6. Transpiration, Stomata, guard cells Flashcards
what is transpiration
transpiration is the lost of water vapour by evaporation through the leaves.
what is the transpiration stream
the transpiration stream is the movement of water through a plant from roots to the xylem to the stomata on the leave surface.
what is the transpiration stream function
As well as supplying water to all the plant cells, the transpiration stream also delivers nutrients salts.
The evaporation of water from the leaves has a cooling effect.
functioning of the transpiration stream
root hairs increase the surface area and have a relatively low water concentration so water moves from the soil into the root hair by osmosis.
water then moves into the cell cortex by osmosis.
Water is then forced from the cortex into the xylem vessels. Ions are pumped into the xylem by active transport, then water follows by osmosis.
This causes pressure that causes water upwards the xylem (root pressure) .
Air spaces in the leaves are saturated with water vapour, this water can evaporate through stoma on the leave surface to the atmosphere.
This allows more evaporation from the cells which causes the cells have to have a lower water concentration, which makes water enter by osmosis from the xylem vessels.
Adhesion & cohesion of water
water itself has properties, that means it can move through the xylem as a continual water column
the ADHESION pf water to the walls of the xylem causes capillarity -> aids transpiration.
the COHESION of water molecules together is strong & allows them to form narrow columns of water that stick together, do not break apart even in the tallest of trees.
the rate of transpiration
the rate of transpiration can be affected by changes in temperature, humidity, windspeed, surface area & leaf etc.
Factors which cause a increase & decrease in transpiration.
Higher temperature INCREASES transpiration.
Greater windspeed INCREASES transpiration.
Larger surface area increases transpiration.
Greater humidity decreases transpiration.
what are stomatal mechanisms
stomata are pores found on the leaf surface .
In grasses (monocotyledons) stomata are found equally on both surfaces.
In plants, like heathers (dicotyledons) stomata are found on the lower surface.
open stomata are required for gas exchange needed for photosynthesis.
Plants use vast quantities of water through transpiration so it makes sense to close the stomata when the plant is not synthesizing.
Guard cells
stomata opening/closing are controlled by a special cell called a guard cell.
Guard cells contain chloroplasts, they also have a sausage shape & a thicker, inner wall.
flaccid guard cell
when water enters the flaccid guard cell, the thicker inner walls do not stretch as much as the outer thinner walls, the cell becomes turgid and the stomata opens.
Turgid guard cells open the stomata.
Flaccid guard cells closes the stomata.
summary : water diffuses into the guard cell which causes stomata to open. On hot/dry days the guard cells has less water, they relax and the stomata closes.
adaptions of xerophytes
xerophytes are plants adapted to survive in dry habitats. These adaptations include;
* Reduced surface area
* Reduction of transpiration by rolled leaves, hairy leaves, stomata in pits, and having pine needles and spines instead of leaves.
* Thick waxy cuticles
* Deep tap roots to seek ground water
* Wide spread surface roots to maximise water collection in case of rain
* Succulent water storing tissues
* Stomata with reversed opening (open at
night, closed during the day)
adaptations of hydrophytes
hydrophytes are plants that are adapted to live in freshwater
Adaptations include;
* Air spaces in stems to give buoyancy
* Long flexible leaf stalks (petioles) to
adjust to changes in water levels.
* Floating leaves with stomata on top surface.
* Little or no xylem
what is mesophytes
mesophytes is the name given to normal plants.
