2.3 plant transport Flashcards
what does photosynthesis produce
glucose through sunlight
atp ?
energy required to move and transport substances
2 types of vascular tissue
xylem and phloem
what is the xylem
tissue in plants that move water and dissolved substances upwards
what is the phloem
plant tissues containing sieve tube elements and companion cells, translocating sucrose and amino acids from the leaves to the rest of the plant
how are plant roots adapted for the uptake of water
give a large surface area for the absorption of water by osmosis
where is the vascular bundle in relation to the plant
the stem
types of tissue (plant)
parenchyma
collenchyma
sclerenchyma
what is the parenchyma
‘packing’ tissue where cells are not specialised, they are alive with thin walls.
where is parenchyma found
in the cortex
what is the collenchyma
modified parenchyma cells, still alive but have irregular shaped thicker cellulose walls
supportive tissue
where is collenchyma found
cortex
what is the sclerenchyma
deads cells (have no cell contents. often stained red, have very thick secondary cell walls which have been lignified
where is sclerenchyma found
in our vascular bundles
absorption of water by the root
when roots make contact with the soil, water moves into the roots via osmosis
where are water and mineral ions found
in the soil
why does water need to get to the xylem
to be distributed around the plant
what are the 3 ways water moves through the root cells and into the xylem tube
- apoplast pathway
- symplast pathway
-vacuolar pathway
what is the pathway of choice for water
apoplast
explain the apoplast pathway
fastest pathway
water moves through cell walls
stops when it hits the endodermis
explain the symplastic pathway
moves through cytoplasm and changes cells through the plasmodesmata
explain the vacuolar pathway
water moves through the vacuoles
what can water not pass through
the impermeable waterproof barrier in the cell wall
what is the endodermis
a single layer of cells around the pericycle and vascular tissue of the root
what does the casparian strip do
stops the flow in the apoplast pathway
what is the casparian strip do
impermeable band of suberin in the cell walls of endodermal cells, blocking the movement of water in the apoplast pathway forcing it to go into the cytoplasm (symplast pathway)
why is the casparian strip important
because water can only enter the xylem from the symplast or vacuolar pathway
explain how water entering the xylem in the root
this cannot happen from the apoplast pathway because the lignin walls are waterproof. water enters xylem via the symplast or vacuolar pathway
what are the 2 transport systems
(water)
- transpiration
-translocation
what is transpiration
movement of water molecules and dissolved mineral ions
where does transpiration happen
in the xylem vessels
does transpiration require energy
no its a passive process
what is translocation
the movement of sugars (sucrose) and amino acids
where does translocation happen
in phloem vessels
sieve and companion cells
does translocation require energy
yes it is an active process
what colour do xylem vessels stain
why?
stain red
because they’re dead
explain the process of xylem vessels
- early on in life each xylem vessel is separate from eachother
- as it matures it dies and the end cell walls break down (forms a long tube of cells)
- the long tube can run all the way up the stem from the roots to the leaves
what is the tube of xylem vessels made of
cellulose
what happens as a result of cellulose not being strong enough
it is strengthened through the addition of lignin making the walls strong
what does lignin appear as
rings/spirals
what are the 2 functions of xylem
1) transport of water and dissolved mineral ions
2) providing mechanical strength and support
explain how xylem tissue is formed
xylem parenchyma (is living) it modifies and turns to xylem tissue which is dead
what is xylem tissue made up of
tracheids and xylem cessles
explain tracheids
walls made from lignin (hard, strong and waterproof) cells can elongate, have tapered endss
the walls have gaps
spindle shapes
what are the gaps in tracheids called
pits
role of pits in tracheids
where water moves through
where are xylem vessels found
in angiosperms only
explain xylem vessels
they are really well adapted
water goes straight up
tracheids vs vessels
water travelling straight up the plant in vessels is so much more efficient than the twisting path through tracheids that angiosperm have become the dominant plant type on earth
2 explanations for water to move from the root endodermis to the xylem
1- an increased hydrostatic pressure in root endodermal cells pushes water into the xylem
2- the decreased water potential in the xylem (below the endodermal cells) draws water in by osmosis across the endodermal cell membrane
how does water move into the xylem
by osmosis down a concentration gradient
what does water coming into the xylem do
