transpiration Flashcards
(17 cards)
explain how water enters a plant root from the soil and travels through to the endodermis
water enters root hair cells by osmosis
because active uptake of mineral ions has created a WP gradient
water moves through cortex
down a WP gradient through symplastic pathway
through cell walls (apoplastic)
describe and explain how water moves via the apoplastic and symplastic pathways from the soil to the xylem in a root
apoplastic- via cell walls, spaces between cells
caused by transpiration pull and cohesion
symplastic- through CSM of epidermis from high to low via osmosis
explain how the structure of the endodermis affects the passage of water by this apoplastic pathway
casparian bonds
which are impermeable
lower water potential in the cytoplasm of endodermis cell
enters symplastic pathway
by osmosis
explain how the active transport of mineral ions into xylem vessels in the roots results in water entering these vessels then being moved up the xylem tissue
water potential in xylem reduced by entry of ions
water potential gradient established between xylem and surrounding cells
plasma membranes of surrounding cells are partially permeable
water enters xylem by osmosis
volume of water in xylem increases
cannot move back due to gradient
pressure in xylem increases and forces water upwards
describe one piece of evidence that supports the root pressure theory and explain how it supports this theory
sap exuding from a cut
only upward force could make this happen
explain how water enters xylem from the endodermis in the root and is then transported to the leaves
in root-
casparian strip blocks apoplast pathway-only allows symplast
active transport of ions to xylem by endodermis
lowers water potential in xylem
water enter xylem by osmosis down a water potential gradient
xylem to leaf-
evaporation/transpiration from leaves
creates cohesion tension- H bonding between water molecules/negative pressure
adhesion-water molecules bind to xylem
creates continuous water column
how to set up a potometer
-cut a leafy shoot from the plant under water then cut a small section from the stem again, this prevents air getting into the xylem and breaking the column of water. do not get leaves wet as this will affect the transpiration rate
-put the potometer under the water and move it from side to side to remove the air from the system and attach the leafy shoot into the potometer under water also
-seal all joins with with water proofing substance like petroleum jelly so that any water loss is from the leafs only
-ensure the environmental conditions dont change around the leaves as this will heavily influence the transpiration rate
-the air bubble introduce into the capillary tube can be timed as it moves across a set distance, thus the rate of water loss can be measured
how is the rate of water uptake calculated in a potometer
volume of water up taken by shoot x pir2 x distance travelled
divided by time taken
in a potometer, why might the actual rate of water uptake being measured not the same as transpiration
potometer assumes that all the water taken up is lost in transpiration but some will be used in photosynthesis, maintaining turgidity of cell and used in hydrolysis reactions
some water may have leaked out of the apparatus
factors affecting transpiration
light- light stimulates the stomata to open allowing gas exchange for photosynthesis, and as a side effect this also increases transpiration. this is a problem for some plants as they may lose water during the day and wilt
temperature- high temperature increases the rate of evaporation of water from the spongy cells and reduces air humidity, so transpiration increases
humidity- high humidity means a higher water potential in the air, so a lower water potential gradient between leaf and air, so less evaporation
wind- wind blows away saturated air from around stomata, replacing it with drier air, so increasing water potential gradient and increasing transpiration
a student places a plant in conditions of high humidity. what happens to the rate of transpiration and why
transpiration rate decreases because the water potential gradient between the leaf and surrounding air is reduced, so less water evaporates from leaf surfaces (stomata)
explain how closing stomata during the day helps a plant in drought conditions
it reduces water loss by limiting evaporation through stomata, conserving water under low availability conditions
why is the cohesion-tension theory important in explaining how water moves up a tall tree
water molecules stick together via cohesion, forming a continuous column pulled up by tension created from transpiration at the leaves
a student accidentally damages the xylem in a plant. predict what happens to the transpiration stream
water movement is disrupted, so the plant cannot transport water effectively to the leaves, leading to wilting and reduced transpiration
if a plant is placed under a bell jar with a drying agent, what would happen to the transpiration rate
it would increase because the surrounding air becomes very dry, steepening the wpg from the leaf to the air
explain how water moves from the soil to the air surrounding the leaves in the transpiration stream, including the roles of cohesion and tension
water enters root hair cells by osmosis due to the lower water potential inside the root cells compared to the toil. it then moves through the cortex of the root, via apoplast and symplast pathways, into the xylem vessels. in the xylem, water is pulled upwards due to transpiration at the leaves, which creates a negative pressure (tension). water molecules are cohesive due to hydrogen bonding, so they stick together and form an unbroken column. as water evaporates from the mesophyll cell walls into the leaf air spaces and diffuses out of the stomata, this pulls more water up through the plant in a continuous stream, driven by the cohesion tension mechanism
a plant kept in warm, dry, windy conditions begins to wilt despite adequate water in the soil. explain physiological cause of wilting and the environmental factors contributing
in warm, dry, windy conditions, the rate of transpiration increases due to higher evaporation rates and stronger water potential gradients between the leaf and air. if the rate of water loss through transpiration exceeds the rate at which water is absorbed by the roots, the cells lose turgor pressure. as a result, the plants non-woody tissues become flaccid and the plant wilts. wilting also helps to reduce surface area exposed to light and air, acting as a short term response to conserve water
