Biology 9700/32 — May/June 2019

Background Concept

A proportional (sometimes called simple) dilution is made by adding a measured volume of a stock solution of known concentration to a measured volume of diluent — here distilled water, W — to give a known final volume of a known lower concentration. The amount of solute is conserved, so the standard relation applies:

where $C_1$ and $V_1$ are the concentration and volume taken from the stock, and $C_2$ and $V_2$ are the concentration and total volume of the diluted solution. Because each new concentration is prepared directly from the 10% stock (not from the previous dilution), errors do not compound as they would in a true serial dilution.

The required final volume is $5.0\,\text{cm}^3$. This is comfortably more than the $2.0\,\text{cm}^3$ needed for the test in step 4 of the procedure, leaving enough for a clean transfer with a syringe.

Understanding the Question

Table 1.2 has one filled row (10%, 5.0 cm³ S + 0.0 cm³ W). The candidate must add four more rows so that the concentration drops by $2\%$ each time, each new solution being made up to $5.0\,\text{cm}^3$ using only S and W. The hazard table reminds the candidate to wear eye protection when using the irritant H later, but does not affect the dilution table itself.

Approach

For each target concentration, calculate the volume of stock needed, then the volume of water that brings the total to $5.0\,\text{cm}^3$:

1. Target concentration $C_2$: 8, 6, 4, then $2\%$.
2. $V_2 = 5.0\,\text{cm}^3$ for each row.
3. $V_1 = \dfrac{C_2 \times V_2}{C_1} = \dfrac{C_2 \times 5.0}{10.0} = 0.5 \times C_2$ (in $\text{cm}^3$).
4. $V_W = 5.0 - V_1$.

Step-by-Step Reasoning

8%: $V_1 = 0.5 \times 8 = 4.0\,\text{cm}^3$ of S; $V_W = 5.0 - 4.0 = 1.0\,\text{cm}^3$ of W.

6%: $V_1 = 0.5 \times 6 = 3.0\,\text{cm}^3$ of S; $V_W = 5.0 - 3.0 = 2.0\,\text{cm}^3$ of W.

4%: $V_1 = 0.5 \times 4 = 2.0\,\text{cm}^3$ of S; $V_W = 5.0 - 2.0 = 3.0\,\text{cm}^3$ of W.

2%: $V_1 = 0.5 \times 2 = 1.0\,\text{cm}^3$ of S; $V_W = 5.0 - 1.0 = 4.0\,\text{cm}^3$ of W.

Every row sums to $5.0\,\text{cm}^3$ and the fraction of S halves as the concentration halves — a quick sanity check.

Key Takeaways

• Proportional dilution from a single stock avoids cumulative dilution error.
• Always check that the two measured volumes add up to the chosen final volume.
• The dilution table also serves as a checklist for the practical work — one row per test-tube, prepared in the labelled beaker.

Common Mistakes

• Treating this as a serial dilution (taking $5\,\text{cm}^3$ of one dilution and adding $5\,\text{cm}^3$ of water to halve the concentration). This would give 5%, 2.5%, 1.25% and 0.625% — not the required 8, 6, 4 and 2% — and would require more than $5\,\text{cm}^3$ per step.
• Recording only one extra row (or none).
• Forgetting to subtract the S volume from $5.0\,\text{cm}^3$ to get the W volume.

Things to Be Careful About

• Use a clean, dry syringe or pipette for each row; cross-contamination would shift all subsequent times.
• Mix each dilution thoroughly before the $2\,\text{cm}^3$ aliquot is taken.
• The volumes in the table are the volumes measured into the beaker, not the volume transferred to the test-tube.

Background Concept

The test for non-reducing sugars has two stages. Sucrose is a non-reducing disaccharide, so it does not react with Benedict's solution directly. In step 6 of the procedure the sucrose is hydrolysed to glucose and fructose by boiling with dilute hydrochloric acid (H); the acid is then neutralised in steps 10–11 with sodium hydrogencarbonate (A). The mixture now contains reducing sugars, and Benedict's solution (added in step 12) reacts with them on heating to give a brick-red precipitate of copper(I) oxide.

