Transport In Plants Flashcards
Reasons for plant transport systems
-Metabolic demands- internal and underground don’t photosynthesise require mineral ions to make proteins
-size - need effective transport to move sub up and down to topmost leaves
-SA:Vol ratio - stems and roots cannot rely on diffusion alone
Dicotyledonous plants
Produce seeds that contain 2 cotyledons - organs that act as food stores for developing embryo
Structure of xylem
Vessels made of dead cells long and hollow in structure and fused end to end
Parenchyma cells pack around xylem storing food and tannin deposits
Tannins
Bitter tasting chemical which plant tissues from attack by herbivores
Lignin
Mechanical strength
Can form rings spirals or solid tubes w non lignified pits for water movement out of the xylem
Xylem function
Movement of water and minerals
Turgor pressure
Hydrostatic pressure as a result of osmosis pressure exerted by the cell surface membrane against the cell wall
Provides a hydrostatic skeleton which supports stems and leaves
Root hair cell adaptations
Large SA:Vol ratio thousands on each root tip
Thin surface layer quick diffusion
Conc of solutes maintains a water potential gradient between soil and cell
Movement of water across the root
Symplast pathway
Apoplast pathway
Symplast pathway
Water moves through continuous cytoplasm of the living plant cells that are connected through the plasmodesmata
As water leaves the root hair cells it drops the water potential allowing for more movement of water
Plasmodesmata
Microscopic channels between neighbouring plant cell through the cell walls enabling transport
Apoplast pathway
Water moves through cell walls and intercellular space
As water moves into the xylem more water is pulled through due to cohesive forces between H2O molecules
Water movement stops at the casparian strip
Casparian strip
Band of waxy suberin which is hydrophobic
Endodermis surrounding vascular tissue
Diverts water moving through apoplast to be diverted through cytoplasm
Prevents toxic solutes moving any further into the plant
Once into vascular tissue water returns to apoplast pathway
Endodermal cell function
Move mineral ions into xylem via active transport which lowers WP in the xylem allowing for water movement via osmosis
Root pressure
Gives water a push up the xylem due to the active pumping of minerals into the xylem
Evidence root pressure is active process
Increases w a rise in temp
If respiration falls RP falls
Xylem sap is forced out at night when transpiration is low
Transpiration
Loss of water vapour from leaves and stems by the opening of stomata when exchanging CO2
Transpiration stream
Water moves across membranes by osmosis and diffusion in apoplast pathway from the xylem through the cells of the leaf
Evaporates from cell walls of mesophyll into air spaces (lowers wp)
Moves through stomata along a diffusion gradient
Adhesion
Water forms hydrogen bonds with carbohydrate walls of the xylem vessels
Cohesion
Water molecules form h bonds with each other and stick together
Capillary action
Combined effects of adhesion and cohesion by which water can rise up xylem against gravity
Cohesion tension theory
Water drawn up the xylem in a continuous stream to replace the water lost by evaporation - transpiration pull
Results in tension in xylem which helps move water across roots from the soil
Evidence for cohesion tension theory
Changes in diameter of trees - day tension is highest diameter lower
Xylem vessel breaks air drawn in rather than water drawn out
Stomata func
Turgid pressure is low guard cells close the pore
When environmental conditions are favourable solutes pumped in incr turgor allowing to open
Cellulose prevent swelling in width
Inner wall less flexible
Factors affecting transpiration
Light - more light longer open
Humidity higher the harder to lose water
Temp - more kinetic energy evaporate quicker and decrease of humidity
Air movement - each leaf layers of hair hold air more wind easier trans
Phloem structure
Sieve tube elements
Long hollow structures no nucleus
Companion cells linked to sieve tubes by plasmodesmata- life support system
Supporting tissues of fibres and sclereids
Phloem function
Transports food in the form of organic solutes around the plant from where they are made
Supplies the sugars and amino acids needed for respiration and synthesis of all useful molecules
Translocation
Transportation of organic molecules in the phloem from source to sink ( tissues that need them)
Assimilates
Products of photosynthesis
Main assimilate is sucrose
Main sources in a plant are green leaves and stem storage organs and stores in seeds when first germinate
Main sinks
Roots that are growing
Meristems that are dividing
Any part of plant that are developing food stores eg seeds fruit etc
Phloem loading
Soluble products moved into phloem via active processes
Main technique by apoplast route
Apoplast route - phloem
Sucrose travels to source through cell walls and intercellular spaces to companion cells
Here H+ ions pumpe out actively and renters cotransporting a sucrose molecule
Travel through plasmodesmata to sieve tubes
Osmosis in phloem loading
Due to low wp water moves in via osmosis which leads to turgor pressure
Moves into the sieve elements reducing pressure in companion cells and move up or down plant by mass flow to low pressures
Forces sap to regions of lower pressure due to turgor pressure increase
Phloem unloading
Unload at any point cells need it
Diffusion of sucrose into cells from phloem
Loss of solutes leads to a rise in wp of the phloem so moves out via osmosis some enters the xylem
Evidence of translocation
Advances in microscopy allow study of companion cells
If mito of comp cells poisoned translocation stops
Flow of sugars in phloem is far faster than diffusion alone - active process of mass flow
Aphids can demonstrate translocation as positive pressure forces sap out
Xerophyte adaptations
- thick waxy cuticle minimise water loss from cuticle
-sunken stomata - in pits to minimise water loss from air movement creating microclimate - reduced number of stomata
- hairy leaves
- Curled leaves - microclimate of humid air
- succulents - store water in specialised parenchyma tissues
- leaf loss
- root adaptations shallow large sa and v deep
Hydrophyte adaptions
-very thin cuticle
- many always open stomata for max gaseous exchange
- reduce structure support by H2O
- wide flat leaves - capture as much light as possible
- small roots but large SA
- air sacs to float
Aerenchyma
Specialised parenchyma tissue many airspace’s for extra buoyancy and forming a low resistance movement of substances to tissues below the water
