1. Module 2 Exchange and transport
Translocation

2. Phloem
Think, Pair, Share

3. Learning Objectives Success Criteria
State examples of source and sinks (Grade E-D)
Define key terms: source, sink & translocation (Grade E-D)
Interpret the mechanism of active loading (Grade C-A)
Evaluate evidence for this mechanism (Grade C-A)
Describe the mechanism of transport in phloem involving active loading at the source and removal at the sink, and the evidence for and against this mechanism
Explain translocation as an energy-requiring process transporting assimilates between the sources and sinks

4. Translocation Definition:
Define key terms: source, sink & translocation (Grade E-D)
Definition:
Transport of assimilates throughout the plant, in the phloem tissue

5. Source & Sink What is a source? What is a sink? Examples: Leaf Cell
State examples of source and sinks (Grade E-D)
Define key terms: source, sink & translocation (Grade E-D)
Source & Sink
What is a source?
Examples:
Leaf Cell
What is a sink?
Root Cell
Leaf cell can be both as can root cell. Leaf sink when growing, source when photosynthesising, root cell can be source as over winter some roots used to store sugars for spring

6. Translocation
Unlike the water in the xylem, the contents of the phloem can move both up or down a plant stem. It helps to identify where the sugar is being transported from (the source), and where to (the sink).
During the summer sugar is mostly transported from the leaves, where it is made by photosynthesis (the source) to the roots, where it is stored (the sink).
During the spring, sugar is often transported from the underground root store (the source) to the growing leaf buds (the sink).
Flowers and young buds are not photosynthetic, so sugars can also be transported from leaves or roots (the source) to flowers or buds (sinks).

7. Active Loading at the Source
Interpret the mechanism of active loading (Grade C-A)
Active Loading at the Source
Companion cells
Have lots of mitochondria to make ATP
ATP used to pump out protons (H+ ions)
This causes a diffusion gradient
Protons diffuse back in through cotransporter proteins, bringing sucrose in with them
This leads to a high concentration of sucrose

8. Active Loading at the Source
Interpret the mechanism of active loading (Grade C-A)
Active Loading at the Source
Sieve Tube Elements
Sucrose moves in through plasmodesmata
Sucrose lowers the water potential, so water moves in by osmosis
This increases the hydrostatic pressure

9. Removal at the Sink Sink
Interpret the mechanism of active loading (Grade C-A)
Removal at the Sink
Sink
Uses up sucrose in respiration or it is stored as starch
Lowers sucrose concentration
Sucrose moves in by diffusion, lowering the water potential
Water moves into the sink by osmosis
This lowers the hydrostatic pressure
This high hydrostatic pressure near the source and low hydrostatic pressure near the sink creates movement in the phloem called mass flow

10. Evidence
Evaluate evidence for this mechanism (Grade C-A)
Active loading and movement of sucrose in the phloem is a theory
Evidence supports this theory:
Summarise the evidence for this theory

11. Learning Objectives State examples of source and sinks (Grade E-D)
Define key terms: source, sink & translocation (Grade E-D)
Interpret the mechanism of active loading (Grade C-A)
Evaluate evidence for this mechanism (Grade C-A)