| How do plants transport organic compounds? | source to sink |
| What are the properties of the phloem? (6) | transport water and food
2 directional
made of sieve element cells which forms a tube
sieve cells are connected with a holey sieve plate
supported by companion cells for loading/un
movement is controlled w hydrostatic P from X |
| What is active transport used for in phloem? | to load organic compounds into pl=hloem sieve tubes at source |
| How does the phloem cause water uptake? | high solute concentrations in source cause water to be take up through osmosis |
| What does the hydrostatic pressure gradient in the phloem do? | causes the material in the phloem to flow towards sinks |
| In what form is sugar transported? (2) | sucrose (disaccharide)
soluble and metabolically inert (still) |
| What makes up the phloem tube? (2) | sieve element cells
companion cells |
| How are sieve tubes specialized? (5) | long and narrow
connected by sieve plates
porous for flow
no nuclei to maximize space for translocation
thick rigid cell wall for hydrostatic pressures |
| What are companion cells used for? | metabolic support to help with loading and unloading materials at source and sink |
| How are companion cells specialized? (3) | infolding plasma membrane = + SA: Vol ratio for more material exchange
many mitochondria = active transport of materials from source to sink
transport proteins in membrane: move materials in / out of sieve tube |
| How can phloem and xylem cells be distinguished? | xylems have a larger diameter |
| Why do sieve element cells need companion cells? (3) | bec sieve cells have no nuclei and few organelles
plamodesmata is a lot bw sieve elements and companion cells
= connects the cytoplasm of the 2 cells |
| How does the arrangement of x + p differ in roots for monos and dis? (5,3) | Monocotyledons
large stele (middle part)
vessels form a radiating circle around it
xylems are placed more internally
phloem places more externally
O = outside
Dicotyledons
small stele
xylem internally placed = forms an X
phloem surrounding it = surrounding gaps |
| How does the arrangement of x + p differ in stems for monos and dis? (2,3) | Monocotyledons
vascular bundles are scattered through the stem
phloem is placed externally o= out
Dicotyledons
vascular bundles arranged in a circle around the stem's centre (pith)
x+p is separated w/ cambium
xylem inside, phloem outside |
| How is ATP used for apoplastic loading of sucrose? (4) | 1) H+ ions are AT out of P cells by proton pumps (hydrolysis of ATP)
2) conc of H+ ions + out of cell = proton gradient
3) H+ ions passively diffuse back into phloem w/ co-transport protein = sucrose movement
4) loads sucrose into sieve tube |
| What does the active transport of solutes cause the sap solution to be.. | hypertonic (more solutes than water) |
| Why does the sap solution being hypertonic cause water to be drawn up? what does this cause?(4) | water is drawn up by the xylem bec the water moves to the higher solute concentrations
bec water is incompressible = hydrostatic p in phloem increases
forces phloem sap to move to areas with lower pressure (mass flow)
= solutes are transported away from source to the sink |
| What happens once the solutes are unloaded in sinks? (3) | sap solution = hypotonic
water is drawn out of phloem = xylem by osmosis
= hydrostatic p at sink is always lower than hydrostatic p at source |
| What happens to the solutes when they are transported to the sink? (2) | metabolised
stores within the tonoplast of vacuoles |
| What are aphids? | insects that feed on the sap in phloem |
| How are aphids specialized to drink phloem? (3) | stylet which pierces plant sieve tube = sap extraction
aided w/ digestive enzymes to soften tissue layers
if stylet is severed = saps continue to flow bec of hydrostatic P |
| How can aphids be used to measure translocation rate? (6) | 1) used to collect sap at various sites of the plant's length
2) plant grown w' leaves in radioactively labelled CO2
3) CO2 becomes radioactively labelled sugars
4) once stylet is severed the sap continues to flow
5) sap is analysed for radioactively labelled sugars
translocation rate is calculated based on time taken for the radioisotope to be detected in diff positions of the plant's length |
| How is the rate of phloem transport determined? | by the conc of dissolved sugars in the phloem |
| What are the factors affecting translocation rate? (4) | photosynthesis rate (light intensity, CO2 conc, temperature)
respiration rate (affected by factors that stress the plant)
transpiration rate (determines how much water enters phloem)
sieve tube's diameter (affects hydrostatic P + differs bw plant species) |
