3.1.3 d transport of water Flashcards
adaptations of root hair cells
microscopic size so can penetrate soil easily
each hair has a large surface area to volume ratio and there are thousands of hairs growing on each root
thin surface layer
concentration of solutes in the cytoplasm of the root hair cells maintains a water potential gradient
how does water get into the xylem from the root hair cells?
symplastic and apoplastic pathway
what is the symplastic pathway?
water moves through the cytoplasm of the living plant cells that is connected through the plasmodesmata by osmosis
the water moves from the high water potential of one cell to the low water potential of the neighbouring cell
what is the apoplastic pathway?
water moving through the cell walls and the intercellular spaces
how does the water move into the xylem?
water moves across the roots until it reaches the endodermis
water in the apoplast pathway can’t go any further because the casparian strip is waterproof so it is forced into the cytoplasm fo the cell joining the water in the symplast pathway
what is the endodermis?
the layer of cells surrounding the vascular tissue in the roots
what is the casparian strip?
band of waxy material called the Suberin which runs around each endodermal cell as a waterproof layer
why is it significant that the water in the apoplast pathway joins the water in the symplast pathway?
the water is forced through the cell surface membrane which means it stops any toxic solutes in the water from being transported to the living tissue as the membranes have no carrier proteins to let them through the membrane
what is root pressure?
an active process
it is a force that helps drive fluids upwards
endodermal cells move mineral ions into the xylem to help move water by osmosis
evidence that active transport is used for root pressure
cyanide stops the production of ATP during photosynthesis, if cyanide is applied to the roots, the root pressure disappears
root pressure increases with rise of temperature and decreases with the fall of temperature
if the level of oxygen or respiratory substrates falls, root pressure falls
what is transpiration?
loss of water from the leaves and the stems of the plant
what is the transpiration stream?
water moving up the xylem from the roots and leaving through the stomata in the leaves
what is the transpiration pull?
water is drawn up the xylem in a continuous stream to replace the water that is lost by evaporation
what is the cohesion-tension theory?
water molecules have dipoles which causes them to have an attraction
water is pulled up the xylem vessels by transpiration in a continuous column of water
what is capillary action?
adhesive forces between water molecules and narrow walls of the xylem that help pull the water up the xylem
evidence for cohesion-tension theory
when a xylem vessel is broken, air is drawn in rather than water coming out
if a xylem vessel is broken, water can no longer move up it
changes in the diameter of trees, when transpiration is at its highest the tree shrinks and vice versa when its at its lowest
factors affecting transpiration
light intensity
temperature
humidity
air movement
soil-water availability
light intensity
by increasing light intensity, the number of stomata that are open increases, this increases the amount of water vapour leaving the leaf which increases the rate of transpiration overall
humidity
when it is more humid, less water will leave the leaf because the concentration gradient of water potential decreases so the rate of transpiration will decrease
temperature
increase in temperature increases the kinetic energy of water molecules so it increases the rate of evaporation from the spongey mesophyll cells into the air spaces in the leaves
increase in temperature also increases the concentration of water vapour that the air can hold because it becomes saturated
air movement
every leaf has a layer of still air that is trapped around it by the shape of the leaf and hairs on the surface of the leaf decrease air movement close to the leaf
the water vapour diffuses out of the leaf and accumulates there which reduces the concentration gradient and reduces the diffusion gradient
air movement will increase the diffusion gradient by removing the water that accumulates there in turn increasing the rate of transpiration
soil-water availability
amount of water available to plants
if it is very dry, plant will be under water stress and the transpiration rate will reduce
what is water stress?
when the demand for water exceeds the available amount of water
how does water leave the stomata?
diffusion