Biology 9700/35 — May/June 2010

Background Concept

Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. When plant tissue is placed in a sucrose solution, water moves out of (or into) the cells depending on the relative water potentials of the cells and the solution. Solutions of equal water potential cause no net movement and therefore no change in mass.

Density is mass per unit volume. A sucrose solution that gains water (becomes more dilute) has a lower mass of sucrose per unit volume, so it becomes less dense. The change in the tissue is the converse of the change in the surrounding solution.

Understanding the Question

The question tells you that plant tissues placed in solutions A and B did not change mass (so their water potentials matched). It then asks you to complete two gaps in sentences that describe what would happen if the tissue did lose water to the solution. The first blank is the verb (gains/loses), the second is the adjective (less/more).

Approach

Use the principle of osmosis to decide which way the water moves, then think about the consequence for the solution.

Step-by-Step Reasoning

• First gap: water moves OUT of the tissue and INTO the solution, so the tissue loses water.
• Second gap: the solution gains water, becoming more dilute; more dilute means less sucrose per unit volume, so the solution becomes less dense.

Key Takeaways

• Osmosis describes NET movement of water down a water-potential gradient.
• Adding water to a solution dilutes it and lowers its density.
• These simple principles underpin the whole of this practical investigation.

Common Mistakes

• Choosing "gains" (the opposite direction of water movement).
• Choosing "more dense" (the opposite change in density — adding water makes the solution less dense, not more).
• Confusing which side is losing and which is gaining water.

Things to Be Careful About

• The two missing words are different parts of speech: one is a verb, one an adjective.
• Read the sentence context before choosing — each gap has its own biological meaning.

Background Concept

The drop method compares the densities of two solutions. A sucrose solution of higher concentration has higher density than one of lower concentration. A small coloured drop injected into a second solution moves according to its density relative to that second solution: a denser drop sinks, a less dense drop rises, and a drop of equal density remains stationary.

Understanding the Question

Fig. 1.3 provides three printed test-tube diagrams, each showing a coloured drop at a marker line inside a sucrose solution. The three tubes are pre-labelled with the relative concentration of the drop and the surrounding solution. You must annotate each diagram to show which way the drop moves (or that it stays still).

Approach

For each tube, decide whether the drop is denser or less dense than the surrounding solution, then indicate the direction of movement with an arrow (or with no arrow if there is no movement).

Step-by-Step Reasoning

• Left tube — "drop more concentrated than solution": The drop is denser, so it sinks. Reposition the drop below the marker line with a downward arrow.
• Middle tube — "drop same concentration as solution": The drop has the same density, so it experiences no net force and stays at the marker line. Do not move it.
• Right tube — "drop less concentrated than solution": The drop is less dense, so it rises. Reposition the drop above the marker line with an upward arrow.

Key Takeaways

• Density increases with sucrose concentration.
• A denser drop sinks; a less dense drop rises; equal density drops remain stationary.
• This principle is the basis for the calibration series you will build in the next part of the question.

Common Mistakes

• Showing all three drops moving in the same direction.
• Reversing the directions (denser drop rising).
• Failing to make the no-movement case for the middle tube explicit.
• Drawing unclear or ambiguous arrows.

Things to Be Careful About

• The middle tube MUST show no movement — equal density means no net force.
• Use clear, unambiguous arrows.
• Keep the marker line visible so the examiner can see the displacement.

Background Concept

To identify the concentration of an unknown sucrose solution using the drop method, you need a series of standards (solutions of known concentration). These are made by diluting a stock solution (here $1.0\ \text{mol dm}^{-3}$ sucrose) with distilled water.

The dilution formula is $C_1 V_1 = C_2 V_2$, where $C_1$ and $V_1$ are the concentration and volume of the stock, and $C_2$ and $V_2$ are the concentration and volume of the diluted solution. With a $1.0\ \text{mol dm}^{-3}$ stock and a target volume of $50\ \text{cm}^3$, the volume of stock required is numerically equal to the desired concentration multiplied by 50, and the volume of water is $50\ \text{cm}^3$ minus that stock volume.

A good calibration series uses at least three (preferably five) concentrations spaced evenly across the likely range of the unknowns.

