Photosynthesis II: The Carbon Reactions

1

• Photosynthesis II The Carbon Reactions
• So what weve seen is how plants convert light
  energy into various forms of chemical energy. In
  order to do this in their various environments,
  it is often necessary for them to cope with
  having sub-optimal conditions such as too much
  light, not enough CO2, or not enough water. But
  wherever plants are found, they have found ways
  to cope with these stresses, and at least survive
  if not thrive and dominate.
• Now theyve made sugar, which is shipped to
  places in the plant that need energy and/or
  carbon as sucrose. Next well see how that is
  done.

2

• Translocation in the Phloem
• What is the phloem?
• What goes through the phloem?
• Where is it going?
• How does it get in the phloem?
• How does it get there?
• How does it get out?

photosynthate
Sink tissue or bust!
3

• Translocation in the Phloem
• The phloem (partly review)
• A. The phloem is usually found toward the
  outside of a
• stem, or toward the underside of a leaf in a
  vein.
• B. The conducting cells are called sieve
  elements.
• 1. Two styles
• a. Gymnosperms (like
• conifers) have sieve cells.
• b. Flowering plants
• (angiosperms) have sieve
• tube elements, so named
• because their cells are
• often connected by larger
• pores, and thus are more
• like open tubes.

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• Translocation in the Phloem
• 2. These are alive at maturity, although
  highly modified.
• a. Have far fewer membranes than most cells,
  having lost their nuclei, rough ER, Golgi, and
  vacuoles.
• b. Sieve cells are connected to their
  neighbors by sieve areas. In seive tube
  elements of angiosperms, the pores can be more
  open, forming sieve plates at the end walls of
  cells. This is why angiosperms have the more
  conductive sieve tubes. Figures 10.3, 10.5

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• Translocation in the Phloem
• 3. Besides having modified plastids and
  mitochondria, as well as smooth ER, sieve tube
  elements have lots of P protein, which
  functions in short-term sealing of wounds. It
  is clustered on the side of the cell, near the
  wall, and springs to action if there is a surge.
• 4. A more long-term solution to damage is
  handled by deposition of a carbohydrate called
  callose. Callose also functions in other
  stress pathways such as heat stress, and in the
  formation of pollen.

6

• Translocation in the Phloem
• C. Companion cells help sieve elements
• 1. They help load photosynthate into the sieve
  cells, and they provide the sieve cells with
  proteins and other metabolites, to make up for
  the fact that it cant make its own.
• 2. Three kinds (Figure 10.7)
• a. Ordinary companion cells have normal
  chloroplasts, and almost all of their
  plasmodesmata are shared with just one sieve
  cell.

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• Translocation in the Phloem
• b. Transfer cells have fingerlike cell wall
  projections into the space that would ordinarily
  be taken up by the cell, to increase the surface
  area of plasma membrane across which solutes can
  pass.
• c. Intermediary cells are not found in all
  plants, and represent a different way of loading
  materials into the phloem. These cells emphasize
  lots of branched plasmodesmatal connections with
  the surrounding cells (phloem parenchyma and
  bundle sheath).

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• Translocation in the Phloem
• D. Other cell types are found in the phloem,
  including parenchyma cells and support cells such
  as fibers.
• E. Vascular bundles
• connect together in
• leaves. These
• connections are
• called anastamoses.
• These Arabidopsis
• sepals and petals show
• anastamoses (C, E), but
• the expression of a viral
• gene interferes with
• normal development.

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• Translocation in the Phloem
• What is transported in the phloem?
• A. Products of photosynthesis
• 1. Sugars are transported in non-
• reducing form, in the circular form of the sugar
  
• with no ketone or aldehyde groups, or as sugar
  alcohols.
• 2. Sucrose is the most commonly translocated
  sugar.
• B. Nitrogen fixed in various organic molecules.
• 1. Most plants transport nitrogen as amino
  acids, especially ones with nitrogen-containing
  side chains.
• 2. Leguminous plants have nitrogen-fixing
  bacteria that live in nodules found in their
  roots. These plants have excess nitrogen,
  fixed from N2 gas, and transport nitrogen as
  ureides.

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• Translocation in the Phloem
• C. Plant hormones, proteins, and some inorganic
  ions are transported in the phloem. Table 10.2
  (dont memorize)
• Where is all this stuff going? Source to sink
  movement.
• A. What are sources and sinks?
• B. What is the pattern of source to sink flow
  (do all source leaves provide photosynthate to
  all sinks, or is there a division of labor)?
• The supply is divided up in a few ways
• 1. Location. Location. Location. Sources
  tend to supply nearby sinks.
• 2. Developmental--young leaves (sinks)
  eventually become sources. Storage organs
  (e.g. root) switch seasonally. Meristems are
  always sinks, as are fruit as they emerge and
  grow.

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• Translocation in the Phloem
• 3. Connections
• a. Sources preferentially supply
• sink areas to which they are
• directly (vertically) connected.
• b. Anastamoses between vascular
• bundles can be used if necessary.

Here you can see how scientists did the
experiments to figure out how all these patterns
work. Figure 10.8
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• Translocation in the Phloem
• Review/Overview
• Phloem is made up of sieve elements, which vary
  from angiosperm to gymnosperm, and companion
  cells which vary from species to species (three
  kinds.)
• Sugars (in non-_______ form), nitrogen carriers,
  hormones, and some inorganic ions are carried in
  the phloem.
• Phloem moves materials from source to sink.
• Sugars are loaded into the sieve elements by one
  of two processes.
• Pressure differences drives bulk flow in the
  phloem.
• Unloading can also occur by two processes.
• The transition from sink to source is gradual.

