transport in plants Flashcards
how does water move through cells to the xylem?
water potential gradient is created as cells further from the roots has less water
what is the symplast route?
- water moves through living spaces of the cytoplasm
- changes cells through the plasmodesmata
what is the vacuolar route?
- slowest route
- similar to the symplast route
- moves through the vacuoles and the cytoplasm
what is the apoplast route?
- movement through the cell wall and intracellular spaces
- adhesive and tension forces acting on the cell wall pulls water up the plant
- fastest route
What is the casparian strip?
- impermeable layer of suberin
- moves apoplast route into the cytoplasm
What is transpiration?
loss of water through leaves
What is the cohesion tension theory?
- water is polar
- hydrogen molecules are slightly attracted to oxygen
- they can cohere and be pulled up a stream
what are the factors affecting the rate of transpiration?
- light intensity
- humidity
- temperature
- air movement
- soil-water availability
how do stomata open?
- water moves into vacuole of guard cells down water potential gradient
- outer wall is flexible so when the guard cells become turgid the stomata opens
how does light open the stomata?
- blue light activates ATPase in guard cells
- catalyses the hydrolysis of ATP, generating energy to operate proton pump
- hydrogen ions are pumped out and reenter with chloride ions
- the increase in solute concentration lowers the water potential so water moves in
what is translocation?
transport of assimilates from source to sink
what are some sources of assimilates?
- green leaves/stems
- storage organs
- food stores in seeds
what are sinks for assimilates?
- meristems
- roots absorbing minerals
- assimilate storage
why are carbohydrates transported as sucrose?
- less reactive than glucose and doesnt affect osmosis
- more energy per mol
what is the assimilate loading method?
- transfer cells (modified companion cells) pump hydrogen ions out of the cytoplasm via proton pump
- concentration of hydrogen ions results in them reentering down the concentration gradient with cotransporters
- this brings in sucrose which move into the sieve tubes via plasmodesmata
how does sucrose unload?
- can leave at any point by diffusion
- turns into another substance eg starch to maintain the concentration gradient
adaptations of xerophytes
- thick waxy cuticle
- sunken stomata
- reduced stomata/leaves
- hairy+curly leaves
- roots`
why do plants need a transport system?
- small sa:v ratio
- diffusion is too slow so the movement of sucrose is not fast enough for the plants metabolic rate
what cells make up cambium tissue?
meristem
explain how the Casparian strip prevents ions from reaching the xylem of the plant by the apoplast pathway
- strip is impervious to water
- forces solution to pass through cell surface membrane
- phospholipid bilayer repels charged particles and it enters the cytoplasm
what method could be used to observe the position of xylem vessels in a leaf stalk of celery?
- cut a thin cross section of tissue
- add eosin and observe under a microscope
what are some similarities and differences between xylem and phloem vessels?
- xylem contains more lignin than phloem
- both made up of more than one type of cell
- xylem has a wider lumen than phloem
describe how a potometer can be used to measure a more accurate rate of transpiration
- use vaseline to make the potometer airtight
- ensure leaves are dry
- cut shoot underwater to maintain transpiration stream
- measure air bubble rate per minute
how is a leaf supported when it has a thin stem and cell wall?
air spaces give it buoyancy and supported surrounding water
explain how glucose produced in photosynthesis is translocated to parts of the plant where glucose is metabolised or stored
Phloem loading
* Glucose is converted to sucrose
* Apoplast route is active
* Proton pump in companion cell which maintains h+ conc grad
* co-transport of H+ and sucrose into companion cell
* Passive loading via symplast route (plasmodesmata)
* entry of sucrose decreases water potential of phloem
* water enters phloem from surrounding cells which results in higher hydrostatic pressure
Mass flow
* Bulk transport of sucrose caused by pressure difference
* Entry / exit of water affects hydrostatic pressure
* Movement from source to sink
Phloem unloading
* Diffusion of sucrose from phloem to surrounding cells
* Sucrose converted back to glucose
* Glucose used for respiration, converted to starch for storage
* Concentration gradient of sucrose maintained between
phloem and cells
* Loss of sucrose / solutes increases water potential of
phloem
* water leaves phloem to surrounding cells / xylem
* results in lower hydrostatic pressure