Understanding the Question

You are given $200\ \text{cm}^3$ of $1.0\ \text{mol dm}^{-3}$ sucrose solution and $200\ \text{cm}^3$ of distilled water. You need to make $50\ \text{cm}^3$ of each standard. You must present a clear plan showing the concentrations chosen, the volume of stock needed for each, and the volume of water needed to make $50\ \text{cm}^3$ total.

Approach

Choose an even range of concentrations covering the expected range of the unknowns. Apply $C_1 V_1 = C_2 V_2$ for each, then subtract the stock volume from $50\ \text{cm}^3$ to find the water volume. Present this as a clear table.

Step-by-Step Reasoning

• Choose five evenly spaced concentrations spanning $0$–$1.0\ \text{mol dm}^{-3}$, e.g. $0.2, 0.4, 0.6, 0.8, 1.0\ \text{mol dm}^{-3}$ (interval of $0.2$).
• For each, calculate the stock volume using $V_1 = C_2 \times 50\ \text{cm}^3 / 1.0 = C_2 \times 50\ \text{cm}^3$:
  • $0.2\ \text{mol dm}^{-3}$: $10\ \text{cm}^3$ stock + $40\ \text{cm}^3$ water
  • $0.4$: $20\ \text{cm}^3$ + $30\ \text{cm}^3$
  • $0.6$: $30\ \text{cm}^3$ + $20\ \text{cm}^3$
  • $0.8$: $40\ \text{cm}^3$ + $10\ \text{cm}^3$
  • $1.0$: $50\ \text{cm}^3$ + $0\ \text{cm}^3$
• Verify that stock + water = $50\ \text{cm}^3$ for each row.

Key Takeaways

• A calibration series must be evenly spaced and span the expected range of the unknowns.
• $C_1 V_1 = C_2 V_2$ is the standard dilution equation.
• Stock + water must equal the total volume required.

Common Mistakes

• Choosing uneven intervals (e.g. 0.1, 0.3, 0.5, 0.7 — intervals of 0.2 followed by 0.4).
• Forgetting to subtract stock volume from total volume to get water volume.
• Picking only one or two concentrations — three is the minimum, but five or more gives better resolution.
• Using awkward numbers (e.g. 0.13) that are difficult to measure accurately.

Things to Be Careful About

• The stock and water volumes for each row must sum exactly to $50\ \text{cm}^3$.
• The concentrations must be achievable by dilution of $1.0\ \text{mol dm}^{-3}$ stock (i.e. each must be ≤ 1.0).
• Present the table with clear column headings including units.

Background Concept

Recording qualitative observations clearly is essential in practical work. The CIE Paper 3 convention is:

• Headings at the top and to the left of the table, each with a quantity and a unit.
• Ruled internal cells but no outer boundary line.
• A key for any symbols used (e.g. ↑ for rises, ↓ for sinks, → for no movement).

Understanding the Question

You have tested drops of unknown solution A and unknown solution B (each coloured blue) in your series of standard sucrose solutions and recorded whether each drop rose, fell or stayed put. You must now record those observations in a properly formatted table.

Approach

Construct a table with the sucrose concentration of each standard as the left-hand column and one column per unknown (syringe A and syringe B). In each cell record the direction of drop movement using clear symbols. Include a key explaining the symbols.

Step-by-Step Reasoning

• The table needs the column headings: "sucrose concentration / $\text{mol dm}^{-3}$", "syringe A", "syringe B".
• Include a key: "↑ = rises, ↓ = falls, → = no movement".
• For each standard concentration, record what happened to the A drop and the B drop.
• The expected pattern (with the unknown concentrations suggested by the mark scheme) is:
  • Unknown A is about $0.7\ \text{mol dm}^{-3}$: in standards less concentrated than A (0.2, 0.4, 0.6), the drop is denser than the surroundings and sinks; in standards more concentrated (0.8, 1.0), it is less dense and rises.
  • Unknown B is about $0.25\ \text{mol dm}^{-3}$: it is denser than only the 0.2 standard (where it sinks) and less dense than all the others (where it rises).

Key Takeaways

• A results table for qualitative observations needs clear headings with units, a key for symbols, ruled cells with no outer boundary.
• The trend of drop movement (falling at low concentrations, rising at high concentrations) identifies the unknown's concentration: it lies between the last "fall" and the first "rise".

Common Mistakes

• Forgetting units in the heading ($\text{mol dm}^{-3}$).
• Drawing an outer boundary line around the whole table.
• Omitting a key.
• Recording only one drop per concentration (the mark scheme expects two observations per concentration — one for A and one for B).

