Transport in plants

Transport

Transport in multicellular plants

Why is a transport system needed?

• Plant cells need a regular supply of nutrients and raw materials just like animal cells.

• They however need these at a lower rate and their needs also differ.

• Multicellular plants have a small surface area to volume ratio.

• Consequently, a transport system is needed to supply some materials to them.

Materials required by the plant

1. Oxygen – all the cells of a plant need oxygen for respiration. Actively photosynthesizing cells however produce more oxygen than they need. Other cells take in oxygen from the environment. Their rate of respiration is however much less than that of mammals and so do not need a rapid supply.

Materials required by the plant cont'd

2. Carbon dioxide – photosynthetic cells require carbon dioxide during daylight.

3. Organic nutrients – cells that do not make their own food require organic nutrients from photosynthetic cells.

4. Inorganic ions and water – all the cells of a plant require inorganic ions and water. These are transported from the soil by roots.

Other needs

• One of the greatest requirements of a plant is sunlight.

• The leaves are adapted to ensure that the cells receive maximum sunlight.

The nature of the transport system in plants

• Plants have a much slower transport system than mammals because their energy requirements are much less than that of mammals.

• Carbon dioxide and oxygen diffuse into and out of a leaf easily and readily.

• Plants have two transport systems; one for inorganic ions and water and another for organic materials.

The dicot roothttp://www.google.com.jm/imgres?q=structure+of+dicot+root&um=1&hl=en&sa=N&rlz=1T4SKPT_enJM414JM415&biw=1280&bih=586&tbm=isch&tbnid=1fdzJWG-pWljBM:&imgrefurl=http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPLANTANATII.html&docid=tXEinYUkvK2vZM&imgurl=http://www.emc.maricopa.edu/faculty/farabee/BIOBK/RanuncROOTXSstele.gif&w=391&h=228&ei=WGCpTrOzFMTKgQegxIDuCw&zoom=1&iact=hc&vpx=302&vpy=150&dur=303&hovh=171&hovw=294&tx=141&ty=72&sig=106135117064292514524&page=1&tbnh=95&tbnw=163&start=0&ndsp=18&ved=1t:429,r:1,s:0

Dicot root

http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/plantreproduction/PlantGrowthStructure/PlantGrowthStructure.htm

Dicot stemhttp://www.google.com.jm/imgres?q=structure+of+dicot+stem&um=1&hl=en&sa=N&rlz=1T4SKPT_enJM414JM415&biw=1280&bih=586&tbm=isch&tbnid=kQBubzw72NEahM:&imgrefurl=http://www.hsc.on.ca/moffatt/bio3a/plantphys/dicotstem.html&docid=8UqnyjMFj3Pr9M&imgurl=http://www.hsc.on.ca/moffatt/bio3a/plantphys/DICOSTEM.JPG&w=1024&h=768&ei=cmGpTvLHM9OcgQfr0IAn&zoom=1&iact=hc&vpx=173&vpy=140&dur=4185&hovh=194&hovw=259&tx=158&ty=110&sig=106135117064292514524&page=1&tbnh=120&tbnw=168&start=0&ndsp=19&ved=1t:429,r:0,s:0

Dicot stemhttp://www.google.com.jm/imgres?q=structure+of+dicot+root&um=1&hl=en&sa=N&rlz=1T4SKPT_enJM414JM415&biw=1280&bih=586&tbm=isch&tbnid=lGLFX__DWA738M:&imgrefurl=http://sci.waikato.ac.nz/farm/content/plantstructure.html&docid=-jcLYUTHhrUyJM&imgurl=http://sci.waikato.ac.nz/farm/images/dicot%252520stem_labelled_web.png&w=400&h=239&ei=2GGpTtqKIoeSgQf9itUX&zoom=1&iact=hc&vpx=653&vpy=149&dur=1949&hovh=173&hovw=291&tx=127&ty=105&sig=106135117064292514524&page=1&tbnh=97&tbnw=163&start=0&ndsp=18&ved=1t:429,r:3,s:0

