Biology • Year 11 • Module 2 • Lesson 20

Autotrophs vs Heterotrophs, Full Synthesis

Build HSC Band 5–6 extended-response technique on the full Module 2 synthesis, comparing organisation, gas exchange, and transport across autotrophs and heterotrophs.

Master · Extended Response

1. Extended response, compare gas exchange in plants and mammals (Band 5–6)

7 marks   Band 5–6

Q1. Compare and explain the mechanisms by which a plant and a mammal each exchange gases with their environment. In your response you must:

  • Name the specific structures used for gas exchange in each organism.
  • Identify the gases being exchanged and the direction of net exchange in each case (for the plant, address both daytime and night-time conditions).
  • Explain the physical principle that drives gas exchange in both organisms.
  • Explain how each organism maintains the concentration/partial pressure gradients necessary for diffusion.
  • Reach a comparative conclusion about similarities and differences in the mechanism.
Stuck? Plan: plant structures (stomata/lenticels) vs mammal structures (alveoli) → gases + direction (day vs night for plant) → diffusion / Fick’s law → gradient maintenance (photosynthesis + respiration for plant; ventilation + blood flow for mammal) → comparison sentence.

2. Full module synthesis, leaf cell vs liver cell (Band 5–6)

8 marks   Band 5–6

Stimulus. A leaf mesophyll cell in a eucalyptus tree and a liver cell in a human both belong to organisms classified as “eukaryotes.” Yet at the cellular level they operate in fundamentally different ways: the leaf cell simultaneously performs photosynthesis and cellular respiration; the liver cell performs cellular respiration and produces urea. Both cells require continuous delivery of materials and removal of wastes, yet the systems that supply them could not be more different.

Q2. Using the stimulus and your knowledge of Module 2, compare the organisation of a plant and a mammal with reference to how each supplies its cells with the materials needed for cellular respiration and removes metabolic wastes. In your answer:

  • Explain how each organism acquires its raw materials (nutrients and gases) from the external environment.
  • Describe the transport system each organism uses to deliver materials to individual cells.
  • Explain how each organism removes metabolic waste products.
  • Use specific named structures throughout (e.g. xylem, phloem, alveoli, stomata, villi, capillaries).
  • Reach a conclusion about what both systems have in common despite their structural differences.
Stuck? Work through Card 4’s five steps (acquire → gas exchange → transport → energy production → waste removal) for each organism in parallel. Both converge on ATP production at Step 4.

3. Evaluate this claim (Band 5–6)

6 marks   Band 5–6

“Plants and animals are completely different organisms with nothing biologically in common. Plants make their own food using the sun and animals eat other things. A plant in full sunlight only releases oxygen and never produces any carbon dioxide. Blood and xylem sap are both just types of ‘plant blood’, both do the same job in the same way.”

Q3. Evaluate this claim. Identify which parts contain biological errors, explain why each is wrong using lesson content, and reformulate the claim into a scientifically accurate statement of no more than three sentences.

Stuck? The claim contains at least four errors. Find each one: (1) “completely different”, (2) O&sub2;-only claim, (3) xylem = “plant blood”, (4) same job / same way. Use Cards 2, 3, and 4 of Lesson 20.
Answers, Do not peek before attempting

Q1, Sample Band 6 response (7 marks), annotated

Both plants and mammals exchange gases by diffusion down concentration or partial pressure gradients. [1, shared physical principle]

In a plant, the primary structures for gas exchange are stomatapores in the leaf epidermis controlled by guard cells, and, secondarily, lenticels in stems. During daylight, when the rate of photosynthesis exceeds cellular respiration, the plant is a net consumer of CO&sub2; (absorbed through stomata) and a net producer of O&sub2; (released through stomata). The gradient driving CO&sub2; in is maintained by the Calvin cycle consuming CO&sub2; inside leaf cells, keeping its concentration low. [1, plant structures + daytime direction with gradient maintenance] At night, photosynthesis ceases because no light energy is available. Cellular respiration continues in all living plant cells, producing CO&sub2; and consuming O&sub2;, making the plant a net CO&sub2; producer and O&sub2; consumer, identical to an animal at all times. [1, night-time behaviour correctly described]

In a mammal, gas exchange occurs at the alveoli in the lungs. O&sub2; diffuses from alveolar air (high partial pressure) into pulmonary capillary blood (lower partial pressure); CO&sub2; diffuses in the reverse direction. At all times, the mammal is a net O&sub2; consumer and CO&sub2; producer, because cellular respiration in every tissue continuously consumes O&sub2; and produces CO&sub2;. Ventilation (breathing) maintains the alveolar O&sub2; concentration high and CO&sub2; concentration low; continuous blood flow through pulmonary capillaries prevents equilibration of the gradient. [1, mammal structures + direction + gradient maintenance by ventilation and blood flow]