The time taken for the first appearance of this colour change depends on the concentration of reducing sugar — and therefore on the original concentration of sucrose. More sucrose → more glucose and fructose after hydrolysis → faster reduction of Cu²⁺ to Cu⁺ by the Benedict's reagent → shorter time for the first appearance of the colour change. This is the basis of the calibration the candidate will use in (a)(iii) and (a)(iv) to estimate the concentration of U.

Understanding the Question

The candidate is asked to record their own times, in an appropriate table, for the five known sucrose concentrations. The word appropriate is what carries the marks: the table must conform to CIE conventions for raw data.

Approach

Lay out the table before the experiment, with a row for each concentration, a column for concentration and a column for time. After each test, write the time in whole seconds. After all five tests, check the trend: it should run from shortest time at 10% to longest time at 2%.

Step-by-Step Reasoning

1. Headings. The left-hand column heading is the independent variable: percentage concentration of sucrose with the unit (%) on a separate line beneath, so the data cells contain only numbers. The right-hand column heading is the dependent variable: time for first appearance of colour change with the unit (s) on a separate line.
2. Rows. One row per concentration, in the same order as in Table 1.2 (10, 8, 6, 4, 2%), so the candidate can read straight across to match the test-tube label.
3. Times. The candidate uses a stopwatch started as the test-tube goes into the boiling water-bath (step 14) and stopped at the first appearance of any orange/red colour (step 15). The result is written as a whole number of seconds; if no change has occurred by 120 s, the entry is "more than 120".
4. Trend check. The times should rise as the concentration falls, because the Benedict's reaction is slower at lower sugar concentrations. Any time that breaks this monotonic trend is an anomaly to be repeated, or to be noted.

Key Takeaways

• A good results table is laid out before the experiment; the only thing the candidate writes during the experiment is the time.
• The first column is always the independent variable; the second is the dependent variable.
• Headings carry the quantity and the unit; the unit goes on a separate line under the quantity so the data cells contain only numbers.
• The expected trend is short time → high concentration; long time → low concentration. Any break in the trend is a likely error and should be rechecked.

Common Mistakes

• Putting the unit in the data cells ("28 s" instead of "28"). This loses the heading mark and looks unprofessional.
• Recording decimal seconds ("28.4") when the stopwatch is being read at the appearance of a colour. The mark scheme requires whole seconds.
• Only recording some of the concentrations, or recording them in a different order so the trend is hard to see.
• Forgetting to record the result as "more than 120" if no change occurs within 120 s — this is the most important data point on the bottom of the range and is what makes the calibration usable in (a)(iii).

Things to Be Careful About

• Always start timing from the moment the test-tube enters the boiling water-bath, not from when the candidate picks up the stopwatch.
• "First appearance of colour change" can be a faint green→yellow→orange transition before the brick-red end-point. Take the first visible departure from blue, not the full brick-red.
• Do not include the 120 s timeout in any mean or further calculation — it is a censored value, not a measurement of 120 s.

Background Concept

The Benedict's test result for U is the raw datum that links parts (a)(ii) and (a)(iv) of the question. By comparing the time taken for U with the times for the five known concentrations, the candidate can place U somewhere on the calibration curve. Whether that position falls inside the 5.5–7.5% range for fusicoccin-treated plants is the basis of the conclusion in (a)(iv).

Understanding the Question

The candidate repeats steps 12–15 with the unknown solution U and writes the time for the first appearance of the colour change in the blank labelled result for U. The mark is for stating the time clearly with its unit.

Approach

Time the reaction with a stopwatch in the same way as for the standards. Read the time at the first appearance of any colour change, and write the value as a whole number of seconds in the blank, with the unit "s". If no change has occurred within 120 s, write "more than 120 s".