http://www.google.com.jm/imgres?q=structure+of+monocot+root&um=1&hl=en&sa=N&rlz=1T4SKPT_enJM414JM415&biw=1280&bih=586&tbm=isch&tbnid=cm1JDfNLEgvbIM:&imgrefurl=http://lylscience.blogspot.com/2010/09/blog-post.html&docid=SEQBlBlem8HKTM&imgurl=http://3.bp.blogspot.com/_T52A1kHq95A/TIeKTZt4QiI/AAAAAAAAAF0/FIKODovXuZI/s1600/I10-22a-root2.jpg&w=454&h=380&ei=PWKpTvLKH8HbgQeXkon8Dw&zoom=1&iact=hc&vpx=674&vpy=134&dur=3538&hovh=205&hovw=245&tx=123&ty=86&sig=106135117064292514524&page=1&tbnh=119&tbnw=142&start=0&ndsp=18&ved=1t:429,r:3,s:0

http://www.google.com.jm/imgres?q=structure+of+a+monocot+stem&um=1&hl=en&sa=N&qscrl=1&nord=1&rlz=1T4SKPT_enJM414JM415&biw=1280&bih=586&tbm=isch&tbnid=1z-lYXcNmbmd6M:&imgrefurl=http://blog.lib.umn.edu/michaels/fall09courseguide/Lecture%25207%2520-%25202008%2520-%2520Roots%2520and%2520Stems.html&docid=PiN7m2j5cGhcKM&imgurl=http://blog.lib.umn.edu/michaels/fall09courseguide/monocot%252520stem.jpg&w=353&h=255&ei=uWKpTriYHcPdgQev0bAT&zoom=1&iact=hc&vpx=851&vpy=141&dur=1716&hovh=191&hovw=264&tx=120&ty=98&sig=106135117064292514524&page=1&tbnh=116&tbnw=160&start=0&ndsp=18&ved=1t:429,r:4,s:0

Transport of water and inorganic ions

• Water enters the plant through the root hairs and moves across the root to the xylem tissue in the centre.

• It then rises up to the leaves through the stem.

Structure of xylem tissue

• Xylem has dual function of support and transport. This is made possible because of their structure.

• In angiosperms, xylem tissue contains:1.Vessel elements2.Tracheids3.Fibres4.Parenchyma cells

Xylem vessels

• Xylem vessels are made up of several vessel elements arranged end to end.

• Vessel elements are elongated.• Vessel elements were originally normal plant

cells in whose walls lignin was laid down. • Lignin is impermeable to water, and as more is

laid down in the wall of a cell, the cell eventually dies.

Xylem vessels cont'd

• The empty space that is left is called a lumen. • Wherever plasmodesmata were in the original

cell wall, no lignin was laid there. • The non-lignified areas form pits which are

gaps in the cell wall. • The pits are crossed by permeable,

unthickened cellulose cell wall.

Xylem vessels cont'd

• The end walls separating neighbouring xylem elements break down and a continuous tube is formed.

• The long, non-living tube is a xylem vessel.

Tracheids

• These are also dead cells with lignified walls. • Their ends are not open and so do not form

vessels. • The ends taper off. • They have pits and so they can help in the

transport of water. • They are the main conducting tissue in

gymnosperms (primitive)

Fibres

• These cells are also elongated and dead. • Their function is to support the plant.

http://www.google.com.jm/imgres?q=xylem+and+phloem&hl=en&sa=X&qscrl=1&nord=1&rlz=1T4SKPT_enJM414JM415&tbm=isch&prmd=imvns&tbnid=XWJ7iVbpaDsZOM:&imgrefurl=http://www.britannica.com/EBchecked/media/379/Cells-of-the-xylem-and-phloem&docid=B6SKBfHZrpPT6M&w=420&h=360&ei=vMtvTsTnGYa6tgeuzZn3CQ&zoom=1&iact=hc&vpx=291&vpy=175&dur=3168&hovh=208&hovw=243&tx=86&ty=106&page=1&tbnh=153&tbnw=179&start=0&ndsp=11&ved=1t:429,r:1,s:0&biw=1280&bih=560

Parenchyma cells

• These are like regular plant cells.• They have unthickened cell walls and the

organelles that would be found in a plant cell. • However, because they are not exposed to

light, they have no chloroplasts. • They have a variety of shapes but are usually

isodiametric (same size in all directions).

Movement from soil to root hair

• Some of the epidermal cells just behind the root tip are drawn out to form long extensions called root hairs.