Things to Be Careful About

• Use a consistent key throughout the table.
• Make sure every (concentration × syringe) combination is filled in.
• Note the boundary where the direction changes — this is where the unknown concentration lies.

Background Concept

When the drop of unknown solution neither rises nor falls, it has the same density (and hence the same concentration and water potential) as the surrounding standard. The concentration at which the drop reverses direction therefore gives the concentration of the unknown.

Understanding the Question

Use your results from part (a)(iv) to estimate the concentrations of unknowns A and B. The answer is a single value in $\text{mol dm}^{-3}$ for each.

Approach

Locate, for each unknown, the pair of adjacent standard concentrations where the drop changes from falling to rising (or vice versa). The unknown's concentration lies between those two values; estimate it by eye.

Step-by-Step Reasoning

• For unknown A: the drop falls in standards $0.2, 0.4, 0.6$ and rises in $0.8, 1.0$. The reversal lies between $0.6$ and $0.8$. Reading from the table, $A \approx 0.7\ \text{mol dm}^{-3}$.
• For unknown B: the drop falls only at $0.2$ and rises at $0.4, 0.6, 0.8, 1.0$. The reversal lies between $0.2$ and $0.4$. Reading from the table, $B \approx 0.25\ \text{mol dm}^{-3}$.

Key Takeaways

• The drop-reversal point gives the unknown's concentration.
• If the reversal is between two standards, the unknown lies between those values; estimate the value by eye.

Common Mistakes

• Quoting a value outside the bracketing range.
• Quoting too many significant figures when the calibration spacing is $0.2$ — one significant figure (or at most two) is appropriate.
• Failing to include the unit $\text{mol dm}^{-3}$.

Things to Be Careful About

• The value must be consistent with your own observations.
• Use the unit $\text{mol dm}^{-3}$.
• If your data are slightly different, give the value supported by your own table.

Background Concept

A calibration graph converts one measured quantity into another. Here you will use it to convert the concentration of unknown A (which you will determine in part (a)(v)) into a water potential.

CIE plotting conventions require:

• Each axis labelled with the quantity and its unit.
• A linear, easy-to-read scale (multiples of 1, 2 or 5 per 2 cm — not 3 or 7).
• The scale using at least half the available grid in both directions.
• All points plotted with a small cross (×) or a dot in a circle (⊙), so the intersection is clear.
• A single straight line of best fit through the points (not dot-to-dot joining).

For a y-axis with negative values, the zero must be at the top, with values becoming more negative going down.

Understanding the Question

Plot the data in Table 1.1 with sucrose concentration on the $x$-axis and water potential on the $y$-axis, then draw the line of best fit.

Approach

• Label the axes with quantity and unit.
• Choose scales: $x$-axis $0$ to $1.0$ in $0.2$ steps; $y$-axis $0$ (at top) to at most $-35$ in steps of $-5$.
• Plot all five data points clearly.
• Draw a single thin, ruled straight line through the points.

Step-by-Step Reasoning

• X-axis label: "sucrose concentration / $\text{mol dm}^{-3}$". A suitable scale is $0$ to $1.0$ with major gridlines every $0.2$.
• Y-axis label: "water potential / $\text{kPa} \times 10^2$". Because the values are negative, place $0$ at the top and label each major gridline with the corresponding negative value (e.g. $0, -5, -10, -15, -20, -25, -30, -35$).
• Plot the five points: $(0.15, -5.0)$, $(0.35, -12.0)$, $(0.55, -19.0)$, $(0.75, -26.0)$, $(1.00, -35.0)$.
• Line of best fit: the points lie almost perfectly on a straight line, so draw a single ruled straight line through all five. Do not join dot-to-dot.

Key Takeaways

• Always label both axes with quantity AND unit.
• Choose a linear scale that uses at least half the grid.
• Plot all points with a small clear cross or dot in a circle.
• A single line of best fit — not joined segments — is required when the relationship is approximately linear.

Common Mistakes

• Reversing the axes (sucrose on $y$, water potential on $x$).
• Forgetting units on either axis label.
• Awkward scale intervals (e.g. 3 per 2 cm).
• Plotting a point at the wrong position.
• Drawing a thick or feathery line.
• Joining the points dot-to-dot rather than drawing a line of best fit.
• Drawing the line through only some of the points.

