04 Transport System in Multicellular Plants

Transport system in Transport system in multicellular plantsmulticellular plants

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Need for transport systemsNeed for transport systems Plants have thin, flat leaves which present large Plants have thin, flat leaves which present large

surface area to the sunsurface area to the sun Relatively easy for CORelatively easy for CO22 and O and O22 to diffuse into and to diffuse into and

out of the leaves, reaching and leaving every cell out of the leaves, reaching and leaving every cell quickly enough so that there is no need for a quickly enough so that there is no need for a transport system for these gasestransport system for these gases

Plants have Plants have twotwo transport systems: transport systems: For carrying mainly water and inorganic ions from roots For carrying mainly water and inorganic ions from roots

to the parts above the groundto the parts above the ground For carrying substances made by photosynthesis from For carrying substances made by photosynthesis from

the leaves to the other areasthe leaves to the other areas However, fluids don’t move as rapidly as blood However, fluids don’t move as rapidly as blood

does in a mammal, nor is there an obvious pump does in a mammal, nor is there an obvious pump such as the heartsuch as the heart

Neither plant transport system carries ONeither plant transport system carries O22 and CO and CO22

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Distribution of xylem and phloem Distribution of xylem and phloem tissue in roots of dicotyledonous tissue in roots of dicotyledonous plantsplants

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Distribution of xylem and phloem Distribution of xylem and phloem tissue in stems of dicotyledonous tissue in stems of dicotyledonous plantsplants

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Distribution of xylem and phloem Distribution of xylem and phloem tissue in stems of dicotyledonous tissue in stems of dicotyledonous plantsplantsSclerenchyma:Sclerenchyma:Plant tissue whose function is to strengthen and support, Plant tissue whose function is to strengthen and support, composed of thick-walled cells that are heavily lignified composed of thick-walled cells that are heavily lignified (toughened). (toughened). Parenchyma:Parenchyma:Plant tissue composed of loosely packed, more or less Plant tissue composed of loosely packed, more or less spherical cells, with thin cellulose walls. Although spherical cells, with thin cellulose walls. Although parenchyma often has no specialized function, it is usually parenchyma often has no specialized function, it is usually present in large amounts, forming a packing or ground present in large amounts, forming a packing or ground tissue.tissue.Collenchyma:Collenchyma:Plant tissue composed of relatively elongated cells with Plant tissue composed of relatively elongated cells with thickened cell walls, in particular at the corners where thickened cell walls, in particular at the corners where adjacent cells meet. adjacent cells meet. It is a supporting and strengthening tissue found in non-It is a supporting and strengthening tissue found in non-woody plants, mainly in the stems and leaves.woody plants, mainly in the stems and leaves.

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Distribution of xylem and phloem Distribution of xylem and phloem tissue in leaves of dicotyledonous tissue in leaves of dicotyledonous plantsplants

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Distribution of xylem and phloem Distribution of xylem and phloem tissue in leaves of dicotyledonous tissue in leaves of dicotyledonous plantsplants

Transpiration Transpiration

The loss of water vapour by diffusion The loss of water vapour by diffusion down a water potential gradient from a down a water potential gradient from a plant to its environmentplant to its environment

Mostly takes place through the stomata Mostly takes place through the stomata on the leaveson the leaves

Transport of water Transport of water Water from soil – root hairWater from soil – root hair Root – xylem tissueRoot – xylem tissue Xylem tissue – stemXylem tissue – stem Stem - leavesStem - leaves

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From soil to root hairFrom soil to root hair Water moves into the root hairs down a Water moves into the root hairs down a

water potential gradient through a partially water potential gradient through a partially permeable membrane into the cytoplasm permeable membrane into the cytoplasm and vacuole of the root hair celland vacuole of the root hair cell

Very fine root hairs provides a large Very fine root hairs provides a large surface area in contact with soil water, surface area in contact with soil water, increasing the rate of water absorbedincreasing the rate of water absorbed

