8.6 Photosynthesis

Photosynthetic Pigments and Light Capture

Photosynthesis converts light energy into chemical energy, producing organic molecules that fuel nearly all life on Earth. While you might associate photosynthesis with plants, this is a microbiology course, so the focus here is broader: cyanobacteria, algae, purple and green bacteria, and other photosynthetic microorganisms all use variations of this process. Understanding how light energy drives carbon fixation is central to understanding microbial metabolism.

The process has two major stages. First, light-dependent reactions capture photons and convert that energy into ATP and NADPH. Then, light-independent reactions (the Calvin cycle) use ATP and NADPH to fix $CO_2$ into organic molecules like glucose.

Distribution of Photosynthetic Pigments

Photosynthetic organisms use pigments to absorb specific wavelengths of light. Different pigments absorb different wavelengths, and organisms combine multiple pigments to capture as much usable light as possible.

Chlorophylls are the primary photosynthetic pigments:

• Chlorophyll a is found in all oxygenic photosynthetic organisms (plants, algae, cyanobacteria). It's the pigment that directly participates in the light reactions.
• Chlorophyll b is found in plants and green algae (Chlorophyta). It acts as an accessory pigment, passing absorbed energy to chlorophyll a.
• Both absorb strongly in the red and blue wavelengths and reflect green light, which is why photosynthetic organisms often appear green.
• Bacteriochlorophylls are found in anoxygenic photosynthetic bacteria (purple and green bacteria). They absorb longer wavelengths, including infrared, allowing these organisms to photosynthesize in environments where visible light is scarce.

Accessory pigments broaden the range of light an organism can use and provide photoprotection:

• Carotenoids (carotenes and xanthophylls) absorb blue and green light and transfer that energy to chlorophylls. They also protect cells from photodamage by quenching reactive oxygen species generated by excess light energy. Found in plants, algae, and many bacteria.
• Phycobilins (phycoerythrin and phycocyanin) absorb green, yellow, and orange wavelengths that chlorophyll captures poorly. Found in cyanobacteria and red algae (Rhodophyta), these pigments enable photosynthesis in deeper water where only shorter-penetrating wavelengths of green and yellow light are available.

Light-harvesting complexes (antenna complexes) organize hundreds of pigment molecules together in the photosynthetic membrane. They funnel captured light energy through a series of pigments until it reaches a reaction center, where the energy drives electron transfer. This arrangement makes light capture highly efficient because many pigment molecules feed energy into a single reaction center.

Photosynthetic Reactions and Products

Light-Dependent Reactions

The light-dependent reactions take place in the thylakoid membranes (in cyanobacteria, these are internal membrane systems; in eukaryotes, they're inside chloroplasts). Their job is to convert light energy into the chemical energy carriers ATP and NADPH.

• Photosynthetic pigments absorb photons and transfer energy to reaction centers in Photosystem II (PSII) and Photosystem I (PSI).
• Water molecules are split at PSII ($2H_2O \rightarrow 4H^+ + 4e^- + O_2$), releasing $O_2$ as a byproduct. This is the source of nearly all atmospheric oxygen.
• Electrons pass through an electron transport chain (including the cytochrome $b_6f$ complex), which pumps protons across the membrane to build a proton gradient.
• ATP synthase uses that proton gradient to generate ATP through photophosphorylation.
• At PSI, electrons are re-energized by light and ultimately reduce $NADP^+$ to NADPH via ferredoxin-$NADP^+$ reductase.

Light-Independent Reactions (Calvin Cycle)

The light-independent reactions occur in the stroma (the fluid surrounding thylakoids). They're sometimes called "dark reactions," but that's misleading since they don't require darkness; they simply don't directly need light. They do require the ATP and NADPH produced by the light reactions.

The Calvin cycle fixes $CO_2$ into organic molecules in three phases:

1. Carbon fixation: The enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) catalyzes the addition of $CO_2$ to ribulose-1,5-bisphosphate (RuBP), a 5-carbon molecule. This produces two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction: ATP and NADPH from the light reactions convert 3-PGA into glyceraldehyde-3-phosphate (G3P). G3P is a 3-carbon sugar that serves as the building block for glucose and other carbohydrates.
3. Regeneration: Most of the G3P molecules are used to regenerate RuBP so the cycle can continue. For every three turns of the cycle (fixing 3 molecules of $CO_2$), one net G3P molecule exits the cycle.

The G3P molecules that exit the cycle are combined through condensation reactions to form glucose ($C_6H_{12}O_6$) and other carbohydrates like starch and cellulose. The overall equation for oxygenic photosynthesis is:

$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$

Cyclic vs. Noncyclic Photophosphorylation

These are two different modes of the light-dependent reactions, and they produce different outputs.

Noncyclic photophosphorylation involves both PSII and PSI:

• Electrons flow in a linear path: $H_2O \rightarrow PSII \rightarrow cytochrome\ b_6f \rightarrow PSI \rightarrow NADP^+$
• Produces both ATP and NADPH
• $O_2$ is released from water splitting at PSII
• This is the "default" pathway that generates the NADPH needed for the Calvin cycle

Cyclic photophosphorylation involves only PSI:

• Electrons excited at PSI pass to ferredoxin but then cycle back to the cytochrome $b_6f$ complex and return to PSI
• Generates ATP only (no NADPH is produced because electrons never reach $NADP^+$)
• No water is split, so no $O_2$ is released
• This pathway is used when the cell needs extra ATP relative to NADPH, such as to power the Calvin cycle's regeneration phase

Electron Transport and Energy Conversion

The photosynthetic electron transport chain is a series of protein complexes embedded in the thylakoid membrane. Its function is to transfer electrons from water to $NADP^+$ while converting light energy into a usable proton motive force.

• PSII absorbs light and extracts electrons from water.
• Electrons move through plastoquinone to the cytochrome $b_6f$ complex, which pumps $H^+$ into the thylakoid lumen.
• Plastocyanin carries electrons from cytochrome $b_6f$ to PSI.
• PSI re-energizes the electrons with a second photon, and ferredoxin passes them to ferredoxin-$NADP^+$ reductase, which produces NADPH.
• The resulting proton gradient (high $H^+$ in the lumen, low in the stroma) drives ATP synthase, coupling electron transport to ATP production.

This is conceptually similar to the electron transport chain in aerobic respiration, but the energy input is light rather than chemical oxidation, and the final electron acceptor is $NADP^+$ rather than $O_2$.