Things to Be Careful About

• Negative y-axis: place 0 at the top; mark each gridline with the negative value.
• The line must be ruled (not freehand-sketched) and thin.
• Each axis must use at least half the available grid.

Background Concept

Once a calibration graph is drawn, an unknown value can be read off by drawing a vertical line from the $x$-value up to the calibration line, then a horizontal line from that intersection across to the $y$-axis. The intersection must be marked clearly so the examiner can see the working.

Understanding the Question

Use your graph from part (b)(i) and the concentration you found for sample A in part (a)(v) to estimate the water potential of A. Show your construction lines on the graph.

Approach

• Vertical line from $A = 0.7\ \text{mol dm}^{-3}$ up to the calibration line.
• Horizontal line from the intersection across to the $y$-axis.
• Read off the $y$-value, including units.

Step-by-Step Reasoning

• For $A \approx 0.7\ \text{mol dm}^{-3}$:
  • Using the linear relation between the tabulated values: slope $= (-35 - (-5))/(1.00 - 0.15) = -30/0.85 \approx -35.3$ per $\text{mol dm}^{-3}$.
  • At $x = 0.7$: $y \approx -5 + (0.7 - 0.15) \times (-35.3) = -5 - 19.4 \approx -24.4$.
• Reading from the graph: approximately $-24$ to $-25$ (in $\text{kPa} \times 10^2$ units), i.e. roughly $-2400\ \text{kPa}$.

Key Takeaways

• A calibration graph converts between variables.
• Construction lines (vertical then horizontal) are the standard way to read off an unknown value.
• Always include units with the final reading.

Common Mistakes

• Forgetting to draw the construction lines on the graph (losing the working mark).
• Quoting the answer without units.
• Misreading the y-axis (e.g. quoting $-24\ \text{kPa}$ when the axis is in $\text{kPa} \times 10^2$ — it should be $-24$ in those units, i.e. $-2400\ \text{kPa}$).

Things to Be Careful About

• The y-axis is in $\text{kPa} \times 10^2$; either quote the value in those units or convert to $\text{kPa}$.
• Make the construction lines obvious on the graph so the examiner can see the working.

Background Concept

To improve the accuracy of a practical procedure, identify specific sources of error and devise ways to reduce each one. In this experiment the main limitations are: (i) the coarse spacing of the standard concentrations, (ii) variation in the amount of dye added (which slightly changes the density of the drop), and (iii) variation in the volume of the drop released.

Understanding the Question

Suggest three improvements that would give a more accurate estimate of the water potential of sample A.

Approach

Walk through each step of the procedure mentally and ask "what could be more precise here?". Focus on (a) the calibration series, (b) the preparation of the coloured drop, and (c) the way the drop is released/observed.

Step-by-Step Reasoning

• Coarse calibration series — currently five standards at $0.2$ intervals. Near the unknown's concentration the reversal point can only be bracketed to ±$0.1$. Preparing more standards tightly clustered around $0.7\ \text{mol dm}^{-3}$ (e.g. $0.60, 0.65, 0.70, 0.75, 0.80$) would narrow the bracket and give a more precise answer.
• Variable dye volume — adding more or less methylene blue slightly changes the drop's density. Use a fixed number of drops from the same pipette each time.
• Variable drop volume — the syringe is pressed by hand, so drop size varies. Fix the syringe (e.g. always use the same setting) so each released drop has the same volume.
• Qualitative observation only — "rises / falls" is binary. Measuring the time for the drop to travel a fixed distance gives a quantitative measurement that can be plotted and compared across standards.

Key Takeaways

• Improvements must address specific limitations of the procedure, not vague "be more careful".
• More standards near the reversal point improve precision.
• Standardising dye and drop volumes removes uncontrolled variation.
• Quantitative measurements (e.g. time) carry more information than qualitative observations.

Common Mistakes

• Vague suggestions ("be more careful", "repeat the experiment", "use better equipment") — these don't address specific limitations.
• Improvements that are not practical in the school lab.
• Confusing improvements with controls (e.g. "use a control" — there is no control needed here, only better resolution and standardisation).

Things to Be Careful About

• Each improvement should be specific and tied to a particular limitation.
• The mark scheme accepts up to three distinct valid improvements.
• "Repeat the experiment" alone is not credited unless it is linked to a specific source of error.