• These root hairs extend into spaces between soil particles.

• There they absorb water. • Many root hairs are formed and these provide a

large surface area in contact with soil water.

http://www.google.com.jm/imgres?q=root+hair+cell&hl=en&sa=X&qscrl=1&nord=1&rlz=1T4SKPT_enJM414JM415&tbm=isch&prmd=imvns&tbnid=n3cpOQZmhHk1UM:&imgrefurl=http://15ipk1.glogster.com/root-hair-cell/&docid=8BbYb_N-2JAClM&w=263&h=122&ei=jcxvTtzAE9CitgfIo42ECg&zoom=1&iact=hc&vpx=593&vpy=230&dur=12537&hovh=97&hovw=210&tx=117&ty=61&page=1&tbnh=95&tbnw=204&start=0&ndsp=10&ved=1t:429,r:2,s:0&biw=1280&bih=560

Movement from soil to root hair cont'd

• This therefore increases the rate at which water is absorbed.

• The root hairs are very delicate and have to be replaced very often.

• Water moves down a water potential gradient. • This gradient is created by the presence of

inorganic ions and organic materials like sugars and proteins in the cell sap of the root hairs.


• This concentration of inorganic ions inside the root hairs is greater than the concentration of inorganic ions in the soil water.

• Mineral ions also move into the root hair cells. • These ions can either move by facilitated

diffusion or active transport. • If the concentration of a particular ion is greater

outside the root hair cell than inside


the root hair cell, then movement is passive by facilitated diffusion.

• If the concentration is greater inside the cell than outside, then movement is by active transport.

• Usually, the ions needed by the plant are in low concentrations in the soil.

• Most movement is therefore by active transport.


• Root hairs are important for the absorption of minerals.

• In some plants, especially trees, there is a symbiotic relationship with fungi called mycorrhizas.

• These serve a similar function to root hairs by helping with the absorption of nutrients from the soil and transporting them.


• Both water and minerals move together even though different processes are employed to move them.

• Mineral salts cannot be absorbed if water is not present.

Movement from root hair to xylem

• Water moves across the cortex into the xylem because the water potential in the xylem vessels is lower than that of the cortical cells.

• The water takes two possible routes across the cortex:

1.Apoplast pathway2.Symplast pathway

Apoplast pathway

• The cell wall is made of fibres of cellulose which form a network.

• The water can soak into the fibres as they would soak into paper.

• Because of this , water can pass from cell wall to cell wall without entering the cytoplasm of cortical cells.

• Movement may be through the intercellular spaces.

Symplast pathway

• The water moves into the cytoplasm or vacuole of the cortical cells.

• It then passes into adjacent cells through the plasmodesmata.

http://www.google.com.jm/imgres?q=apoplastic+pathway+of+water&um=1&hl=en&sa=N&rlz=1T4SKPT_enJM414JM415&tbm=isch&tbnid=UjZWOFIe2GiPeM:&imgrefurl=http://en.wikibooks.org/wiki/A-level_Biology/Transport/multicellular_plants&docid=GeyYlqhPyy3jGM&w=466&h=249&ei=Is1vTt7MDtG9tgfyueCACg&zoom=1&iact=hc&vpx=150&vpy=280&dur=4463&hovh=164&hovw=307&tx=195&ty=104&page=1&tbnh=91&tbnw=170&start=0&ndsp=21&ved=1t:429,r:14,s:0&biw=1280&bih=560

Movement from root hair to xylem cont'd

• The cells in the outer layer of the stele, the endodermis, have suberin in their cell walls.

• This suberin forms the Casparian strip and makes the endodermis impenetrable to water.

• The suberin is laid down as the plant gets older.

• Some cells do not have suberin deposits. These are called passage cells.

Movement from root hair to xylem cont'd

• Water can then travel symplastically and then pass through passage cells.

• The water then continues to move through the pericycle and into the xylem.

• Symplastic movement of water gives a plant control over what ions pass into its xylem vessels since everything passes the plasma membrane.

Movement of water up the xylem vessel

• To fully explain how water moves up the xylem vessels, transpiration must first be considered.

Transpiration (movement from leaves to atmosphere)

• Mesophyll cells are not tightly packed and have air spaces among them.

