Biology 5090/41 — October/November 2025

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

questions

marks

minutes

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

Q115MMediumUse of Techniques, Apparatus and MaterialsPlanning Experiments and InvestigationsObservations and MeasurementsAnalysis, Conclusions and EvaluationExperimental Contexts

Apple juice is made by cutting up and crushing apples. When apple tissue is cut up and crushed, it can become a darker colour and go brown.

Some students investigated the effect of vitamin C on the colour change in apples when they are cut up and crushed.

The students followed these instructions:

Label a large test-tube and add $5\text{ cm}^3$ of vitamin C solution.
Label a second large test-tube and add $5\text{ cm}^3$ of distilled water.
Label a third large test-tube and do not add any liquid.
Cut three cubes of apple each $1\text{ cm} \times 1\text{ cm} \times 1\text{ cm}$ . Do not include any skin or core.
Cut each cube into many small pieces. Take care to keep the pieces from each cube separate.
Add the pieces of one cube to the test-tube containing vitamin C solution.
Add the pieces of the second cube to the test-tube containing distilled water.
Add the pieces of the third cube to the test-tube with no added liquid.
Use a glass rod to gently crush the pieces of apple in the bottom of each test-tube.
Clean the glass rod between using it in each test-tube.
Start timing.

(a)

11M

(i)

Describe the most appropriate piece of apparatus to measure the required volumes of vitamin C solution and distilled water.

______

2M Similar

(ii)

Explain why the students were told to clean the glass rod.

______

1M Similar

(iii)

The students noted that the apple tissue in each test-tube was white when they started timing.

They observed the colour of the apple tissue in each test-tube at 5, 10 and 15 minutes and recorded this in their notebooks.

Fig. 1.1 shows one of their notebooks.

Fig. 1.1

Complete the headings in Table 1.1 and use the observations in Fig. 1.1 to complete the table.

Table 1.1

______	______
	vitamin C	distilled water	no added liquid
0	white	white	white
5
10
15

3M Similar

DifficultyMedium-Easy

Worked solution

Answer

time / minutes	colour of apple : vitamin C	colour of apple : distilled water	colour of apple : no added liquid
0	white	white	white
5	white	white	yellow
10	white	pale yellow	dark yellow
15	white	dark yellow	brown

Final answer

See working.

Detailed explanation

Walkthrough

The question provides handwritten notes in Fig. 1.1 and asks to complete Table 1.1. The rows are labelled 0, 5, 10, 15, which correspond to the time in minutes. The columns are the three treatments: vitamin C, distilled water, and no added liquid. The observations are colours (white, pale yellow, dark yellow, yellow, brown). The first heading must be time with units (minutes), and the second set of headings must describe what is being measured (colour of apple).

Key Takeaways

When completing a table from raw notes, identify the independent variable (time) and the dependent variable (colour change) to determine the headings. Ensure units are included for quantitative headings like time.

Common Mistakes

Forgetting the units in the time heading (e.g., writing "time" instead of "time / minutes").
Misreading the notebook notes: for example, writing "yellow" for distilled water at 5 minutes instead of "white".
Not entering a colour in every cell; the table must be fully populated.

Things to Be Careful About

The mark scheme awards marks for correct headings, a colour in all cells, and the correct colours. Pay close attention to the distinction between "white", "pale yellow", "dark yellow", and "yellow" as written in the notebook. The "no added liquid" tube turns yellow at 5 minutes, whereas the "distilled water" tube remains white at 5 minutes.

Techniques used

record qualitative observations in a table with correct headings and units

(iv)

Using your observations in Table 1.1, state what can be concluded about the effect of vitamin C on the colour change when apple is cut up and crushed.

______

1M Similar

(v)

Using your observations in Table 1.1, state what can be concluded about the effect of distilled water on the colour change when apple is cut up and crushed.

______

2M Similar

(vi)

The students were aware that there may have been sources of error in the method used which affected their observations.

Suggest one possible source of error and suggest how it could be overcome.

______

2M Similar

DifficultyMedium-Easy

Worked solution

Answer

Source of error	Improvement
It is difficult to objectively assess or describe the colours.	Use a colour chart to match the colour of the apple tissue.

Final Answer

See working.

Final answer

See working.

Detailed explanation

Walkthrough

The experiment relies on subjective visual observation of colour changes (white, pale yellow, dark yellow, brown). A major source of error is that different people might describe the same shade differently. An improvement is to use a standard colour chart. Other valid errors include: the apple cubes not being crushed to the same extent (improvement: crush for the same amount of time or cut into a specific number of pieces), or the time taken to add the apple to the tubes varying (improvement: have multiple people add the apple simultaneously).

