2.3b - adaptations for transport in plants Flashcards
How is water transported?
water from the soil enters a plant through the root hair and then travels across the cortex of the root into the xylem vessels through the root into the stem and finally onto the leaves
Epidermis layer?
root hair cells have extensions on the outer epidermis layer
which extend outwards between the soil particle and increases the surface area for exchange of water and minerals
How does water enter the root hairs?
by osmosis
as surrounding soil solution has a higher water potential than there is inside the cytoplasm and vacuole of root hair cell
Alternatively?
water may enter root without entering cell at all
moving from cell to cell via fully permeable cell wall until reaches the centre of the root
How does water travel from the root hair cells?
across the cortex of the root into the xylem vessels down a water potential gradient
2 routes water can take through the cortex?
Apoplast pathway
Symplast pathway
Apoplast pathway?
water travels from cell wall to cell wall and through interconnecting cell spaces without ever entering the cell
Symplast pathway?
water enters the cell by osmosis
across selectively permeable membrane
and travels between adjacent cells via fine strands of cytoplasm called plasmodesmata
Importance of these pathways varies?
from one plant species to another and also depends on the environmental conditions in which the plant is found
When water reaches the stele?
apoplast pathway is abruptly stopped by a thick waterproof, waxy sybstance vcalled the suberin
suberin forms an impenterable band - casperian strip in the cell walls of the endodermis layer and forces any water or solutes to travel via the symplast pathway
arrangement gives plant control over the substances that enter the xylem as they must first cross selectively permeable membrane + prevents solutes freely entering xylem vessel
Within the spongy mesophyll?
the layers are irregularly packed and have large air space between them
the water in the cells seeps into the cell walls, so the cells are always wet
some of this water evaporates into the air spaces, so the air spaces become saturated with water vapour
Air spaces?
are in direct contact with the surrounding atmospheric air through stomatal pores.
If there is a water potential gradient between the air in the air spaces and the surrounding atmosphere, water can diffuse out of the leaf through the stomata.
Transpiration def?
loss of water vapour from the leaf surface
As water evaporates from the cell walls of the mesophyll cells?
water moves into them to replace it
water comes from xylem vessels in top of the leaf
as water is removed from the top of the xylem vessel in the leaf, the hydrostatic pressure is reduced at the top of the xylem
therefore a hydrostatic pressure gradient exists between the bottom and top of the xylem vessel, causing water to move by mass flow from high to low pressure
Transpiration stream?
the continuous movement of water molecules from the roots to the leaf
plant does not have to provide any energy for this - therefore passive process
Xylem vessels?
xylem vessels - made of many long narrow cells called xylem elements which are stacked end to end
Each xylem began as living cell=however died during differentiation
xylem vessels contain no living material
walls of xylem vessels are made up of cellulose and a strong waterproof substance called lignin
lignin = important in keeping water inside the xylem vessels but also contributes to supporting the plant
end walls of xylem vessels break down so stack of xylem elements form a continuous tube which runs all the way through the plant
xylem vessels are usually arranged in bundles
water can move between adjacent xylem vessels through pits in their walls
small gaps where no lignin has been deposited leaving just cellulose cell wall
originally plasmodesmata
How water travels from roots to leaf in the xylem?
as water enters xylem vessels in root, hydrostatic pressure is slightly raised
therefore pressure gradient exists between the top and bottom of the xylem vessels, causing water to move up in the xylem vessel by mass flow
increased pressure in the xylem vessels in the roots is thought to be generated by the endodermal cells actively transporting mineral cells into the xylem
creating a force known as root pressur
How does the water move?
as a continuous column
because the water molecules are attracted to each other ( cohesion)
andalso to the sides of the xylem vessels
( adhesion)
column of water - stretched as column = placed under tension
theory of mechanism = cohesion tension theory
Capillarity?
another force that may contribute to the rise of water in the xylem
water rises up narrow tubes by large capillary action but this force is probably of more relevance in small plants than large trees
Humidity?
in low humidity, there is a steep water potential gradient between the leaf and the air
Transpiration rates = therefore greater in low humidity conditions
Light intensity?
