Chapter 9 : Transport in plants Flashcards
what is the reason for transport systems in plants?
- Plants have high metabolic demands: this means that leaf cells in a plant make glucose but they are needed elsewhere / hormones need to be transported to where they have an effect
- Plants are relatively large in size: so need effective transport system to move substances up to the top most leaves / down to the root tips
- Plants have a small SA:V ratio: so cannot rely on diffusion alone to supply everything they need
What is the vascular system?
consists of the xylem and phloem
what is the vascular cambium and what does it consist of
cylindrical layer of meristematic cells which lies between the xylem and the phloem responsible for producing secondary vascular tissues (xylem + phloem)
where is the vascular cambium found
in the stem and roots
Describe the structure of the vascular bundle in the leaf
palisade mesophyll layer on top, xylem on the inside and phloem on the outside, midrib of leaf on outer edge
how does the structure of the vascular bundle in the leaf relate to its function
midrib of dicot leaf is main vein carrying the vascular tissue through the organ, this supports the structure of the leaf
Describe the structure of the vascular bundle in the root
outer exodermis, epidermis, endodermis and x shaped xylem on the inside surrounded by the phloem
how does the structure of the vascular bundle in the root relate to its function
vascular bundle is in the middle to help the plant withstand the tugging strains caused by the wind (because stems and leaves are blown
Describe the structure of the vascular bundle in the stem
outer epidermis, cortex, vascular bundles arranged in circle with phloem on the outside and xylem on the inside, and parenchyma in the centre
how does the structure of the vascular bundle in the stem relate to its function
vascular bundles are around the edge to give strength and support
how does the longitudinal section of a stem look like
cortex, long column of phloem on the outside and then long column of xylem on the inside. (vascular cambium between xylem + phloem)
parenchyma in the centre
describe the structure of the xylem vessel
are part of the xylem tissue
They are made up of dead hollow cells with no end cell walls
The cells have no organelles or cytoplasm,
The cell walls contain non lignified pits
The cell wall also contains tough, secondary walls called lignin,
xylem parenchyma packs around the vessels storing bitter tannins
How does the structure of a xylem vessel relate to its function
The cells have no organelles or cytoplasm, which creates more space inside the vessel for transporting water + mineral ions.
The cell walls contain non lignified pits which allows water and mineral ions to move into and out of the vessel (to supply other cells)
The cell wall also contains a tough, secondary walls called lignin, which strengthens the xylem vessel and provides structure and support to the plant.
xylem parenchyma packs around the plant storing tannins - a bitter tasting chemical that protects plant tissues from herbivores
In which direction does water and mineral ions flow in the xylem
upwards from the roots to the shoots and leaves
What are sieve tube elements
sieve tube elements are the transporting vessels that make up the phloem tissue
describe the structure of a sieve tube element
contain no organelles and are only filled with phloem sap (sucrose)
contain perforated walls called sieve plates allowing phloem contents (sucrose) to pass through and to allow cells to join end to end
contain plasmodesmata with attached companion cells
how does the structure of a sieve tube element relate to its function
specialised to carry out its sole function of carrying phloem sap around the plant
describe the structure of companion cells
form plasmodesmata with sieve tube elements (linking their cytoplasm)
maintain their nucleus and all other organelles
how does the structure of companion cells relate to their function
are very active cells that function as a life support system for the sieve tube elements which have lost their normal cell functions
can provide the energy for active transport of solutes
in what direction does the materials in the phloem flow in a plant
can go both up and down the plant (travelling from source- where it is made, to sink- where it is needed for respiration)
in what pathways can water move across the root and to the xylem ?
how ?
the apoplast pathway- the movement of water through the cell walls and intercellular spaces
water molecules are pulled across by the cohesive forces amongst the water molecules creating tension
the symplast pathway: moving through the continuous cytoplasm of plant cells connected via plasmodesmata water moves from cell to cell via osmosis,
water potential of cytoplasm in previous cells falls + a continuous movement of water from the soil into cell is enabled.