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• Translocation in the Phloem
• How do sugars get into the phloem? Phloem
  loading
• A. Sugars are loaded into the sieve elements at
  the tiniest veins. Figure 10.13
• B. Some species load sugar via the apoplast,
  some via the symplast. These are quite different
  methods. Figure 10.14

The distance from synthesis of the sugar
(directly from triose phosphates or from glucose
that is released from starch within the
chloroplast at night) to a sieve element is
rarely more than a few cells.
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• Translocation in the Phloem
• 1. Loading sugar (sucrose) into sieve elements
  via the apoplast (space within the cell walls)
  requires energy.
• a. Because companion cells in these plants
  (ordinary companion and transfer cells)
  contain many plasmodesmatal connections to
  sieve elements and few/none to other cells,
  they can be thought of as a functional unit
  together. Sucrose loaded into companion
  cells will easily move into sieve elements
  through plasmodesmata.
• b. Sucrose is released from phloem parenchyma
  cells into the apoplast space. Then it has to
  get inside either a companion cell or a sieve
  element.

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• Translocation in the Phloem
• c. Since there is almost always higher
  sucrose concentration in sieve elements of
  source tissue than in the apoplast, this
  process has to be active transport.
• d. It has been shown that
• H-sucrose symporters Figure 10.16
• are used to actively load
• sucrose from the apoplast
• space into the companion
• cell/sieve element complex.
• e. In some plants there are
• different symporters on the
• PMs of sieve elements
• than on those of companion
• cells.

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• Translocation in the Phloem
• 2. Some plants have intermediary cells, and
  these load sugars via the symplast pathway.
• a. Since there are plasmodesmal connections
  between the bundle sheath or phloem parenchyma
  cells and the intermediary cell, sucrose can
  flow from one to the other.
• Polymer trapping Figure 10.17
• b. It is thought, with experimental support,
  that the sugar is then modified to raffinose
  or further to stachyose, both of which may be
  too large to flow back through the
  plasmodesmata that sucrose came through.
• c. The only way these transport sugars can go
  is into the sieve element, since the
  plasmodesmata are large enough for this path.

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• Translocation in the Phloem
• 3. These two methods are preferred by
  different types of plants
• a. Apoplast-loading plants are more often
  herbaceous plants and/or plants in arid or
  temperate climates.
• b. Symplast-loading plants are usually woody
  plants and/or ones that live in tropical
  regions.
• Now sugar is in the phloem, how does it move?
• Pressure differences drive flow in the phloem.
• A. We know that the materials move way too fast
  to be powered by diffusion.

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• Translocation in the Phloem
• B. Bulk flow is driven by pressure differences.
  If this is what is happening, then
• Should there be path be clear, or have
  obstructions?
• Should you ever see materials flowing different
  directions in the same tube?
• Should it take a lot of energy during transit?
• Should there be measurable differences in the
  pressure within phloem tubes from source tissue
  to sink tissue?
• 1. At first scientists did electron microscopy
  and thought that P-proteins were blocking sieve
  plates. Turns out this is an artifact of their
  sample preparation technique. Tubes are now
  known to be relatively unblocked.
• 2. Transport that has been observed is always
  in a single
• direction within one tube. Figure 10.11

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• Translocation in the Phloem
• 3. Inhibiting energy utilization in transport
  tissue has no long-term effect on
  transportation.
• In this experiment, the petiole (connects leaf to
  stem) was chilled to inhibit energy utilization
  (respiration). Fig. 10.12
• 4. Pressure differences in sieve tubes between
  source and sink have been measured, and are
  shown to be sufficient.
• C. So how does it work? Figure 10.10
• But I mentioned that unloading in the phloem is
  one of the keys.
• How does that happen?

20

• Translocation in the Phloem
• How do I get out of here?!?! Phloem unloading
• A. Unloading, too can occur symplastically or
  apoplastically, and both these pathways are
  found in the same plant, at different stages of
  the sink-to-source transition.
• 1. Young sink tissue usually unloads materials
  symplastically, where all the cells from SE/CC
  to the sink cell (e.g. dermal cell in a rapidly
  dividing leaf) share symplastic connections.
  Figure 10.18A

Unregulated, flow by passive diffusion
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• Translocation in the Phloem
• 2. Other tissues, such as developing seeds and
  stomatal guard cells, lack plasmodesmatal
  connections to surrounding cells, and so must
  use an apoplastic unloading system. A couple
  of ways this could work
• a. Type 1 Apoplastic unloading occurs right
  at the SE/CC complex. No evidence for this.
• b. Type 2 occurs further away from the SE/CC
  complex.
• All types of apoplastic unloading may require
  energy. In
• soybean embryos it has been shown that both
  transport into
• the apoplast and import into the sink cell are
  active processes.

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• Translocation in the Phloem
• Tissues transition from sink to source gradually.
• A. The cessation of sucrose import begins at
  the tip of the leaf, when the leaf is about 25
  expanded (in dicots), and is completed by the
  time the leaf is about 40 - 50 expanded.
• Here, a (fully) source leaf from squash was
  incubated with
• radioactive CO2 for two hours. A little while
  later, upper
• leaves (pictured below) were removed and exposed
  to an
• autoradiograph. Figure 10.19

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• Translocation in the Phloem
• B. Cessation of sucrose import (sink activity)
  and beginning of sucrose export (source activity)
  may be independently regulated.
• In order to do each process, the cell needs
  certain different enzyme and/or especially
  different transport molecules. All these are
  developmentally regulated, and evidence from
  albino tobacco leaves indicates that one side of
  the transition may be independent of the other.