Fig. 1.1 is a transverse section through a leaf from the maize plant, Zea mays . Maize is a C4 plant. Fig. 1.1 (a) On Fig. 1.1, use label lines and letters to show: A – a cell in the epidermal layer B – a cell that contains PEP carboxylase. (i) Identify the cell type labelled C in Fig. 1.1. C = ______ (ii) Explain how the leaf anatomy shown in Fig. 1.1 adapts the C4 plant to maintain a high rate of photosynthesis at high temperatures. (c) Fig. 1.2 shows the results of an…

(a) A → epidermal cell; B → mesophyll cell · (b)(i) bundle sheath (cell) · (b)(ii) See working · (c)(i) See working · (c)(ii) See working

Biology 9700/43 — October/November 2020

Cambridge A-Level · A Level Structured Questions · worked solutions for every part, with the mark scheme

questions

100

marks

120

minutes

Topics Genetic Technology · Inheritance · Selection and Evolution · Classification, Biodiversity and Conservation · Control and Coordination · (outdated) Crop Plants · +3 more

Q1(outdated) Crop PlantsPhotosynthesisFree sample

Fig. 1.1 is a transverse section through a leaf from the maize plant, Zea mays.

Maize is a C4 plant.

Fig. 1.1

(a)

On Fig. 1.1, use label lines and letters to show:

A – a cell in the epidermal layer

B – a cell that contains PEP carboxylase.

DifficultyMedium-Easy

Worked solution

Answer

A — label line drawn to a cell in the upper or lower epidermis.
B — label line drawn to a mesophyll cell (PEP carboxylase is located in the cytoplasm of mesophyll cells in $\text{C}_4$ plants).

Final answer

A → epidermal cell; B → mesophyll cell

Detailed explanation

Background Concept

In a $\text{C}_4$ plant leaf, the vascular bundle (xylem and phloem) is surrounded by a ring of large bundle sheath cells. This ring arrangement is called Kranz anatomy (German for 'wreath'). Around the bundle sheath, mesophyll cells are arranged in a more loosely packed layer, and the whole structure is enclosed by upper and lower epidermis. $\text{C}_4$ photosynthesis uses two cell types to fix $\text{CO}_2$ : PEP carboxylase in the mesophyll cell cytoplasm fixes $\text{CO}_2$ into oxaloacetate (a 4-carbon compound), which is transported to the bundle sheath where rubisco runs the Calvin cycle. The two cell types therefore carry out complementary halves of carbon fixation.

Understanding the Question

You are looking at a transverse section of a maize (Zea mays) leaf — a $\text{C}_4$ plant. You need to add two label lines: A to a cell in the epidermal layer, and B to a cell that contains PEP carboxylase. The key piece of knowledge is that PEP carboxylase is in the cytoplasm of mesophyll cells, not in the bundle sheath.

Approach

Identify the cell types visible in the micrograph from the outside in: epidermis (outermost single layer of cells at the top and bottom of the section), mesophyll (loosely arranged cells just inside the epidermis), bundle sheath (ring of large cells immediately around the vascular bundle). Then add label A to any epidermal cell and label B to any mesophyll cell.

Step-by-Step Reasoning

PEP carboxylase is the initial $\text{CO}_2$ -fixing enzyme in $\text{C}_4$ photosynthesis and is found in the cytoplasm of mesophyll cells.
Bundle sheath cells, in contrast, contain rubisco and run the Calvin cycle — they do not contain PEP carboxylase.
The epidermis is the outermost single layer of cells (often with a thickened cuticle on its outer face) and is the obvious target for label A.
Therefore: A points to an epidermal cell, and B points to a mesophyll cell.

Key Takeaways

In a $\text{C}_4$ leaf, PEP carboxylase is in mesophyll cells; rubisco is in bundle sheath cells.
The anatomical division of labour between these two cell types is what allows $\text{C}_4$ plants to concentrate $\text{CO}_2$ around rubisco and minimise photorespiration.
Kranz anatomy is the visible ring of bundle sheath cells around each vascular bundle.

Common Mistakes

Pointing label B at a bundle sheath cell — this would be wrong because bundle sheath cells contain rubisco, not PEP carboxylase.
Confusing the epidermis with the bundle sheath — they are very different layers (epidermis is the outer single cell layer; bundle sheath is the ring of large cells around the vein).
Pointing the label at a sub-epidermal cell rather than a true epidermal cell.

