Biology • Year 11 • Module 2 • Lesson 21
Module 2 Review, Organisation of Living Things
Apply IQ1, IQ2 and IQ3 knowledge to real data sets, experimental scenarios, and cross-system comparisons.
1. Interpret transpiration-rate data from a potometer experiment
A student used a potometer to measure the rate of water uptake (mm/min) by a cut leafy shoot under four conditions. The experiment was conducted at constant temperature. 7 marks
| Condition | Water uptake rate (mm/min) |
|---|---|
| Still air, low light | 0.6 |
| Still air, high light | 1.4 |
| Moving air (fan), high light | 2.9 |
| Moving air (fan), high light, petroleum jelly on lower leaf surface | 0.3 |
1.1 Describe the effect of increasing light intensity on water uptake rate (compare rows 1 and 2). 2 marks
1.2 Using lesson content, explain the biological mechanism by which high light intensity increases transpiration rate. 3 marks
1.3 The petroleum jelly condition (row 4) almost eliminates water uptake. Explain this observation and identify what this tells you about the location of stomata on this leaf. 2 marks
2. Interpret blood composition data across four sites in the human body
The table below shows the partial pressure of O2 (pO2, mmHg) and the relative concentration of glucose (arbitrary units) measured in blood at four anatomical sites. 8 marks
| Site | pO2 (mmHg) | Glucose (A.U.) |
|---|---|---|
| Pulmonary artery (entering lungs) | 40 | 5.0 |
| Pulmonary vein (leaving lungs) | 100 | 5.0 |
| Hepatic portal vein (entering liver after meal) | 75 | 9.8 |
| Femoral vein (draining leg muscles during exercise) | 20 | 2.1 |
2.1 Explain why pO2 rises from 40 mmHg to 100 mmHg as blood passes through the lungs. Reference the partial pressure gradient and Fick’s law. 3 marks
2.2 The femoral vein pO2 is only 20 mmHg during exercise. Explain why active muscle tissue causes such a large drop in blood pO2. 2 marks
2.3 Glucose concentration is high in the hepatic portal vein after a meal (9.8 A.U.) but low in the femoral vein (2.1 A.U.). Identify where glucose is absorbed from the meal, and explain why it is lower in blood draining active muscle. 3 marks
3. Apply to a new scenario, xerophyte adaptations in an arid environment
A research team studying desert plants identifies three species with different structural adaptations to reduce water loss. All three are autotrophs. 9 marks
Species X: Small, needle-like leaves with a thick waxy cuticle and sunken stomata in furrows. Species Y: No leaves; photosynthesis occurs in thick, water-storing stem cells. Stomata open only at night. Species Z: Broad leaves with dense hairs (trichomes) covering both surfaces.
3.1 For Species X, explain how each of the three listed features (thick waxy cuticle, needle-like leaves, sunken stomata) reduces water loss. Refer to the boundary layer and Fick’s law in your answer. 4 marks
3.2 Species Y opens its stomata only at night. Explain the trade-off this creates for photosynthesis, and identify what name is given to this photosynthesis pathway. 2 marks
3.3 Explain how dense leaf hairs (trichomes) on Species Z reduce transpiration rate. 2 marks
3.4 All three species are autotrophs. State one structural feature that all autotrophic land plants must retain, even in arid environments, to carry out photosynthesis, and explain why it cannot be permanently sealed. 1 mark
Q1.1, Effect of light on water uptake (2 marks)
Increasing light intensity from low to high increased the water uptake rate from 0.6 mm/min to 1.4 mm/min [1], more than doubling the rate [1]. (Accept: approximately 2.3-fold increase.)
Q1.2, Mechanism of light-induced transpiration increase (3 marks)
High light intensity triggers K+ ion pumping into guard cells [1]. This lowers the water potential of guard cells, causing water to enter by osmosis, increasing turgor pressure and opening the stomatal pore wider [1]. The wider pore increases the cross-sectional area of the diffusion pathway, so more water vapour can diffuse out per unit time according to Fick’s law (rate ∝ SA × gradient / thickness) [1]. Note: the energy for evaporation comes from heat, not directly from light.
Q1.3, Petroleum jelly observation (2 marks)
Petroleum jelly blocks the stomata on the lower surface, almost completely preventing water vapour from diffusing out of the leaf [1]. This tells us that stomata are predominantly located on the lower (abaxial) surface of this leaf species [1].
Q2.1, O2 rise in lungs (3 marks)
The alveolar air has a pO2 of approximately 100–105 mmHg, while blood entering the lungs (pulmonary artery) has a pO2 of only 40 mmHg [1]. This concentration gradient drives O2 diffusion from alveolar air into blood by Fick’s law [1]. The thin alveolar wall (one-cell thick, approximately 0.5 μm) and enormous surface area (~250 m2) maximise diffusion rate, loading haemoglobin in red blood cells until blood pO2 equilibrates near 100 mmHg [1].
Q2.2, Low pO2 in femoral vein during exercise (2 marks)
Active skeletal muscle cells consume O2 rapidly through cellular respiration to produce ATP for contraction [1]. This lowers the pO2 inside muscle cells to approximately 20–30 mmHg, creating a steep gradient that drives O2 diffusion from blood (pO2 ~95 mmHg at arteriole) out to muscle, dropping blood pO2 dramatically by the time it reaches the draining vein [1].
Q2.3, Glucose sites (3 marks)
Glucose is absorbed from the small intestine (specifically the villi of the small intestine after enzymatic digestion of carbohydrates) [1], and travels via the hepatic portal vein directly to the liver, explaining the high 9.8 A.U. reading after a meal [1]. Glucose is lower in the femoral vein because active leg muscles are consuming glucose as a substrate for cellular respiration (glycolysis and aerobic respiration) to produce the ATP needed for contraction [1].
Q3.1, Species X features (4 marks)
Thick waxy cuticle: The waxy cuticle is impermeable to water, so water can only leave through stomata rather than diffusing freely through the epidermis, reducing the overall pathway for evaporation [1]. Needle-like (small) leaves: Reduce total surface area available for transpiration and also reduce the area exposed to warming by solar radiation (which would increase evaporation rate by raising temperature) [1]. Sunken stomata: The recessed crypts trap a humid boundary layer directly adjacent to the stomatal pore, reducing the effective water vapour concentration gradient between the leaf interior and the surrounding atmosphere; according to Fick’s law, a smaller gradient means a lower diffusion rate, so less water is lost [1 + 1 for referencing boundary layer and Fick’s law explicitly].
Q3.2, Night stomata and CAM (2 marks)
Opening stomata only at night allows CO2 to be absorbed when temperatures are cooler and transpiration rates are lower, reducing water loss [1]. The trade-off is that CO2 cannot be fixed directly by the Calvin cycle at night (no light); instead, CO2 is stored as organic acids overnight and released during the day for photosynthesis with stomata closed. This pathway is called CAM (Crassulacean Acid Metabolism) photosynthesis [1].
Q3.3, Trichomes on Species Z (2 marks)
Dense hairs trap a layer of still, humid air close to the leaf surface (thickening the boundary layer) [1]. This reduces the water vapour concentration gradient between the stomata and the free atmosphere, lowering the rate of diffusion of water vapour out of the leaf according to Fick’s law [1].
Q3.4, Feature autotrophs cannot seal (1 mark)
Stomata must remain openable (not permanently sealed) because CO2 must be able to enter the leaf to serve as the inorganic carbon source for photosynthesis; permanently sealed stomata would block CO2 entry and prevent the Calvin cycle from operating.