Topic 9: PLANT BIOLOGY (HL)

9.1 Transport in the xylem of plants

Transpiration
Transpiration is the inevitable consequence of gas exchange in the leaf.

Modelling water transport 
Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.

Using a potometer (PRACTICAL)
Measurement of transpiration rates using potometers.

Effect of humidity on transpiration
Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.

Xylem structure helps withstand low pressure
The cohesive property of water and the structure of the xylem vessels allow transport under tension.

Data-based questions: The Renner experiment

Tension in leaf cell walls maintain the transpiration stream
The adhesive property of water and evaporation generate tension forces in leaf cell walls.

Active transport of minerals in the roots
Active uptake of mineral ions in the roots causes absorption of water by osmosis.

Replacing losses from transpiration
Plants transport water from roots to leaves to replace losses from transpiration.

Adaptions for water conservation
Adaption of plants in deserts and in saline soils for water conservation.

Drawing xylem vessels
Drawing the structure of primary xylem vessels in sections of stems based on microscope images.

Nature of science
Use models as representations of the real world: water transport mechanisms in the xylem can be modeled using apparatus and materials that will show similarities in structure to plant tissue.

9.2 - Transport in the phloem of plants

Translocation occurs from source to sink
Plants transport organic compounds from sources to sinks.

The transport of organic solutes in plants (sap) is known as translocation.
sources - where sap is made, stored or absorbed.
sinks - where sap is used or stored.
[structure of phloem]

Phloem loading
Active transport is used to load organic compounds into phloem sieve tubes at the source.
Sucrose is not available for plants to metabolize during respiration and is therefore used to transport carbohydrates as it will not metabolize. This makes sucrose the most prevalent solute in sap that is transported in the phloem.

[image of phloem loading]
Pressure and water potential differences play a role in translocation

Data-based questions: Explaining water movement

Phloem sieve tubes
Experiment using aphid stylets

Data-based questions

Radioisotopes as important tools in studying translocation

Data-based questions: Radioactive labelling [1]