9.1 Transport in the xylem of plants Flashcards
What is the primary organ of photosynthesis?
Plant leaves
What is the role of carbon dioxide and oxygen in photosynthesis?
Carbon dioxide is used as a raw material
Oxygen is produced as a waste product
What two gases are need to sustain photosynthesis?
the exchange of Co2 and oxygen
What are stomatas?
Pores through the epidermis (the outer layer of tissue in a plant) of the leaves
Why are stomatas needed in plants?
- absorption of carbon dioxide is essential
- but waxy cuticle has very low permeability to it
- stomata are pores that allow for the absorbtion of Co2
What is the problem for plants shown in this image?
if stomata allow carbon dioxide to be absorbed, they will usually also allow water vapour to escape
What substances do plants need to exchange with their surrounding and through what part of the plant? (4)
H2O
* osmosis (roots)
* Evaporation (stomata)
CO2
* Diffisuion (stomata)
O2
* diffusion (stomata)
Mineral ions
* active transport (root)
What is transpiration?
- the loss of water vapour from the leaves and stems of plants
- the inevitable consequence of gas exchange in the leaf
How do plants minimize water loss?
through stomata using guard cells
What are guard cells?
- cells that are found in pairs, one on either side of a stoma
- controls the aperture of the stoma and can adjust from wide open to fully closed
What are guard cells?
- cells that are foundin pairs, one on either side of a stoma
- controls the aperture of the stoma and can adjust from wide open to fully closed
What is the exception of stomata being found in nearly all groups of land plants for at least part of the plant’s life cycle?
a group called the liverworts
What are 2 sorts of transport in plants?
Xylem and phloem
What does the xylem transport and which direction?
Water and soluble mineral ions travel upwards
What does phloem transport and which direction?
Assimilates (sugars, amino acids from photosynthesis) and travels up and down
How are xylem and phloem arranged?
in vascular bundles
What are 3 models of water transport in the xylem?
- Porous pots
- Capillary tubes
- Blotting or filter paper
What does porous pots model?
Model evaporation from leaves
* water fills pores within the pot demonstrating adhesion to the clay molecules within the pot
* As the water is drawn into the pot, cohesion causes water molecules to be drawn up the glass tubing
What does capillary tubes model?
The capillary action
* water dyed with red is shown to be climbing the capillary tube due to the adnesion and cohesion property of water
* the mercury on the right shows none of these qualitites so there it does not climb the glass
What does using filter or blotting paper model?
Capillary action
* the ability of adhesive forces to result in the movement of water is demonstrated in this image
* a folded paper towl with one end immersed in water will transport water into an empty container by capillary action
What and how does a potometer measure?
measures water uptake
consists of a leafy shoot in a tube (right), a reservoir (left of shoot), and a graduated capillary tube (horizontal)
- A bubble in the capilarry tube marks the zero point
- as plant takes up water through its roots, the bubble will move along the capillary tube
- the progress of the bubble is timed along with the distance travelled
- the tap below the reservoir allows the bubble to reset and carry out new measurements
Equation to measure the volume of the capillary tube (cylinder)?
What does the structure of xylem vessels allow for?
allows them to transport water inside plants very efficiently
What are xylem vessels?
- long continuous tubes
- elongated dead cells but contain living parenchyma cells which act as packing tissue to separae and support the vessels
- thickened walls impregnated with a polymer called lignin
What is Lignin and its role?
Lignin (a polymer)
* deposited in xylem vessel walls in rings or spirals (allow felexibility)
* strengthens the walls so they can withstand very low pressures without collapsing
What are xylem vessels formed from?
from files of cells, arranged end-to-end
Why is lignin needed to provide strength to the xylem structure?
the pressure inside xylem vessels is usually much lower than atmospheric pressure so lignin provides a rigid structure that prevents the xylem vessels from collapsing
What happens to the cell wall, plasma membranes and contents of a cell in a flowering plant?
