Phloem Definition and characteristics of phloem 3.0 / 5 based on 1 rating ? BiologyHuman, animal and plant physiologyAS Created by: Morgan BainesCreated on: 13-10-11 17:45 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 1 of 6 Can transport up/down the stem Phloem tissue consists of two types of cell: sieve tube elements companion cells 2 of 6 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 3 of 6 4 of 6 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 5 of 6 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 6 of 6
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