3.3 Transport in plants

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When setting up a potometer, why is it important to cut the shoot underwater?
The xylem is under tension, so if the shoot was cut in the air, air would be drawn into the stem, breaking the column of water up the xylem. Cutting it underwater ensures the column of water is maintained
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Why is the potometer sealed with waterproof jelly?
Stops air being drawn into the xylem and prevents the breaking of the water column, as well as stopping water from leaking out that would cause inaccurate results
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What assumption is made if a potometer is used to work out rate of transpiration?
That all the water taken up by the plant is also transpired by the plant
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If a potometer was used to compare transpiration rate between two species of plant, what should be kept the same?
SA of the shoot cut as well and SA of the leaves
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What is the name of the plant transport system and why is this transport system needed rather than just relying on diffusion alone?
Vascular system; to meet metabolic demands, to transport materials from the top to the bottom and vice versa (esp. for growing), SA:V ratio is too disproportionate to rely on diffusion alone
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What are the two types of specialised vascular tissue that the transport system is made up of?
1. Xylem tissue: water and minerals, one way transport from the roots up 2. Phloem tissue: dissolved products of photosynthesis e.g. sucrose, multi-directional
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Name some of the adaptations of the xylem?
Dead cells aligned end to end to form continuous columns, narrow tubes w/ small lumen so column doesn't break and capillary action effective, bordered pits, lignification in patterns to allow flexibility
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What are xylem vessel elements?
The stacks of cells that form a continuous tube as their cell walls disappear, dead due to lignification, contains bordered pits
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What are tracheids?
Dead cells with lignified walls, do not have open ends so don't form vessels, tapered ends and certain plants rely more heavily on these than vessel elements
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What are the role of xylem fibres?
Help support and provide strength as they are lignified elongated cells
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What are the parenchyma cells?
Living cells, un-lignified, cell walls, can store water in vacuoles, making the plant turgid so provide support, act as packing tissue to separate/support tissues, no chloroplasts
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How is the flow of water not impeded in xylem vessels?
No cross-walls, no cell contents e.g. nucleus or cytoplasm as they are dead and lignification prevents collapse
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What is the main role of xylem vessels and what direction does it transport materials
Transports water and dissolved mineral ions e.g. K, Ca+ in one directions; from the roots to the leaf
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What is the main role of phloem vessels and what direction does it transport materials in?
Transports assimalates in both directions i.e. to growing parts of the plant or to the roots for storage
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What are assimalates and some examples of ones transported in the vascular system?
Soluble organic substances and examples would be sucrose and amino acids
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What are the sieve tubes in the phloem?
Made up of sieve tube elements lined up, sieve plates with large pores where STEs meet, few mitochondria/ER, no nucleus/ribosomes, only a thin later if cytoplasm, leaving space for mass flow
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What are the companion cells?
Normal plant cells w/ high numbers of mitochondria+ribosomes, plasmodesmata connecting to STEs to supply ATP/ other materials STEs cannot supply as they lack the organelles. Sit between STEs to support/supply them
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Name some differences between xylem and phloem
Xylem dead phloem living, xylem carries water/minerals to leaves phloem carries assimalates to growing parts AND storage parts, xylem lignified phloem normal cellulose cell walls, xylem no cytoplasm phloem does, xylem no cross walls phloem does
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What is transpiration?
The loss of water vapour from the aerial parts of a plant
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What is the transpiration stream?
The flow of water through a plant from the roots to the leaves via xylem vessels, where at the end water vapour leaves the leaf through open stomata
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What is a plasmodesmata?
A thin thread of cytoplasm that passes through the cell walls of adjacent cells, essentially cell junctions that water can pass through
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What are the three pathways water moves from the root hair cell through the cortex to get to the xylem?
1. Apoplast: most water, through spaces in cell walls, never enters plasma membrane/cytoplasm 2. Symplast: water enters cytoplast through plasma membrane from one plasmodesmata to next 3.Vacuolar: Symplast but also through vacuoles of cells
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Outline the pathway of water from the RHC to the xylem
RHC actively transport mineral ions in = more -ve Ψ=osmosis in, apoplastic/symplastic/vacuolar cortex by Ψ gradient=osmosis, blocked at epidermis carparian stripp (suberin), forced into symplastic, osmosis into xylem as Ψ more -ve in xylem (minerals)
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What are the main forces essential to drive the movement of water up the stem?
Root pressure, transpiration pull and capillary action
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Describe root pressure
Epidermis actively transport mineral ions in to medulla, reduces Ψ, water moves in by osmosis, increasing hydrostatic pressure. This can push water up a few meters, doesn't account for how it gets all the way to the top
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Describe transpiration pull
Water vapour evaporates from the top constantly, reduces hydrostatic pressure at the top, creating a pressure gradient that causes water to move up the xylem in an unbroken column
