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Transport in Plants
Description
A-Levels Biology f211 Mind Map on Transport in Plants, created by Gemma Bradford on 12/05/2013.
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biology f211
biology f211
a-levels
Mind Map by
Gemma Bradford
, updated more than 1 year ago
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Gemma Bradford
over 11 years ago
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Resource summary
Transport in Plants
Xylem
Transports water and mineral ions up a plant's stem to leaves
Root - xylem found in centre as a x
Provides support for root
Stem - xylem found toward the outside
Reduces bending
Leaf - xylem makes up network of veins
Supports leaves
Adaptations
Xylem vessels are long tube like structures
Formed from vessel elements joined end to end
No end walls
Allows water to pass up through middle of cell easily
Dead cells, containing no cytoplasm
Woody substance lignin thickens cell walls
Stops walls collapsing inwards
Lignin in spiral patterns
Flexibility
Prevents stem from breaking
Amount of lignin increases as cell gets older
Supports xylem walls
Small pits in walls where there is no lignin
Allows water and mineral ions to move in/out of vessels
Phloem
Transports dissolved substances - sugars (like sucrose) up and down a plant
Root - phloem found in centre outside of xylem
Supports root
Stem - phloem on outside
Reduces bending
Leaf - phloem on inside making up network of veins
Supports leaves
Adaptations
Formed of cells arranged in tubes
Purely transport tissue
Contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
Not used for support
Sieve tube elements
Living cells forming tube for transporting sugars through plant
Joined end to end to form sieve tubes
Sieve parts are end walls
With holes to allow sugars to pass through them
Sieve plates
No nucleus, thin layer of cytoplasm and few organelles
Sieve tube elements cannot survive on their own
Cytoplasm of adjacent cells is connected through holes in sieve plates
Companion cells
Companion cell for every sieve tube element
Helps it survive
Carries out living functions for both itself and sieve tube elements
Provides energy to actively transport sugars
Multicellular organisms
Small surface are to volume ratio
Uptake water, minerals and sugars
Get rid of waste substances
Diffusion would be too slow without a transport system
Water transport
Entering plant
1) Water enters through root hair cells
2) Passes through root cortex and endodermis
3) Reaches xylem
Water moves down a water potential gradient
Soil around roots has high water potential
Leaves have lower water potential
Water constantly evaporates from them
Water potential difference creates a gradient
Keeps water moving through plant from roots to leaves
Through roots
Water travels by osmosis through roots via root cortex, into xylem by pathways
Symplast pathway
Goes through living parts (cytoplasm) of cells
Cytoplasm of neighbouring cells connect through plasmodesmata
Small channels in cell walls
Apoplast pathway
Goes through non-living parts of cells (cell walls)
Walls absorbant = easy diffusion
When reaching endodermis cells in root, path blocked by casparian strip
Waxy, waterproof strip in cell walls
Now takes symplast pathway
After this water moves into xylem
Useful as water has to go through cell membrane first
Control if substances in water get through
Through leaves
Water leaves xylem and moves into cells by apoplast pathway
Water evaporates from cell walls into spaces between cells in leaf
Stomata open = water moves out of leaf as water vapour into air
Down water potential gradient
Transpiration
Up plant
Water movement from roots to leaves = transpiration stream
Cohesion
Water molecules stick together
Causes column of water in xylem to move upwards as one
Tension
As water evaporates from leaves, this creates suction at top of xylem
Pulls more water up into leaf
Movement of water causes more water to enter stem through root cortex cells
Adhesion
Water molecules attracted to walls of xylem vessels
Helps water rise up through vessels
Air bubbles in xylem can block column of water from reaching cells
Transpiration
Evaporation of water from a plant's surface
Happens as a result of gas exchange
Plant need to open stomata to let in CO2 for photosynthesis, to produce glucose
Opening stomata lets water out
Higher concentration of water inside leaf than in surrounding air
Water moves out down water potential gradient
Factors affecting transpiration rate
Light
More light = faster transpiration rate
Stomata open wider the lighter it gets
Allowing more water to evaporate
Dark = stomata closed
Little transpiration
