9 - Transport in plants Flashcards
1
Q
need for plants to have transport systems
A
- metabolic demands: products of PHS need to go to other areas than leafs. Waste products need to be removes. Hormones need to travel from where they r made to effector. Mineral ions from root need to be transported to cells to make proteins
- size:
- SA:Vol ratio: it is not simple in plants, diffusion alone does no supply the cells with everything they need
2
Q
what is a dicotyledonous plant
A
makes seeds that contain two cotyledons
2
Q
what is cotyledon
A
organs that act as food stores for the developing embryo, and form the first leaves when the seed germinates
3
Q
vascular bundles
A
arrangement of transport tissues in herbaceous dicots
- contains xylem and phloem
4
Q
structure of TS stem of young herbaceous plant
A
epidermis, cortex, vascular bundles and parenchyma
- vascular bundles are around the edge to give strength and support
5
Q
structure of TS root of young herbaceous plant
A
root, exodermis, epidermis, cortex, endodermis and vascular bundle
- xylem arranged like a cross in the centre, with the phloem in the
acute corners
- vascular bundles in the middle to help the plant withstand tugging strains that result as the stem and leaves are blown in the wind
6
Q
structure and function of xylem
A
- non-living tissue that transports water and mineral ions and gives support
- flows up to the laves
- xylem vessels are long hollow structures, made by joining several columns of cells fusing end to end
- it has parenchyma that packs around the vessels storing food and tannin (bitter chemical that prevents herbivory)
- xylem fibres are long lignified cells that provide mechanical strength
- lignin can form spirals or rings, with undignified areas called bordered pits (where water leaves)
7
Q
structure and function of phloem
A
- living tissue that transports substances needed by cells like sucrose and AA
- can go up and down
- sieve tube elements are cells that join end to end to form hollow structure
- cells arent lignified
- areas between cells become perforated o form sieve plates
- tonoplasts break down to form sap
- companion cells are linked to STE by plasmodesmata, and supply the companion cells
- contains sclereids
8
Q
why is water important for plants
A
- turgor pressure as a result of osmosis provides a hydrostatic skeleton to support stem and leaves
- turgor drives cell expansion
- loss of water by evaporation keeps plants cool
- mineral ions and products of PHS are transported in aqueous solutions
-water is needed for PHS
9
Q
what is a root hair cell
A
- exchange surfaces in plants where water is taken in from the soil
- a root hair is a thin extension of the root hair cell
10
Q
how are root hairs adapted
A
- microscopic size so penetrate between soil
- has large SA:V ratio
- has thin surface layer (cell wall and PM) so osmosis happens quickly
- conc of solutes in cytoplasm maintains water potential gradient between the soils and water
11
Q
how does water move into root hair cells
A
- by osmosis
- soil water has low conc of solutes and high water potential, sap and cytoplasm have a low water potential
12
Q
movement of water across root
A
symplast
apoplast
13
Q
symplast pathway
A
- continuous cytoplasm of the living cells that is connected through plasmodesmata
- root hair cell has higher ψ than the next cell along, so water diffusing into root has made the cytoplasm more dilute, so it moves to the next cell by osmosis.
- this repeats until the xylem
- as water leaves root hair cell ψ falls, maintaining ψ gradient
14
Q
apoplast pathway
A
- movement of water through the apoplast
-> the cell walls and intercellular spaces - water fills between the open network of fibres in the cellulose cell wall
- as water moves into the xylem, more water molecules are pulled through the apoplast dues to cohesion
- the pull of the water into the xylem, up the plant, by cohesions, creates tension = continuous flow of water
15
Q
movement of water xylem
A
- water moves by symplast and apoplast pathways until it reaches endodermis
- it has the casparian strips
-> it is a band of waxy material called suberin that runs around each endodermal cell forming waterproof layer - water in apoplast cant go further so it enters the symplast pathway as it is forded into the cytoplasm of the cell
-this removes any toxic solutes as water must pass through selectively permeable membranes - ψ in endodermal cells is high compared to cells in xylem. so more water moves into xylem by osmosis down ψ gradient through the symplast pathway
- endodermal cells move mineral ions into the xylem to increase its ψ, it also causes root pressure - giving water a push up the xylem
- ## once in vascular bundle it enters the apoplast pathway to entre the xylem
16
Q
evidence for the role of active transport in root pressure
A
- some poisons affect the mitochondria and prevent production of ATP, if is is applied to root cells, there is no energy, so root pressure drops
- root pressure increase with a rise in temp and falls with fall in temp, suggesting a chemical reaction
-if O2 levels fall root pressure falls
17
Q
xerophytes
A
plants wit adaptation to survive in dry habitats
18
Q
ways of conserving water
A
- thick waxy cuticle to reduce transpiration
- sunken stomata - reduced air movement, forming a microclimate of still humid air, reducing ψ gradient, reducing transpiration
- reduced numbers of stomata
- hair leaves - forming a microclimate of still humid air, reducing ψ gradient, reducing transpiration
- curled leaves - forming a microclimate of still humid air, reducing ψ gradient, reducing transpiration
- succulents - have specialised parenchyma that store water
- leaf loss
- root adaptations - long tap roots or a mass of widespread shallow roots with a large surface area
- avoiding the problems- becomes dormant or die, leaving seeds
19
Q
Hydrophytes
A
plants with adaptation to survive in watery habitats
20
Q
adaptations of hydrophytes
A
- very thin waxy cuticle
- many open stomata - maximises gas exchange
- reduced structure to the plant as water supports it
- wide, flat leaves to capture lots of light
- small roots - water can diffuse directly into stem and leaves
- large surface area of stems and roots under water
- air sacs to float
- aerenchyma - specialised parenchyma with large air spaces (made by apoptosis). makes leaves buoyant. forms a low resistance internal pathway for the movement of substances to tissues below the water
21
Q
transpiration
A
the loss of water vapour from the stems and leaves as a result of evaporation from inside the leaf and diffusions down a concentration gradient out the stomata
- stomata open to allow gas exchange, gain CO2 and remove O2.
