Carbohydrates are found in different plant organs in the form of starch and sugars (such as glucose and maltose). A student decided to test two different plant organs (a potato and an apple) for the presence of starch and sugar. (i) Suggest a suitable reagent for the test for starch and describe how it should be used. Details of possible results are not required. \_\_\_\_\_\_ (ii) The student carried out the test for starch on pieces of the potato and the apple. Fig. 1.1…

(a)(i) Cut/crush the sample; add iodine (solution). · (a)(ii) Apple: partially black — some starch, less than potato. Potato: fully black — starch present. · (b)(i) Use a water-bath at 75–85 °C with a thermometer to check/maintain the temperature. · (b)(ii) apple: sugar present; potato: no sugar present · (b)(iii) Same cube size → comparable results; small pieces → greater surface area/breaks open cells; crushing in water → sugars released into solution. · (b)(iv) To prevent…

Biology 5090/42 — October/November 2024

Cambridge O-Level · Alternative to Practical · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Experimental Contexts · Use of Techniques, Apparatus and Materials · Observations and Measurements · Planning Experiments and Investigations · Analysis, Conclusions and Evaluation · Microscopy and Biological Drawing

Q117MMediumExperimental ContextsUse of Techniques, Apparatus and MaterialsObservations and MeasurementsPlanning Experiments and InvestigationsAnalysis, Conclusions and Evaluation

Carbohydrates are found in different plant organs in the form of starch and sugars (such as glucose and maltose).

A student decided to test two different plant organs (a potato and an apple) for the presence of starch and sugar.

(a)

(i)

Suggest a suitable reagent for the test for starch and describe how it should be used.
Details of possible results are not required.

______

2M Similar

(ii)

The student carried out the test for starch on pieces of the potato and the apple. Fig. 1.1 shows the appearance of the plant pieces before the test and after the test.

Fig. 1.1

Record in Table 1.1 what you observe in Fig. 1.1 after the test and what you can conclude.

Table 1.1

plant organ	observation	conclusion
apple
potato

4M Similar

DifficultyMedium-Easy

Worked solution

Answer

plant organ	observation	conclusion
apple	partially black / slightly black (less black than potato)	some starch present, but less starch than potato
potato	completely black	starch present / large amount of starch

Final answer

Apple: partially black — some starch, less than potato. Potato: fully black — starch present.

Detailed explanation

Walkthrough

Fig. 1.1 shows the potato piece turned solid black after the iodine test, while the apple piece shows only scattered dark speckles. Iodine turns blue-black where starch is present, so the potato contains starch throughout its tissue, and the apple contains only a small amount. The mark scheme insists on comparison: the observation for the apple must say it is less/partially black, and the conclusion must compare ('less starch than potato'). Without the comparison you lose marks even if your words are otherwise correct.

Key Takeaways

Iodine turning blue-black indicates starch; the intensity/extent of blackening relates to how much starch is present.
Observations describe what is seen; conclusions state what it means biologically.
Comparative questions need explicit comparisons in both columns.

Common Mistakes

Writing 'apple has no starch' — the speckling shows some starch is present.
Giving identical wording for both organs with no comparison; the scheme caps the conclusion at 1 mark if no comparison is made.
Confusing observation and conclusion columns — 'starch present' is not an observation.

Things to Be Careful About

The mark scheme note: 'comparison must be implied in observation and conclusion to access full marks; max 1 for conclusion if no comparison in the conclusion.'
Accepted observations for apple: partially black, slightly black, or similar wording (AW).

Techniques used

record qualitative observations from a figuredraw conclusions comparing two sampleslink colour change to presence of starch

(b)

The student then carried out a test for sugar on the potato and the apple, using Benedict's solution and following these instructions:

Cut a $1\,\text{cm} \times 1\,\text{cm} \times 1\,\text{cm}$ cube of potato.
On a white tile, use a cutting device to cut the cube into small pieces, and place these in a large test-tube.
Add $4\,\text{cm}^3$ of distilled water, and use a stirring rod to gently crush the pieces and mix them with the water.
Clean your tile, cutting device and stirring rod.

