BIOLOGY AS LEVELREVISION 02

7. Plant TransportAnd Phytotomy

Phytotomy

Root System- RootsShoot System- Node – Wear plants grow outward- Internodes- Leaves

Why do plants need transport system?

Small surface area: volume ratio – diffusion is inefficient

Large amount of nutrient requirement for growth

Large amount of nutrient requirement for repair

Roles of the plant transport system

Move substances from area of absorption – to area of uses (root – xylem - leaf)

Move substances from area of production (SOURCE) to area needed for metabolism (SINK) (leaf – phloem – stem for growth/ cell wall building)

Move substances from area of production to area of storage (leaf – phloem - root)

Transport of Carbon dioxide

Diffusion through plant cells (phospholipids membrane as it is non-polar)

Carbon Dioxide --- absorbed through stomata (controlled by guard cells)

Plant leaves have large surface area to receive as much of the gas as possible – diffusion from the air

Transport of Oxygen

Plant Tissues

Dermal Tissues: Protection, prevent water loss

- Epidermis, PeridermGround Tissues: metabolism, Storage, Growth- Parenchyma, Collenchyma, SclerenchymaVascular Tissue: Transport- Xylem and Phloem

Epidermis

Continuous layer on the outside of the plantOne-Cell thickProvides protectionWaxy Cuticle – made of cutin – protects organ

from frying out of water lossLeaves: Stomata – for gas exchangeRoots: Root Hair

Parenchyma Thin-walled cell – they are packing tissues Isodiametric (allow tight packing) Metabolically active The turgidity helps support plant Storage of starch Has air spaces between cells – allow gas

exchange Water and minerals are transported

between walls and the living content of the cell

Made up the cortex in roots/ stems Pith in stems

Mesophyll

Meso = middle, Phyll = LeavesSpecialized parenchyma cells –

photosynthesisPALISADE MESOPHYLL – near to the upper

surface – hence has more chloroplastSPONGY MESOPHYLL

Palisade Mesophyll

Chlorenchyma – A parenchyma specialized for photosynthesis

Many chloroplast Large vacuole – storage Starch grains Arranged end-on to pack in as

much of the cell as possible Elongated – located nearer to

the surface to receive maximum sunlight

Spongy Mesophyll

Aerenchyma – parenchyma that is specialized for gas exchange/ diffusion

CollenchymaParenchyma cell is modified to form

collenchymaExtra cellulose deposited at the corners of each

cellAdds extra strengthThe midrib is the collenchymaRidged stemsThe layer just below the epidermisCelery – mostly collenchyma

Collenchyma

Allows plant to bend in the wind (more deposition of cellulose)

Cells are living/ non-lignified – allows flexibility/ stretching

Usually not found in roots – they are not exposed to wind

Sclerenchyma

Dead cells with rigid lignified wallsThey cannot stretch1. Fibres (Long, narrow, thick walled, narrow

lumens, tapering ends) – mechanical strength – protection to non growing parts [XYLEM/ PHLOEM]

2. Schlereids – shorter/ fatter than fibre – provides mechanical strength – exist isolated in cortex, pith, xylem/phloem or in groups in testa/ walnut shells

Endodermis

Once cell thick layerBefore the PericycleSurrounds the vascular tissue in stems and

roots

Pericycle

One or several cell thickBetween the endodermis and the vascular

tissueNew roots grow out of this In stems – formed from sclerenchyma cells –

dead lignified cell

Vascular Tissue

XylemPhloem

Xylem Vessel element Vessel elements – long tube like

structure No end-wall between cells No cytoplasm – no organelles (More

room/ uninterrupted flow) Lignified – support the xylem as it

doesn’t have the turgidity provided by vacuole – withstand negative pressure – waterproofing

Pits – lateral movement of water Angiosperm – vessels very important

– large leaves = high water losses

Xylem vessels - Tracheid

Dead hollow cells – narrower lumens than xylem vessel elements

Found in conifers – do not lose as much waterNarrow lumen = more capillarityTapering end walls – provide mechanical

strength

Phloem

Sieve tube elementsCompanion cellsParenchyma – provide turgidityFibres- Provide support/ protection

Sieve Tube Elements

Living, no lignifiedTubular – linked end to endPerforated end wallsThin cytoplasmFew organelles, no nucleusCellulose Cell WallsPlasmodesmata connecting