22
Q
what is the transpiration stream
A
the movement of water through plant from the roots until it is lost by evaporation from the leaves
- water entres the roots by osmosis and is transported up the xylem until it reaches the leaves
- here it moves by osmosis across membranes and by diffusion in the apoplast pathway from the xylem through the cells leaf.
- it evaporates from the freely permeable cellulose cell walls of the mesophyll cells, into the air spaces
- is then leaves by the stomata.
23
Q
steps of the transpiration stream
A
- water evaporates from the surface of the mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient
- loss of water in the mesophyll cell lowers its ψ, so water moves into the cell from adjacent cells by osmosis, by the apoplast and symplast pathway
- this is repeated across the leaver to the xylem
- water molecules form H bonds with the carbohydrates in the walls of the xylem (adhesion) as well as with each other. Together it results in capillary action. water is drawn up the xylem in a continuous stream to replace the water lost by evaporation. (transpiration)
- the transpiration pull results in tension in the xylem, helping move water across the soil
24
cohesion- tension theory
- the model of water moving from the soil in a continuous stream up the xylem
25
capillary action
water can rise up a narrow tube against the force of gravity
26
evidence for cohesion tension theory
- changes in diameter of trees, during the day transpiration is at its highest and tension too-the trees diameter shrinks
- when xylem is broken, air is drawn into the xylem rather than water leaking out
- if xylem is broken air is pulled in the plant can no longer move water up the stem as the continuous stream of water molecules has been broken
27
stomata - controlling the rate of transpiration
- when turgor is low, the guard cell closes the pore
- when environment conditions are good, the guard cell pumps solutes in by active transport - increasing turgor
- cellulose hoops prevent the cells from swelling in width
- the inner wall is less flexible than the outer, so it becomes bean shaped and opens the pore
- when water becomes scarce, hormonal signals from the root trigger turgor loss from the guard cells
28
factors affecting transpiration
- light is required for PHS, in light the stomata open for gas exchange. IN the dark they close. Increasing light means more stomata open, so more transpiration
- high relative humidity lowers transpiration rate as there is a reduced ψ gradient between in the leaf and outside
- temperature increase KE of water molecules, so rate of evaporation increases. An increase in temp increases the conc of water vapour the external air can hold (it decreases ψ and relative humidity)
- air movement-
- soil water availability.
29
what is translocation
what is assimilates? give example
- transporting organic compounds in the phloem form sources to sinks
- it is usually an active process, up and down the plant
- the products of PHS
- glucose - however it is transported in the form of sucrose
30
the main sources of assimilates
- green leaves and stems
- storage organs like tubers and tap roots that unload there stores at the beginning of the growth period
- food stores in seeds when they germinate
31
the main sinks in a plant
- roots that growing or actively absorbing mineral ions
- meristems that are actively dividing
- any part of the plant that are laying down food stores, like developing seeds, fruits of storage organs
32
outline steps of translocation
Phloem Loading
- soluble products of PHS are moved into the phloem
- symplast of apoplast
- apoplast pathway
-> when sucrose reaches companion cells it actively transports H+ ions out by a proton pump, if comes back in down a conc grad through a co-transporter which also brings along sucrose
33
Phloem loading - apoplast route
- sucrose from the source travels through the cell walls and inter-cell spaces to the companion cells and sieve elements by diffusion down a conc grad
- sucrose is moved into the companion cell actively
-H+ ions are pumped out of the companions cells into the surrounding tissue using ATP
- H+ ions return to the companion cell, down a conc grad, by a co-transporter protein - where sucrose is co-transported
- increase of sucrose in companion cells and sieve tube elements
- water moves in by osmosis, building up turgor in the phloem - this forces sap to regions of lower pressure in the sinks
34
companion cells adaptations for phloem loading
- many infoldings in cell membrane to increase surface area for AT of sucrose
- many mitochondria to supply ATP needed for proton pumps
35
Phloem unloading
- sucrose in unladed from the phloem to any point into the cells that need it
- it diffuses into the surrounding cells
- the loss of solutes rises of the phloem, so water moves out by osmosis
36
evidence for translocation
- microscopy allows us to see adaptations of companion cells
- if mitochondria of companion cells is poisoned, translocation stops
- the flow of sugars in the phloem is much faster than diffusion alone, so there must be an active process