MicorrhizasMicorrhizas associations formed by fungi located in or on associations formed by fungi located in or on

roots which serve a similar function to root hairsroots which serve a similar function to root hairs act like a mass of fine roots which absorb act like a mass of fine roots which absorb

nutrients, especially phosphatenutrients, especially phosphate Some plants growing on poor soils are unable to Some plants growing on poor soils are unable to

survive without these fungisurvive without these fungi The fungi receive organic nutrients from the The fungi receive organic nutrients from the

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From root hair to xylemFrom root hair to xylem Water moves down the water potential gradient across the Water moves down the water potential gradient across the

root (water potential inside xylem vessels < water potential root (water potential inside xylem vessels < water potential in root hairs)in root hairs)

Two possible routes through the cortex:Two possible routes through the cortex: Apoplast pathwayApoplast pathway – water seeps across the root from cell wall – water seeps across the root from cell wall

to cell wall without entering cytoplasm of cortical cellsto cell wall without entering cytoplasm of cortical cells Symplast pathwaySymplast pathway – water moves into the cytoplasm or – water moves into the cytoplasm or

vacuole of cortical cell and into adjacent cells through vacuole of cortical cell and into adjacent cells through plasmodesmataplasmodesmata

Apoplast pathway barred at stele (Apoplast pathway barred at stele (endodermisendodermis have a have a thick, waterproof, waxy band of thick, waterproof, waxy band of suberinsuberin in cell walls – in cell walls – Casparian stripCasparian strip) due to impenetrable barrier to water) due to impenetrable barrier to water

Only way to cross the endodermis is through the cytoplasm Only way to cross the endodermis is through the cytoplasm of the cellsof the cells

Suberin deposits become more extensive as endodermal Suberin deposits become more extensive as endodermal cells get older except in cells get older except in passage cellspassage cells (gives plant control (gives plant control over inorganic ions and may help with generation of root over inorganic ions and may help with generation of root pressure)pressure)

Once across the endodermis, water continues to move down Once across the endodermis, water continues to move down water potential gradient across the pericycle and towards water potential gradient across the pericycle and towards the xylem vesselsthe xylem vessels

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From xylem to leafFrom xylem to leaf Water constantly moves out of xylem Water constantly moves out of xylem

vessels down a water potential gradient vessels down a water potential gradient either into the mesophyll cells or along either into the mesophyll cells or along their cell walls as water evaporates from their cell walls as water evaporates from the cell walls the cell walls

The removal of water from top of xylem The removal of water from top of xylem vessels reduces the hydrostatic pressure vessels reduces the hydrostatic pressure (pressure exerted by a liquid)(pressure exerted by a liquid)

Pressure difference causes water to move Pressure difference causes water to move up the xylem vessels (water up a straw)up the xylem vessels (water up a straw)

Xylem vessels have strong, lignified walls Xylem vessels have strong, lignified walls to stop from collapsing due to tensionto stop from collapsing due to tension

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Movement of water up through xylem Movement of water up through xylem vessels is by vessels is by mass flow mass flow (all water (all water molecules move together as a body of molecules move together as a body of liquid)liquid)

Helped by water molecules attracted to Helped by water molecules attracted to each other (each other (cohesioncohesion) and to the lignin in ) and to the lignin in the walls of xylem vessels (the walls of xylem vessels (adhesionadhesion))

If an air bubble forms in the continuous If an air bubble forms in the continuous column of water, column breaks and column of water, column breaks and difference in pressure cannot be difference in pressure cannot be transmitted through the vessel (air lock)transmitted through the vessel (air lock)

Adaptive features:Adaptive features: Small diameter of xylem vessels – prevents Small diameter of xylem vessels – prevents

breaksbreaks Pits in vessel walls Pits in vessel walls

Allow water to move out (bypass air lock)Allow water to move out (bypass air lock) Air bubbles cannot pass through pitsAir bubbles cannot pass through pits Allows water to move out of xylem vessels to Allows water to move out of xylem vessels to

surrounding living cellssurrounding living cellsALBIO9700/2006JK

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Root pressureRoot pressure Plants may also increase pressure Plants may also increase pressure

difference by difference by raisingraising the water pressure at the water pressure at the the basebase of the vessels of the vessels