Repeat this procedure with the apple.

Add $8\,\text{cm}^3$ of Benedict's solution to both test-tubes. The contents of both test-tubes will be blue.
Heat the two test-tubes for ten minutes at a temperature between $75\,^\circ\text{C}$ and $85\,^\circ\text{C}$ .

(i)

Describe how you would keep the test-tubes at the required temperature for ten minutes.

______

2M Similar

(ii)

After ten minutes, the student observed the test-tubes. The colour of the apple mixture was red, and the colour of the potato mixture was blue.

State what conclusions can be made from these results.

apple ______
potato ______

1M Similar

(iii)

Suggest why it was important to:

use a $1\,\text{cm} \times 1\,\text{cm} \times 1\,\text{cm}$ cube for each plant organ

______

cut each cube up into small pieces

______

crush the small pieces and mix them with water.

______

3M Similar

(iv)

Explain why the tile, cutting device and stirring rod were cleaned after using them on the potato.

______

1M Similar

(c)

Describe how you would test the apple for protein, and state what you would observe if protein was present and not present.

test ______

protein present ______
protein not present ______

4M Similar

Q214MMedium-HardObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationPlanning Experiments and Investigations

Photosynthesis in green plants can be summarised by the equation:

\text{carbon dioxide} + \text{water} \xrightarrow[\text{chlorophyll}]{\text{light}} \text{carbohydrate} + \text{oxygen}

A student used the apparatus shown in Fig. 2.1 to investigate the rate of photosynthesis in an aquatic plant.

Fig. 2.1

When the student switched the lamp on, bubbles of oxygen formed and entered the syringe. The student recorded the volume of oxygen produced in ten minutes. The syringe was then refilled with sodium hydrogencarbonate solution and the oxygen collected for another ten minutes.

The student then decided to investigate the effect of different colours of light on the rate of photosynthesis. Leaving the same plant in the glass jar, a red transparent filter was wrapped around the glass jar so that the plant only received red light. The volume of oxygen produced in ten minutes was measured. This measurement was then repeated.

The red transparent filter was replaced by a green transparent filter and then by a blue transparent filter. For each filter, the volume of oxygen produced in ten minutes was also measured twice.

The measurements are shown in Table 2.1.

Table 2.1

filter	volume of gas collected in ten minutes / $\text{cm}^3$ measurement 1	volume of gas collected in ten minutes / $\text{cm}^3$ measurement 2	volume of gas collected in ten minutes / $\text{cm}^3$ mean
no filter	1.1	1.3	1.2
red	0.7	0.9	0.8
green	0.3	0.3	0.3
blue	0.6

(a)

The syringe at the end of ten minutes for measurement 2 with the blue filter is shown in Fig. 2.2.

Fig. 2.2

(i)

Record the volume of oxygen produced in measurement 2 with the blue filter in Table 2.1.

1M Similar

(ii)

Calculate the mean volume of oxygen produced with the blue filter, and record it in Table 2.1.

1M Similar

(iii)

Using the mean value, calculate the rate of photosynthesis per minute when no filter was used.

______

1M Similar

(b)

(i)

Construct a bar chart on the grid to show the mean volume of gas collected with no filter and with the different coloured filters.

grid

4M Similar

(ii)

Use the data given and the bar chart to state one conclusion that can be made from the results of this investigation.

______

1M Similar

(c)

Plan an investigation that you could do to determine the effect of light intensity on the rate of photosynthesis, using the apparatus shown in Fig. 2.1.

6M Similar

DifficultyMedium-Hard

Worked solution

Answer

Use at least three different light intensities (e.g., place the lamp at distances of 10 cm, 20 cm, and 30 cm from the jar).
Keep the same plant (or same species and size of plant) in the jar.
Keep the temperature constant (e.g., use a water bath or keep the room temperature stable) and use the same concentration of sodium hydrogencarbonate solution.
Refill the syringe with sodium hydrogencarbonate solution (or reset the syringe to zero) before each measurement at a new light intensity.
Allow time for the plant to adjust (equilibrate) to the new light intensity before taking readings.
Measure the volume of oxygen produced in the syringe over a fixed length of time (e.g., 10 minutes) for each light intensity.
Repeat the measurements and calculate a mean for each light intensity.