- cell wall material in some areas between adjacent cells in the file is largely removed
- plasma membranes and contents of the cells break down
What happens to a a mature xylem cells and what does this mean for the flow of water?
they are non living
Flow of water along them must be a passive process
What does xylem vessels contain that act as packing tissue to separate and support the vessels?
living xylem parenchyma cells
What do xylem parenchyma do?
- act as packing tissue
- living tissue
- connects two tissues together
How is water cohesive?
are polar and the partialy negative charge on the oxygen atom in one water molecule attracts the hydrogen atom in the neighbouring water molecule
How is water adhesive to cell walls of xylem?
water is attracted to the hydrophilic parts of the cell walls of xylem
Due to water’s connection between molecules and the cell wall, what does it allow?
water can be pulled up from the xylem in a continuous stream
What are bordered pits?
Gaps in the cell wall where lignification is not complete
What do bordered pits allow for?
water to pass between adjacent xylem vessels and into surrounding tissues
What happens when water evaporates from the surface of the wall in a leaf?
Adhesion causes water to be drawn through the cell wall from the nearest available supply (xylem vessels in the veins of the leaf) to replace the water lost by evaporation
How does adhesion during evaporation affect the pressure in the xylem when its already low?
the force of adhesion between water and cell walls in the leaf is strong enough to suck water out of the xylem, further reducing its pressure
What does the low pressure in the xylem of a plant generate?
a pulling force called transpiration-pull
What is transpiration-pull?
a pulling force generated by low pressure in the xylem
is transmitted through the water in the xylem vessels down the stem and to the ends of the xylem in the roots
strong enough to move water upwards, against the force of gravity
is a passive process
What is transpiration-pull?
a pulling force generated by low pressure in the xylem
is transmitted through the water in the xylem vessels down the stem and to the ends of the xylem in the roots
strong enough to move water upwards, against the force of gravity
is a passive process
How is transpiration pull a passive process?
- the energy needed is coming from the thermal energy (heat) that causes transpiration
- the pulling of the water upwards in xylem vessels depends on the cohesion that exists between water molecules
What is cavitation?
when the column of liquid would break in xylem vessels due to some liquids not being able to resist the very low pressures
* does occasionally happen even with water, but it is unusual
What is cavitation?
when the column of liquid would break in xylem vessels due to some liquids not being able to resist the very low pressures
* does occasionally happen even with water, but it is unusual
What are root hairs made up of?
root hair cells
How is water able to be absorbed via osmosis into root hair cells?
the solute concentration inside the root cells is greater than that in the water in the soil
How is the structure of root hair cells adapted for their function? (5)
- Vacuole stores mineral ions to decrease water potential -> increase rate of osmosis
- Thin cell walls -> short diffusion distance
- Long extensions -> increase surface area for absorption of water and minerals
- Lots of mitochondria -> to release energy for active transport
- Cell membrane contain transport proteins for mineral ions
What is the difference between vessel element and tracheid?
Vessel element:
* One continuous vessel
* end walls are fused
* faster rate of water transfer
Tracheid:
* 2 separate vessels that meet
* connected by bordered pits where the water can be transferred through
* slower rate of water transfer
How is water lost from transpiratin replaced?
- Water leaving through stomata by transpiration is replaced by water from the xylem
- Water in the xylem climbs the stem through the pull of transpiration combined with the forces of adhesion and cohesion
- water moves from the soil into roots by osmosis due to active transport of the minerals into the roots
- once the water is in the root it travels ito the xylem through cell walls (apoplast pathway) and through cytoplasm (symplast pathway)
What are plasmodesmata?
pits in the cellulose cell wall
What are the 3 pathways of water through a plant?
- Apoplast pathway
- symplast pathway
- Vacuolar pathway
What is the apoplast pathway?
- water does not pass through any cell membranes into cells
- fastest movement of water
What is the symplast pathway?
- water passes through cell membranes and cell walls (plasmodesmata)
What is the vacuolar pathway?
- water passes thrugh cell membranes and tonoplast (vacuole membrane)
- Slowest route
What is an example of indirect active transport?
