Plant Transport

 

The main elements in this section are:
  • The substances that are transported in the phloem and the xylem
  • Where the phloem and xylem tissues are located in various parts of plants
  • The mechanisms by which substances are moved through phloem and xylem
  • Adaptations of plants to hot and dry conditions.

 

Comparison of xylem and phloem transport


tissues transport is through

Materials moved

Driving force

Xylem

mainly water and mineral salts

transpiration

Phloem

mainly organic food e.g. sucrose and hormones

 hydrostatic pressures

 

Substance

Xylem Sap

Phloem Sap

Sucrose

0

154,000

Amino Acids

700

13,000

Nitrate

10

0



Position of xylem and phloem tissue in the root

The central area of the root is called the stele, both the xylem and the phloem tissues are located here. Sometimes we refer to the clusters of xylem and phloem cells as the vascular bundles. The majority of the root is the cortex - consisting of parenchyma cells which tend to be larger than most other plant cells and can be involved in storage. The outer layer of root cells make up the epidermis - this is where root hair cells are found though none are visible in this micrograph (so I included the diagram on the right to remind you of what they look like). Root hair cells massively increase the surface area of the root. They are responsible for the uptake of water (by osmosis) and mineral ions (mainly by active transport) from soil into the root.

This micrograph shows the central portion of the root. The circular band of cells around the stele is the endodermal layer (endodermis). This is where cells with suberin in their cells walls form the Casparian strip. The Casparian strip is a waterproof layer which blocks the passage of substances traveling through the cell wall. The xylem vessels are found in a star shaped formation in the centre of the stele, with phloem vessels found in between the points of the stars.

 

Position of xylem and phloem tissue in the stem

  • A = Flower
  • B = Stem
  • C = Leaf
  • D = Roots
  • E = Phloem
  • F = Xylem

The diagrams of the stems clearly show the xylem and phloem are arranged in vascular bundles. These vascular bundles are located around the edge of the stem. The centre of the stem (or trunk) is called the pith. In the vascular bundles the phloem tissue is closer to the outside of the stem and this is why in ringing experiments where the bark is peeled off the movement of water is unaffected while the movement of sugars is stopped.

 

 

Structure of Xylem and Phloem tissues

 

Xylem Vessels: Responsible for conduction of water & mineral salts (and support of the plant)

  • Single cells, which are elongated and lignified

  • Dead with empty lumens

  • Long, tubular structures

  • Formed by fusion of several cells end to end in a row

  • Lignin deposited as rings or spirals in the cell wall providing ridgidity

 

Phloem Sieve tube elements + Companion cells: Responsible for translocation of solutions of organic solutes

Sieve Tube Elements:  
  • Living cells which contain obstructions to flow of solution (i.e. sieve plates and, to a lesser extent, the cytoplasm)

  • No nucleus

  • Cell walls at ends develop into sieve plates

Companion Cells:
  • Closely associated with each sieve element

  • Dense cytoplasm with all the usual cell organelles

  • Metabolically very active deduced from large numbers of mitochondria present

  • Very close link with sieve tube elements. If companion cell dies, so do some elements. In leaves they function as transfer cells, absorbing sugars and transferring them to sieve tube elements

 

 

 

Transport of water and mineral ions across the root

Water uptake into the plant is by osmosis into root hair cells - the presence of these cells massively increases the surface area of the root for absorption to occur over. Mineral elements exist in the form of ions. Uptake is greatest in region of the root hairs. Uptake occurs mainly by active transport into the root hair cells. 
 

Movement of water across the root

diagram from ultranet.com/~jkimball/BiologyPages

This diagram shows the movement of water across the root. There is a water potential gradient across the root which means the water is actually transported by osmosis. The two main pathways are the apoplast and the symplast. Basically the apoplast is through the cell walls - these are freely permeable to all small molecules and the plant has no control over the movement of substances through them (they can also be seen as non-living). The symplast can also have the subdivision of the vacuolar pathway. It is important to note that the Casparian strip effectively blocks the apoplast pathway at the endodermis therefore all water enters the stele through the symplast - this prevents leakage of water from xylem vessels and aids the development of root pressure (an upwards force pushing water up the stem).

 

 

Transport of water and mineral ions from the root to the leaves

3 pathways:
  • Root pressure (-minor)
  • Capillarity (-minor)
  • Cohesion tension (major)
Root pressure is caused by the mineral ions which are actively transported into xylem vessels in the root by endodermal cells. This makes the water potential of the xylem more negative and causes water to enter the xylem by osmosis. Capillarity is responsible for some water creeping up the xylem vessels simply because they are very narrow.

