Photosynthesis doesn't end at glucose. Understanding where the products go, how they move through the plant, and how scientists came to understand the process is the full picture NESA expects you to know.
Content from this lesson that appears directly in HSC Biology exams
One of the most commonly tested mechanisms in plant biology — appears in Section II as a 3–5 mark explanation question in most HSC papers. Must describe all three components: evaporation at stomata, cohesion in xylem, tension pulling water up.
Tracing where glucose goes after photosynthesis — respiration, starch, sucrose transport, cellulose, biosynthesis. Appears as 2–3 mark short answer and as the basis for data interpretation questions.
NESA explicitly requires evaluation of how models developed over time. Scientists, experiments, and what each revealed appear in 3–4 mark secondary source analysis questions (also core content for L19).
Two-stage summary (light-dependent and light-independent) without full biochemistry. Tested as 2–3 mark questions requiring location, inputs, and outputs of each stage — not the full electron transport chain.
Core Content
Overview only — location, inputs, and outputs at each stage
Photosynthesis occurs in two connected stages inside the chloroplast. The first stage captures light energy and converts it into chemical energy. The second stage uses that chemical energy to build glucose from CO₂.
| Feature | Stage 1 — Light-dependent | Stage 2 — Light-independent |
|---|---|---|
| Location | Thylakoid membranes (grana) | Stroma of chloroplast |
| Requires light? | Yes — directly | No — but depends on Stage 1 products; stops when light is absent |
| Key inputs | H₂O, light energy | CO₂, ATP, NADPH |
| Key outputs | ATP, NADPH, O₂ | Glucose, ADP, NADP⁺ |
| What happens | Water split by light; energy captured in ATP and NADPH; O₂ released | CO₂ fixed into organic molecules using ATP and NADPH; glucose built |
Tracing the five fates of the primary photosynthesis product
Glucose produced in the chloroplast stroma during the light-independent reactions is not simply stored — it is immediately distributed to wherever it is needed. Understanding these fates is what the syllabus means by "trace the development and movement of products of photosynthesis."
| Fate | Process | Where / Purpose | Reversible? |
|---|---|---|---|
| 1. Cellular respiration | Glucose oxidised in mitochondria → ATP + CO₂ + H₂O | All living plant cells — immediate energy for growth, transport, reproduction | Yes — ATP is continuously produced and consumed |
| 2. Starch storage | Glucose polymerised into starch (condensation reaction) | Temporarily in chloroplasts; long-term in roots, seeds, tubers (potato, carrot) | Yes — starch broken back to glucose by amylase when needed |
| 3. Sucrose transport | Glucose + fructose → sucrose; loaded into phloem sieve tubes | Transported from leaves (source) to all non-photosynthetic tissues (sink) — roots, growing tips, fruit, seeds | Yes — sucrose unloaded and converted back at sink tissues |
| 4. Cellulose synthesis | Glucose polymerised into cellulose (different linkage to starch) | Cell walls of all plant cells — structural support, rigidity | No — cellulose is not readily broken down by plants |
| 5. Biosynthesis | Carbon skeletons from glucose modified with N, P, S from minerals → amino acids, lipids, nucleotides | Growth — building proteins, membranes, DNA, RNA throughout the plant | Partially — proteins broken down during senescence |
How sucrose moves from source to sink
Once glucose is converted to sucrose in photosynthetic cells, it must be transported to every non-photosynthetic part of the plant. This transport occurs through phloem sieve tubes and is explained by the pressure-flow hypothesis (also called the mass flow hypothesis).
| Term | Definition | Examples |
|---|---|---|
| Source | Any tissue that produces or releases sucrose into the phloem | Mature photosynthetic leaves; starch-storing organs releasing glucose during germination |
| Sink | Any tissue that consumes or stores sucrose from the phloem | Growing roots, shoot tips, developing fruit and seeds, storage organs (tubers) |
How water travels from roots to leaves against gravity
Water must travel from soil through roots, up the stem, and into leaves — often against gravity and over heights of 100+ metres in tall trees. No pump drives this movement. Instead, three interconnected physical properties create a passive but powerful mechanism.
| Component | What it is | Role in water movement |
|---|---|---|
| Transpiration | Evaporation of water from leaf cells through open stomata into the atmosphere | Creates the driving force — water evaporating from mesophyll cell walls lowers water potential in the leaf, pulling water from adjacent cells |
| Cohesion | The attraction between water molecules due to hydrogen bonding — water molecules "stick" to each other | Creates an unbroken column of water throughout the xylem — when water evaporates from the top, cohesion means the entire column is pulled upward rather than breaking |
| Tension | The negative pressure (pulling force) created in the xylem when water evaporates from the top | The tension created by evaporation at the top is transmitted down the entire water column via cohesion — pulling water up from the roots |
The tallest trees on Earth — coast redwoods over 115 metres — pull water to their canopies using transpiration-cohesion-tension alone. No pump, no energy expenditure by the plant. The mechanism works only because water molecules are so cohesive that an unbroken column can sustain tension equivalent to hanging a 200-metre column of water from the tree canopy. The moment the column breaks (a cavitation event), that section of xylem becomes blocked with air and is permanently non-functional. This is why trees can be damaged by prolonged drought — cavitations accumulate until water supply collapses.