Minerals binding to clay particles being absorbed
1. Hydrogen ions are actively transported out of the vacules of root cells and into the soil
2. H+ ions displace minerals from clay particles, which diffuse into root (move along gradiant)
Clay: negatively charge
Mineral ions: positively charged
Clay and mineral ions (K+, Na+, Mg2+) are attrached and bind
H+ released by the root cells displace the mineral ions
What is a casparian strip?
A strip of suberin (waxy substance) between cortex and xylem
* blocks apoplast pathway ensuring water and minerals pass through the cell membrane (which is partially permeable rather than fully permeable like cell wall)
* Contains starch granules -> need ATP for active transport of mineral ions from cytoplasm of cells into the xylem
What does the caspiarian strip containing starch granules suggest?
It needs ATP for active transport of mineral ions from cytoplasm of cells into the xylem
What does the caspiarian strip stop?
it stops water flow in apoplast pathway (in cell wall) so it must pass through the symplast pathway or vacuolar pathway
What characteristics do cacti have that allows it to survive dry habitats? (6)
- spines - leaves that are so reduced in size
- stem - contain water storage tissue and become swollen after rainfall
- Pleats - allow the stem to expant and contract in volume rapidly
- epidermis of cactus stem has a thick waxy cuticle
- stomata - more widely spaced than in leaves, usually open at night when its much cooler and transpiration occurs more slowly
- CO2 - absorbed at night and stored in the form of malic acid (4-carbon compound), released during the day to allow photosynthesis even when stomata is closed
What are the 3 xerophytic adaptation?
- closing stomata when water availability is low will reduce water loss and so reduce the need to take up water
- low water vapour potential inside leaf cells by maintaining a high salt concentration in the cells - reduces evaporation of water as the water vapour potential gradient between the cells and the leaf air spaces is reduced
- a very long tap root that can reach water deep underground
Xerophyte and halophyte adaptations
table
What is Crassulacean acid metabolism?
The system that xerophytes use
* CO2 is absorbed and stored in a form of malic acid (4-carbon compound) at night
* CO2 is released from the malic acid during the day to allow photosynthesis when the stomata is closed
What is Marram Grass (ammophila arenaria)?
A xerophyte
Describe the process of water movement through. a plant
- Water enters the leaf through the xylem
- moves by osmosis into the cells of the spongy mesophyhll
- it may also pass along the cell walls via the apoplast pathway
- water evaporates from teh cell walls fo the spongy mesophyll
- water vapour moves by diffusion out of the leaf through the open stomata
What is the water vapour potential gradient?
the difference in the concentration of water vapour molecules in the leaf compared to the outside of the leaf
Why is a water vapour potential gradient important?
Water vapour diffusing out of the leaf through the open stomata relies on the gradient
There must be a higher water vapour potential inside the leaf than outside
How may evaporation within the leaf tissue occur?
Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
What is ABA?
Abscisic acid
A plant hormone that is released by dehydrated mesophyll cells when a plant begins to wilt from water stress
What does ABA trigger?
the efflux of K+ ions from guard cells, decreasing water potential within the cells
What does the loss of turgor make the stomatal pore do?
Close, as the guard cells become flaccid and block the opening
How is water loss regulated by ABA?
- Plant begins to wilt from water stress
- dehydrated mesophyll cells release the plant hormone ABA Abscisic acid
- ABA triggers the efflux of K+ ions from guard cells, decreasing water potential within the cells
- A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
How does light intensity affect transpiration?
- as ligh intensity increases, the number of stomata open increases so rate of traspiration increases
- when all the stomata are open, the rate of transpiration will remain constant even when light intensity increases
How does air movement (wind) affect transpiration?
As wind speed increases, water vapour evaporated from the stomata is moved away from the air surrounding the leaf, maintaining a steep water vapour potential gradient, increasing the rate of transpiration
How does temperature affet transpiration?
as temperature increases, water molecules have more kinetic energy so evaporate faster, maintaining a steeper water vapour potential gradient, increasing the rate of transpiration
How does relative humidity affect transpiration?
The more humid the air, the higher the concentration of water vapour in the air, reducing the water vapour potential gradient between the air spaces in the leaf and the surrounding air, reducing the rate of transpiration
