3.3.4.2 mass transport in plants Flashcards
Xylem
Tissue that transports water in the stem and leaves of plants
Cohesion-tension theory of water transport in the xylem
Theory: how water moves up the xylem against gravity via transpiration stream
Water evaporates from the leaves via the stomata due to transpiration
Reducing water potential in the cell and increasing water potential gradient
Water drawn out of xylem, creating tension
Cohesive forces between water molecules pull water up as a column
Water lost enters roots via osmosis
Water is moving up, against gravity
Water is also cohesive so sticks to edges of the column
Adaptations of xylem
Waterproof preventing water loss
Rigid so less likely to collapse under low pressure
Phloem
Tissue that transports organic substances in plants
Translocation
Movement of solutes from source to sink
Translocation at source
High concentration of solute
Active transport loads solutes from companion cells to sieve tubes of the phloem
Lowering water potential inside sieve tubes
Water enters sieve tubes by osmosis from xylem and companion cells
Increasing pressure inside sieve tubes at the source end
Translocation at sink
Low concentration of solute
Solutes removed to be used up
Increasing water potential inside sieve tubes
Water leaves tubes via osmosis
Lowering pressure inside sieve tubes
Mass flow
Pressure gradient from source to sink
Pushes solutes from source to sink
Solutes used or stored at sink
Adaptations of phloem
Sieve tube elements have no nucleus and few organelles
Companion cell for each sieve tube element to carry out the living functions for the sieve cells
Use of tracers
Supply plant with radioactive tracer (e.g. 14C) in CO2 to a photosynthesising lead by pumping the radioactive CO2 into a container surrounding the leaf
14C is incorporated into the organic substances produced by the leaf
Organic substances undergo translocation
Autoradiography - plant killed and placed in a photographic film, film turns black where the radioactive substance is present
Identifies where radioactive substance has moved to and thus where the organic substances have moved to via translocation from source to sink
Can show this over time by taking autoradiographs at different times
Aphid
Aphids pierce the phloem using mouthpiece
Releasing sap from plants
Flow of sap higher at source than at sink
Evidence of a pressure gradient, higher pressure near source
Metabolic inhibitor
Add a metabolic inhibitor to phloem
Translocation stops
Proves active transport is involved
As it requires ATP to move against a concentration gradient
Potometer to investigate effect of named environmental variable on rate of transpiration
Potometer estimates the transpiration rate by measuring water uptake
- assume water uptake is directly related to water loss of leaves
Method w/ potometer
Cut a shoot underwater
- to prevent air entering xylem > interrupt water flowing in column, stopping transpiration
Assemble potometer with capillary tube and submerged in a beaker of water
Insert shoot underwater
Ensures apparatus is watertight and airtight
Dry leaves and allow time for shoot to acclimatise
Shut off tap to reservoir
Remove end of capillary tube from the water beaker until one hair bubble has formed, then put the tube back into the water
Record position of the air bubble
Use a stopwatch to record time
Record distance moved per unit time
Rate of air movement = estimate of transpiration rate
Change one variable at a time and keep all other variables constant
Variables
Wind
Humidity
Light
Temperature
How light affects transpiration rate
The higher the light intensity, the faster the transpiration rate
Because stomata open in light to let CO2 for photosynthesis
Allowing more water to evaporate faster
Stomata close when it’s dark so there is a low transpiration rate
How temperature affects transpiration rate
The higher the temperature, the faster the transpiration rate
Water molecules gain kinetic energy as temperature increases
Move faster
Water evaporates faster
How humidity affects transpiration rate
The lower the humidity, the faster the transpiration rate
Because as humidity increases, more water is in the air so it has a higher water potential
Decreasing the water potential gradient from leaf to air
Water evaporates slower
How wind affects transpiration rate
The windier, the faster the transpiration rate
Wind blows away water molecules from around the stomata
Decreasing the water potential of the air around the stomata
Increasing the water potential gradient
Water evaporates faster
Describe the mass flow hypothesis (5)
At source, sucrose actively transported to phloem sieve cells
From companion cells
Sieve cell wp lowered at source so water enters from xylem by osmosis
HP increases causing mass movement of sucrose towards sink
Sucrose actively transported into sink and converted to starch or glucose
