specified time each day.

One consequence of a lack of discrete boundaries is that ecosystems tend to overlap one another. This allows organisms to move in and out of multiple ecosystems with relative ease. A prime example of this situation would be the world’s major oceans. In reality, they are a single large body of water that humankind has subdivided based on climate and geographical location. This body of water is an extremely large marine ecosystem made up of many smaller, overlapping ecosystems. Whales, other sea mammals, and birds routinely winter in warmer waters but move north to their breeding grounds during the spring. However, geography and climate are not the only things that define an ecosystem. A reef ecosystem differs from the open sea surrounding it, in that certain sea creatures inhabit the reef and never swim or otherwise move away from the reef. Meanwhile, some predator fish such as sharks may visit the reef at a specific time of day but spend the rest of their time in the open ocean. Similar patterns occur in both land and estuarine-based ecosystems.

The organisms within an ecosystem are classified as either producers or consumers. Producers are autotrophic, meaning that they are capable of capturing and storing energy for themselves. The word autotroph literally means “self-feeder”. Autotrophs can be further classified as either chemoautotrophs or photoautotrophs depending upon the source of energy that they capture. Chemoautotrophs (chemotrophs) capture energy from inorganic chemical sources, usually gases such as hydrogen sulfide. Photoautotrophs (phototrophs) capture light energy. Chemoautotrophs typically are algae. Photoautotrophs tend to be plants. Both types of autotrophs convert the captured energy into organic (carbon-containing) compounds that can be used for energy by other organisms.

Consumers are heterotrophs, meaning that they must obtain energy by consuming other organisms. Heterotrophs can be divided into four major classes: herbivores, carnivores, omnivores, and detritivores (decomposers). Herbivores consume only plants or algae. Omnivores and detritivores (decomposers) consume both plants and animals. Carnivores consume other animals. Detritivores (decomposers) differ from herbivores, carnivores, and omnivores in that they consume dead organic material rather than living organisms.

There are three types of detritivores (decomposers): scavengers, bacteria, and fungi. Scavengers are opportunists. They feed on the remains of organisms killed by predators after carnivores and omnivores have eaten from the carcass. Scavengers come from all parts of the animal kingdom, including insects. Detritivores (decomposers) serve a significant role in nature, especially bacteria and fungi. Bacteria and fungi complete the cycle of returning nutrients to their inorganic forms for conversion by breaking down the remaining dead tissues of plants and animals through decomposition.

Ecosystems have a structure of feeding relationships called trophic levels. A trophic level is a group of organisms that is identified by the organisms’ food source. The supporting trophic level for all other levels in an ecosystem consists of primary producers. Primary producers are responsible for converting non-organic energy sources into organic energy sources. The primary class of organic energy source is carbohydrates; however, the proteins and lipids (fats and oils) that make up the majority of cell structural components in all organisms are also organic chemicals and thus contain energy that can be used by an organism’s consumer. Once converted into organic forms, energy moves from one trophic level to the next as organisms are consumed and digested. Figure 1 illustrates the organization of trophic levels within an ecosystem and the path that energy takes as it moves from one trophic level to the next.

As energy moves through the system, it is transformed from light (solar) energy to chemical energy and heat. Heat represents energy lost to the ecosystem. Heat is not a true loss as energy cannot be destroyed (Law of Conservation of Energy). However, it is a loss in terms of no longer being in a form that can be used by organisms in the ecosystem. An important characteristic of energy flow through an ecosystem is that the flow is unidirectional. As Figure 1 illustrates, energy always moves up through the different levels and does not return to the lower levels from the upper ones. This is significantly different from the flow of nutrients within an ecosystem, as nutrients cycle from inorganic forms to organic forms back to inorganic forms as they pass through the ecosystem.

