Biology Chapter 7

Light and Dark Reactions in Photosynthesis: Nature's Energy Factory

Introduction to Photosynthesis

• Complex process converting light energy into chemical energy.

• Takes place in chloroplasts of plant cells.

• Produces glucose and oxygen from carbon dioxide and water.

• Two main stages: Light-dependent (Light Reactions) and Light-independent (Dark Reactions/Calvin Cycle).

The Chloroplast Structure

• Double membrane organelle containing thylakoids and stroma.

• Thylakoids: membrane sacs stacked in grana, housing chlorophyll and other pigments.

• Stroma: fluid-filled space surrounding thylakoids.

Light-Dependent Reactions (Light Reactions)

• Occur in thylakoid membranes, require direct sunlight.

• Convert light energy to chemical energy (ATP and NADPH).

• Split water molecules (photolysis) to release oxygen gas.

  • Photosystems: Two types (PSI and PSII) with hundreds of pigment molecules.

• Main Points:

  • Linear electron transport chain in membranes, creating a proton gradient (H+).

  • Produces ATP through chemiosmosis and generates NADPH.

Photolysis and Oxygen Production

• Light excites electrons in PSII, which move along an electron transport chain, causing H+ ion creation.

• Splits H2O to replace lost electrons, releasing oxygen.

ATP Synthase and Chemiosmosis

• H+ gradient moves through ATP Synthase to produce ATP.

• Electrons in PS I help turn NADP to NADPH.

Dark Reactions: The Calvin Cycle

• Occurs in stroma, does not require sunlight.

• Uses ATP and NADPH from light reactions to fix carbon dioxide into glucose.

• Three main stages:

  1. Carbon fixation: CO2 combines with RuBP, catalyzed by RuBisCO.

  2. Reduction Phase: ATP and NADPH convert PGA to G3P (some G3P exits for glucose synthesis).

  3. Regeneration of RuBP: Uses ATP to convert some G3P back to RuBP.

Products and Their Uses

• Glucose: Primary energy source.

• Oxygen: Released to atmosphere.

• ATP: Energy currency.

• NADPH: Electron carrier.

Environmental Factors Influencing Photosynthesis

• Light intensity, temperature, CO2 concentration, water availability, and mineral nutrients.

Types of Photosynthesis in Plants

• C3 Plants: Most common (e.g., wheat, rice); direct use of rubisco; less efficient in heat.

• C4 Plants: Adapted to hot climates (e.g., corn, sugarcane); uses PEP carboxylase; more efficient water use.

• CAM Plants: Adapted to arid environments (e.g., cacti, pineapples); opens stomata at night to conserve water, stores CO₂ for daytime use.

Water’s Role in Photosynthesis

Water is essential to photosynthesis because it provides electrons, protons, and oxygen. Its contributions include:

1. Electron Source: Water molecules undergo photolysis (splitting by light) in the thylakoid membrane of chloroplasts, producing electrons that replenish those lost by chlorophyll in Photosystem II (PSII).

   • Where it goes: These electrons move through the electron transport chain (ETC) to Photosystem I (PSI), helping to generate ATP and NADPH.

2. Proton Supply: Water provides protons (H⁺) that contribute to a proton gradient in the thylakoid lumen, driving ATP synthesis.

   • Where it goes: Protons are pumped into the lumen and later pass through ATP synthase, generating ATP.

3. Oxygen Release: Oxygen is a byproduct of water splitting, diffusing out of the chloroplast.

   • Where it goes: It exits through the stomata and is used by organisms for cellular respiration.

Light Reactions & Calvin Cycle Interconnection

1. Light Reactions (Occur in the Thylakoid Membrane)

   • Light energy excites chlorophyll in PSII, starting electron transport.

   • Electrons pass through the ETC, powering proton pumping into the lumen.

   • ATP is formed via chemiosmosis, and NADP⁺ is reduced to NADPH.

2. Calvin Cycle (Occurs in the Stroma)

   • Uses ATP and NADPH from light reactions to fix CO₂ into organic molecules.

   • Produces G3P, which forms glucose and other carbohydrates.

   • Regenerates RuBP to sustain the cycle.

Link Between the Two: Light reactions supply ATP and NADPH to fuel the Calvin cycle, which in turn regenerates ADP and NADP⁺, feeding them back into the light reactions.

Photosynthesis & Cellular Respiration as a Cycle

Using the equations:

Photosynthesis:
6CO2+6H2O+Light→C6H12O6+6O26CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂6CO2​+6H2​O+Light→C6​H12​O6​+6O2​

Cellular Respiration (Aerobic):
C6H12O6+6O2→6CO2+6H2O+ATPC₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATPC6​H12​O6​+6O2​→6CO2​+6H2​O+ATP

• Photosynthesis stores energy in glucose, while cellular respiration releases it.

• Oxygen from photosynthesis is used in respiration, while CO₂ and water from respiration return for photosynthesis.

• ATP produced in respiration fuels metabolic activities, indirectly supporting plant growth and continued photosynthesis.

Electromagnetic Spectrum & Photosynthesis

The electromagnetic spectrum consists of different wavelengths of light, where shorter wavelengths (e.g., gamma rays, X-rays) have more energy than longer ones (e.g., radio waves).

• Visible light (400-700 nm) is used in photosynthesis, with blue (450 nm) and red (680 nm) light being most effective.

• Humans use other parts:

  • Infrared: Thermal imaging

  • Ultraviolet: Sterilization, tanning

  • Microwaves: Cooking

  • X-rays: Medical imaging

ATP Formation in Light Reactions vs. Aerobic Respiration