Cohesion tension. Water evaporating from the leaves drives the movement of water from the roots. The evaporation creates a WP gradient and the properties of water and the xylem vessels allow this pressure to be transmitted all the way to the roots even in the tallest trees. This is known as the cohesion tension theory of water transport. Conditions always allow evaporation of water from the leaves. When water evaporates from a leaf mesophyll cell, this cell’s WP will become more negative. Water from an adjacent cell will then move into this cell by osmosis as a result of the WP difference between them. This ‘chain’ of WP differences continues back to the xylem sap. Water moves smoothly and continuously along this WP gradient. The continual movement of water from the roots to the leaves is often called the Transpiration Stream. This can be thought of as the movement of water from a less negative to more negative water potential. Water moves from less negative in soil to the more negative air surrounding leaves. . This provides a very steep gradient from soil to air and is one of the driving forces of the transpiration stream.


Cohesion-Tension explained

  • Xylem vessels are full of water.

  • Tension is set up in water column as water leaves xylem in the leaves.

  • Tension transmitted back to root due to cohesion of water molecules. Water has high cohesion because it is a polar molecule and H-bonding occurs between water molecules.

  • The column of water under tension does not break because of adhesion = water molecules tend to ‘stick’ to xylem walls - this supports the column.

 

Transport of Organic Substances by Phloem

Photosynthetic products must obviously be transported to non-photosynthetic tissues of the plant.


Evidence that movement is through Phloem

  • Metabolic inhibitors - when introduced to phloem, translocation is halted therefore suggesting active processes involved.

  • Radioactive Labeling - Plants supplied with 14CO2 and a light source. Radioactivity later detected in phloem.

  • Aphids - these feed on translocating sugars by penetrating sieve tubes with modified mouthparts (stylets). Aphid is anaesthatised and body removed leaving stylets embedded in phloem. Sieve tube contents continue to exude from stylet, can be collected and analysed.

 

The role of aphids in phloem research

photographs from ultranet.com/~jkimball/BiologyPages

The reason aphids are used is because normally when a sieve tube is punctured with a measuring probe, the holes in its end walls quickly plug up. However, aphids can insert their mouth parts without triggering this response. The above left diagram shows when it punctures a sieve tube, sap enters the insect's mouth parts under pressure. above right diagram shows phloem sap will continue to exude from the mouthparts after the aphid has been cut away from them.

 

Munch’s Mass Flow (Pressure) Hypothesis

This is the major hypothesis used to explain movement in phloem though it has its limitations.

 

diagram from ultranet.com/~jkimball/BiologyPages

This diagram shows the pressure flow theory of phloem transport - normally we would use hydrostatic pressure as the alternate to turgor pressure (the AQA spec B examiners like the phrase hydrostatic pressure!)
The major steps in the mass flow theory are as follows:  
  • 1.Active transport/active movement of sugar (sucrose) at the source into phloem cells
  • 2.causes the water potential of phloem contents to become more negative. 
  • 3.Therefore water follows by osmosis from adjacent cells.
  • 4.This means the hydrostatic pressure in phloem increases
  • 5.which causes mass flow. 
  • 6.At another part of the plant (a sink e.g. the roots) the sugars are removed from the phloem by active transport so the gradients are maintained 
(if you can learn this paragraph it could score you 4-5 marks!) 

 

 

Adaptations of Plants to dry conditions (Xerophytic plants)

These adaptations all reduce water loss from the plant

  • Stomata sunken in pits creates local humidity/decreases exposure to air currents;

  • Presence of hairs creates local humidity next to leaf/decreases exposure to air currents by reducing flow around stomata;

  • Stomata mainly located on underside of leaf so less exposed to air currents/heat from sun;

  • Stomata close midrib so more sheltered from air currents;

  • Stomata close together so diffusion shells overlap;

  • Thick waxy cuticle makes more waterproof impermeable to water;

  • Double palisade layer increases diffusion distance;

  • Stomata on inside of rolled leaf creates local humidity/decreases exposure to air currents because water vapour evaporates into air space rather than atmosphere e.g. British Marram grass

  • Less stomata decreases transpiration as this is where water is lost;

These two diagrams show Marram grass leaves - this is a British grass found on sand dunes which possesses: rolled leaves, leaf hairs and sunken stomata. These adaptations make it resistant to dry conditions and of course sand-dunes which drain very quickly retain very little water.