9.2.11 Outline the Role of Phloem in Active Translocation of Sugars (sucrose) and amino acids from source (photosynthetic

tissue to storage organs) to sinks (fruits, seeds, roots)

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Nutrient Transport

–Movement from Source to Sink• A process of phloem transport moves sugars

through a plant from a source to a sink.• A source is any cell in which sugars are produced

by photosynthesis.• A sink is any cell where the sugars are used or

stored (fruits, roots, seeds).

Vascular TissuePHLOEM Tissue• Phloem Cells are alive.• Sieve Tube elements (cells) lacks

nucleus & cytoplasm• Sieve tube elements are

connected to each other via sieve plates

• Sieve plates have holes which allow movement of water and dissolved organic molecules

• Companion Cells has nucleus and dense cytoplasm

• Companion cells are attached to sieve tube element via plasmodesmata

Sieve plate

Phloem Translocation Summary• 1.Translocation moves the organic molecules

(sugars, amino acids) from their source through the tube system of the phloem to the sink. Phloem vessels still have cross walls called sieve plates that contain pores.

• 2. Companion cells actively (active transport) load sucrose (soluble, not metabolically active) into the phloem.

• 3. Water follows the high solute in the phloem by osmosis. A positive pressure potential develops moving the mass of phloem sap forward.

• 4. The sap must cross the sieve plate. Current hypothesis do not account for this feature.

• 5. The phloem still contains a small amount of cytoplasm along the walls but the organelle content is greatly reduced.

• 6. Companion cells actively unload (ATP used) the organic molecules

• 7. Organic molecules are stored (sucrose as starch, insoluble) at the sink. Water is released and recycled in xylem.

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STEP 1•Sugars produced during photosynthesis (source cells) are actively (using ATP) pumped into the sieve tubes.•This loading of sugar is accomplish by Active Transport.

Sugar molecules

Movement of water

Movement of sugar

Phloem Xylem

Source cell

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STEP 2

•As sugar concentrations increase in the sieve tubes, water from the xylem moves into the sieve tube by osmosis.

Movement of water

Movement of sugar

Sugar molecules

Phloem Xylem

Source cell

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Movement of water

Movement of sugar

STEP 3•This movement of water causes a positive pressure to build up in the sieve tube that results in a bulk flow of phloem sap.• This movement does not require energy.

Sugar molecules

Phloem Xylem

Source cell

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STEP 4•The pressure diminishes by the removal of sugar from the sieve tube at the sink. •This removal of sugar is accomplished via Active Transport (uses ATP)•The sugar is converted into starch and starch in insoluble and exerts no osmotic affect.

Xylem Phloem

Movement of water

Movement of sugar

Sink cell

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STEP 5 •Xylem recycles the relatively pure water by carrying it from the sink back to the source.

Movement of water

Movement of sugar

Xylem Phloem Sink cell

Transport in Plants Summary• 1. Source produces organic molecules• 2. Glucose from photosynthesis produced• 3.Glucose converted to sucrose for transport• 4. Companion cell actively loads the sucrose• 5. Water follows from xylem by osmosis• 6. Sap volume and pressure increased to give

Mass flow• 7. Unload the organic molecules by the

companion cell• 8. Sucrose stored as the insoluble and

unreactive starch• 9. Water that is released is picked up by the

xylem• 10. water recycles as part of transpiration to re

supply the sucrose loading

Watch the below animations on translocation

Click on the links below to access animations:• http

://www.youtube.com/watch?v=-b6dvKgWBVY&feature=related

• http://www.youtube.com/watch?v=MxwI63rQubU&feature=related