How does water move into the root from the soil
water moves into the root hair cells from the soil
these cells are well adapted for exchange
soil water has a low conc of dissolved minerals +
a high water potential whereas the cytoplasm of a root hair cell has a high concentration of solutes
so water moves in by osmosis
how are root hairs adapted for their role as exchange surfaces
- microscopic size allowing them to penetrate soil particles
- each hair has a large SA:V ratio (to take in water)
- thin surface layer where diffusion and osmosis can take place quickly
- concentration of solutes maintains water potential gradient between soil water and the cell
Describe the movement of water towards the xylem
water moves across the root in apoplast and symplast pathways
until the endodermis is reached (layer surrounding vascular tissue in the roots) - the Casparian strip on the endodermal cells form a waterproof layer
water in the apoplast pathway is forced into the symplast pathway:
water mast pass through selectively permeable membranes excluding the toxic solutes in soil water entering the cytoplasm
what is the casparian strip made of
a waxy waterproof material known as suberin
how does water enter the xylem
solute conc in cytoplasm of endodermal cells is low / dilute compared to xylem
endodermal cells move mineral ions into the xylem by active transport (forming ATP)
this leads to water potential of the xylem being lower than the endodermal cells
this increases the rate of water moving into the xylem by osmosis down a water potential gradient from endodermis
describe the process of water being transported out of the leaves (transpiration stream)
water molecules evaporate from the surface of the mesophyll cells into surrounding air spaces and move out of the stomata into outside air by diffusion (down a conc gradient)
loss of water by mesophyll cells —> lowers water potential and water moves into this cell from an adjacent cell via osmosis, along apoplast and symplast pathways
this occurs across the leaf leading up to the xylem, water moves out of the xylem via osmosis
water molecules form hydrogen bonds with carbohydrates in xylem walls (adhesion) water molecules also form hydrogen bonds with each other (cohesion) leading to capillary action
this transpiration pull in the xylem leads to tension helping to move water across the roots from the soil
what is the transpiration pull
water being drawn up the xylem in a continuous stream to replace water lost by evaporation
consists of mechanisms like cohesion tension and adhesion
what is capillary action
the process by which water rises up a narrow tube against the force of gravity
what is the transpiration stream
the movement of water from the roots to the leaves
why does transpiration happen
it is a consequence of gaseous exchange which must occur because a plant must photosynthesise (take in co2 and give out o2)
how does light intensity affect transpiration rate
- increasing light intensity increases numbers of open stomata (because gas exchange is needed for photosynthesis)
- this increases the rate of water vapour diffusing out and so increasing the evaporation from the surfaces of the leaf
how does temperature affect transpiration rate
- increasing temperature increases transpiration rate
- an increase in kinetic energy occurs therefore increasing the rate of evaporation from the spongy mesophyll cells into air spaces of the leaves
- also increases water potential gradient inside and outside of the leaf (because there is a decrease in humidity)
how does humidity affect transpiration rate
- increasing humidity will decrease transpiration rate
- this is because there is a reduced water vapour potential gradient between the inside of the leaf and the outside of the leaf
how does air flow affect transpiration rate
- the windier it is the faster the transpiration rate
- lots of air movement blows water which may have been trapped by hairs surrounding the stomata, this increases water potential gradient and therefore the rate of transpiration
how does soil water availability affect transpiration rate
- if there is a large amount of water in the soil it leads to an increase in transpiration rate
- if it is very dry the plant will be under water stress and rate of transpiration is reduced
what is a potometer
- it is a special piece of apparatus used to estimate transpiration rates
- it actually measures water uptake by a plant, but its assumed that water uptake by the plant is directly related to water loss by the leaves
how is a potometer used
- Cut shoot underwater to prevent air from entering the xylem, cut it at a slant to increase surface area available for water uptake
- Assemble the potometer and insert shoot underwater (so no air enters)
- Remove apparatus from the water but keep end of capillary tube submerged
- Check apparatus is air tight (seal joints with petroleum jelly)
- dry leaves and allow time for shoot to acclimatise + shut tap
- remove end of the capillary tube from water until one air bubble has formed, put end of the tube back in water
- record starting position of the bubble start stop watch and record distance moved by bubble using ruler (per unit of time) rate of air bubble movement is transpiration rate