Key Takeaways

When evaluating a method, look for subjective measurements (like colour) or inconsistencies in preparation (like crushing). The improvement must directly address the error identified.

Common Mistakes

Pairing an error with an unrelated improvement (e.g., "too few repeats" with "use a colour chart").
Vague improvements like "be more careful" or "measure more accurately". The mark scheme rejects these.
Suggesting "human error" as the source of error.

Things to Be Careful About

The mark scheme explicitly states "source of error must match improvement". Choose the easiest valid pair to write. The colour chart / subjective colour pair is the most straightforward for this specific experiment.

Techniques used

evaluate experimental methods and suggest improvements

(b)

(i)

The students then measured the pH of the vitamin C solution and the pH of the distilled water using universal indicator paper, with these results:

distilled water: pH 6
vitamin C solution: pH 4.

Describe how you would use universal indicator paper to determine the pH of a liquid.

______

2M Similar

(ii)

When apple tissue is cut up and crushed, an enzyme comes into contact with oxygen in the air. In the presence of oxygen, the enzyme catalyses the reaction of a colourless molecule in the cells of the apple tissue to cause the change in colour.

Suggest how this might explain the effect of:

distilled water ______

vitamin C ______

2M Similar

DifficultyMedium

Worked solution

Answer

Distilled water: reduces the amount of oxygen available (or stops oxygen from getting to the apple tissue), so the enzyme cannot catalyse the reaction.
Vitamin C: is an acid (has a low pH), which inhibits or denatures the enzyme (stops it from working), so the colour change does not occur.

Final Answer

See working.

Final answer

See working.

Detailed explanation

Walkthrough

The stem states that cutting the apple causes an enzyme to contact oxygen, which catalyses a reaction that changes the colour. We need to explain why distilled water and vitamin C affect this.

Distilled water: Submerging the apple in water displaces air, reducing the amount of oxygen that can reach the enzyme. Without oxygen, the enzyme cannot catalyse the colour-changing reaction. This explains why the change is slower than in air (no added liquid) but still happens.
Vitamin C: The results in part (b)(i) show vitamin C has a pH of 4, making it acidic. Enzymes have an optimal pH; if the pH is too low (too acidic), the enzyme's active site can be altered (denatured) or inhibited, stopping it from working. Since the enzyme is stopped, no colour change occurs, even though oxygen is present.

Key Takeaways

When explaining experimental results, link the physical conditions (submersion in water, addition of acid) to the biological mechanisms (oxygen availability, enzyme denaturation/inhibition) described in the question stem.

Common Mistakes

Saying "vitamin C kills the apple". Apples don't have living cells that need to be killed for this reaction; it's an enzyme-catalysed chemical reaction.
Saying "distilled water dilutes the enzyme". Dilution doesn't stop an enzyme; lack of substrate (oxygen) does.
Forgetting to mention oxygen for distilled water or pH/denaturation for vitamin C.

Things to Be Careful About

The mark scheme gives two routes for vitamin C: it is an acid/low pH + inhibits/denatures enzyme, OR it is more acidic so denatures the enzyme to a greater degree. Both are acceptable. For distilled water, the key is reducing oxygen availability. Ensure both parts of the question are answered.

Techniques used

explain enzyme activity in the context of pH and oxygen availability

Q218MMedium-HardObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationUse of Techniques, Apparatus and MaterialsPlanning Experiments and Investigations

Yeast is used to produce bread. Flour is mixed with water and yeast to produce dough. The yeast breaks down starch in the flour to produce sugar. The yeast uses the sugar for respiration, giving off bubbles of carbon dioxide. The bubbles of carbon dioxide are trapped in the dough, making it increase in volume before it is baked.

(a)

Some students tested the effect of yeast in four doughs, each made from a different type of flour – wheat, rye, buckwheat and maize. The effect of the yeast in the dough was evaluated by measuring how much the dough increased in volume.

The same mass of each type of flour was mixed with the same volume of water and the same mass of yeast to form a dough. Four $100\text{ cm}^3$ measuring cylinders were labelled and dough was added to each. The volume of the dough in each of the cylinders was measured and recorded at 15-minute intervals. Some of the results are shown in Table 2.1.

10M

(i)

Fig. 2.1 shows the measuring cylinder with dough made from wheat flour after 30 minutes.
Record the volume of the dough in Table 2.1.

Fig. 2.1

1M Similar

(ii)

Complete Table 2.1 by calculating the total increase in volume of each dough after 45 minutes.

