Photosynthesis: Metabolic Pathways and Light Reactions (General Biology Study Notes)

Topic 9: Metabolic Pathways – Photosynthesis

Overview

Photosynthesis is a fundamental metabolic pathway in photoautotrophic organisms, converting light energy into chemical energy. This process sustains the biosphere by providing organic molecules and oxygen, and is covered in Chapter 10 (Concepts 10.1–10.5).

10.1 Photosynthesis Feeds the Biosphere

Role of Photosynthesis

• Photosynthesis builds reduced organic molecules from CO2 and H2O.

• It is an anabolic (building) and endergonic (energy-requiring) process.

• Performed by photoautotrophs (e.g., plants, algae, cyanobacteria), which are producers in the biosphere.

• Heterotrophs (consumers) depend on photoautotrophs for food and oxygen.

• Approximately 50–80% of global O2 comes from phytoplankton.

10.2 Photosynthesis Converts Light Energy to Chemical Energy

Overall Chemical Equation

The process of photosynthesis can be summarized by the following equation:

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

• 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 theory).

• CO2 enters via stomata; water via roots.

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

• Thylakoids: Membranous sacs stacked into grana; site of light reactions.

• Thylakoid membrane contains chlorophyll and other pigments.

10.3 The Nature of Sunlight

Electromagnetic Spectrum

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

• Shorter wavelengths have higher energy; longer wavelengths have lower energy.

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

Photon Interactions

• When a photon hits matter, it can be reflected, transmitted, or absorbed.

• Absorption depends on the energy of the photon and the molecule it interacts with.

10.3 Spectrophotometers and Pigment Absorption

Measuring Absorbance

• Spectrophotometers measure how much light is absorbed by pigments at different wavelengths.

• If light is not absorbed, it is reflected (e.g., leaves appear green because they reflect green light).

• Different pigments absorb different wavelengths.

Absorption and Action Spectra

• Absorption spectrum: Graph of pigment’s light absorption versus wavelength.

• Chlorophyll a and chlorophyll b have slightly different absorption spectra, maximizing light capture.

• Carotenoids: Accessory pigments aiding in photoprotection.

• Action spectrum: Shows photosynthetic activity at different wavelengths.

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.

  • Re-emit energy as a lower-energy photon (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: Main pigment; absorbs blue-violet and red light.

• Chlorophyll b: Accessory pigment; broadens absorption spectrum.

• Structure: Porphyrin ring (light-absorbing), hydrocarbon tail (anchors in membrane).

• Multiple pigments allow for greater energy absorption from sunlight.

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) absorb light best and pass electrons to the primary electron acceptor.

10.3 The Linear Flow of Electrons in Light Reactions

Stepwise Electron Flow

1. Photosystem II (PSII) comes first. Light is captured and transferred to P680, which absorbs energy and ejects electrons.

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

3. P680+ (oxidized) is a strong oxidant and regains electrons by splitting water, releasing O2 and H+ ions (oxygen evolution).

4. Electrons move through the ETC, with carriers like plastoquinone (Pq), cytochrome complex, and plastocyanin (Pc).

5. Protons build up in the thylakoid lumen, creating a proton motive force used to make ATP via ATP synthase (chemiosmosis).

6. Electrons are passed to Photosystem I (PSI), where they are re-energized by light absorbed by P700.

7. Photoexcited electrons are transferred to a second ETC, ending with the protein ferredoxin (Fd) and NADP+ reductase, forming NADPH.

8. Both photosystems are required to fully energize electrons for NADPH production.

Summary Table: Key Components of Light Reactions

Component	Function
Photosystem II (PSII)	Absorbs light, splits water, initiates electron flow
Electron Transport Chain (ETC)	Transfers electrons, pumps protons, creates proton gradient
ATP Synthase	Uses proton gradient to synthesize ATP
Photosystem I (PSI)	Re-energizes electrons, transfers to NADP+
NADP+ Reductase	Forms NADPH from NADP+ and electrons

Additional info:

• The Calvin cycle uses ATP and NADPH from the light reactions to fix CO2 into sugars.

• ATP and NADPH are recycled; the Calvin cycle can occur in the dark as long as these molecules are available.

• Photosynthetic efficiency is maximized by the arrangement of pigments and the structure of the chloroplast.