transport in plants + exchange surfaces Flashcards
mass flow definition
the bulk movement of fluids down a pressure or concentration gradient
3 reasons why plants need specialised transport systems
- to move products of photosynthesis, water and O2 from source to sink
- most plants are large and substances have to travel huge distances from root to tip
- SA:V is very low overall
outline the process of phloem loading
- sucrose is actively loaded into phloem companion cells
- sucrose can diffuse through plasmodesmata into sieve tube elements
- as sucrose conc increases in phloem, water potential decreases causing water to move in laterally from xylem via osmosis
- this increases hydrostatic pressure causing phloem sap to move towards sink via mass flow
- at the sink sucrose moves back into companion cells + is used up, maintaining conc gradient
- water rejoins xylem
source definition
where a substance is made or stored
sink definition
where a substance is needed e.g. roots which cannot photosynthesise, storage organs, growing regions
how is sucrose actively loaded into companion cells
a protein in companion cell membrane uses ATP to pump H+ out of cytoplasm into cell wall, then H+ reenters through cotransporter proteins with sucrose
why is sucrose used to transport sugars around plants
glucose is too reactive, sucrose isn’t so ensures all glucose makes it to sink where it is needed
what happens when sucrose gets to the cells where it is needed
it is usually converted into:
- glucose for respiration
- starch for storage
what is the function of the phloem
facilitates the transport of assimilates from source to sink, either up or down the plants
assimilates definition
substances made in plants as a result of photosynthesis, e.g. dissolved ions, sugars, etc
how are assimilates transported in phloem
assimilates dissolve in water to form sap which moves through phloem
outline the structure of the phloem
made of living cells
- sieve tube elements
- companion cells
- phloem fibres / sclerenchyma
- parenchyma
3 functions + structure of sieve tube elements
- main conducting cells so have strong cellulose walls
- also have a very thin layer of cytoplasm and few organelles which provides more space for assimilates
- have pores on end plates which allows liquid to easily flow
4 functions + structure of companion cells
- provides metabolic support so linked to sieve tube elements by plasmodesmata
- also has very dense cytoplasm with many organelles
- provides ATP for movement so contain many mitochondria
- also lots of proteins in plasma membrane, allow for the creation of conc gradients + supplying substances to sieve tube elements + receiving substances from neighbouring cells
3 functions + structure of phloem sclerenchyma
- made of dead cells with tapered ends
- provides structural support so reinforced with lignin in walls which is water proof
- also prevents wilting but costly to make
3 functions + structure of phloem parenchyma
- storage cells for food, etc so have a dense cytoplasm and a large vacuole
- thin cellulose wall allows them to closely pack together
- elongated + tapered + cylindrical
what is the function of the xylem
facilitates the transport of water around the plant
transpiration definition
the evaporation of water from the xylem out of the leaves
how does water leave the plant
through the stomata
how is water loss in plants controlled
the stomata can open and close, this is controlled by guard cells (found in pairs)
how do the guard cells control the opening + closing of the stomata
through turgor pressure
- epidermal cells surrounding guard cells pump in K+ ions to lower water potential, these can also diffuse back out to increase water potential
- when guard cells fill with water, they swell and become turgid, inner wall next to stomata is thicker than outer wall, causing the thicker wall to bend, opening the stomata
- when guard cells lose water they become flaccid, and stomata close
what is important about guard cell walls
the inner wall next to stomata is thicker than the outer wall
4 ways plants conserve water
- waxy cuticle
- stomata on underside of leaf
- closeable stomata
- roots that grow down to water in soil
what 4 main factors affect the rate of transpiration
- humidity
- temperature
- light intensity
- air movement
how does humidity affect the rate of transpiration
high air humidity decreases rate
this is because there is more water in the air which decreases the water potential gradient
how does temperature affect the rate of transpiration
high temp increases rate
this is because molecules have more kinetic energy, increasing rate of evaporation
also at high temps the concentration of water held by air increases
how does light intensity affect the rate of transpiration
high light intensity increases rate
this is because stomata open in response to light, also light warms leaf increases rate of evaporation
how does air movement affect the rate of transpiration
lots of air movement increases rate
this is because wind moves air away from stomata which maintains a steep water potential gradient
outline the structure of the xylem
made of dead cells
- vessel cells
- tracheids
- xylem fibres / sclerenchyma
- parenchyma
5 functions + structure of xylem vessel cells
- water conducting cells so have open rounded edges and no end plates so mass flow of water in uninhibited
- also ensures that cohesive/adhesive forces of water aren’t disrupted
- walls are reinforced with lignin as must withstand high hydrostatic pressure
- large and multicellular
- pits in walls allow for lateral movement of water between xylem and phloem
functions + structure of tracheids
- water conducting cells
- must withstand hydrostatic pressure so reinforced with lignin in walls
- lots of lignin in walls means there is a narrow lumen, maintaining pressure?