Step-by-Step Reasoning

1. Add $4\,\text{cm}^3$ of Benedict's solution to the test-tube labelled U (step 12) and shake gently to mix (step 13).
2. Place the test-tube in the boiling water-bath and start the stopwatch immediately (step 14).
3. Watch the lower part of the liquid against a white background; stop the stopwatch at the first appearance of any orange or red colour (step 15).
4. Read the time in whole seconds and write it in the blank, e.g. result for U = 40 s.

Key Takeaways

• The result for U is the bridge between the calibration table and the conclusion; treat it with the same care as any of the standards.
• Always quote a unit — the mark scheme credits result + unit as a single mark.

Common Mistakes

• Forgetting the unit "s". The mark scheme says "states the result for U + units"; both are required.
• Timing to the full brick-red end-point rather than the first appearance of the colour change, which is what the standards were timed to.
• Recording decimal seconds when the standard protocol used whole seconds.

Things to Be Careful About

• Use the same water-bath, the same volume of Benedict's solution and the same lighting as for the standards, so the comparison in (a)(iv) is fair.
• If no change occurs within 120 s, write "more than 120 s" rather than 120 s; recording 120 s would suggest a low but detectable concentration, not an undetectable one.

Background Concept

The Benedict's test is being used here as a quantitative rather than a qualitative test. The time for the first appearance of the colour change is inversely related to the concentration of sucrose in the sample. The candidate's five standards (10, 8, 6, 4, 2%) form a calibration ladder; U's position on that ladder is estimated by where its time falls between the standards.

Fusicoccin is a fungal toxin that stimulates the plasma-membrane H⁺-ATPase of companion cells, increasing sucrose loading into the phloem. The problem states that, in this experiment, fusicoccin-treated plants have a phloem sucrose concentration of 5.5–7.5%. The conclusion in this part is simply a matter of deciding whether U's estimated concentration lies inside that band.

Understanding the Question

The candidate must (i) estimate the concentration of sucrose in U from their calibration, (ii) state whether that concentration is in the 5.5–7.5% range, and (iii) give a reason that explicitly references the concentration of sucrose in U. The answer is binary (yes/no) but must be supported by the data.

Approach

1. Read off the time for U from (a)(iii).
2. Compare it with the times in the table from (a)(ii). The standards straddle the 5.5–7.5% band at 6% and 8%, so the question reduces to whether U's time falls between the times for the 6% and 8% standards.
3. State yes or no, and quote the corresponding concentration range of sucrose in U as the reason.

Step-by-Step Reasoning

Case 1 — U came from a fusicoccin-treated plant. The time for U lies between the time for the 6% standard and the time for the 8% standard. The candidate should then interpolate to estimate U's concentration (e.g. 40 s might sit closer to the 6% standard at 45 s than to the 8% standard at 35 s, giving an estimated 6.5%). The estimated concentration is in the 5.5–7.5% band, so the answer is yes, and the reason is that the concentration of sucrose in U is between 5.5% and 7.5%.

Case 2 — U did not come from a fusicoccin-treated plant. The time for U is shorter than the 6% standard (concentration above 7.5%) or longer than the 8% standard (concentration below 5.5%, or possibly no reaction within 120 s). The estimated concentration is outside the band, so the answer is no, and the reason is that the concentration of sucrose in U is above 7.5% (or below 5.5%).

In both cases the reason must explicitly reference the concentration of sucrose in U, not just the time. The mark scheme requires "reference to the concentration of sucrose in U".

Key Takeaways

• A quantitative biochemical test is read by interpolation between standards, not by extrapolation beyond them.
• A conclusion is only as good as the justification that supports it; here, the justification is the estimated concentration of sucrose in U.
• A "yes/no" question still requires a reason — the reason is what earns the mark.