generates an upwards push, the root pressure on water already in the xylem
explain the uptake of minerals
1) minerals absorbed into root hair by active transport
2) mineral ions can also move along the apoplastic pathway in solution
what does the casperian strip do to mineral ions
prevents the movement in the cell wall so mineral ions enter the symplast pathway in the cytoplasm by AT and are diffused or actively transported into the xylem
what does AT allow
(mineral ions)
the plant to absorb ions at the endodermis
explain the movement of water from root to leaf
water moves down a water potential gradient, air has a very low water potential and soil water has a very high water potential so water moves from roots to leaves in the xylem
3 mechanisms of the movement of water from root to leaf
1) cohesion tension theory (transpiration)
2) capillarity
3) push of water up the xylem (root pressure)
explain cohesion tension theory/ transpiration
evaporation of water into the atmosphere from the stomata of the plants creates a pulling force drawing the water column up the xylem (tension) the water molecules are strongly attracted to eachother (cohesion)
explain adhesion
occurs as a result of attraction between the charges of the water molecules and the hydrophillic lining of the vessels
adhesion holds the column of water in place and contributes to water movement
explain capillarity
movement of water of narrow tubes by capillary action, only operates over short distances, up to a meter. only makes a small contribution to water movement in plants more than a few CM high
capillarity
HOW?
cohesion between water molecules generates surface tension which combines with water’s attraction to the xylem walls draws the water up
explain root pressure
upward force on water in roots, derived from osmotic movement of water into the root xylem.
in the root, endodermis cells actively transport minerals ions into the xylem, reducing its water potential
summary of root pressure
- water is drawn in
- hydrostatic pressure increases
- water is pushed upwards
what is the site of transpiration
stomata
what is transpiration
the loss of water by evaporation, which is water vapour from leaves and shoots of plants. this causes the transpiration stream
what is the transpiration stream
continual flow of water in at the roots, up the stem to the leaves and out to the atmosphere
what does the rate of transpiration depend on
these factors interact with eachother
1- genetic factors: genes controlling the number, distribution and size of stomata
2- x4 genetic factors: temperature, humidity, air movement. these x3 affect the water potential gradient between water vapour in the leaf and the atmosphere
and light, by controlling the degree of stomatal opening
harmful aspect of transpiration
water loss
useful aspects of transpiration
water uptake (photosynthesis)
water distribution (turgidity)
ion distribution and evaporative cooling
what is humidity
a measure of number of water molecules in the air. the air inside a leaf is saturated with water vapour, so it’s relative humidity is 100%
what is still air
accumilation of water around the stomata
what does still air do
reduces water potential gradient
how do we measure the rate of transpiration
using a potometer, this actually measure the rate of water absorption, but this should the same as the rate of transpiration
what do you need to do to set up a potometer correctly
cut the shoot under water (prevent air bubbles forming in the xylem)
keep the leaves dry
set up apparatus under water
ensure all joints are airtight
what does the reservoir do
specified prac
returns the bubble to zero/starting point
what does the air bubble show
specified prac
movement of the bubble indicates the volume of water taken up by the shoot
rate of water uptake =
speed of movement of air bubbles x cross-sectional area of capillary tube
3 groups of plants classified according to their adaptations to water ability
mesophytes
hydrophytes
xerophytes
describe mesophytes conditions
live in habitats with adequate water supply
what adaptations do mesophytes have
close stomata in the night to decrease water loss
shed their leaves in the winter to survive unfavourable times (frost)
bulbs to survive winter
xerophytes conditions
live in conditions where water is scarce
explain xerophytes
highly specialised, the water lost via transpiration is greater than water taken up by roots
example of a xerophyte
marram grass
describe living conditions of hydrophytes
water plants, they grow submerged or partially submerged in water
example of a hydrophyte
water lily
adaptations of water lily
-no waxy cuticle because the plants are submerged in water so water loss isn’t a problem
- stomata are on the upper surface, the underside of the leaf is submerged. stomata must be on the upper leaf to allow gas exchange
-plant is surrounded by water, little need for highly developed transport tissues, xylem is poorly developed.
