Transport in Plants Flashcards
What is transpiration?
the evaporation of water from a plants surface.
this happens through the stomata
What is the function of the stomata?
they open to allow the entry of CO2 for photosynthesis
How is the rate of transpiration affected by light?
- during daylight the stomata open to let CO2 enter
- increases rate if transpiration as water evaporates from the mesophyll cells and diffuse out
How does temperature affect the rate of transpiration?
- increase in temp increases rate of transpiration
- water molecules are provided with more kinetic energy so the evaporate more rapidly m
How does humidity affect the rate of transpiration?
- increase in humidity increases water potential in air
- this decreases rate of transpiration
- water potential gradient for diffusion of water decreases
Hoe does air movement affect the rate of transpiration?
- Air movement removes water vapour from the leaf surface
- This increases the water potential gradient and rate of transpiration
How does still air affect rate of transpiration?
- in still water vapour will build up around the leaf
- this decreases the water potential gradient thus rate if transpiration
What is a Xerophyte?
a plant that possesses a range of adaptations that limit water loss via transpiration
What is the purpose of the thick cuticle on a Xerphyte?
- provides a long diffusion pathway
this reduces the rate if transpiration
What is the function of the Hairs in Xerophyte leaves?
- hairs on leaf surface trap layer of still air
- this becomes saturated with water vapour which reduces the water potential gradient for water loss
What is the purpose of the rolling up if leaves in Xerophytes?
- traps layer if still air which becomes saturated with water vapour
- reduces water potential gradient for water loss
- thus reducing transpiration rate
How does the position of stomata reduce water loss in Xerophytes?
- they’re positioned in epidermal pits/grooves beneath leaf’s surface
- this reduces exposure to air
- this trapped air becomes saturated with water vapour
- reduces water potential gradient for evaporation
Describe the structure of the Xylem
- no cell contents
- hollow tubes
- cell wall strengthened by lignin which makes cell rigid and provides support
Describe the Cohesion-Tension theory.
- water moves across leaf down water potential gradient
- water is drawn from xylem, this creates tension in xylem
How is the water column in the xylem maintained?
by cohesive and adhesive forces
What is cohesion?
attention of water molecules to each other by hydrogen bonds
What is adhesion?
attraction of the water molecules to the xylem walls
How is water taken from the soil?
- upward movement of water from the xylem in the roots maintain water potential gradient across root cells
- water is taken from soil via osmosis
Describe the evidence for Cohesion-Tension theory.
as rate if transpiration increases the diameter of a tree trunk decreases
- evaporation from leaves draws water from xylem by osmosis
- so water is pulled up xylem creating tension
- the tension pulls in the walls of xylem vessels
What is a potometer?
measures rate of transpiration
Why is the rate of water uptake not the same as rate of transpiration?
some of the water taken up by plants is often used to maintain turgidity and a small % is used in photosynthesis
What measurements do you need when calculating rate if transpiration in a potometer?
- distance travelled by air buble
- radius of lumen of the capillary tubing
- time taken for air bubble to move
What is the calculation for the volume of water taken up in a potometer?
π x r^2 x d
pi x radius squared x distance moved
What is needed for the potometer to work?
- no air bubbles
- rubber bung must have air tight seal
Why should the shoot be cut underwater?
to prevent air entering
How can mineral ions be traced?
radioactive isotopes can act as tracers to provide evidence that mineral ions travel in xylem
Why are the xylem and phloem separated using a wax cylinder?
to prevent lateral (sideways) transport
How is evidence for the movement of ions shown?
- roots are supplied with potassium ions(radioactive)
- plant is left for a few hours and amount of radioactivity in xylem and phloem tissues is measured and compared
What does the change in radioactivity in xylem and phloem show?
radioactivity in xylem is higher which shows transport of the potassium ion
small amount of radioactivity in phloem is from lateral transport where the wax cylinder isn’t present
What is translocation?
Transport of photosynthetic products
- occurs in phloem
How are carbohydrates transported in the Phloem?
in the form of sucrose
In what form are proteins transported in the phloem?
in the form if amino acids
In what form are lipids transported in the phloem?
as fatty acids and glycerol
Where are photosynthetic products transported to?
- there produced in leaves during photosynthesis and transported up and down stem of plant into the sinks
What are the sinks if plants?
- growing areas, young keaves and roots/shoot tips
- roots
- developing fruits and storage areas
What are some properties of Sieve elements in the phloem?
- joined end to end to form sieve tubes
- end walls of sieve plates are sieve plates as the possess pores
- no nucleus & cytoplasm only has few organelles
Describe a companion cell
- next to each sieve element
- have dense cytoplasm and many mitochondria
What is the method for translocation in the phloem?
- mass flow of organic substances* occurs down an hydrostatic pressure gradient
How does sucrose enter phloem?
actively transported into sieve tubed by companion cell
- lowers WP of sieve tube so water enters from xylem creating high HS pressure
What is the function of sugar in the sinks?
Sugars are being used in respiration for growth
Or they’re stored as insoluble starch
What happens when sucrose is transported into the sink?
- increases WP in sieve tubes and water moves back into xylem
- so HS pressure is lower in sink
What is the pathway products take in translocation?
- actively transported into sieve tubes
- low WP water enters from xylem
- product transported to sink
- this increases WP so water moves back to xylem
- low HS pressure in sink
How is evidence for transport in Phloem shown?
Ringing
- removing ring of phloem around a plant to prevent transport
Why does swelling form after ringing?
- due to the build up of photosynthetic products from the leaves which were prevented from being transported
Describe an experiment where radioactive isotopes demonstrate translocation using ringing?
- two plants of similar growth one ringed one not
- leaf below ring supplied with isotope and left in the sun for abit
- transport of radioactivity detected using autoradiography
What does the plant not ringed show in autoradiography?
- shows transport of photosynthesis products particularly in growing regions (young leaves root/shoot tips)
How can some products get transported in a ringed plant?
via transport through the xylem (lateral transport)
What is some evidence supporting the mass flow hypothesis?
- cutting stem if plant results in phloem sap being released
this indicates hydrostatic pressure in sieve tubes - lowering temperature reduces rate of translocation
shows active transport is involved
What is some evidence against the mass flow hypothesis?
- specific structure of sieve tubes and plates is not required for mass flow
- in young phloem tissue substances have been observed moving in the opposite direction in the same sieve tube
