3.4 Gas exchange and mass transport in plants Flashcards
What tissues make up a leaf?
epidermal tissue
spongy mesophyll tissue
xylem and phloem tissues
palisade mesophyll tissue
What adaptions do leaves have to maximise photosynthesis?
epidermal tissue covers the whole leaf - prevents water leaving
upper epidermis is transparent for light
the palisade layer is near the top of the leaf to get the most light
What is the purpose of the vascular bundle in the leaf?
provides suppport for the leaf
What does the xylem do?
transports water and other nutrients from the roots to the leaves - transpiration
What does the phloem do?
transports the sucrose made during photosynthesis away from the leaves
What is the purpose of the air spaces in the spongy mesophyll?
increases the diffusion of gases through the leaf
What happens to O2 in the leaf when it is dark?
it diffuses into the leaf because it is used for resp
What is the purpose of the upper epidermis?
transparent to allow light to pass through and protects against mechanical damage
What is the purpose of the lower epidermis?
protects leaf tissue from mechanical damage
What is the purpose of the waxy cuticle?
waterproof (impermeable to water) - prevents water leaving by evapouration, reducing water loss
What is the purpose of the palisade mesophyll?
main site of photosynthesis - has the highest density of chloroplasts
What is the purpose of the spongy mesophyll layer?
site of gas exchange - contains vascular bundle
What is the purpose of the guard cells?
they open and close the stomata
What is the purpose of the stomata?
allows entry and exit of gases
Is CO2 moving into or out of the plant?
into the plant for photosynthesis, it is exchanged with the environment
Why is there a net movement of O2 out of the plant?
the rate of photosynthesis is greater than the rate of resp
What are xerophytes?
plants that are adapted to environments where water supply is short
What are the two examples of xerophytes?
marram grass and cacti
How are cacti adapted to their environment?
spines - reduces SA for stomata = fewer stomata —> lower rate of transpiration
thick - storing water, suuculent stem
folded - allows it to expand and contract = maximises storage of water
small SA:VOL = round
How is tension created in the plant?
by transpiration
What is cohesion?
bonding (attraction) between the same molecules - water bonds to itself
What is adhesion?
attraction/bonding between different molecules = water and lignin
Why are the water molecules adhesive to the lignin plant wall?
there is a hydrophyllic lining on the walls
What is transpiration?
the loss of water vapour from the stomata by evapouration
What factors affect transpiration?
light intensity
temp
humidity
wind
What is the cohesion-tension theory?
how water moves against gravity up the plant, a combination of cohesion and adhesion of water molecules and root pressure
How are water molecules cohesive?
water is dipolar - H bonds can form between water molecules so they stick together and travel up the xylem
How is root pressue created in the plant?
as water moves into the roots by osmosis it increases the volume of liquid inside the root = pressure inside the root increases
forces water above it upwards
How is water moved up the xylem (method)?
- water evapourates out the stomata on leaves. This loss in water volume creates a lower pressure
- water is lost by transpiration, more water is pulled up the xylem to replace it and it and moves due to negative pressure
- due to H bonds between water molecules, they are cohesive - creates a transpiration stream
- water molecules also adhere to the walls of the xylem - helps pull the continuous column of water up
- as this collumn of water is pulled up the xylem it creates tension, pulling the xylem in to become narrower
What is the purpose of packing cells?
to fill space so that all cells touch each other for diffusion
What are features of a TS stem?
epidermis
cortex
phloem
vascular cambium
xylem
pith
What are features of a root?
phloem
epidermis
exodermis
pericycle
endodermis
xylem
vascular cambium
Which features of a stem and root are parenchyma cells?
cortex
pith
What are parenchyma cells?
packing cells
Why is the vascular bundle in the middle of the root?
for support as it is subject to the pull of gravity
What is the structure of the xylem?
Structure:
lignified
non-living
How does light intensity affect transpiration rate?
increased light intensity –> increased rate of transpiration
it stiumulates stomatal opening
more stomata open = more water diffuses out as there is a higher SA for diffusion
How does temperature affect rate of transpiration?
increased temp = increased transpiration rate
increased temp increases evapouration of water out of mesophyll cells into air spaces
How does humidity affect the rate of transpiration?
increased humidity = decreased transpiration rate
as water vapour leaves the plant, the leaves are still saturated from the air so reduces the diff grad
How does air movement affect transpiration rate?
increased air movement = increased transpiration rate
removes humid air from the leaf’s surface - increases diffusion grad
What is capillary action of water?
the ability of liquid to move through a narrow space against gravity
What is the root pressure theory for movement of water up the stem?
ions pumped into root tissue - water potential grad formed between root and soil
more water moves into the roots by osmosis = increase in hydrostatic pressure = forces water upwards
What ions are taken up by a plant and why?
