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Module 2 Exchange and transport 1.2.19 Translocation Phloem Think, Pair, Share Learning Objectives • 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 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) • Define key terms: source, sink & translocation (Grade E-D) Translocation • Definition: – Transport of assimilates throughout the plant, in the phloem tissue •State examples of source and sinks (Grade E-D) •Define key terms: source, sink & translocation (Grade E-D) • What is a source? – Examples: • Leaf Cell • What is a sink? – Examples: • Root Cell Source & Sink 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). •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 •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 •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 • Evaluate evidence for this mechanism (Grade C-A) Evidence • Active loading and movement of sucrose in the phloem is a theory • Evidence supports this theory: • Summarise the evidence for this theory 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)