- tapered + closed ends
- small and unicellular
- pits in walls allow lateral movement of water between xylem and phloem
function of xylem sclerenchyma
structural support
function of xylem parenchyma
storage cells for food, etc
an advantage of the xylem being a bundle of very narrow vessels rather than one wide vessel
- greater heights of liquid are achieved due to capillarity
- smaller tubes mean grater contact with vessel walls so greater adhesive forces
outline adaptations of lignin
lignin is made up of fibres arranged in a spiral
this provides the plant with flexibility to grow + allows it to cope with external pressures e.g. wind
xylem vessels aren’t completely lignified, which allows stem to flex and move slightly e.g. in the wind
what are the 3 pathways water takes through roots
- apoplast pathway
- symplast pathway
- vacuolar pathway
outline the apoplast pathway
water travels through plant cell walls
- this is possible due to the open structure of cellulose
- cohesion means water molecules stick together, so as water is carried away by xylem vessels water molecules move along apoplast pathway
outline the symplast pathway
water moves through cell cytoplasm and plasmodesmata
- as water potential of root cells > water potential of xylem, water moves down water potential gradient
- this pathway is quite slow as organelles obstruct pathway
outline the vacuolar pathway
water moves through plasmodesmata and through cortex cell vacuoles
- water travels down water potential gradient from soil to xylem via osmosis
why is the water potential in the xylem so low
this is because cells in the root endodermis actively pump mineral ions into xylem to decrease water potential, allowing water to passively move towards/in xylem via osmosis - THIS IS ROOT PRESSURE
- this also increases pressure which forces water upwards
- as this is an active process, if respiration is inhibited e.g. by cyanide or lack of O2 root pressure will completely stop
what is the casparian strip
a band of suberin which runs around the walls in the endodermis
- suberin is a waterproof material
- this means water from apoplast pathway must switch to symplast pathway
- allows water moving inot xylem to be regulated by cell membrane
once water has entered the xylem how does it move
as a continuous stream due to forces of cohesion + adhesion
what 3 processes are involved in the movement of water up the stem of a plant
- root pressure
- capillarity
- cohesion-tension theory/capillary action
capillarity definition
the tendency of liquid in a tube to rise or fall due to surface tension
what causes capillarity
adhesive forces between xylem vessels and water molecules, which allows water to be pulled up xylem
outline cohesion tension theory
the cohesive force between water molecules pulls other molecules upwards as in the xylem it is a continuous stream
this puts an inwards pressure on the column of water, called tension
- this tension produces a narrower column of water
capillary action definition
the process of a liquid flowing in a narrow space in opposition to or without the assistance of external forces like gravity
what happens if a xylem vessel is broken
the continuous stream of water is broken so no water can be taken up
can water still reach the leaves if a xylem vessel is blocked
yes
this is because pits in the xylem allow for lateral movement of water, it can travel through phloem
vascular bundle definition
the arrangement of xylem + phloem tissues + sometimes cambium that form fluid-conducting channels in herbaceous dicots
vascular system definition
a system of transport vessels in plants + animals
what makes up the vascular bundle in stem from outside to in
sclerenchyma
phloem
cambium
xylem
cambium definition
meristematic tissues that can differentiate into xylem and phloem as the plant grows
herbaceous definition
having a fleshy or soft stem
dicotylendonous definition
plants that produce 2 seed leaves
what is the difference between mono- or di-cotylendonous plant leaves
mono - vessels are parallel to the leaves
di - vessels branch out from a central vessel
5 reasons why plants need water
- to maintain turgidity of cells
- to transport nutrients around plant
- to create an aq environment for reactions to occur
- to cool plants by evaporation
- for photosynthesis
from outside to inside what tissues are present in a dicot root section
epidermis + root hair cells
exodermis
cortex
endodermis
pericycle
phloem
xylem
what is the pericycle
the part of the root where lateral roots grow, made up of meristematic tissue
what is the function of the endodermis
contains a ring of suberin which is impermeable to water - casparian strip
what is the function of the cortex
stores a large amount of starch
what shape is the xylem tissue
star shaped
what is the function of root hairs
uptake of water from roots, lots of root hairs have a high SA:V which increases rate of water uptake by osmosis
what must water potential of soil be relative to roots for water uptake to occur
soil water potential must be less negative / greater than root water potential
remember pure water has a water potential of 0
how does the plant ensure water potential of roots is more negative/less than soil
mineral ions from soil are actively pumped into root
1 example of what plants use glucose for
respiration
1 example of what plants use lipids for
cell membranes
1 example of what plants use proteins for
enzymes for reactions
1 example of what a plant uses nucleic acids for
DNA replication
do all plant cells contain chloroplasts
no
root cells don’t, as there is no light underground so cannot photosynthesise
what 3 conditions could cause stomata to close
- high temperatures
- high CO2 concentration
- water stress
how are palisade cells adapted for their function
contain lots of chloroplasts + packed tightly together to maximise light absorption for photosynthesis
