4.2: Energy flow
Teaching time: 3 hours Practical time: 1 hour
Key Vocabulary
Retrieval practice and prior learning
Review 2.8 cell respiration
Review 2.9 photosynthesis
Sunlight and ecosystems
Essential idea: Ecosystems require a continuous supply of energy to fuel life processes and to replace energy lost as heat.
Nature of science: Use theories to explain natural phenomena—the concept of energy flow explains the limited length of food chains. (2.2)
U1: Most ecosystems rely on a supply of energy from sunlight.
Exercise 1: Read page 213 of Allott & Mindorff and answer the following questions:
What is the initial source of energy for most ecosystems? (Can you remember the definition of an ecosystem? Can you research any ecosystems that don't require sunlight?)
How is sunlight energy harvested?
What is the difference between the words "autotroph" and "producer"?
How are heterotrophs indirectly dependent on sunlight energy?
Why do the redwood forests of California produce more energy for consumers than the Sahara desert?
Pyramids of energy
S1: Quantitative representations of energy flow using pyramids of energy.
Guidance:
Pyramids of number and biomass are not required. Students should be clear that biomass in terrestrial ecosystems diminishes with energy along food chains due to loss of carbon dioxide, water and other waste products, such as urea.
Pyramids of energy should be drawn to scale and should be stepped, not triangular. The terms producer, first consumer and second consumer and so on should be used, rather than first trophic level, second trophic level and so on.
Exercise 2: Construct a labelled pyramid of energy following the steps below.
Click the Model Ecosystems simulation below
Read the left hand side information and instructions
Follow steps 1-5 on the simulation
Once you have your energy values by clicking on pyramid of energy construct a stepped pyramid using graph paper:
Choose a scale e.g. 1000 energy units = 1cm
Decide on the height of each bar
Calculate the length of each bar using the experimental data
Draw the bar for the producers at the bottom or your graph paper
Draw the subsequent bars on top
Exercise 3: Complete the following steps
U2: Light energy is converted to chemical energy in carbon compounds by photosynthesis.
Indicate on your pyramid where light energy enters the ecosystem
U3: Chemical energy in carbon compounds flows through food chains by means of feeding.
Indicate where feeding occurs on your pyramid
U4: Energy released from carbon compounds by respiration is used in living organisms and converted to heat.
U5: Living organisms cannot convert heat to other forms of energy.
U6: Heat is lost from ecosystems.
Indicate on your diagram how energy is lost from ecosystems
U7: Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels.
Read the appropriate pages of your textbook and answer the following questions:
Why is energy lost between trophic levels in an ecosystem?
Why does this restrict the length of food chains within food webs?
The distinction between energy flow in ecosystems and cycling of inorganic nutrients should be stressed. Students should understand that there is a continuous but variable supply of energy in the form of sunlight but that the supply of nutrients in an ecosystem is finite and limited.
