# Staff answers, descriptive feedback and next teaching

Keep these worked models separate from learner first and later pages. A model illustrates a supported answer, not words a learner must reproduce. Accept a correct alternative explanation with the same conditions, quantities and limits. Record source reading, prompting, calculator, response mode and any partially completed table separately from the quality of the biological reasoning. Access support alone does not make a response wrong.

## Ordinary learner tasks

**11:** Leaf L and Bacterium B both list membrane, cytoplasm and ribosomes (wall is another valid shared item); their DNA organisation differs by membrane-bound nucleus versus nucleoid region. Plant wall is outside membrane. Heads face both watery spaces; tails face inward. “Simplified inventory, not exhaustive specimen evidence” is a valid limit. Use W01–W02 if the learner treats an unlabelled feature as absent.

**12:** Channel: selective hydrophilic route. Glycoprotein: carbohydrate-bearing protein with possible recognition/adhesion/signalling role. Cholesterol: regulates fluidity/permeability in this animal model, with condition-dependent effects. Carrier: binds and changes conformation, potentially passive or energy coupled. Every protein is not a pump; cholesterol does not always increase fluidity. If functions are swapped, ask the learner to identify the route or carbohydrate feature before revisiting the full caption.

**13:** Q: facilitated diffusion inward, 8 to 2 mmol/L, suitable passive carrier, no cellular energy supplied to that movement. R: stated ATP-coupled active pump inward, from lower outside 1 toward higher inside 5 mmol/L. Oxygen: simple diffusion inward, 6 to 2 mmol/L, permeable bilayer. Net direction summarises opposing molecular movements. Ion direction is unresolved without electrical/charge information. Reteach a neutral channel case before the ion qualifier if needed.

**14:** File O: initial net water inward toward the 0.30 mol/L side under water-permeable/solute-impermeable, equal-temperature/equal-initial-pressure conditions. Water, not solute, moves by osmosis. Final volume/time/real-cell outcome unavailable. V is endocytosis by wrapping and internal vesicle formation; X is exocytosis by internal vesicle fusion/release. If all cases are called diffusion, sort by the actual action: single molecules through a route versus membrane enclosure/fusion.

**16:** S edge 30 µm: area 6 × 900 = 5,400 µm²; volume 27,000 µm³; ratio 0.20 µm⁻¹; centre distance 15 µm. Compared with P, S has 9 times the total area, 27 times the volume and one third the ratio. Teach separate area and volume lines if the learner doubles both together. Practise dimensional division if the ratio lacks µm⁻¹.

**17:** Cube: 24 u², 8 u³, 3 u⁻¹, 1 u shortest centre distance. Box: 28 u², 8 u³, 3.5 u⁻¹, 0.5 u. Thinner model offers shorter nearest path and more surface at equal volume; actual exchange also depends on permeability, gradients, demand and internal arrangements. If “all paths are shorter” appears, locate the nearest face explicitly.

**18:** Cube edge 6 mm, depth 1 mm: core 4 mm; total 216 mm³; core 64 mm³; penetrated fraction 152/216 = 19/27 ≈ 70.3704%. With d_start = 0 mm and t_start = 0 min, interval-average front speed = Δdepth/Δtime = (d_end − d_start)/(t_end − t_start) = (1 − 0) mm / (5 − 0) min = 0.20 mm/min. One value is volume fraction at a time, the other an interval-average change in depth divided by elapsed time. No universal linear-depth law or molecule flux follows. If only one layer is subtracted, mark both opposite faces before recalculating.

**19:** Proposed independent variable: edge size. Dependent measure: time in seconds to a specified centre endpoint. Suitable constants include material, external concentration, temperature, stirring and endpoint rule. Card H means 104 s and 164 s, ranges 8 s each; A's supplied list is shorter, with a 60 s mean difference and no overlap here. Only three invented values per group; actual observations and detailed method are absent. Improve independent samples, resolution/calibration or reproducible endpoint. If a learner writes “experiment completed,” ask them to distinguish authored data, a plan, actual observations and a future school practical.

