Biology • Year 11 • Module 2 • Lesson 7
Plant Structure: Macroscopic and Microscopic
Apply structure-function reasoning to real data on leaf adaptations, microscopy selection, and environmental trade-offs in plant anatomy.
1. Interpret stomata density data
A student counted the number of stomata visible in a fixed field of view (1 mm2) on the upper and lower epidermis of five Australian plant species. The results are shown below. 8 marks
| Species | Stomata on upper surface (per mm2) | Stomata on lower surface (per mm2) | Habitat |
|---|---|---|---|
| Eucalyptus globulus | 0 | 220 | Wet sclerophyll forest |
| Acacia pycnantha | 0 | 185 | Dry sclerophyll / mallee |
| Spinach (Spinacia oleracea) | 14 | 178 | Cultivated, moist soil |
| Mulga (Acacia aneura) | 0 | 92 | Semi-arid, low rainfall |
| Waterlily (Nymphaea sp.) | 460 | 0 | Aquatic, floating leaves |
1.1 Describe the general pattern in stomata distribution between the upper and lower epidermis across the five species. 2 marks
1.2 Explain why stomata are concentrated on the lower surface in most land plants. Use the concept of water loss in your answer. 3 marks
1.3 The waterlily has all its stomata on the upper surface. Explain how this is an adaptation to its aquatic environment and what would happen if the stomata were on the lower surface. 3 marks
2. Interpret graph, leaf thickness and CO2 diffusion rate
The stylised graph below shows how the maximum rate of CO2 delivery to palisade mesophyll cells changes with leaf thickness, holding all other variables constant. 6 marks
Stylised model, illustrative of the CO2 diffusion distance principle from Card 4 of the lesson.
2.1 Describe the trend shown by the curve. 2 marks
2.2 Explain, using the concept of diffusion distance, why thicker leaves have a lower relative CO2 delivery rate to palisade mesophyll cells. 2 marks
2.3 Using the graph, explain how this supports the lesson's claim that the thin profile of a leaf is a structural adaptation. 2 marks
3. Select the right imaging technology
A plant biology researcher wants to investigate plant structures at different levels of detail. For each investigation below, recommend the most appropriate imaging technology from: light microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), confocal microscope. Justify each choice by explaining what the technology can show that the others cannot. 8 marks, 2 per question
3.1 Observing the arrangement of cell layers in a leaf cross-section (cuticle, epidermis, palisade mesophyll, spongy mesophyll) in a stained slide.
Technology:
Justification:
3.2 Examining the three-dimensional surface structure of stomatal pores and the texture of the surrounding guard cells on a leaf surface.
Technology:
Justification:
3.3 Investigating the membrane structure inside chloroplasts in palisade mesophyll cells, specifically the thylakoid membranes and grana stacks.
Technology:
Justification:
3.4 Tracking the movement of a fluorescently labelled sugar molecule through phloem tissue in a living leaf.
Technology:
Justification:
4. Apply to a new scenario, cactus vs. rainforest tree leaf
Australian desert cacti (introduced) have replaced their leaves with spines. The photosynthesis function is performed instead by the green stem. A rainforest tree such as a Ficus species has large, broad, thin leaves. 6 marks
4.1 Explain, using the trade-off between photosynthesis and water loss, why the cactus has evolved spine-leaves rather than broad flat leaves. Use at least two plant structure terms from the lesson. 3 marks
4.2 A broad leaf such as a Ficus leaf is described as having a high surface area-to-volume ratio. Explain two specific advantages this gives the leaf for functioning as a photosynthetic organ. 2 marks
4.3 Both the cactus stem and the Ficus leaf must still exchange gases. Identify one structural feature both share that enables gas exchange, and state its function. 1 mark
Q1.1, Stomata distribution pattern (2 marks)
In most land plant species (Eucalyptus, Acacia, Mulga), stomata are concentrated almost entirely on the lower (abaxial) surface, with none or very few on the upper surface [1]. The waterlily is an exception, all its stomata are on the upper surface and none are on the lower surface [1].
Q1.2, Why stomata on lower surface (3 marks)
The lower surface of a leaf is shaded from direct sunlight, so it is cooler [1]. A cooler surface reduces the water vapour pressure gradient between the inside of the leaf and the surrounding air, which reduces the rate of evaporative water loss through open stomata [1]. Positioning stomata on the lower surface therefore allows gas exchange (CO2 in, O2 out) to continue while minimising water loss, a critical adaptation for land plants that cannot afford excessive transpiration [1].
