Transport In Plants - KD Bio

Transport inplants

Homework for Monday: Correct potometer questions Complete transport in plants worksheet

Transpiration – the loss of water from a plantthrough evaporationDid you know? A 15m maple tree in the USA was estimated to have177,000 leaves, with a total surface area of 675square metres On a summers day, it lost 220 litres of water perhour through transpiration through the leaves The roots then needed to absorb this much from thesoil each hour to prevent the plant from wilting

Transpiration mostly happensthrough the stomata, althoughsome is lost through the leafcuticle and a little through thestem Internal and external factorsinfluence the rate oftranspiration:Internal factors Leaf surface area – more transpiration will occurwith a larger leaf surface area Stomatal density – if lots of stomata are presentthen more transpiration will occur Cuticle thickness – if the cuticle is thick then lesstranspiration will occur

External factors If it’s light, the stomata are open (Light intensity) – this affects thestomatal aperture (size of opening)as more light encourages morephotosynthesis, which needs carbondioxide so stomata open more toallow it to diffuse in (i.e. Morewater vapour loss) Air currents – wind blows moist air awayand maintains the concentration gradientfor water to evaporate (no wind createsmoist air pockets around stomata so lesstranspiration) Temperature – the higher the temperature,the faster water molecules will evaporate

Humidity (amount of water vapour in the air) – dryair has a low water potential and thus evaporation(transpiration) is high i.e. a gradient between leafsurface and the air exists. Moist air has a high waterpotential and thus less transpiration occurs Soil water availability – more soil water available willallow for more photosynthesis so the stomata willopen more to get carbon dioxide and thus more waterwill be lost. Also, the guard cells will be more turgiddue to having lots of water and they will openCarbon dioxideenters, while waterand oxygen exit,through a leaf’sstomata:

AS 2 Practical demo – the bubble potometer.

Potometer PPQ answers (a) Assume that the rate of water uptake is equal tothe rate of transpiration(b) - Prevents air from entering the xylem andcreating an air lock- Remove capillary tube from water/expose to airand air is drawn in as shoot takes up water- Allows air bubble to be moved back to the origin- Allow the rates of transpiration to acclimatise tothe surrounding conditions

(c) Transpiration reduced when plant is covered byclear bag as air currents reducedAnd more humid air outside stomatal poresDiffusion gradient is therefore reduced / diffusionshells build upEven less transpiration when black plastic bag isadded as stomata close in the darkOnly cuticular traspiration can now happen (much lesswater lost in this way)

(ii) 90mm long X 0.8 mm2 72 mm372 / 10 7.2 mm3 minute -1(d) Different shoots have different sizes andsurface areas with different stomatal densities andnumbers of leaves

There are 2 systems of transport in plants: Xylemtransporting water and mineral ions Phloemtransporting sugars and soluble organicproducts of photosynthesis

Plant tissues in relation to water (and ion)transport and translocation:

The root - regions include: Piliferous layerCortexEndodermisSteleThe piliferousregion contains theroot hair cells

Important diagram – note the position of the vasculartissues:Draw a block tissue diagram of this!

Piliferous region

The tissues of the rootEpidermis – the outer layer of cells. Root hair cells(elongated epidermal cells) project into soil andincrease surface area to absorb water and mineralsCortex – a layer of UNDIFFERENTIATED CELLS(parenchyma /packing tissue) between the epidermisand the vascular (conducting) tissue. In the leaf, thistissue can photosynthesise. CORTEX CELLSUSUALLY HAVE SMALL AIR SPACES BETWEENTHEM AND CAN BE RICH IN STARCH GRAINS.Endodermis – thin, SINGLE layer of cells surroundingthe vascular tissue (xylem and phloem). Theendodermis contains a waterproof Casparian stripmade of suberin

The stele – the endodermis and the vascular tissuethat it surroundsXylem tissue – many dead, hollow vessels carryingwater and dissolved mineral ions up the plant to theleaves and other organsPhloem – tubular living cells that carry dissolvedorganic materials made through photosynthesis e.g.sucrose and amino acids, around the plantPiliferous region – the section of a young root whichhas root hairs as part of its epithelium (where waterand mineral absorption occurs)

The stem – regions include: EpidermisCortexVascular scularbundle

PhloemXylem

Some extra tissues found in the stem:Vascular bundle – describes the “veins” in a plantthat consist of xylem, phloem and cambiumCambium – a third type of tissue in the vascularbundle (between the xylem and phloem). Cambiumcells can divide to make new xylem cells to the insideor phloem cells to the outsidePith – found at the very centre of the stem (insidethe vascular region). Like the cortex, the pith is madeof unspecialised packing cells called parenchyma.They can store products of photosynthesis and theirturgidity provides non-woody plants with support

The leaf – regions include: Epidermis ( waxy cutilcle)Palisade mesophyllSpongy mesophyllVascular bundles (veins)Stomata guard cells

The uptake of mineral ions by root hairs: Minerals can enter the roothairs passively by diffusion oractively by active transport(most ion uptake is through this) If the conc. of the ions is higherinside the root hair than in thesoil, then energy from ATP isused to pump the ions in to theroot against the concentrationgradient Factors that affect the rate ofrespiration will therefore affectthe rate of ion uptake e.g.?Oxygen supply, temperature, presence of inhibitorse.g. cyanide