Slots 15 and 20 use the separate fresh models below. W01–W20 include full solved context answers. Home and supplement files include bounded feedback for every offer; this public feedback is not secure assessment material.

## Check E · full first worked model

**Part 1:** U moves net inward by facilitated diffusion: outside 6 mmol/L exceeds inside 2, the bare bilayer excludes U and the suitable carrier is explicitly passive. No cellular energy is supplied to that transport. V's specified ATP-coupled protein pump carries it inward by active transport, from outside 3 toward inside 7 mmol/L, against the stated concentration difference. ATP coupling is given for this case; not all active transport uses ATP directly. Net inward describes the difference between opposing movements, not a claim that every molecule follows a single arrow.

**Part 2:** Heads face the water outside and in the cytoplasm; tails face the bilayer interior toward other tails. A channel can offer a selective hydrophilic crossing route. A glycoprotein has an external carbohydrate chain and can contribute to recognition, adhesion or signalling. Cholesterol among tails helps regulate fluidity and permeability in this animal model, with condition-dependent effects. The word map is simplified and unscaled; it gives neither complete composition nor a micrograph or measured mobility. A channel's presence alone does not prove active transport.

**Part 3:** Initially water moves net inward toward the pouch's 0.40 mol/L impermeant-solute side from outside 0.15 mol/L. This follows from water permeability, solute impermeability, equal temperature and equal initial hydrostatic pressure. The inside has lower water potential under those simple conditions. Osmosis moves water, not the impermeant solute. Final volume, duration or real-cell survival cannot be inferred.

## Check E · full later worked model

The first equal-pressure scenario supported the inward prediction. In the new scenario pressure is unknown and may differ, so the solute values alone no longer make net inward direction certain. Water movement depends on the full water-potential gradient, which includes pressure effects. Request the pressure difference or full water-potential values under the stated conditions before predicting certain net direction. The conclusion may be expressed conditionally: if pressure were equal, the inward prediction would follow; pressure inequality could change or oppose it. U's passive-carrier inward transport, V's ATP-coupled inward pump and the membrane component roles remain supported by their unchanged sources. Do not erase a valid first conclusion or claim pressure must have reversed the direction.

## Check F · full first worked model

**Part 1:** A edge 3 mm: area 6 × 9 = 54 mm²; volume 27 mm³; ratio 2 mm⁻¹; centre distance 1.5 mm. B edge 6 mm: area 216 mm²; volume 216 mm³; ratio 1 mm⁻¹; distance 3 mm. B has 4 times the total area and 8 times the volume, so half the surface per unit volume. The larger cube does not have less total surface.

**Part 2:** With depth 0.5 mm from every face, A's core edge is 2 mm and B's 5 mm. A fraction = 1 − 8/27 = 19/27 ≈ 70.3704%; B fraction = 1 − 125/216 = 91/216 ≈ 42.1296%. In both supplied layer models, d_start = 0 mm at t_start = 0 min. Interval-average front speed = Δdepth/Δtime = (d_end − d_start)/(t_end − t_start) = (0.5 − 0) mm / (2 − 0) min = 0.25 mm/min. The smaller model has a larger fraction reached, not a faster supplied interval-average front speed. Neither shows a measured intrinsic diffusion coefficient, flux or universal linear time relation.

**Part 3:** The separate endpoint file gives A mean 44 s and range 8 s; B mean 92 s and range 8 s. B's mean is 48 s longer; A's listed values are all shorter with no overlap in these small lists. Temperature and stirring are missing, so attributing the whole difference solely to size is unjustified. Record and standardise relevant conditions, define the endpoint, use independent samples and report resolution/uncertainty. The authored table is not an actual learner investigation. Keep the endpoint table distinct from the imposed layer-time calculation.

