lec 14- sugar transport in plants

Question 1

what is translocation?

Answer 1

is the movement of sugars through a plant by bulk flow from sources to sinks through the phloem

Question 2

what is a source?

Answer 2

a tissue where sugar enters the phloem

Question 3

what is sink

Answer 3

a tissue where sugar exits the phloem

Question 4

what are the sugar concentrations in sources and sinks?

Answer 4

sources have high sugar concentration and sinks have low sugar concentrations

Question 5

what is bulk flow?

Answer 5

movement of fluid due to a pressure gradient

Question 6

what is the phloem and what does the phloem do?

Answer 6

phloem are the leaf veins and they transport and remove sugars in a plant

Question 7

how does sugar move in the source (phloem loading)?

Answer 7

sugar moves from photosynthesizing mesophyll cells into phloem of minor veins

Question 8

how do aphids prove the pressure gradient between source and sink?

Answer 8

aphids puncture the phloem and sap oozes out, aphids puncture xylem and air is sucked in. source has high pressure and sink has low

Question 9

are leaves always sources?

Answer 9

no, they start out as a sink then become a source

Question 10

on what sides and ends do sources supply to the sink?

Answer 10

on the same side and ends

Question 11

how does the location of sources and sinks vary with time of year?

Answer 11

-early in the growing season, storage cells in roots and stems are sources while developing leaves are sinks
-during the growing season, mature leaves and stems are sources while meristems, developing leaves, flowers, seeds, and fruits and storage cells in roots are sinks

Question 12

what is the anatomy of a phloem cell?

Answer 12

a phloem mother cell divides to form:
a sieve tube element and a companion cell
multiple sieve tube elements connected form a sieve tube

Question 13

what resistance do sieve tubes have?

Answer 13

low resistance to create pressure gradient

Question 14

what do phloem proteins do (P-proteins)?

Answer 14

quickly move as flow rate increases to block sieve plate pores to reduce loss and heal wounds

Question 15

why are sieve tube elements considered living and what do they lack and have?

Answer 15

-lack: nuclei, vacuole, golgi bodies, and ribosomes
-have: plasma membrane and dilute cytoplasm, plasmodesmata to companion cells, and sieve plate pores between sieve tube elements

Question 16

how do companion cells support sieve elements?

Answer 16

-mediate sugar loading of sieve elements
-plasmodesmata are used to transfer to sieve elements
-provide sieve elements with proteins and metabolites
-abundant mitochondria may provide energy to sieve elements

Question 17

how are sieve tubes specialized for transporting?

Answer 17

-plasma membrane allows generation of turgor pressure
-absence of organelles allows bulk flow
-sieve plate pores allow intercellular flow of water and solutes
-sieve plate pores can also restrict flow which allows a pressure gradient to build up along the sieve tube

Question 18

what type of sugars are transported in the phloem?

Answer 18

non-reducing sugars because they are less reactive and ideal

Question 19

what type of pressure causes phloem transport?

Answer 19

positive pressure

Question 20

what causes high pressure at sources?

Answer 20

sugar is loaded into phloem in source leaves, water diffuses down gradient into phloem causing high pressure resulting in bulk flow towards sinks

Question 21

what causes low pressure at sinks?

Answer 21

sugar is unloaded at sinks, water diffuses against concentration gradient out of phloem causing low pressure resulting in water and dissolved sugar pushed as bulk flow toward sink regions

Question 22

what is the pressure flow model at the source?

Answer 22

1. sugar is actively loaded into sieve cell/companion cells
2. water potential decreases in sc/cc complex
3. water potential in nearby xylem is higher
4. water will diffuse down its gradient from xylem to phloem
5. turgor pressure builds up in sc/cc complex, pushing liquid towards cells/regions with lower pressure

Question 23

what is the pressure flow model at the sink?

Answer 23

1. sugars are actively unloaded from the sc/cc complex into sink cells
2. water potential increases in sc/cc complex
3. water potential in nearby xylem is lower
4. water will diffuse down gradient from phloem to xylem
5. turgor pressure reduced
6. the difference in pressure between source and sink generates a net flow from source to sink of water and its solutes

Question 24

how fast do sugars move?

Answer 24

1 meter per hour (diffusion takes 1 meter per 32 years in a sieve tube)

Question 25

what are the three types of phloem loading?

Answer 25

1. apoplastic loading
2. passive symplastic loading
3. polymer trapping symplastic loading

Question 26

what is apoplast?

Answer 26

cell wall space

Question 27

what is symplast?

Answer 27

cytoplasm of cells connected by plasmodesmata

Question 28

what is apoplastic loading?

Answer 28

-sucrose in mesophyll cells move via plasmodesmata into the apoplast then transporters move sucrose into companion cell
-sucrose movement in companion cells across plasma membrane then into the cytosol of sieve element by plasmodesmata

Question 29

how do the transporters move sucrose in apoplastic loading?

Answer 29

SWEETs carry sucrose across the plasma membrane into the apoplast down the gradient, sucrose uptake from the apoplast is driven by SUC2 and other symporters that use a proton gradient across membrane to load sucrose into cell against concentration gradient

Question 30

what is symplastic loading?

Answer 30

sugar movement via plasmodesmata

Question 31

what is passive symplastic loading?

Answer 31

sugar is synthesized in mesophyll cells, concentration gradient starts in mesophyll cells rather than companion cells, used by trees like conifers

Question 32

what is polymer trapping symplastic loading?

Answer 32

uses specialized intermediary cells, express enzymes for synthesis of raffinose and stachyose, have plasmodesmata bundle-sheath cells that prevent backflow of larger raffinose molecules, raffinose and stachyose polymer trapping, loading of phloem, used by cucumbers (cucurbitaceae)

Question 33

how are mutants used to identify genes/proteins involved in phloem function?

Answer 33

-arabidopsis mutants lack a gene called SUC2, gene encodes for symporter
-mutant plants grow slowly and their chloroplast accumulate huge amount of starch, sink tissue in mutants recieve less sucrose while more sucrose in source, opposite of wild-type plants

Question 34

explain phloem unloading (sinks):

Answer 34

-in sugar beets, phloem unloading in young, growing leaves occurs by simple diffusion
-root cells have a large vacuole that stores sucrose, surrounded by a membrane called the tonoplast (active transport, tonoplast contains proteins, pumps the work to accumulate sucrose in the vacuole)

Question 35

how have sinks been manipulated in breeding?

Answer 35

been manipulated to grow bigger to have more cells with sugar
-accumulate more sugar