Module 3.3 Transport in Plants Flashcards
Xylem tissue
Transports water + soluble mineral ions upwards
Phloem tissues
Transport assimilates such as sugars up or down
Vascular bundles in the root
Found in centre
Xylem in X shape
Phloem between arms of xylem’s X shape
Ring of endodermis around vascular bundle
Meristomatic pericycle located on inside of endodermis
Vascular bundles in the stem
Found near outer edge of stem
Xylem on inside of vascular bundle
Phloem on outside
Cambium between xylem and phloem (meristomatic cells)
Vascular bundles in leaves
Form veins
Xylem above phloem
Xylem is wider and often stained darker
Leaf structure🍃
Waxy cuticle Upper epidermis Palisade layer Xylem Phloem Spongy mesophyll Air spaces Lower epidermis Stomata Guard cells
🌟Transpiration process
Water evaporates from mesophyll cells (surface) in leaf - water vapour formed
Water vapour diffuses from high water pot. to lower water pot. out the leaf via the stomata
Water drawn from mesophyll cells via symplast/apoplast pathways - replaces water lost (osmosis down water pot. grad.)
Water moves out of xylem vessels via osmosis to replace water lost
Low hydrostatic pressure at top of xylem
Water moves from high pressure (roots) to low pressure down pressure gradient under tension i.e. Water moves from roots to top of xylem
This movement is mass flow
Cohesion between water molecules causes long unbroken column of water - transpiration stream
Water moving up the xylem is helped by
Root pressure
Capillary action
The transpiration pull
Adaptions of root hair cell
Large SA for osmosis and mineral uptake
Thin walls = short diffusion path
Lots of mitochondria = lots of energy for active trans.
Lots of aquaporins for uptake of water
Why does water enter the root hair cell?
Minerals are actively transported in
This decreases the water pot. of the root hair cell below that of the soil
Water move in via osmosis down a water pot. grad. and across the plasma membrane
Apoplast pathway
Water travels through gaps between the cellulose fibres
Minerals transported
Doesn’t cross membranes!
Symplast pathway
Water travels through the cytoplasm
Can move through plasmodesmata
Vacuolar pathway
Basically the exact same as the symplast pathway but water can also cross through vacuoles
The casparian strip
On the cell walls of the cells of the endodermis
Made of suberin
Block apoplast pathway
Water must take symplast pathway
Minerals actively transported through via carrier proteins
Water pot. lowered
Water moves in via osmosis
🌟Xylem vessel structure
Continuous hollow tubes (no contents) - less resistance to water flow, more space
Walls impregnated with lignin - strengthens walls (prevents collapse), waterproofs wall (reduces lateral movement of water), Increases adhesion - increases capillarity
Spiral pattern of lignin - flexibility
Bordered pits - allows lateral movement of water to get around a blockage
Narrow lumen - more effective capillary action
🌟Structure of sieve tube elements in phloem tissues
Small cytoplasm + most organelle absent - less resistance, more space
Sieve plates - allows sucrose through
Joined end to end to form tube - continuous transport
Bi-directional flow - sucrose can go up and down
Living - active processes can take place
🌟Structure of companion cells in phloem tissue
Lots of mitochondria - lots of respiration, allows active processes to occur e.g. active loading of sucrose into sieve tubes
Nucleus - controls companion cell and sieve tube element
Plasmodesmata - allows continuation of cytoplasm between companion cell and sieve tube element
Water always moves from
A region of higher water potential to a region of lower water potential
The importance of transpiration
Transports minerals up the plant
Maintains cell turgidity
Supplies water for growth, cell elongation and photosynthesis
Keeps the plant cool on a hot day
🌟How does light intensity affect rate of transpiration?
In light, the stomata open to allow gaseous exchange for photosynthesis. Higher light intensity increases the rate of transpiration because there will be a larger surface area for water to evaporate out
🌟How does temperature affect the rate of transpiration?
It increases the rate of transpiration in three ways:
1) increases the rate of evaporation from the cell surfaces - the water vapour potential in the leaf rises
2) increases the rate of diffusion through the stomata because the water molecules have more kinetic energy
3) decreases the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf
🌟How does relative humidity affect rate of transpiration?
Higher humidity will decrease the rate of water loss. There will be a smaller water vapour potential gradient between the air spaces in the leaf and the outside air.
🌟How does wind affect rate of transpiration?
Wind will carry away water vapour as soon as it has diffuses out of the leaf. This maintains a high water vapour potential gradient.
🌟How does water availability affect rate of transpiration?
If there is little water in the soil then the plant cannot replace the water that it has lost.
