Photosynthesis: Structure, Function, and Mechanisms

Photosynthesis Overview

Introduction to Photosynthesis

Photosynthesis is the process by which light energy is converted into chemical energy stored in organic molecules, with oxygen produced as a waste product. This process is fundamental to life on Earth, providing the organic molecules and oxygen required for cellular respiration.

• Key Point: Photosynthesis converts light energy into chemical energy in the form of sugars.

• Key Point: The products of photosynthesis (organic molecules and O2) are used in cellular respiration, while CO2 and H2O from respiration are reactants for photosynthesis.

• Overall Equation: $\mathrm{CO_2 + 2H_2O + \text{light energy} \rightarrow CH_2O + O_2 + H_2O}$ The direct product is a three-carbon sugar (CH2O), which can be polymerized into glucose.

Autotrophs vs. Heterotrophs

Classification of Organisms by Energy Source

Organisms are classified based on how they obtain organic molecules and energy.

• Autotrophs: "Self-feeders" that synthesize organic molecules from inorganic sources using light (photoautotrophs) or chemical energy (chemoautotrophs).

• Heterotrophs: Obtain organic compounds by consuming other organisms or their waste.

Group	Energy Source	Examples
Photoautotrophs	Light + CO2 + H2O	Most plants, algae, cyanobacteria
Chemoautotrophs	Inorganic chemical reactions	Certain bacteria
Heterotrophs	Organic compounds	Animals, fungi, most bacteria

Chloroplasts – Sites of Photosynthesis

Structure and Function of Chloroplasts

Chloroplasts are double-membrane organelles where photosynthesis occurs. They evolved from photosynthetic bacteria and contain specialized structures for capturing light energy.

• Outer & Inner Membranes: Enclose the organelle.

• Stroma: Fluid containing enzymes, DNA, and ribosomes.

• Thylakoids: Flattened sacs stacked into grana; host light-harvesting complexes.

• Chlorophyll: Resides in thylakoid membranes, absorbing light.

The Nature of Sunlight

Electromagnetic Radiation and Energy

Light is a form of electromagnetic radiation. Photons are discrete packets of energy, and their energy is inversely related to wavelength.

• Visible Spectrum: 380–750 nm (detected as colors).

• Higher Energy: Shorter wavelength (e.g., violet).

• Lower Energy: Longer wavelength (e.g., red).

Photosynthetic Pigments

Types and Functions of Pigments

Pigments absorb specific light wavelengths; unabsorbed light is reflected or transmitted, giving leaves their green color.

• Chlorophyll a: Primary pigment; peaks in blue (~430 nm) and red (~660 nm).

• Chlorophyll b: Accessory pigment; slightly shifted peaks, broadening absorption.

• Carotenoids: Absorb blue-violet light; protect against excess light.

• Chlorophyll d & f: Found in cyanobacteria; absorb far-red light for low-light habitats.

Pigment	Absorption Peaks (nm)	Role
Chlorophyll a	~430, ~660	Primary energy capture
Chlorophyll b	~453, ~640	Extends range
Carotenoids	~450-500	Photoprotection
Chlorophyll f	~710	Low-light photosynthesis

An Overview of Photosynthesis: Cooperation of the Light Reactions and the Calvin Cycle

Integration of Photosynthetic Processes

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle. These processes cooperate to convert light energy into chemical energy and synthesize sugars.

• Light Reactions: Occur in the thylakoid membranes; produce ATP, NADPH, and O2.

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix CO2 into sugars.

Photosystems – Reaction Centers & Light-Harvesting Complexes

Structure and Function of Photosystems

A photosystem consists of a reaction-center complex surrounded by light-harvesting complexes. There are two types: Photosystem II (PSII) and Photosystem I (PSI).

• Photosystem II (PSII): Reaction-center chlorophyll a = P680 (absorbs 680 nm). First step: water splitting, O2 release, electrons to electron transport chain.

