Mass transport in plants Flashcards
label where the atriaventrical node and the sino atrial node are on the heart
What is transpiration?
the loss of water from the plants surface- especially the leaves
How is the xylem adapted for the transport of water?
- no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily
- Lignin (thick woody substance), helps to supports the xylem vessels and stops them collapsing inwards
How does water move through the leaf?
if moves down the water potential gradient from the air spaces through the stomata to the air, when the stomata are open
The water lost by diffusion from air spaces is replaced by water evaporating from moist cell walls of mesophyll cells.
The water lost from the mesophyll cells is replaced by water reaching the mesophyll cells from the xylem either via cell walls or via the cytoplasm
Describe transpiration
- water evaporates from the moist cell walls and accumulates in the spaces between the leaf and the cell
- when the stomata open, water moves out of the leaf down the water potential gradient - there is more water inside the lead than there is in the air outside
How does water move up the xylem?
- loss of water at the leaves as water vapour evaporates out of the stomata (transpiration) = loss in water volume creates a lower pressure
- This negative pressure creates tension (suction), which pulls more water into the leaf
- water moves from the top of the xylem into the leaf by osmosis (transpirational pull) due to moving down the pressure gradient (there was a negative pressure created by transpiration) → water is able to do this due to its’ cohesive nature (hydrogen bonds between water molecules) which means when some water molecules are pulled into the leaf the other molecules follow
- Water molecules also adhere (stick) to the walls of the xylem = pull the water column upwards
- As this column of water is pulled up the xylem it creates tension, pulling the xylem to become narrower
What are the two things that allow water to move against gravity? explain them
root pressure = due to the uptake of mineral ions, the water potential of the root becomes lower than the soil = water diffuses into the roots = increases the pressure of roots, so water gets pushed upwards
cohesion tension = water attracted to each other due to hydrogen bonds so when water moves in a direction, the rest of the water molecules moves in that direction too.
What are the 4 main factors that affect transpiration rate and explain them?
- Humidity = negative correlation, more water vapour in the air will make the water potential more positive outside of the leaf, therefore reduces the water potential gradient
- temperature = positive correlation more heat means more kinetic energy, faster moving molecules and therefore more evaporation
- wind = positive correlation, more wind will blow away humid air containing water vapour, therefore maintaining the water potential graident
- Light intensity = positive correlation, more light causes more stomata to open to let co2 in for photosynthesis = larger surface area for evaporation
What is the purpose of companion cells in the phloem?
They provide ATP required for active transport of organic substances
What is the purpose of companion cells in the phloem?
They provide ATP required for active transport of organic substances
How is the phloem adapted for the transport of solutes?
- sieve tube elements= living cells with perforated end walls , no nucleus and few organelles to allow for continuous flow of solutes
- Companion cells (one for each sieve tube elements)= provide ATP required for active transport of organic substances
What is a source cell and what is a sink cell?
source cell = photosynthesising leaf cell
sink cell = respiring cell
How does sucrose transport from the source to the sieve tube element?
- Photosynthesis occurring in the chloroplasts of leaves creates organic substance (eg. sucrose) creating a high concentration of sucrose at the site of production
- sucrose, hence, diffuses down its concentration gradient into the companion cells via facilitated diffusion
- active transport of H+ occurs from the companion cell into the spaces within the cell walls using energy
- this creates a concentration gradient and therefore the H_ moves own their concentration gradient via carrier proteins into the sieve tube elements
- Co-transport of sucrose with the H+ ions occurs via protein co-transporters to transport the sucrose into sieve tube elements
Describe the mass flow hypothesis.
1) sucrose is actively transported from the companion cells into the phloem
2) This reduces the water potential of the phloem
3) Water, therefore, enters the sieve tube elements from the surrounding xylem vessels via osmosis
4) this increases the hydrostatic pressure within the phloem closest to the source, as there is lower hydrostatic pressure within the phloem of the sink, the sucrose, moves down the pressure gradient towards the sink
5) at the sink the sugars are used as a respiratory substrate so there is a lower concentration of them in the cells, therefore the sucrose diffuses from the sieve tube elements into the companion and sink cells here. some water also moves into the neighbouring xylem cells
6) the removal of water decreases the volume in the sieve tube element and therefore the hydrostatic pressure decreases
What is the evidence that supports cohesion theory?
- Diameter changes during the day
When the tension in the xylem vessels are at its greatest during the day, transpiration is at its highest, so the plant’s diameter shrinks. This is due to the xylem vessels caving in slightly due to being pulled inwards by the negative pressure.
- Broken xylem vessel process
If a xylem vessel becomes broken, as the pressure is lost, hence the cohesive forces break, water cannot be pulled up through the vessels, so air is drawn into the vessel rather than water leaking out. If the water was under pressure it would leak out. This means that water works under tension
