The first law at Higher Level: The same law as SL, followed through the Southern Ocean, from phytoplankton to blue whales. At HL it underpins the energy accounting you will do with productivity and ecological efficiency.
Practise this as you read
- Account for every unit of energy: stored, or out as heat.
- Name the form of energy before and after each change.
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Energy is never made or destroyed: The first law of thermodynamics is a rule that holds everywhere. As energy flows through an ecosystem it changes form, but the total amount stays the same: energy is conserved.
The points to remember
- The first law of thermodynamics: energy cannot be created or destroyed.
- It can only be transformed into another form, or transferred to another place.
- So all the energy that enters an ecosystem is either stored there or leaves it.
- Energy is never 'used up': in the end it becomes heat, which spreads out and leaves.
Remember it as: Energy in = energy stored + energy out.
Real example: a blue whale can eat about four tonnes of krill in a day. None of that energy is destroyed: it becomes whale body, swimming, and heat.
Not 'used up': Never write that energy is 'used up' or 'lost' and stop there. Say what it became: usually heat.
The second law, that every change wastes some energy as heat, has its own page (2.2.8).
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As energy moves along a food chain it keeps changing form. Each change of form is a transformation.
The transformations to know
- Light to chemical: photosynthesis in producers stores light energy in glucose.
- Chemical to chemical: an animal turns the food it eats into its own body tissue.
- Chemical to kinetic: muscles turn the energy in food into movement.
- Chemical to heat: respiration in every living thing releases heat.
- A transfer moves energy without changing its form, such as food passing from zebra to lion (see 1.2.4).
Remember it as: Light, chemical, movement, heat: it changes form, it never disappears.
Real example: in the Southern Ocean, phytoplankton turn sunlight into chemical energy; krill eat them and turn that energy into swimming and heat.
Name both forms: For each transformation, name the form before and after: 'light energy to chemical energy', not just 'photosynthesis'.
The first law also holds in the air above us. A question may ask you to compare how energy moves through the atmosphere and through an ecosystem.
The same in both
- In both, the Sun is the source of the energy.
- In both, energy is not created or destroyed, only transformed.
- In both, sunlight is reflected, absorbed or passed on.
- In both, energy finally leaves as radiated heat.
| What happens to sunlight | In the atmosphere | In an ecosystem |
|---|---|---|
| Reflected by | clouds | light surfaces such as ice and snow |
| Absorbed by | clouds and greenhouse gases | the ground, water, soil and living things |
| Turned into chemical energy | never | by photosynthesis in producers |
| Lost as heat by | greenhouse gases radiating it | respiration all along the food chains |
| Passes through living things | no | yes |
Greenhouse gases absorb heat and radiate it again, some back to the ground and some out to space.
Pair each contrast: Write each difference as a pair: 'in the atmosphere, heat is radiated by greenhouse gases, whereas in an ecosystem it is released by respiration'. Real example: the dark green canopy of the Amazon absorbs most of the sunlight that reaches it.
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How this comes up: Paper 2, Section B (a): outline how a named food chain shows the first law of thermodynamics [4].
In the Southern Ocean, phytoplankton are eaten by krill, and krill are eaten by blue whales.
Outline how energy flow along this food chain illustrates the first law of thermodynamics.
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