Plant transport Flashcards
Explain where the vascular bundle is in the stem
Outer epidermis, then the cortex.
Phloem on outside of vascular bundle, then xylem on inside.
The vascular bundle is split by the vascular cambium and covered by a sclerenchyma cap
Explain where the xylem and phloem are inside the root
RHC on the exodermis, then the epidermis then endodermis then the phloem and xylem
What is the structure of the xylem?
Dead, hollow, lignified tube.
Lignin provides structural strength and waterproofing.
Has non-lignified pits that allow movement of water.
What are the roles of the xylem?
-Transport water and mineral ions(transpiration stream/pull)
-Provide structural support
What is the xylem parenchyma?
A cell which stores food+ contains tannins that area chemical defence method against herbivores
What is the structure of the phloem?
Live cell
Made up of sieve tube elements which have sieve plates between them
Companion cell(support phloem life) bordering it, connected cytoplasm by plasmodesmata
What is the role of the phloem?
Transport assimilates(sugars as sucrose) from source to sink.
Does this by active transport.
What are the 4 steps involved in water entering the xylem?
- Water and mineral ions enters endodermis via osmosis from soil
- H2o moves via apoplast route or symplast route across the endodermis until it reaches the casperian strip where it is forced to take the symplast route
- Mineral ions are actively transported into xylem which reduces xylems water potential, so water follows by osmosis.
- This generates root pressure which pushes water up the plant
What is the symplast route?
When the water travels through the cytoplasm by diffusion and across the plasmodesmata by osmosis
What is the apoplast route?
When the water travels through the cell walls, by tension
Why does the casperian strip force the water to take the symplast route?
Because the casperian strip is made of suberin, a waterproof layer in the endodermis, because its waterproof the water cant cross it so it moves into the cytoplasm
This means it has to cross the plasma membrane, which affectively filters out the ‘bad’ toxins etc. It is a physical defence
Explain the 4 steps of assimilate transportation
- H+ ions are actively transported out of the companion cell into the cytoplasm, this creates a H+ conc grad
- H+ ions move back into companion cell via facillitated diffusion from a co-transport protein, sucrose is co-transported
- Increased sucrose concentration in the companion cell, so sucrose diffuses through the plasmodesmata into the sieve tube element
- The presence of the solute sucrose decreases the water potential of the sieve tube element, so water moves out of the CC by osmosis vis the plasmodesmata. Creating turgor pressure for mass flow
Name the main sources of assimilates in plants
Green leaves and green stems,
Storage organs like tap roots that are unloading their stores at the beginning of the growth period
Food stores in seeds when they germinate
Name the main sinks in a plant
Roots that are growing
Meristems that are actively dividing
Any plants of plant that are laying down food stores like fruits
What is transpiration?
The evaporation of water vapour via the stomata
What factors increase transpiration rate
Increased temp, H2o vapour have more kinetic energy=more movement=more likely to leave via stomata
Decreased humidity and increased air movement, maintains H2o vapour conc grad.
Increased light intensity, increased photosynthesis so stomata open for gas exchange.
Explain the transportation stream/pull
Stream=The movement of water from the roots through the plant till its lost by evaporation through the stomata.
Pull=Idea that when 1 water vapour molecule leaves another is ‘pulled’ up to restore concentration(there is a continuous stream of this)
What is cohesion?
The H+ bonds formed between water molecules and their tendency to stick together due to their polar nature
What is the cohesion tension theory?
The idea that water moves from soil in a continuous stream up the xylem and across the leaf
Name 3 pieces of evidence for the cohesion tension theory
-Changes in trunk diameter, at night its transpiration is at its lowest so the xylem vessels tension is also at its lowest so trunk diameter increases
-When the xylem vessel is broken, air is drawn in rather than water leaking out
-When xylem vessel draws air in, the continuous streak of water held together by cohesive forces between WATER MOLECULES is broken so water cant be transported
What is adhesion?
The H+ bonds created between the water molecules and xylem walls due to the waters polar nature
How do you measure transpiration?
Using a potometer, but this is only am estimate as it measures water uptake rather than the actual water evaporated.
Explain the set up of a potometer
There is capillary tube that the air bubble travels across
Position of air bubble can be reset turning the tap on the reservoir attached to the capillary tube which allows water in
There is also the freshly cut shoot which is in the capillary tube
What precautions are taken when using a potometer?
The shoot is cut underwater, to reduce chance of air bubble being drawn into xylem and breaking transpiration stream
Air tight seals(vaseline), ensure no water vapour escapes from anywhere other than the capillary tube
