Photosynthesis: Light Reactions, Carbon Fixation, and Alternative Pathways

Overview of Photosynthesis and Energy Conversion

• Photosynthesis converts electromagnetic light energy into chemical energy stored in carbohydrate bonds.

• Autotrophs ("self-feeders") produce organic carbohydrates from inorganic ions and molecules; heterotrophs ("different-feeders") must consume sugars from other organisms.

• Overall photosynthetic reaction:   $CO_2 + H_2O + \text{Light energy} \rightarrow (CH_2O)_n + O_2$

• Cellular respiration reaction (photosynthesis reversed):   $(CH_2O)_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O + \text{Energy}$

• Source of released oxygen:

  • Cornelius van Niel demonstrated through purple sulfur bacteria ($CO_2 + 2\,H_2S + \text{Light energy} \rightarrow (CH_2O)_n + H_2O + 2\,S$) that oxygen comes entirely from splitting $H_2O$, not $CO_2$.

• Two interdependent reaction modules:

  • Light-capturing reactions: Split $H_2O$ into $O_2$, transfer high-energy excited electrons to $NADP^+$ forming $NADPH$, and synthesize $ATP$.

  • Calvin cycle: Uses $ATP$ and $NADPH$ to reduce $CO_2$ into biologically usable sugars.

Chloroplast Architecture and Pigment Function

• Chloroplast structure:

  • Bounded by outer and inner membranes; photosynthetic plant cells typically contain $40\text{--}50$ chloroplasts.

  • Encloses thylakoids (flattened vesicle-like sacs) grouped into stacks called grana, an internal thylakoid lumen, and the surrounding fluid matrix called the stroma.

• Light physics:

  • Light behaves as waves (defined by wavelength) and discrete packets called photons.

  • Photon energy is inversely proportional to wavelength ($\text{visible light} = 400\text{--}710\,\text{nm}$; shorter wavelengths carry higher energy).

• Primary photosynthetic pigments:

  • Chlorophyll $a$ and chlorophyll $b$: Absorb red and blue light and reflect/transmit green light; contain a light-absorbing porphyrin ring head with a central $Mg$ atom and an isoprenoid tail anchored in the thylakoid membrane. Chlorophyll $a$ contains a $-CH_3$ group, whereas chlorophyll $b$ contains a $-CHO$ group.

  • Carotenoids ($\beta$-carotene and xanthophylls): Accessory pigments absorbing blue and green light; quench free radicals to protect chlorophylls from oxidative damage.

• Electron excitation and fates:

  • Blue photons excite electrons to energy level $2$; red photons excite electrons to energy level $1$.

  • Fluorescence and heat release (~$2\%$ of absorbed photons in isolated chlorophyll).

  • Resonance energy transfer between antenna complex pigments.

  • Reduction-oxidation reactions at the photosystem reaction center (~$98\%$ of energy drives photosynthesis).

Light-Capturing Reactions and Electron Transport

• Enhancement effect:

  • Photosynthetic rates dramatically accelerate when cells are exposed simultaneously to red light ($653\,\text{nm}$) and far-red light ($713\,\text{nm}$), revealing two interacting photosystems.

• Photosystem II (PSII, $P680$):

  • Light-harvesting complex routes resonance energy to the $P680$ chlorophyll pair.

  • High-energy electrons reduce pheophytin, which transfers electrons to plastoquinone (PQ).

  • PQ shuttles electrons across the thylakoid membrane to the cytochrome complex, translocating protons into the lumen to establish a $1000\text{-fold}$ proton gradient.

  • Proton-motive force powers photophosphorylation via ATP synthase, keeping $ATP$ inside the chloroplast for carbohydrate manufacture.

  • Electrons lost from the reaction center are replenished by water-splitting enzymes:     $2\,H_2O \rightarrow 4\,H^+ + 4\,e^- + O_2$

• Photosystem I (PSI, $P700$):

  • Absorbs photons to re-energize electrons delivered by plastocyanin (PC).

  • Shuttles electrons through ferredoxin to $NADP^+$ reductase to reduce $NADP^+$ into $NADPH$.

