IB DP Biology ยท B3.2 Plant Transport
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Click any labelled tissue in the transverse section of a dicotyledonous leaf. The diagram shows the distribution of tissues involved in gas exchange and transpiration, including the vein / vascular bundle, xylem, phloem, mesophyll, epidermis and stomata.
Water enters root hair cells by osmosis, travels through the root cortex, enters the xylem, and is pulled upwards by transpiration. In the leaf, water evaporates from the wet surfaces of spongy mesophyll cells into the air spaces, then diffuses out through the open stomata. Click a part of the diagram to see what it does.
IB expects students to draw and label a plan diagram of a transverse section of a dicotyledonous leaf. A plan diagram is low power and simplified. It should show tissue regions, not individual cells, chloroplasts, shading or tiny details.
Select a label, then click the matching number on the diagram. You need all ten labels correct.
Stomata are pores in the lower epidermis, each flanked by two kidney-shaped guard cells. When guard cells take up water and become turgid, they bow outwards and open the pore. When they lose water, they go limp and close it. Move the sliders below to see this happen.
Low internal COโ signals "photosynthesis is happening - keep stomata open."
Plants in different habitats have evolved very different leaf structures. Compare three plant types below, then try the stomatal density practical to measure these differences quantitatively.
Complete the leaf cast protocol before the microscope unlocks. Students have to choose the correct epidermis, make the clear nail varnish cast, lift the imprint with tape, mount it on a slide, focus the microscope, then count repeated fields of view.
Select the leaf surface you would sample, then complete each preparation step in order.
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Apply what you have learned. Work through the cards below, predict before revealing, and use the potometer data you collected to do the calculations.
Core: predict whether the rate increases or decreases, then explain using one keyword: light, temperature, humidity, wind or leaf area.
Standard: keep control variables constant, run three repeats, calculate a mean rate, and describe the pattern using data.
Challenge: evaluate limitations, identify anomalies, compare stomatal density with water uptake rate, and explain the trade-off between gas exchange and water loss.
Bubble distance = 24 mm. Time = 6 min. Capillary radius = 0.5 mm.
Distance per hour = 24 รท 6 ร 60 = 240 mm/hour.
Cross-sectional area = ฯrยฒ = ฯ ร 0.5ยฒ = 0.785 mmยฒ.
Volume per hour = 240 ร 0.785 = 188.4 mmยณ/hour.
Convert to cmยณ: 188.4 รท 1000 = 0.188 cmยณ/hour.
Vocabulary and quick reference for the B3.2 plant transport content covered in this simulation.
Transpiration is the loss of water vapour from a plant, mainly from the leaves through the stomata. It is a consequence of having stomata open for gas exchange.
The transpiration stream is the continuous flow of water from roots โ xylem โ leaves โ atmosphere. It also delivers mineral ions to the leaves.
Cohesion-tension theory explains how water moves up the xylem against gravity. Water molecules cohere through hydrogen bonding. As water evaporates from leaves, tension is created that pulls the entire water column upwards.
Light opens stomata for gas exchange; transpiration rises in light. Temperature increases evaporation from leaf surfaces and increases the kinetic energy of water molecules. Wind removes the layer of humid air around the leaf, maintaining a steep concentration gradient for diffusion. Humidity reduces the concentration gradient between the leaf and air - high humidity slows transpiration. Leaf surface area directly scales the rate.
Mesophytes live where water is moderately available. Standard leaf structure.
Xerophytes live in dry conditions. Thick waxy cuticle, sunken stomata, reduced surface area, leaf hairs, fewer stomata per mmยฒ - all reduce water loss.
Hydrophytes live in or on water. Floating hydrophytes such as water lilies often have stomata on the upper surface because that side is exposed to air. Submerged hydrophytes such as Elodea are different and may have no functional stomata.
Typical values per mmยฒ: Mesophyte lower epidermis about 150, xerophyte lower epidermis about 60, and floating hydrophyte upper epidermis about 300. These are averages. There is wide variation by species and even between leaves on the same plant.
B3.1.7: Leaf adaptations for gas exchange include waxy cuticle, epidermis, air spaces, spongy mesophyll, stomatal guard cells and veins.
B3.1.8: Students should draw and label a plan diagram showing the distribution of tissues in a transverse section of a dicotyledonous leaf.
B3.1.9: Transpiration is a consequence of gas exchange through open stomata, and the rate is affected by light, temperature, humidity, wind and leaf area.
B3.1.10: Stomatal density is determined from micrographs or leaf casts. Repeated counts improve reliability and show natural variation in biological material. A consistent border rule prevents double counting or biased sampling.