Q1.3, Waterlily stomata on upper surface (3 marks)
The waterlily leaf floats on the water surface, so its lower surface is in contact with water, not air [1]. Stomata on the lower surface would be submerged and could not exchange gases with the atmosphere. Having all stomata on the upper surface (exposed to air) allows effective gas exchange [1]. Water loss is not a concern because the plant has unlimited access to water from below, so the usual constraint of reducing stomatal exposure to sun does not apply [1].
Q2.1, Describe the trend (2 marks)
As leaf thickness increases, the relative CO2 delivery rate to palisade mesophyll decreases [1]. The decline is steep at lower thicknesses and becomes shallower (levels off) at greater thicknesses, forming a downward-curving relationship [1].
Q2.2, Diffusion distance explanation (2 marks)
In a thicker leaf, palisade mesophyll cells are further from the stomata [1]. Because CO2 enters through stomata and diffuses through the spongy mesophyll air spaces, a greater diffusion distance means CO2 takes longer to reach the palisade cells and the concentration gradient is steeper, the rate of delivery to palisade cells is lower per unit time [1].
Q2.3, Link to thin leaf adaptation (2 marks)
The graph shows that the highest CO2 delivery rates occur at leaf thicknesses below about 0.2 mm, in the range typical of real leaves [1]. This supports the lesson's claim that the leaf's thin profile is adaptive: by minimising the diffusion distance between stomata and the palisade mesophyll, the thin leaf allows rapid CO2 delivery, sustaining a high photosynthesis rate [1].
Q3, Imaging technology selection (2 marks each)
3.1 Light microscope. Justification: a light microscope resolves to approximately 200 nm, which is sufficient to distinguish individual cell layers, their shapes, and the presence or absence of chloroplasts. The sample can be stained to enhance contrast. SEM only shows surfaces; TEM requires extremely thin sections and shows ultrastructure, not whole tissue layers.
3.2 Scanning electron microscope (SEM). Justification: SEM produces high-resolution 3D surface images by detecting electrons scattered from the coated surface of the specimen. This is the only technology that reveals surface topography, the shape and aperture of stomatal pores and the texture of guard cells. Light microscopy lacks the resolution for surface detail; TEM shows internal cross-sections only.
3.3 Transmission electron microscope (TEM). Justification: thylakoid membranes are approximately 5–10 nm thick, far below the ~200 nm resolution limit of a light microscope. TEM resolves to approximately 0.1 nm and can clearly image individual membrane bilayers, grana stacks, and thylakoid lumen. SEM shows surfaces only, not internal organelle structure.
3.4 Confocal microscope. Justification: confocal microscopy allows imaging of fluorescently labelled molecules in living cells and can produce 3D optical sections through tissue without physical cutting. This makes it the only technique that can track the movement of a fluorescent sugar molecule through living phloem without killing the tissue. Light microscopy lacks the optical sectioning capability and fluorescence resolution; SEM and TEM require dead specimens.
Q4.1, Cactus spine adaptation (3 marks)
In a desert environment, water is scarce and temperatures are high. Broad flat leaves would maximise surface area for photosynthesis but also maximise transpiration, evaporative water loss through stomata and the cuticle [1]. By replacing leaves with spines, the cactus eliminates the leaf's large surface area, drastically reducing transpiration while retaining some protection from herbivores [1]. The green stem takes over photosynthesis: it has chloroplasts in its cortex but presents far less surface area per unit volume, so water is conserved. The trade-off between photosynthesis rate and water loss is shifted in favour of water conservation in an arid environment [1].
Q4.2, Two advantages of high SA:V ratio for photosynthesis (2 marks)
First, a large surface area maximises the number of palisade mesophyll cells that receive direct sunlight, increasing the rate of photosynthesis per unit time [1]. Second, the thin profile (high SA:V) minimises the diffusion distance from stomata to any mesophyll cell, so CO2 can reach photosynthetic cells quickly, sustaining a high photosynthesis rate without a CO2 bottleneck [1].
Q4.3, Shared gas exchange structure (1 mark)
Both have stomata (or lenticels in the stem), pores flanked by guard cells that open to allow CO2 to diffuse in and O2 to diffuse out, enabling gas exchange for photosynthesis and respiration. [1]