4.2: Energy Flow

*Outline the flow of energy through a food chain.* Understanding: Chemical energy in carbon compounds flows through food chains by means of feeding.

1. Light energy is converted by an autotrophs to chemical energy stored in carbon compounds (such as glucose) through photosynthesis. 2. Energy is transferred to other organisms through feeding; producers are eaten by primary consumers, these by secondary consumers, these by tertiary consumers... 3. Cellular respiration releases energy from the carbon compounds to produce ATP for use by organisms for metabolism, growth, repair, and/or movement 4. Approximately 90% of the energy at each trophic level is lost as heat, biomass not consumed (i.e. bones/hair) or biomass lost as waste (i.e. in feces/urine) *Energy is NOT recycled!*

*Define "food chain."* Understanding: Chemical energy in carbon compounds flows through food chains by means of feeding.

A food chain is a *sequence of organisms through which nutrients and energy pass as one organism eats another.* Food chains show which organisms eats which organism. Food chains always begin with a producer and follow the flow of energy through trophic levels.

*State how energy in carbon compounds enters most biological communities.* Understanding: Most ecosystems rely on a supply of energy from sunlight.

In most ecosystems, chemical potential energy stored in carbon compounds enters biological communities through *photosynthesis in producers.*

*Draw a food chain, labeling the producer, primary consumer, secondary consumer and tertiary consumer.* Understanding: Chemical energy in carbon compounds flows through food chains by means of feeding.

When drawing a food chain, begin with a producer. Draw an arrow to the organism(s) that eats the producer, called the primary consumer. Draw additional organisms and arrows to represent the chain of feeding, from the primary consumer to the secondary consumer and from secondary consumer to the tertiary consumer.

*Outline the energy conversions performed by living organisms.* Understanding: Living organisms cannot convert heat to other forms of energy.

*Living organisms can perform various energy conversions:* Light energy -> chemical energy (photosynthesis) Chemical energy -> kinetic energy (muscle contraction) Chemical energy -> electrical energy (neuron action potential) Chemical energy -> heat energy (cell respiration)

*Define "food web."* Understanding: Chemical energy in carbon compounds flows through food chains by means of feeding.

A food web represents *the interconnected feeding relationships within an ecological community.*

*Define "trophic level."* Understanding: Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels.

A trophic level is *the group of organisms within an ecosystem which occupy the same level in a food chain." The first level contains the producers. The producers are consumed by the second-trophic level organisms—the herbivores (aka primary consumers). At the third trophic level, secondary consumers eat the herbivores; and at the fourth trophic level, tertiary consumers eat the secondary consumers. These categories are not strictly defined, as many organisms feed on several trophic levels. A separate trophic level, the decomposers (saprotrophs), consists of organisms such as bacteria and fungi that break down dead organisms and waste materials into nutrients usable by the producers.

*State the function of ATP.* Understanding: Energy released from carbon compounds by respiration is used in living organisms and converted to heat.

Adenosine triphosphate (ATP) is a nucleotide that functions as the *main source of energy for most metabolic processes.* ATPs are consumed by energy-requiring (endergonic) processes and produced by energy-releasing (exergonic) processes in the cell.

*State the reason why heat created by living organisms is eventually lost from the ecosystem.* Understanding: Heat is lost from ecosystems.

All energy released by metabolism for use in cell activities will ultimately be lost from an ecosystem because *living organisms cannot turn this heat into other forms of usable energy.*

*Describe the shape of a pyramid of energy.* Skill: Quantitative representations of energy flow using pyramids of energy.

An energy pyramid is a graphical depiction of energy flow in a community. The different levels represent different trophic levels, with producers at the bottom. The energy pyramid shape shows how the amount of useful energy that enters each trophic level — chemical energy in the form of food — decreases as it is used by the organisms in that level. The top trophic level is the smallest because *only part of the energy in one trophic level will become part of the next trophic level.*

*State the meaning of the arrow in a food web or chain.* Understanding: Chemical energy in carbon compounds flows through food chains by means of feeding.

Arrows in food chains and webs represent the flow of energy and nutrients through the trophic levels.

