Phloem

Definition and characteristics of phloem

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Definition of Phloem

Phloem is a plant transport tissue that carries the products of photosynthesis (e.g. sugars) to the rest of the plant

·       hollow columns of dead cells

·       lined end to end with lignin- strengthens

·       Provides support for plant

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  • Can transport up/down the stem

Phloem tissue consists of two types of cell:

  • sieve tube elements
  • companion cells
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Sieve Tubes

Sieve Tubes

  • not true cells- contain very little cytoplasm and no nucleus
  • Lined end to end to form a tube- plants transports sugars (usually sucrose) using tube
  • Sucrose is dissolved in water to form sap
  • Unlike xylem vessels, tube contains cross-walls at intervals

Cross walls:

  • Are perforated by many pores to allow the sap to flow

Hence...

  • Cross walls are called sieve plates
  • Tubes are called sieve tubes

Sieve Tubes

  • thin walls
  • five or six sided usually
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(http://leavingbio.net/FLOWERING%20PLANTS_files/image048.jpg)

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Companion Cells

  • Located inbetween sieve tubes
  • Each have large nucleus and dense cytoplasm
  • They have numerous mitochondria (site of aerobic respiration) to produce the ATP needed for active processes
  • The cytoplasm of the companion cells and sieve tube elements are linked through many plasmodesmata

PLASMODESMATA:

  • They are gaps in the cell walls allowing communication and flow of minerals between the cells
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What companion cells do

  • Carry out metabolic processes needed by the sieve tube elements

- this includes using ATP as source of energy to load sucrose into sieve tubes

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