• Electron transport configurations:

  • Noncyclic electron flow (Z scheme): Linear pathway ($H_2O \rightarrow \text{PSII} \rightarrow \text{PQ} \rightarrow \text{cytochrome complex} \rightarrow \text{PC} \rightarrow \text{PSI} \rightarrow \text{ferredoxin} \rightarrow NADP^+$). Plastocyanin shuttles over $1000\,\text{electrons/s}$.

  • Cyclic electron flow: PSI diverts excited electrons back through ferredoxin to PQ and the cytochrome complex, generating supplementary $ATP$ via proton-motive force without producing $NADPH$ or oxygen.

Carbon Fixation and the Calvin Cycle

• Gas exchange mechanisms:

  • Leaf cuticle forms a waxy barrier that impedes water loss and gas movement.

  • Stomata (microscopic pores flanked by two guard cells) open to facilitate $CO_2$ intake and $O_2$ diffusion along concentration gradients.

• Three phases of the Calvin cycle (located in chloroplast stroma):

  • 1. Fixation: Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) fixes $CO_2$ onto the 5-carbon acceptor ribulose-1,5-bisphosphate (RuBP):     $3\,\text{RuBP} + 3\,CO_2 \rightarrow 6\,\text{3-phosphoglycerate (3PGA)}$

  • 2. Reduction: 3PGA is phosphorylated by $ATP$ and reduced by $NADPH$:     $6\,\text{3PGA} + 6\,\text{ATP} + 6\,\text{NADPH} \rightarrow 6\,\text{glyceraldehyde-3-phosphate (G3P)}$     One net $G3P$ exits the cycle to build carbohydrates.

  • 3. Regeneration: Remaining G3P molecules use $ATP$ to restore RuBP:     $5\,\text{G3P} + 3\,\text{ATP} \rightarrow 3\,\text{RuBP}$

• Inefficiency and photorespiration:

  • Rubisco has 8 catalytic active sites but exhibits slow kinetics and competes between $CO_2$ and $O_2$.

  • When oxygen binds RuBP, photorespiration produces 3PGA and 2-phosphoglycolate, consuming $ATP$ and liberating $CO_2$, thereby reversing carbon fixation.

• Sugar utilization:

  • G3P is converted via gluconeogenesis into glucose and fructose, synthesizing sucrose in the cytosol or forming starch reserves inside the chloroplast.

Carbon-Concentrating Mechanisms: C4 and CAM Pathways

• $C_4$ pathway (spatial separation):

  • Adaptations found in plants such as corn to suppress photorespiration in hot, dry environments.

  • Phosphoenolpyruvate (PEP) carboxylase fixes $CO_2$ to PEP ($3\text{-carbon}$) in mesophyll cells to create a $4\text{-carbon}$ organic acid.

  • The $4\text{-carbon}$ compound travels via plasmodesmata to bundle-sheath cells, decarboxylating to deliver concentrated $CO_2$ to Rubisco while recycling a $3\text{-carbon}$ compound to regenerate PEP.

• Crassulacean acid metabolism (CAM) pathway (temporal separation):

  • Found in succulent plants and cacti to minimize daytime transpiration.

  • Stomata open at night; PEP carboxylase incorporates $CO_2$ into $4\text{-carbon}$ organic acids.

  • Stomata close during the day; stored organic acids release $CO_2$ to supply Rubisco and the Calvin cycle fueled by light reactions.

Questions & Discussion

• Dr. Sean Overa developed an artificial photosynthetic system which coupled electricity harvested from solar panels to artificially catalyze the conversion of $CO_2$ and water into $O_2$ and small organic molecules for the growth of food-producing organisms. What high-energy molecules in photosynthesis are replaced by electricity in this system?

  • Electricity replaces $ATP$ and $NADPH$, the high-energy intermediate chemical carriers produced during natural light-capturing reactions.

• Multiple Choice Question: During the Calvin cycle, $CO_2$ is converted into more complex molecules such as sugars. Where does the energy for this reaction originally come from?

  • a. ATP and NADH are hydrolyzed to create the energy necessary for carbohydrate synthesis

  • b. The energy is captured through light energy to create temporary energy storage molecules such as ATP and NADPH

  • c. The bonds in carbon dioxide are broken releasing the energy to drive the reaction

  • d. Water is broken down into $O_2$ and energy is released

  • Correct response: b. The energy is captured through light energy to create temporary energy storage molecules such as ATP and NADPH.