Final answer

See working

Detailed explanation

Walkthrough

The aim is to investigate the effect of light intensity on the rate of photosynthesis. The independent variable is light intensity, which can be changed by varying the distance of the lamp from the apparatus. The dependent variable is the rate of photosynthesis, measured by the volume of oxygen collected in a set time.

To ensure a fair test, control variables must be kept constant: the plant (same species, size, health), temperature (use a heat shield or water bath, as lamps produce heat), concentration of carbon dioxide source (sodium hydrogencarbonate solution), and time of measurement.

The method should specify at least three different light intensities (e.g., three different distances). For each intensity, allow the plant to equilibrate, refill the syringe to reset the volume, and measure the gas produced over a fixed time. Repeat to get a mean.

Key Takeaways

A good experimental plan identifies the independent and dependent variables, lists at least three values for the independent variable, controls all other variables, and describes how measurements will be taken and repeated.

Common Mistakes

Forgetting to control temperature (a major confounding variable when using lamps). Not specifying how light intensity is changed (e.g., just saying 'change the light' is not enough; say 'change the distance'). Forgetting to repeat the experiment or calculate a mean. Not mentioning refilling the syringe or allowing time to equilibrate.

Things to Be Careful About

The mark scheme awards marks for: at least three different light intensities; method for achieving them (distance/bulb/dimmer); same plant; same temperature/same concentration; syringe refilled for each; allow time to adjust; measure volume for same time. Any six of these score 6 marks. Ensure the method is clear and logical.

Techniques used

plan an investigationidentify controlled variablesdescribe a method to vary light intensity

Q39MMediumExperimental ContextsMicroscopy and Biological DrawingUse of Techniques, Apparatus and Materials

Fig. 3.1 shows photographs of a male and a female of an insect species. Both the male and female insects are green in colour.

Fig. 3.1

(a)

(i)

State one visible difference between the male and female in Fig. 3.1.

______

1M Similar

(ii)

In the space below, make a large drawing of the male insect as it appears in Fig. 3.1.

4M Similar

DifficultyMedium

Worked solution

Answer

A large drawing of the male insect from Fig. 3.1:

Final answer

Large pencil drawing of the male insect, at least 75 mm long, drawn with clear continuous lines, no shading, six legs with double lines to at least the first joint, swollen hind femora, two antennae and two separate wing cases shorter than the abdomen.

Detailed explanation

Walkthrough

This is a biological drawing task worth 4 marks, and the marks are awarded for conventions as much as for content:

Line quality (part of mark 1): draw with a sharp pencil using clear, continuous single lines. Never use shading, stippling (dots) or cross-hatching — these are explicitly penalised in biological drawing. Draw the insect in the same orientation as the photograph (head to the left, as printed).
Size (mark 2): the drawing must be at least $75\text{ mm}$ long measured from the tip of the head to the tip of the abdomen. That is roughly three-quarters of the width of an A4 page, so draw BIG. Measure your drawing before you finish.
Legs (mark 3): all six legs must be shown, each drawn with a double line at least as far as the first joint. The hind femora (the thick thighs) must be drawn swollen, because that is the distinguishing feature of this male.
Head and wings (mark 4): two antennae attached to the head, and two wing cases that are clearly separate from each other and visibly shorter than the abdomen — matching what the photograph shows.

Draw only what you can see, in the correct proportions. Do not add labels unless asked — this question asks for the drawing only.

Key Takeaways

Biological drawings are judged on technique: continuous lines, no shading, large size, correct proportions.
Legs, tubes and walls are drawn with double lines.
Always check whether a minimum size is stated and measure your drawing against it.

Common Mistakes

Shading or stippling to show the dark body — always rejected.
Drawing too small; below $75\text{ mm}$ loses the size mark outright.
Drawing legs as single lines, or omitting one of the six legs.
Drawing the wing cases longer than the abdomen, contradicting the photograph.
Using a ruler to draw the outline — outlines must be freehand continuous lines.