By active secretion of solutes (active By active secretion of solutes (active transport) into water in xylem vessels in transport) into water in xylem vessels in rootroot

Solutes lowers water potential, draws in Solutes lowers water potential, draws in water and increases water pressurewater and increases water pressure

Water transport is a passive process Water transport is a passive process fuelled by transpiration from the fuelled by transpiration from the leavesleaves

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From leaf to atmosphere - From leaf to atmosphere - transpirationtranspiration

Air inside leaf (spaces around Air inside leaf (spaces around mesophyllmesophyll) ) is usually saturated with water vapour is usually saturated with water vapour from water around mesophyll cell wallsfrom water around mesophyll cell walls

If there is a water potential gradient If there is a water potential gradient between the air inside leaf and outside, between the air inside leaf and outside, water vapour will diffuse out through small water vapour will diffuse out through small pores (pores (stomatastomata) – ) – transpirationtranspiration

Increase in the water potential gradient Increase in the water potential gradient between the air spaces in the leaf and the between the air spaces in the leaf and the air outside will increase rate of air outside will increase rate of transpirationtranspiration

Transpiration cools leavesTranspiration cools leaves

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Factors that affect transpiration Factors that affect transpiration raterate

Temperature, wind speed, light intensity or Temperature, wind speed, light intensity or humidityhumidity

Rate of water vapour leaving leaves vs. Rate of Rate of water vapour leaving leaves vs. Rate of water taken up by stemwater taken up by stem

High proportion of water taken up is lost by High proportion of water taken up is lost by transpirationtranspiration

Rate at which transpiration is happening directly Rate at which transpiration is happening directly affects the rate of water uptakeaffects the rate of water uptake

Potometer Potometer Completely water-tight and airtight (no leakage of water Completely water-tight and airtight (no leakage of water

and no air bubbles break the continuous water column)and no air bubbles break the continuous water column) Submerge in waterSubmerge in water Cut the end of the stem with a slanting cutCut the end of the stem with a slanting cut

Position of the meniscus at set time intervals is Position of the meniscus at set time intervals is recorded. Plot a graph of distance moved against recorded. Plot a graph of distance moved against time. Compare rates of water uptake under time. Compare rates of water uptake under different conditions. different conditions.

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Xylem vessel elementXylem vessel element Tissue functions in support and transportTissue functions in support and transport Angiosperms (flowering plants except Angiosperms (flowering plants except

conifers) contain several different types of conifers) contain several different types of cell:cell: Vessel elementsVessel elements and and tracheidstracheids are involved are involved

in transport of water in transport of water FibresFibres are elongated cells with lignified walls are elongated cells with lignified walls

that help support the plant (dead cells)that help support the plant (dead cells) Parenchyma cellsParenchyma cells are ‘standard’ plant cells are ‘standard’ plant cells

except they don’t usually have chloroplasts except they don’t usually have chloroplasts (shapes vary but often isodiametric)(shapes vary but often isodiametric)

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Xylem vesselsXylem vessels Vessels are made up of many elongated Vessels are made up of many elongated

vessel elementsvessel elements arranged end to end arranged end to end LigninLignin is laid down in the cell wall is laid down in the cell wall As it builds up, cell dies and an empty As it builds up, cell dies and an empty

space (space (lumenlumen) is left) is left Lignin not laid down at plasmodesmata Lignin not laid down at plasmodesmata

areas leaving ‘gaps’ called areas leaving ‘gaps’ called pitspits (not open (not open pores; crossed by permeable, pores; crossed by permeable, unthickened cellulose cell wall)unthickened cellulose cell wall)

The end walls of neighbouring vessel The end walls of neighbouring vessel elements break down completely to form elements break down completely to form a continuous non-living tube (a continuous non-living tube (xylem xylem vesselvessel))

