As 1 2 3 Plant Revision

7/21/2019 As 1 2 3 Plant Revision

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Transport in Plants

• Explain the need for transport systems inmulticellular plants in terms of size and surfacearea:volume ratio;

• Describe, with the aid of diarams andphotoraphs, the distribution of xylem andphloem tissue in roots, stems and leaves ofdicotyledonous plants;

• Describe, with the aid of diarams andphotoraphs, the structure and function of xylemvessels, sieve tube elements and companioncells;



Transport in Plants

• Plants need a transport

system so that cells deep

within the plants tissues

can receive the nutrients

they need for cellprocesses

• The problem in plants is

that roots can obtain

water, but not suar, andleaves can produce suar,

but can!t et water from

the air



"hat substances need to be

moved#

• The transport system

in plants is called

vascular tissue

• $ylem tissuetransports water and

soluble minerals

•Phloem tissuetransports suars



The %ascular Tissues

• $ylem and phloem

are found toether in

vascular bundles, that

sometimes containother tissues that

support and

strenthen them



&oot vs' stem vs' leaf The vascular bundle

differs dependin on ifit is a root or stem



&oot• The vascular bundle is found

in the centre• There is a lare central core ofxylem( often in an x(shape

• This arranement providesstrenth to withstand thepullin forces to which roots

are exposed• )round the vascular bundleare cells called the endodermiswhich help to et water into thexylem vessels

• *ust inside the endodermis is

the periycle which containsmeristem cells that can divide+for rowth



-tem• The vascular bundles are found near the outer ede of the stem

• The xylem is found towards the inside of each vascular bundle, the

phloem is found towards the outside• .n between the xylem and phloem is a layer of cambium

• /ambium is a layer of meristem cells that divide to ma0e new xylemand phloem



1eaf

• The vascular bundles

+xylem and phloem form

the midrib and veins of the

leaf

• ) dicotyledon leaf has a

branchin networ0 of

veins that et smaller as

they branch away from the

midrib• "ithin each vein, the

xylem can be seen on top

of the phloem



Phloem

$ylem

-tem



) 2 $ylem

3 2 Phloem

/4D 2 5pper41ower epidermis

1eaf



$ylem vessel wall

Phloem

&oot

Endodermis

$ylem vessel

lumen

-tarch

rains





-tructure of $ylem

• 5sed to transport

water and minerals

from roots to leaves

• /onsists of tubes forwater, fibres for

support and livin

parenchyma cells



$ylem vessels

• 6bvious in dicotyledonous plants

• 1on cells with thic0 walls containin linin

• 1inin waterproofs walls of cells and strenthens

them• /ells die and ends decay formin a lon tube

• 1inin forms spiral, annular rins or bro0en rins+reticulate

• -ome linification is not complete and pores areleft called pits or bordered pits, allowin water tomove between vessels or into livin parts



)daptations of $ylem to 7unction

• $ylem can carry water and minerals from roots

to shoot tips because:

• 8ade of dead cells formin continuous column

• Tubes are narrow so capillary action is effective

• Pits allow water to move sideways

• 1inin is stron and allows for stretchin

• 7low of water is not impeded as: there are noend walls, no cell contents, no nucleus, linin

prevents tubes collapsin



-tructure of Phloem

• 7unction to transport

suars from one part

to another

• 8ade of sieve tubeelements and

companion cells



-ieve Tubes

• -ieve tube elements not true cells as they

have little cytoplasm

• 1ined up end to end to form a tube

• -ucrose is dissolved in water to form a

sap

• Tubes +0nown as sieve tubes have a fewwalls across the lumen of the tube with

pores +sieve plates



/ompanion cells

• .n between sieve tubes

• 1are nucleus, dense

cytoplasm

• 8any mitochondria toload sucrose into sieve

tubes

• 8any plasmodesmata

+aps in cell walls

between companion cells

and sieve tubes for flow

of minerals



"ater route between cells

• )poplast: between cell

walls of neihbourin

cells

• -ymplast: throuh

plasma membrane and

plasmodesmata to

cytoplasms from cell to

cell

• %acuolar: same assymplast, but also

throuh vacuoles



"ater upta0e from the soil

• Epidermis of roots contain root hair cells

• 8inerals absorbed by active transport

usin )TP

• 8inerals reduce the water potential in the

cell cytoplasm +more neative so water is

ta0en up by osmosis





8ovement across the root• )ctive process occurrin at the endodermis +layer of cells surroundin the

xylem, some containin waterproof strip called casparian strip

• /asparian strip bloc0s the apoplast pathway +between cells forcin waterinto the symplast pathway +throuh the cytoplasm• The endodermis cells move minerals by active transport from the cortex into

the xylem, decreasin the water potential +more neative, thus water movesfrom the cortex throuh the endodermal cells to the xylem by osmosis

• ) water potential radient exists across the whole cortex, so water is movedalon the symplast pathway +throuh cytoplasm from the root hair cells

across the cortex and into the xylem



/asparian -trip

• 3loc0s the apoplast pathway +cell walls

• "ater and dissolved nitrate ions have to pass

into the cell cytoplasm throuh cell membranes

• There are transporter proteins in the cellmembranes that actively transport nitrate ions

into the xylem lowerin the water potential +more

neative

• "ater enters xylem down concentration radient

and cannot pass bac0



"ater movement up stem

• &oot pressure: minerals move into xylem byactive transport, forcin water into xylem andpushes it up the stem