with Companion Cells

Sieve Tube Elements

Bidirectional flows of solutes/ hormonesPerforated walls – allow movement of

substancesFew organelles with no nucleus/ thin

cytoplasm – no impediment of phloem sap’s flow

Cellulose cell wall – allows exchange of substances

Plasmodesmata – exchanges of substance with companion cells

Companion Cells

Has nucleus and a lot of other organelles

Nucleus – control activities both of the cell + sieve tube elements

Ribosomes – production of enzymes/ co-transporters/ carriers proteins

Mitochondria – produces ATP for active transport

Stem Vs. Root STEM1. Vascular bundle in a ring –

provide flexibility/ support2. Sclerenchyma – vascular

bundle cap3. Collenchyma – cortex

beneath epidermis – flexible support against wind

4. Chlorenchyma/ Palisade – under epidermis – for new growth – may have stomata

5. No endoderm

Stem Vs. Roots Roots1. Vascular bundle in the central –

reduces damage from friction with soil

2. No sclerenchyma – soil provides support

3. No collenchyma – no wind to withstand

4. No Chlorenchyma – not exposed to sunlight

5. No stomata – most gas exchange occur with root hair cells

6. Endodermis surrounds vascular bundle

TRANSPIRATION

Transpiration

The loss of water vapor at the surface of the leaf through the stomata by the process of diffusion down the water potential gradient. The loss of water vapor from plants to the environment.

Water movement through a leaf

Transpiration at the surface of the leaf – reduces water potential in the leaf

Water EVAPORATES from the mesophyll cell wall into the air space

Root

Xylem is in the centerRoot hair is where water is absorbedWater moves through the cortex – enters the

xylemDue to water potential gradient

Root – The 2 Pathways

Water can take two routes through the root cortex

1. The Apoplast pathway: Cell wall is made of fibre crisscrossing each other – water can soak in easily. Hence water seep from wall to wall without entering the cytoplasm

2. The Symplast pathway: Water moves into cytoplasm/ vacuole of cortical cell by osmosis – move into adjacent cells through plasmodesmata

Root – Entering the Xylem

Sometimes – mineral ions secreted into the xylem water – reducing the water potential gradient = ROOT PRESSURE

In the roots – xylem in the center Water moves through the cortex following the water

potential – through symplast/ apoplastic pathway It reaches the endodermis where there is a suberin

layer on the cell wall called the Casparian strip This is waterproof

Root – Entering the Xylem

This forces water to move through cell membrane – may help in generation of root pressure or help in controlling what’s going into the xylem

When plants grow old, some cells become fully suberized – leaving only the passage cells that can allow water to pass through

Water moves through the Pericycle and into the pits and into the xylem

Root hair increases surface area for water absorption Mycorrhiza has a similar function – it is a fungi that receives

nutrient from plant while helping it in water transport (mutualism)

Xylem

Water moves in continuous column Major force: Hydrostatic pressure Move by mass flow Water molecules – attracted by hydrogen bonding Cohesion and adhesion – allows this type of movement Air lock happens when there’s an air bubble that breaks

the flow of water up the tube – the small diameter of the lumen prevents this

Pits connect xylem to the other cells

Leaf

Transpiration is the loss of water from the leaves to the atmosphere

Evaporation of water from the leavesReduces water potential in the leaf

Leaf

Cells: Mesophyll (not tightly packed) – many air spaces

Air inside usually saturatedSir inside has contact with air outside through

stomataPotential gradient causes movement of air

out

Xerophytes Adaptations

Reduction of surface area: Needle leavesSwollen stems for water storageReduction in water potential gradient: Sunken

stomata, infold of cell membranes, leaves folding, Trichomes

Translocation

Translocation

Movement of assimilates (substances which plants make) from source to sinks

In this case – mostly sucroseTransported in sieve elements helped by

companion cells, parenchyma and fibre

Phloem Sap

Content: Sucrose, Potassium ions, Amino acids, Chloride ions, Phosphate ions, Magnesium ions, Sodium ions, ATP

Usually it is hard to extract phloem sap There is a clotting technique When the phloem is cut, the sap surges up to the cut

but is blocked by the sieve plate The sieve plate is then sealed with carbohydrate callose The speed of flow and the amount triggers this

mechanism

How Translocation happens

Moves as mass flow 1 m per hour on average ACTIVE TRANSPORT from source (organ of production) to sink (organ

that needs the sucrose) At source: caused by active loading of sucrose into the phloem vessel Causes water to diffuse into phloem down the water potential

gradient gradient Raises hydrostatic pressure At sink: Active unloading of sucrose Causes water to move out Lowers the pressure – maintains the gradient

How Translocation happens

Photosynthesis produces triose sugar Water moves through the mesophyll – apoplastic or symplastic Here, the companion cell pumps Hydrogen ions into its wall –

using ATP Excess of hydrogen – causes hydrogen to move back by their

concentration gradient through carrier protein – Co-transporter protein

This Co-transporter transports sucrose with it Sucrose then moves into sieve tube element via plasmodesmata At unloading points – same method is used – enzyme invertase

converts sucrose into glucose and fructose – reducign the concentration of glucose – setting up the gradient

Sieve Tubes Vs. Xylem

Active transport and Passive transportLiving cells required, non living cells requires

(membranes needed to control entry/ loss of solutes)

Lignified cell wall for xylemEntirely empty tube for xylem – flow

unimpeded – strong wallsSieve plates which allow self healing