Photosynthesis

Introduction

Photosynthesis is the vital process used by plants, algae, and cyanobacteria to convert light energy into chemical energy, primarily in the form of glucose (sugar). This conversion uses carbon dioxide and water, releasing oxygen as a byproduct. Photosynthesis is the ultimate source of metabolic energy for most life on Earth, as the sugars produced provide fuel for plants and are a food source for animals.

Types of Photosynthesis

• Light-Dependent Reactions

• Light-Independent Reactions

Light-Dependent Reactions

Overview

The light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts, where chlorophyll captures light energy to split water, releasing oxygen, and producing ATP and NADPH using photosystems II and I, the electron transport chain, and ATP synthase.

Concepts

• Occur on thylakoid membranes; products (ATP, NADPH) are used in the Calvin cycle in the stroma.

• Use Photosystem II (PSII) and Photosystem I (PSI) arranged in a Z-scheme of electron flow.

• Photolysis of water supplies electrons and releases O2.

• Electron transport drives H+ pumping → proton gradient → ATP synthase makes ATP (photophosphorylation).

• NADP+ is the final electron acceptor → NADPH (reducing power).

• Cyclic flow around PSI can add extra ATP when needed (no NADPH/O2 made).

Key Events

1. Light absorption by chlorophyll in PSII and PSI.

2. Water splitting (photolysis) at PSII → electrons + H+ + O2.

3. Electron transport chain (ETC) shuttles electrons PSII → PSI.

4. H+ gradient formation across thylakoid membrane.

5. ATP synthesis via ATP synthase (chemiosmosis).

6. NADPH formation when PSI-excited electrons reduce NADP+.

Detailed Steps

1. Oxygen-evolving complex splits H2O; lost PSII electrons are replaced; O2 released to air; H+ added to lumen.

2. Electron travels via plastoquinone (PQ) → cytochrome b6f → plastocyanin (PC); pumps H+ into the lumen.

3. Photon hits PSI (P700) → PSI electron excited to its primary acceptor.

4. PSI electron passes to ferredoxin (Fd).

5. Ferredoxin-NADP+ reductase (FNR) reduces NADP+ → NADPH (in stroma).

6. Proton gradient (from photolysis + b6f pumping) drives ATP synthase → ATP formed in stroma.

7. (Optional balance) If extra ATP is needed, electrons can cycle from Fd → b6f → PSI, boosting ATP only (cyclic photophosphorylation).

Terms Used in Light-Dependent Reaction

• ATP synthase

• Chemiosmosis

• Chlorophyll

• Cyclic photophosphorylation

• Cytochrome b6f complex

• Electron Transport Chain (ETC)

• Ferredoxin (Fd)

• NADP+ / NADPH

• NADP+ reductase (FNR)

• Non-cyclic photophosphorylation

• Oxygen (O2)

• Photosystem

• Photon

• Photosystem I (PSI / P700)

• Photosystem II (PSII / P680)

• Plastocyanin (PC)

• Plastoquinone (PQ)

• Proton gradient (H+ gradient)

• Thylakoid membrane

Non-cyclic Photophosphorylation

In non-cyclic photophosphorylation, electrons flow in a straight line from water through PSII and PSI to NADP+, producing ATP, NADPH, and O2.

• Pathway: Electrons flow: water → PSII → PSI → NADP+

• Reactants: Water, NADP+

• Products: ATP, NADPH, and O2 (from water splitting)

• Purpose: Provides both energy (ATP) and reducing power (NADPH) for the Calvin cycle.

• Main feature: Involves both Photosystem II (P680) and Photosystem I (P700).

NOTE: The special pair of photosystem I is called P700, while the special pair of photosystem II is called P680.

Cyclic Photophosphorylation

In cyclic photophosphorylation, electrons cycle around PSI, producing ATP only (no NADPH or O2).

• Reactants: None (uses recycled electrons)

• Products: ATP only (no NADPH, no O2)

• Purpose: Provides extra ATP when the Calvin cycle needs more ATP than NADPH.

• Main feature: Involves only Photosystem I (PSI).

NOTE: The special pair of photosystem I is called P700, while the special pair of photosystem II is called P680.

Why Cyclic Reaction Involves Only Photosystem I (PSI)

1. Main function of cyclic electron flow: The goal is to make extra ATP when the cell needs more energy but no additional NADPH or oxygen.

2. Why only PSI is involved:

   • In the non-cyclic pathway (Z-scheme), both PSI and PSII are used:

     • PSII splits water → releases O2, electrons, and H+.

     • PSI excites electrons again → helps form NADPH.

   • In the cyclic pathway, electrons from PSI do not move forward to make NADPH; instead, they are sent back to the electron transport chain, recycling and creating a proton gradient (H+ build-up) → drives ATP synthase → makes ATP only.

3. Why PSII is skipped: PSII's job is mainly to split water and provide new electrons.

Light-Independent Reactions (Calvin Cycle)

Overview

The light-independent reactions (Calvin cycle) do not require light directly, but depend on products of the light-dependent reactions to fix CO2 into glucose (sugars).

Concepts

• Occur in the stroma of chloroplasts.

• Use CO2 as a carbon source.

• Rely on ATP (energy) and NADPH (reducing power).

• Main enzyme: RuBisCO (catalyzes carbon fixation).

• Cycle regenerates its starting molecule (RuBP) to keep running.

Key Events

1. Carbon fixation – CO2 is attached to RuBP by RuBisCO.

2. Reduction – ATP and NADPH are used to reduce 3-PGA into G3P (a sugar).

3. Regeneration of RuBP – Some G3P is recycled to regenerate RuBP.

4. Carbohydrate formation – Remaining G3P exits the cycle to eventually form glucose and other carbohydrates.

Step-by-Step Events in the Calvin Cycle (with 6 CO2 as reactants)

1. Carbon Fixation

   • 6 CO2 molecules combine with 6 RuBP (5-carbon each).

   • Catalyzed by RuBisCO, producing 12 molecules of 3-PGA (3-carbon each).

2. Reduction

   • 12 ATP are used to phosphorylate the 12 molecules of 3-PGA → 12 molecules of 1,3-bisphosphoglycerate (1,3-BPG).

   • 12 NADPH reduce them → 12 molecules of G3P (glyceraldehyde-3-phosphate).

3. Carbohydrate Output

   • Out of the 12 G3P formed, 2 G3P exit the cycle → used to form glucose and other sugars.

4. Regeneration of RuBP

   • The remaining 10 G3P + 6 ATP regenerate 6 RuBP (cycle restarts).

Summary per Turn

• Reactants: 6 CO2 + 18 ATP + 12 NADPH

• Products: 1 glucose (from 2 G3P) + 18 ADP + 12 NADP+ + Pi

Terms Used in Light-Independent Reaction

• Reduction phase

• Carbohydrate synthesis

• RuBisCO (Ribulose bisphosphate carboxylase/oxygenase)

• Regeneration phase

• G3P (glyceraldehyde-3-phosphate)

• RuBP (ribulose-1,5-bisphosphate)

Key Equations

• Overall photosynthesis equation:

• Calvin cycle summary:

Comparison Table: Non-cyclic vs. Cyclic Photophosphorylation

Example Application

Photosynthesis is essential for life on Earth, providing oxygen for respiration and organic molecules for energy. The Calvin cycle is the pathway by which plants fix atmospheric CO2 into sugars, which are then used by plants and animals for growth and metabolism.