- only change one variable at a time for experiment. other conditions must be kept constant
what are xerophytes
- Plants with adaptations to conserve water/ prevent excess water loss by transpiration (because they exist in dry climates)
- these include cacti and marram grass
how are xerophytes adapted to conserve water
(include examples of cacti and marram grass)
-have a thick waxy cuticle - to minimise water loss by transpiration
-contain a sunken stomata- reduce air movement, create a microclimate of still humid air reducing water vapour potential gradient (found in cacti and marram grass)
-reduced numbers of stomata- reduce rate of water loss by transpiration—> can also reduce gas exchange capabilities
-reduced leaves- reducing leaf area can also reduce rate of water loss/ SA:V ratio is reduced minimising water loss by transpiration
- hairy leaves- create a microclimate of still humid hair reducing water vapour potential gradient and evaporation of water from stomata
- curled leaves- confines stomata to microclimate of still humid air, reducing diffusion of water vapour from the stomata (found in marram grass)
- long roots- to receive as much water as possible
what are hydrophytes
- plants with adaptations to cope with growing in water/ permanently saturated soil
- include water-lilies, water-cress
how are hydrophytes adapted to the availability of water in their surroundings
- have a very thin waxy cuticle - do not need to conserve water as there is plenty available
- many always open stomata on the upper surface (of leaf)- maximising gaseous exchange, there is no risk of loss of turgor (due to water loss) in plant cells
- wide flat leaves- capture as much light as possible for photosynthesis
- small roots- water can diffuse directly into the stem and leaf because of its abundance so no need for it
- large surface area of roots and stem underwater- maximises area for photosynthesis
- air sacs- to enable hydrophyte leaves/ flowers to float on the surface
what is aerenchyma
- specialised parenchyma which has formed by apoptosis of normal parenchyma
- it makes the leaves and stem more buoyant, forms a low resistance internal pathway for movement of substances like oxygen to tissue below the water
- helps plant cope with anoxic conditions, by transporting oxygen in the tissues
what are sources and sinks + how are they significant in plant transport
sources are where the organic compounds formed from photosynthesis are made and sinks are the tissues that require them
plants typically transport the organic substances from source to sink
What are assimilates and what is the main assimilate in plants
the products of photosynthesis that are transported are known as assimilates and the main assimilate transported around the plant is sucrose.
What are the main sources of assimilates in plants
green leaves, green stems, storage organs like tubers and tap roots, food stores in seed when they germinate
What are the main sinks in plants
roots (that are actively growing and dividing + absorbing mineral ions)
meristems that are actively dividing
parts of the plants laying down food stores e.g. storage organs
Describe the process of phloem loading
sucrose travels through the apoplast route from sources to companion cells and sieve elements - via diffusion down a conc gradient
proton pump, pumps out a proton outside companion cell (using ATP)- a cotransporter then takes both the proton and sucrose into the companion cell (down proton gradient)
increases sucrose concentration in companion cells and sieve elements via linked plasmodesmata
How are companion cells adapted to take in as much sucrose as possible
they have many infoldings in their membrane which increase the surface area for active transport of sucrose into the cell cytoplasm
also have many mitochondria to supply ATP for transport pump
How do the assimilates move towards the sources
Build up of sucrose in companion and sieve tube elements lead to an increase in turgor pressure (water moves in by osmosis)
sucrose moves out of the companion cells into tubes of sieve tube elements reducing pressure and moves up or down the plants to the sinks by mass flow
Describe the process of phloem unloading
Sucrose is unloaded from the phloem via diffusion to surrounding cells.
sucrose is then rapidly converted into another substance e.g. glucose for respiration- so that the concentration gradient of sucrose is maintained between phloem + surrounding cells
loss of solutes from phloem leads to rise in water potential in phloem - this water moves out to surrounding cells by osmosis/ drawn into transpiration stream
What is the evidence for translocation
if mitochondria in companion cells are poisoned translocation stops
flow of sugars in phloem is 10,000 times faster than it would be by diffusion alone, suggesting an active process
(presence of aphids) positive pressure that forced the sap out of the phloem through the stylet + pressure therefore flow rate is lower closer to the sink than it is near to the source.
concentration of sucrose is also higher near to the source than sink
what is the effect of light intensity on translocation
Light intensity affects photosynthesis, influencing the amount of sucrose available for translocation.
there is an increase in the sucrose concentration for transport.