• Transpiration Pull: loss of water at leavesreplaced by water movin up xylem' /ohesion(tension theory( cohesion between watermolecules and tension in the column of water+which is why xylem is strenthened with linin

means the whole column of water is pulled up inone chain• /apillary action: adhesion of water to xylem

vessels as they are narrow



9ow water leaves the leaf

• Throuh stomata

• Tiny amount throuh the waxy

cuticle

• "ater evaporates from the

cells linin the cavity betweenthe uard cells, lowerin water

potential and meanin that

water enters them by osmosis

from neihbourin cells, which

is replaced by furtherneihbourin cells and so on



Transpiration

• 1oss of water vapour from upper parts of theplant

• "ater enters leaf from xylem and passes to

mesophyll cells by osmosis• "ater evaporates from surface of mesophyll

cells to form water vapour +air spaces allowwater vapour to diffuse throuh leaf tissue

• "ater vapour potential rises in air spaces, sowater molecules diffuse out of the leaf throuhopen stomata



Transpiration: three processes

• 6smosis from xylem to mesophyll cells

• Evaporation from surface of mesophyll

cells into intercellular spaces

• Diffusion of water vapour from intercellular

spaces out throuh stomata



"ater use in plant

• Photosynthesis

• /ell rowth and elonation

• Turidity• /arriae of minerals

• /ools the plant



8easurin transpiration

• Potometer is used to

estimate water loss



7actors affectin transpiration

• 1eaf number: more leaves, more transpiration• umber, size, position of stomata: more and lare, more

transpiration, under leaf, less transpiration• /uticle: waxy cuticle, less evaporation from leaf surface

• 1iht: more as exchane as stomata are open• Temperature: hih temperature, more evaporation, more

diffusion as more 0inetic enery, decrease humidity somore diffusion out of leaf

• 9umidity: hih humidity, less transpiration

• "ind: more wind, more transpiration• "ater availability: less water in soil, less transpiration

+e'' in winter, plants lose leaves



Too much water loss

• 1ess turidity

• on(woody plants wilt and die

• 1eaves of woody plants die first then it willdie if water loss continues



$erophytes

• -maller leaves reducin surface area e'' pine tree• Densely pac0ed spony mesophyll to reduce surface area, so less

water evaporatin into air spaces• Thic0 waxy cuticle e'' holly leaves to reduce evaporation• /losin stomata when water availability is low

• 9airs on surface of leaf to trap layer of air close to surface whichcan become saturated with water, reducin diffusion

• Pits containin stomata become saturated with water vapourreducin diffusion

• &ollin the leaves so lower epidermis not exposed to atmospherealso traps air which becomes saturated

• 8aintain hih salt concentration to 0eep water potential low andprevent water leavin



8arram rass1eaf rolled up to trap

air inside

Thic0 waxy cuticle to

reduce water

evaporation from the

surface

Trapped air in the

centre with a hih

water potential +less

neative

9airs on lower

surface reduce

movement of air-tomata in pits to

trap air with moisture

close to the stomata



8ovement of -uars

• Translocation: movement of assimilates +suars

and other chemicals throuh the plant

• -ource: a part of the plant that releases sucrose

to the phloem e'' leaf • -in0: a part of the plant

that removes sucrose from

the phloem e'' root



-ucrose Enterin the Phloem

• )ctive process +re<uires enery• /ompanion cells use )TP to transport hydroen

ions out of their cytoplasm• )s hydroen ions are now at a hih

concentration outside the companion cells, theyare brouht bac0 in by diffusion throuh specialco(transporter proteins, which also brin thesucrose in at the same time

• )s the concentration of sucrose builds up insidethe companion cells, they diffuse into the sievetubes throuh the plasmodesmata +apsbetween sieve tubes and companion cell walls



-ucrose movement throuh phloem

• -ucrose enterin sieve tube lowers the water

potential +more neative so water moves in by

osmosis, increasin the hydrostatic pressure

+fluid pushin aainst the walls at the source• -ucrose used by cells surroundin phloem and

are moved by active transport or diffusion from

the sieve tube to the cells' This increases water

potential in the sieve tube +ma0es it lessneative so water moves out by osmosis which

lowers the hydrostatic pressure at the sin0



8ovement alon the phloem

• "ater enterin the phloem at the source,

movin down the hydrostatic pressure

radient and leavin at the sin0 produces

a flow of water alon the phloem thatcarries sucrose and other assimilates'

This is called mass flow' .t can occur

either up or down the plant at the sametime in different phloem tubes



Evidence for translocation

• &adioactively labelled carbon from carbon dioxide can appear in thephloem

• &inin a tree +removin a rin of bar0 results in suars collectinabove the rin

• )n aphid feedin on the plant stem contains many suars when

dissected• /ompanion cells have many mitochondria• Translocation is stopped when a metabolic poison is added that

inhibits )TP• p9 of companion cells is hiher than

that of surroundin cells

• /oncentration of sucrose is hiher atthe source than the sin0



Evidence aainst translocation

• ot all solutes move at the same rate

• -ucrose is moved to parts of the plant at

the same rate, rather than oin more

<uic0ly to places with low concentrations

• The role of sieve plates is unclear



5seful &evision -ites

• http:44scienceaid'co'u04

• http:44www's(cool'co'u04

• http:44www'spar0notes'com4bioloy4

http://scienceaid.co.uk/

http://www.s-cool.co.uk/

http://www.sparknotes.com/biology/

http://www.sparknotes.com/biology/

http://www.s-cool.co.uk/

http://scienceaid.co.uk/

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