🌟How to set up a potometer
Set it up underwater
Get rid of all air bubbles
Cut the stem underwater and at an angle
Put the stem into the potometer and take the potometer out of the water, ensuring it is full of water and has no air bubbles
Dry the leaves
Allow time to acclimatise
Introduce an air bubble into the capillary tube
Measure rate of transpiration e.g. by how long it takes the bubble to move a set distance
Most terrestrial plants are adapted to reduce water loss by…
Having a thick waxy cuticle - reduces water loss due to evaporation through the epidermis
Stomata often found on the underside of the leaf - reduces evaporation
Most stomata closed at night - no light for photosynthesis
Deciduous plants (i.e. plants that lose their leaves in winter) - temp. may be too low for photosynthesis
🌟Adaptions of xerophytes
Leaf rolled - traps air inside, air becomes humid, water vapour collects, inc. water potential in space, dec. water potential grad. between air space and outside the leaf, reduces water loss
Thick waxy cuticle on upper epidermis - hydrophobic, reduces evaporation
Stomata in sunken pits - reduces air movement, air becomes humid, water vapour collects, inc. water potential in space, dec. water potential grad. between air space and outside the leaf, reduces water loss
Hairs - traps air inside, air becomes humid, water vapour collects, inc. water potential in space, dec. water potential grad. between air space and outside the leaf, reduces water loss
Spongy mesophyll - v dense, few air spaces, less SA for water loss
Small leaves - small SA, fewer stomata, less evaporation, less transpiration
Stomata shut in day but open at night - warmer during the day, more transpiration occurs during the day so shutting the stomata in the day reduces this loss
Cacti adaptations 🌵
Corrugated surface - allows expansion to store even more water
Spines NOT leaves - reduces SA and total SA, less water lost by transpiration
Green stem - for photosynthesis
Roots - widespread, max. water uptake if it rains
Adaptions of hydrophytes
Air spaces (large) - buoyancy, keeps leaves afloat so they are in the air and can absorb sunlight
Stomata on upper epidermis - exposed to air for gas exchange
Leaf stem has many large air spaces - buoyancy, allows O2 to diffuse quickly to the roots for aerobic respiration
Active loading (how sucrose enters the phloem from the source)
H+ ions actively transported out of the companion cell using ATP
This creates a conc. grad. for H+ ions into surrounding tissue
The high conc. of H+ ions causes facilitated diffusion back into the companion cell. Sucrose is carried w/H+ ions through the cotransporter (protein) and into the plasma membrane. This process actively loads sucrose
There is a higher conc. of sucrose in the companion cell compared to the sieve tube element causing it to diffuse through the plasmodesmata into the sieve tube element down the conc. grad.
Any growing part of a plant =
A sink e.g. Flowers or growing tips
Starch storage molecules
Can be a source and a sink
🌟How sucrose moves along the phloem at the source
Sucrose actively loaded into sieve tube elements @source
Reduces water pot. in sieve tube element
Water enters sieve tube elements by osmosis
Hydrostatic pressure of sieve tube element near source increases
🌟How sucrose moves along the phloem at the sink
Sucrose unloaded at the sink by diffusion or active transport and used in respiration or is stored
Water pot. of sieve tube element increases
Water moves into the sink from the sieve tube element via osmosis down the water pot. grad
The hydrostatic pressure in the sieve tube element near the sink decreases
Water in the sieve tube element at the source moves down the hydrostatic pressure gradient from source to sink
This creates a mass flow of sucrose and other assimilates along the phloem either up or down the plant
🌟If a ring is cut around the bark of a tree, a swelling may occur above the ring. Why?
The phloem is in the bark so sucrose cannot pass the cut
The area above the cut acts as a sink so water moves into cells
Damage causes more cell divisions to produce cells to store sugars
Cuts cause infection
🌟Evidence for translocation: How we know the phloem is used
Radioactively labelled CO2 supplies for photosynthesis appear in the phloem
Aphids that feed on plant stems insert their “mouth” into the phloem
Sugars collect above the ring when a tree is ringed to remove the phloem
🌟Evidence for translocation: ATP is needed
Companion cells have many mitochondria
Translocation stops if a poison stopping ATP production is given
Flow of sugars is v high - ATP must be used - diffusion would be much slower
🌟Evidence for translocation
The pH of companion cells is higher than the surrounding cells (and H+ ions reduce pH)
The conc. of sucrose is higher in the source than in the sink
🌟Evidence against translocation
Not all solutes in the phloem move @ the same rate
Sucrose is moved to all parts of the plant at the same rate and doesn’t move to places with the lowest conc. faster
The role of sieve plates is unclear
🌟Why can a potometer not accurately measure rate of transpiration?
Transpiration = loss of water by evaporation via the stomata
A potometer measures rate of water uptake to replace water loss
Some water will be used by the plant for photosynthesis and other processes rather than all evaporating from the leaves - not actual rate measured
Uptake by the detached shoot may not be the same as the uptake for the whole plant
Precautions for setting up a potometer to get a valid reading/readings?
Set up underwater so no air bubbles
Ensure shoot is healthy
Cut stem underwater
Ensure equipment is watertight
Allow time to acclimatise
Keep conditions constant
Dry leaves
🌟Summarise how water moves from the soil to the xylem
Minerals actively transported into root hair cells through carrier proteins
Water moves via osmosis from the soil into root hair cells across the cell surface membrane through aquaporins down the water potential gradient
Water transported via the apoplast pathway or via the symplast pathway (cytoplasm, through plasmodesmata)
At the endodermis, the casparian strip (made of suberin) blocks the apoplast pathway –> forces water to enter via the symplast pathway
Water pot. most neg. in the xylem due to active transport of minerals into it –> causes water to move into the xylem from the cells of the endodermis and cortex
🌟How does number of leaves affect water loss by transpiration?
More leaves = more water loss
Larger SA over which water can evaporate out of plant
More stomata
🌟How does number and size of stomata affect water loss via transpiration?
More/bigger stomata = more water loss
Larger SA over which water can evaporate out of plant via stomata
🌟How does the waxy cuticle affect water loss via transpiration?
The waxy cuticle reduces water loss
Hydrophobic
🌟Translocation/active loading
H+ ions actively transported (ATP required) out of companion cells
Diffusion gradient of H+ ions created
H+ ions move back into companion cells via facilitated diffusion through a co-transporter carrier protein along w sucrose
Sucrose has been actively loaded into the companion cell
High conc. of sucrose in the companion cell compared to the sieve tube element –> diffuses into sieve tube element down conc. grad. through plasmodesmata
🌟How the sieve tube elements are adapted to allow mass flow to occur
Elongated elements, joined end to end to form a column
Sieve plates with pores in end walls - allow sucrose through
Little cytoplasm + no nucleus - less resistance to transport