during light hours, stomata open to allow gaseous exchange for photosynthesis
wider the stomata - easier for water to be lost by transpiration
some plants will sacrifice their photosynthetic rate during their hottest parts of the day by partially closing their stomata to prevent wilting
Temperature?
increasing temp increases kinetic energy of water molecules
this therefore increases rate at which water molecules evaporate from the cell into the air spaces and also at rate at which water molecules diffuse out of the leaf
furthermore, water potential of atmosphere becomes lower as its temp is raised and it can hold more moisture
Air movement?
transpiration in still air results in the accumulation of a layer of saturated air at the surface of leaves
offers considerable resistance to the movement of water vapour through stomata
as it lowers the water potential gradient and thus reduces the rate of transpiration
movement of the surrounding air reduces the thickness of the layer of saturated air
results in increased transpiration
Comparing rates of transpiration using a potometer?
potometer actually measure the rate of water uptake but approx 99% of the water taken up by a leafy shoot is lost via transpiration, there is a strong correlation between the rate of uptake and the rate of transpiration
Potometer can be used to compare the rate of water uptake in different leafy shoots in response to the same environmental conditions
How is a potometer set up?
cut leafy shoot underwater
( the shoot could be cut with a slanted cut to prevent the formation of an air lock )
completely fill the apparatus with water, ensuring there are no air bubbles
insert the leafy shoot in the apparatus under water
remove the potometer and shoot from the water + seal joints with petroleum jelly ( vaseline - ensuring that its airtight +
watertight )
carefully dry leaves
introduce an air bubble into capillary tube
Before starting the experiment, allow the apparatus sufficient time to acclimatise before taking any readings. Using the water reservoir, bring the air bubble back to the start point prior to commencing the experiment
measure the distance the bubble moves in a given
time
using the water reservoir, bring the air bubble back to the start point and repeat the measurement a number of times and calculate a mean
the experiment may be repeated to compare the rates of water uptake under different conditions
It is important to remember to leave the apparatus for a short while when a variable has been altered and to reset the air bubble to the start position before taking any readings
What can plants be classified in ?
into 3 groups
Mesophytes - plants living in adequate water conditions
Xerophytes - plants living in conditions where water is scarce
Hydrophytes - water plants
Plants grow in temperate regions are mesophytes?
water they lose by transpiration is readily replaced by uptake from the soil, they do not require any special means of conserving water
if a plant loses too much water, it wilts and the leaves will droop
surface area of the leaves = greatly reduced and this will reduce the efficiency of photosynthesis
Mesophytes?
flourish in habitats with adequate water supply
most plants of temperate regions are Mesophytes and most importantly, most of our crop plants = mesophytes
They are adapted to grow best in well - drained soils and moderately dry air
Mesophytes lose a lot of water but excessive water loss is prevented by closure of the stomata
water uptake during the night compensates for the water lost during the day
mesophytes need to survive unfavourable times of the year, particularly when the ground = frozen
many trees and shrubs shed their leaves before the onset of winter
the aerial parts of many non-woody plants die off as a result of frost or cold winds but their underground organs survive e.g bulbs
most annual mesophytes survive the winter as
dormant seeds
Xerophytes?
they are plants that shoe xeromorphic adaptations
these plants have adapted to living in conditions of low water availability and have developed modified structures to prevent excessive water loss
they may live in hot, dry desert regions ; cold regions where the water is frozen for much of the year, exposed windy locations
Example of xerophyte?
marram grass
colonises sand dunes around the coast
sand dune makes it difficult for a mesophyte to survive there because there is no soil, rapid drainage of rain water occurs, there are high wind speeds, salt spray and a lack of shade from the sun
Marram grass adaptations?
rolled leaves - large thin walled epidermal cells at the bases of the grooves shrink when they lose water from excessive transpiration, causing the leaf to roll up
this reduces the surface area of the leaf which transpiration can occur
sunken stomata - stomata are found in grooves on the inner side of the leaf
they are located in pits or depressions so that humid air is trapped outside the stomata
this reduces the water potential gradient between the leaf and the atmosphere and reduces the rate of diffusion of water
Hairs - stiff, interlocking hairs trap water vapour and reduce water loss
the thicker the cuticle
the lower the rate of cuticular transpiration
Hydrophytes?