Common Mistakes

• Stating yes or no with no reason, or with a reason that just restates the time for U (e.g. "because it took 40 s"). The mark scheme requires a reference to the concentration of sucrose in U.
• Extrapolating beyond the 6%–8% standards to claim a concentration outside the calibration range (e.g. "the concentration is 4%" when the time for U is shorter than for the 4% standard). The conclusion is not valid outside the calibrated range.
• Confusing time for the colour change with concentration in the reason.

Things to Be Careful About

• The 2% interval between standards is too coarse to give a tight estimate of U's concentration; the candidate should acknowledge this and answer in terms of the band the time falls into (e.g. "between 6% and 8%") rather than claim a more precise value.
• The 5.5–7.5% band straddles two standards, so the answer is unambiguous only if U's time sits clearly between them; if it sits very close to one or the other, the candidate should say so and qualify the answer.

Background Concept

The resolution of any quantitative comparison is set by the spacing of the standards. With 2% steps between 2% and 10%, any unknown whose time lies between two adjacent standards can only be assigned to a 2%-wide band. If the band straddles the threshold of interest (here the 5.5–7.5% range for a fusicoccin-treated plant), the answer to (a)(iv) is robust, but the concentration of U can only be given to ±1%. Narrowing the steps halves the band and tightens the answer.

Proportional dilution from a single 10% stock is the cleanest way to make a closely-spaced series: for 1% steps, the volume of S in $5\,\text{cm}^3$ of diluted solution is $0.5\,\text{cm}^3$ per 1% concentration.

Understanding the Question

The candidate has answered (a)(iv) using 2%-step standards, which only allows the concentration of U to be assigned to a 2%-wide band. The question asks how the independent variable (the range of sucrose concentrations used for the calibration) could be changed to give more confidence in the answer.

Approach

Two improvements are needed:

1. Reduce the spacing of the standards so the band the time falls into is narrower.
2. Keep the method of making the dilutions reliable — proportional dilution from a single stock is the obvious choice.

Step-by-Step Reasoning

• Replace the 2% steps with 1% steps. New standards at 1, 2, 3, 4, 5, 6, 7, 8 and 9% (plus the original 10%) would allow the time for U to be assigned to a 1%-wide band.
• Make each new concentration by proportional dilution from the 10% stock, not by serial dilution. The volume of S in $5\,\text{cm}^3$ of a 1%-step standard is $0.5\,\text{cm}^3$ per 1% concentration; the rest is water.
• More standards means more test-tubes and more work, so the candidate should keep the total number of standards manageable (about 6–8 is usually the practical maximum within the time available).

Key Takeaways

• Resolution of a quantitative test is set by the spacing of the standards, not by the precision of the stopwatch.
• Proportional dilution from a single stock avoids cumulative error and is the right method whenever a closely-spaced series is needed.
• A calibration should be denser around the threshold of interest; here that is the 5.5–7.5% band for a fusicoccin-treated plant.

Common Mistakes

• Suggesting a wider range rather than a denser one (e.g. adding 12% and 14% standards). This does not improve the resolution around 5.5–7.5%.
• Suggesting more repeats of the existing standards rather than more standards at finer intervals. This improves reliability but not resolution.
• Suggesting "more accurate equipment" (e.g. a more precise stopwatch). The time is already being read to ±1 s; the bottleneck is the spacing of the standards.

Things to Be Careful About

• More standards means more test-tubes; the candidate should consider whether the apparatus and time allow for them.
• The narrower the interval, the smaller the expected difference in time between adjacent standards; the candidate should check that the time difference is still large enough to measure reliably.

Background Concept

A control is a treatment that differs from the experimental treatment in only one respect — here, the presence of fusicoccin. Everything else (the volume of liquid added, the time of application, the growing conditions) is held constant. By comparing the experimental plant (F) with the control plant (G), the experimenter can be confident that any difference in phloem sucrose concentration is due to the fusicoccin and not to some other variable.

Understanding the Question

The question describes two plants given identical treatments except that plant G received water in place of fusicoccin solution. The candidate must state the reason for treating G with water.