- stems and leaves have large air spaces (provides buoyancy to the plant tissue when submerged)
- water is a support medium, little or no lignified support tissues are needed
adaptations of marram grass
-rolled leaves: large, thin walled epidermal cells at the bases of the grooves shrink when they lose water from excessive transpiration, causing the leaf to roll inwards, reduces transpiration
-reduced leaf area/ less area for evaporation
-hairs: stiff, interlocking hairs trap water vapour and reduces the water potential gradients
- thick waxy cuticle: a waxy covering which reduces water loss by evaporation from the epidermal tissue. No stomata, outer epidermis small thick-walled cells
- sunken stomata- stomata are found in the grooves on the inner side of the leaf, they allow water vapour to accumulate above the stomatal pore. increases the humidity which reduces the water potential gradient between the inside of the leaf and the air chamber. reduces the rate of transpiration
what is translocation
the transport of soluble organic materials, sucrose and amino acids
what and where is transported in the phloem
products of photosynthesis, away from the site of synthesis in the leaves to all other parts of the plant
what is the source
leaves
what are the products of photosynthesis used for
growth and stroage
why is sugar transported as sucrose and not glucose
because it is less reactive than glucose is
4 types of tissue cells in the phloem
companian
sieve tubes
phloem fibres
phloem parenchyma
explain the structure of phloem
contains sieve elements which end in sieve plates, these plates contain pores through which cytoplasmic filaments extend linking cells. not other organelles are in the sieve elements. companion cells contain mant mitochondria which release ATP
they also contain organelles for protein synthesis.
what does ATP help in the phloem
helps sucrose enter the sieve tube elements
what are both sieve tube elements and companion cells
alive
sieve tube elements
contain no nucleus and very little cytoplasm or organelles leaving room for mass flow of sap.
how do sieve tube elements organise themselves
stack end to end and are separated by sieve plates.
what do sieve plates allow
movement between sieve elements
role of companion cells
carry out metabolic processes, make proteins and ATP for sieve tube elements
how are companion cells connected to sieve tube elements
by plasmodesmata
explain what ringing experiments have provided
evidence to support translocation in the phloem
briefly describe what was found after this experiment
phloem was removed, analysis revealed that sucrose accumulated above the cut ring. provided evidence that sucrose was transported to this region of the stem by translocation.
there was also no sucrose below the ringed area. sucrose couldn’t be transported here as the phloem tissue had been removed
evidence to support translocation in the phloem
aphids have a stylet, it is inserted directly to the sieve tube allowing the aphid to feed on the sugary sap, stylet is lasered off. analysis shows the sap contains products of photosynthesis- sucrose and amino acids
explain radioactive labelling
technique showed sucrose is transported upward and downwards.
what is the mass flow theory- what does it suggest
is the theory by which we think solute transport occurs in plants, from the source (highest conc.) to the sink (lowest conc)
suggests there is a PASSIVE MASS FLOW of sucrose
summary of mass flow theory
- initially high hydrostatic pressure in the source because dissolved sucrose reduces the water potential and draws water in by osmosis this increases the water potential
- sucrose is pushed into the phloem, due to the high hydrostatic pressure. this increases the water pressure and reduces the water potential
-water flows along the sieve tube element by osmosis from high to low hydrostatic pressure - sucrose is removed from the sieve tube by the surrounding cells at the sink and increases the water potential in the sieve tube
- water moves out of the sieve tube and reduces the hydrostatic pressure- waters lost by osmos
arguments against mass flow theory
- rate of translocation is 10,000 times faster than it would be if substances were moving by diffusion
- sieve plates with tiny pores act as a barrier impeding flow
- sucrose and amino acids move at different rates and in different directions in the same in the same phloem tissue
-phloem tissue has a high rate of oxygen consumption, and translocation is stopped when a respiratory poison such as cyanide enters the phloem
-companion cells contain lots of mitochondria and produce ATP but the mass flow hypothesis fails to suggests a role for the companion cells
