3.3 Transport in Plants (✓)

plants with 2 seed leaves and branching pattern of veins in leaf

layer of dividing cells

to transport assimilates up and down the plant

tissue consisting of cells specialised for transporting fluids by mass flow

transport of water and soluble mineral ions up the plant

plants not very active so demand can be met by diffusion

water and sugars

provide strength and support

xylem and phloem as well as other tissues like collenchyma and sclerenchyma

at centre

xylem: X-shaped central core
phloem: between arms of X shape

provide strength to withstand pulling forces roots are exposed to

special sheath around vascular bundle which helps get water into xylem

layer of meristem cells just inside endodermis

near outer edge of stem

separate and discrete

young: separate
older: continuous ring

provides strength and flexibility to withstand forces

layer of meristem cells in between xylem and phloem which can divide to produce new xylem and phloem

midrib and veins of leaf

xylem

staining tissue, allowing coloured water to be taken up and cut

cells that help load sucrose into sieve tubes

make up tubes in phloem tissue that carry sap

sieve plates

-vessel to carry water and mineral ions
-fibres for support
-living parenchyma cells acting as packing tissue to separate + support vessels

1) lignin impregnates walls making walls waterproof
2) this kills the cells
3) end walls and contents of cell decay leaving long column of dead cells w no contents

strengthens vessel to keep it open even when water in short supply

spiral, annular (rings), reticulate (network of broken rings)
prevents vessel being too rigid and allows flexibility

gaps in cell walls which align between 2 adjacent vessels allowing water to be transferred or transferred to living parts of plant

where lignificatiom not complete

water column doesnt break easily and capillary action can be effective

-no cross walls
-no cell contents
-lignin thickening preventing collapse

sieve tube elements and companion cells

no nucleus and very little cytoplasm leaving space for mass flow of sap

thin

-large nucleus
-dense cytoplasm
-numerous mitochondria

perforated cross-walls allowing movement of sap from one element to next

gaps in the cell wall containing cytoplasm that connects 2 cells acting as special cytoplasmic bridges

yes

water passing through spaces in cell walls and between cells (not through membrane)

mass flow rather than osmosis

yes

water enters cell cytoplasm through membrane and then passes through plasmodesmata to next cell

water enters through membrane into cytoplasm and is able to enter and pass through vacuole before leaving to another cell

tendency of water molecules to move from one place to another

0

decrease, fewer free water molecules available

from higher water potential to lower

cell takes up water by osmosis

when a cell is full of water

strong cellulose wall

when the water inside the cell starts to exert pressure on the cell wall

water influx reduces

lose water by osmosis

-cytoplasm and vacuole shrink
-cytoplasm no longer pushes against cell wall so cell no longer turgid
-plasmolysis
-tissue now flaccid

plasma membrane loses contact with wall

device that measures rate of water uptake as leafy stem transpires

loss of water vapour from aerial parts of plant (stomata in leaves)

waxy cuticle

during day since photonsynthesis requires sufficient light

1) enters leaf through xylem, moving by osmosis or apoplast pathway to spongy mesophyll cells
2) water evaporates from spongy mesophyll cell walls
3) vapour moves by diffusion out of leaf through open stomata, relying on water vapour gradient where water vapour potential must be higher inside from outside

-transports useful mineral ions up plant
-maintains cell turgidity
-supply water for growth, cell elongation and photosynthesis
-suppky water to evaporate which cools plant

higher light intensity increases transpiration rate
because stomata open for gaseous exchange for photosynthesis

higher temp increases rate of transpiration
because increase evaporation from cell surfaces so water-vapour potential in leaf rises
because increase rate of diffusion through stomata because water molecules have more kinetic energy
because decrease in relative water vapour potential in air allowing more rapid diffusion

Higher relative humidity in air decrease transpiration rate
because smaller water vapour potential gradient between in leaf and air

more air movement increases transpiration rate
because air movement carries away water vapour maintaining high water vapour potential gradient

low water availability decreases rate of transpiration
because not enough water to replace water lost so stomata close and leaved wilt

attraction between water molecules and walls of xylem vessel

attraction between water molecules caused by hydrogen bonds

movement of water from soil, through plant, to air surrounding leaves driven by water potential gradient between soil and air by leaves

