lec 14- sugar transport in plants Flashcards
what is translocation?
is the movement of sugars through a plant by bulk flow from sources to sinks through the phloem
what is a source?
a tissue where sugar enters the phloem
what is sink
a tissue where sugar exits the phloem
what are the sugar concentrations in sources and sinks?
sources have high sugar concentration and sinks have low sugar concentrations
what is bulk flow?
movement of fluid due to a pressure gradient
what is the phloem and what does the phloem do?
phloem are the leaf veins and they transport and remove sugars in a plant
how does sugar move in the source (phloem loading)?
sugar moves from photosynthesizing mesophyll cells into phloem of minor veins
how do aphids prove the pressure gradient between source and sink?
aphids puncture the phloem and sap oozes out, aphids puncture xylem and air is sucked in. source has high pressure and sink has low
are leaves always sources?
no, they start out as a sink then become a source
on what sides and ends do sources supply to the sink?
on the same side and ends
how does the location of sources and sinks vary with time of year?
-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
what is the anatomy of a phloem cell?
a phloem mother cell divides to form:
a sieve tube element and a companion cell
multiple sieve tube elements connected form a sieve tube
what resistance do sieve tubes have?
low resistance to create pressure gradient
what do phloem proteins do (P-proteins)?
quickly move as flow rate increases to block sieve plate pores to reduce loss and heal wounds
why are sieve tube elements considered living and what do they lack and have?
-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
how do companion cells support sieve elements?
-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
how are sieve tubes specialized for transporting?
-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
what type of sugars are transported in the phloem?
non-reducing sugars because they are less reactive and ideal
what type of pressure causes phloem transport?
positive pressure
what causes high pressure at sources?
sugar is loaded into phloem in source leaves, water diffuses down gradient into phloem causing high pressure resulting in bulk flow towards sinks
what causes low pressure at sinks?
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
what is the pressure flow model at the source?
- sugar is actively loaded into sieve cell/companion cells
- water potential decreases in sc/cc complex
- water potential in nearby xylem is higher
- water will diffuse down its gradient from xylem to phloem
- turgor pressure builds up in sc/cc complex, pushing liquid towards cells/regions with lower pressure
what is the pressure flow model at the sink?
- sugars are actively unloaded from the sc/cc complex into sink cells
- water potential increases in sc/cc complex
- water potential in nearby xylem is lower
- water will diffuse down gradient from phloem to xylem
- turgor pressure reduced
- the difference in pressure between source and sink generates a net flow from source to sink of water and its solutes
how fast do sugars move?
1 meter per hour (diffusion takes 1 meter per 32 years in a sieve tube)
