Biology 5090/31 — May/June 2017

Walkthrough

This part records what you actually observed with your own strips, so there is no single printed answer — the marks are awarded for how you record it. The apparatus in Fig. 1.1 holds the potato strip horizontally with one end pinned level with the cork, and the free end hangs over graph paper. A turgid strip stays stiff and hardly bends, so its unpinned end sits far out and high on the paper. A flaccid strip is soft and flops downwards, so its unpinned end sits lower and nearer the cork.

To score all three marks you must: (1) draw a small × for EACH strip — two crosses in total; (2) label them clearly S1 and S2 so the examiner knows which is which; (3) place the S1 cross HIGHER than the S2 cross, because S1 (the solution that made cells turgid) leaves the strip firmer and less bent. The crosses must be small and sit at the actual position of the unpinned end of your own strip.

Key Takeaways

• Recording observations on a printed figure must be done neatly and labelled — an unlabelled cross cannot be credited.
• Strip flexibility links directly to water movement: turgid = firm = high position; flaccid = floppy = low position.

Common Mistakes

• Drawing only one cross, or drawing large blobs instead of small × symbols.
• Forgetting the labels S1 and S2 — this loses a whole mark.
• Putting S2 higher than S1, which contradicts the biology (S2 cells lose water and go flaccid).

Things to Be Careful About

The mark scheme reads 'S1 mark higher than S2' as a required point — the relative height IS a marking point, not just good practice. Use a sharp pencil and keep the crosses small.

Walkthrough

After 20 minutes in their solutions, the two strips feel different when you bend them gently. Solution S1 has a water potential such that water enters the potato cells by osmosis, so the cells become turgid — the strip feels firm, hard and stiff. Solution S2 causes water to leave the cells, so they become flaccid — the strip feels soft and bends easily.

You need exactly one description per row. Any wording meaning 'stiff' scores for S1 ('less flexible', 'firm', 'turgid', 'hard'), and any wording meaning 'bendy' scores for S2 ('more flexible', 'soft', 'flaccid').

Key Takeaways

• Turgid cells push against the cell wall, making tissue rigid; flaccid cells do not, so tissue is limp.
• Flexibility is a quick, observable indicator of whether cells have gained or lost water.

Common Mistakes

• Writing vague answers like 'it changed' or 'it was different' — no credit without a comparative descriptor.
• Reversing the rows: S1 makes cells turgid, not flaccid.
• Describing colour or length when the question asks specifically about FLEXIBILITY.

Things to Be Careful About

The mark scheme accepts AW (alternative wording) here, so precise synonyms are fine — but each entry must describe flexibility/stiffness, not some other property.

Walkthrough

A controlled variable is anything (other than the independent variable — the sucrose concentration) kept the same for both strips so that any difference in the results is due to the solutions alone. Read back through the method in part (a): every 'same' instruction is a controlled variable.

The scheme lists six possibilities and asks for any two:

• strips cut to the same size/dimensions/surface area/volume;
• strips fully submerged/covered by the solutions;
• left for the same time (20 minutes);
• standard blotting procedure;
• aligned on the cork the same way;
• strips cut from the same potato.

Any two of these earn the two marks. The safest choices are the ones most obviously stated in the method: identical dimensions and identical time.

Key Takeaways

• Controlled variables come straight from the 'keep everything else the same' instructions in a method.
• Comparability means the ONLY difference between the two set-ups is the concentration of sucrose solution.

Common Mistakes

• Naming the sucrose concentration itself — that is the INDEPENDENT variable, not a controlled one.
• Naming the flexibility of the strips — that is the dependent variable (what is measured).
• Giving generic answers like 'temperature' when the method never mentions controlling temperature.

Things to Be Careful About

Give exactly two variables — extra ones gain nothing, and a wrong one (like the independent variable) may cost the mark if it replaces a correct one.

Walkthrough

This is the explanation behind everything observed in (a)(i) and (a)(ii). Osmosis is the diffusion of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. Every potato cell is surrounded by a partially permeable cell membrane, so water can pass in or out depending on the relative water potentials of the cell contents and the surrounding solution.

Solution S1 must be more dilute than the cell sap (higher water potential than the cytoplasm), so water flows INTO the cells. The vacuoles swell, the cytoplasm presses on the cell wall, and the cells become turgid — the strip stiffens and bends less.

Solution S2 must be more concentrated than the cell sap (lower water potential), so water flows OUT of the cells into the solution. The cells lose water, become flaccid, and the strip goes soft and floppy.

The four marks map onto four points: (1) mention movement of water; (2) name osmosis; (3) water INTO S1's cells → firmer; (4) water OUT OF S2's cells → softer.

Key Takeaways

• Always name osmosis explicitly — describing water movement alone does not earn the osmosis mark.
• Direction of water movement depends on comparing water potentials: water moves from higher to lower water potential.
• Turgor pressure against the inelastic cell wall is why turgid tissue is firm.

Common Mistakes

• The mark scheme states 'Ig diffusion' — saying 'diffusion' instead of osmosis gains nothing for the osmosis point.
• 'R if osmosis and active transport in the same answer' — never suggest active transport here; water moves passively by osmosis.
• Explaining only one strip — both directions (in for S1, out for S2) are needed for full marks.
• Saying water moves 'to equalise concentrations' without naming osmosis or the membrane.