*List three groups of autotrophs.* Understanding: Most ecosystems rely on a supply of energy from sunlight.

Autotrophs are organisms that produces complex organic compounds from inorganic substances, generally using energy from light or inorganic chemical reactions. Common autotrophs are: -plants -algae -cyanobacteria

*Explain why there is a limited number of organisms in a food chain.​* Nature of Science: Use theories to explain natural phenomena- the concepts of energy flow explains the limited length of food chains.

Because only a small proportion (10 %) of energy can pass from one trophic level to the next, the *consumers at the top of an energy pyramid have much less energy available to support them than those closer to the bottom.* *The amount of useful energy left can’t support another level.* There is not enough energy for a 4th/5th/later stages of a food chain. Most of the energy that enters a community is ultimately lost to the living world as heat released by cell respiration, biomass not consumed (i.e. bones/hair) or biomass lost as waste (i.e. in feces/urine).

*Define "biomass."* Understanding: Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels.

Biomass is the *mass of living biological organisms in a given area or ecosystem at a given time.*

*Outline three reasons why the amount of energy decreases at higher trophic levels.* Understanding: Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels.

Energy decreases as it moves up trophic levels because *energy is lost as metabolic heat* at each trophic level. Additionally, some chemical energy is *excreted in feces* at each trophic level. Lastly, *not all of the biomass at each level is consumed* during feeding, the unconsumed biomass is a source of loss of chemical energy from the trophic level.

*State the unit used for communicating the energy in each trophic level of a food chain.* Understanding: Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels.

Energy flow in an ecosystem is measured as energy per unit area per unit time: *kJ m-2 yr-1* "kilojoules per square meter per year" kJ = kilojoules, a measure of energy m-2 = square meter; a measure of area yr-1 = year; a measure of time

*Draw a pyramid of energy given data for an ecosystem.​* Skill: Quantitative representations of energy flow using pyramids of energy.

Energy pyramids should be *drawn as stepped,* not pyramidal. *Each bar of the energy pyramid should be labeled* with the trophic level it represents and the relative amount of energy in the trophic level.. The width of the bars should substantially decrease at each trophic level*(at least 1/5), to depict the average 10% of energy moved between trophic levels.

*Outline how ATP is formed, referencing exothermic and endothermic reactions.* Understanding: Energy released from carbon compounds by respiration is used in living organisms and converted to heat.

Formation of ATP: *ADP + Pi --> ATP + H2O* (this reaction is *endothermic,* with the energy input coming from the oxidation of carbon compounds such as carbohydrates and lipids) Hydrolysis of ATP *ATP + H2O --> ADP + Pi* (this reaction is *exothermic*)

*Outline the reason why respiration releases heat.* Understanding: Energy released from carbon compounds by respiration is used in living organisms and converted to heat.

Heat energy is released as glucose burns. This is combustion. During cell respiration *glucose is broken down gradually by a series of reactions*, each catalysed by a different enzyme. This releases energy in small amounts so that it can be used by cells. However, *metabolism is far from efficient at capturing the energy in glucose in the form of ATP.* A large fraction of the energy is lost as heat in the process of making ATP.

*Outline how light energy is converted to chemical energy.​* Understanding: Light energy is converted to chemical energy in carbon compounds by photosynthesis.

Light energy is converted to chemical energy through *photosynthesis.* Producers absorb wavelengths of light using photosynthetic pigments and convert this light energy into chemical energy.

*List three reasons why living organisms need energy for cell activities.* Understanding: Energy released from carbon compounds by respiration is used in living organisms and converted to heat.

Maintaining a living system requires an input of energy for cell activities such as *synthesizing large molecules* (such as DNA and proteins), *active transport* of molecules and ions across the cell membrane, *movement of structures within the cell* (such as vesicles and chromosomes) and *contraction of proteins* (such as during cytokinesis).

*State the average amount of energy passed through each trophic level of a food chain.* Understanding: Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels.

Only a fraction of the energy available at one trophic level is transferred to the next trophic level; the fractions can vary between 1-15%, with an *average value of 10%.*