Both organisms rely on the same physical mechanism, diffusion down gradients, consistent with Fick’s law, and both structures feature thin membranes and large surface areas to maximise exchange rate. [1, Fick’s law / structural similarity] The key difference is that in the plant, the direction of net exchange reverses between day and night (because photosynthesis adds a second gas exchange process only present in light), while in the mammal the direction is constant regardless of time of day. [1, key comparative difference: light-dependent reversal in plant vs constant in mammal] Both organisms also use their respective transport systems (xylem/phloem for the plant; cardiovascular system for the mammal) to connect their exchange surfaces to individual cells, ensuring every cell receives O&sub2; for cellular respiration. [1, linking gas exchange to transport system]

Marking criteria.

  • 1 markIdentifies diffusion (down concentration or partial pressure gradients) as the shared physical principle, consistent with Fick’s law.
  • 1 markNames stomata (and optionally lenticels) as plant structures; correctly describes daytime direction (CO&sub2; in, O&sub2; out net) and explains gradient maintenance (Calvin cycle removes CO&sub2;, light reactions split water releasing O&sub2;).
  • 1 markCorrectly describes night-time behaviour: photosynthesis ceases, only respiration occurs, net CO&sub2; out and O&sub2; in, same direction as a heterotroph.
  • 1 markNames alveoli (or gills / equivalent) for mammals; correctly describes direction (O&sub2; in, CO&sub2; out at all times) and explains gradient maintenance by ventilation and blood flow.
  • 1 markReferences Fick’s law / large SA / thin membranes as shared structural features enabling diffusion.
  • 1 markMakes an explicit comparative statement distinguishing how the plant’s net exchange reverses with light conditions while the mammal’s remains constant.
  • 1 markLinks gas exchange to the organisms’ respective transport systems in a closing synthesis statement.

Q2, Sample Band 6 response (8 marks), annotated

Despite being separated by billions of years of evolution, a eucalyptus leaf cell and a human liver cell share one requirement: a continuous supply of O&sub2; and organic substrate for cellular respiration, and removal of the waste products CO&sub2; and urea. How each organism meets this requirement is fundamentally different.

(i) Acquiring raw materials: The plant acquires carbon by fixing atmospheric CO&sub2; through stomata in leaves, using light energy captured by chlorophyll to drive the Calvin cycle inside chloroplasts. Water and minerals (NO&sub3;−, K+, Ca2+, PO&sub4;3−) are absorbed from soil by root hairs using active transport, regulated by the Casparian strip. [1, plant acquisition: CO&sub2; via stomata, water/minerals via root hairs] The mammal acquires organic nutrients by ingesting food, digesting it physically (mechanical breakdown) and chemically (enzymes: amylase, protease, lipase), and absorbing the resulting monomers (glucose, amino acids, fatty acids) across the large surface area of villi and microvilli in the small intestine. O&sub2; is acquired at the alveoli by diffusion. [1, mammal acquisition: digestion, villi, alveoli]

(ii) Internal transport: The plant uses two separate vascular tissues. Xylem transports water and dissolved minerals passively from root to leaf by cohesion-tension (transpiration pull driven by solar energy). Phloem transports sucrose and amino acids from source leaves (where organic molecules are made) to sink tissues (growing roots, fruits) by active loading at the source and pressure-flow to the sink, requiring ATP. [1, xylem function; 1, phloem active loading + source-to-sink] The mammal uses a closed cardiovascular system: the heart pumps blood through arteries to capillary beds in every tissue, where O&sub2; and glucose diffuse into cells and CO&sub2; and metabolic waste diffuse out; veins return blood to the heart. A single transport medium (blood) carries all substances simultaneously. [1, closed cardiovascular system, heart pump, capillary exchange]

(iii) Waste removal: The plant releases CO&sub2; as a respiratory waste product through stomata (net release at night; masked by photosynthesis during the day). Plants do not produce urea because they recycle nitrogenous compounds; some metabolic wastes are stored in vacuoles. [1, plant CO&sub2; via stomata; no urea] The mammal expels CO&sub2; through alveolar ventilation. The liver produces urea via deamination of excess amino acids; urea enters blood and is filtered by the kidneys into urine for excretion. [1, mammal: CO&sub2; via lungs, urea via liver→kidneys]

Despite these structural differences, both systems apply the same underlying physical principles, diffusion, osmosis, and concentration gradients, to supply every cell with the materials it needs for cellular respiration. Both converge on the same endpoint: ATP production in mitochondria, powering all cellular processes. [1, unifying conclusion: shared physical principles, convergence on ATP production]

Marking criteria.