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TracheidsTracheids Dead cells with lignified walls but Dead cells with lignified walls but

without open ends (don’t form vessels)without open ends (don’t form vessels) Elongated cells with tapering endsElongated cells with tapering ends They have pits in the walls so water They have pits in the walls so water

can pass from one tracheid to the nextcan pass from one tracheid to the next Main conducting tissue only in Main conducting tissue only in

‘primitive’ plants i.e. ferns and conifers‘primitive’ plants i.e. ferns and conifers In the root, water which has crossed In the root, water which has crossed

the cortex, endodermis and pericycle the cortex, endodermis and pericycle moves into the xylem vessels through moves into the xylem vessels through the pits in their walls and then moves the pits in their walls and then moves up the vessels towards the leavesup the vessels towards the leaves

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Xerophytes Xerophytes Plants that live in places where water is in short Plants that live in places where water is in short

supply requiring adaptations to reduce the rate of supply requiring adaptations to reduce the rate of transpirationtranspiration The The marram grassmarram grass leaves can roll up, exposing a tough, leaves can roll up, exposing a tough,

waterproof cuticle to the air outside the leaf, while the waterproof cuticle to the air outside the leaf, while the stomata open into the enclosed, humid space in the stomata open into the enclosed, humid space in the middle of the ‘roll’. Hairs trap a layer of moist air close to middle of the ‘roll’. Hairs trap a layer of moist air close to the leaf surface (reducing diffusion gradient)the leaf surface (reducing diffusion gradient)

OpuntiaOpuntia is a cactus stems that store water. Leaves are is a cactus stems that store water. Leaves are reduced to spines, which reduce surface area for reduced to spines, which reduce surface area for transpirationtranspiration

Sitka spruceSitka spruce have leaves in the form of needles have leaves in the form of needles (reducing surface area available for water loss). Covered (reducing surface area available for water loss). Covered in a layer of waterproof wax and have sunken stomatain a layer of waterproof wax and have sunken stomata

Phlomis italicaPhlomis italica have ‘trichomes’ that act as a physical have ‘trichomes’ that act as a physical barrier to the loss of water barrier to the loss of water

The The cardoncardon has swollen, succulent stems that store has swollen, succulent stems that store water and photosynthesise. The stems are coated with water and photosynthesise. The stems are coated with wax and leaves are extremely smallwax and leaves are extremely small

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Plant adaptations to arid Plant adaptations to arid conditionsconditions

AdaptationAdaptation ExamplesExamples

Needle-like leaves, bipinnate Needle-like leaves, bipinnate leaves, phyllodesleaves, phyllodes

Mulga (acacias), she-oaks Mulga (acacias), she-oaks (casuarinas), hakeas, Xanthorrhoea (casuarinas), hakeas, Xanthorrhoea (grass trees)(grass trees)

Photosynthetic stemsPhotosynthetic stems She-oaks (She-oaks (CasuarinaCasuarina))

Woody fruitsWoody fruits Banksias, hakeasBanksias, hakeas

Waxy leavesWaxy leaves Saltbush (Saltbush (AtriplexAtriplex))

Ephemeral growthEphemeral growth Ephemeral plants e.g. paper daisies Ephemeral plants e.g. paper daisies ((HelipterumHelipterum), yellow tops (), yellow tops (SenecioSenecio) )

Partially deciduousPartially deciduous EucalyptsEucalypts

Leaf curlingLeaf curling Hummock grass (Triodia)Hummock grass (Triodia)

Sunken stomataSunken stomata Hakeas Hakeas

Water storageWater storage Baobab tree, parakeelyasBaobab tree, parakeelyas

Hanging leavesHanging leaves Eucalypts (sclerophylls)Eucalypts (sclerophylls)

Hairy or shiny leavesHairy or shiny leaves Banksias, paper flowers (Thomasia)Banksias, paper flowers (Thomasia)

Water-directing leaves and Water-directing leaves and stemstem

Mulga (acacia), Xanthorrhoea (grass Mulga (acacia), Xanthorrhoea (grass trees)trees) ALBIO9700/2006JK

Mulga (acacias)Mulga (acacias)

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CasuarinasCasuarinas

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HakeasHakeas

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BanksiasBanksias

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Banksia fruitsBanksia fruits