Temperature
Higher temp = faster transpiration rate
Warmer water molecules have more energy = evaporate from cells inside leaf faster
Increases water potential gradient between inside and outside of leaf
Water evaporates out of leaf faster
Humidity
Lower humidity = faster transpiration rate
Water potential gradient increases between leaf and air if air surrounding plant is dry
Lower in air, higher in leaf
Moves down water potential gradient
Increase in water potential gradient = increases transpiration
Wind
Increase in wind = faster transpiration rate
Air movement blows away water molecules from around stomata
Water potential in air is now more lower than leaf
Increases water potential gradient
Increases rate of transpiration
Potometers
Estimates transpiration rates
Measures water uptake by a plant
Use to estimate how different factors affect transpiration rates
Process
1) Cut shoot underwater at a slant
Prevents air entering xylem which can block water column
Increases surface area for water uptake
2) Assemble potometer in water and insert shoot underwater
3) Remove potometer from water, but keeping end of capillary tube in beaker of water
4) Check potometer is watertight and airtight
5) Dry leaves
Make sure water potential around leaves isn't higher than it should be
6) Allow time for shoot to acclimatise, then shut tap
7) Remove end of capillary tube from beaker until one air bubble has formed
Then put end of tube back into water
8) Record starting position of air bubble
9) Start stopwatch and record distance the bubble moves per unit of time
Keep conditions constant throughout experiment
Factors etc
Xerophytic plants
Examples
Cacti
Pine trees
Prickly pears
Stomata are sunk in pits
Trapping water vapour
Reduce transpiration by lowering water potential gradient
Hair layer on epidermis
Traps moist air round stomata
Reduces water potential gradient between leaf and air
Curled leaves
Traps moist air
Lowers exposed surface area for losing water
Protects stomata from wind
Less stomata
Fewer places for water vapour to diffuse out of leaf
Thick waxy layer on epidermis
Layer is waterproof
Water cannot pass through to evaporate
Spines for leaves
Reduces surface area for water loss
Translocation
Movement of assimilates in plant
Sugars
Requires energy
Happens in phloem
Moves assimilates from source to sink
Source = where assimilates are produced
High concentration
Sink = where assimilates are used up
Low concentration
Enzymes maintain concentration gradient from source-sink
Changing dissolves substances at sink into something else/by breaking them down
= lower concentration at sink than source
Examples
Sucrose
Source = leaves
Sink = food storage organs and meristmems in roots/stems/leaves
Potatoes
Sucrose converted to starch in sink
Lower concentration of sucrose at sink than in phloem
Mass flow
Hypothesis
1) Source
Active transport loads assimilates into sieve tubes of phloem
Lowering water potential inside sieve tubes
Water enters tubes by osmosis
Creates high pressure inside sieve tubes at source end of phloem
2) Sink
Assimilates removed from phloem to be used up
Increases water potential inside sieve tubes
Lowers pressure inside sieve tubes
3) Flow
Result of pressure gradient from source end to sink end
Gradient pushes assimilates along sieve tubes to where they are needed
Evidence
Support
Removing ring of bark from woody stem = bulge forms above ring
Includes phloem but not xylem
Fluid in bulge has higher concentration of sugars than below the ring
Sugars cannot move past the area where bark has been removed
Evidence downward flow of sugars
Investigating pressures in phloem using aphids
Aphids pierce phloem, bodies removed, mouthparts left behind allowing sap to flow out
Sap flows out quicker nearer leaves than further down stem
Evidence for pressure gradient
Metabolic inhibitors in phloem = translocation stops
Inhibitors stop ATP production
Evidence active transport is involved
Against
Sugar travels to many different sinks
Not just to one with highest water potential
As model suggests
Sieve plates would create barrier to mass flow
A lot of pressure would be needed for assimilates to get through at reasonable rate
Experiment
Showing mass flow hypothesis
Two containers A and B
Lined with selectively permeable membrane like cells
Top tube connecting A and B represents phloem
Bottom tube connecting A and B represents xylem
A = source end
Contains concentrated sugar solution
B = sink end
Contains weak sugar solution
Water enters A by osmosis
Increases pressure = sugar solution flows along top tube
Pressure increases in B
Forcing water out and back through bottom tube
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