9.2 transport in the phloem of plants

Question 1

what is the function of the phloem

Answer 1

to efficiently conduct sugar molecules (from source to sink – use/storage)

Question 2

what is the main sugar conducted in phloem

Answer 2

sucrose

Question 3

structure of phloem

Answer 3

1. sieve tubes (colums of cells)
2. each cell in sieve tube: sieve element
3. sieve element: limited cytoplasm + few mitochondria (nucleus, vacuole, etc broken down)

Question 4

structure of sieve elements

Answer 4

• largely filled with phloem sap, contained in a single plasma membrane
• have perforated end walls (sieve plates) – allow sap to flow cell to cell
• sieve elements connected end to end to form a sieve tube

Question 5

composition of phloem sap

Answer 5

water, dissolved sucrose + other carbohydrates, amino acids, proteins, some minerals and plant hormones

Question 7

how are sieve elements and companion cells connected

Answer 7

pores called plasmodesmata (singular: plasmodesma)

Question 8

function of reduced organelles in sieve elements of phloem

Answer 8

frees the lumen to conduct a large volume of sap

Question 9

function of companion cells in phloem sieve tubes

Answer 9

metabolic support cells – provide biomolecules (eg enzymes) to maintain life functions in sieve elements

Question 10

function of plasmodesmata

Answer 10

openings – allow communication and support from companion cells (to sieve elements)

Question 11

function of structure of sieve plate

Answer 11

has pores thru the horiznotal cells joining sieve elements – allow sap to flow freely

Question 12

what is translocation

Answer 12

where the sap (actively loaded with carbohydrates) flows from source to sink
(supplies sugar a hundred times faster than diffusio! wow!)

Question 12

function of cell membrane in phloem

Answer 12

contains specialised protein pumps – control composition of sap

Question 12

2 types of sources and sinks (Each)

Answer 12

sources
- photosynthetic tissues (eg mature leaves)
- storage organs unloading their sotres at the start of growing season (eg germinating seeds)

sinks
- actively respiring tissues (eg roots)
- organs developing food stores (eg developing fruits)

Question 13

movement: xylem vs phloem

Answer 13

xylem: unidirectional
phloem: bidirectional

Question 14

what is phloem loading

Answer 14

process by which soluble carbohydates enter phloem
- requires active transport
- contents transported: sucrose, amino acids, plant hormones

Question 15

why is sucrose transported in phloem

Answer 15

non reducing sugar = less reactive = will not be used up by cells = successful transport from source to sink

disaccaride = contains more energy = more efficient

Question 16

process of translocation (9) this is really long so im putting it in the essay doc too

Answer 16

1. sucrose produced by source
2. companion cells actively loaded (using ATP)
3. sucrose diffuses thru plasmodesmata into sap of sieve tube elements
4. active loading of sucrose incr solute concentration in sieve tubes
5. water then moves from xylem vessels to sieve tubes via osmosis
6. at sink, companion cells unload sugars from sieve tube
7. decr solute concentration allows water to return to xylem
8. decr in water in sieve tube = lower hydrostatic pressure near sink = allows sap to flow
9. phloem sap flows from source to sink (down hydrostatic pressure gradients)

Question 17

why is there difficulty analysing rate of translocation

Answer 17

• sap flows rather slowly
• phloem is sensitive to disruption (severed sieve tubes are closed off quickly)

Question 18

how is tranlocation rate derived using aphids 4

Answer 18

1. aphids are evolved to pierce phloem tubes with long stylets w/o triggering defensive responses in the plant
2. stylet is surgically seperated from the aphid
3. phloem sap continues to be pushed out bc of high hydrostatic pressure in sieve tubes
4. sap can be analysed for chem composition or using radioactive labelling (for direction and rate)

Question 19

how to calculate rate of translocation using radioactively-labeled co2

Answer 19

rate of translocation =
dist travelled by C-labelled sugars (cm) / time it took (hours)

Question 20

4 factors affecting translocation rates

Answer 20

rate of translocation = conc of dissolved sugars in the phloem
1. rate of photosynthesis
2. rate of cellular respiration
3. rate of transpiration
4. diameter of sieve tubes

Question 21

xylem vs phloem

Answer 21

1. movement: transpiration // translocation
2. due to: evaporation and cohesion tension = transpirational pull // hydrostatic pressure gradients
3. materials: water, dissolved minerals // sucrose and other organics
4. direction: unidirectional // bidirectional
5. cellular structure: dead cells (xylem vessels) // living cells (sieve tube elements)
6. special features: lignin-thickened cell wall // connected by plasmodesmata to companion cells

Question 22

how are phloem and xylem arranged in a DICOT STEM

Answer 22

next to each other in a ring of vascular bundles
- phloem closer to surface
- phloem narrower

Question 23

how are phloem and xylem arranged in a DICOT ROOT

Answer 23

• vascular tissue in centre
• xylem forms and X shape
• phloem in spaces around it

Question 24

how are phloem and xylem arranged in a MONOCOT ROOT

Answer 24

• xylem forms a ring inside phleom ring

Question 25

how are phloem and xylem arranged in a MONOCOTS

Answer 25

vascular bundles are scattered throught the stem
- phloem always faces the outside

Question 26

whats the benefit of having xylem vessels close to phloem sieve tubes

Answer 26

water more easily supplied to the phloem