• Water from the mesophyll cells evaporates into the air spaces making the air spaces saturated with water vapour.

• There is direct contact between the air inside the leaf and the air outside the leaf through the stomata.

Transpiration cont'd

• If there is a water potential gradient, then water will move out of the leaf.

• This loss of water vapour from the leaf is called transpiration.

• Transpiration rate will increase if a steep gradient is created.

Transpiration cont'd

• Several things can increase the rate of transpiration.

• These include:1.Low humidity2.Rise in temperature3.Increased wind speed4.Opening of the stomata

Movement of water up xylem vessels cont'd

• Movement of water up the xylem is dependent on the difference in pressure at the top and bottom of the vessel.

• As water leaves the leaf by transpiration, water constantly leaves the top of the xylem vessel down a gradient.

• The water either moves into mesophyll cells or along their cell walls.


• As water leaves the top of the xylem vessels, hydrostatic pressure is reduced.

• The hydrostatic pressure is greater at the bottom of the vessel than at the top.

• This causes water to move up the vessel. • The water in the xylem vessel is under great

tension and would cause the vessel to collapse if the lignin wasn’t present.


• Movement through the vessels is by massflow.• This means that all the water molecules move

together as a body of liquid. • This is due to cohesion and adhesion.• Cohesion is the attraction of water molecules

to each other. • Adhesion is the attraction of water molecules

to the lignin in the walls of the xylem vessels.


• If an air bubble becomes trapped (an air lock develops)in the column then the difference in pressure is not transmitted.

• This prevents the water moving upwards. • Development of air locks is prevented by the

small diameter of the vessel. • If however one develops, the pits allow the

water to move to another xylem vessel.


• The pressure difference between the top and bottom of the plant is maintained by increasing the pressure at the bottom of the vessels (root pressure).

• Root pressure is increased when ions are actively secreted into the root cells.

• This lowers the water potential and water moves in in great amounts.


• The contribution of root pressure is very small and not significant as water will continue to move up the xylem of a dead plant.

• Water movement in plants is passive and is fuelled largely by transpiration.

NB. As water is being moved, minerals salts move with it also

Movement of water from xylem to leaf cells

• Water diffuses from the top of the xylem vessels to the cells of the leaf.

• This is down a concentration gradient the creation of which is explained before.

Movement of organic materials

• Organic materials eg. Sugars are made by the plant.

• These materials are also called assimilates. • The method of transporting soluble organic

substances is called translocation. • Phloem tissue is responsible for translocation.

Phloem tissue

• Phloem tissue contains:1.Sieve elements2.Companion cells3.Parenchyma cells

Sieve elements

• A sieve element is a living cell• It contains a cell wall, a plasma membrane,

cytoplasm, endoplasmic reticulum and mitochondria.

• It doesn’t have a nucleus and ribosomes. • The layer of cytoplasm is very thin. • The end walls have a very special feature.

Sieve elements cont'd

• A sieve plate is formed where two sieve elements meet.

• The plate is made up of the end walls of two elements perforated by holes.

• Many elongated sieve elements, joined at their end walls forms a sieve tube.

• A sieve tube is a continuous column.

Sieve elements cont'd

• If the elements are viewed under the microscope, strands of protein can be seen passing through the pores.

• These strands however are produced in response to the damage caused when the tissue was cut during slide preparation;

Companion cells

• A companion cell is found lying close to every sieve tube element.

• Companion cells have the same structure as regular plant cells.

• They however have a large number of mitochondria and ribosomes.

• Many plasmodesmata make contact between the cytoplasm of the companion cell and the sieve element.

Contents of sieve tubes

• The liquid inside sieve tubes is called phloem sap or just sap.

Collecting sap for analysis

• This is a difficult process.• As soon as the tissue is cut, the elements

respond by immediately secreting protein. • Within hours it secretes a carbohydrate called

callose. • Castor oil plant is unusual and the sap

continues to flow even after the phloem is cut for some time.

Collecting sap for analysis cont'd

• In other plants, aphids are used to collect the sap.

• Aphids feed by sticking their stylet into the phloem.

• If the stylet is cut near to the insect’s head the sap continues to flow.

• The flow is slow and so the plant’s phloem ‘clotting’ mechanism is not employed.