Approach

Recognise that the substitution of water for fusicoccin makes G a control, and state this clearly. One word — control — is enough to earn the mark, but a full sentence explains the role of the control and is the more complete answer.

Step-by-Step Reasoning

• Both plants receive the same volume of liquid ($10\,\text{cm}^3$) and are grown in the same conditions.
• The only difference is the presence of fusicoccin.
• Therefore G acts as a control, against which the effect of fusicoccin on F can be judged.

Key Takeaways

• A control matches the experimental treatment in every respect except the variable under test.
• Without a control, any observed difference could be due to handling, to the added water, or to any other unintended variable.

Common Mistakes

• Saying that G was a control but not explaining why the control is needed (the comparison with F).
• Confusing the control with a "repeat" or a "replicate". A control is a different treatment, not a repeat of the same one.
• Saying that G was given water "to keep it alive". This is true but does not answer the question, which is about the experimental design.

Things to Be Careful About

• The control must be as similar as possible to the experimental treatment. Here, the volume of water ($10\,\text{cm}^3$) is matched to the volume of fusicoccin solution ($10\,\text{cm}^3$); the growing conditions are identical.

Background Concept

A scatter graph with a line of best fit is the right way to display two continuous variables when the candidate wants to see the trend and read off intermediate values. The conventions of CIE graph plotting — labelled axes with units, sensible scales, small crosses for the points, and a ruled line of best fit — are there to make the graph readable and unambiguous.

Here the data are the rate of mass flow through the phloem measured every 35 min after treatment with fusicoccin. The graph is the basis of (b)(ii) (reading off the time at which the rate reached $20\,\text{cm}^3\,\text{min}^{-1}$) and of (b)(iii) (describing the effect of fusicoccin).

Understanding the Question

The candidate is given the data in Table 1.3 and a blank grid in Fig. 1.1. The task is to plot the data on the grid using a sharp pencil, with the conventions of axis labels, sensible scales, accurate points and a line of best fit.

Approach

1. Decide which variable goes on which axis. The independent variable (time, set by the experimenter) goes on the x-axis; the dependent variable (rate of mass flow, measured) goes on the y-axis.
2. Choose scales. Both axes should use a scale that uses at least half the grid in the direction of plotting, and that is "easy to read" — i.e. each labelled gridline is a round number of units.
3. Plot each point as a small cross or a small dot in a circle, using a sharp pencil so the mark is small and the coordinates are unambiguous.
4. Draw a single, continuous line of best fit through the points. The line should pass through the centre of the cloud of points, with roughly equal numbers of points on either side.

Step-by-Step Reasoning

x-axis. Time runs from 0 to 175 min, with five data points between 35 and 175. The 50-to-2-cm scale (one large square = 25 min, two large squares = 50 min) places the 0 at the left-hand edge of the grid, the 50 at the second labelled gridline, the 100 at the fourth, the 150 at the sixth and the 200 at the eighth. The 175-min point sits three-quarters of the way between the 150 and 200 labels. This scale uses most of the grid and the labels are round numbers.

y-axis. Rate of mass flow runs from 0 to 23.75 $\text{cm}^3\,\text{min}^{-1}$, with all five data points above 5. The 5-to-2-cm scale (one large square = 2.5 $\text{cm}^3\,\text{min}^{-1}$, two large squares = 5 $\text{cm}^3\,\text{min}^{-1}$) places the 0 at the bottom of the grid, the 5 at the second labelled gridline, the 10 at the fourth, the 15 at the sixth, the 20 at the eighth and the 25 at the tenth. This scale uses the full height of the grid and the labels are round numbers.

Points.