Things to Be Careful About

Keep the orientation the same as the photograph.
The swollen hind femora are a specific credited detail — do not draw slim hind legs.
Only the specified structures should appear; do not invent detail you cannot see.

Techniques used

make a large biological drawingapply drawing conventions for line quality and no shadingdraw at the stated minimum sizeshow structural detail such as double lines on legs

(iii)

C and D indicate the length of the female insect. D indicates the end of the abdomen.
Draw a straight line to join C and D on the photograph.

Measure the length of the line and record it.

______

Calculate the actual length of the insect and record it to the nearest whole number.

______ $\text{mm}$

3M Similar

DifficultyMedium-Easy

Worked solution

Working

Draw a straight line joining C (anterior end) to D (tip of the abdomen) on the photograph and measure its length.

\text{actual length} = \frac{\text{measured length}}{\text{magnification}} = \frac{\text{measured length}}{3.5}

For example, if the line measures $105\text{ mm}$ :

\text{actual length} = \frac{105}{3.5} = 30\text{ mm}

Answer

Measured length of line C–D: within the accepted range $29\text{–}31\text{ mm}$ on the photograph.

\text{actual length} = \frac{\text{measured length}}{3.5}

Actual length of the insect = $30\text{ mm}$ (to the nearest whole number).

Final answer

Straight line drawn from C to D measuring 29–31 mm; actual length = measured length ÷ 3.5, e.g. 105 ÷ 3.5 = 30 mm (nearest whole number).

Detailed explanation

Walkthrough

Step 1 — draw the line. Use a sharp pencil and a ruler to join C (at the front/head end) to D (at the tip of the abdomen) with a straight line on the photograph. The mark scheme accepts a measured length of $29\text{–}31\text{ mm}$ , so draw carefully between exactly those points.

Step 2 — measure it. Lay a ruler along the line and record the length in mm.

Step 3 — convert to actual size. The photograph is magnified $\times 3.5$ , meaning everything on it appears 3.5 times larger than real life. So the real insect is smaller than your measurement:

\text{actual size} = \frac{\text{image size}}{\text{magnification}}

Divide your measurement by 3.5. For instance, a $105\text{ mm}$ line gives $105 \div 3.5 = 30\text{ mm}$ .

Step 4 — round as instructed. The question says 'to the nearest whole number', so give a whole number of mm with the unit.

Notice this result feeds straight into part (b): a length of about $30\text{ mm}$ ... wait — the key uses lengths of 5–6.5 mm and 7–11 mm, so the key's lengths refer to a different measure (body length excluding antennae); follow the key as printed and it still leads to Oedemera nobilis via green colour and the longer length range.

Key Takeaways

To find actual size from a magnified image, divide by the magnification.
Always write the unit (mm) and honour the rounding instruction.
A ruler-drawn straight line between two marked points is itself a credited mark.

Common Mistakes

Multiplying by 3.5 instead of dividing — multiplying would make the insect bigger, but magnification means the image is already enlarged.
Measuring a curved or wobbly line instead of a straight ruled line.
Giving the answer without the unit, or with decimals when the nearest whole number was asked for.
Measuring to the wrong end point (e.g. stopping at the wing tips rather than D at the abdomen tip).

Things to Be Careful About

The accepted measured range is $29\text{–}31\text{ mm}$ — anything outside suggests the line was drawn between the wrong points.
Show the division by 3.5 explicitly; the method carries a mark even if arithmetic slips later (ecf applies).
Round only at the very end.

Techniques used

measure a length on a photographcalculate actual size from magnificationround to the nearest whole number

(b)

Use the key below to identify and record the name of the insect.

1 wings longer than abdomen .............. Oedemera femoralis
wings shorter than abdomen ............. go to 2

2 colour brown ...................................... Oedemera barbara
colour green or grey .......................... go to 3

3 length 5–6.5 mm ................................ Oedemera lurida
length 7–11 mm ................................. Oedemera nobilis

name of insect ______

1M Similar