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Grass treesGrass treesSaltbushSaltbush

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Yellow topsYellow tops

Paper daisiesPaper daisies

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EucalyptsEucalypts

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Hummock grassHummock grass

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Baobab treeBaobab tree

ParakeelyasParakeelyas

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Translocation Translocation Transport of soluble organic substances Transport of soluble organic substances

within a plant, for example sugars within a plant, for example sugars ((assimilatesassimilates))

Transported in Transported in sieve elements sieve elements which are which are found in found in phloem tissuephloem tissue along with along with several other types of cells including several other types of cells including companion cellscompanion cells (parenchyma and fibres) (parenchyma and fibres)

Phloem sap moves by Phloem sap moves by mass flowmass flow To create the pressure differences needed To create the pressure differences needed

for mass flow in phloem, the plant has to for mass flow in phloem, the plant has to use energy (use energy (activeactive process) process)

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The pressure difference is produced The pressure difference is produced by by active loadingactive loading of sucrose into of sucrose into the sieve elements (in the the sieve elements (in the photosynthesising leaf)photosynthesising leaf)

This decreases the water potential in This decreases the water potential in the sap inside sieve elementthe sap inside sieve element

Water follows sucrose into the sieve Water follows sucrose into the sieve element (osmosis – down water element (osmosis – down water potential gradient)potential gradient)

At other points, sucrose is removed At other points, sucrose is removed and water follows by osmosisand water follows by osmosis

In the leaf water moves into sieve In the leaf water moves into sieve tube, in the root water moves outtube, in the root water moves out

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Pressure difference created causing Pressure difference created causing water flow from the high pressure water flow from the high pressure area to the low pressure area, taking area to the low pressure area, taking with it any soluteswith it any solutes

SourceSource: any area of a plant in which : any area of a plant in which sucrose is loaded into the phloemsucrose is loaded into the phloem

SinkSink: any area where sucrose is taken : any area where sucrose is taken out of the phloemout of the phloem

Sap flows both upwards and Sap flows both upwards and downwards in phloem (xylem always downwards in phloem (xylem always upwards)upwards)

Can only flow one way in any Can only flow one way in any particular sieve tube at any one time particular sieve tube at any one time

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Loading of sucrose into phloemLoading of sucrose into phloem

Sucrose in solution moves from mesophyll cell to Sucrose in solution moves from mesophyll cell to the phloem tissuethe phloem tissue

May move by the symplast pathway (moving from May move by the symplast pathway (moving from cell to cell via plasmodesmata) or apoplast pathway cell to cell via plasmodesmata) or apoplast pathway (travelling along cell walls)(travelling along cell walls)

Companion cells and sieve elements work in Companion cells and sieve elements work in tandemtandem Sucrose is loaded into a companion cell by active transportSucrose is loaded into a companion cell by active transport HH++ are moved out of the companion cells using ATP are moved out of the companion cells using ATP Large excess of HLarge excess of H++ outside outside Can move back into cell down concentration gradient Can move back into cell down concentration gradient

through protein which acts as carrier for both Hthrough protein which acts as carrier for both H++ and and sucrosesucrose

Sucrose molecules are carried through this Sucrose molecules are carried through this co-co-transportertransporter molecule into companion cell against molecule into companion cell against concentration gradient for sucroseconcentration gradient for sucrose

Sucrose molecules can then move from the companion cell Sucrose molecules can then move from the companion cell into the sieve tube (through plasmodesmata)into the sieve tube (through plasmodesmata)

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Unloading of sucrose from phloemUnloading of sucrose from phloem

Occurs into any tissue which requires Occurs into any tissue which requires sucrose but mechanism still unknownsucrose but mechanism still unknown

Probably by diffusionProbably by diffusion In tissue, enzymes convert sucrose In tissue, enzymes convert sucrose

into something else (e.g. invertase into something else (e.g. invertase hydrolyses sucrose to glucose and hydrolyses sucrose to glucose and fructose)fructose)