Things to Be Careful About

Make sure label A ends clearly in a single cell of the outermost layer.
The label line should not cross other features of the micrograph unnecessarily.

Techniques used

identify C4 leaf cell types from a micrographrecognise Kranz anatomy in a transverse sectionlocate epidermal and mesophyll cells

(b)

(i)

Identify the cell type labelled C in Fig. 1.1.

C = ______

DifficultyEasy

Worked solution

Answer

C = bundle sheath (cell)

Final answer

bundle sheath (cell)

Detailed explanation

Background Concept

In a $\text{C}_4$ leaf, the vascular bundle (xylem and phloem) is tightly wrapped by a ring of large, often thick-walled cells called bundle sheath cells. This ring is the defining feature of Kranz anatomy ('wreath anatomy'). The bundle sheath cells contain many chloroplasts and are the site of the Calvin cycle in $\text{C}_4$ plants.

Understanding the Question

The cell labelled C in Fig. 1.1 is one of the large cells immediately surrounding the vascular bundle. Identifying it correctly is a direct test of recognising Kranz anatomy in a maize leaf.

Approach

Look at the position of the labelled cell: it sits inside the ring of mesophyll cells and directly against the xylem and phloem of the vascular bundle. This is precisely the position occupied by a bundle sheath cell.

Step-by-Step Reasoning

The vascular bundle is the cluster of xylem and phloem vessels in the centre of the section.
The cells immediately surrounding the vascular bundle form a continuous ring — this is the bundle sheath.
The cell labelled C is one of these ring-forming cells.
Therefore C is a bundle sheath cell.

Key Takeaways

The bundle sheath is the ring of cells immediately around the vascular bundle.
Kranz anatomy = bundle sheath ring + surrounding mesophyll, characteristic of $\text{C}_4$ plants.

Common Mistakes

Calling C a 'mesophyll cell' — the mesophyll is outside the bundle sheath ring, not inside it.
Calling C a 'vascular cell' or 'xylem cell' — the labelled cell is around the vascular tissue, not part of it.
Calling C a 'parenchyma cell' — too vague; the specific term is bundle sheath cell.

Things to Be Careful About

The label C in the figure clearly points to a cell in the ring around the vein, so the answer is unambiguous as long as the candidate knows what a bundle sheath cell is.

Techniques used

identify a cell type from a micrographrecognise Kranz anatomy in a transverse section

(ii)

Explain how the leaf anatomy shown in Fig. 1.1 adapts the C4 plant to maintain a high rate of photosynthesis at high temperatures.

DifficultyMedium

Worked solution

Answer

Any four of:

Mesophyll cells form a ring around the bundle sheath cells (Kranz anatomy).
The bundle sheath cells are not in direct contact with the air spaces in the leaf, so $\text{O}_2$ cannot reach them.
$\text{O}_2$ therefore cannot bind to RuBP / rubisco in the bundle sheath cells.
This decreases / stops photorespiration.
$\text{CO}_2$ is highly concentrated in the bundle sheath cells, so rubisco acts as a carboxylase and not an oxygenase.
AVP (e.g. PEP carboxylase has a high affinity for $\text{CO}_2$ and works efficiently at high temperatures where rubisco's affinity for $\text{CO}_2$ falls).

Final answer

See working

Detailed explanation

Background Concept

At high temperatures, rubisco (the enzyme that normally fixes $\text{CO}_2$ in the Calvin cycle) increasingly binds $\text{O}_2$ instead of $\text{CO}_2$ . This is the oxygenase reaction, which produces 2-phosphoglycolate instead of glycerate-3-phosphate (GP), and the plant must recycle 2-phosphoglycolate at a metabolic cost — this wasteful process is photorespiration. $\text{C}_4$ plants minimise photorespiration by spatially separating the two stages of carbon fixation: PEP carboxylase in mesophyll cytoplasm fixes $\text{CO}_2$ into oxaloacetate (a 4-carbon compound), which is shuttled to the bundle sheath where it releases $\text{CO}_2$ . The bundle sheath thus contains a much higher concentration of $\text{CO}_2$ than ordinary air, which favours rubisco's carboxylation reaction over its oxygenation reaction.

Understanding the Question

The question asks you to explain how the Kranz anatomy visible in Fig. 1.1 helps a $\text{C}_4$ plant keep photosynthesising fast at high temperatures. The link you need to make is: leaf structure → high $\text{CO}_2$ / low $\text{O}_2$ in the bundle sheath → rubisco works in carboxylase mode → photorespiration is reduced → high rate of photosynthesis at high T.

