Biology · Year 11 · Module 2 · Lesson 8
HSC Exam Practice
Photosynthesis, Products, Movement and Function
Short answer
1.Short answer
State the location and two key outputs of Stage 1 (light-dependent reactions) of photosynthesis.
A student labels Stage 2 of photosynthesis as the "dark reactions" and states that they occur only in darkness. Identify the error in this statement and explain why Stage 2 can occur in daylight.
Identify the five fates of glucose produced during photosynthesis in a leaf cell. For each fate, state whether it is reversible.
Explain the role of companion cells in phloem loading during the pressure-flow hypothesis.
Data response
2.Data response, leaf gas exchange under varying light intensity
The graph below shows the rate of net CO₂ uptake by a single leaf at different light intensities. Net CO₂ uptake represents the difference between photosynthesis and respiration. At zero net CO₂ uptake, the leaf has reached its light compensation point.
(a) At zero light intensity, the net CO₂ change is −2 μmol m⁻² s⁻¹. Explain what this tells you about what is happening in the leaf cells at this point. (2 marks)
(b) Describe the relationship between light intensity and net CO₂ uptake shown by the graph. Include reference to the compensation point and the plateau. (3 marks)
(c) Using Stage 2 of photosynthesis, explain why the rate of net CO₂ uptake eventually plateaus at high light intensities rather than continuing to increase. (2 marks)
Extended response
3.Extended response
Explain the transpiration-cohesion-tension theory of water movement in plants. In your response, describe the role of each component and explain how they work together to move water from the soil to the leaf canopy. You may use a labelled diagram to support your answer.
Using examples from the historical development of photosynthesis understanding, evaluate the claim that scientific knowledge progresses because each generation of scientists builds on and corrects the work of previous scientists. In your answer, refer to at least three named scientists and their experiments.
Biology · Year 11 · Module 2 · Lesson 8
Answer Key & Marking Guidelines
Section 1 · Short answer · 2 marks · Band 3
Sample response. Stage 1 (light-dependent reactions) occurs in the thylakoid membranes (grana) of the chloroplast. The two key outputs are: (1) ATP and NADPH, energy carriers passed to Stage 2; and (2) oxygen (O₂), released as a by-product of water splitting (photolysis).
Marking notes. 1 mark for location (thylakoid membranes / grana, "inside the chloroplast" alone is insufficient); 1 mark for two correct outputs (must name both ATP/NADPH as a pair OR oxygen, any two of the three are acceptable but naming only one output scores 0).
Section 1 · Short answer · 3 marks · Band 3–4
Sample response. The error is the claim that Stage 2 occurs only in darkness. Stage 2 (light-independent reactions) does not directly require light but it does not occur only in the dark, it occurs continuously during daylight hours. It only stops in darkness because Stage 1 stops producing the ATP and NADPH that Stage 2 requires. The term "dark reactions" is misleading because these reactions run at their maximum rate during full daylight, as long as Stage 1 is providing energy carriers.
Marking notes. 1 mark for identifying the specific error (Stage 2 does not occur only in darkness / "dark reactions" is misleading); 1 mark for explaining that Stage 2 requires ATP and NADPH from Stage 1; 1 mark for explaining that Stage 2 stops in darkness only because Stage 1 stops producing ATP/NADPH (not because Stage 2 itself requires darkness).
Section 1 · Short answer · 5 marks · Band 3–4
Sample response. (1) Cellular respiration, glucose oxidised in mitochondria → ATP; reversible (ATP produced and consumed continuously). (2) Starch storage, glucose polymerised into starch; reversible (starch broken back to glucose by amylase). (3) Sucrose transport, glucose converted to sucrose, loaded into phloem; reversible at sink tissues. (4) Cellulose synthesis, glucose polymerised into cellulose for cell walls; not reversible. (5) Biosynthesis, carbon skeletons + N, P, S → amino acids, lipids, nucleotides; partially reversible (proteins broken down during senescence).
Marking notes. 1 mark per correctly named fate paired with a correct reversibility statement. Fates must be distinguishable (e.g. "cellular respiration" not just "energy release"). Reversibility: respiration = reversible; starch = reversible; sucrose/phloem = reversible; cellulose = not reversible; biosynthesis = partially reversible. Accept "yes/no/partial" responses.
Section 1 · Short answer · 2 marks · Band 4
Sample response. Companion cells use ATP-driven active transport to load sucrose from surrounding mesophyll cells into adjacent phloem sieve tube elements. This lowers the water potential inside the sieve tube, drawing water in from adjacent xylem by osmosis and creating the high turgor pressure at the source end that drives bulk flow through the phloem.
Marking notes. 1 mark for stating that companion cells actively (ATP-requiring) load sucrose into sieve tube elements; 1 mark for explaining the osmotic consequence, water enters by osmosis, creating high turgor pressure at the source.
Section 2 · Data response · 2 marks · Band 4
Sample response. At zero light intensity, photosynthesis is not occurring (no light energy available). The −2 μmol m⁻² s⁻¹ net CO₂ change means the leaf is releasing CO₂, which tells us that cellular respiration is occurring in darkness, the mitochondria are consuming glucose and releasing CO₂ at a rate of 2 μmol m⁻² s⁻¹.