This decreases its concentration and This decreases its concentration and maintains concentration gradientmaintains concentration gradient

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Evidence for the mechanism of phloem Evidence for the mechanism of phloem transporttransport

Phloem protein is not present in living, Phloem protein is not present in living, active phloem tissueactive phloem tissue

The rate of transport in phloem is about The rate of transport in phloem is about 10,000 times faster than diffusion10,000 times faster than diffusion

Considerable evidence for the active Considerable evidence for the active loading of sucrose into sieve elements in loading of sucrose into sieve elements in sources such as leaves:sources such as leaves: Phloem sap always has a relatively high pH Phloem sap always has a relatively high pH

(around 8) – expected if H(around 8) – expected if H++ is being actively is being actively transported out of the celltransported out of the cell

There is a difference in electrical potential There is a difference in electrical potential across the plasma membrane (-150mV) – across the plasma membrane (-150mV) – consistent with excess of Hconsistent with excess of H++ outside the cell outside the cell compared with insidecompared with inside

ATP is present in phloem sieve elements in ATP is present in phloem sieve elements in large amounts – expected as it is required for large amounts – expected as it is required for active transport of Hactive transport of H++ out of cell out of cell

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Sieve elementsSieve elements Sieve tubesSieve tubes are made up of many elongated are made up of many elongated

sieve elements, joined end to end vertically to sieve elements, joined end to end vertically to form a continuous columnform a continuous column

A living cell (cellulose cell wall, plasma membrane A living cell (cellulose cell wall, plasma membrane and cytoplasm containing ER and mitochondria)and cytoplasm containing ER and mitochondria)

Amount of cytoplasm is very small and only forms Amount of cytoplasm is very small and only forms a thin layer lining the inside of the wall of the cella thin layer lining the inside of the wall of the cell

No nucleus and ribosomesNo nucleus and ribosomes Sieve plateSieve plate: made up of end walls of 2 meeting : made up of end walls of 2 meeting

sieve elements, perforated by large poressieve elements, perforated by large pores In living phloem, pores are always open, In living phloem, pores are always open,

presenting little barrier to the free flow of liquids presenting little barrier to the free flow of liquids through themthrough them

Contents of phloem sieve tubes (Contents of phloem sieve tubes (phloem sapphloem sap) )

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Companion cellsCompanion cells Each sieve element has one Each sieve element has one

companion cell lying beside itcompanion cell lying beside it Cellulose cell wall, plasma Cellulose cell wall, plasma

membrane, cytoplasm, small vacuole membrane, cytoplasm, small vacuole and nucleusand nucleus

Number of mitochondria and Number of mitochondria and ribosomes is larger than normal ribosomes is larger than normal (metabolically very active)(metabolically very active)

Numerous plasmodesmata pass Numerous plasmodesmata pass through their cell walls (direct through their cell walls (direct contact between cytoplasms of contact between cytoplasms of companion cell and sieve elementcompanion cell and sieve element

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Difference between sieve elements and xylem Difference between sieve elements and xylem vesselsvessels

Xylem vessels are dead, translocation Xylem vessels are dead, translocation through phloem sieve tubes involves active through phloem sieve tubes involves active loading of sucrose at sources requiring living loading of sucrose at sources requiring living cellscells

Xylem vessels have lignified cell walls, Xylem vessels have lignified cell walls, whereas phloem tubes do not (lignin in cell whereas phloem tubes do not (lignin in cell walls kills the cell)walls kills the cell)

Water flow through dead xylem vessels Water flow through dead xylem vessels unimpeded and strong walls support the unimpeded and strong walls support the plantplant

The end walls of xylem elements disappear The end walls of xylem elements disappear completely, whereas those of phloem sieve completely, whereas those of phloem sieve elements form sieve plates (probably elements form sieve plates (probably supporting structures/allows phloem to seal supporting structures/allows phloem to seal up rapidly if damaged/prevents entry of up rapidly if damaged/prevents entry of microorganisms which feed on the nutritious microorganisms which feed on the nutritious sap or cause disease) sap or cause disease)

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