Things to Be Careful About

Use the exact word osmosis. Link each direction of water movement to the observed change in flexibility — the '+' structure of the mark scheme means the water movement AND its effect are needed together.

Walkthrough

Use the key under Fig. 1.3: a plasmolysed cell shows a shrunken oval of cytoplasm inside the cell boundary, while a non-plasmolysed cell is filled solid grey. Work through the grid systematically — row by row, left to right — ticking off each cell so none is counted twice or missed. There are 7 cells with the shrunken oval (plasmolysed) and 21 filled grey (non-plasmolysed), giving 28 cells in total.

Key Takeaways

• Always use the printed key to decide what counts as each category.
• Systematic counting (row by row) avoids double-counting in a grid of cells.

Common Mistakes

• Misreading the key and counting the oval cells as non-plasmolysed.
• Skipping cells or counting boundary cells twice — check the total adds up to 28.

Things to Be Careful About

Both numbers must be correct for the single mark; the pair 7 and 21 is credited together.

Walkthrough

The question asks for the plasmolysed cells as a percentage of the TOTAL number of cells, not of the non-plasmolysed cells. Total = 7 + 21 = 28. Divide 7 by 28 and multiply by 100: 7 ÷ 28 = 0.25, and 0.25 × 100 = 25%. Writing the answer as 25.0% matches the precision shown in the mark scheme.

Note the mark scheme's rule: the correct answer with no working still gains both marks, but if your answer is wrong, one mark is available for correct working — so always show the substitution.

Key Takeaways

• percentage = (part ÷ whole) × 100, where 'whole' is stated in the question — here the total of ALL cells.
• Showing working protects a mark even if arithmetic slips.

Common Mistakes

• Dividing by 21 (the non-plasmolysed count) instead of 28 — this gives 33.3%, which is wrong.
• Forgetting to multiply by 100.
• Not showing working and then making an arithmetic slip, losing both marks.

Things to Be Careful About

Read 'total' carefully — it means plasmolysed + non-plasmolysed. Give the unit (%) as printed on the answer line.

Walkthrough

Graph questions on Paper 3 are marked on conventions, and each convention is a separate mark:

1. Axes fully labelled — write 'concentration of sucrose solution / mol per $\text{dm}^3$' along the x-axis and 'percentage of cells that were plasmolysed' along the y-axis, units included. An unlabelled axis loses the first mark even if everything else is perfect.
2. Correct orientation and scales — the independent variable (concentration) goes on the x-axis and the dependent variable (% plasmolysed) on the y-axis. Both scales must be LINEAR (equal steps per square), start at 0 at the origin, and use at least half the grid in each direction. A scale of 2 squares per 0.1 mol per dm³ and 2 squares per 10% works well on this grid.
3. Plotting — all five points must be plotted within half a small square of their true position. Use a sharp pencil and small, neat crosses.
4. Joining — the question specifies ruled STRAIGHT lines between consecutive points (not a smooth curve), and the line must STOP at the last point (0.8, 100). Extrapolating beyond the data loses the fourth mark.

Key Takeaways

• Independent variable on x, dependent variable on y, always.
• Linear scales, zero at the origin, at least half the grid used.
• Follow the joining instruction exactly: here ruled straight lines, not a curve.
• Never extend a line beyond the plotted data.

Common Mistakes

• Swapping the axes (putting % on the x-axis).
• Non-linear scales (e.g. 0, 0.2, 0.5, 0.8 equally spaced).
• Using a smooth curve when the question demands ruled straight lines.
• Extrapolating the line past (0.8, 100) towards the top of the grid.
• Missing axis units — '/ mol per dm³' must appear.

Things to Be Careful About

Plot points to within half a small square. Check each point against the table before drawing the line — one misplotted point costs the plotting mark outright.

Walkthrough

50% lies between the plotted values 18% (at 0.4) and 75% (at 0.6), so this is interpolation — reading a value BETWEEN plotted points, which is legitimate (unlike extrapolation beyond the data). The method:

1. Find 50 on the y-axis and rule a horizontal line across until it meets the straight segment joining (0.4, 18) and (0.6, 75).
2. From that meeting point, rule a vertical line down to the x-axis.
3. Read the concentration where the vertical line lands. On this segment the line rises 57 percentage points over 0.2 mol per dm³, so 50% falls just over halfway: about 0.55 mol per dm³.

The mark scheme awards one mark for the construction lines being SHOWN on the graph and one for the value read correctly from the candidate's own working, with a tolerance of ± half a small square. This means your answer must be consistent with YOUR OWN lines — a slightly different scale gives a slightly different read-off, and both can score.

Key Takeaways

• Interpolation (between points) is valid; extrapolation (beyond the last point) is not.
• Construction lines must actually be drawn — the examiner marks the working on the graph, not just the final number.
• The accepted value comes from the candidate's own graph, within ± half a small square.

Common Mistakes

• Giving a value without drawing the horizontal and vertical construction lines — this loses the working mark.
• Reading from the wrong segment (e.g. between 0.6 and 0.8).
• Extrapolating instead of interpolating.
• Quoting a value inconsistent with the drawn lines.

Things to Be Careful About

Include the unit 'mol per $\text{dm}^3$' in the answer — the mark scheme requires it. Keep construction lines ruled and light but visible.