Transport in Plants

The Need for Transport Systems

Metabolic Demands

  • Some parts of the plant make their own glucose and oxygen through photosynthesis 
  • Internal and underground parts of the plant cannot undergo photosyntehsis and so need glucose and oxygen transported to them 
  • Hormones and mineral ions are examples of molecules which need transporting around plants 

Size

  • Plants vary in size and they require transport systems to move substance up and down from the tips of the shoots to the tips of the roots 

Surface Area to Volume Ratio

  • The leaves of plants are often adapted to have a large SA:V ratio. 
  • When the entire plant is considered it has a relatively small SA:V ratio. 
  • Plants cannot rely on diffusion alone.
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Transport System in Dicotyledonous Plants

Dicotyledonous Plants

  • They make seeds that contain two cotyledons. Cotyledons are organs that act as food stores for the developing embryo plant. They form the first leaves when the seed germinates. 
  • They have a series of transport vessels running through them. This is called the vascular system
  • The vascular system is made of xylem and phloem

Root

  • Vascular bundles uare in the middle of the plants, this helps withstand the tugging strains from the wind.

Image result for vascular bundles in root

Leaf

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Vascular bundles in Stem

Stem

  • Vascular bundles are around the edge to give strength and support 

Image result for vascular bundles in stem

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Vascular bundles in Root

Root

  • Vascular bundles uare in the middle of the plants, this helps withstand the tugging strains from the wind.

Image result for vascular bundles in root

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Vascular bundles in Leaf

Leaf

  • Midrib of a dicot leaf is the main vein, this carries the vascular tissue.
  • It helps to support the structure of the leaf
  • There are many branching veins; these help with both support and transport.

Image result for leaf cross section diagram

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Structure and Function of Xylem

Xylem

  • Largely composed of non-living tissue
  • Involved in the transport of water and mineral ions around the plant. 
  • Xylem vessels are long, hollow structures made by several columns of cells fusing together. 

Two other tissues associated with Xylem: 

  • Xylem Paranchyma = Thick walled, packed around Xylem vessels, stores food and contains tanin deposites. 
  • Xylem Fibres = Long cells, lignified secondary walls providing extra mechanical strength do not transport water.

Image result for xylem vessel diagram

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Structure and Function of Phloem

Phloem

  • Living tissue that transports food. (Ph - loem = F - ood)
  • Supplies cells with sugars and amino acids required for cellular respiration
  • Two ways- can go both up and down a plant
  • Main transporting vessels are sieve tube elements, hollow and long like the xylem but not lignifie. 
  • Areas between cells are perforated walls these form sieve plates, they let the content of the phoem flow through 
  • As pores appear in cells, some organelles break down. 
  • The phloem becomes a tube filled with phloem sap and the mature phloem cells have no nucleus. 
  • Companion cells form with the sieve tube elements, they are linked to each other by the plasmodesmata.

Image result for phloem vessel diagram

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Water Transport in Plants

Water Transport in Plants 

Water is important for the structure and metabolism in plants:

  • Turgor Pressure provides a hydrostatic skeleton to support the stems and leaves. It occurs due to osmosis 
  • Turgor also drives cell expansion, it enables plant roots to force their way through hard surfaces
  • Loss of water by evaporation helps plants to cool
  • Mineral ions and photosynthetic products are transported in aqueous solutions
  • Water is a necessary raw material for photosynthesis
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Movement of Water into Roots

  • Root hair cells are exchange surfaces where water is taken into the plant from the soil. 

They are very well adapted:

  • microscopic size- they can easily squeeze between soil particles
  • large SA:V ratio
  • thin surface later- short diffusion pathway for fast diffusion and osmosis
  • water potential gradient- concentration of solutes in the cytoplasm maintains a water potential gradient between the soil water and cell

Image result for root hair cell diagram

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Movement of Water across the root

Symplast Pathway

  • The symplast is the contiunous cytoplasm of the living plant cells that is connected through the plasmodesmata.
  • Water travels through the symplast by osmosis.
  • Root hair cells has a higher water potential than the next cells along, water has diffused in from the soil and has diluted the cytoplasm.
  • Water moves from the root hair cell into neighbouring cells via osmosis, the process continues from cell to cell across the root until it reaches the xylem. 
  • Water leaves the root hair cell by osmosis causing the water potential of the cytoplasm to fall again, this help to maintain a steep water potential gradient. 
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Movement of Water across the root

Apoplast

  • Movement of water through the apoplast.
  • The apoplast is the cell walls and intercellular spaces. 
  • Water fills the spaces between the loose open network of fibres in the cellulose cell wall.
  • When water molecules move into the xylem more molecules are pulled throguh the apoplast behind them. This is due to waters cohesive properties. 
  • The pull from water moving up the xylem and up the plant along with the with the cohesive forces forces between the water molecules creates a tension that means that the flow of water in continuous. 
  • This continuous flow of water water through the open structure of the cellulose wall has little or no resistance. 
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Movement of Water into the Xylem

