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