Chapter 10: Transportation in the Phloem & Respiration

Question 1

phloem

Answer 1

• 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

Question 2

where do sugars move to?

Answer 2

from mature leaves to areas of growth and storage (e.g., immature leaves,
roots, fruits, seeds)

Question 3

in plants with secondary growth, where is the phloem?

Answer 3

• 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

Question 4

girdling

Answer 4

removal of phloem (and bark) in a ring around the stem, disrupts connection between
roots and leaves killing the tree

Question 5

what cells is the phloem made up of?

Answer 5

• sieve cells
• companion cells
• parenchyma cells
• fiber and sclereids
• sometimes laticifers

Question 6

parenchyma cells

Answer 6

• store and release photosynthetic products

Question 7

fibers and sclereids

Answer 7

strengthen and protect tissue complex

Question 8

laticifers

Answer 8

• used for protection

Question 9

mature sieve cells

Answer 9

• 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

Question 10

sieve plates

Answer 10

• 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

Question 11

companion cells

Answer 11

• 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

Question 12

what are the three types of companion cells?

Answer 12

• ordinary
• transfer
• intermediary

Question 13

ordinary companion cells

Answer 13

relatively few plasmdesmatal connections to cells except
to its own sieve element; symplastically isolated from surrounding cells (involved in
apoplastic loading)

Question 14

transfer cells

Answer 14

– 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

Question 15

intermediary cells

Answer 15

numerous plasmodesmata connecting them to bundle sheath
and sieve elements, a lot of continuous cytoplasmic connections to solute transport
(involved in symplastic loading)

Question 16

how does phloem move?

Answer 16

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

Question 17

what does the transport pathway depend on for source to sink

Answer 17

• 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)

Question 18

how can scientists observe phloem?

Answer 18

• use aphid stylets as natural syringes

- Scientists remove the aphid body and collect the exudate from the still inserted stylet

Question 19

what is the most abundant substance in the phloem?

Answer 19

• water

- dissolved in the water include carbohydrates, amino acids, proteins, horomones, inorganic ions, and RNA

Question 20

what are the sugars that are in the phloem?

Answer 20

• non-reducing sugars: sucrose, raffinose, stachyose, and verbascose
• reducing sugars: glucose, mannose and fructose; not transported in the phloem bc too reactive

Question 21

Pressure Flow Model

Answer 21

• 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.

Question 22

what are the four predictions the pressure flow model makes?

Answer 22

1. sieve pores are open
2. flow is not bidirectional
3. flow does not require energy
4. a pressure gradient exists such that pressure is higher near the source and lower near the sink

Question 23

what are sieve cells filled with?

Answer 23

• a solution of sugars and organic molecules called sap

Question 24

P-proteins

Answer 24

• 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

Question 25

wound callose

Answer 25

• 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)

Question 26

short-distance transport pathway

Answer 26

• 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

Question 27

Apoplastic loading

Answer 27

• requires ATP
• h-ATPase transport H into the apoplast and a sucrose-H symporter transports sucrose from the apoplast into the SE-CC complex.

Question 28

how is apoplastic loading regulated?

Answer 28

• Regulated by solute potential in the sieve element, turgor pressure,
  sucrose concentration in the apoplast, and the availability of symporters
  in the cell membrane

Question 29

symplastic loading

Answer 29

• 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

Question 30

what happens to sugars once they leave the sink?

Answer 30

• 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

Question 31

what else is loaded and unloaded at sources and sinks?

Answer 31

• proteins and signaling molecules

Question 32

what is respiration?

Answer 32

• 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

Question 33

what respiration aspects are unique to plants?

Answer 33

1. used to change their temperature - can heat themselves up
2. cannot pee so plants have to rapidly break down waste products to prevent toxic buildup
3. cannot move - need to attract pollinators or prevent herbivores - creates secondary molecules from respiration to do this

Question 34

process of respiration

Answer 34

• 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

Question 35

in a functioning cell, where is reduced carbon derived from?

Answer 35

• reduced carbon is mainly derived from sucrose, triose phosphates, and other sugars, lipids, organic acids, and sometime proteins

Question 36

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?

Answer 36

• glycolysis
• the pentose phosphate pathway
• citric acid cycle
• oxidative phosphorylation