• (35 min, 6.00 $\text{cm}^3\,\text{min}^{-1}$): x = 35 (just past the 0–50 first quarter of the grid); y = 6.00 (just above the 5 label).
• (70 min, 10.25 $\text{cm}^3\,\text{min}^{-1}$): x = 70 (just past the 50–100 halfway); y = 10.25 (just above the 10 label).
• (105 min, 15.00 $\text{cm}^3\,\text{min}^{-1}$): x = 105 (just past the 100–150 first quarter); y = 15.00 (on the 15 label).
• (140 min, 19.50 $\text{cm}^3\,\text{min}^{-1}$): x = 140 (just before the 150 label); y = 19.50 (just below the 20 label).
• (175 min, 23.75 $\text{cm}^3\,\text{min}^{-1}$): x = 175 (three-quarters of the way between 150 and 200); y = 23.75 (just below the 25 label).

Line of best fit. The five points lie close to a straight line; a straight line of best fit is appropriate. The line has a positive slope of approximately $0.13\,\text{cm}^3\,\text{min}^{-1}$ per min, and a y-intercept (extrapolated) of about $1.5\,\text{cm}^3\,\text{min}^{-1}$ — which matches the pre-treatment rate given in (b)(iii).

Key Takeaways

• Always place the independent variable on the x-axis and the dependent variable on the y-axis.
• Choose scales that use most of the grid in the direction of plotting and that are easy to read (round-number labels).
• Plot points as small crosses or dots in circles, with a sharp pencil.
• A line of best fit should be a single ruled line, with roughly equal numbers of points on either side, and not necessarily passing through any individual point.

Common Mistakes

• Swapping the axes (rate on the x-axis, time on the y-axis). This is a non-standard choice and loses the axis mark.
• Using awkward scales (e.g. 1 small square = 3 min). The mark scheme requires the 50-to-2-cm scale on the x-axis and the 5-to-2-cm scale on the y-axis.
• Plotting the points as large dots or smudges, making the coordinates ambiguous.
• Joining the points dot-to-dot. The mark scheme allows either plot-to-plot or a line of best fit; dot-to-dot is acceptable but a line of best fit is the better choice for these data because the points are roughly linear.
• Drawing a line that misses most of the points (e.g. drawn from the first to the last point and ignoring the middle three). A line of best fit passes through the centre of the cloud of points, not through the extreme points.

Things to Be Careful About

• The x-axis scale is set so that two large squares = 50 min; do not change this to two large squares = 35 min, which would be awkward.
• The y-axis scale is set so that two large squares = 5 $\text{cm}^3\,\text{min}^{-1}$; do not change this to two large squares = 4 $\text{cm}^3\,\text{min}^{-1}$, which would be awkward.
• The line of best fit should be drawn with a sharp pencil and a ruler. Freehand curved lines are not appropriate for a linear trend.

Background Concept

The line of best fit is a model of the relationship between time and rate of mass flow. Once drawn, it can be used to read off the value of either variable corresponding to any chosen value of the other. To do this, the candidate draws a construction line from the chosen value on one axis, perpendicular to that axis, until it meets the line of best fit; then a second construction line, perpendicular to the first, back to the other axis. The two construction lines should meet on the line of best fit, and the value is read from the second axis.

Understanding the Question

The candidate is asked to find the time at which the rate of mass flow was $20\,\text{cm}^3\,\text{min}^{-1}$, and to show on the graph how the answer was obtained. The "show on the graph" instruction is the source of the second mark: the construction lines must be visible on the answer.

Approach

1. Find the value 20 on the y-axis.
2. Draw a horizontal line from this point to the right, until it meets the line of best fit.
3. From that intersection, draw a vertical line down to the x-axis.
4. Read the time value where the vertical line meets the x-axis.

Step-by-Step Reasoning

• The y-value 20 is at the eighth large square on the y-axis (each large square = 2.5 $\text{cm}^3\,\text{min}^{-1}$).
• The horizontal line from y = 20 crosses the line of best fit at approximately x = 144 min. Interpolating between (140, 19.50) and (175, 23.75):

• The vertical line from the intersection meets the x-axis at approximately 144 min.

