Xylem and phloem
- Created by: Nicola Carter
- Created on: 03-01-13 13:11
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- Xylem and phloem
- Structure of xylem
- Xylem is used to transport water and minerals from the roots up to the leaves and other parts of the plant.
- Xylem tissues consists of tubes to carry the water and dissolved minerals, fibres to help support the plant and living parenchyma cells
- Xylem vessels
- In dicotyledonous plants, the obvious features of xylem and the xylem vessel elements.
- These are long cells with thick walls that have been impregnated by lignin.
- As the xylem develops the lignin waterproofs the walls of the cells.
- As a result the cells die, and their end walls and contents decay
- This leaves a long column of dead cells with no contents - a tube with no end walls (xylem vessels)
- As a result the cells die, and their end walls and contents decay
- The lignin strengthens the vessel walls and prevents the vessel from collapsing. This keeps the vessels open even at times when water may be in short supply
- The lignin thickening forms patterns in the cell wall. These may be spiral, annular (rings) or reticulate (a network of broken rings)
- This prevents the vessel from being too rigid and allows flexibility of the stem or branch
- In some places this lignification is not complete. It leaves pores in the wall of the vessel, which are called pits or bordered pits.
- These allow water to leave one vessel and pass into another adjacent vessel, or pass into the living parts of the plants
- In dicotyledonous plants, the obvious features of xylem and the xylem vessel elements.
- Adaptations of xylem to its function
- Xylem tissue can carry water and minerals from roots to the very top of the plant because:
- it is made from dead cells aligned end-to-end to form a continuous column
- the tubes are narrow so the water column doesn't break easily and capillary action can be effective.
- pits in the lignified walls allow water to move sideways from one vessel to another
- lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the pant grows and enables the stem/branch to bend
- The flow of water in impeded because:
- there are no end walls
- there are no cell contents
- there is no nucleus/cytoplasm
- lignin thickening prevents the walls from collapsing
- Xylem tissue can carry water and minerals from roots to the very top of the plant because:
- Structure of phloem
- The function of the phloem is to transport sugars from one part of the plant to another. Could be up or down the stem
- Phloem tissue consists of two types of cell;the sieve tube elements and the companion cells.
- Sieve tubes
- Not true cells, contain very little cytoplasm and no nucleus
- Lined up end-to-end to form a tube, in which the plant transports sucrose
- Unlike xylem vessels, this tube contains cross-walls at intervals.
- These cross-walls are perforated by many pores to allow the sap to flow.
- Hence the cross-walls are called sieve plates and the tubes are called sieve tubes. The sieve tubes have very thin walls and are usually 5/6 sided
- Companion cells
- In between the sieve tubes are small cells, each with a large nucleus and dense cytoplasm
- Have numerous mitochondria to produce the ATP needed for active processes.
- The companion cells carry out the metabolic processes needed by the sieve tube elements. Includes using ATP as a source of energy to load sucrose into the sieve tubes.
- The cytoplasm of the companion cells and the sieve tube elements are linked through many plasmodesmata.
- These are gaps in the cell walls allowing communication and flow of substances between the cells
- Structure of xylem
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