Approach

Work through the causal chain: mesophyll ring around bundle sheath → bundle sheath not in direct contact with air → $\text{CO}_2$ concentrated in bundle sheath / $\text{O}_2$ kept out → rubisco binds $\text{CO}_2$ not $\text{O}_2$ → no photorespiration → high rate of photosynthesis maintained at high temperatures.

Step-by-Step Reasoning

The mesophyll cells form a tight ring around the bundle sheath. This is the visible Kranz anatomy in Fig. 1.1.
Because the bundle sheath is enclosed by mesophyll, the air spaces inside the leaf do not directly bathe the bundle sheath. So $\text{O}_2$ does not easily reach the bundle sheath cells (and $\text{O}_2$ produced by the bundle sheath chloroplasts cannot easily diffuse away).
With little $\text{O}_2$ around rubisco, the oxygenase reaction is suppressed — $\text{O}_2$ cannot bind to RuBP at the active site of rubisco.
As a result, photorespiration is reduced or stopped.
Additionally, the $\text{C}_4$ pump (PEP carboxylase in mesophyll cytoplasm) delivers $\text{CO}_2$ into the bundle sheath at high concentration, so rubisco sees much more $\text{CO}_2$ than $\text{O}_2$ and the carboxylation reaction is strongly favoured. PEP carboxylase also has a much higher affinity for $\text{CO}_2$ than rubisco, so it still works efficiently at the higher temperatures where rubisco's affinity for $\text{CO}_2$ falls.
Without photorespiration siphoning off fixed carbon, the plant can maintain a high net rate of photosynthesis even at temperatures where $\text{C}_3$ plants suffer.

Key Takeaways

Kranz anatomy in $\text{C}_4$ leaves creates a high- $\text{CO}_2$ , low- $\text{O}_2$ microenvironment around the bundle sheath chloroplasts.
This suppresses the oxygenase activity of rubisco and therefore photorespiration.
The result is a high net rate of photosynthesis at high temperatures, where $\text{C}_3$ plants lose carbon to photorespiration.

Common Mistakes

Stating only that ' $\text{C}_4$ plants don't photorespire' without linking this to the anatomy shown in the figure.
Saying ' $\text{CO}_2$ cannot reach the bundle sheath' — this is the opposite of the truth; $\text{CO}_2$ is pumped INTO the bundle sheath; it is $\text{O}_2$ that is excluded.
Failing to mention the role of PEP carboxylase in concentrating $\text{CO}_2$ .
Not mentioning photorespiration by name — the question specifically asks about how the plant maintains a HIGH RATE at high T, which is the photorespiration argument.

Things to Be Careful About

The question specifically asks how the ANATOMY adapts the plant, so at least one of the answer points should refer to the mesophyll ring around the bundle sheath (don't just talk about biochemistry).
Mark scheme point 1 is 'decrease / stop, photorespiration' — be explicit that photorespiration is reduced, not just generally 'inefficient photosynthesis'.

Techniques used

relate leaf anatomy to functionexplain how Kranz anatomy reduces photorespirationapply knowledge of C4 carbon fixationlink structure to a high rate of photosynthesis at high temperature

(c)

Fig. 1.2 shows the results of an experiment comparing the rate of carbon dioxide uptake in a C3 plant (Chenopodium album) and a C4 plant (Amaranthus retroflexus) in high and low carbon dioxide ( $\text{CO}_2$ ) conditions.

The rate of $\text{CO}_2$ uptake is used to measure the rate of photosynthesis.

Fig. 1.2

(i)

Using Fig. 1.2, compare the rates of photosynthesis in high $\text{CO}_2$ conditions in the C3 and C4 plants.

DifficultyMedium

Worked solution

Answer

Any four of:

In both plants the rate of $\text{CO}_2$ uptake increases as temperature rises from 5 °C up to about 25 °C.
At low temperatures (below about 22 °C) the $\text{C}_3$ plant has the higher rate.
At high temperatures (above about 22 °C) the $\text{C}_4$ plant has the higher rate.
The $\text{C}_3$ plant's optimum temperature is about 25 °C, while the $\text{C}_4$ plant's optimum is 30–35 °C.
The $\text{C}_3$ plant has a lower maximum rate (peak ≈ 33 µmol m⁻² s⁻¹) than the $\text{C}_4$ plant (peak ≈ 45 µmol m⁻² s⁻¹).
$\text{C}_3$ has a lower optimum temperature than $\text{C}_4$ .
Data quote — e.g. at 25 °C the $\text{C}_3$ rate (~33 µmol m⁻² s⁻¹) exceeds the $\text{C}_4$ rate (~30 µmol m⁻² s⁻¹); at 35 °C the $\text{C}_4$ rate (~45) far exceeds the $\text{C}_3$ rate (~25).

