Mass Transport in Plants Flashcards
What is transpiration?
The evaporation of water from the leaves, through the stomata
Xylem structure
- Continuous hollow tube of dead cells, transporting water and minerals up plant
- Rings of lignin (protein) to strengthen xylem cell wall
- No cell walls between dead cells
- Pits in walls
Cohesion-tension theory
- Transpiration of water through stomata, lowering water potential of mesophyll cells / lowering pressure at the top of the xylem
- Water pulled up xylem creating tension as polar water molecules cohere to each other and water molecules adhere to walls of xylem
- Water molecules sucked up to leaves, forming continuous water column
Factors affecting rate of transpiration
- Temperature (higher temperature, higher rate)
- Light (more light, more stomata open, higher rate)
- Humidity (less humid, steeper water potential gradient, higher rate)
- Wind (more wind, steeper water potential gradient, higher rate)
How to calculate rate of transpiration with a potometer
1) shoot cut under water
2) potometer filled with water
3) potometer removed and all joints sealed with waterproof jelly
4) air bubble introduced
5) (distance bubble moves x cross sectional area of capillary tube) / time
6) open tap on the reservoir to push bubble back to start of scale
Xylem dissection
- Transverse cut at base of stem
- Place end in coloured water
- Transverse cross section to observe location of coloured fluid
Phloem dissection
- Grow plant in radioactive carbon dioxide
- Carbon14 dioxide is converted into organic compounds (eg. Sucrose)
- Transverse cross section of stem
- Observe location of carbon14 compounds
Phloem structure
- Hollow sieve tube cells, transporting glucose and amino acids up and down plant
- Sieve tube plates
- Companion cells, providing support and ATP
What is translocation?
The movement of solutes/assimilates from source to sink
What happens that the source?
- Active transport of sucrose from companion cell into sieve tube cell, decreasing water potential in phloem
- Water moves in by osmosis from companion cell and xylem
- Creates high hydrostatic pressure in the phloem
- Mass flow to respiring cells
What happens at the sink?
- Sucrose unloads from phloem by active transport
- Sink uses or hydrolyses sucrose
- Creates low concentration of sucrose, increasing water potential in phloem
- Water moves out by osmosis
- Decreases pressure in the phloem
How is radioactive labelling used as evidence for mass flow?
- Use radioactive C14 lable
- Grow plants in C14 atmosphere
- Measure C14 as it moves down stem
How are ringing experiments used as evidence for mass flow?
- Remove bark in ring from tree trunk (phloem)
- Solutes can’t move up or down
- Bulge forms above ring
- Fluid above ring has more solutes than below
- Evidence solutes are moving down
