Chapter 10: Transportation in the Phloem & Respiration Flashcards
phloem
- the tissue that transports the products of photosynthesis (sugars) and other substances (e.g., hormones, proteins, RNA, ions) throughout the plant
- the phloem and xylem extend throughout the plant body
where do sugars move to?
from mature leaves to areas of growth and storage (e.g., immature leaves,
roots, fruits, seeds)
in plants with secondary growth, where is the phloem?
- phloem is typically outside the vascular cambium opposite of the xylem
- only the innermost layer of cells is active, the outer layers are crushed by the
expansion of the newest layer while xylem (wood) grows inward from the
cambium increasing stem diameter as the plant grows
girdling
removal of phloem (and bark) in a ring around the stem, disrupts connection between
roots and leaves killing the tree
what cells is the phloem made up of?
- sieve cells
- companion cells
- parenchyma cells
- fiber and sclereids
- sometimes laticifers
parenchyma cells
- store and release photosynthetic products
fibers and sclereids
strengthen and protect tissue complex
laticifers
- used for protection
mature sieve cells
- directly involved in translocation
- do not have nuclei, tonoplasts, microfilaments, microtubules, Golgi bodies, or ribosomes
- do have mitochondria, plastids, and smooth ER
- non-lignified walls
- stack to form sieve tubes
sieve plates
- in angiosperms
- on end walls (where they stack up to form a tube) differentiate into larger pores
- gymnosperms do not have sieve plates, all sieve areas are the same across the cell
- does have sieve areas covered in ER
companion cells
- sieve cells rely on these because they do not have things like nuclei, etc.
- companion cells regulate metabolic activity
- a mother cells divides to form a sieve cell and a companion cell
- connected via plasmodesmata
- critical role in maintenance such as providing protein, ATP, transport of sugar from mesophyll to sieve cells
what are the three types of companion cells?
- ordinary
- transfer
- intermediary
ordinary companion cells
relatively few plasmdesmatal connections to cells except
to its own sieve element; symplastically isolated from surrounding cells (involved in
apoplastic loading)
transfer cells
– like ordinary cells, but with finger-like wall ingrowths facing away
from the sieve elements
Increases surface area for solute transport from the apoplast (involved in
apoplastic loading
intermediary cells
numerous plasmodesmata connecting them to bundle sheath
and sieve elements, a lot of continuous cytoplasmic connections to solute transport
(involved in symplastic loading)
how does phloem move?
source to sink
- sources are areas capable of producing photosynthates in excess of their own needs - mature leaves or even storage organs
- sinks are any non-photosynthetic tissue that rely on other organs for growth - roots, tubers, developing fruits, immature leaves
what does the transport pathway depend on for source to sink
- proximity of the organs
- development (roots and shoots are key in growth)
- vascular connections (leaves will supply sinks to which are directly connected)
- modification of pathways (wounding or pruning can change pathways)
how can scientists observe phloem?
- use aphid stylets as natural syringes
- Scientists remove the aphid body and collect the exudate from the still inserted stylet
what is the most abundant substance in the phloem?
- water
- dissolved in the water include carbohydrates, amino acids, proteins, horomones, inorganic ions, and RNA
what are the sugars that are in the phloem?
- non-reducing sugars: sucrose, raffinose, stachyose, and verbascose
- reducing sugars: glucose, mannose and fructose; not transported in the phloem bc too reactive
Pressure Flow Model
- Ernst Munch in 1930
- sap moves by bulk flow and is driven by a pressure gradient
- Companion cells load solutes into sieve elements causing a decrease in sieve element water potential. Water flows into the sieve element following the more negative water potential via osmosis. This generates a positive pressure potential in the sieve element causing phloem sap to move through the sieve tube via mechanical pressure driven bulk flow. Phloem loading can be apoplastic or symplastic. Apoplastic loading requires ATP – H+-ATPase transports H+ into the apoplast and a sucrose-H+ symporter transports sucrose from the apoplast into the SE-CC complex. Symplastic loading happens via polymer loading, wherein raffinose and stachyose are synthesized from the sucrose in intermediary cells. These larger molecules cannot reverse their path back through the small plasmodesmata to the bundle sheath, so they diffuse through the larger plasmodesmata leading to the sieve cell due to a chemical potential diffusion gradient. Unloading is just loading in reverse at the sink.
what are the four predictions the pressure flow model makes?
- sieve pores are open
- flow is not bidirectional
- flow does not require energy
- a pressure gradient exists such that pressure is higher near the source and lower near the sink
what are sieve cells filled with?
- a solution of sugars and organic molecules called sap
P-proteins
- seal damaged sieve elements by plugging up sieve plate pores
- Because phloem cells are under positive pressure, the contents of
the cells (including the P-proteins) surge towards a cut or puncture
and the P-proteins are trapped in the sieve plate pores sealing off
the damaged element
wound callose
- over time callose is synthesized in the sieve element and deposited in the sieve plate pore
- Callose can be deposited and broken down in response to transient stress (e.g., temperature, drought, mechanical
stimulation)
short-distance transport pathway
- when sucrose moves from the photosynthetically active cells to the cells nearest to the sieve element-companion complex
- only a few diameters long and transport is symplastic
Apoplastic loading
- requires ATP
- h-ATPase transport H into the apoplast and a sucrose-H symporter transports sucrose from the apoplast into the SE-CC complex.
how is apoplastic loading regulated?
- Regulated by solute potential in the sieve element, turgor pressure,
sucrose concentration in the apoplast, and the availability of symporters
in the cell membrane
symplastic loading
- occurs in plants with intermediary cells - polymer trapping
- sucrose synthesized in the mesophyll diffuses from the bundle sheath cells into the intermediary cells via small plasmodesmata
- in the intermediary cells, raffinose and stachyose are synthesized from the sucrose and are larger molecules that cannot reverse their path into the small plasmodesmata but can diffuse into the larger plasmodesmata leading to sieve cells
what happens to sugars once they leave the sink?
- phloem unloading can occur symplastically or apoplastically
- all developing leaves begin as sinks and sugars are unloaded into the major veins
- once the plant develops and expands, the veins are space further apart and minor veins develop
- once minor veins are developed loading can begin at minor veins and major veins stop unloading
what else is loaded and unloaded at sources and sinks?
- proteins and signaling molecules
what is respiration?
- photosynthesis in reverse
- breakdown and dispersal of sugars into ATP
- this ATP is used for metabolism, protein synthesis, and DNA synthesis
- all living organisms use aerobic respiration
what respiration aspects are unique to plants?
- used to change their temperature - can heat themselves up
- cannot pee so plants have to rapidly break down waste products to prevent toxic buildup
- cannot move - need to attract pollinators or prevent herbivores - creates secondary molecules from respiration to do this
process of respiration
- during respiration, free energy is released and temporarily stored as ATP that can be used for cell maintenance and development
- the oxidation of sucrose and the reduction of O2 to water
in a functioning cell, where is reduced carbon derived from?
- reduced carbon is mainly derived from sucrose, triose phosphates, and other sugars, lipids, organic acids, and sometime proteins
To prevent heat damage to cellular structures by a large release of energy, the cell mobilizes the
free energy in sucrose in multi-step reactions, what are they?
- glycolysis
- the pentose phosphate pathway
- citric acid cycle
- oxidative phosphorylation