Final answer

See working

Detailed explanation

Background Concept

Photosynthesis is affected by several limiting factors including temperature, light intensity and $\text{CO}_2$ concentration. As temperature rises, the rate of the light-independent reactions (Calvin cycle) increases because the enzymes involved (notably rubisco) work faster — up to an optimum, beyond which the enzymes begin to denature. $\text{C}_3$ plants are particularly sensitive to high temperatures because rubisco's affinity for $\text{CO}_2$ falls relative to $\text{O}_2$ as temperature rises, so photorespiration increases sharply. $\text{C}_4$ plants, with their $\text{CO}_2$ -concentrating mechanism, are buffered against this and so can maintain high rates of photosynthesis at much higher temperatures than $\text{C}_3$ plants.

Understanding the Question

You are asked to compare the $\text{C}_3$ (Chenopodium album) and $\text{C}_4$ (Amaranthus retroflexus) curves in HIGH $\text{CO}_2$ only. Two of the curves in Fig. 1.2 represent these conditions (the two labelled 'high $\text{CO}_2$ conditions'). You need to pick out specific features — where each curve rises, peaks and falls, the maximum rates, and the temperatures at which one curve overtakes the other.

Approach

Read both curves carefully across the temperature axis. Note (a) the shape of each curve (rise then fall), (b) the temperature of the peak (optimum), (c) the height of the peak (maximum rate) and (d) the relative positions of the two curves at low vs high temperatures.

Step-by-Step Reasoning

At 5 °C the $\text{C}_3$ rate is about 10 µmol m⁻² s⁻¹ and the $\text{C}_4$ rate is about 6 µmol m⁻² s⁻¹ — $\text{C}_3$ already has the higher rate at low temperature.
Both curves rise as temperature increases. The $\text{C}_3$ curve rises steeply and reaches its peak at 25 °C (~33 µmol m⁻² s⁻¹); the $\text{C}_4$ curve rises more slowly initially and continues rising past 25 °C, reaching its peak at 30–35 °C (~45 µmol m⁻² s⁻¹).
The two curves cross at around 22 °C — below this, $\text{C}_3$ > $\text{C}_4$ ; above this, $\text{C}_4$ > $\text{C}_3$ .
Above the optimum, the $\text{C}_3$ curve falls sharply (down to ~20 µmol m⁻² s⁻¹ at 40 °C) while the $\text{C}_4$ curve falls only slightly (down to ~38 µmol m⁻² s⁻¹ at 40 °C).
Summary of comparisons:
- Optimum temperature: $\text{C}_3$ ≈ 25 °C vs $\text{C}_4$ = 30–35 °C ( $\text{C}_3$ has a lower optimum).
- Maximum rate: $\text{C}_3$ ≈ 33 vs $\text{C}_4$ ≈ 45 µmol m⁻² s⁻¹ ( $\text{C}_3$ has a lower maximum).
- At low T (< 22 °C): $\text{C}_3$ > $\text{C}_4$ .
- At high T (> 22 °C): $\text{C}_4$ > $\text{C}_3$ .

Key Takeaways

$\text{C}_3$ plants outperform $\text{C}_4$ plants at low temperatures.
$\text{C}_4$ plants outperform $\text{C}_3$ plants at high temperatures.
$\text{C}_4$ plants have both a higher optimum temperature and a higher maximum rate in high $\text{CO}_2$ conditions.
This difference reflects the $\text{C}_4$ plant's resistance to photorespiration, which becomes severe in $\text{C}_3$ plants at high temperature.

Common Mistakes

Comparing the $\text{C}_3$ plant in high $\text{CO}_2$ with the $\text{C}_4$ plant in low $\text{CO}_2$ (or vice versa) — the question restricts the comparison to HIGH $\text{CO}_2$ only.
Saying ' $\text{C}_3$ is always lower' — this is wrong below 22 °C.
Failing to give a data quote — at least one of the four marking points should include numerical support.
Just describing one curve rather than comparing — every point should refer to both plants.