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Describe capillary action
As xylem vessels are narrow, waters natural adhesive properties attract to the lignin and aids pulling water up the sides of the vessel
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What two intermolecular forces does movement of water up the stem rely on?
Cohesion: attraction of water molecules to each other H bonds, allows long unbroken chains of water molecules for transpiration pull Adhesion: waters attraction to the sides of the vessels, makes capillary action possible
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What is cohesion tension theory?
The theory that states water moves up in one long unbroken column and can maintain this through bordered pits and the constant loss of water at the leaves
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How does water then leave the plant at the leaves
Transpiration: Water enters leaf from xylem, osmosis through spongy mesophyll via symplast/apoplast, osmosis to palisade cells (photosynth possible), some evaporates into internal air spaces, H2O(g) potential higher outside, moves down H20 pot. grad.
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What factors affect transpiration
Relative humidity, wind speed, temperature, light intensity, water availability
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What piece of apparatus can measure transpiration
A potometer
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Briefly outline the steps of setting up a potometer
Leafy shoot cut underwater diagonally (SA), potometer filled w/ H2O w/ no air bubbles, leafy shoot fitted to rubber tube to potometer, waterproof jelly sealed, air bubble introduced into capillary tube, distance moved measured for rate or volume
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What is 1. A mesophyte, 2. A xerophyte, 3. a halophyte and 4. a hydrophyte
1. adapted to adequate water i.e. not too much not too little 2. dry/arid areas 3. salty areas 4. very aqueous habitat
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What are some common adaptations of terrestrial plants to reduce water loss?
Waxy cuticles on leaves, stomata on underside of leaf, closed stomata at night, deciduous plants lose leaves in winter when water availability low
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What are some adaptations of xerophytes?
Reduced number stomata, swollen stems/leaves, huge root systems increase SA, CO2 intake at night to close stomata day, thick waxy cuticles, hairs on leaf surface, maintaining high salt conc. inside so Ψ gradient less steep to reduce transpiration
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Name some adaptations of marram grass
Leaf rolled longitudinally to trap air becomes humid reduces Ψ gradient, thick waxy cuticles, stomata located inner side of rolled leaf, sunken pits covered by hair reduce air movement, dense spongy mesophyll w/ little air spaces so less SA for evapo
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Name some adaptations of cacti
Fleshy swollen stems to store water, ribbed/fluted stems to allow expansion, leaves reduced to spines reduce SA, stem green for photosynthesis as leaves are spines, widespread root systems
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What issues would a hydrophyte face?
Getting O2 to submerged tissues, keeping afloat so exposed to sunlight for photosynthesis, no Ψ gradient for water to leave and stops mineral ion transport
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Name some adaptations of water lillies
Many large air spaces within the leaf allowing flotation to expose themselves to sunlight for photosynthesis, stomata located upper epidermis to expose to air so gaseous exchange continues
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What is translocation
The active mechanisms of transport of assimilates (mainly sucrose, amino acids and lipids) via the phloem in both directions
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Suggest a reason why glucose is transported as sucrose instead of glucose and what is sucrose
A disaccharide (glucose+fructose) making it more energy efficient, water soluble unlike starch but does not affect Ψ like glucose would
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What are the main stages of translocation>
Loading at the source, travelling through the stems via the sieve tubes and unloading at the sink
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Outline active loading at the source
Actively pump H+ out of companion cells, electrochem. grad. H+ can only move back to companion cotransport prot. with sucrose secondary active transport, sucrose conc companion high, down conc. grad into STE (via plasmodesmata)
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What are sources and sinks?
Source: Any part of the plant that loads sucrose into sieve tube. Sink: Any part of the plant that removes sucrose from sieve tube
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Why could the sources and sinks change seasonally?
Summer leaves (source) do loads of photosynthesis so excess sugars sent to the roots (sink) for storage but in early spring sugar needed for bud/flower/fruit growth, becoming sinks and roots become sources
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What is mass flow theory
How solute transport occurs in a plant, loaded at sources so Ψ more -ve, water moves in by osmosis, hydrostatic pressure increases at source, moves down pressure gradient to sink
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What evidence supports mass flow theory
Using radioactive tracers (C14) and radioactive imaging ringing experiments to remove phloem forms bulge, puncturing stem sap oozes out showing P, phloem sap high pH showing H+ transported out, loads of mitochondria in STE supplying ATP for active t
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Other cards in this set

Card 2

Front

Why is the potometer sealed with waterproof jelly?

Back

Stops air being drawn into the xylem and prevents the breaking of the water column, as well as stopping water from leaking out that would cause inaccurate results

Card 3

Front

What assumption is made if a potometer is used to work out rate of transpiration?

Back

Preview of the front of card 3

Card 4

Front

If a potometer was used to compare transpiration rate between two species of plant, what should be kept the same?

Back

Preview of the front of card 4

Card 5

Front

What is the name of the plant transport system and why is this transport system needed rather than just relying on diffusion alone?

Back

Preview of the front of card 5
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