Bio 351 - Exam 3 (phloem & respiration)

Flow of photoassimilates

Through phloem from sources to sinks

source

Mature leaves or storage organs that produce more photoassimilates than they use

sink

Roots, stems, and fruits that consume photoassimilates for metabolism

Why aren’t mature leaves sinks?

They have a slow growth rate with little metabolic demands

phloem

Living STEs that are consistently regenerated for long-distance transport, bidirectional (different sieve tubes within the same bundle)

vascular bundle makeup

inner xylem with outer phloem surrounded by a bundle sheath

vascular cambium

produces xylem inward and phloem outward

sieve tube elements (STEs)

elongated conducting cells connected end to end by sieve plates that lack a nucleus and rely on companion cells

sieve tube elements

How are damaged sieve plates sealed?

P-proteins plug plates to prevent pressure release that could affect phloem sap, protein crystals from ruptured plastids seal tubes

companion cells

Have nuclei, mitochondria, chloroplasts. Reduced cytoplasmic contents and smooth ER minimize resistance to sap flow

transfer cells

Parenchyma cells associated with phloem that transfer photoassimilates b/t mesophyll cells

Where does G3P go after exiting the chloroplast?

To the cytosol for starch storage or sucrose synthesis

C allocation refers to what?

Whether G3P is devoted to starch or sugars

How is C mainly transported?

As sucrose because its soluble, osmotically effective, and non-reducing

reducing sugars

not translocated because ketone group is reactive

non-reducing sugars

disaccharides that don’t have reactive groups and if they do it forms sugar alcohol

pressure flow model for phloem transport

Mass flow due to pressure gradient (+ hydrostatic pressure from source to sink), linked with transpirational water flow from the xylem, passive

phloem loading (pressure-flow model)

At source, photoassimilates are transported into STEs. Decreases solute potential so water diffuses in from the xylem and pressure gradient drives solutes towards sink tissues

phloem unloading (pressure-flow model)

At sink tissues, change in water potential causes water to exit towards the xylem

symplastic phloem loading

Assimilates travel from mesophyll cells to sieve elements via plasmodesmata (passive), rely on conversion of sucrose to larger sugars to get trapped within the phloem (polymer trap model)

RFO’s in polymer trap model

After plasmodesmata diffusion, glucose enters intermediary cells, changes to large non-reducing sugars that cannot diffuse back into mesophyll cells. Creates concentration gradient

apoplastic phloem loading

sugars move symplastically through plasmodesmata to bundle sheath, are released into cell wall space (apoplast), and actively transported across PM of companion cells via sugar transport proteins

What does apoplastic phloem loading do?

Lowers solute potential increasing water entry into phloem so that sugars accumulate in STEs

How do sugars get into the cell wall and then the cytosol in active phloem transport?

1. H+-ATPase (in companion cell PM) uses ATP to pump protons into apoplast creating high H+ in the apoplast and low H+ in the cytosol = electrochemical gradient

2. Sucrose-H+ symporters use gradient to transport sucrose into companion cell, sucrose uptake occurs against its own conc. gradient by moving with H+

Where does respiration occur?

plastids, cytosol, mitochondria

“metabolic currency” forms

hexose-P and triose-P (G3P) link photosynthesis, glycolysis, and biosynthesis

Purpose of respiration in plants

Supplies ATP and NADH/NADPH reducing power. Designed to manage C allocation rather than only ATP, C retained in biomass

How does resp. differ in plant organs?

Buds have high rates whereas older tissues are lower

Respiration biosynthetic roles

C skeletons (AAs, nucleotides, organic acids, secondary metabolites), shared intermediated with N-assimilation, plant growth

Cytosol role in respiration

Where C arrives from photosynthesis, storage, and phloem. Hexose-P pools are C-decision hubs where some is reconverted to sucrose

Plastids role in respiration

Where photosynthesis produces G3P, pentose phosphate pathway generates NADPH for biosynthesis

Mitochondria role in respiration

Organic acids from cytosol enter to fuel TCA cycle (releases CO2, NADH, FADH2), oxidative phosphorylation (uses these to make ATP). Primary metabolites accumulate (malate, citrate, 2-oxoglutarate)

Characteristics of mitochondria

Continuously change to match metabolic demand (high demand in growing regions), close to plastids for metabolite exchange, function within other organelles

mitochondria electrochemical gradients

Due to metabolite transport across cristae that forms a proton motive force, ATP export/ADP import, and organic acid movement

roles of metabolites in mitochondria

C storage, pH buffering, ion balance, transport C b/t tissues

Malate roles

Major transport C form b/t plastids, mito., and cytosol. Redox shuttle b/t compartments, osmoregulation in guard cells

2-oxoglutarate roles

Connects resp. to N assimilation to support AA biosynthesis

How is energy released from stored photoassimilates?

glycolysis, citric acid cycle, respiratory e- transport chain

glycolysis

Creates pyruvate from carbohydrates, begins with sucrose, flexible and branched, many rxns are reversible (ex. too much sugar=convert back to starch)

tricarboxylic acid cycle (TCA or Krebs cycle)

Convert pyruvate to acetyl-CoA and e-, which is passed to electron carriers NADH and FADH2. Intermediates are citrate, 2-oxoglutarate, malate

respiratory (mitochondria) e- transport chain

Electron transport used as energy to create ATP, O is terminal e- acceptor. Need membrane, H+ gradient, ATP synthesis

Hexose-P fates

• Glycolysis for energy and organic acids

• OPPP pathway for NADPH

• Sucrose (transport)

• Starch (storage)

• Cell wall synthesis or other paths

Fermentation in plants vs animals

Animals→ occurs because O2 is low, more urgent

Plants→ When soils are flooded and O2 is low but not required since plants can recycle NADH from pyruvate reduction

Oxidative pentose phosphate pathway (OPPP)

Produces NADPH since plants cannot only rely on chloroplasts. In cytosol and plastids to allow NADPH formation in dark roots and non-photosynthetic tissue, suppressed in light

TCA intermediates in plants vs animals

Animals→ Intermediates cycled quickly

Plants→ Cycle strategically, biosynthesis and metabolism first, energy second

malic enzyme

Allows mitochondria to oxidize malate directly to pyruvate, NADH generation without glycolytic input

PEP carboxylase

Links resp to N-assimilation, converts PEP to oxaloacetate without C loss (essential for ammonium assimilation)

Alternative oxidase (AOX)

When plant is receives more light than it can process, e- bypass processing complexes to prevent over-reduction of ETC and ROS. Limits ATP production and generates heat

Role in thermogenesis in emerging skunk cabbage