3.1.3 Transport In Plants Flashcards
Why do multicellular plants need transport systems
Metabolic demands: internal/undergound parts of plant don’t photosynthesise, need exchange of substances, hormones and mineral ions need transporting
Size: need effective transport systems to move substances up and down from roots to leaves
SA: small SA:V, can’t rely on diffusion alone to supply cells
Function of xylem
Transport of water and mineral ions
Support
Structure of xylem
Non-living tissue
Long, hollow structures made by several columns of cell fusing end to end
Lignified walls to provide mechanical strength (rings, spirals or bordered pits)
Function of phloem
Transports food in form of organic solutes
Structure of phloem
Living tissue
Can be transported up/ down
Sieve tubes elements are cells joined end to end to form long, hollow structure
Not lignified
Between cells, sieve plates which let contents flow through
No nucleus
Companion cells, linked to sieve tube elements by plasmodesmata, have organelles
How are root hairs adapted to be exchange surfaces
Large SA:V due to hairs
Has thin surface layer, diffusion and osmosis can take place quickly
Concentration of solutes in cytoplasm maintains water potential gradient between soil and cell
What is the symplast
Cytoplasm of cells connected through plasmodesmata
What is the symplast pathway
Water moves through symplast by osmosis
- root hair cell has higher water potential than adjacent cell so water diffuses down water potential gradient, continues until xylem is reached
- as water leaves root hair cells, water potential falls again which maintains the water potential gradient between soil and root hairs
What is the apoplast
Cell walls and intercellular spaces
What is the apoplast pathway
Water fills spaces between fibres in cellulose cell walls
As water molecules move into xylem, water molecules are pulled through behind due to cohesive forces between water molecules
- the pull and cohesive forces creates tension meaning there’s continuous flow of water, no resistance from cellulose wall
How does water move into the xylem
Water reaches endodermis (layer of cells surrounding vascular tissue of roots) after travelling through either pathway
- casparian strip is a band of Suberin that creates a waterproof layer around endodermal cells
- this forces water in apoplast pathway into cytoplasm which means no toxic solutes in the soil water reach living tissues (no carrier proteins in CSMs)
- endodermal cells have lower solute concentration than xylem and mineral ions are actively transported into xylem
- rate of osmosis increased into xylem
How is water brought up the xylem (not transpiration)
Active pumping of minerals into xylem established root pressure
Gives water a push up the xylem
What is transpiration
Loss of water vapour from leaves and stems by diffusion due to stomata being open
Inevitable consequence of gas exchange
Explain gas exchange in plants
CO2 diffuses into leaf cells down a concentration gradient from air spaces in the leaf
O2 diffuse out of cells into spaces by diffusion down concentration gradient
Enter/ leave through stomata, opened and closed by guard cells
What is the transpiration stream
Water reaches leaves from xylem, moved by osmosis across membranes and by diffusion in apoplast pathway from xylem through cells of the leaf
- evaporates into air spaces in leaf
- water vapour moves into external air through stomata along diffusion gradient
- loss of water lowers water potential of leaf cells, water moves into cell from adjacent cells by osmosis (symplast and apoplast) and from xylem to leaf cells
What maintains the transpiration stream
Cohesion-tension theory
Adhesion: water molecules form hydrogen bonds with carbohydrates in walls of narrow xylem vessels
Cohesion: water molecules form hydrogen bonds with eachother
Capillary action: due to cohesion and adhesion, water can rise up a narrow tube against gravity, so water is pulled up to replace water lost by evaporation
Tension created in xylem: helps move water from soil to roots
How to measure the rate of transpiration
- cut shoot underwater at a slant
- assemble potometer underwater
- keep end of capillary tube submerged in beaker of water
- ensure apparatus is watertight and airtight (can grease apparatus to ensure this)
- dry leaves, allow shoot to acclimatise, shut tap
-remove capillary tube from beaker until one air bubble forms - record starting point
- start stopwatch, record distance moved per unit time, estimation of transpiration rate
How is stomatal opening controlled
By turgor
- turgor is low, asymmetric configuration of guard cell walls closes the poor
- when environmental conditions are favourable, guard cells pump solutes by active transport and turgor is increased
- cellulose hoops stop cells swelling in width, length extended, inner wall less flexible so cells become bean-shaped and pore is opened
- hormonal signals can trigger turgor loss when water is low
How does light affect rate of transpiration
In low light, most stomata will close
Increasing light increases open stomata and rate of water vapour diffusing out, and evaporation from leaf, therefore rate of transpiration increases
How does humidity affect rate of respiration
Affects water vapour potential gradient between inside leaf and outside air
High humidity = low rate of transpiration
How does temperature affect rate of transpiration
Increase in temperature increases KE of water molecules, increases rate of evaporation into air spaces from spongy mesophyll cells
Increase in temperature increases concentration of water vapour that external air can hold before it becomes saturated
How does air movement affect rate of respiration
Each leaf had layer of still air trapped due to shape and hairs which decreases air movement
This means water vapour accumulates, water vapour potential gradient reduced
So: air movement increases rate of transpiration
How does soil-water availability affect transpiration
Lower availability = lower rate of transpiration
What is translocation
Transport of organic compounds (mainly sucrose) in phloem from sources to sinks