1) roots outer layer of cells (epidermis) contain root hair cells which absorb water and mineral ions
2) water moves across root cortex down water potential gradient to endodermis of vascular bundle
3) any water travelling by apoplast pathway must enter symplast pathway as apoplast pathway blocked by Casparian strip

endodermis

layer of cells around medulla and xylem containing granules of starch

dissolved in water that has to pass into cell due to casparian strip then plasma membrane transporter proteins actively pump mineral ions from cytoplasm of cortex cells into medulla+ xylem

due to water potential being lower in medulla and xylem because mineral ions are pumped into xylem

apoplast pathway of endodermal cells blocked by casparian strip and water potential gradient prevents it from moving through cytoplasm

yes

minerals in medulla draw water into medulla where pressure builds up so water forced into xylem, pushing it up

-water molecules are attracted to eachother by forces of cohesion holding them together in a long chain
-so when some are lost at the top the whole chain is pulled up
-this creates tension in column of water (why lignin strengthening is needed)

cohesion-tension theory

water column maintained through another vessel via bordered pits

adhesion pulls water up sides of vessel due to xylem being very narrow

plant adapted to living in water or where ground is very wet

plant adapted to living in dry conditions

-reduce loss of water
-replace water lost

-waxy cuticle
-stomata are on the under surface of leaves reducing evaporation from heat from direct sun
-stomata closed at night
-deciduous plants lose leaves in winter when ground is frozen and temp too low for photosynthesis

-leaf rolled longitudinally so air trapped inside becomes humid which reduces water loss and can roll tighter in very dry conditions
-thick waxy cuticle on outer side of rolled leaf
-stomata on inner side so protected by enclosed air
-stomata in pits in lower epidermis that are folded and covered by hairs helping reduce air movement
-spongymesophyll very dense so less SA for evaporation

-store water in stems which are often ribbed or fluted to allow expansion
-spines not leaves to reduce SA so less water lost
-stem green for photosynthesis
-wide spread roots

-closing stomata when low water availability
-low water potential in leaf by maintaining high salt concentration which reduces evaporation as gradient between cells and air spaces reduced
-very long tap root to reach underground water

-getting oxygen to submerged tissue
-keeping afloat to keep leaves in sunlight

-large air spaces in leaf to keep afloat for sunlight
-stomata on upper epidermis for gas exchange
-leaf stem with large air spaces for buoyancy and fast diffusion of oxygen to roots for respiration

structures that release water droplets which then evaporate from leaf surface

to maintain transpiration stream in water and humid conditions

part of plant where materials are removed from transport system (eg roots recieving sugar)

part of plant that loads materials into transport system (eg sugar from leaves photosynthesis)

transport of assimilates throughout plant

substances made by plant using substances absorbed from environment (eg sugar and amino acids)

phloem

1) H+ ions actively transported out of companion cell
2) increases concentration outside cell and decreases concentration inside cell
3) H+ ions diffuse back into cell due to concentration gradient through special cotransporter proteins which only allow H+ in if accompanied by sucrose molecules
4) as concentration of sucrose in companion cell increases, it diffuses through plasmodesmata into sieve tube

Cotransporter proteins allowing only movement of H+ ions into cell if accompanied by sucrose molecules

secondary active transport, result of active transport of H+ out and moves sucrose against its concentration gradient

solution of assimilates

by mass flow

-difference in hydrostatic pressure between 2 ends of tube resulting in pressure gradient
-water enters at source increasing pressure and leaves at the sink reducing pressure

sucrose entering sieve-tube element makes water potential in sieve-tube more negative so water molecules enter by osmosis from surrounding tissue

sucrose converted to starch for storage

provide sucrose from stored starch

sucrose used for growth

diffuse out via plasmodesmata or active transport

removal of sucrose cause water potential in sieve tube to increase so water moves out to surrounding cells

no but can flow in opposite directions in different sieve tubes at once