TRANSPORT IN PLANTS Flashcards
what is the structure of xylem?
made up of elongated xylem elements which form a continuous tube
walls are formed of dead tissues and lignin
lignin is strong which prevents collapse, prevents water loss and provides support
an empty lumen with no cytoplasm
xylem walls have pits that allow water to move in and out of the xylem vessels
what is the structure of phloem; sieve tube?
made up of living elongated sieve elements forming a continuous tube
each sieve element contain: cell wall, cell membrane, cytoplasm, ER, mitochondria
each sieve element doesn’t contain: nucleus and ribosomes
a sieve plate is a wall formed when the end walls of 2 sieve elements meet
a sieve plate has sieve pores to:
allow free movement of liquids through them
support and prevent collapse of sieve tube
allow phloem to heal itself if damaged
what is the structure of phloem; companion cells?
each sieve element has at least one companion cell
companion cells are involved in the production of ATP for active processes
they contain: cytoplasm cell wall cell membrane mitochondria ribosomes nucleus small vacuole
they have a lot of plasmodesmata passing through their walls making contact with the cytoplasm of the other companion cells and sieve tubes
what is plasmodesmata?
cytoplasm that passes through the cell walls of plant cells and allows communication between them
how is water transported from the soil into the roots?
water is absorbed from the soil through the root hairs by osmosis, down the water potential gradient
the more root hairs, the more surface area. this increases the rate of absorption of water
how is water transported from the root hairs to the xylem?
water is absorbed by the root hairs and travels through the cortex to the xylem
water moves into the xylem as the water potential inside the xylem is lower than in the root hairs
two pathways through the cortex:
- apoplast pathway: water moves across the root, from cell wall to cell wall without entering the cytoplasm
- symplast pathway: water moves into the cytoplasm of cortical cells and moves to adjacent cells through plasmodesmata or cell membranes and walls
normally symplast pathway is more important but when transpiration rate increases, water moves through apoplast pathway
when the water reaches the endodermis, it meets the casparian strip
the casparian strip is a layer of waterproof suberin
to cross the strip, water must enter the symplast pathway
after endodermis, water continues to move down water potential gradient and enters the xylem through pits
what are the two pathways for water to move through the cortex?
apoplast pathway: water moves across the root, from cell wall to cell wall without entering the cytoplasm
symplast pathway: water moves into the cytoplasm of cortical cells and moves to adjacent cells through plasmodesmata or cell membranes and walls
what is the casparian strip and what happens there?
the casparian strip is a layer of waterproof suberin
to cross the strip, water must enter the symplast pathway
after endodermis, water continues to move down water potential gradient and enters the xylem through pits
what is transpiration?
transpiration is the loss of water vapor to the atmosphere by diffusion from the leaves leaving through pores
water evaporates from the mesophyll cell wall and moves into the air spaces in the spongy mesophyll and diffuses out of the leaf through the stomata down the water potential gradient
what are the factors that affect transpiration?
humidity: less humid, more transpiration as there is more water potential inside the leaf than outside the leaf
wind: higher wind speed, more transpiration as wind moves water vapor away from the leaf
temperature: higher temperature, more transpiration as water molecules gain more kinetic energy and diffuse faster
what are stomata?
Stomata are pore-like openings on the bottom of the leaf that allow carbon dioxide and oxygen to diffuse into and out of the leaf
when rate of transpiration is high, stomata will partially close to prevent the plant from drying out, even if this reduces the rate of photosynthesis
what is a transpirational pull?
the evaporation of water from the cell walls of mesophyll cells, makes water constantly move out of the xylem of the leaf down the water potential gradient
the hydrostatic pressure at the top of the xylem vessel becomes lower than the bottom of the vessel
this pressure difference causes the water to move up the xylem vessel
the movement of water up the xylem is by mass flow helped by:
- cohesion between water molecules
- adhesion between water molecules and lignin of xylem walls
what happens due to root pressure?
root pressure surrounding the xylem vessels use energy to pump minerals and ions across its membrane into the xylem by active transport
this lowers the water potential in the xylem therefor drawing more water in from the roots increasing the hydrostatic pressure at the bottom of the xylem vessel
what are xerophytes?
plants that are adapted to live in dry habitats with a short water supply
what are the adaptations for xerophytes?
small needle-like leaves: reduce surface area for evaporation and decrease rate of transpiration
rolled leaves: keeps humid air around stomata to decrease rate of transpiration
thick waxy cuticle: impermeable to water to decrease rate of transpiration
low number stomata: to decrease rate of transpiration
sunken stomata: keeps humid air around stomata to decrease rate of transpiration
trichomes: maintains humid air around stoma to decrease rare of transpiration
long roots: maximise water intake
