Photosynthesis: Metabolic Pathways and Light Reactions (Chapter 10.1–10.5)

Topic 9: Metabolic Pathways – Photosynthesis

Overview

Photosynthesis is a fundamental metabolic pathway that sustains life on Earth by converting light energy into chemical energy. This process occurs in photoautotrophic organisms, primarily plants, algae, and some bacteria, and is essential for the biosphere's energy flow and carbon cycling.

10.1 Photosynthesis Feeds the Biosphere

Role of Photosynthesis

• Photosynthesis captures solar energy and converts it into reduced organic molecules (e.g., glucose) from CO2 and H2O.

• Only about 1% of sunlight is captured by photosynthetic organisms.

• Photosynthesis is an anabolic and endergonic process (requires energy input).

• Photoautotrophs (e.g., plants, algae) produce organic matter and oxygen, supporting heterotrophs (consumers).

• Phytoplankton contribute 50–80% of global O2 production.

10.2 Photosynthesis Converts Light Energy to Chemical Energy

Overall Reaction

• Photosynthesis involves the reduction of CO2 and oxidation of H2O.

$\text{Energy} + 6\text{CO}_2 + 6\text{H}_2O \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• CO2 is reduced to form glucose.

• H2O is oxidized to form O2.

Stages of Photosynthesis

• Light Reactions (in thylakoid membranes): Capture light energy, generate ATP and NADPH, and release O2.

• Calvin Cycle (in stroma): Uses ATP and NADPH to fix CO2 into sugars.

Chloroplast Structure

• Surrounded by a double membrane (endosymbiont origin).

• CO2 enters via stomata; water absorbed by roots.

• Internal fluid: stroma (site of Calvin cycle).

• Third membrane: thylakoids (site of light reactions), stacked into grana.

• Thylakoid space: site of proton gradient formation.

10.3 The Nature of Sunlight

Electromagnetic Spectrum

• Visible light is a small part of the spectrum (approx. 380–750 nm).

• Light behaves as both a wave and a particle (photon).

• Shorter wavelengths (e.g., blue) have higher energy; longer wavelengths (e.g., red) have lower energy.

Photon Interactions

• Photons can be reflected, transmitted, or absorbed by matter.

• Absorption depends on the energy of the photon and the properties of the molecule.

10.3 Spectrophotometers and Pigment Absorption

Measuring Absorbance

• Pigments absorb specific wavelengths; unabsorbed light is reflected (e.g., green leaves reflect green light).

• Different pigments absorb different wavelengths, allowing efficient use of sunlight.

Absorption and Action Spectra

• Absorption spectrum: Graph of pigment light absorption vs. wavelength.

• Action spectrum: Shows photosynthetic activity at different wavelengths.

• Chlorophyll a and b have slightly different absorption spectra; carotenoids are accessory pigments.

Pigment	Main Absorption Peaks (nm)	Role
Chlorophyll a	~430, ~662	Main photosynthetic pigment
Chlorophyll b	~453, ~642	Accessory pigment, broadens absorption
Carotenoids	~400–500	Photoprotection, accessory pigment

10.3 Pigments as Light Receptors

Photon Absorption and Energy Transfer

• Absorption of a photon excites an electron to a higher energy state.

• Excited electrons can:

  • Dissipate energy as heat

  • Emit lower-energy photons (fluorescence)

  • Transfer energy to another molecule (energy transfer, not electron transfer)

• Photosynthetic pigments are arranged to maximize energy transfer within the chloroplast.

10.3 Chlorophylls: The Plant Pigments

Structure and Function

• Chlorophyll a: Contains a porphyrin ring with a central Mg2+ atom and a hydrophobic tail.

• Multiple pigments allow absorption of a broader range of wavelengths.

• Chlorophylls are embedded in thylakoid membranes.

10.3 Pigments Organized into Photosystems

Photosystem Structure

• Photosystems are complexes of reaction centers surrounded by light-harvesting complexes.

• Energy from photons is transferred among pigment molecules to the reaction center.

• Special chlorophyll molecules (P680 in PSII, P700 in PSI) are located in the reaction center.

• Electrons are passed to a primary electron acceptor upon excitation.

10.3 The Linear Flow of Electrons in Light Reactions

Stepwise Electron Flow

1. Photosystem II (PSII) absorbs light; energy is transferred to P680, which ejects an electron.

2. The primary electron acceptor captures the electron and shuttles it to the electron transport chain (ETC).

3. P680+ (oxidized) is reduced by electrons from water, producing O2 and H+ ions (oxygen evolution).

4. Electrons move through the ETC (plastoquinone, cytochrome complex, plastocyanin), creating a proton gradient in the thylakoid lumen.

5. Proton motive force drives ATP synthesis via ATP synthase (chemiosmosis).

6. Electrons reach Photosystem I (PSI), are re-energized by light, and transferred to P700.

7. Photoexcited electrons are passed to ferredoxin (Fd) and then to NADP+ reductase, forming NADPH.

8. Both photosystems are required to energize electrons sufficiently to reduce NADP+ to NADPH.

Component	Function
Photosystem II (PSII)	Initial light absorption, water splitting, O2 release
Electron Transport Chain	Transfers electrons, builds proton gradient
Photosystem I (PSI)	Re-energizes electrons, reduces NADP+ to NADPH
ATP Synthase	Uses proton gradient to synthesize ATP

Key Equations

$\text{Light Reactions:}\quad 2\text{H}_2\text{O} + 2\text{NADP}^+ + 3\text{ADP} + 3\text{P}_i + 2\text{H}^+ + \text{light} \rightarrow 2\text{NADPH} + 2\text{H}^+ + 3\text{ATP} + \text{O}_2$

Summary of Outputs

• ATP and NADPH are produced in the light reactions and used in the Calvin cycle.

• O2 is released as a byproduct.

Additional info:

• The Calvin cycle consists of three phases: carbon fixation, reduction, and regeneration of RuBP.

• ATP and NADPH produced in the light reactions are consumed in the Calvin cycle to synthesize glucose.