HSC Exam Practice

Sample response. An autotroph is an organism that produces its own organic molecules from inorganic sources, using an external energy source such as light (e.g. plants via photosynthesis). A heterotroph is an organism that obtains organic molecules by consuming other organisms or their products.

Marking notes. 1 mark for autotroph: must reference production of own organic molecules from inorganic sources. 1 mark for heterotroph: must reference obtaining organic molecules by consuming organisms or products. A definition that only states “makes its own food” or “eats other organisms” without specifying organic molecules from inorganic sources scores 0 for that term.

Sample response. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy required). Photosynthesis occurs in the chloroplast of plant and algal cells.

Marking notes. 1 mark for correct balanced equation (all six components; must have arrows not equals sign). 1 mark for chloroplast as the location. Award the equation mark for a correct word equation if the symbol equation is wrong but all reactants and products are present.

Sample response. Autotrophs: a green plant (e.g. Eucalyptus) and a cyanobacterium (e.g. Anabaena) / algae (e.g. Spirogyra). Heterotrophs: a mammal (e.g. a human) and a fungus (e.g. Agaricus bisporus).

Marking notes. 1 mark for two correctly named autotrophs from different groups (e.g. plant + alga, or plant + cyanobacterium). 1 mark for two correctly named heterotrophs from different groups (e.g. animal + fungus, or animal + bacterium). Named genus/species not required; common names acceptable if unambiguous.

Sample response. Photosynthesis produces glucose, but glucose itself cannot be used directly to power cellular processes. Cellular respiration in the mitochondria breaks down glucose using oxygen to produce ATP, the only form of energy that cells can directly use to power active transport, protein synthesis, cell division, and other processes. Because ATP cannot be directly produced by photosynthesis alone, all living cells including autotrophs must continuously perform cellular respiration to convert glucose into usable ATP energy.

Marking notes. 1 mark for identifying that glucose produced in photosynthesis cannot be used directly by cells. 1 mark for stating that cellular respiration converts glucose into ATP (usable energy). 1 mark for identifying that ATP is required for cellular processes and is produced by cellular respiration (not photosynthesis), explaining why respiration is necessary even in autotrophs.

Sample response. The statement is correct. At night, photosynthesis stops completely because light is unavailable to drive the reactions. Only cellular respiration continues in the plant, consuming O₂ and releasing CO₂the same gas exchange pattern as an animal.

Marking notes. 1 mark for correctly stating the student is right (not awarding marks if the student says it is wrong). 1 mark for the justification: at night photosynthesis stops (no light), so only cellular respiration occurs, producing CO₂ and consuming O₂.

Sample response. Chamber A (plant): CO₂ rises slightly in the initial dark period (~430 to ~460 ppm), then falls sharply during the light period (~460 to ~260 ppm), then rises again in the dark (~260 to ~420 ppm), a V-shaped pattern. Chamber B (mouse): CO₂ increases continuously and approximately linearly throughout the entire 24 hours (~350 to ~650 ppm), regardless of light conditions.

Marking notes. 1 mark for correctly describing Chamber A (V-shape or equivalent: rise in dark, fall in light, rise in dark) with at least one approximate data value cited. 1 mark for correctly describing Chamber B (continuous increase, unaffected by light) with at least one data value cited.

Sample response. During the light period, the plant (autotroph) is performing both photosynthesis and cellular respiration simultaneously. Photosynthesis consumes CO₂ as a reactant to produce glucose, while respiration produces CO₂ as a byproduct. Because the rate of photosynthesis exceeds the rate of respiration during bright daylight, more CO₂ is consumed than produced, resulting in a net decrease in chamber CO₂. The mouse (heterotroph) cannot photosynthesise, it has no chloroplasts and no mechanism to fix CO₂. Only cellular respiration occurs, continuously consuming O₂ and releasing CO₂ at a roughly constant rate throughout the day. Because the mouse’s CO₂ production is unaffected by light, its chamber CO₂ rises steadily.

Marking notes. 1 mark for identifying that the plant performs both photosynthesis and respiration simultaneously during the day. 1 mark for explaining that photosynthesis rate exceeds respiration rate, resulting in net CO₂ consumption and a decrease in chamber CO₂. 1 mark for correctly identifying that the mouse cannot photosynthesise (no chloroplasts), so only respiration occurs in Chamber B. 1 mark for explaining that the continuous and light-independent CO₂ rise in B is due to uninterrupted cellular respiration with no photosynthetic CO₂ consumption.

Sample response. Autotrophs such as plants and algae produce their own organic molecules from inorganic sources, carbon dioxide and water, using light energy captured by chlorophyll in chloroplasts. Photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) converts light energy into chemical energy stored in glucose, releasing O₂ as a byproduct. Their nutrient requirements therefore include inorganic CO₂, H₂O, light, and minerals; during daylight their net gas exchange is CO₂ in and O₂ out (because photosynthesis rate exceeds respiration rate). Heterotrophs such as humans, fungi, and most bacteria cannot fix CO₂ and cannot photosynthesise. They obtain organic molecules entirely by consuming other organisms or their products, digesting them to acquire both carbon and energy. Their gas requirements are always O₂ in and CO₂ out, at all times and in all light conditions, because only cellular respiration occurs. The key structural difference between the two groups is that autotrophs possess chloroplasts (the site of photosynthesis) while heterotrophs do not; both groups possess mitochondria. Despite their differences, autotrophs and heterotrophs share the fundamental biochemical process of cellular respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Both break down glucose in mitochondria to produce ATP, and both require O₂ and release CO₂ and H₂O during this process. The distinction between autotrophs and heterotrophs is therefore not in how glucose is used (both use it the same way), but in how it is obtained: autotrophs produce it from inorganic CO₂; heterotrophs consume it from other organisms. At night, when photosynthesis stops in autotrophs, their gas exchange becomes identical to that of heterotrophs, O₂ in, CO₂ out, confirming that the shared process of cellular respiration is the unifying biochemical foundation of all living organisms.

Marking criteria.

• 1 markCorrectly defines autotroph (organic molecules from inorganic sources, using light or chemical energy) with a named example.
• 1 markCorrectly defines heterotroph (organic molecules from consuming other organisms/products) with a named example.
• 1 markAccurately describes the role of photosynthesis in an autotroph (converts light energy to glucose; inputs CO₂ + H₂O; outputs glucose + O₂; occurs in chloroplasts).
• 1 markAccurately describes autotroph gas requirements: CO₂ in + H₂O for photosynthesis; net CO₂ in / O₂ out during daylight; O₂ in / CO₂ out at night.
• 1 markAccurately describes heterotroph gas requirements: always O₂ in / CO₂ out, regardless of light; explains absence of photosynthesis as the reason.
• 1 markIdentifies the key structural difference: autotrophs have chloroplasts; heterotrophs do not; both have mitochondria.
• 1 markIdentifies shared cellular respiration (same equation, same organelle, mitochondria, same substrates/products) as the fundamental biochemical commonality.
• 1 markUses precise lesson terminology throughout and draws a correct overall conclusion: the difference is in how glucose is obtained, not how it is used; all living organisms share cellular respiration.