Chapter 9 - Transport In Plants Flashcards
Name three reasons why multicellular plants need transport systems
Metabolic demands
Size
Surface are : volume
Metabolic demands for plant transportation
Internal and underground parts of the plant need to transport
- Oxygen and glucose for photosynthesis
- Remove waste products from metabolism reactions
- Hormones to areas where they have an affect
- Mineral ions around the body from the root, to make proteins for enzymes and cell structure
Size demands for plant transportation
Large plants need effective transport systems that will transport substances (assimilates) from the roots to the leaves
Surface area: volume demands for plant transportation
Quite a small sa:v
So can’t rely solely on diffusion to supply cells with everything they need
Function of xylem
Transport of mater and mineral ions in solution from bottom to top
Describe the xylem
- Made from columns of vessel elements fusing together end to end
- Non living tissue, dead cells
- Walls thickened with lignin - support, structure, waterproof (spirals, rings)
- Bordered pits - water ions move into and out of
What does xylem tissue contain:
Parenchyma
- packs around xylem vessels
- stores food
- contains tannin (protects from attack)
Fibres
- long cells with lignified secondary walls
- provide extra mechanical strength
Function of phloem
Transports organic solutes such as sugars and amino acids , needed for cellular respiration and synthesis, around the plant from the leaves where they are made by photosynthesis
What are the main transporting vessels of the phloem
What does the phloem tissue consists of
Phloem sieve tube elements
- no nucleus, thin layer of cytoplasm, few organelles
- many cells joined end to end to form hollow structure
- sieve plates let solutes pass through
Companion cells
- carry out living function for both of them
- provide the energy for the active transport of solutes
Fibres, parenchyma and sclereids
- support and provide strength
Positioning of transport tissue in herbaceous dicot
Stems
Roots
Leaves
Near the outside
- strength and support, reduces bending
Xylem in centre (x)surrounded by phloem
- gives strength against tugging forces
Large central vein containing vascular bundle
- support for thin leaves
Similarities between phloem and xylem
Transport materials around plant
Made up of cells joining end to end forming long hollow structures
Sieve tube elements and xylem vessel elements don’t have nucleus
Differences between phloem and xylem
Xylem is non living tissue
Xylem transports water and supports plant - phloem transports organic solutes
Xylem transports only up - phloem does up and down
Phloem doesn’t have lignified walls
Uses of water in plants
- turgor pressure supplies support for leaves, stem and drives cell expansion
- loss of water keeps plant cool
- raw material for photosynthesis
- mineral ions and products of photosynthesis transported in it
How are root hairs adapted as exchange surfaces
- small size so can penetrate easily between soil particles
- large SA:V
- thousands of hairs
- thin surface layer - easy diffusion
- concentration of solutes in cytoplasm maintains a water potential gradient (sugars, mineral ions, amino acids)
What is the symplast pathway
What causes continuous flow
Travels through cytoplasm and plasmodesmata (small channels in cell walls)
Via osmosis
- root hair cell has higher WP than the next cell
- result from water diffusing in from cell
- made cytoplasm more dilute
- continues to the other cells by diffusion
- when water leaves root hair cell, WP in cytoplasm falls
- creating WP concentration gradient
What is the apoplast pathway
What causes continuous flow
Goes through cell walls
Move from high hydrostatic pressure to low hydrostatic pressure
Mass flow
- as water enters xylem, water is drawn through apoplast pathway
- due to cohesive forces that creates tension
- meaning there is a continuous flow
How does water travel from the roots hair to endodermis layer
- water enters root hair cell
- passes through the root cortex by both symplast and apoplast pathways
- when apoplast reaches endodermis it is blocked by the waxy casparian strip
- forces water to take symplast route so it has to pass through the selectively permeable cell surface membrane, filtering out toxic solutes as there is no carrier protein
Travelling from endodermis to xylem
- xylem is more concentrated in solutes, so active transport is required to pump minerals
- water travels by osmosis
- producing root pressure - this gives water a push up the xylem
Evidence for active transport in endodermis
- cyanide (prevent atp production) causes root pressure to disappear
- root pressure increases with temperature
- if levels of respiratory substances fall so does root pressure
Differences between symplast and apoplast
Apoplast composed of non living parts of plant - symplast is living
Apoplast water movement occurs by passive diffusion - symplast occurs by osmosis
Apoplast shows less resistance to the water movement
Apoplast movement is rapid
Process of transpiration
Plant opens stomata for gaseous exchange
This let’s out water vapour by diffusion
Factors that affect transpiration
More Light
- stomata open more because photosynthesis increases
Higher Temperature
- water molecules have higher energy so evaporate from cells faster
Lower humidity
- water potential gradient between leaf and air increased
Faster wind
- blows away water molecules around stomata
More Soil water availability
How water moves from xylem and out at leaves
- xylem transports water around plant
- water leaves xylem and moves into cells by apoplast pathway
- water evaporates from mesophyll cells into air space
- water leaves into external air when stomata open
Transpiration definition
Loss of water from a plants surface
Transpiration stream
Movement of water through a plant
How does water move up against the force of gravity
Adhesion
- water forms hydrogen bonds with carbohydrates in the walls of narrow xylem vessels
- helps water rise up through xylem
Cohesion
- water molecules form hydrogen bonds with each other and move as a mass
- water follows up the xylem
Tension
- transpiration pull produces tension
- pulls more water into the leaf
Evidence for cohesion tension theory
Tree diameter decreases when tension in xylem is highest
When xylem is cut, air is drawn into xylem instead of water leaking out
How does a stomata open and close
- when conditions are favourable, guard cells actively pump solutes, to increase turgor pressure.
- because the inner wall of the guard cell is less flexible than the outer, the cell swells and opens the pore
- when water less water, hormonal signals from root close the cell
What is translocation
The movement of dissolved substances from sources to sinks
Differences between source and sink
Source
- the supplier of carbohydrates needed by cells of plant
- e.g : leaves, provide sucrose
Sink
- area of plant that needs assimilates in phloem
- e.g : roots, meristems
The apoplast route from source to sink
- sucrose moves into companion cells and sieve tube elements by active transport
- hydrogen ions are pumped out of companion cells using ATP
- they return down a concentration gradient via a co-transport protein carrying sucrose as well
- build up of sucrose in sieve and companion draws water in by osmosis
- causing a build up of turgor pressure
- assimilates move into sieve tube elements and are carried to the sink
- assimilates leave by diffusion
Phloem unloading
- leaves by diffusion
- converted into another substance to maintain a concentration gradient
What are the limitations of the use of a photometer
Not all water is lost by transpiration
- photosynthesis
- maintain turgid
The plant is dying so might not take up as much water
Using a photometer
Cut the shoot under water and at a slant
Assemble the photometer under water
Take photometer out of water - leave capillary in water beaker
Make sure it is air tight
Dry the leaves
Allow for plant to acclimatise
Shut the tap
Bring capillary tube out of water until an air bubble has formed then put back
Record starting position
Adaptations of marram grass to reduce water loss
Sunken stomata
- sheltered from wind
Hairs on epidermis
- traps moist air - reduces wp gradient
Roll their leaves
- traps moist air
- protects stomata from wind
Thick waxy cuticle
- reduces evaporation
Adaptations of cacti to reduce water loss
Have spines instead of leaves
- reduces SA of water loss
Close their stomata
Adaptations of water lilies to survive in water
Air spaces in tissues
- can float obtain maximum light for photosynthesis
- act as oxygen store
Stomata on upper leaf
- maximise gas exchange
Have flexible leaves
- prevent damage by water current