Process of translocation

• For the food to be transported, it has to be loaded from its site of manufacture to the phloem.

• Triose sugars are converted into sucrose. • The sucrose in solution moves across the cells

of the leaves into the phloem tissue.• The sucrose is actively loaded into the phloem

element from the companion cell.

Process of translocation cont'd

• The sucrose moves in solution and can therefore move by the apoplast or symplast pathway.

• Both the companion cell and the sieve elements work together.

• Sucrose is loaded into the companion cell by active transport.

Loading of sucrose into phloem

• Hydrogen ions are actively moved out of the companion cell, using ATP as an energy source.

• The hydrogen ions then move back into the companion cell down the concentration gradient.

• As the hydrogen ions move back in, sucrose moves into the companion cell as well.

Loading of sucrose into phloem cont'd

• This is possible because the same protein that acts as a carrier for hydrogen ions is also a carrier for sucrose molecules (co-transporter molecule).

• Even though sucrose is moving against a concentration gradient, no ATP is required because of the presence and movement of hydrogen ions.

Loading of sucrose into phloem cont'd

• The sucrose molecules then move through the plasmodesmata into the sieve tube.

Movement in the phloem

• Movement in the phloem is by mass flow. • The pressure difference is actively created (in

the xylem, the pressure difference is passively created).

• When sucrose molecules are actively loaded into the phloem, the water potential in the phloem is reduced.

• Water thus moves into the phloem by osmosis.

Movement in the phloem cont'd

• When the sucrose is removed from the phloem by other cells like those of a fruit, water again follows by osmosis.

• The movement of the water out of the phloem creates a pressure difference.

• Hydrostatic pressure is low in the lower part of the leaf and high in the area of the leaf where the sieve tube is found.

Movement in the phloem cont'd

• The pressure difference causes water to flow from a high pressure area to a low pressure area taking solutes with it.

Unloading of sucrose from the phloem

• It is still unclear as to how sucrose is unloaded from the phloem.

• It is believed that it is unloaded by diffusion. • As soon as it enters the cells an enzyme

converts it into something else thus maintaining a concentration gradient. Eg. Invertase hydrolyses sucrose to glucose and fructose.

Translocation cont'd

• Wherever in the plant sucrose is loaded into the phloem, that area is called a source.

• Wherever in the plant sucrose is unloaded from the phloem, that area is called a sink.

Benefits of having a sieve plate

• They probably act as support preventing the phloem from collapsing. The support for the xylem comes from its lignified walls.

• They allow damage to be rapidly overcome because of the ‘clotting mechanism’.

• The ‘clotting’ prevents loss of important nutrients. It also prevents entry of pathogens.

Evidence for mass flow

• Initially there was much controversy about whether movement in the phloem was by mass flow.

• One reason for the controversy is the phloem protein blocking the pores that is observed when phloem is looked at.

• Now it is known that these proteins only form when the phloem is damaged.

Evidence for mass flow cont'd

1. The rate of movement in phloem is approximately 10000 times faster than it would be if movement was by diffusion.

2. The rate of transport measured match closely with the pressure differences measured at source and sink (providing the pores are unobstructed).


• There is also much evidence for the active loading of sucrose into sieve elements.

• The work done so far has been tedious and inconclusive.

• There is circumstantial evidence to support active loading however.


• These include:1.The pH of phloem sap is approximately 8. This

is what is expected if Hydrogen ions are actively transported out of the companion cell.

2.The difference in electrical potential across the plasma membrane is approximately -150mV.


• This is consistent with an excess of ions outside of the companion cell compared with inside.

3. ATP is in large amounts in phloem companion cells. This would be needed if active transport is taking place.

Similarities between movement of manufactured food and movement of water.

• Both move by mass flow along a pressure gradient.

• Both move through tubes formed by cells stacked end to end.

Differences between xylem and phloem

Xylem• Vessels are made of dead

cells• Vessels have lignified cell

walls• The end walls disappear

completely

• Have pits

Phloem• Elements are made from

living cells• Phloem tubes do not have

lignified cell walls• The end walls form sieve

plates. They do not disappear completely

• Do not have pits. They have plasmodesmata instead

Transport in plants

Technology

Transcript of Transport in plants