Marking notes. 1 mark for identifying that no photosynthesis is occurring (no light) and the value represents cellular respiration only; 1 mark for correctly interpreting the negative value as CO₂ release (not uptake) meaning respiration exceeds or is the only process occurring.
Section 2 · Data response · 3 marks · Band 4
Sample response. As light intensity increases from zero, net CO₂ uptake increases, the curve rises steeply at first. At the compensation point (approximately 100 μmol m⁻² s⁻¹ light intensity), net CO₂ uptake reaches zero, meaning the rate of photosynthesis exactly equals the rate of respiration. Above the compensation point, photosynthesis exceeds respiration and net CO₂ uptake becomes positive, continuing to rise as light intensity increases until the curve plateaus at approximately +8 μmol m⁻² s⁻¹ at high light intensities (above about 400 μmol m⁻² s⁻¹).
Marking notes. 1 mark for describing the increasing trend with reference to at least one data value; 1 mark for correctly explaining the compensation point (photosynthesis = respiration, net = 0); 1 mark for correctly identifying and describing the plateau at high light intensities.
Section 2 · Data response · 2 marks · Band 5
Sample response. The plateau occurs because Stage 2 (light-independent reactions / Calvin cycle) has become the limiting factor. Even though Stage 1 is producing maximum ATP and NADPH at high light intensities, the enzymes in Stage 2 that fix CO₂ and synthesise glucose (e.g. RuBisCO) are working at their maximum rate. The rate of CO₂ fixation cannot increase further regardless of additional light, so net CO₂ uptake plateaus.
Marking notes. 1 mark for identifying that Stage 2 / enzyme activity / CO₂ availability has become the limiting factor at high light intensities; 1 mark for explaining that the Stage 2 enzymes are saturated or that CO₂ cannot be fixed any faster regardless of increased ATP/NADPH production.
Section 3 · Extended response · 4 marks · Band 4–5
Sample response. Transpiration is the evaporation of water from mesophyll cell walls through open stomata into the atmosphere. This lowers the water potential of mesophyll cells, pulling water from xylem vessels in the leaf veins into these cells by osmosis and removing water from the top of the xylem column. Cohesion refers to the strong hydrogen bonding between water molecules, this keeps the water column in the xylem intact as a continuous unbroken thread; when water is removed from the top, cohesion means the entire column is pulled upward. Tension is the resulting negative pressure transmitted down the entire xylem column via cohesion, it lowers water potential at the root xylem below that of soil water. Water then enters root hair cells by osmosis and moves across the root cortex into the xylem. Together, transpiration provides the driving force, cohesion transmits that force as tension down the column, and the resulting tension at the root drives osmotic uptake from the soil, a continuous, passive process requiring no ATP.
Marking criteria: 1 mark, Transpiration defined (evaporation at stomata; lowers leaf water potential; removes water from xylem top). 1 mark, Cohesion defined (hydrogen bonding; keeps column intact; allows tension to be transmitted). 1 mark, Tension defined (negative pressure; transmitted via cohesion to root; draws water from soil by osmosis at root hair cells). 1 mark, Three components linked as a continuous passive mechanism with no ATP required.
Section 3 · Extended response · 5 marks · Band 5–6
Sample response. The claim is well supported by the historical evidence from the development of photosynthesis understanding. Scientific knowledge did not arise from a single experiment, it was built incrementally across three centuries, with each scientist building on, extending, and sometimes correcting previous work. Van Helmont (1648) established that plant mass does not come from soil, using a quantitative experiment with a willow tree, but incorrectly concluded that water alone was the source, failing to account for CO₂. Priestley (1771) then discovered that plants could restore air exhausted by combustion, correctly identifying that plants produce something (O₂) that restores breathability, but he did not recognise the role of light and sometimes failed to replicate his own results. Ingenhousz (1779) built directly on Priestley's work and identified the missing variable: light. He showed that the process only occurred in sunlight, and that in darkness plants "corrupted" the air, thereby separating photosynthesis from respiration for the first time. De Saussure (1804) later corrected van Helmont's water-only conclusion by showing CO₂ uptake was essential. Finally, Calvin (1950s) mapped the complete biochemical pathway of the light-independent reactions using radioactive ¹⁴C tracing. Each contribution was built on what came before: Ingenhousz needed Priestley's sealed-jar method; Calvin needed the two-stage framework established by earlier researchers. The claim is therefore valid, scientific progress in photosynthesis was driven by a sequence of incremental experiments, each revealing one piece of the puzzle, correcting errors, and leaving new questions for subsequent researchers to answer.
Marking criteria: 1 mark, States an evaluative position (the claim is well supported / valid). 1 mark, Correctly describes what at least two named scientists discovered AND identifies how the second built on or corrected the first. 1 mark, Names a third scientist and correctly identifies their contribution. 1 mark, Explains how at least one experiment corrected an error in a previous model. 1 mark, Reaches an explicit, well-reasoned generalisation about how photosynthesis understanding was built incrementally across multiple scientists.