## Check F · full later worked model

**Valid KEEP:** “A's listed times are shorter and its mean is 48 s lower, but I cannot attribute that difference to size alone because conditions were missing.” Keep this bounded first claim and add that the new notes identify different temperature settings, 20°C versus 30°C, and unstandardised stirring.

**Valid revision:** If the first claim said size alone caused the difference, replace it with the numerical comparison and the confound qualification. Do not assert the notes prove temperature or stirring caused a particular amount of the difference. A better plan compares sizes at a common stated temperature, same stirring/material/solution and a reproducible endpoint, using independent samples and recorded measurement resolution. The area, volume, ratio and imposed-layer results remain unchanged. Keeping a sound cautious conclusion is not inferior to making an edit.

## Descriptive rubric · Check E

This is an original formative feedback guide, not QCAA reporting standards or a school ISMG. Use each dimension independently, and name evidence from the actual response.

| Dimension | Clear and supported | Developing | Next teaching priority |
| --- | --- | --- | --- |
| Mechanism and net movement | Correct U/V mechanisms and directions, explicit route/concentration/energy evidence, and net versus individual distinction. | Mechanisms or directions mostly correct, but a route, energy condition or net distinction is omitted. | Uses a protein to imply active transport, reverses a gradient, or treats all particles as following one arrow. Rebuild one neutral case from permeability and energy first. |
| Membrane structure/function | Correct heads/tails and all three requested component roles, with conditional cholesterol wording and a genuine model limit. | Correct orientation and several jobs; one role or limit is weak or overbroad. | Confuses channel/glycoprotein/cholesterol or treats the drawing as full specimen evidence. Match one component to its stated feature before explaining its job. |
| Osmosis conditions | Correct initial inward water prediction with water/solute permeability, equal temperature/pressure and unavailable final outcome. | Direction is right but conditions or moving substance are incompletely stated. | Solute is said to move by osmosis, or concentration is used as a universal rule. Label water and all conditions before drawing the arrow. |
| Later judgement | Preserves the valid first scenario, qualifies the pressure-unknown one through water potential, requests the missing information and identifies an unchanged result. | Notices pressure uncertainty but gives an incomplete explanation or does not identify what remains valid. | Retains a certain new arrow without evidence or calls the original equal-pressure answer wrong. Compare the two scenario condition lists explicitly. |

## Descriptive rubric · Check F

| Dimension | Clear and supported | Developing | Next teaching priority |
| --- | --- | --- | --- |
| Geometry and units | Both areas, volumes, ratios and centre distances correct, with units and absolute-area versus ratio distinction. | Method is sound with a local arithmetic/unit omission. | Area and volume scale alike or ratio is treated as total area. Calculate them on separate lines and compare factors. |
| Penetration versus speed | Correct cores/fractions and common 0.25 mm/min, with a clear distinction between volume fraction and interval-average front speed. | Core/fraction method is mostly sound; interpretation or one arithmetic step is incomplete. | Subtracts only one face, permits a negative core, or calls percentage a speed. Mark both faces and label each measure/unit. |
| Supplied-data analysis | Correct means/ranges and bounded number comparison, with missing condition, improvement and conceptual/actual distinction. | Summaries or comparison mostly accurate but a limitation or improvement is vague. | Claims causation/statistical significance/practical completion from the authored list. Identify measure and source status before a broader claim. |
| Later evaluation | Valid KEEP or revision with temperature/stirring confounds, unchanged geometry and a concrete controlled repeat plan. | Identifies a confound but does not link it to attribution or specify a useful improvement. | Treats a confound as proof of its effect, demands an edit of an already valid claim, or discards geometry. Separate arithmetic, condition evidence and causal confidence. |

## Four reserved recheck keys

**E1:** N facilitated diffusion outward, 9 inside to 3 outside, passive carrier/no supplied cellular energy. P active pump outward, 2 inside toward 5 outside, ATP coupling. Heads face water; tails inward; channel selective route, glycoprotein recognition/adhesion/signalling, animal cholesterol conditional fluidity/permeability. The original pouch's water moves net outward, 0.12 inside toward 0.28 outside, under all stated conditions. No final outcome follows. Unknown-pressure version cannot retain a certain concentration-only arrow; request full water-potential/pressure information. Use E rubric dimensions with these specific directions.

