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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