2.2.4 Transport systems in plants

what is translocation

transport of assimilates from source to sink in the phloem

what is a source + give examples

site of photosynthesis

• green leaves/stem

• food stores in seeds

• storage organs

what is a sink + give examples

site of assimilate storage

• roots

• dividing meristem

• developing seeds

outline the process of translocation: LOADING of sucrose into phloem via apoplast route

• companion cells contain lots of mitochondria that provide the energy used to actively pump protons out into surrounding tissue

• protons move back into CC, down the conc gradient via a co-transporter protein

• sucrose is co-transported with the protons into the CC —> inc sucrose conc in CC + decreases water potential

• sucrose diffuses into STE via plasmodesmata from an area of high to low conc

• Water diffuses into CC by osmosis (down wp gradient)

• this increases turgor pressure

• water diffuses into STE

outline how sucrose is unloaded from phloem

• sucrose diffuses from phloem into surrounding tissue

• this increases the water potential inside STE

• water diffuses out of STE by osmosis down wp gradient —. some water enters transpiration stream

outline LOADING of sucrose into phloem via symplast route

PASSIVE route

• sucrose diffuses from source to STE down conc gradient

• this decreases wp in STE

• so water diffuses into STE by osmosis down wp gradient

• this generates a hydrostatic pressure that moves sucrose through phloem —> MASS FLOW HYPOTHESIS

define transpiration

the loss of water vapour by diffusion from the underside of the leaf via stomata

State the 3 forces that play a role in transpiration and explain their importance

1. cohesion - water molecules are attracted to each other due to hydrogen bonds

2. tension - evaporation of water molecules from walls of PMC to mesophyll air spaces —. generates a suction that pulls more water into leaf

3. adhesion - water molecules are attracted to cellulose walls of the xylem vessels + hydrophilic parts of lignin

describe transpiration at the leaves

• Water evaporates from the cellulose cell walls of the mesophyll cells + into mesophyll air spaces

• Water vapour then diffuses to the sub-stomatal air spaces where the wp is highest

• water diffuses down wp gradient through the open stomata to the surrounding air

Describe how water moves through the xylem during transpiration

• Xylem vessels transport water up the stem from the roots to the leaves due to a difference in water potential at the top of the stem compared to the bottom

• water is drawn up via capillary action

• water moves up xylem via cohesion forces in a continuous column

• water leaves the xylem via apoplast pathway

what is the apoplast pathway

• water moves through cellulose cell walls

• cell walls are freely permeable —> little resistance to mass flow of water so most water travels via this pathway

what is the symplast pathway

• water travels through cytoplasm and moves from 1 cell to the next via the plasmodesmata

• when water reaches endodermis, there’s a layer of suberin that’s waterproof —> forms casparian strip

• forces all water to travel through symplast pathway —> toxins are filtered out as membrane is partially permeable

what is the vacuolar pathway

water moves via vacuole

what adaptations do RHC have?

• extended cytoplasm —> increase SA for uptake of water + mineral ions

• thin cell wall —> short diffusion distance for water + ions

• lots of mitochondria —> provides energy for AT of ions

• no chloroplasts —> no PHS occurs

how does stomata open + close

• abscisic acid binds to receptors on guard cell membranes

• Ca2+ channels open and Ca2+ enter

• activates proton pumps

• influx of H+ generates a PMF

• PMF opens voltage-gated K+ channels

• K+ leave via facilitated diffusion

• w.p. of cytosol increases

∴ water leaves via osmosis down w.p.g.

• cells become flaccid –> closed stoma

EQ: Compare and contrast gaseous exchange in stomata and lenticels.

PAG: investigating transpiration rates

Control: type of plant, temp, light intensity, wind flow, humidity

• cut shoot underwater at a 45o angle

• connect potometer to shoot underwater

• ensure apparatus is airtight using vaseline as a seal

• dry surface of leaves —> reduces humidity

• allow shoot time to acclimatise

• allow air bubble to form

• record initial position of bubbl

• time using a stopwatch and measure distance travelled by bubble per unti time

• rate of water uptake = vol of water/time

• vol of water absorbed = area of tubing x distance moved