Mass Transport In Plants Flashcards
What is transpiration
The loss of water vapour in the stomata via evaporation
What are the 4 factors of transpiration
1.light=more light causes more stomata to open
2.temperature=more kinetic energy so faster moving molecules so more evaporation
3.humidity=more water potential in the air so water potential more outside leaf- reduces water potential gradient
4.wind=more wind blows humid air containing water vapour maintains water potential gradient
Describe the cohesion theory of water transporting in the xylem
-water molecules lost from leaf because of transpiration and diffusion of water through stomata
-lowers water potential of leaf cells
-water pulled from xylem creating tension
-water molecules cohere together by hydrogen bonds
-forming continuous water column
-adhesion of water molecules to walls of xylem
Describe root pressure and how that forces water upwards
-as water moves into the root by osmosis it increases volume of liquid inside the root
-pressure in the root increases forcing water above it upwards
What does the xylem transport in plants
-water
What does the phloem transport in plants
-organic ions ie sucrose
What are the 2 key cells in the phloem that makes it adapted for it’s function
-sieve plates
-companion cells
Describe the sieve plate elements of the phloem
● No nucleus / few organelles → maximise space for / easier flow of organic substances
Describe the companion cells elements of the phloem
Many mitochondria
High rate of respiration to make ATP for active transport of organic ions
Describe the mass hypothesis for the mechanism of translocation in plants
-in the source of the leaf sucrose are actively transported into phloem by companion cells
-this lowers water potential of sieve cells and the tube
-water enters by osmosis
-this creates an increase in hydrostatic pressure causing mass movement towards sink and root
-at the sink sugars(sucrose) used are converted in root for respiration
How is transpiration measured in xylem
Using a potometer
Using understanding of mass flow hypothesis to explain how pressure is created in phloem tube
-sucrose actively transported in phloem
-lowering water potential
-from the xylem water is transported In the phloem by osmosis
What are the 2 methods of investigating translocation
-ringing experiments
-tracer experiments
Describe the tracer experiment
- Leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope 14C
- Radioactive carbon incorporated into organic substances during photosynthesis
- These move around plant by translocation
- Movement tracked using autoradiography or a Geiger counter
Describe the ringing experiment of translocation
- Remove phloem by removing a ring of bark
- Bulge forms above ring
- Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem
- Tissues below ring die as cannot get organic substances
Describe the function of xylem tissue
Transports water and mineral ions through the stem, up the plant to leaves of plants
Suggest how xylem tissue is adapted for its function
● Cells joined with no end walls forming a long continuous tube → water flows as a continuous column
● Cells contain no cytoplasm / nucleus → easier water flow / no obstructions
● Thick cell walls with lignin → provides support / withstand tension / prevents water loss
● Pits in side walls → allow lateral water movements
Describe how to set up a potometer
- Cut a shoot underwater at a slant → prevent air entering xylem
- Assemble potometer with capillary tube end submerged in a beaker of water
- Insert shoot underwater
- Ensure apparatus is watertight / airtight
- Dry leaves and allow time for shoot to
acclimatise - Shut tap
- Form an air bubble - quickly remove end
of capillary tube from water
Describe how a potometer can be used to measure the rate of transpiration
- Record position of air bubble
- Record distance moved in a certain amount of time (eg. 1 minute)
- Calculate volume of water uptake in a given time:
● Use radius of capillary tube to calculate cross-sectional area of water (πr2)
● Multiply this by distance moved by bubble - Calculate rate of water uptake - divide volume by time taken
Suggest limitations in using a potometer to measure rate of transpiration
● Rate of water uptake might not be same as rate of transpiration
○ Water used for support / turgidity
○ Water used in photosynthesis and produced during respiration
● Rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant
○ Shoot in potometer has no roots whereas a plant does
○ Xylem cells very narrow
Hoe does light intensity effect transpiration rate
Increases rate of transpiration
-● Stomata open in light to let in CO2 for photosynthesis
● Allowing more water to evaporate faster
● Stomata close when it’s dark so there is a low transpiration rate
How does temperature effect transpiration rate
Increases rate of transpiration
● Water molecules gain kinetic energy as temperature increases
● So water evaporates faster
How does wind intensity effect transpiration rate
Increases rate of transpiration
● Wind blows away water molecules from around stomata
● Decreasing water potential of air around stomata
● Increasing water potential gradient so water evaporates faster
How does humidity effect transpiration rate
Decreases rate of transpiration
● More water in air so it has a higher water potential
● Decreasing water potential gradient from leaf to air
● Water evaporates slower
