7) Transport in Plants Flashcards
what is adhesion ?
the formation of hydrogen bonds between carbohydrates in the xylem vessel walls and water molecules
what is cohesion ?
- formation of hydrogen bonds between water molecules.
what is the apoplastic pathway ?
water moves through the intercellular spaces between cellulose molecules in the cell wall
what is the casparian strip ?
waterproof strip surrounding endodermal cells of the root that blocks the apoplastic pathway, forcing water through the symplast route.
what is the symplastic pathway ?
- water enters the cytoplasm through the plasma membrane and moves between adjacent cells via plasmodesmata.
- water diffuses down its water potential gradient by osmosis.
what is the function of the xylem
- carries dissolved minerals and water up the plant
- structural support
- food storage
what is the function of phloem
transport organic compounds (sucrose) from source to the sink
where is xylem and phloem in the roots
xylem : centre of vascular bundle
phloem : on the edges of the centre
where is xylem and phloem in the stems
xylem : inside (closest to centre of stem)
phloem : outside (closest to epidermis)
where is xylem and phloem in the leaves
xylem : upper side (closest to upper epidermis)
phloem : lower side (closest to lower epidermis)
phloem sieve tube adaptations
-cell membranes that retain sucrose & other assimilates within cells
- few cell contents to reduce resistance to flow of phloem sap
- sieve plates to hold sieve tubes together/resist internal pressure
- sieve pores to allow ease of flow between sieve tube elements
adaptations of companion cell
- many mitochondria to provide energy to move solutions into sieve tubes
- many plasmodesmata to allow easy movement of phloem sap into/out of sieve tubes
- pump & co-transporter proteins in cell membranes for absorption of sucrose (from apoplast pathway from mesophyll cells)
- some plasmodesmata shared with mesophyll cells for transport of sucrose via symplast pathway
how is xylem vessel suited to their function
- transport of water & mineral ions
- elongated cells / tubes for transport
- hollow/ no cytoplasm or contents SO more space for greater volume for flow
- cellulose lining SO adhesion of water molecules
- lignified walls to prevent collapse
- lignified walls SO waterproof/prevents loss of water
- pits to allow sideways movement
how does sieve tube element help translocation
- little cytoplasm/no vacuole/no tonoplast
- pores in sieve plate provide little resistance
- sieve plate prevents cell collapsing
- plasmodesmata allows loading
- cellulose walls prevent tm expansion
- mitochondria for ATP for repair
how do companion cells help in translocation
- assimilates (sucrose/amino acids) loaded/pumped
- pumping H+
- co-transport (H+ carry sucrose with them)
- mitochondria provide ATP for active transport
describe the apoplastic pathway
- water enters cell wall
- water moves through the cell wall
- water may move from cell wall to cell wall through intracellular spaces OR directly
describe the symplastic pathway
- water enters cytoplasm by osmosis through cell membrane
- water moves into the sap in the vacuole through tonoplast by osmosis
- water may move from cell to cell through plasmodesmata OR adjacent cell membrane/walls
5 features of xerophytic plants + their effects
- thick cuticle to leaf epidermis : prevents water loss thru external wall of epidermis.
- layer of hairs on epidermis : traps moist air over leaf & reduces diffusion.
- reduction in no of stomata : reduces outlets through which moist air can diffuse.
- stomata in pits/groves : moist air trapped outside stomata, reducing diffusion.
- leaf rolled/folded : reduces area from which transpiration can occur.
Suggest how the permeability of the tonoplast changes and explain how this change
could result in the rupture of the tonoplast
- increased permeability to water
- increased protein channels (in the tonoplast)
- water potential in vacuole is lower than that of cytoplasm
- more water enters (vacuole) by osmosis
- vacuole increases, in volume / pressure (until it bursts)
why is water the main component of xylem sap and phloem sap
- solvent for assimilates / mineral ions
- hydrogen bonds for cohesion & adhesion (xylem)
- to build up hydrostatic pressure for mass flow/translocation (phloem)
Explain how water moves up xylem vessels in the trunks of trees
- diff in water potential
- continuous columns of water
- transpiration pull/ cohesion tension
- hydrogen bonding between molecules / cohesion
- adhesion
role of cotransporter proteins in companion cells
- movement of sucrose with, protons / H+ ions, into companion cells.
- (sucrose moved) from, apoplast / cell wall / mesophyll cell ;
- sucrose moves against its concentration gradient (into companion cell) ;
- (needs cotransporter protein because) sucrose is polar so cannot pass through membrane
Suggest why mineral ions are found within phloem sieve tubes and state how they are
transported within phloem sieve tubes
- they enter from xylem through plasmodesmata from companion cells
- dissolved in water
- transported as part of mass flow in phloem
Suggest and explain how the presence of mineral ions in the root hair cell can increase
the absorption of water by the root hair cells
- mineral ions dissolve in water
- they decrease w.p in cytoplasm
- w.p gradient steep
- water enters by osmosis
how is sucrose transported into phloem sieve tubes
- mass flow
- from companion cell
- sucrose dissolved in water/lowers w.p
- transport from high hydrostatic pressure to low
roles of hydrogen bonding in the movement of water through xylem vessels
- gives adhesion between water molecules and cellulose = formation of continuous column
- gives cohesion between water molecules
- transpiration pull
how is tension created in the xylem vessels
- transpiration pull
- water lost during transpiration
- cohesion & adhesion
why are lignified walls in the xylem important ?
- prevents collapse
- thickening for support
- waterproofing properties
- allows formation of hollow cells
- maintains column of water (adhesion of water to hydrophilic groups in lignin)
Describe and explain the mechanism that is responsible for the movement of phloem sap in sieve tubes
- diffuse into phloem sieve tube by plasmodesmata.
- w.p in sieve tube decreases.
- water enters by osmosis
- hydrostatic pressure increases
- mass flow
- unloading at sink decreases hydrostatic pressure