Table 2.1

time / minutes	volume of dough in measuring cylinder / $\text{cm}^3$
	wheat flour	rye flour	buckwheat flour	maize flour
0	20	21	21	20
15	24	23	21	21
30		27	22	22
45	45	32	23	22
total increase in volume after 45 minutes	25

2M Similar

DifficultyMedium-Easy

Worked solution

Working

Wheat flour: $45 - 20 = 25$
Rye flour: $32 - 21 = 11$
Buckwheat flour: $23 - 21 = 2$
Maize flour: $22 - 20 = 2$

Answer

time / minutes	volume of dough in measuring cylinder / $\text{cm}^3$
	wheat flour	rye flour	buckwheat flour	maize flour
0	20	21	21	20
15	24	23	21	21
30	38	27	22	22
45	45	32	23	22
total increase in volume after 45 minutes	25	11	2	2

Final answer

11, 2, 2

Detailed explanation

Walkthrough

The total increase in volume after 45 minutes is calculated by subtracting the initial volume at 0 minutes from the final volume at 45 minutes for each flour type:

For rye flour: $32 - 21 = 11\text{ cm}^3$
For buckwheat flour: $23 - 21 = 2\text{ cm}^3$
For maize flour: $22 - 20 = 2\text{ cm}^3$

Each calculated value is entered into its respective column in the bottom row of Table 2.1.

Key Takeaways

Increase in volume = (Volume at 45 minutes) - (Volume at 0 minutes).

Common Mistakes

Writing units (such as $\text{cm}^3$ ) in the body cells of the table (explicitly rejected by mark schemes when headers already include units).
Subtracting consecutive intervals (e.g. 45 min minus 30 min) instead of calculating the total increase from 0 minutes.

Things to Be Careful About

Ensure values are placed correctly in each column corresponding to the right flour type.

Techniques used

calculate change in volume from raw data

(iii)

On the grid draw a bar chart to show the total increase in volume of the four doughs after 45 minutes.

Fig. 2.2

4M Similar

(iv)

The students wanted to know why the effect of yeast in the four doughs varied.

Table 2.2 shows some of the biological molecules in the four flours.

Table 2.2

biological molecule	mass / $\text{g}$ per $100\text{ g}$ flour
	wheat flour	rye flour	buckwheat flour	maize flour
starch	54.5	54.1	59.0	74.0
gluten (a protein)	8.9	3.1	0.0	0.0
fibre	10.6	13.7	10.4	4.3

Using Table 2.2, your bar chart and the information about the process of bread production, suggest and explain how these biological molecules affect the total increase in volume of bread dough.

______

3M Similar

DifficultyMedium

Worked solution

Answer

Gluten (protein) content affects the total increase in volume, as higher gluten content corresponds to a greater increase in volume (wheat has the most gluten and greatest volume increase, followed by rye, while buckwheat and maize have no gluten and the lowest volume increase).
The dough with more gluten is able to trap more bubbles of carbon dioxide gas produced by yeast respiration.
Starch and fibre have little or no effect on the volume increase because their amounts do not correlate with the differences in volume increase (e.g. maize has the highest starch content but the lowest volume increase).

Final answer

Gluten content correlates with volume increase by trapping carbon dioxide bubbles, whereas starch and fibre show no correlation with volume increase.

Detailed explanation

Walkthrough

Identify correlation with gluten: Wheat contains $8.9\text{ g}$ gluten per $100\text{ g}$ and showed the largest increase ( $25\text{ cm}^3$ ). Rye has $3.1\text{ g}$ gluten and increased by $11\text{ cm}^3$ . Buckwheat and maize have $0.0\text{ g}$ gluten and showed minimal increases ( $2\text{ cm}^3$ ). Therefore, gluten content directly matches the trend in volume increase.
Explain the mechanism: The prompt states that yeast respiration produces carbon dioxide bubbles that are trapped in dough. Gluten provides elasticity to trap these bubbles, causing the dough to expand.
Evaluate other molecules: Starch content is highest in maize ( $74.0\text{ g}$ ), yet maize had the lowest volume increase. Fibre content is highest in rye ( $13.7\text{ g}$ ) and similar in wheat and buckwheat, showing no direct pattern with volume increase. Hence, starch and fibre do not determine the dough expansion.

Key Takeaways

Making a valid scientific conclusion requires looking at both positive correlation and the lack of correlation among variables.

Common Mistakes

Concluding that starch is the main factor simply because starch is broken down into sugar for yeast respiration, without checking the data in Table 2.2.

