transpiration Flashcards
(35 cards)
transpiration
the loss of water vapour from the aerial and upper parts of the plant mostly through the stomata in the leaves
where does transpiration occur
in the underside of the leaf (many stomata in the epidermis)
what does the waxy cuticle limit
water evaporating from the upper surface of the leaf
when does most water vapour leave
it leaves the stomata which opens allowing gas exchange for photosynthesis.
when is majority of the water vapour lost and why
since photosynthesis occurs when there is sufficient light, the majoirty of water vapour is lost during the day
what is a potometer
a device that can measure the rate of water uptake as a leafy stem transpires
what is pathway of water leaving the leaf
1) water enters the leaf through the xylem and moves by osmosis into the cells of the spongy mesophyll. it may also pass along the cell via the apoplast pathway
2) water evaporates from the cell walls of the spongy mesophyll
3) water vapour moves by diffusion out of the leaf through the open stomata. this relies on a difference in the concentration of water molecules in the leaf compared with outside the leaf (water vapour potential gradient)
where must the less negative water potenial be
there must be a less negative (higher) water potential inside the leaf than outside
what is an inevitable consequence of gaseous exchange
transpiration
what must happen as water vapour must be lost from the leaf
it must be replaced from below. this draws water up the stem as a transpiration stream
what does the transpiration stream do
- transports useful mineral ions up the plant
- maintains cell turgidity
- supplies water for growth, cell elongation and photosynthesis
- supplies water that as it evaporates, can keep the plant cool on a hot day
how do most plants control their water intake
by opening and closing their stomata
how do plants open and close their stomata
it happens when water levels change in the guard cells around each stoma. this occurs passively by osmosis or by active transport of solutes
what can cause transpiration rates to vary
environmental factors
what is the effect of light intensity on transpiration
high light intensity results in the stomata opening so that gaseous exchange can occur providing plants with carbon dioxide for photosynthesis. open stomata increases rate that water vapour diffuses out the leaf so rate of transpiration is faster
how does temperature affect light intensity
high temperature increases kinetic energy of water molecules which increases the rate of evaporation from mesophyll cells to air spaces in the leaf. high temperature also increases saturation point (how much water it can hold) of the air outside the leaf. it causes a steeper water potential gradient making transpiration faster
how does relative humidity affect transpiration
high atmospheric humidity will reduce the water potential gradient between the air spaces in the leaf and the air outside so the rate of transpiration is slower (diffusion of water vapour occurs more slowly). low humidity = iless water vapour molecules outside the leaf so water potential gradient is steeper and transpiration is faster
how does wind speed affect transpiration
higher wind speed increases the layer of still air around the stomata (where diffused water vapour accumulates). it causes water potential between air spaces in leaf and air outside to become steeper. rate of transpiration increases
how does water availability affect transpiration
low rainfall and dry conditions, transpiration rate increases to a certain point as it plays an important role in cooling the leaves. the evaporating water from cell walls absorbs heat energy from cells reducing temperature in low rainfall so water potential is steep. if little water is available in soil, the rate of transpiration exceeds the rate of uptake of water, the plant wilts and closes its stomate and decreases the rate of transpiration
how are leaves adapted
they have a large surface area for capturing sunlight and carrying out photosynthesis. their surfaces are covered with a waxy cuticle making them waterproof. it prevents water loss from evaporation
what is the process of transpiration
carbon dioxide moves the from the air into the leaf and oxygen moves out of the leaf by diffusion down the concentration gradient through the stomata. when the stomata opens for gas exchange water vapour is out and lost
what do guard cells do
surround the stomatal opening to open and close the stomata
why do stomata open and close
to control the amount of water lost in a plant but during the day a plant needs to take in carbon dioxide for photosynthesis and at night when no oxygen is being produced by photosynthesis it needs to take in oxygen for cellular respiration
how is the rate of transpiration measured
potometers
