3.3 Transports in plants Flashcards
Why do plants need transport systems?
Metabolic demands - Photosynthesis
Size
Sa:Vol ratio
Transport system in dicotyledonous plants:
Contains vascular system:
Phloem
Xylem
Draw a root tissue of a dicotyledonous plant, annotate
Exodermis
Cortex
Endodermis
Cambium
Xylem = + shape structure
Phloem = triangle shape structures surrounding xylem
Draw and annotate a stem tissue of a dicotyledonous plant
Epidermis
Cortex
Phloem - (outside tissue of vascular bundle)
Cambium - separating phloem and xylem
Xylem - (inside tissue of vascular bundle)
Draw and annotate a leaf tissue of a dicot leaf
Upper epidermis
Palisade mesophyll
Spongy mesophyll
Inside spongy mesophyll - xylem - towards top, phloem - underneath xylem
Lower epidermis
What is the xylem function
Transport of water and mineral ions
Support
What is the structure of Xylem
Hollow tube
Non-living tissue
Lignified walls - spiral shape
+ bordered pits
Xylem parenchyma - around xylem vessels
What is the purpose of lignified wall in the xylem
provide structure
What is the purpose of Xylem parenchyma in xylem
storing food
Tannin deposits - bitter substance - prevent against animal attacks
What is and what is the purpose of bordered pits in the xylem
un-lignified areas in the xylem, that allow for water to leave the xylem and move to other areas of the plant
What is the purpose of phloem?
Transport assimilates up and down the the cell
What is the structure of the phloem?
Contains sieve tube elements
Perforated walls form sieve plates
No nucleus, tonoplast, or other organelles
supporting tissues - fibres and sclereids
What is the function of companion cells
Cells linked to sieve tube elements, by plasmodesmata
Active cells ‘life support system’ - maintain organelles
How is water transported in plants?
Turgor/hydrostatic pressure
Why is water needed in plants
Photosynthesis
How are root hair cells adapted for efficient exchange
Microscopic hairs - large SA:V
Hairs have thin surface layers
High concentration of solutes in cytoplasm, maintaining a water potential gradient
Where does water exchange into a root of a plant
Root hair cells
How does water move into the root hair cell
Via osmosis
Down a water potential gradient
What is the symplast pathway
Water moves through the cytoplasm of plants
How is water moved in the symplast route
Water diffuses in, making the cytoplasm more dilute. so water moves into the next cell along. This makes the water potential fall, maintaining a steep concentration gradient
What is the apoplast pathway
Water moves between intercellular spaces
How is water moved in the apoplast route
Water molecules are pulled through due to cohesive forces between the water molecules, creating tension
How does water move close to the xylem
Water moves to endodermis, across the casparian strip, into the xylem
What is the casparian strip
Waterproof wax band that runs around cells
What is the function of the casparian strip
Forces water into the symplastic route. This filters the water, removing any toxic solutions, as cell surface membranes have no carrier proteins
How does water move into the xylem
endomeral cells move mineral ions into the xylem via active transport, Water potential in xylem cells is much lower, water move into xylem via osmosis
What happens after water enters the xylem
Water returns to apoplast pathway,
pumping of minerals into the xylem and the osmosis of water into the xylem results in root pressure, giving water a push up the xylem
Evidence for role of active transport in root pressure:
Some poisons that affect mitochondria are applied to roots, removing root pressure
Root pressure increases with a rise in temperature
If oxygen or respiratory substrates fall, root pressure falls
What is transpiration
Loss of water vapour from the leaves
What structure allows for gas loss in plants
Stomata
Opened and closed via guard cells
Why are guard cells important
Control amount of water loss
Varies day and night
seasonally
What is the transpiration stream
stream of water, moving from roots to leaves,
Water leaving via stomata:
Water molecules evaporate out of mesophyll cells into air spaces, moving out of stomata, down a concentration gradient, this lowers the water potential of the cell, water moves in from other cells via osmosis.
Capillary action:
Water molecules for hydrogen bonds with carbohydrates in xylem vessels (adhesion), and hydrogen bonds with other water molecules, so stick together (cohesion), resulting in capillary action
Transpiration pull:
Water is drawn up into xylem in a continuous stream to replace water lost via evaporation
Cohesion tension theory:
water moving from the soil in a continuous stream up the xylem and across the leaf
Evidence for cohesion tension theory
Changes in diameter of tress - during the day, transpiration is at the highest, xylem is tense, so tree shrinks.
At night, transpiration is at the lowest, xylems are not under tension, diameter increases.
If a xylem vessel is broken and put in water, air is drawn into xylem, water can no longer be drawn up, continuous stream is broken
Factors impacting transpiration:
Light intensity
Humidity
Temperatures
water availability
Wind/air movment
How do stomata close and open
When turgor is low, guard cell walls close the pores
When conditions are good, solutes are actively pumped, increasing their turgor
How are guard cells adapted
Inner walls of guard cells are more flexible then the outer walls
Cellulose hoops prevent width extension, so they extend lengthways
What is translocation
Movements of assimilates around the plant
Where do assimilates need to move in a plant
From source to sink
What is the main assimilate needed to be transported in plants
Sucrose
Sources:
Leaves and stems
Storage organs unloading their stores at the beginning of a growth period
Germination of food stores in seed
Sinks:
Roots that are growing and actively absorbing mineral ions
Meristems that are dividing
Parts of plants that are using food stores (seeds, frits or storage)
Active loading:
At the source, companion cells actively pump hydrogen ions into the source tissue, increasing the concentration. Hydrogen ions diffuse back into companion cells via facilitated diffusion along with sucrose molecules (Co-transport). Sucrose molecules build up in companion cells, then move into sieve cells via facilitated diffusion, across the plasmodesmata and cytoplasm bridge
Mass flow: At the source
At the source, the active loading of sucrose into sieve tissue lowers the water potential in the sieve tube. Water moves into sieve tubes via osmosis from xylem and source. Increasing the volume and thus hydrostatic pressure withing sieve tissues.
Mass flow: At the sink
At the sink, sucrose moves into the sink tissue, via simple diffusion, across plasmodesmata into companion cells, then facilitated diffusion into sink, increasing water potential. Water movves out of sieve tues, into xlem, decreasing volume of water and hydrostatic pressure.
Why is Mass flow important
Sucrose and water is forced from the source tissue into the sink, quicker then normally, down a hydrostatic pressure gradient
Evidence for translocation:
Microscopy
If mitochondria are poisoned, translocation stops
Flow of sugars is very fast compared to diffusion
Aphids show that there is pressure in the phloem that forces the sap out, pressure is lower at the sink then nearer the source
What are xerophytes
Plants that live in low water availability areas
hot, dry, breezy
Cold, icy
Ways of conserving water:
Thick, waxy cuticle
Sunken stomata - reduce air movement
Reduced number of stomata
Reduced leaves
Hairy leaves - catches lost water, creating humidity, reducing excess water loss
Curved leaves - increases humidity
Leaf loss
Deep roots or widespread, shallow roots
Adaptations of marram grass:
Curled leaves
Sunken stomata
Hairs
Vertical and horizontal roots
What are hydrophtes
Plants that live in water
Adaptations of hydrophtes
Thin or little waxy cuticle
Open stomata / inactive guard cells
Stomata on upper surface
Reduced structure - supported by water
Wide flat leaves
Small roots
Large SA of stems and roots underwater
Air sacs / Aerenchyma - tissue with many air spaces
Adaptations of water lillies:
Stomata on upper surface
REduced structure
Wide, flat leaves
