9 - Transport in plants

Question 1

need for plants to have transport systems

Answer 1

• 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

Question 2

what is a dicotyledonous plant

Answer 2

makes seeds that contain two cotyledons

Question 2

what is cotyledon

Answer 2

organs that act as food stores for the developing embryo, and form the first leaves when the seed germinates

Question 3

vascular bundles

Answer 3

arrangement of transport tissues in herbaceous dicots
- contains xylem and phloem

Question 4

structure of TS stem of young herbaceous plant

Answer 4

epidermis, cortex, vascular bundles and parenchyma

• vascular bundles are around the edge to give strength and support

Question 5

structure of TS root of young herbaceous plant

Answer 5

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

Question 6

structure and function of xylem

Answer 6

• 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)

Question 7

structure and function of phloem

Answer 7

• 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

Question 8

why is water important for plants

Answer 8

• 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

Question 9

what is a root hair cell

Answer 9

• exchange surfaces in plants where water is taken in from the soil
• a root hair is a thin extension of the root hair cell

Question 10

how are root hairs adapted

Answer 10

• 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

Question 11

how does water move into root hair cells

Answer 11

• by osmosis
• soil water has low conc of solutes and high water potential, sap and cytoplasm have a low water potential

Question 12

movement of water across root

Answer 12

symplast
apoplast

Question 13

symplast pathway

Answer 13

• 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

Question 14

apoplast pathway

Answer 14

• 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

Question 15

movement of water xylem

Answer 15

• 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

Question 16

evidence for the role of active transport in root pressure

Answer 16

• 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

Question 17

xerophytes

Answer 17

plants wit adaptation to survive in dry habitats

Question 18

ways of conserving water

Answer 18

• 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

Question 19

Hydrophytes

Answer 19

plants with adaptation to survive in watery habitats

Question 20

adaptations of hydrophytes

Answer 20

• 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

Question 21

transpiration

Answer 21

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.

Question 22

what is the transpiration stream

Answer 22

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.

Question 23

steps of the transpiration stream

Answer 23

• 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

Question 24

cohesion- tension theory

Answer 24

• the model of water moving from the soil in a continuous stream up the xylem

Question 25

capillary action

Answer 25

water can rise up a narrow tube against the force of gravity

Question 26

evidence for cohesion tension theory

Answer 26

• 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

Question 27

stomata - controlling the rate of transpiration

Answer 27

• 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

Question 28

factors affecting transpiration

Answer 28

• 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.

Question 29

what is translocation
what is assimilates? give example

Answer 29

• 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

Question 30

the main sources of assimilates

Answer 30

• 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

Question 31

the main sinks in a plant

Answer 31

• 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

Question 32

outline steps of translocation

Answer 32

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

Question 33

Phloem loading - apoplast route

Answer 33

• 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

Question 34

companion cells adaptations for phloem loading

Answer 34

• many infoldings in cell membrane to increase surface area for AT of sucrose
• many mitochondria to supply ATP needed for proton pumps

Question 35

Phloem unloading

Answer 35

• 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

Question 36

evidence for translocation

Answer 36

• 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