Transport In Plants Flashcards
Location of xylem/phloem in the ROOT , STEM and LEAF?
ROOT: vascular bundle in centre
xylem is in centre core of bundle/ phloem on edges of centre core
- to withstand pulling strains when plants transport water upwards/grows
STEM : vascular bundle on outside
- xylem on the inside and phloem on outside- support that reduces bending
LEAF : xylem/phloem form network of veins- support
- xylem found on upper side of vascular bundles
- phloem found on lower side of vascular bundles
Function/structure of phloem tissue?
Transport organic compounds ,like sucrose, from source to the sink
CONTAINS:
Phloem fibres
Phloem parenchyma
Sieve tube elements
Companion cells
Function/structure of xylem tissue?
- vascular tissue that transports dissolved minerals/water around plant
Structural support/food storage
Structure:
Tracheids- long narrow cells with pits (allow lateral movement of water )
Vessel elements- large with thickened (strength) cell walls/no end plates (allow mass flow of water as cohesive/adhesive forces not disrupted)
Xylem parenchyma / Sclerenchyma cells
Made up of dead cells- does disrupt mass flow of water
Small diameter of vessels - prevent water column from breaking /assist capillary action
lignified/cellulose cell wall
What are sieve tube elements in phloem tissues ?
Structure and function?
Living cells - form tube for transporting solutes through plant
- joined end to end to form sieve tubes
Sieve plates with sieve pores (on end walls) - allows continuous movement of compounds
No nucleus, few organelles - maximise space for translocation
Thin cytoplasm - reduce friction to facilitate movement of compounds
cellulose cell wall - strengthen wall to withstand hydrostatic pressures that move assimilates
Companion cells structure and function ?
Each sieve tube elements have companion cells associated with it
- control metabolism of their sieve tube member / involved in loading/unloading sugars into phloem
nucleus /organelles present - provide metabolic suppport to sieve element
Transport proteins- moves substance into/out sieve tube element
Mitochondria- provide ATP for active transport of substance into/out companion cells
Plasmodesmata - links sieve tube elements , so substance can move from companion cells into sieve elements
REQUIRED PRACTICAL: dissecting plant stems
- Use scalpel to cut cross section of stem (transverse or longitudinal) as thin as possible
- Use tweezers to put cut sections in water - prevent drying out
- Transfer each section to dish containing a stain (toluidine blue O)/ leave for a minute
Stains lignin in xylem vessel walls blue-green - Rinse in water/mount onto slide
How does water move through roots into xylem?
Apoplast pathway: goes through non living parts of cells (cell walls)
- water can diffuse through cell walls
When water reaches the endodermis cells in root , path is blocked by waxy strip in cell walls ,CASPARIAN STRIP - must take symplast pathway
- useful bc can control which substances can get to xylem/generate root pressure , as membrane is partially permeable
Symplast pathway : goes through living parts of cells - cytoplasm/plasmodesmata
Cytoplasm of neighbouring cells connected by plasmodesmata (channels in cell walls)
- water moves by osmosis into cells (across cell walls)
- slower than Apoplast pathway
Which transport pathway is more common in plants and why?
Apoplast pathway
- provide least resistance
What is the transpiration stream ?
Movement of water from roots to the leaves
- water evaporates from leaves through stomata - transpiration
- creates tension ,so pulls more water into leaf cells/ moves through Apoplastic (cell wall) or symplastic pathway (cytoplasm)
- water lost by transpiration lowers WP in the air spaces surrounding the mesophyll cells
- water evaporates from mesophyll cell wall into air spaces - cause TRANSPIRATION PULL
Water moves through mesophyll cell wall - water moves out of xylem, through non lignified areas (PIT), into mesophyll cells BY OSMOSIS
- causes water to move up the xylem vessels (due to the cohesive and adhesive properties of the water)
- adhesion allows water to move by CAPILLARY ACTION
This movement is called transpiration stream
What is transpiration ?
Water leaves xylem through NON LIGNIFIED AREA /moves into leaf cells by the APOPLAST PATHWAY
- water evaporates from cell walls into air spaces
- when stomata open , water diffuses out of leaf into surrounding air (down water potential gradient)
- loss of water = transpiration
What are four factors effecting transpiration rate?
LIGHT: stomata are open when light , for photosynthesis - co2 can diffuse into leaf to produce glucose
- faster transpiration rate, bc water is lost
TEMPERATURE : water molecules have more KE, so evaporate from cells FASTER /creates steeper water potential gradient for faster diffusion
HUMIDITY : when air is saturated with water vapour, water potential grad is decreased , so REDUCED RATE OF TRANSPIRATION
WIND : windier it is , faster transpiration rate
- blows away water molecules from leaf surface , around stomata , maintaining steep water potential grad
- as water may accumulate on surface of leaf —> create local area of humidity = lower conc grad
REQUIRED PRACTICAL : investigating rate of transpiration using potometer
- Cut a shoot at a slant (INCREASE SA) , underwater to prevent air entering xylem
- Assemble potometer in water /insert shoot underwater , so no air can enter xylem
- Remove apparatus from water/but keep end of capillary tube submerged in water - make sure apparatus is AIRTIGHT/WATERTIGHT
- Dry the leaves of shoot - moisture present could effect rate of transpiration
- Remove a capillary tube from beaker of water to allow single air bubble to form/place back in water
- Record starting position of air bubble
- Start stopwatch and record distancemoved by bubble per (minute/hour) - rate of bubble movement = rate of transpiration
- Can repeat with different conditions (temp,light, humidity and wind)
Xerophytes: what are they/adaptations?
