Transport In Plants
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- Created by: Kirsty_hodnett
- Created on: 15-04-15 10:53
Vascular Tissue
- vascular bundle: 2 tissues: xylem & phloem
- xylem in roots is central- resists pull so helps anchor root
- xylem in stems is arranged in bundles-flexible support but resistant to strain of bending
- vascular tissues in leaves is a network of veins for flexible strength and resistance to tearing
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Structure Of Xylem
- cells: vessels & tracheids
- form a system of tubes through which water can travel
- cellulose walls add lignin- strong hard substance
- lignin builds up and cell contents dies leaving a hollow tube
- lignin is impermeable to water and solutes
- xylem transport and give mechanical strength
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Water Uptake by the Roots
- large amounts of water lost through leaves in transpiration
- greatest uptake happens in the roots, where surface area is increased by root hairs
- soil water contains weak solution of mineral salts & has high water potential
- vacuole of root has strong solution of dissolved solutes so it has a lower water potential
- this allows water to move in down a concentration gradient via osmosis
- then they travel across the cortex in 3 pathways
- apoplast- cell wall
- symplast- cytoplasm & plasmdesmata
- vacuolar- vacuoles
- apoplast is the fastest
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Root Pressure
- xylem in centre surrounded by endodermis
- endodermis has suberin which makes a casparian *****
- suberin is waterproof and prevents use of apoplast pathway
- water must travel through the symplast pathway
- active transport of salts lowers the water potential in the xylem to allow water to move in via osmosis down a concentration gradient
- the water potential gradient produced makes a force called root pressure
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Uptake Of Minerals
- taken up via active transport through the roots from the soil
- minerals move along apoplast pathway in solution
- enter cytoplasm when reach casparian *****
- ions must then be taken up by active trasport past the casparian band
- plant can be selective
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Movement of Water from Roots to Leaves
- transpiration pulls water up the stem
- passive process
- water travels up stem xylem to leaves where it evaporates out as water vapour
- transpiration draws up water molecules beneath it along same pathways as roots
- transpiration pull= large cohesive forces between molecules and adhesive forces between water molecules and hydrophilic lining of the vessels
- maintain column of water in the xylem
- cohesion tension theory
- capillarity contributes to rise of water in xylem- water rises up narrow tubes by capillary action
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Transpiration
- continuous losing of water to the atmosphere
- 99% of water absorbed is lost through the leave
- evaporation of water from the inside of leaves through stomata is transpiration
- balance water uptake with water loss
- loses too much water- wilts, cannot regain turgor and dies
- stomata need to be open in the day for photosynthesis and gaseous exchange
- most of the water is lost through stomata
- 5% through epidermis but this is reduced by waxy cuticle
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Factors Affecting Transpiration
- temperature
- rise in temp provides additional kinetic energy for the movement of water molecules
- additional energy speeds up evaporation and rate of diffusion of watervapour through the stomata to the atmosphere
- WP of atmosphere is lowered as its temperature rises- can hold more moisture
- humidity
- air inside leaf is saturated with water vapour
- humidity outside the leaf varies
- water potential gradient between leaf and atmosphere is great and when stomata are open water vapour rapidly diffuses from leaf
- air movement
- transpiration in still air makes a layer of saturated air around leaf
- resistance to diffusion of water vapour
- reduces rate of transpiration
- movement of air increases transpiration
- light intensity
- controls degree of stomatal opening
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Potometry
- potometers actually measure water uptake rates but since this water is mainly lost through transpiration anyway the rates are classed as the same
- potometer can be used to measure water uptake by a leafy shoot under different conditions or used to compare different types of shoots
- set up
- cut leaf shoot underwater
- fill apparatus completely with water so no air bubbles
- use rubber tubing underwater to connect shoot to potometer
- lift up potometer/shoot seal joints with waterproof jelly
- dry leaves
- introduce air bubble
- measure air bubble travel in set intervals
- use water reservoir to reset bubble
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Adaptations of Flowering Plants Mesophytes
mesophytes
- habitats of adequate water supply
- mainly crops of temperate regions
- grow best in well drained soils and moderate dry air
- loss of water prevented by closing stomata
- water uptake at night makes up for losses during the day
- many shed their leaves before winter
- aerial parts can die off non wood plants but underground organs survive
- annual mesophytes survive winter as dormant seeds
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Adaptations of Flowering Plants Xerophytes
xerophytes
- low water availabilit
- hot dry arid cold conditions
- marram grass colonised sand dunes, with no soil, rapid drainage high wind speeds salt spray and lack of shade
- marram grass adaptations
- rolled leaves- thin walled epidermal cells in grooves shrink in lack of water causing leaf to roll into itself reducing the surface area from which transpiration can occur
- sunken stomata- found in grooves on inner side of leaf in deep pits trapping humid air reducing wpg reducing rate of diffusion of water
- hairs- stick and interlocking to trap water vapour reducing wpg
- thick cuticle- waxy covering on leaf surface reducing water loss lowering cuticlar transpiration
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Adaptations of Flowering Plants Hydrophytes
hydrophytes
- submerged or partially submerged in water
- water lilly
- rooted to mud at bottom of pond with floating leaves on the surface of water
- water supports them- no need for lignified tissues
- little need for transport tissues so xylem is poor
- little to no cuticle
- stomata on upper surface
- stems and leaves have large air spaces for bouyancy and reservoir of co2 and o2
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Translocation
- products of photosynthesis transported in phloem away from synthesis site (source) to other parts where they are used- growth /storage (sinks)
- transport of soluble organic materials eg sucrose/ amino acids
phloem
- sieve tubes adapted for longitudinal flow of material, formed from sieve elements placed end to end
- ends perforated by pores= sieve plates
- cytoplasmic filaments containing phloem proteins extedn through the pores between sieve tubes
- sieve tubes have no nucleus or organelles so are accompanied by companion cells with dense cytoplasm, large central nucleus and an abundance of mitochondria
- connected to sieve tube by plasmodesmata
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Transport In Phloem
- ringing experiments
- cylinder sof outer bark removed from tree tissue (therefore phloem) contents above and below analysed later
- radioactive tracing
- aphids mouth stylet removed from head when feeding from sap
- high pressure sap leaks through tube and is collected and analysed
- found transocation to be too rapid a process to be explained by diffusion
- radioisotope labelling
- CO2 with radioactive carbon supplied to plant leaf
- carbon then fixed in a sugar in photosynthesis and its translocation can be traced using autoradiography
- source and sink tissues placed on photgraphic film and developed
- presence of radioactivity shows as fogging
- found sugars are transported in both upwards and downwards
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Theories Of Translocation
mass flow hypothesis
- passive mass flow of sugars from phloem in leaf where there is highest concentration to other areas of lower concentration such as growing tissues
- sugar made at leaves causes WP to become more negative and water flows in, building hydrostatic pressure and forcing the sugars into the phloem joined to the sink
- mass flow of solutes into the sink forced water out into xylem bringing water back to th leaves
arguments
- rate of transport is way more faster than diffusion
- doesnt explain the existence of sieve plates which in this theory would act as a barrier
- sucrose and amino acids move at different rates and directions in the same tissue
- phloem has high rate of O2 consumption, translocation is stopped in the presence of respiratory inhibitors
- companion cells have load of mitochondria for energy but MFH has no explanation for this
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New Theories
- active processes are involved
- observed streaming in cytoplasm of individual sieve tubes= bi-directional movements
- different solutes transported in different routes= protein filaments pass through sieve pores
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