Key Takeaways

• A line of best fit is a model; it can be used to read intermediate values between the plotted points, but it should not be extrapolated far beyond them.
• Construction lines must be visible on the graph paper; the second mark is for the visible lines, not just for the answer.
• The answer is read from the second axis, not estimated in the head.

Common Mistakes

• Forgetting to draw the construction lines, so the second mark ("shows on graph from y-axis to x-axis") is not earned.
• Reading the answer from the printed x-axis labels rather than the construction line; off-by-one errors (143 or 145) are common if the line is not used.
• Extrapolating beyond the right-hand end of the line of best fit. The value 20 $\text{cm}^3\,\text{min}^{-1}$ lies between the 140 and 175 data points, so no extrapolation is needed.

Things to Be Careful About

• Use a sharp pencil and a ruler for the construction lines so they are unambiguous.
• The construction lines should be drawn lightly so they are clearly distinct from the line of best fit itself.
• Read the time to the nearest minute; the mark scheme accepts a small tolerance (143–145 min).

Background Concept

Mass flow in the phloem is driven by the difference in hydrostatic pressure between the source (e.g. a leaf) and the sink (e.g. a root or a fruit). Anything that increases sucrose loading at the source raises the solute concentration inside the sieve tube, which draws in water osmotically, which raises the hydrostatic pressure, which drives faster mass flow towards the sink. Fusicoccin is known to stimulate the plasma-membrane H⁺-ATPase of companion cells, increasing the proton-motive force that drives sucrose–H⁺ cotransport; the question is testing whether the candidate can link this mechanism to the data.

Understanding the Question

The candidate is given a single pre-treatment rate ($1.5\,\text{cm}^3\,\text{min}^{-1}$) and a table of post-treatment rates that rise steadily. The task is to describe the effect of fusicoccin.

Approach

Compare the pre-treatment rate with the post-treatment rates. The pre-treatment rate is the baseline; the post-treatment rates are higher and rising. The candidate's job is to state the direction of the effect.

Step-by-Step Reasoning

• Pre-treatment: $1.5\,\text{cm}^3\,\text{min}^{-1}$ (one reading).
• Post-treatment: 6.00, 10.25, 15.00, 19.50, 23.75 $\text{cm}^3\,\text{min}^{-1}$ at 35, 70, 105, 140 and 175 min respectively.
• Every post-treatment rate is higher than the pre-treatment rate; the highest is about 16 times the pre-treatment rate.
• The trend across the post-treatment rates is a steady increase with time.

The mark-scheme point is the simple statement fusicoccin increases the rate of mass flow; a candidate who also quantifies the increase (e.g. "from 1.5 to 23.75 $\text{cm}^3\,\text{min}^{-1}$") demonstrates stronger understanding but the one-word statement is enough to earn the mark.

Key Takeaways

• A "describe the effect" question on a graph wants a comparison of the treatment with the control (here, the pre-treatment reading).
• The direction of the effect is the minimum acceptable answer; quantitative description is better.
• A trend within the data (the rate continues to rise across the 175 min of sampling) is a separate observation and not required by this part.

Common Mistakes

• Stating only that the rate "changes" or "increases over time" without comparing to the pre-treatment rate. The mark scheme requires an explicit statement that fusicoccin increases the rate.
• Stating that the rate increases "linearly" without supporting it. The data are close to linear, but the candidate should not over-claim without showing the working (a line of best fit in (b)(i) and an interpolation in (b)(ii)).
• Confusing the rate of mass flow with the rate of sucrose loading. The data are about mass flow, which is the consequence of loading, not the loading itself.

Things to Be Careful About

• The pre-treatment rate is a single number, not a data point on the graph. The candidate should compare to it explicitly.
• The unit of the rate is $\text{cm}^3\,\text{min}^{-1}$, not $\text{cm}^3$ (which would be a volume) or $\text{min}^{-1}$ (which would be a frequency).