Not all of the energy from the Sun which reaches the Earth is captured by plants. Of the 1022 joules of energy that reaches the earth on a daily basis, only one percent, or 1020 joules, is captured by plants. The remaining 99 percent is reflected or absorbed by bare soil, rocks, and water. The percentage of energy from the Sun that is captured varies from ecosystem to ecosystem. There are several reasons for this variation. Organisms differ in number and concentration among ecosystems. A rainforest ecosystem will capture and convert significantly greater amounts of light energy than a desert ecosystem due to the differences in plant populations. In addition, light intensity varies due to geographic location and season. Ecosystems located along the equator receive a constant level of light throughout the year. However, light intensity varies with the season for ecosystems located North and South of the Equator, with the greatest variation due to season occurring at the North and South Poles.

Light level can also be affected by the presence of dust particles or clouds in the atmosphere, like dust and water vapor decrease the amount of light that can reach the Earth from the Sun. The effect of dust in the atmosphere is most noticeable after catastrophic events such as volcanic eruptions or large wildfires. A series of volcanic eruptions in the early 1800s led to the famous “year without a summer” in Europe and North America in 1816. The dust resulting from the massive eruptions of volcanoes such as Mt. Tambora greatly decreased the light energy reaching the Earth. This massive dust cloud caused a summer of freezing weather that prevented crops and other plants from growing.

As with energy capture, energy transfer between trophic levels is inefficient. On average, only 10 percent of the energy present in one level is transferred to the next. This is known as a “10% rule”. The overall effect of decreasing energy levels from the Sun to the detritivores (decomposers) is described by scientists as a pyramid of production and can be illustrated as shown in Figure 2.

Inefficiency in the energy transfer between trophic levels is due to more than just the loss due to heat. Heat is a byproduct of cellular respiration, which is an exothermic process. An exothermic reaction is one in which heat is released. Cellular respiration provides organisms with another form of chemical energy (in the form of high-energy phosphate bonds) as the result of breaking down carbohydrates, lipids, and proteins. The compounds which contain the high-energy phosphate bonds provide fuel for the cell’s maintenance and growth. This energy is consumed by the cell and is not available to transfer to the next level of the ecosystem.

Not all energy losses between levels occur as a result of metabolism. One loss occurs in the form of standing crops at the producer level. Some carbohydrates, proteins, and lipids are unavailable to herbivores and omnivores because they are tied up in wood or other organic compounds that cannot be consumed. Others may be in organic form but inaccessible due to location, such as petroleum or peat. Another type of loss is the failure of the consumer to assimilate, or take up, the nutrients and energy in consumed food as a result of incomplete digestion or because the food passes through the gut too quickly for all of the digested nutrients to be absorbed. On average, approximately 50% of consumed food remains undigested; however, this varies from species to species depending on the food source. Energy must be absorbed by organisms and stored through growth and reproduction in order to be transferred to the next level.

Ecologists refer to stored energy within a level or ecosystem as biomass. Biomass is the weight of all the organisms within an ecosystem minus the weight of the water they contain. It is measured in units of grams per square meter (g/m2). The close relationship between the amount of biomass in a trophic level and the amount of energy contained by a trophic level means that the efficiency of biomass transfer mirrors that of energy transfer. Thus biomass can also be represented by a pyramid as shown in Figure 3a. However, unlike the pyramid of production, the biomass pyramid represents the standing crop of biomass within each level. Therefore, the relationship between the biomass of producers to consumers depends on how rapidly the consumers eat the biomass generated by the producers. In some marine ecosystems, the producer biomass (phytoplankton) is outweighed significantly by the herbivore biomass (zooplankton), leading to an inverted pyramid as shown in Figure 3b. In a terrestrial ecosystem, such a relationship would result in the desertification of the region in a fairly rapid fashion, but phytoplankton reproduce at such a rapid rate that they support the greater zooplankton biomass easily. In fact, this inverse relationship is necessary. Otherwise, the mass of phytoplankton would block sunlight from filtering into the depths of the water and prevent the growth of other aquatic plants that feed larger marine herbivores.

In this CELL, students will explore how energy and biomass are related through models and experimentation. Students will examine how both energy and biomass within an ecosystem decrease as they move up through the levels of an ecosystem. Students will model the inefficiency of energy transfer by examining energy use during cellular respiration. Finally, students will explore the impact decreasing the amount of plant biomass in an ecosystem impacts the number of levels and animals per level that can be supported within that ecosystem.