Plants that are adapted to dry/arid conditions
E.g cacti and marram grass
Sunken stomata - slows transpiration , bc sheltered from wind , so decreased water potential gradient
- also traps moist air to area of water loss - reduce diff gradient
Hairs on epidermis - traps moist air around stomata , reducing WP grad - slow transpiration rate
Leaves curled/needle or spine like - reduced transpiration due to less SA
Thick /waxy layer on epidermis - reduce water loss by evaporation
Reduced no. Stomata - less water loss
Hydrophytes : adaptations /what are they?
Plants adapted to living in freshwater - face challenge of low CO2 level in day/low levels of O2 at night
AIR SPACES in tissues - help plant float /act as store of oxygen for respiration
STOMATA ON UPPER SURFACE of floating leaves - maximise gas exchange
FLEXIBLE LEAVES /STEMS- prevent damage from water currents
What is translocation ?
Transport of assimilates from source to source to sink - requires ATP
Source eg:
leaves/stem
Storage organs
Sink eg: meristems
Growing roots
Storage organs
Why is sucrose transported around plants?
- Efficient energy transfer /storage
- Less reactive than glucose - no intermediate reactions occur as it gets transported
- Soluble - transported in sap
Process of loading assimilates at the source ?
ACTIVE LOADING
In companion cell, ATP used to actively transport H+ ions out of cytoplasm into surrounding cells , VIA A PROTON PUMP
- sets up conc grad- more H+ ions outside
- H+ ion binds to co transporter protein on companion cells/re enters cell down conc grad
- sucrose molecule binds to co transport protein at same time .
Movement of H+ ion into cell will mean sucrose molecule moves into cytoplasm of companion cell as well (against conc grad)
- sucrose molecules moved into sieve plates VIA PLASMODESMATA
Role of enzymes in loading/unloading of assimilates ?
Maintain conc grad from source to sink by changing dissolved substances at the sink
(Break them down into something else )
- ensures always a lower conc at sink
E.g invertase breaks down sucrose into glucose, so lower conc of sucrose at sink
What is the mass flow hypothesis ?
Hypothesis to explain movement of assimilates in the phloem tissue from source to sink
SOURCE : active transport used to actively load solutes (sucrose/sugar) into sieve tubes , at the source
- lowers water potential in sieve tubes , so water enters tubes by osmosis
- creates high pressure inside sieve tubes at source end of phloem
SINK : at sink end , solutes are removed from phloem to be use up
- increase water potential in sieve plates, so water leaves by osmosis
- lower pressure inside sieve tubes
Results in pressure gradient from source end to sink end
- pressure gradient pushes solutes along sieve tubes
How is the xylem and phloem similar and different?
SIMILARITIES : solutes carried in solution
- cells are joined end to end
- both use mass flow/pressure gradient
DIFFERENCES: phloem has companion cells /xylem don’t
Phloem have sieve plates/ xylem don’t
Phloem are made from living cells/xylem made form dead cells
What do mass transport systems help?
- bring substances quickly from one exchange site to another
- maintain diff gradients at exchange sites /between cells /suroundinngs
- ensure cell activity by keeping fluid environment of cells within suitable metabolic range
Why do plants not have a specialised transport system for O2 and CO2?
- have adapatations that give HIGH SA:V ratio for diffusion
- leaves/stem have chloroplasts which produce O2/use up CO2
- low demand for O2 as plant tissues have ((low metabolic rate**
Advantage of transpiration?
- provide cooling effect
- Transpiration stream helpful in uptake of mineral ions
- turgor pressure of cells (due to water moving up plant) provides support to leaves - allowing larger SA of leaf/stem
How does the CASPARIAN strip prevent ions reaching xylem by Apoplast pathway?
- strip is impervious to water /solutions
- forces water/solutions to pass through plasma membrane
Resolution?
The ability to see more detail /separate 2 objects
How is transport in phloem similar to and different from transport in xylem?
SIMILAR : solutes carried in solution in both
Both use mass flow
DIFFERENT: transport in phloem take place in different directions but transport in xylem only take place up the plant
- phloem transport uses living cells and xylem does not
Explain how some parts of plant can act both as a source and sink?
Certain parts can store and release carbohydrates when needed
E.g root /leaf
Process of sucrose unloading at the sink?
- sucrose diffuse from phloem into sink
- Sucrose moved into other cells - maintain conc grad between cells and phloem
Or converted into other forms - e.g :
GLUCOSE for respiration
STARCH - for storage - Loss of assimilates increases water potential of PHLOEM
So water leaves phloem by osmosis to surrounding cells/xylem
- leads to low hydrostatic pressure
Assumption made when using potometer?
Water loss = uptake
Extra precautions when using potometer?
Keep shoot supported —> avoid breaking of water column
Don’t allow air bubble to move too far/use syringe to move bubble —> so air bubble doesn’t enter xylem