Background Concept

Phloem loading is the process by which sucrose, manufactured in the mesophyll of a photosynthesising leaf, is transferred into the sieve-tube elements of the phloem against a concentration gradient. The current model is the apoplastic loading via companion cells:

1. Sucrose diffuses from the mesophyll through the symplast (plasmodesmata) to the companion cell, or is loaded apoplastically by a sucrose–H⁺ symporter (SUT or SUC transporter).
2. Inside the companion cell, the sucrose is concentrated by the activity of the plasma-membrane H⁺-ATPase, which pumps H⁺ out of the cell using ATP. The resulting proton-motive force drives the sucrose–H⁺ symporter.
3. The concentrated sucrose then moves through plasmodesmata from the companion cell into the sieve-tube element, where it raises the solute concentration and draws in water osmotically from the xylem. The turgor pressure in the sieve tube rises, driving mass flow towards the sink.

Fusicoccin is a fungal toxin that binds to and stimulates the H⁺-ATPase, increasing the proton-motive force and therefore the rate of sucrose loading. The question is testing whether the candidate can name two steps in this mechanism.

Understanding the Question

The candidate is asked to suggest how fusicoccin increases the loading of sucrose into phloem sieve tubes. The command word suggest means the candidate can offer a plausible mechanism based on biological knowledge; the mark scheme credits any two of the four steps above.

Approach

Recall the four steps of the apoplastic loading mechanism, and select any two that fit the wording of the question. The steps most directly related to fusicoccin's known action are steps 1 and 2 (binding to a receptor and stimulating the H⁺-ATPase), but the question credits any of the four.

Step-by-Step Reasoning

• Plasmodesmata. Sucrose passes from the companion cell into the sieve-tube element through plasmodesmata. This is the route of loading, not the energy-supplying step.
• Receptor binding. Fusicoccin binds to a receptor on the plasma membrane of the companion cell. This is the first step in the chain of events triggered by fusicoccin.
• H⁺ pumping. The companion cell uses ATP to pump H⁺ (protons) out of the cell into the apoplast, via the H⁺-ATPase. Fusicoccin stimulates this pump.
• Cotransport. The H⁺ then re-enters the companion cell down its electrochemical gradient through a cotransporter (a symporter) that carries sucrose with it. This is the energy-coupling step that concentrates sucrose inside the cell.

The mark scheme allows any two of these four points; a strong answer gives three or all four.

Key Takeaways

• Fusicoccin acts on the companion cell, not on the sieve-tube element itself.
• The effect on the companion cell is to stimulate the H⁺-ATPase, raising the proton-motive force that drives the sucrose–H⁺ symporter.
• The loading is energy-coupled: the ATP used to pump H⁺ out of the cell is what ultimately concentrates the sucrose inside the cell.
• Once concentrated in the companion cell, the sucrose passes into the sieve-tube element through plasmodesmata, not through the plasma membrane.

Common Mistakes

• Saying that fusicoccin directly transports sucrose into the sieve tube. Fusicoccin acts on the H⁺-ATPase; the sucrose is moved by the cotransporter and by diffusion through plasmodesmata.
• Saying that the H⁺-ATPase transports sucrose. The H⁺-ATPase transports protons; the sucrose is carried by a separate cotransporter.
• Confusing active transport (against a gradient, using ATP) with facilitated diffusion (down a gradient, through a channel). The H⁺-ATPase is active; the cotransporter is passive (the energy is provided by the H⁺ gradient).
• Saying that water moves into the sieve tube by active transport. Water moves in by osmosis, driven by the lowered water potential created by the concentrated sucrose.

Things to Be Careful About

• The mechanism applies to the apoplastic loading pathway, which is the pathway used by most herbaceous plants. Some plants (e.g. Cucurbita) use a symplastic pathway that does not involve the companion-cell H⁺-ATPase; the question is about the apoplastic pathway, in which fusicoccin acts.
• The cotransporter is a symporter: H⁺ and sucrose move in the same direction (into the cell). An antiporter would move them in opposite directions and is not the right answer here.