Things to Be Careful About

The question asks for COMPARISON — every marking point should say something about both plants, not just describe one.
A correct answer usually has 2–3 points about the $\text{C}_3$ plant (its shape, optimum, maximum) and 2–3 points about the $\text{C}_4$ plant, plus a data quote.

Techniques used

read values from a line graphcompare two curves at specified temperature rangesidentify the optimum temperature of each curveextract comparative data quotes

(ii)

Using Fig. 1.2, compare the rates of photosynthesis in low and high $\text{CO}_2$ conditions in the C4 plant between $30^{\circ}\text{C}$ and $35^{\circ}\text{C}$ and suggest an explanation for this difference.

DifficultyMedium-Easy

Worked solution

Answer

Comparison:

Between 30 °C and 35 °C the $\text{C}_4$ plant's rate of $\text{CO}_2$ uptake is higher / faster in high $\text{CO}_2$ than in low $\text{CO}_2$ (e.g. ~45 vs ~30 µmol m⁻² s⁻¹); both curves reach their maximum rate in this temperature range.

Explanation:

$\text{CO}_2$ is the limiting factor in the low $\text{CO}_2$ conditions, so increasing $\text{CO}_2$ increases the rate.

Final answer

See working

Detailed explanation

Background Concept

A limiting factor is the variable that, at a given moment, is restricting the rate of a process. For photosynthesis, the three classic limiting factors are light intensity, $\text{CO}_2$ concentration and temperature. If $\text{CO}_2$ is in short supply, raising the temperature further will not increase the rate of photosynthesis because the dark reactions cannot proceed without more $\text{CO}_2$ substrate. Conversely, if $\text{CO}_2$ is abundant, the dark reactions run as fast as the temperature-controlled enzymes allow.

Understanding the Question

You are asked to compare the $\text{C}_4$ plant in low $\text{CO}_2$ and high $\text{CO}_2$ specifically between 30 °C and 35 °C, and then explain the difference. The two relevant curves are the dashed line ( $\text{C}_4$ in low $\text{CO}_2$ ) and the dash-dot line ( $\text{C}_4$ in high $\text{CO}_2$ ).

Approach

Read the values for the two $\text{C}_4$ curves in the 30–35 °C window, note the difference, and then identify which of the two variables ( $\text{CO}_2$ in this case) is the factor holding the rate back in the lower curve.

Step-by-Step Reasoning

Between 30 °C and 35 °C the $\text{C}_4$ plant in high $\text{CO}_2$ reaches its maximum of ~45 µmol m⁻² s⁻¹ (the curve plateaus at the top).
The $\text{C}_4$ plant in low $\text{CO}_2$ also reaches its maximum in this range but at a much lower value of ~30 µmol m⁻² s⁻¹.
So in the high- $\text{CO}_2$ condition the rate is higher (≈ 45 vs ≈ 30 µmol m⁻² s⁻¹).
The two curves both peak in this temperature range, which means temperature is not the limiting factor at 30–35 °C — temperature has been 'saturated' in the sense that it is no longer the bottleneck.
The only variable that has been deliberately changed between the two curves is $\text{CO}_2$ , so $\text{CO}_2$ must be the limiting factor in the low- $\text{CO}_2$ condition. Increasing $\text{CO}_2$ from low to high therefore increases the rate.

Key Takeaways

When two curves for the same plant differ only in $\text{CO}_2$ , $\text{CO}_2$ is the limiting factor.
The peak of both curves at 30–35 °C tells you that temperature is no longer the limiting factor in that range — it is the combination of light and $\text{CO}_2$ that controls the maximum rate.

Common Mistakes

Saying 'temperature is the limiting factor' — the curves peak in the 30–35 °C range, so temperature is not the limiting factor there; the rates are at their maximum.
Giving the explanation without the comparison (or vice versa) — both are needed for the two marks.
Saying the rate is 'slower because the plant is not getting enough light' — light intensity is not the variable being changed in the experiment.
Saying 'because plants need $\text{CO}_2$ ' — too vague; the specific point is that $\text{CO}_2$ is the limiting factor.

Things to Be Careful About

A data quote helps: 'in high $\text{CO}_2$ the rate is ~45 µmol m⁻² s⁻¹ compared with ~30 µmol m⁻² s⁻¹ in low $\text{CO}_2$ ' is a strong answer.
Keep the comparison to the 30–35 °C range; do not stray into other temperature ranges.

Techniques used

read values from a line graph at a specified temperature rangecompare two conditions for the same plantidentify a limiting factor

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