• Photosystem I (PSI): Reaction-center chlorophyll a = P700 (absorbs 700 nm). Receives electrons from PSII; reduces NADP+ to NADPH.

Feature	PSII	PSI
Primary chlorophyll	P680 (680 nm)	P700 (700 nm)
Position in electron flow	First	Second
Main product	O2, electrons to ETC	NADPH

Linear Electron Flow (LEF) – From Light to ATP & NADPH

Mechanism of Electron Transfer

LEF transfers electrons from water to NADP+, generating a proton gradient that drives ATP synthesis.

1. Photon absorption by PSII antenna → energy reaches P680.

2. Excited electron transferred to primary acceptor (P680).

3. Water splitting (photolysis) supplies electrons, releases O2, and adds H+ to lumen.

4. Electron transport chain moves electrons to PSI, pumping H+ into lumen.

5. Proton gradient powers ATP synthase (chemiosmosis) → ATP formed.

6. Photon absorption by PSI antenna → energy reaches P700.

7. Excited electron from P700 transferred to ferredoxin (Fd).

8. NADP+ reductase uses electrons + H+ to reduce NADP+ → NADPH.

Resulting products of the light reactions: O2 (byproduct), ATP, NADPH — used in the Calvin cycle.

Photosynthesis as a Redox Process

Redox Reactions in Photosynthesis

Photosynthesis reverses respiration: water is oxidized (donates electrons) while CO2 is reduced (accepts electrons). Light supplies the energy for this endergonic reaction.

• Oxidation: $\mathrm{H_2O \rightarrow \frac{1}{2}O_2 + 2H^+ + 2e^-}$

• Reduction: $\mathrm{CO_2 + 2H^+ + 2e^- \rightarrow CH_2O}$

Detailed Look at Photosystem II

Steps in Photosystem II Function

Photosystem II (PSII) is the first protein complex in the light-dependent reactions; it captures photons, oxidizes water, and passes high-energy electrons to the primary electron acceptor.

• Light absorption: Photons excite chlorophyll a (P680).

• Charge separation: The excited electron is transferred to the primary electron acceptor.

• Water oxidation: The oxygen-evolving complex splits two H2O molecules, producing O2, 4 H+ (released into the thylakoid lumen), and electrons that replenish P680.

• Electron transport: Electrons travel via plastoquinone (PQ), cytochrome b6f, and plastocyanin toward Photosystem I.

• Proton motive force: The H+ released into the lumen contributes to the electrochemical gradient used by ATP synthase.

Chemiosmosis: Chloroplasts vs. Mitochondria

Comparison of Energy Conversion Mechanisms

Chemiosmosis is the process by which ATP is synthesized using a proton gradient. Both chloroplasts and mitochondria use chemiosmosis, but differ in electron donors, energy sources, and by-products.

Feature	Chloroplast	Mitochondrion
Electron donor	Water (oxidized in PSII)	Organic substrates (e.g., NADH, FADH2 from the TCA cycle)
Energy source	Light (photons)	Chemical (oxidation of metabolites)
Proton pumping direction	From stroma → thylakoid lumen	From matrix → intermembrane space
ATP synthase orientation	Protons re-enter stroma → ATP synthesis	Protons re-enter matrix → ATP synthesis
By-product of electron donor	O2 (released to atmosphere)	CO2 (released to atmosphere)

Summary

• Chloroplasts: Use light energy to drive electron flow from water, producing ATP and O2.

• Mitochondria: Use chemical energy from organic molecules to drive electron flow, producing ATP and CO2.

Example:

In plant cells, chloroplasts perform photosynthesis during the day, while mitochondria carry out cellular respiration continuously.

Additional info: The notes above cover the foundational concepts of photosynthesis, including its overall process, the role of chloroplasts, the nature of sunlight, pigment function, the cooperation of light reactions and the Calvin cycle, and the mechanisms of electron flow and chemiosmosis.