Photosynthesis, Products, Movement and Function

Q1, Sample Band 5 response (6 marks), annotated

The transpiration-cohesion-tension theory explains how water moves passively from the soil through a plant to the leaves without any ATP-powered pump.

Transpiration is the evaporation of water from the cell walls of mesophyll cells in the leaf and its exit through open stomata into the drier atmosphere. This lowers the water potential of mesophyll cells, causing water to move from the xylem in leaf veins into these cells by osmosis. The removal of water from the top of the xylem is the driving force. [1, transpiration defined with location and driving force]

Cohesion refers to the strong attraction between water molecules due to hydrogen bonding. When water is removed from the top of the xylem by transpiration, these cohesive forces prevent the water column from breaking apart, the entire column is pulled upward as one continuous unit. [1, cohesion defined with hydrogen bonding and role in keeping column intact]

Tension is the negative pressure (pulling force) created in the xylem vessels as water is removed from above. Via cohesion, this tension is transmitted down the entire length of the xylem column from the leaf to the root, lowering water potential in the root xylem. [1, tension defined and linked to transmission via cohesion]

Water enters the plant at root hair cells by osmosis, the tension in the root xylem lowers water potential below that of the surrounding soil water, so water moves by osmosis from the soil into root hair cells and across the root cortex, eventually entering the xylem. [1, osmosis at root hair cells correctly described]

Together, the three components create a continuous passive mechanism: transpiration provides the driving force at the leaf, cohesion keeps the water column intact so tension can be transmitted, and tension at the root draws in new water from the soil. The process requires no ATP, it is entirely driven by the physical properties of water and the water potential gradient created by evaporation. [1, three components linked as a continuous mechanism; 1, correctly identifies passive/no ATP with reason]

Marking criteria:

• 1 markTranspiration defined (evaporation at stomata / leaf surface) as the driving force that creates a water potential gradient.
• 1 markCohesion defined (hydrogen bonding between water molecules) and its role in keeping the water column intact.
• 1 markTension defined (negative pressure in xylem) and described as transmitted from leaf to root via cohesion.
• 1 markWater entry at root hair cells by osmosis explained (tension lowers water potential → soil water enters by osmosis).
• 1 markAll three components linked as a continuous, integrated mechanism.
• 1 markMechanism correctly identified as passive / no ATP required, with a valid reason (driven by evaporation / water potential gradient / physical properties of water).

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

Similarity: Both xylem and phloem are vascular tissues that form continuous bundles running from roots through stems to leaves, and both function to transport substances throughout the plant. [1, similarity correctly identified]

Direction: Xylem transports water and dissolved inorganic ions unidirectionally, always upward from roots to leaves, driven by the transpiration-cohesion-tension mechanism. In contrast, phloem transports sucrose bidirectionally, from any source tissue to any sink tissue, which may be upward (leaf to shoot tip) or downward (leaf to root) depending on the location of demand. [1, direction comparison with xylem unidirectional upward vs phloem bidirectional]

Substance transported: Xylem transports an inorganic solution, water and dissolved mineral ions (e.g. nitrate, phosphate). Whereas phloem transports organic solutes, primarily sucrose, dissolved in phloem sap. [1, substance comparison: inorganic vs organic]

Energy requirements: Xylem transport is entirely passive, no ATP is consumed by the plant; the driving force is the tension created by transpiration at the leaf. In contrast, phloem transport requires active loading of sucrose into sieve tubes by companion cells using ATP, and active unloading at sink tissues also requires energy. [1, energy comparison: passive vs ATP-requiring]

Living state of cells: Xylem vessels and tracheids are dead at functional maturity, their cell contents are removed, leaving hollow tubes that offer unobstructed flow. Whereas phloem sieve tube elements must remain living because the active membrane transport of sucrose during loading and unloading requires functional cell membranes; companion cells (alive) support the sieve tube elements. [1, cell state: dead xylem vs living phloem]

Driving mechanism: Xylem uses the transpiration-cohesion-tension mechanism, a physical, passive process. Phloem uses the pressure-flow hypothesis, sucrose is actively loaded at source, water follows by osmosis creating high turgor pressure, and the solution flows by bulk flow from high pressure (source) to low pressure (sink). [1, driving mechanism comparison, pressure-flow hypothesis named]

[+1 for consistent use of connective language (whereas/in contrast/both/however) throughout the response; +1 for a well-structured, logical response that addresses all criteria and reaches an explicit comparison conclusion, e.g. "The fundamental difference is that xylem is a passive dead-cell conduit driven by physics, while phloem is an active living-cell system driven by biochemistry."]