grow submerged or partially submerged in water
e.g water lily which is rooted to the mud and the bottom of a pond and has floating leaves on the surface of the water
Hydrophytes = adapted as followed
as water is a supportive medium, they have little or mno lignified support tissues
surrounded by a water, there is little need for transport tissue, so xylem is poorly developed
leaves have little or no cuticle
stomata are found on the upper surface of the leaves
stem and leaves have large air spaces, forming a reservoir of oxygen and carbon dioxide
these gases also provide buoyancy to plant tissues when submerged
What is translocation?
term used to describe the transport of soluble organic substances within a plant in the phloem tissue
these are assimilates such as sugars that have been made by the plant itself during photosynthesis
main substance transported in the phloem is sucrose
What does phloem tissue consist of?
sieve elements and companion cells
these cells work closely together to achieve translocation
Sieve elements?
are joined end to end to form a continuous column
( a sieve tube)
each sieve element is a living cell and contains a cell wall, a plasma membrane , endoplasmic reticulum and mitochondria
however, only has a small amount of cytoplasm and is devoid of a nucleus and ribosomes
How is a sieve plate formed?
when the ends of 2 sieve elements meet
this is made of the walls of both sieve elements perforated by large pores
What does each sieve element have?
at least 1 companion cell lying close beside it
What does companion cells have?
a typical plant cell structure
however, the number of ribosomes and mitochondria are much larger than usual and the cells are highly metabolically active
vacuole, however remains small and does not form a large central vacuole
there = a close association between the companion cells and the adjacent sieve elements with numerous plasmodesmata links through the cell walls, providing a direct pathway between the cytoplasm of the companion cell and the cytoplasm of sieve element
Translocation process?
it occurs by mass flow
the pressure gradients have to be established using some of the plants own energy
therefore, it is an active process
sucrose is actively loaded into the sieve elements at the place from which sucrose is to be transported
as sucrose is loaded into the sieve elements, the potential inside the sieve elements is decreased and water follows by osmosis
elsewhere along the sieve tube, sucrose may be removed by other cells
since these cells are usually using sucrose, these cells typically have a low sucrose concentration
sucrose diffuses out of the phloem into these cells
once again, this lowers the water potential of the recipient cell and water follows by osmosis
therefore, water enters the sieve tube in the leaf and water flows out of the sieve tube at any point where sucrose is unloaded
creating a hydrostatic pressure difference between the sieve tube in the leaf ( high pressure ) and elsewhere ( low pressure , in the roots )
pressure difference allows fluid in sieve tube to move by mass flow
What is a source ?
any region of the plant from which sucrose is loaded into the phloem
Sucrose?
an area that unloads the sucrose out of the phloem
Where does translocation occur?
from the source to the sink
Sinks?
can be anywhere in the plant, both above and below the photosynthesising leaves
therefore, phloem sap movement is bidirectional
( contrast with the unidirectional movement in the xylem)
Loading of sucrose into the phloem?
some of the sugars that are made in photosynthesis are converted into sucrose
sucrose = soluble and dissolves in the water in the cell
it can move out of the mesophyll cell and across the leaf by either the apoplast or symplast pathways
sucrose is actively loaded into companion cells in a convoluted way
Hydrogen ions are actively transported out of the companion cells, using ATP as an energy source
these hydrogen ions accumulate outside the cell to form a large gradient
they can only move back into the companion cells down their conc gradient - using a co-transporter protein
( a protein which carries both H ions and sucrose at the same time)
in this way, the sucrose molecules are carried into the companion cells against its conc gradient
sucrose molecules can then move into this adjacent sieve tube through the interconnecting plasmodesmata
Unloading of sucrose from the phloem?
sucrose moves out of the phloem by facilitated diffusion
once in the tissues, the sucrose is converted into something else by enzymes
( e.g enzyme invertase converts sucrose into glucose and fructose )
decreases the sucrose concentration inside the cell and maintains a steep conc gradient from phloem into the tissue
