3.3.4.2 mass transport in plants Flashcards
What are the two types of vascular tissue in plants?
Xylem and phloem
Role of xylem
transports water & mineral ions in a solution in one direction, UP (from roots to leaves)
Features of xylem vessels
- Long tube like structures
- Hollow thick-walled tubes
- formed from dead cells
- can be spiral, annular, reticulate or pitted
- provide structural support to stem
Role of phloem
transporting organic substances like sugars in both directions in plant
Cohesion definition (for transpiration)
When water molecules stick to other water molecules
Adhesion definition
When water molecules stick to the walls of the xylem vessels
Explain the cohesion-tension theory.
- water evaporated from the surface of mesophyll cell in leaf
- diffuses out of stomata
- replaced by water pulled up from xylem vessels. (Move between vessels via pits)
- tension is created which pulls more water into the leaf
- water molecules are cohesive so the whole column of water in the xylem moves upwards
- water then enters the stem via the roots
Upward movement so it maintains water potential gradient.
What are the three components of
phloem vessels?
● Sieve tube elements= form a tube to transport
sucrose in the dissolved form of sap.
● Companion cells= involved in ATP production for
active loading of sucrose into sieve tubes.
● Plasmodesmata= gaps between cell walls where
the cytoplasm links, allowing substances to flow.
Name the process whereby organic
materials are transported around the
plant.
Translocation
process of transpiration
- water evaporates from a plants surface and build up in air spaces of leaf.
- stomata open—-> water vapour diffuses out of leaf (down a concentration gradient)
factors that affect transpiration rate
light intensity
humidity
temperature
wind
light intensity on transpiration rate
Increased light=increased rate of photosynthesis
light causes stomata to open to let in CO2 for photosynthesis.
Therefore, increase rate of transpiration
temperature on transpiration rate
Increased temp= increase rate of transpiration
more energy so evaporate out of leaf faster increases concentration gradient, making water diffuse out of the leaf faster.
humidity on transpiration rate
Increased humidity=decrease in transpiration
Decreased humidity= increase in transpiration
wind on transpiration rate
More wind= increased transpiration rate
what is translocation?
movement of solutes (e.g. sucrose) to where they’re needed in a plant - requires energy to move solutes from source to sink
what is the source?
where the assimilates (products of photosynthesis) are produced (there is a high concentration)
e.g. leaves
what is the sink?
where the solutes are used up (low concentration)
enzyme role in translocation
maintain a concentration gradient from source to sink by changing the solutes at the sink (e.g. using them up or breaking them down) so that there is always a lower concentration at the sink
Root Pressure Theory
Evidence to say water is pushed up from roots.
- cut near base of stem- water leaks
- Active transport- pumps ions across endodermis into xylem of roots
- enters xylem by osmosis pushing upwards creating root pressure
process of using a potometer
- cut shoot underwater & at a slant
- assemble the potometer underwater & insert shoot underwater
- remove apparatus from underwater but keep end of capillary tube submerged
- check apparatus is water & airtight
- dry leaves & allow time for shoot to acclimatise then close tap
- remove submerged end of capillary tube until 1 air bubble has formed then put tube back in water
- record starting position for air bubble
- start a stopwatch & record distance moved by bubble per unit time - rate of bubble movement is estimated transpiration rate
why is the shoot cut underwater (potometer)
prevent water entering xylem (prevents cohesion)
what is the reason for sealed joints (potometer)
prevent water& air entering xylem
why is the shoot cut at a slant (potometer)
increases surface area
evidence for cohesion theory
- broken xylem vessel doesn’t leak water so water not under pressure
- broken xylem vessels means that the water column and cohesive forces between water molecules are broken - stops flow of water
- tree trunk diameter decreases when transpiration rate is high due to negative pressure caused by cohesion between water molecules
mass flow hypothesis process
SOURCE
- active transport is used to actively load solutes from companion cells into sieve tube elements at the source
- lowers water potential of sieve tubes so water enters by osmosis from xylem & companion cells
- high pressure created inside sieve tubes at the source
SINK
- solutes are removed from phloem and used up
- increases water potential so water moves out by osmosis
- low pressure created inside sieve tubes
FLOW
- a pressure gradient between source & sink is created
- the gradient pushes solutes from source to sink along sieve tubes
- solutes used up at sink
mass flow evidence - supporting
- pressure in sieve tubes- leaks when cut
- concentration of sucrose higher in sources than in sink
- downward flow in daylight but slows/stops in dark
- increased sucrose in leaves is followed by increased sucrose in phloem
- metabolic poison/lack of O2 inhibits translocation
- companion cells produce lots of ATP
mass flow evidence - against
- function of sieve plates unknown - do not affect mass flow other than forming a barrier
- not all solutes move at same rate
- sugar travels to many different sinks not just those with the highest water potential
ringing experiements
- if a ring of bark (including phloem) is removed from a woody stem, a bulge forms above the ring
- the fluid from the bulge has a higher concentration of sugars than the fluid below the ring - sugars cannot flow past areas where bark has been removed
tracer experiment
- radioactive isotopes (14C) is used to make radioactive 14CO2
- a plant is grown in presence of 14CO2 will carry out photosynthesis and incorporate 14C into its glucose & sugar molecules
- radioactive molecules can be traced round the plant using autoradiography
- thin cross sections of plant are produced and exposed to xray radiation using xray film
why is there a layer of oil (potometer)
To stop water evaporating from surface
aphids experiment
- aphids use their mouthpiece to pierce the phloem and extract sugary sap
- the contents show daily variations of sugar concentration in leaves which is mirrored in short time by identical changes in phloem sugar concentration
puncture experiment
Xylem- air gets sucked into the xylem
phloem- cell sap oozes out