How science built our understanding — experiment by experiment
Understanding photosynthesis was not a single discovery — it was built incrementally over 300 years through a series of carefully designed experiments, each revealing one piece of the puzzle. NESA requires you to understand how this scientific knowledge developed and what evidence each key experiment provided. This content also forms the foundation of L19 (Secondary Source Analysis).
| Scientist | Date | Experiment / Observation | What it revealed | What it did NOT explain |
|---|---|---|---|---|
| Jan Baptist van Helmont | 1648 | Grew a willow tree in a pot for 5 years. Tree gained 74 kg; soil lost only 57 g. Concluded that plant mass came from water. | Plant mass does not come from soil — the soil contribution is negligible. Mass must come from somewhere else (he assumed water). | Did not identify the role of CO₂ or light; incorrectly concluded water alone explained plant growth |
| Joseph Priestley | 1771 | Showed that a plant placed in a sealed jar with a candle could restore air that had been "exhausted" by combustion — a mouse placed with the plant survived; without the plant it died. | Plants produce something that restores the ability of air to support combustion and life — now understood to be O₂. | Did not know what the substance was; did not understand the role of light (his experiments sometimes failed in the dark) |
| Jan Ingenhousz | 1779 | Repeated Priestley's experiments under different light conditions — showed plants only "purify" air in sunlight; in darkness they "corrupt" it. | Light is essential for the process that produces O₂. In darkness, plants produce CO₂ (respiration) — separating photosynthesis from respiration experimentally for the first time. | Did not identify CO₂ as a reactant or glucose as a product |
| Nicolas-Théodore de Saussure | 1804 | Used careful quantitative measurements to show that plants gain more mass than can be accounted for by water alone — identified CO₂ uptake as essential to plant growth. | CO₂ is a reactant in photosynthesis; plant dry mass is partly built from carbon derived from atmospheric CO₂ — corrected van Helmont's water-only hypothesis. | Did not identify the internal mechanism or the two-stage nature of photosynthesis |
| Melvin Calvin | 1950s | Used radioactive ¹⁴C-labelled CO₂ and paper chromatography to trace the path of carbon through the light-independent reactions, identifying the intermediates of what became the Calvin cycle. | The specific biochemical pathway by which CO₂ is fixed into organic molecules (the Calvin cycle / light-independent reactions). Won the Nobel Prize in Chemistry 1961. | Full electron transport chain detail came from subsequent research |
| Robin Hill | 1939 | Showed isolated chloroplasts could produce O₂ in light without CO₂ — proving O₂ comes from water splitting, not from CO₂. | O₂ produced in photosynthesis originates from water (photolysis of water) — confirmed by later isotope labelling experiments using ¹⁸O-labelled water. | Did not fully elucidate the mechanism of water splitting (photosystem II detail) |
Activities
A plant produces a large amount of glucose on a sunny afternoon. Trace what happens to this glucose over the next 24 hours, considering: immediate needs during the day, what happens at night when photosynthesis stops, how non-photosynthetic tissues are supplied, and how the plant uses glucose for long-term growth.
Refer to all five fates. Use specific tissue names — chloroplast, phloem, root, cell wall.
In your book, draw a diagram of a plant showing the transpiration-cohesion-tension mechanism. Your diagram should show: water evaporating from stomata, cohesive water column in xylem, and water entering root hair cells from soil. Then answer the questions below.
Type here or answer in your book.
Answer the following questions about the historical development of photosynthesis understanding.
Type here or answer in your book.
Assessment
Select the best answer — feedback shown immediately
1. The light-independent reactions of photosynthesis are sometimes called "dark reactions." Which statement about this is most accurate?
2. In the transpiration-cohesion-tension theory, what is the direct driving force that initiates water movement up the xylem?
3. Which scientist was the first to demonstrate experimentally that light is essential for the process by which plants produce oxygen?
4. Sucrose is transported from a leaf to a developing fruit via the phloem. In the pressure-flow hypothesis, what creates the high pressure at the source (leaf) end of the phloem?
