AP Syllabus focus:
‘Autotrophs capture physical or chemical energy; heterotrophs obtain energy by consuming organic matter from autotrophs.’
Energy flow in ecosystems begins when certain organisms capture energy from the environment and store it in organic molecules. That stored chemical energy then powers all other organisms through feeding and metabolism.
Core idea: how energy enters and moves through ecosystems
Autotrophs: the entry point for new energy
Autotrophs convert physical energy (light) or chemical energy (inorganic redox reactions) into chemical energy stored in organic molecules (biomass).
This diagram summarizes chemosynthesis as an alternative pathway for primary production in ecosystems without sunlight (e.g., hydrothermal vents and cold seeps). It shows how microbes use chemical energy from reduced inorganic compounds to fix into organic molecules that then support higher trophic levels. Source
This conversion is what makes energy available to the rest of the ecosystem.
Autotroph: An organism that synthesises organic molecules from inorganic carbon (e.g., ) using energy from light or inorganic chemical reactions.
Key features of autotroph-driven energy capture:
Photosynthetic autotrophs use light energy to build sugars and other carbon compounds.
Chemoautotrophs use energy released by oxidising inorganic substances (e.g., ammonia, hydrogen sulfide) to fix carbon.
The energy stored as biomass is the “fuel” heterotrophs can access by eating or absorbing organic matter.
Heterotrophs: accessing stored chemical energy
Heterotrophs cannot capture external energy to build all needed organic molecules from inorganic carbon. Instead, they obtain energy and carbon by consuming organic matter produced by autotrophs (directly or indirectly).
Heterotroph: An organism that obtains energy and organic carbon by consuming or absorbing organic molecules produced by other organisms.
Heterotroph roles closely tied to energy flow:
Consumers ingest other organisms or their parts, transferring chemical energy through feeding.
Decomposers and detritivores break down dead organic material and wastes; they obtain energy from these molecules while reducing complex organics into simpler compounds.
Energy in biomass vs energy used for life processes
Even though autotrophs introduce energy into ecosystems, organisms use much of that energy for cellular work rather than building new biomass. This matters because only energy stored as new biomass is readily available to other organisms at the next feeding step.
Primary productivity (biomass formation by autotrophs)
Autotrophs allocate captured energy into:
Growth and storage (adds to biomass)
Maintenance and activity (released as heat during respiration)
A common way to express how much autotroph biomass is actually added (and thus available to heterotrophs) is:
= Rate of biomass energy stored after autotroph respiration; typically per area per time (e.g., kJ m yr)
= Total rate of energy capture and conversion to chemical energy by autotrophs; per area per time
= Energy used in autotroph respiration; per area per time
Why energy transfer is inefficient (and why that’s normal)
Energy transfer from autotroph biomass to heterotroph biomass is limited because:
This figure compares ecological pyramids (numbers, biomass, and energy) across trophic levels, making the ‘bottom-heavy’ nature of energy flow visually explicit. It supports the idea that only a fraction of energy stored as producer biomass becomes available to higher trophic levels, with progressively less energy supporting each level. Source
Not all biomass is eaten.
Not all consumed material is digested/assimilated (some is egested/excreted).
Much assimilated energy is used in respiration for movement, transport, and homeostasis, leaving less for growth.
What to be able to do for AP Biology
Distinguish energy capture (autotrophs) from energy acquisition by consumption (heterotrophs).
Explain that energy enters ecosystems primarily through autotrophs and then moves as chemical energy in organic molecules.
Connect productivity to how much energy becomes biomass that can support heterotrophs.
Describe why energy available to organisms decreases at each feeding step due to metabolic energy loss as heat.
FAQ
Mixotrophs can do both, depending on conditions.
They may photosynthesise when light is available and switch to consuming organics when light or nutrients limit photosynthesis.
Classification is context-dependent: describe the strategy used in the scenario.
Energy source availability is key.
Light penetration, turbidity, and seasonality favour or limit photoautotrophy.
Presence of reduced inorganic compounds (e.g., near vents, anoxic sediments) supports chemoautotrophy.
They often estimate energy indirectly.
Measure dry mass production and convert using energy content (calorimetry-derived conversion factors).
Use gas exchange (e.g., $CO_2$ uptake) or oxygen production as proxies for productivity.
They are powered by chemical disequilibria.
Oxidation of inorganic molecules releases energy that can be captured to fix carbon.
Ecosystem size is constrained by the supply rate of those inorganic substrates, not by light.
Isotope ratios can act as tracers.
Distinct $\delta^{13}C$ signatures can separate carbon fixed by different autotrophic pathways or sources.
Consumers reflect the isotopic composition of their diet with predictable fractionation patterns.
Practice Questions
State the key difference between an autotroph and a heterotroph in how each obtains energy and carbon. (2 marks)
Autotroph captures physical/chemical energy to synthesise organic molecules from inorganic carbon (1)
Heterotroph obtains energy and organic carbon by consuming/absorbing organic matter made by autotrophs/other organisms (1)
(5 marks) In an ecosystem, autotrophs have a GPP of kJ m yr and respire kJ m yr.
Explain what NPP represents and why NPP constrains heterotroph biomass.
Uses relationship (1)
NPP is the rate of energy stored as new autotroph biomass after respiration (1)
Heterotrophs depend on autotroph organic matter as their energy/carbon source (1)
Energy used in autotroph respiration is not available to heterotrophs (lost as heat/metabolic work) (1)
Additional reduction occurs because heterotroph transfer is inefficient (not all eaten/assimilated; respiration by heterotrophs) (1)