Once inside the root hairs, mineral ions move acrossthe cortex to the xylem by the symplast (bydiffusion) and apoplast (by diffusion and activetransport) routes After passing across the endodermis via thesymplast route, mineral ions are thought to enter thexylem by a combination of active pumping anddiffusion

Water uptake involving osmosis Water enters the root hair from the soil by osmosisand passes across the cortex to the xylem tissue The root hair cells in the piliferous region lack awaxy cuticle so that is why osmosis occurs here andnowhere else in the root (and have large SA) The root hair cells have a lower water potential thanthe soil due to the solutes in their vacuoles (e.g.mineral ions); thus water moves in, and follows aconcentration gradient: When water enters a root hair cell, its waterpotential rises compared to the adjacent cells Water moves into the next cell by osmosis The water potential in the xylem is lower than thatin the root cortex cells, so water travels across thecortex to the xylem vessels at the centre of the root

Root hairs have: Thin walls (short diffusionpathway) Large surface area tovolume ratio forabsorptionWater travelsthrough the cortex ofthe root to the xylemalong 2 differentroutes.The symplast and apoplast pathways

What we will learn.Understand the apoplast and symplastpathways through plant tissues:-The apoplast pathway along cellulose cell walls-The symplast pathway through protoplastsconnected by plasmodesmata-The apoplast and symplast pathways in theroot and leaf;-The role of the endodermis in ensuring thesymplast pathway into the stele

Water moves through theprotoplasts of the cortexcells How? Water can move throughthe plasmodesmata thatconnect the cytoplasm ofone cell to the next

Water moves through thecellulose fibrils of cellwalls of the cortex cells As water passes throughthe spaces between thecellulose fibres thecohesive forces betweenthe water molecules meansmore water is pulledalong the apoplasticroute as it offers lessresistance than thesymplast route

Qu: Does water move straight into the cellwalls of epidermal cells when travelling bythe apoplast route or does it enter thecytoplasm first and then travel through thewalls?Water goes directly into cell walls , which iswhy apoplast pathway is lots quicker thansymplast – cellulose fibrils in cell wall offerno resistance to the movement of water,unlike the cytoplasm in the protoplast

The apoplast route is blocked at the endodermis The cell walls of the endodermis are impregnatedwith waxy suberin This forms a band of wax around the cells calledthe casparian strip As suberin is waterproof the Casparian strip stopswater passing along the cell walls To pass through the CS water must cross the cellmembrane and pass into the cytoplasm and continuealong the symplast route into the steleWhy is this useful?

By forcing the water and ions through the symplastpathway before entering the xylem, the Casparianstrip ensures the plant has control over what and howmuch water and dissolved mineral ions enter thexylem i.e. important for regulating the plant’scellular metabolismCasparianstrip

Water and ions entering the endodermis via thesymplast pathway thus come under the control ofthe cells metabolism Ions are actively pumped into the xylem, whichcreates a lower water potential and then waterfollows along this water potential gradient

June 06

The movement of water (and dissolved ions)through the xylem tissue-Cohesion-tension theory-Transpiration creating a negative pressurewithin leaf xylem vessels resulting in thetranspiration stream-The cohesive and adhesive forces of water-The root pressure hypothesisFirst we need to look at the structure of the xylem.

Homework for tomorrow:Finish plant transport notes

is made of XYLEM VESSELS – these cells transportmost of the water and mineral ions These are formed from a column of unspecialisedcells which lose their end walls In these cells a secondary cell wall, impregnatedwith lignin is formed inside the primary cell wall This increases mechanical strength and isimportant if the vessels are to withstand thestrong pressures that occur during water transport The lignin stops the xylem vessels from collapsingwhen under tension during water transport

How xylem tissue forms:

Lignin is impermeable so materials cannot pass intothe xylem cells and the protoplasm dies (what’sinside the cell membrane) i.e. mature xylem vesselsare dead! The cells are hollow and nothing restricts the flowof water so less pressure is required to move thewater than in living cells No lignin is laid down where plasmodesmata werepresent in the original cell walls These non-lignified areas are known as pits andthey allow water to pass sideways between onexylem vessel and the next

Homework for Monday:Complete past paper questionHomework for Thursday:Revise for transport in plants test

NEW XYLEM ORIGINATES FROM THEMERISTEMATIC REGION BETWEEN THE XYLEMAND PHLOEM CALLED THE CAMBIUM.Different types of lignin deposition in xylemvessels occurs depending on the age of the xylemand where it is found in the plant:

2 TYPES OF XYLEM TISSUE CAN EXISTTOGETHER ALTHOUGH PROTOXYLEM IS MOREASSOCIATED WITH NEW GROWTH ANMETAXYLEM WITH MORE MATURE TISSUE1. Protoxylem – the first xylem todevelop behind the root andshoot tips. Lignin is added inrings or spirals to form annularvessels (rings) and spiral vessels– this is to allow the vessels toelongate along with other tissuein the growth regions.

2. Metaxylem – is produced in themore mature parts of the plantand the walls are fully lignified(with the exception of pits). Wesee reticulated and pittedpatterns of lignification here- Reticulated vessels arethickened by interconnectingbars of lignin- Pitted vessels have a uniform(always the same) thickness,except at pores seen as pits –these allow for rapid movementof water and ions out of thevessels to the surrounding cells

Secondary xylem allows woody plants to increasetheir girth each year This is known as secondary thickening This seasonal growth shows up as annual rings A ring formed in the previous year transports littlewater, its main function being to support the plant’sincreasing biomass