**E2:** J simple diffusion inward through permitted bilayer, 4 outside to 1 inside, no cellular energy supplied to that transport. K active inward ATP-coupled pump, 2 outside toward 6 inside. Component/orientation/limit answers as above. Equal impermeant-solute values with equal temperature/initial pressure support no initial net water movement, with continuing opposing molecular motion. Unknown pressure means the equal-solute values alone cannot guarantee no net water movement. Use E rubric, recognising that this equal case has a different correct outcome.

**F1:** C edge 4: area 96 mm², volume 64 mm³, ratio 1.5 mm⁻¹, distance 2 mm; D edge 8: 384 mm², 512 mm³, 0.75 mm⁻¹, distance 4 mm. C core 2 mm, fraction 1 − 8/64 = 87.5%; D core 6 mm, fraction 1 − 216/512 = 57.8125%. In both models d_start = 0 mm at t_start = 0 min; Δdepth/Δtime = (d_end − d_start)/(t_end − t_start) = (1 − 0) mm / (5 − 0) min = 0.20 mm/min, an interval average. Endpoint means 74 s and 134 s; ranges 8 s each; mean difference 60 s, no overlap in listed sets. Unknown then disclosed different temperatures 18°C/28°C and stirring prevent size-only attribution. Keep or revise appropriately, retaining geometry and specifying a common-temperature/stirring independent-sample endpoint plan. Use F rubric with these values.

**F2:** G edge 2: area 24 mm², volume 8 mm³, ratio 3 mm⁻¹, distance 1 mm. H edge 4: area 96 mm², volume 64 mm³, ratio 1.5 mm⁻¹, distance 2 mm. G core 0 mm/fraction 100%; H core 2 mm/fraction 87.5%. In both models d_start = 0 mm at t_start = 0 min; Δdepth/Δtime = (d_end − d_start)/(t_end − t_start) = (1 − 0) mm / (4 − 0) min = 0.25 mm/min, an interval average; full supplied penetration does not establish flux or final cellular response. Means 90 s and 100 s; ranges 20 s; 10 s mean difference with overlapping lists. Later common temperature/stirring removes those particular information gaps, but 10 s endpoint resolution and three supplied values still limit confidence. Improve endpoint measurement/resolution and independent data, without inventing statistical significance. Valid careful KEEP is allowed. Use F rubric with a focus on overlap/full-penetration/measurement limits.

## A next teaching menu

- **Route or energy confusion:** give one neutral case with the concentration values hidden; determine the possible mechanism from permeability and energy, then reveal values for direction. Use E1 only after that feedback and a delay.
- **Osmosis without conditions:** make a four-condition checklist on a fresh ordinary pouch case. Compare equal pressure with unknown pressure, then reserve E2 for later evidence.
- **Area/volume confusion:** calculate edge 5 and 10 µm in two separate columns, compare factors, then explain the ratio in words. Calculator support is compatible with biological interpretation.
- **Fraction called speed:** label central edge, volume fraction and Δdepth/Δtime on separate lines; ask which is dimensionless and which has mm/min. Do not demand a sophisticated diffusion coefficient calculation that was not taught.
- **Causal or practical overclaim:** underline exactly what was supplied, measured, planned or unknown. Write one numerical comparison and one next actual evidence request. A school-approved investigation remains separate future work.

Use one next move at a time. Log the observed response and follow-up; a successful answer to one recheck does not establish the full topic, investigation repertoire or course objectives.

All front-speed worked results above use the explicitly stated zero initial layer at zero time. The model's interval average is not a constant future speed, intrinsic diffusion coefficient or molecular flux.