Marking criteria:

• 1 markCorrectly identifies one similarity (both are vascular tissues / both transport materials throughout the plant).
• 1 markCompares direction: xylem unidirectional upward vs phloem bidirectional.
• 1 markCompares substance: xylem = inorganic (water + ions) vs phloem = organic (sucrose).
• 1 markCompares energy: xylem = passive (no ATP) vs phloem = active loading/unloading (ATP required).
• 1 markCompares cell state: xylem = dead at maturity vs phloem = living sieve tube elements (+ companion cells).
• 1 markNames and briefly explains the pressure-flow hypothesis for phloem.
• 1 markUses precise connective language (whereas / in contrast / however / both) at least twice.
• 1 markResponse is logically structured and reaches an explicit comparative conclusion.

Q3, Sample Band 6 response (7 marks), annotated

The claim is incorrect in its overall conclusion, though it rightly notes that van Helmont's specific conclusion was wrong. [1, evaluative stance stated]

Van Helmont's experiment correctly established that plant mass does not come primarily from soil, the fact that 74 kg of tree growth corresponded to only 57 g of soil loss was a genuinely important finding that ruled out the soil as the main source of plant mass. This was a valid scientific conclusion supported by quantitative evidence. [1, correctly identifies what the experiment established]

His incorrect conclusion was that water alone was responsible for the 74 kg mass gain. He did not account for the role of CO₂, which makes up the majority of plant dry mass (plants are approximately 45% carbon by dry mass, derived from atmospheric CO₂). This error was later corrected by Nicolas-Théodore de Saussure (1804), who used quantitative measurements to show that plants absorb CO₂ and that this accounts for most of their dry mass gain, going beyond what water alone could explain. [1, incorrect conclusion identified; 1, de Saussure named as the scientist who corrected it]

Despite this error, van Helmont's experiment made a genuine contribution to the development of the photosynthesis model in two ways. First, it disproved the prevailing belief that plants simply extracted their mass from soil, clearing the way for researchers to look elsewhere for the source of plant mass. Second, the quantitative approach, weighing both the tree and the soil before and after, established a rigorous experimental method that later scientists like de Saussure followed and extended. [1, contribution despite error: disproved soil theory; 1, contribution: introduced quantitative method]

This example illustrates a key pattern in how scientific models develop: each experiment narrows the range of possible explanations even if its specific conclusion is later revised. Scientific knowledge is built incrementally, each researcher builds on what came before, corrects previous errors, and opens new questions. An incorrect conclusion does not negate the value of the data, the method, or the questions the experiment raises. The claim that "scientific experiments producing incorrect conclusions have no value" is therefore wrong, in science, a well-designed experiment that disproves a previous model (even if it cannot yet name the correct answer) is of high value. [1, correct generalisation about incremental model development]

Marking criteria:

• 1 markStates an overall evaluative judgement (the claim is incorrect, or "partly right but misleading").
• 1 markCorrectly identifies what van Helmont's experiment established (soil is not the main source of plant mass, supported by quantitative data).
• 1 markCorrectly identifies what his conclusion incorrectly claimed (water alone was the source, failed to account for CO₂).
• 1 markNames de Saussure (or another appropriate scientist) as the one who corrected the error and explains the correction.
• 1 markExplains at least one way van Helmont's experiment contributed positively despite the error (disproved the soil hypothesis / introduced quantitative methods).
• 1 markConnects to a second contribution or uses the Priestley/Ingenhousz/Calvin sequence to support the generalisation.
• 1 markReaches a correct generalisation about how scientific models develop (incrementally, building on and correcting previous models, with each step narrowing possibilities even if not fully correct).