5. A plant is kept in complete darkness for 48 hours. Which of the following correctly describes what happens to the plant's starch stores?
Explain mechanisms — not just outcomes
6. Explain the transpiration-cohesion-tension theory of water movement in plants. In your answer, describe the role of each component and explain how they work together to move water from soil to leaf. 4 MARKS
Name each component → explain mechanism → link to the next component
7. Ingenhousz's 1779 experiments were a significant advance over Priestley's 1771 experiments. Explain what Ingenhousz discovered and why his contribution represented an advance in the scientific model of photosynthesis. 3 MARKS
8. Compare the transport of water in xylem and sucrose in phloem. In your answer, identify one similarity and three differences, referring to direction of flow, energy requirements, and the living state of the transport cells. 4 MARKS
Use: whereas / however / both / in contrast
1. C — "Dark reactions" is misleading. Stage 2 runs continuously during daylight as long as Stage 1 produces ATP and NADPH. It only stops in darkness because Stage 1 stops when light is absent — not because Stage 2 itself requires darkness.
2. A — Transpiration (evaporation at stomata) is the driving force. It lowers water potential in leaf cells, pulling water from xylem, which creates tension transmitted down the entire water column via cohesion. No active pumping occurs in xylem transport.
3. D — Ingenhousz (1779) was first to show light is essential. Priestley showed plants could restore air but did not identify the light requirement. Ingenhousz showed the process only occurred in sunlight — a crucial additional discovery.
4. B — Active loading of sucrose into phloem sieve tubes by companion cells lowers the water potential inside the phloem. Water then enters from adjacent xylem by osmosis, increasing turgor pressure at the source end. This pressure gradient drives bulk flow toward the lower-pressure sink end.
5. C — In darkness, photosynthesis stops and no new glucose is produced. Cellular respiration continues 24/7 and requires glucose. The plant mobilises starch stores (breaking starch → glucose via amylase) to supply the glucose needed for respiration, so starch stores decrease.
Transpiration: Water evaporates from mesophyll cell walls and exits through open stomata into the atmosphere. This reduces the water potential of mesophyll cells, causing water to move from the xylem in leaf veins into these cells by osmosis, removing water from the top of the xylem column.
Cohesion: Water molecules are strongly attracted to each other via hydrogen bonds. When water is removed from the top of the xylem, these cohesive forces mean the entire water column is pulled upward as a single continuous unit — rather than breaking apart.
Tension: The removal of water from the top of the xylem creates a negative pressure (tension) in the xylem vessels. Via cohesion, this tension is transmitted all the way down the xylem column to the roots, lowering water potential there and causing water to enter root hair cells from the soil by osmosis.
Together: These three components create a continuous passive mechanism — transpiration provides the driving force, cohesion keeps the water column intact so tension can be transmitted, and the resulting tension pulls water from the soil all the way to the leaf canopy without any energy expenditure by the plant.
Priestley (1771) had demonstrated that plants could restore air that had been exhausted by combustion — producing something that allowed a candle to burn and a mouse to survive. However, he did not understand the role of light and his experiments sometimes failed when conducted in the dark.
Ingenhousz (1779) discovered that this restorative process — the production of oxygen — only occurred when plants were exposed to sunlight. In darkness, plants actually "corrupted" the air by producing CO₂ (cellular respiration).
This was a significant advance for two reasons: it established light as an essential requirement for photosynthesis (contributing to the understanding that photosynthesis uses light energy), and it experimentally separated photosynthesis from cellular respiration for the first time — showing that plants perform both processes but in different conditions.
Similarity: Both xylem and phloem form continuous vascular bundles that run from roots through stems to leaves, and both function in transporting materials throughout the plant.
Difference 1 — Direction: Xylem transports water and inorganic ions unidirectionally upward from roots to leaves, driven by transpiration. In contrast, phloem transports sucrose bidirectionally — from any source tissue to any sink tissue — which may be upward (leaves to shoot tips) or downward (leaves to roots) depending on demand.
Difference 2 — Energy: Xylem transport is entirely passive — no ATP is required by the plant; the driving force is transpiration creating tension. Whereas phloem transport requires active loading of sucrose into sieve tubes by companion cells using ATP, and active unloading at sink tissues.
Difference 3 — Cell state: Xylem vessels and tracheids are dead at maturity — their cell contents are removed, leaving hollow tubes for unobstructed water flow. Whereas phloem sieve tube elements must remain living because active membrane transport of sucrose (loading and unloading) requires functional cell membranes and companion cell support.
Tick when you've finished all activities and checked your answers.