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Transport Tissue in Plants -Vascular Tissue

Xylem- Transports water and mineral ions

Phloem- Transports organic molecules (sugars and amino acids)

The uptake of water by a plant begins at the root. The root hairs greatly increase S.A. for water and mineral ion absorption.

Water is drawn into the root by osmosis. As the water potential of the cell sap is lower than the water potential of the surroudning soil water.

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Water travels across the cortex by 2 routes:

Appoplast  - along the cellulose cell walls

Symplast- through the cytoplasm of adjacent cells, cells connected by plasmodestmata 

Most water flows by the appoplast pathway as it offers less resistance.

Water is forced to pass through the endodermis via the symplast route since the casparian strip prevents passage through the cell walls. 

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/sym_apoplast.gif)

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The Movement of Water through the Stem

  • Water is essentially pumped into the xylem in the root causing a root pressure
  • Water moves up the plant because of the water potential gradient
  • There are cohesive forces between the water molecules themselves (hydrogen bonds)
  • The are also adhesive forces between water and the cell walls.
  • These forces of choesion and adhesion are caused by the polarity of the water molecules
  • The whole water colunmn is pulled upwards due to a negative pressure (tension) as water is drawn out of the xylem vessels in the leaf. Resulting in the 'transpiration stream'

Cohesion- tension theory - water is not pushed from below, but pulled from above as water evaporates from mesophyll cells of leaves by transpiration.Involves forces of adhesion and cohesion.The cohesion-tension theory requires that the column of water in the xylem is continuous and remains unbroken.

Capillarity theroy-  adhesive forecs between water and cell walls.

Root pressure theory- water is pushed up by a force generated in the root.

The root prssure and capillarity alone are not enough for water to travel up the stem, they reduce forces necessary for transpiration pull.

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Movement through the leaf

Water vapour evaporates off the surface of the spongy mesophyll cells and diffuses through the air spaces out via the stomata. 

(http://www.bbc.co.uk/staticarchive/0896f222a3d9836459643a0d0ac28ea26a8a8304.gif)

Transpiration- is the loss of water from a plant to the atmosphere.

While most water vapour difffuses out of stomata some water is lost through the waxy cuticle- Cuticle Transpiration 

You could measure the rate of water loss for each by close one off- sealing it with vasaline.

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Translocation

'Translocation' describes the movement of organic substances in the phloem

Transloaction is biodirectional, occurs from source to sink, and requires energy.

Source: organ where sugar is produced in photosynthesis or where sugar is produced by the break down of starch (leaves)

Sink: organ that uses or stores carbohydrate (flowers, buds, roots)                                          Note: suagr can be moved up to buds or down to the roots.

Evidence that movement in the phloem involves energy expenditure 

  • metabolic poisins which stop respiration also stop translocation.
  • companion cells have high rates of metabolic activity (they contain many mitochondria)

Evidence Movement is biodirectional/2way in phloem

Using radioactively labelled sucrose shows that sucrose can move up and down the stem. (radioactivity is detected in the shoot tip and in the roots below)

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Phloem Structure

Phloem tissue consists of a no. of cells types.The cells primarily involved with transport are the 'seive tube elements'

These lie end-to- end to form a continuous stack called the 'sieve tube'

The end walls are perforted with sieve pores to form sieve plates.

Sieve tube elements have no nucleus and have only a reduced amount of cytoplasm (around the edge of the cell) to allow better movement of sugars.    

Thus each sieve tube element has one or more companion cell.

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Phloem Structure- companion cells

companion cells:

  • are linked to the sieve tube elements by plasmodesmata
  • have a dense cytoplasm rich in mitochondria and other organelles 
  • have a high metabolic rate
  • don't transport materials but support the sieve tube elements keeping them alive

(http://leavingbio.net/FLOWERING%20PLANTS_files/image048.jpg)

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Xylem Structure

There are many types of cells in xylem but the main cell involved in water and ion transport is the xylem vessels

  • they have no end walls and no cell contents
  • a column of vessels form a long continuous tube up the plant, ideal 4 water transport
  • the secondary walls are thickend by an impermeable substance called lignin.
  • xylem vessels are dead when fully formed

why are the xylem vessels lignified?

  • To prevent the xylem vessels collapsing under the negative pressure (tension) created as a result of transpiration. 
  • offers support for the whole plant
  • It is waterproof- prevents leakage
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Patterns of lignification in xylem vessels

Protoxylem (found in growing regions-root and shoot tips)

  • Annular or spiral thickening
  • to allow xylem vessels to elongate with other tissues in the growth region.

Metaxylem (mature xylem)

  • have a more complete covering of lignin-greater deposition
  • reticulate or pitted pattern
  • pits allow lateral movement of water between surrounding cells n xylem vessels.
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