Things to Be Careful About

Ensure you refer to carbon dioxide bubbles being trapped, as outlined in the question context.

Techniques used

correlate biological composition with experimental resultsrelate gas production and retention to dough volume

(b)

Describe how you would show that each of the four flours contained starch.

______

2M Similar

(c)

The students made more dough from wheat flour, yeast and water.

Using this dough, plan an investigation based on the method in 2(a) to determine the effect of temperature on the increase in volume of dough at temperatures between $20\,^\circ\text{C}$ and $80\,^\circ\text{C}$ .

Predict your expected results.

investigation plan ______

expected results ______

6M Similar

DifficultyMedium-Hard

Worked solution

Answer

Investigation plan:

Divide the prepared wheat dough equally (same mass or starting volume, e.g. $20\text{ cm}^3$ ) into measuring cylinders.
Set up at least three different temperatures between $20\,^\circ\text{C}$ and $80\,^\circ\text{C}$ (e.g. $20\,^\circ\text{C}$ , $40\,^\circ\text{C}$ , $60\,^\circ\text{C}$ and $80\,^\circ\text{C}$ ) using thermostatically controlled water-baths.
Place a measuring cylinder with dough into each water-bath.
Measure and record the volume of the dough at regular time intervals (e.g. every 15 minutes) for a set duration such as 45 minutes.
Calculate the total increase in volume after 45 minutes for each temperature.
Repeat the investigation at least 3 times for each temperature and calculate the mean increase in volume.
Plot a graph of total volume increase on the y-axis against temperature on the x-axis.

Expected results:

As temperature increases from $20\,^\circ\text{C}$ up to an optimum temperature (around $35\text{--}45\,^\circ\text{C}$ ), the total increase in volume will increase because yeast respiration and enzyme activity increase.
Above the optimum temperature (towards $80\,^\circ\text{C}$ ), the total increase in volume will decrease / stop because yeast enzymes become denatured and yeast cells are killed.

Final answer

Plan: Equal volumes of dough placed in water-baths at 3+ temperatures between 20 °C and 80 °C; volume recorded at set intervals (45 min) to find volume increase; repeat 3 times and find mean. Prediction: Volume increase rises to an optimum then decreases at higher temperatures due to enzyme denaturation.

Detailed explanation

Walkthrough

To score full marks on a 6-mark planning question:

Standardising the sample: Divide the dough equally by mass or initial volume into identical measuring cylinders.
Independent variable: Choose at least 3 distinct temperatures strictly within the specified range ( $20\,^\circ\text{C}$ to $80\,^\circ\text{C}$ ), e.g. $20\,^\circ\text{C}$ , $40\,^\circ\text{C}$ , $60\,^\circ\text{C}$ , $80\,^\circ\text{C}$ .
Apparatus for temperature control: Use water-baths to maintain constant temperatures.
Dependent variable measurement: Measure dough volume at fixed time intervals (e.g. every 15 minutes up to 45 minutes) and calculate total increase in volume.
Reliability and Processing: Repeat at least 3 times at each temperature, calculate the mean, and plot volume increase against temperature.
Prediction: State that rate/volume increase rises with temperature up to an optimum (around $35\text{--}45\,^\circ\text{C}$ ) due to increased kinetic energy, then decreases at high temperatures because yeast enzymes denature.

Key Takeaways

Planning questions require: independent variable range, method of control, dependent variable measurement, control variables, repeats/mean, and a scientific prediction.

Common Mistakes

Stating temperatures outside the specified range (e.g. $0\,^\circ\text{C}$ or $100\,^\circ\text{C}$ ). The prompt specified between $20\,^\circ\text{C}$ and $80\,^\circ\text{C}$ .
Forgetting to mention water-baths to maintain the temperatures.
Omitting repeats or calculation of the mean.

Things to Be Careful About

Ensure both the investigation plan and the expected results sections are fully answered, as 1 mark is allocated specifically to the prediction.

Techniques used

plan an investigation with controlled variablesspecify independent variable range and control temperature using a water-bathpredict the effect of temperature on enzyme-controlled respiration

Q37MMediumExperimental ContextsObservations and MeasurementsPlanning Experiments and InvestigationsAnalysis, Conclusions and EvaluationMicroscopy and Biological Drawing

A student investigated a plant that had some stems with flowers and some stems with no flowers. The student picked one leaf from a stem with flowers and one leaf from a stem with no flowers.
Fig. 3.1 shows the two leaves.

Fig. 3.1

(a)

(i)

Complete Table 3.1 to compare the size and shape of the leaves.