  • Water moves across the root in the apoplast and symplast pathways until it reaches the endodermis.
  • The endodermis is the layer of cells surrounding the vascular tissue. 
  • It is particularly noticeable in the roots due to the effect of the Casparian *****. 
  • The casparian ***** is a waxy band of material called suberin, it runs around each of the endodermal cells forming a layer that is water proof. 
  • At this point, water in the apoplast pathway can go no further and it is forced into the cytoplasm of the cell, it will then join water in the symplast pathway. 
  • This diversion is important because water must pass throguh selectively permeable cell surface membranes, this excludes any potentially- toxic solutes in the soil water from reaching living tiisues. 
  • This is because the membranes would have no carrier proteins to admit them.
  • Solute concentration in the cytoplasm of the endodermal cells is relatively dilute to that in the xylem. 
  • Endodermal cells move mineral ions into the Xylem but active transport. This means that the water potential of the xylem is lower than that of the endodermal cells. 
  • Rate of water moving into the xylem by osmosis is increased. 
  • In the Vascular bundle water returns to the apoplast pathway
  • Active pumping of mineral into the Xylem by osmosis causes root pressure. 
  • Root pressure gives water a push up the xylem. 
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Process of Transpiration

  • CO2 moves from the air into the leaf and oxygen moves out of the leaf by diffusion down concentration gradients through Stomata. 
  • The Stomata are opened and close by Guard cells. 
  • When the Stomata open to allow gases between the air inside and outside of the leaf, water vapour also moves out by diffusion
  • The loss of water from the leaves an stems of plants is called transpiration. 
  • Stomata open and close to control the amount of water lost by a plant. some stomata have to be open all of the time.
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Transpiration Stream

  • Water vapour moves into the air through the stomata along a diffusion gradient. This is the transpiration stream.
  • Transpiration stream moves water up from the roots of a plant to the highest leaves. 

1) Water molecules evaporate from the surface of mesophyll cells into the air spacesz in the leafiand move out of the stomate into the surrounding air, it diffuses down a concentration gradient.

2) Water potential of the cell is lowered by the loss of water through evaporation

3) Water moves into the cell from an adjacent cell by osmosis along the apoplast and symplast pathways. 

4) It repeats across the leaf to the xylem and water moves out of the xylem by osmsosis into the leaf cells. 

5) Water can form hydrogen bonds with the carbohydrates in the walls of the xylem vessels, this isiadhesion. They can form them with each other also, this is cohesion. 

6)The adhesion and cohesion form capillary action- the process of water moving up the xylem.

7) Water is drawn up the xylem by the transpiration pull, this is to replace the water lost byievaporation.

8) Transpiration pull causes tension in the xylem, this helps move water across the roots fromithe soil

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Cohesion- Tension Theory

Evidence:

  • When tension in the xylem is high it can cause a plant to shrink and the diameter to change. It does this based on the rate of transpiration.
  • Breaking the xylem vessel causes air to be drawn in to the xylem vessel rather than water leaking out. After this the plant can no longer mocve water up the stem this is because the continuous stream of molecule has been broken. 
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Factors affecting Transpiration

Light:

  • Increasing light intensity increases the rate of diffusion of water vapour. Evaporation is increased. So transpiration rate is increased.

Humidity:

  • High humidity lowers transpiration rate as the water vapour potential gradient is decreased. 

Temperature:

  • Increases kinetic energy of water and so increases the rate of evaporation to air spaces in the leaf
  • Increases the concentration of water vapour that the external air can hold before its saturated.

Air movement:

  • Each leaf has a trapped layer of still air around it. Water vapour that diffuses out accumulates here, so the potential gradient is increased. This reduces the diffusion gradient. Air movement increases diffusion gradient and rate of transpiration. 

Soil-water availability: 

  • If the soil is very dry then the plant will be under water stress and the rate of transpiration is reduced.

 

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Sources to Sinks

Translocation is the process in which plants transport organic compounds in the phloem from sources to sinks.

Sources:

  • Green leaves and stems
  • Storage organs that are unloading their stores at the beginning of a growth period. 
  • Food stores in seeds when they germinate. 

Sinks:

  • Roots that are growing or actively abosrbing mineral ions
  • Actively dividing meristems. 
  • Parts of the plant which are in the process of laying down food stores.
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Process of Translocation- Phloem loading

  • Phloem loading can occur in two ways. The symplast route (laregly passive) or the apoplast route (active). 

Symplast route:

  • Sucrose moves through the cytoplasm of mesophyll cells and into sieve tubes via diffusion through the plasmodesmata. Sucrose then ends up in the sieve elements and water follows by osmosis. This creates a pressure of water that moves sucrose through the phloem by mass flow. 

Apoplast Route

  • In companion cells sucrose is moved into the cytoplasm, across the membrane, actively. H+ ions are actively pumped out of the companion cells and into the surrounding tissue via ATP.
  • They return to the companion cell down the concentration gradient via a co-transport protein.
  • Sucrose is co-transported. The sucrose concentration increases. 
  • Companion cells have folded membranes to increase Surface Area and many mitochondria. 
  • Water moves in by osmosis due to the build up of sucrose, this leads to turgor pressure.
  • The water carrying assimilated moves into tubes of sieve elements reducing the pressure in the companion cells and moves up or down by mass flow. 
  • Solute accumulation in source phloem leads to an incrgions ease in pressure that forces sap to regions of lower pressure in the sinks 
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Process of Translocation- Phloem unloading

  • Sucrose is unloaded at any point, to the cells that need it. 
  • This is mainly done through diffusion of sucrose from the phloem into surrounding cells. 
  • Sucrose moves into other cells by diffusion or it is converted into another substance. 
  • This means that the concentration gradient of sucrose is maintained between the contents of the phloem and the surrounding cells. 
  • The loss of solutes from the phloem increases the water potential in it therefore water moves out into the surrounding cells by osmosis. 
  • Some of the water that has carried the solute to the sink is drawn into the transpiration stream in the xylem. 
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