  • 1 markPlant raw material acquisition: CO&sub2; via stomata and water/minerals via root hairs / Casparian strip.
  • 1 markMammal raw material acquisition: digestion (physical + chemical) and absorption via villi; O&sub2; via alveoli.
  • 1 markXylem: passive transport of water and minerals by cohesion-tension from root to leaf.
  • 1 markPhloem: active loading of sucrose at source; pressure-flow transport to sinks; requires ATP.
  • 1 markMammalian cardiovascular system: heart pumps blood through arteries to capillary beds; exchange of O&sub2;, glucose, CO&sub2;, wastes; veins return blood.
  • 1 markPlant waste: CO&sub2; via stomata (net at night); no urea produced; some wastes in vacuoles.
  • 1 markMammal waste: CO&sub2; expelled via ventilation at alveoli; urea produced by liver (deamination), excreted by kidneys.
  • 1 markSynthesis conclusion: both systems use diffusion/osmosis/concentration gradients and converge on ATP production via cellular respiration in mitochondria.

Q3, Sample Band 6 response (6 marks)

The claim contains multiple biological errors. [1, overall evaluative judgement]

Error 1, “completely different with nothing in common”: Plants and animals share fundamental cellular processes: both use mitochondria for cellular respiration, both require O&sub2; and produce CO&sub2; continuously, both use diffusion and osmosis for transport at the cellular level, and both are organised into cells, tissues, organs and systems. [1, refutes “nothing in common” with specific shared features]

Error 2, “a plant in full sunlight only releases oxygen and never produces CO&sub2;”: This is the most common HSC misconception. A plant performs cellular respiration continuously in all living cells, 24 hours a day, which produces CO&sub2; and consumes O&sub2;. In bright sunlight, photosynthesis exceeds respiration so there is net O&sub2; release, but CO&sub2; is still being produced by respiration at the same time. [1, correctly refutes the “no CO&sub2; in sunlight” misconception with continuous respiration explanation]

Error 3, “blood and xylem sap are both just plant blood”: Xylem sap carries water and inorganic minerals only; blood carries O&sub2; (bound to haemoglobin), CO&sub2;, glucose, amino acids, urea, hormones and many other substances. Xylem uses passive cohesion-tension; blood is pumped actively by the heart. Xylem is dead at maturity; blood vessels are living tissues. The two systems are structurally and functionally very different. [1, refutes the “same job / same way” claim with specific structural and compositional differences]

Error 4, incomplete transport picture: The claim ignores phloem, which transports sucrose and amino acids in plants, a function much more analogous to aspects of blood transport (organic molecules delivered to living cells). [1, identifies the missing phloem component as an additional inaccuracy]

Defensible reformulation: “Plants (autotrophs) and animals (heterotrophs) differ in how they acquire energy and organic nutrients, but both share fundamental cellular processes including cellular respiration, which requires a continuous supply of O&sub2; even in plants. A plant in sunlight still produces CO&sub2; via respiration; only the net gas exchange is altered because photosynthesis dominates. Plant transport uses two separate vascular tissues (xylem for water and minerals; phloem for organic molecules), while mammalian transport uses a single closed cardiovascular system with a muscular heart, making them structurally distinct but both serving the common purpose of supplying all cells with the materials needed for respiration.” [1, scientifically accurate reformulation covering all corrected errors]

Marking criteria.

  • 1 markStates an overall evaluative judgement (e.g. “the claim contains multiple biological errors”).
  • 1 markCorrectly refutes “nothing in common” by naming at least two shared biological features (e.g. cellular respiration, mitochondria, diffusion, cell organisation).
  • 1 markCorrectly refutes the “only releases O&sub2; in sunlight” error: respiration is continuous and produces CO&sub2; in all light conditions; only net O&sub2; release is observed in bright light.
  • 1 markCorrectly refutes the “xylem = plant blood” error with specific compositional or structural differences (water/minerals vs blood substances; passive vs active pumping; dead vs living tissue).
  • 1 markIdentifies at least one further inaccuracy (e.g. phloem ignored; xylem and blood do not perform the same job in the same way).
  • 1 markProvides a scientifically accurate reformulation of the claim that corrects all major errors and uses precise lesson terminology.

HSCScience • Biology • Year 11 • Module 2 • Lesson 20 • Worksheet 3 (Master)