Summary and Exam Tips for Transport in the Phloem of Plants
Transport in the phloem of plants is a subtopic of Plant Biology, which falls under the subject Biology in the IB DP curriculum. Translocation is the process of moving organic compounds like sugars and amino acids from sources (photosynthetic tissues such as leaves) to sinks (roots, fruits, seeds) via the phloem. Sugars are mainly transported as sucrose due to its solubility and metabolic inertness. The phloem sap is a nutrient-rich fluid that moves through the plant.
The movement is driven by pressure and water potential differences. At the source, active transport of solutes into the phloem creates a hypertonic solution, drawing water from the xylem by osmosis, increasing hydrostatic pressure. This pressure forces the sap to flow towards the sink, where pressure is lower due to the unloading of sucrose and water loss by osmosis.
Phloem loading involves active transport of H ions out of the phloem, creating a gradient that facilitates sucrose cotransport. At the sink, solutes are unloaded, making the sap hypotonic and allowing water to return to the xylem, maintaining the pressure gradient.
Identification of xylem and phloem in micrographs is crucial for understanding plant structure. Experiments using aphid stylets and radioactively-labelled carbon dioxide help measure phloem transport rates, providing insights into the efficiency of translocation.
Exam Tips
- Understand the Source-Sink Concept: Clearly differentiate between sources and sinks and their roles in translocation.
- Hydrostatic Pressure Mechanism: Be able to explain how pressure differences drive the movement of phloem sap.
- Active Transport and Osmosis: Know how active transport and osmosis contribute to phloem loading and unloading.
- Micrograph Identification: Practice identifying xylem and phloem in microscope images to understand plant anatomy.
- Experiment Analysis: Familiarize yourself with experiments using aphid stylets to measure phloem transport rates, as these are often tested in exams.
