💙 3.3.4.2 Mass Transport In Plants Flashcards
What is the main force that pulls water up the stem of a plant?
It is the evaporation of water from leaves and it is a process called transpiration
What does the cohesion tension theory describe?
It describes how water is drawn up from the ground and then up through the plant
What does the xylem do?
It is a vessel which transports water from the roots to the leaves
What are some structural features of the xylem?
- Long, dead cells with no end walls
-Continuous water columns
-no cytoplasm/no organelles to obstruct water flow
-strong lignin to withstand tension
Describe what happens in the cohesion tension theory
- water evaporates/transpires from leaves
- Reduces water potential in cells
- Water potential/osmotic gradient across cells in the leaf
- Water is drawn out of the xylem due to the negative pressure i.e. there is tension on water in the xylem
- Many hydrogen bonds which means the water is cohesive
- it is water up the xylem in a continuous column
What is the evidence of the cohesion tension theory?
Tree trunks change diameter according to the rate of transpiration. During the day when the transpiration is a greatest, there is more tension (more negative pressure) in the xylem and in the trunk there is a decreasing diameter.
What are the four factors that affect the rate of transpiration?
- High temperature
- Wind (air movement)
- Humidity
- light intensity
How does high temperature affect the rate transpiration?
It increases the rate of transpiration. This is due to the water molecules having more kinetic energy some more evaporation occur.
How does wind (air movement) affect the rate of transpiration?
It increases the rate of transpiration. This is due to the moist air being removed from around the leaf. This increases the water potential gradient so water transpires more quickly.
How does humidity affect the rate of transpiration?
It decreases the rate of transpiration. This is due to the increased water potential in the air which decreases the water potential gradient between the leaf and the air. Water is then lost quickly.
How does light intensity affect the rate of transpiration?
It increases the rate of transpiration. This is due to more photosynthesis taking place so the stomata are open for gaseous exchange. This allows more water to be lost by transpiration.
What can you use to measure the rate of transpiration?
Use a potometer
What do you need to measure in order to calculate the rate of transpiration?
The distance the bubble moved, the time taken and the radius of the tube ( so you can calculate the volume ) to measure in mm3 / cm3 per minute.
What are some precautions you need to take (8) when measuring rate of transpiration?
1- seal joints- ensure they are air tight and water tight
2- cut shoot under water
3- cut shoot at a slant
4- dry off leaves
5- insert into apparatus under water
6- ensure no air bubbles are present - shut tap
7- note where bubble is at start / move bubble to the start position
8- reset bubble to start using reservoir
What does potometer measure rather than transpiration?
Water uptake
Why does the potometer measure water uptake rather than transpiration?
1) some water is used in photosynthesis
2) water used during hydrolysis/released during condensation reactions
What is the job of a phloem?
It is a vessel that transports organic substances in a plant
What are some structural properties of phloem?
- made up of living cells
- they run close to xylem vessels
- they have sieve pores, for ease of flow
- thick walls to withstand pressure
- companion cells have many mitochondria ( contain ATP - release energy )
- sieve element has few organelles for ease of flow
Describe the process of mass flow for transport of sugars through sieve tubes (translocation)
- an active mechanism which uses ATP
- transports sugar from source to sieve tube elements (phloem)
- by companion cells
- the water potential is lowered in phloem due to solubility of solute e.g sucrose
- water moves via osmosis into phloem from the xylem down its water potential gradient
- high hydrostatic pressure created at the source end gradient
- mass flow of water and sugars from high to low pressure ( down pressure gradient ) - from source to sink
- sucrose unloaded at sink via active transport
- water follows by osmosis
- excess water re enters the xylem
- 2 way flow (this can occur in both directions)
Give evidence for the mass flow of organic substances (translocation) hypothesis.
1-if a tree is pierced then sap is forced out - demonstrating that there is a pressure forcing out organic substances
2-if phloem is removed (ringing) then no organic substances can pass beyond this point (radioactive Co2 will be incorporated in those organic substances and then ‘tracked’ )
