22: Carbon Assimilation & Phloem Notes

Carbon Assimilation & Phloem

Objectives

• Understand the structure and function of leaf tissues.
• Understand the structure and function of phloem.
• Understand the pressure-flow model of phloem transport.

Carbon Cycle

• Vegetation fixes approximately 123 billion tonnes of carbon yearly, according to Beer et al. (2010) in Terrestrial gross carbon dioxide uptake, Science 329: 834-838.
• Most carbon fixed by plants returns to the atmosphere through respiration.
• A small percentage of fixed carbon is trapped, entering longer geological cycles, like the formation of sedimentary rock.
• Fossil fuels represent carbon trapped by past photosynthesis, now being released.

Endosymbiont Origin of Chloroplasts

• Endosymbiosis occurred approximately 1.5-1.2 billion years ago.
• The molecular fundamentals of photosynthesis pre-date eukaryotic plants.
• Primary endosymbiosis involves a phagotrophic plant ancestor engulfing cyanobacteria.

Monocot & Dicot Leaves

• Monocot leaves, such as maize (Zea mays), have parallel veins.
• Dicot leaves, such as sunflower (Helianthus annuus), have netted veins.

Leaf Structure

• Epidermis: Lacks chloroplasts
• Palisade mesophyll: Contains many chloroplasts
• Spongy mesophyll: Has large air spaces

Amplification of Incident Light by Leaf Structure

• Epidermis:
  • The convex surface focuses light.
  • Mesophyll receives 2-3 times more intense light than what falls on the leaf.
• Palisade mesophyll:
  • Columnar cells channel light.
• Spongy mesophyll:
  • Air spaces scatter light.
  • Increases the time each photon spends in the leaf.
  • Photon density inside the leaf is greater than outside.

Stomata

• $CO_2$ enters the leaf through pores called stomata (singular: stoma).
• $O<em>2$ and $H</em>2O$ exit via stomata.

Stomata Structure

• Stomatal pore is surrounded by guard cells and subsidiary cells.
• Guard cells have thick walls.

Regulation of Stomata

• Stomata close when guard cells lose turgor pressure.
• Generally, stomata are open during the day and closed at night.
• Regulation occurs via active ion transport to conserve $H_2O$, especially in drought conditions.

Summary of Photosynthesis

• Equation: $6CO<em>2 + 6H</em>2O \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Photosynthesis is the ultimate source of all food and oxygen.
• Reference: Morris et al. (2016).

Calvin Cycle

• Primary carbon fixation cycle.
• The 3-carbon product is a result of Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase), which is the $CO_2$-fixation enzyme.
• Rubisco:
  • Evolved in prokaryotes approximately 2.45 billion years ago.
  • Fixes approximately 123 billion tonnes of carbon per year.
  • It's the most abundant protein on Earth.

Fixed Carbon Export

• Fixed carbon is exported from the mesophyll.
• Transport routes:
  • Symplastic route: via plasmodesmata.
  • Apoplastic route: via cell walls.

Sources and Sinks

• Source: photosynthate-exporting organ or storage organs.
• Sink: photosynthate-importing organ.
• Transport occurs from sources to sinks.

Phloem Location

• Phloem is located in vascular bundles.
• Example shown: Sunflower stem (Helianthus annuus).
• Other tissues present: Epidermis, Cortex, Fibers, Cambium, Xylem, Pith

Companion Cells

• Each sieve tube element (SE) has a companion cell (CC).
• CC contains a nucleus and mitochondria.
• CC provides SE with proteins and energy for transport.
• CC connected by plasmodesmata to SE.

Sieve Tube Elements

• Conducting cells: sieve tube elements.
• Sieve tube element: lacks a nucleus and mitochondria.
• End walls are perforated, forming cytoplasmic connections between stacked cells, creating a 'tube'.

Composition of Phloem Sap (from Castor Bean)

• Sugars: 80.0-106.0 $mg \cdot mL^{-1}$
• Amino acids: 5.2 $mg \cdot mL^{-1}$
• Organic acids: 2.0-3.2 $mg \cdot mL^{-1}$
• Protein: 1.45-2.20 $mg \cdot mL^{-1}$
• Potassium: 2.3-4.4 $mg \cdot mL^{-1}$
• Chloride: 0.355-0.675 $mg \cdot mL^{-1}$
• Phosphate: 0.350-0.550 $mg \cdot mL^{-1}$
• Magnesium: 0.109-0.122 $mg \cdot mL^{-1}$

Sugars Transported

• Sugars transported are 'non-reducing'.
• Reducing sugars (e.g., glucose, fructose, galactose) are too reactive (can be oxidized).
• Most common transported sugar is sucrose (disaccharide, non-reducing).
• Others consist of sucrose bound to galactose(s) or non-reducing sugar alcohols.

Pressure-Flow Model: Loading

• Sucrose accumulates against the concentration gradient.
• Proton pumps generate a $H^+$ gradient by active transport.
• Sucrose is driven across plasma membranes by co-transport with $H^+$ ions.

Pressure-Flow Model: Loading (Continued)

• At sources, sugars are loaded into sieve tubes.
• Water follows by osmosis from source cells and xylem.
• Pressure is created.

Pressure-Flow Model: Unloading

• At the sink, sugars are unloaded out of the phloem.
• Water follows by osmosis.
• Phloem sap flows from high to low pressure.
• Flow rate (approximately 1 m h-1) is faster than diffusion of the same molecules.

Phloem as a Signal Transducer

• Phloem transmits ‘signal’ molecules.
• Day-length sensor in leaves produces ‘FT protein’.
• FT moves in the phloem to buds, triggering flowering.

Phloem - A 'Green Cable'

• The Venus flytrap utilizes rapid electrical signaling to close its modified leaves when prey touches sensory hairs.
• Trigger mechanism: Prey touching sensory hairs generate an action potential (AP).
• Phloem-mediated signal: AP propagates via $Ca^{2+}$, $K^+$, and $Cl^-$ ion fluxes, causing rapid leaf closure (approximately 100 ms).
• Hydraulic response: Turgor pressure changes in motor cells at the hinge drive trap movement.

Summary

• Leaf structure amplifies incident light.
• Leaf internal tissues include palisade and spongy mesophyll.
• $CO<em>2$ enters and $O</em>2$ & $H_2O$ exit via stomata.
• Angiosperm phloem has sieve-tube elements with companion cells.
• The pressure-flow model explains phloem transport from sources to sinks.
• Flowering signals move in the phloem.