# Ten optional extensions

Choose an extension for curiosity or a particular teaching need. Each has its own complete supplied source and feedback. None is required for the first-response checks, and none is an actual biological investigation. Use words, directed marks or a table instead of drawing if that suits you.

## S01 · Audit the word “all”

**Source:** Three invented captions say, “All plant cells have chloroplasts”; “All membrane proteins are pumps”; “All particles stop at equilibrium.” A typical photosynthetic leaf model lists chloroplasts, but a nonphotosynthetic root model does not; a passive channel and a recognition glycoprotein are listed; stable equal neutral-solute concentrations allow ongoing opposing movement.

Rewrite each caption and cite its counterexample. Explain why replacing “all” with “some” without a reason is weaker than a specific correction. **Feedback:** Not every plant cell contains chloroplasts; membrane proteins have diverse roles; equal stable conditions may give no net movement while particles continue to cross. The listed models do not constitute a survey of all cells. **Next:** Ask what evidence would justify a broader claim.

## S02 · An analogy with a stopping point

**Source:** An invented gallery uses “gate” for a selective channel and “name badge” for a glycoprotein. Real channels provide molecular pathways and glycoproteins can contribute to recognition; molecules have no intention and the membrane is not staffed by people.

Write two helpful sentences using the analogy, then two sentences explaining where it breaks. **Feedback:** Selective passage and recognition are useful comparisons. A gatekeeper who consciously chooses cargo or a badge that works without molecular interactions is misleading. **Next:** Replace one analogy sentence with a literal biological statement; neither form demonstrates an actual transport event.

## S03 · A concentration that is not an amount

**Source:** Model solution A contains 2 mmol of neutral solute in 1 L; B contains 2 mmol in 2 L. Concentration = amount/volume. A permitted neutral substance has a passive route between them under simple comparable conditions.

Calculate concentrations and predict initial net direction. Explain why equal amounts do not settle equal concentrations. **Feedback:** A is 2 mmol/L; B is 1 mmol/L; initial net movement is A toward B for the specified route. Changing volumes would alter the values. **Next:** Ask why this rule alone cannot settle a charged-ion case or an impermeable substance. These are arithmetic models, not solution-making instructions.

## S04 · Pressure is part of the source

**Source:** Two model compartments both have 0.20 mol/L impermeant neutral solute and equal temperature; water crosses. Pressure is different, but its direction and magnitude are not provided.

Assess “Equal solute means certainly no net water movement.” Write the missing-information request. **Feedback:** Equal solute concentration removes that particular solute difference, but unknown pressure differences can change water potential. No certain net result follows without the pressure/full water-potential information. **Next:** Compare with a fully stated equal-pressure case, keeping initial predictions distinct from final volumes. Do not turn the example into hydration advice.

## S05 · What counts as a repeat?

**Source:** Plan A reads the same sample three times at one endpoint. Plan B independently prepares three samples under the same conditions, using the same endpoint rule. Neither plan has been run.

Explain what each can help assess and why they are not interchangeable. Suggest a record that separates sample identity from repeated readings. **Feedback:** Repeated readings can examine reading variability; independent samples can reveal between-sample variation. Neither automatically removes bias or confounds. **Next:** Ask what the scientific question requires before selecting a repeat design. A written plan is not practical evidence.

## S06 · Make a graph on paper

**Source:** Geometric cube edges are 2, 4 and 8 mm. SA/V values are 3, 1.5 and 0.75 mm⁻¹, respectively. These are formula results, not measured rates.

Make a graph with edge on the horizontal axis and SA/V on the vertical. Use true numerical spacing, or describe the coordinates if drawing is difficult. Explain the trend without labelling the vertical axis “diffusion speed.” **Feedback:** Plot (2, 3), (4, 1.5), (8, 0.75); ratio decreases as edge increases for similar cubes. The inverse relation is not a straight line through the points. **Next:** Predict edge 6 mm using 6/a rather than a visually guessed average; answer 1 mm⁻¹.

## S07 · A unit conversion checkpoint

**Source:** 1 mm = 1,000 µm. A geometric cube edge is 0.01 mm. Area = 6a² and volume = a³.

Calculate area, volume and ratio in mm units; then express edge as µm and repeat. Explain why numerical ratios change while geometry does not. **Feedback:** Edge 10 µm. Area 0.0006 mm² = 600 µm²; volume 0.000001 mm³ = 1,000 µm³; ratio 600 mm⁻¹ = 0.6 µm⁻¹. Area conversions square and volume conversions cube the length factor. **Next:** Practise one labelled unit cancellation if a ratio is given without dimensions.

## S08 · A reached layer cannot exceed all the volume

**Source:** Cube edge 2 mm; supplied layer depth 1.2 mm from every face at a stated time. Central edge = max(a − 2d, 0), not a negative physical edge.

Use the rule to find the reached fraction. Explain why an uncorrected negative central edge would produce nonsense. **Feedback:** max(2 − 2.4, 0) = 0; fraction = 1 − 0³/2³ = 1 = 100%. A negative central length is not an untouched volume and cannot produce more than full penetration. **Next:** Name what remains unknown: concentration distribution, actual amount transported and biological response. No physical test is implied.

## S09 · Preserve the inconvenient value

**Source:** Invented times are 42, 43, 44 and 70 s. A learner wants to delete 70 because it does not match their prediction. No method fault is documented.

Calculate the mean/range with every value. Propose a responsible next step. **Feedback:** Mean = 199/4 = 49.75 s; range = 70 − 42 = 28 s. Retain the value unless a justified, documented rule supports exclusion. Inspect method/sample records and seek further independent evidence; report any exclusion transparently. **Next:** Explain why repeats and documentation matter more than making a neat expected pattern. These are invented numbers, not grounds to infer actual misconduct.

## S10 · A proposal for the next actual investigation

**Source:** A school is considering a supervised actual or genuine virtual comparison of model sizes. No apparatus, approval, samples or results exist in this paper slice. Possible measures include seconds to a defined centre endpoint, depth at a specified time, or amount crossing per time.

Write a planning brief choosing one measure and its units, the size variable, three controls, independent repeats, a recording table, one resolution issue and a safety/access question for the teacher. State what work remains before observations can be claimed. **Feedback:** A coherent plan identifies one dependent measure without swapping it for another. A qualified teacher must choose a suitable approved system, manage risk, prepare access and collect actual evidence. **Next:** Connect a future real result to the syllabus's investigation requirement; a paper brief alone does not meet it.