Table 3.1

feature of leaf	leaf from stem with flowers	leaf from stem with no flowers
size
shape

2M Similar

DifficultyMedium-Easy

Worked solution

Answer

feature of leaf	leaf from stem with flowers	leaf from stem with no flowers
size	large / larger	small / smaller
shape	longer than wide, pointed oval with just one point at the tip	widest at the base, with three pointed sections (lobes)

Final answer

See working

Detailed explanation

Walkthrough

The table asks for a comparison of size and shape between two leaves photographed at different magnifications. The left leaf is shown at $\times 1$ and the right leaf at $\times 2$ , so although the right leaf looks bigger on the page, it is actually the smaller leaf — you must mentally divide its image size by 2 before comparing. So the flowering-stem leaf is larger, and the non-flowering-stem leaf is smaller.

For shape: the left leaf is an unlobed oval that tapers to a single point — longer than it is wide. The right leaf is the classic ivy-type leaf: broadest at the base where it joins the petiole, with three pointed lobes fanning out from that base.

Key Takeaways

Always check magnification before comparing sizes on photographs; apparent size is not actual size.
Comparisons should be written as paired statements naming both leaves.
Shape descriptions should use precise terms: 'pointed oval', 'lobed', 'widest at the base'.

Common Mistakes

Saying the lobed leaf is bigger because it appears bigger in Fig. 3.1 — ignoring the $\times 2$ magnification label.
Writing vague shapes such as 'one is rounder' instead of describing the outline ('three pointed sections', 'widest at base').
Describing only one leaf rather than comparing both columns.

Things to Be Careful About

Each row earns one mark for a correct comparison, so fill in both cells of each row.
Use comparative language ('larger', 'smaller') or clear absolute descriptions — either scores, but the pair must be consistent.

Techniques used

compare two specimens from photographsrecord qualitative observations in a table

(ii)

Suggest what the student needs to do to be certain that the two leaves they picked are representative of the leaves on the two types of stem.

______

2M Similar

(b)

In the space below, make a large drawing of the leaf from the stem with no flowers as it appears in Fig. 3.1.

3M Similar

DifficultyMedium

Worked solution

Answer

Draw the ivy-type leaf from the stem with no flowers exactly as it appears in Fig. 3.1:

sharp pencil, clean continuous lines, no shading or stippling, no ruled lines, labels not overlapping the drawing;
drawn at least 80 mm high, in the correct proportions and orientation;
show the notch in the right lobe at the base, at least three side veins branching from the top of the petiole (one running into each lobe), and draw the petiole as a double line.

Final answer

Large drawing of the five-lobed leaf, at least 80 mm high, continuous pencil lines, no shading, notch in right basal lobe, three side veins radiating from the top of the petiole, petiole as a double line

Detailed explanation

Walkthrough

Biological drawing marks are awarded for technique as much as content. The mark scheme splits into three blocks:

Drawing conventions — use a sharp pencil so lines stay fine and clean; every outline is one continuous unbroken line; absolutely no shading, stippling or cross-hatching (tone is not allowed in scientific drawing); no ruled lines anywhere; and any labels must not overlap the drawing itself.
Size and accuracy — the drawing must be at least 80 mm tall (measure it before you finish) and must reproduce the proportions and orientation of the photograph: a five-lobed ivy-type leaf, pointed tip uppermost, petiole at the bottom.
Specific detail — the scheme names three features: the notch cut into the right-hand lobe near the base (visible in Fig. 3.1); at least three side veins leaving the top of the petiole, one heading into each lobe; and the petiole drawn as a double line, because tubes and stalks are always drawn with two parallel lines.

Key Takeaways

Learn the standard drawing rules: sharp pencil, continuous single lines, no shading, generous size, correct proportions.
Stalks, midribs and tubes get double lines; veins branch from the top of the petiole.
Always include the specific detail named in the question or visible in the specimen — here, the notch in the right lobe.

Common Mistakes

Shading or stippling to show the dark leaf surface — instantly loses the convention mark.
Drawing too small: under 80 mm loses the size mark even if everything else is right.
Single-line petiole, or side veins omitted or not reaching the lobes.
Using a ruler for outlines — ruled lines are rejected.
Copying the wrong leaf: the question asks for the lobed leaf from the stem with no flowers, not the oval one.

Things to Be Careful About

Measure your drawing height against the 80 mm requirement before moving on.
Keep labels outside the outline with clean label lines touching the correct structure.
Draw what you see, including imperfections like the notch — idealised symmetry loses the detail mark.

Techniques used

make a large biological drawingapply drawing conventions for line quality and no shadingshow structural detail of veins and petiole