Mg 2+ = main component of chlorophyll
K+ = synthesise proteins
PO4 3- = for membranes
What is evidence for active transport in plants?
ion conc in root tissue is greater than in the soil - against the conc grad
resp inhibited added to root reduces uptake - reduces ATP
what environments do xerophytes live in?
high rain fall but high temps so rain is quickly evapourated
or coastal areas = salty soil has lower water potential of the soil solution reducing the water potential between the soil and the root hair cells - slows osmosis
What are all the xerophytic adaptions?
thick waxy cuticle
leaf curling
hairs on leaves
sunken stomata
reduced leaves
fewer stomata
How does a thick waxy cuticle prevent transpiration?
forms a waterproof barrier - impermeable to water
How does leaf curling prevent transpiration?
traps a layer of moist air next to the surface reducing the water potential between leaf and air
How does hairs on the leaf prevent transpiration?
traps moist air next to the leaf’s surface, reduces the water potential between air and leaf- keeps air around stomata humid
How does sunken stomata prevent transpiration?
sheltered from the air movements and traps moist air next to the leaf , reduced water potential grad between leaf and air
What are the features of a xylem tissue? (TS)
tracheid
parenchyma cells
fibre
lignified wall
What is the role of the phloem?
transports the products of photosynthesis - sucrose and amino acids, from source to sink
What are examples of storage organs of plants?
bulbs and tubers
Describe the phloem structure.
living tissue - no lignin
stacked end to end
what does it mean that translocation is bidirectional?
the movement of sucrose and amino acids can be upwards or downwards depending where the source and sink are located
Why do tubers become sources in winter?
flowers and leaves have dropped off
they have all the sucrose stored for growth
Why isn’t translocation done simply by diffusion alone?
diffusion alone is too slow for the rates of translocation found
What is hydrostatic pressure?
liquid pushing against its container
Why will there always be a constant flow of sucrose?
photosynthesis and resp - no equilibrium is reached
What are the criticisms of Munch’s mass flow hypothesis?
sucrose and amino acids transported at different rates in the same tissue
sucrose and amino acids translocate in different directions at the same time in the same tissue
it is not passive - companion cells have lots of mitochondira = supports sieve tube with ATP , high O2 consumption in phloem - high rate of resp
resp inhibitor added = rate of translocation decreased
What is the process of translocation?
- sucrose is actively transported into the phloem from the source by using ATP from the companion cells
- water moves into the phloem by osmosis from the xylem
build up of hydrostatic pressure in the phloem - mass flow of sap down a hydrostatic pressure grad
- sucrose is actively unloaded into respiring sink cells
- water moves back into xylem from the phloem by osmosis
What is the process of sucrose entering the phloem?
- H+ ions are actively pumped out of the companion cell into the source tissue by a hydrogen pump using ATP
accumulation of H+ ions outside of the companion cell - grad of H+ ions to passively move back into the companion cell - co-transport of H+ ion and sucrose down the H+ conc grad
- build up of sucrose in companion cell higher than the seive tube element
- passive movement of sucrose by facili. diff into the seive tube element
The whole process is active because of the ATP used initally in step 1
Describe the sieve tube element structure.
living cells
no nucleus
contains few organelles
end walls are perferated for continuous mass flow of sap
forms a hollow tube for max mass flow
What is the role of companion cells?
provides ATP required for ACT of organic substances
what is a source?
where the organic substances are produced
What is a sink?
where the organic substances are used up
How are tracers used to investigate translocation?
uses radioactive CO2 which is visible on x-ray
photosyn –> glucose –> sucrose which is transported
you are able to see where the sucrose is transported due to the radioactive C
measure rate of translocation
What is the method of using tracers for investigating translocation?
- expose 1 leaf to radioactive CO2 (14CO2)
- 14CO2 is taken up by photosyn
- after varying exposing times, put the plant in liquid nitrogen so it dies and all the reactions stop so the molecules are fixed in place
- x-ray plant to produce autoradiograph which shows the location of any compounds containing 14C
How is ringing used to investigate translocation?
remove a ring of stem/stalk which removes the phloem but the xylem remains
bulging above removed section = shows sap moves downwards as phloem is removed
underneath doesnt bulge = growth has not occured as there was no resp, no sap for resp
How are aphids used to investigate translocation?
they feed on sap by their stylets
place aphids on different parts of the stem
stylet is cut off and left in phloem
gather the sap from stylet and test for sucrose conc
collect sap from different points of the stem
What is the aphids stylet?
specialised mouth part to pierce stem to access the phloem to drink sap
Why can’t you insert a needle in the phloem to get sap sample?
the sap would get stuck - aphids have enzymes that prevent this in their stylet
What are the features of phloem tissue? (TS)
companion cell
cell wall
cell membrane
cytoplasm
vacuole
nucleus
sieve tube cell
What are sieve tube cells?
elongated cells stacked to form a series of tubes
Why are sieve plates perferated?
allows cytoplasmic connections = connects one cell to another
describe the structure of a companion cell
adjacent to sieve tube elements
large nucleus
lots of organelles
permeable - has plasmadesmata
How do you calculate the rate of water uptake?
rate of watee uptake (mm3S-1)=
speed of air bubble movement (mms-1) x cross-sec area of capillary tube (mm2)
How does the potometer expt work to estimate transpiration rate?
as water is transpired, the shoot absorbs water from the potometer, the air bubble moves along the capillary tube
How is the equipment of the potometer set up?
shoot is cut from plant underwater
do not want any air in the system so everything is done underwater however the leaves need to be dry - water can block pores preventing transpiration
Why is petrolium jelly used in the potometer expt?
blocks joints in apparatus - makes them air tight
What 3 measurements are taken in the potometer expt?
distance travelled by air bubble (mm)
time it takes bubble to travel (secs)
diameter of capillary tube (mm)
Why does the diameter of a plant drecrease over the day?
water is being pulled up the xylem = adhesion pulls the xylem walls in making the plant stem narrower
Why is there an increased rate of transpiation with an increase in temperature?
water molecules have more kinetic energy to move out of the leaf
stomata open allowing for gas exchange
