Biology • Year 11 • Module 4 • Lesson 2
Autotrophs, Heterotrophs and Saprotrophs
Lock in the three feeding strategies, the key terms, and the difference between energy flow and matter cycling in ecosystems.
1. Term–definition match
The eight definitions below are shuffled. In the right-hand column write the matching term from this list: autotroph, heterotroph, saprotroph, producer, decomposer, nutrient cycling, chemoautotroph, detritivore. 8 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | An organism that obtains organic molecules by consuming other organisms or their products. | |
| 1.2 | An organism that externally digests dead organic matter and absorbs the soluble products. | |
| 1.3 | An autotroph that forms the base of most food webs by converting light energy into chemical energy stored in glucose. | |
| 1.4 | The movement and exchange of organic and inorganic matter back into the production of living matter. | |
| 1.5 | An organism that produces organic molecules from inorganic inputs using energy from light or chemicals. | |
| 1.6 | An organism (typically a saprotroph) that breaks down dead organic material and releases inorganic nutrients. | |
| 1.7 | An autotroph that obtains energy from chemical reactions rather than light — important in deep-sea vent communities. | |
| 1.8 | A heterotroph that ingests dead matter and digests it internally (e.g. earthworms, some crustaceans). |
2. Classify the organisms
For each organism, tick one box to classify its feeding strategy and write one piece of evidence from the lesson that supports your choice. 10 marks (1 per row)
| Organism | Autotroph | Heterotroph | Saprotroph | Evidence from lesson |
|---|---|---|---|---|
| Cyanobacteria on a billabong surface | ||||
| Fungi growing on a fallen eucalypt log | ||||
| A kangaroo grazing on grass | ||||
| Coral reef algae (zooxanthellae) | ||||
| Soil bacteria decomposing leaf litter | ||||
| A wedge-tailed eagle hunting a rabbit | ||||
| Chemosynthetic bacteria at a hydrothermal vent | ||||
| A tapeworm living in a kangaroo intestine | ||||
| Eucalyptus trees in dry sclerophyll forest | ||||
| Earthworms consuming dead plant material |
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version in the space provided. 8 marks (1 for T/F, 1 for correction where needed)
3.1 Plants are the only organisms that can act as autotrophs in an ecosystem. T / F
3.2 Saprotrophs and detritivores use the same feeding mechanism. T / F
3.3 Energy flows through ecosystems and is eventually lost as heat; matter is recycled continuously. T / F
3.4 Without decomposers, nutrients released from dead organisms would remain locked in organic compounds, reducing producer growth. T / F
4. Function recall
Answer each question in 1–2 sentences using precise lesson terms. 8 marks (2 each)
4.1 What is the function of autotrophs in terms of energy entry into a food web?
4.2 What is the function of saprotrophs in nutrient cycling?
4.3 Why do food webs rarely extend beyond four or five trophic levels?
4.4 What feature of fungal hyphae makes fungi particularly effective as saprotrophs?
5. Complete the nutrient cycling diagram
The boxes below show the three groups of organisms in an ecosystem nutrient cycle. Fill in the blanks and answer the questions below. 6 marks
(____________ )
e.g. plants, algae
(____________ )
e.g. herbivores, carnivores
(____________ )
e.g. fungi, bacteria
↪ Decomposers release nutrients (N, P, K) back to producers — completing the cycle
5.1 Fill in the three feeding-strategy terms in the brackets in the diagram above.
5.2 What enters this cycle from outside (i.e. what energy source powers the whole system)?
5.3 What eventually happens to all the energy that enters via producers?
Q1 — Term–definition matches
1.1 heterotroph • 1.2 saprotroph • 1.3 producer • 1.4 nutrient cycling • 1.5 autotroph • 1.6 decomposer • 1.7 chemoautotroph • 1.8 detritivore
Marking notes. 1 mark per correct match (8 marks total). Saprotroph and detritivore are both acceptable for 1.6 if a student uses them interchangeably, but full credit requires distinguishing them where both terms appear.
Q2 — Classify the organisms
Cyanobacteria → Autotroph (photosynthesis; form mats in aquatic ecosystems). Fungi on log → Saprotroph (external digestion of dead organic matter). Kangaroo → Heterotroph (consumes other organisms). Zooxanthellae → Autotroph (photosynthetic; provide up to 90% of coral energy). Soil bacteria decomposing litter → Saprotroph (external digestion and absorption of nutrients). Wedge-tailed eagle → Heterotroph (carnivore; consumes other organisms). Hydrothermal vent bacteria → Autotroph (chemoautotroph; chemical energy source not light). Tapeworm → Heterotroph (absorbs digested nutrients from host; internal consumer / parasite). Eucalyptus → Autotroph (photosynthetic plant). Earthworms → Heterotroph / detritivore (ingest dead matter, digest internally).
Marking notes. 1 mark per row for correct classification plus a valid evidence phrase. Do not penalise if a student calls earthworms saprotrophs — accept with a note about the internal/external digestion distinction.
Q3 — True / false with correction
3.1 False. Correction: algae, cyanobacteria and some bacteria are also autotrophs; in aquatic ecosystems algae and cyanobacteria are often the primary producers rather than plants.
3.2 False. Correction: saprotrophs externally digest dead matter and absorb nutrients, whereas detritivores ingest dead matter and digest it internally (e.g. earthworms).
3.3 True.
3.4 True.
Marking notes. 1 mark for T/F identification + 1 mark for a correct, specific correction where the statement is false.
Q4.1 — Function of autotrophs as energy entry point
Autotrophs capture light energy and convert it into chemical energy stored in organic molecules (e.g. glucose via photosynthesis). They are the entry point for almost all energy into food webs — every joule of energy in the web originally came from an autotroph. [1 mark for energy entry; 1 mark for mechanism/photosynthesis reference]
Q4.2 — Function of saprotrophs in nutrient cycling
Saprotrophs externally digest dead organic matter using secreted enzymes, releasing inorganic nutrients (nitrogen, phosphorus, potassium) back into the soil or water. These nutrients can then be taken up again by producers, maintaining ecosystem productivity. [1 mark for external digestion/release of inorganic nutrients; 1 mark for stating producers can reuse them]
Q4.3 — Why food webs have 4–5 trophic levels
At each trophic level transfer approximately 90% of energy is lost as heat through metabolic processes, leaving only about 10% available for the next level. By the fourth or fifth level, so little energy remains that it cannot support a viable population of higher predators. [1 mark for the ~90% heat loss figure; 1 mark for explaining insufficient energy remains for higher levels]
Q4.4 — Fungal hyphae and surface area
Fungal hyphae are extremely thin (2–10 µm diameter), giving fungi an enormous surface-area-to-volume ratio. This maximises the contact area over which digestive enzymes can be secreted and through which nutrients can be absorbed from dead organic matter. (A single cm³ of soil can contain over 100 m of fungal hyphae.) [1 mark for identifying large SA:V ratio from thin hyphae; 1 mark for linking this to increased enzyme secretion/nutrient absorption]
Q5 — Nutrient cycling diagram
5.1 Producers = autotrophs; Consumers = heterotrophs; Decomposers = saprotrophs. [1 mark each, 3 marks total]
5.2 Light energy (sunlight), captured by producers via photosynthesis. [1 mark]
5.3 Energy is lost as heat at each trophic level through metabolic processes (cellular respiration). It cannot be recycled and must be continuously replenished by producers capturing new light energy. [1 mark for “lost as heat”; 1 mark for noting it cannot be recycled]