Chapter 10 — Photosynthesis (BSC2010c)

Autotrophs vs. Heterotrophs

Autotrophs- Synthesize their own organic molecules; “producers,” base of food webs.
Heterotrophs- Depend on external organic molecules; “consumers,” derive energy and carbon from ingested food.

Chloroplasts & Leaf Anatomy

Chloroplasts- Endosymbiotic origin; cellular sites of photosynthesis.
Leaf Structure- Mesophyll (main site); Stomata (gas exchange: $CO<em>2$ in, $O</em>2$ & $H_2O$ out).
Chloroplast Ultrastructure- Stroma (aqueous matrix); Thylakoids (membranous sacs, stacked into grana), containing chlorophyll.

Overall Photosynthetic Equation & Relationship to Respiration

Balanced summary: $6CO<em>2 + 12H</em>2O + \text{light energy} \;\longrightarrow\; C<em>6H</em>{12}O<em>6 + 6O</em>2 + 6H_2O$
Conceptual mirror image of aerobic respiration, but with distinct pathways.

Water-Splitting Discovery

Released $O<em>2$ originates from water ( $H</em>2O$ ), not $CO<em>2$ . $2H</em>2O \;\xrightarrow{\text{photolysis}}\; 4H^+ + 4e^- + O_2$

Photosynthesis as a Redox Process

Electrons are boosted by photons from water to $CO_2$ to generate sugar; represents reverse electron flow relative to respiration.

Bifurcation into Two Major Stages

Light Reactions ("photo" stage)- Occur in thylakoids; convert solar energy to chemical energy (ATP & NADPH).
Calvin Cycle ("synthesis" or dark reactions)- Occurs in stroma; incorporates $CO_2$ into carbohydrate (G3P) using ATP & NADPH.

The Nature of Light & Pigments

Light behaves as waves and photons; visible light ( $\sim380\text{–}750\,\text{nm}$ ) powers photosynthesis.
Pigments- Chlorophyll a (primary, absorbs violet-blue & red);
- Chlorophyll b (accessory);
- Carotenoids (photoprotective & light-harvesting).

Photoexcitation & Energy Transfer

Photon absorption elevates an electron to an excited state; energy is transferred among pigments towards the reaction center.

Photosystems — Molecular Antennas

Contain Light-harvesting complexes (LHCs) and a Reaction-center complex (chlorophyll a + primary electron acceptor).
Photosystem II (PS II): reaction-center chlorophyll $P_{680}$ .
Photosystem I (PS I): reaction-center chlorophyll $P_{700}$ .

Linear Electron Flow (Non-cyclic)

Steps: Photon excites PS II -> electron captured by primary acceptor -> water-splitting replenishes $P<em>{680}^{+}$ (releasing $O</em>2$ ) -> electrons move via ETC (pumping $H^+$ ) -> excite PS I -> electrons reduce $NADP^+$ to $NADPH$ .
Result: ATP (via chemiosmosis) + NADPH + $O_2$ .

Cyclic Electron Flow (CEF)

Involves PS I only; produces additional ATP (via proton pumping) but no NADPH or $O_2$ .

Chemiosmosis: Chloroplast vs. Mitochondrion

Shared Principle: ETC-driven proton-motive force (pmf) coupled by ATP synthase to ATP synthesis.
Differences in proton reservoir (thylakoid lumen vs. intermembrane space) and energy source (light vs. organic oxidation).

The Calvin Cycle — $CO_2$ Reduction to Carbohydrate

Consumes three $CO_2$ molecules per net G3P produced.
Phase 1 (CO 2 Fixation)- Rubisco catalyzes $CO</em>2$ + RuBP -> 2 x 3-PGA.
Phase 2 (Reduction)- 3-PGA phosphorylated by ATP and reduced by NADPH to G3P; one G3P exits.
Phase 3 (Regeneration of RuBP)- Remaining G3P + ATP rearrange to regenerate RuBP.

Integration & Significance

Light Reactions supply ATP & NADPH to Calvin cycle; cycle returns ADP, Pi, $NADP^+$ to thylakoids.
Converts $\approx 10^{11}$ tons of $CO_2$ annually into organic matter, storing solar energy.
Plants perform both photosynthesis (chloroplast) and respiration (mitochondria).

Practical, Ethical, & Philosophical Perspectives

Drives agricultural engineering, climate change mitigation, and sustainable energy inspiration (artificial photosynthesis).

Key Numerical, Statistical & Formulaic References

$6CO<em>2 + 12H</em>2O + h\nu \;\longrightarrow\; C<em>6H</em>{12}O<em>6 + 6O</em>2 + 6H_2O$
Calvin cycle stoichiometry per G3P: $9\;ATP + 6\;NADPH + 3\;CO<em>2 \;\longrightarrow\; G3P + 9\;ADP + 8\;P</em>i + 6\;NADP^+ + 3\;H_2O$
Visible spectrum: $\approx 380\text{–}750\,\text{nm}$ .