Understanding Photosynthesis

• Photosynthesis is the process where plants, algae, and some bacteria convert light energy from the sun into glucose (C₆H₁₂O₆)
• This happens mainly in the chloroplasts of plant cells and is key to sustaining life on Earth

Photosynthesis Equation

• The overall photosynthesis reaction is: 6 CO₂ + 12 H₂O + sunlight → C₆H₁₂O₆ + 6 H₂O + 6 O₂
• CO₂ (carbon dioxide) is absorbed from the atmosphere
• H₂O (water) is absorbed from the soil
• C₆H₁₂O₆ (glucose) is the main product, providing energy for plant growth
• O₂ (oxygen) is released as a byproduct into the atmosphere

Photosynthesis as Redox Reaction

• Photosynthesis is a redox (reduction-oxidation) reaction
• This involves both reduction and oxidation of molecules

Redox Reaction Definition

• Oxidation occurs when a molecule loses electrons
• Reduction occurs when a molecule gains electrons
• The reaction formula is: 6 CO₂ + 12 H₂O + sunlight → C₆H₁₂O₆ + 6 H₂O + 6 O₂

Oxidation

• Water (H₂O) is oxidized to produce oxygen (O₂)
• During the light-dependent reaction, water molecules are split (photolysis)
• This releases electrons (e⁻), protons (H⁺), and oxygen (O₂)
• Electrons lost from water are transferred through the electron transport chain (ETC)

Reduction

• Carbon dioxide (CO₂) is reduced to form glucose (C₆H₁₂O₆)
• This happens in the Calvin cycle (light-independent reaction)
• NADPH, donates high-energy electrons to help convert CO₂ into glucose

Importance of Photosynthesis

• Energy Production: Converts solar energy into chemical energy (glucose) for plant growth
• Oxygen Production: Releases oxygen as a byproduct that is essential for respiration in animals and humans
• Reduces Carbon Dioxide: Balances atmospheric CO₂ levels and reduces global warming
• Foundation of the Food Chain: Provides energy to plants, serving as food for herbivores and other organisms

Three Major Events of Photosynthesis

• Light Absorption: Chlorophyll in the thylakoid membrane absorbs sunlight, exciting electrons and splitting water molecules into oxygen, protons, and electrons
• ATP and NADPH Formation: Excited electrons pass through the electron transport chain, leading to the production of ATP and NADPH, which store energy for the Calvin cycle
• Glucose Formation (Calvin Cycle): In the stroma, ATP and NADPH are used to convert carbon dioxide into glucose through enzyme-controlled reactions

Chloroplast

• The chloroplast is the organelle responsible for photosynthesis in plants and algae
• Photosynthesis happens in the chloroplasts, specifically in the mesophyll cells of plant leaves

Chlorophyll

• Chlorophyll inside chloroplasts absorbs sunlight
• It absorbs light energy from the sun, especially in the red and blue wavelengths and reflects green light
• Without chlorophyll, plants wouldn't be able to produce glucose (food) and oxygen
• Leaves turn yellow, orange, and red in autumn because chlorophyll breaks down, revealing other pigments like carotenoids and anthocyanins

Photosystems

• A photosystem is a protein-pigment complex embedded in the thylakoid membrane
• A light-harvesting complex (antenna complex) contains pigments like chlorophyll a, chlorophyll b, and carotenoids that absorb light energy
• A reaction center (chlorophyll a molecule) transfers energy and excites electrons
• An electron acceptor molecule (Primary Electron Acceptor) accepts high-energy electrons from the reaction center

Two Types of Photosystems

• Photosystem II (PSII, P680)
• Photosystem I (PSI, P700)
• The numbers 680 and 700 represent the wavelength of light (in nanometers) that each photosystem absorbs best

Light Dependent Reaction

• Location: Thylakoid membrane
• Main Purpose: Convert light energy into ATP and NADPH, which are used in the Calvin cycle
• Reactants: Light, Water (H₂O), NADP⁺, ADP + Pi
• Products: ATP, NADPH, Oxygen (O₂)

Steps

• 1st Photoexcitation: Chlorophyll pigments in PSII absorb light energy from the sun, exciting electrons to jump from ground state

• Photolysis (Splitting of Water): Water molecules (H₂O) are split to replace electrons lost in Photosystem II, releasing oxygen (O₂) and protons (H⁺)

  Reaction: 2 H₂O → 4 H⁺ + 4 e⁻ + O₂

• Electron Transport Chain (ETC) with Diffusion: Excited electrons pass through the ETC from PSII to PSI; protons (H⁺) are pumped into the thylakoid lumen, creating a proton gradient (diffusion)

• ATP Synthesis (Chemiosmosis): Protons (H⁺) flow back into the stroma through the enzyme ATP synthase, producing ATP from ADP + Pi

• 2nd Photoexcitation and NADP⁺ Reduction: In PSI, light excites electrons and are passed to NADP⁺ Reductase, combining them with NADP⁺ and H⁺ to form NADPH

Light Independent Reaction (Calvin Cycle)

• Location: Stroma of the chloroplast
• Main Purpose: Convert carbon dioxide (CO2) into glucose (C6H12O6) using energy from ATP and NADPH
• Reactants: Carbon dioxide (CO2), ATP, and NADPH
• Products: Glucose (C6H12O6), ADP, NADP+, and Inorganic phosphate (Pi)

Steps

• Carbon Fixation: 3 CO2 molecules enter the stroma; Rubisco fixes 3 CO2 with ribulose-1,5-bisphosphate (RuBP), forming an unstable 6-carbon compound that breaks down into two molecules of 3-phosphoglycerate (3-PGA)
• Reduction Phase: ATP and NADPH convert 3-PGA molecules into a higher energy form, producing glyceraldehyde-3-phosphate (G3P), a three-carbon sugar phosphate
• Phosphorylation of 3-PGA: Each 3-PGA is phosphorylated by one ATP, forming 1,3-bisphosphoglycerate (1,3-BPG); six 3-PGA molecules require 6 ATP molecules; the enzyme phosphoglycerate kinase catalyzes the reaction by converting ATP to ADP
• Reduction of 1,3-BPG to G3P: NADPH donates electrons and a hydrogen ion (H+), reducing 1,3-BPG to glyceraldehyde-3-phosphate (G3P); six 1,3-BPG molecules require 6 NADPH molecules where the enzyme glyceraldehyde-3-phosphate dehydrogenase catalyzes this step by converting NADPH to NADP+
• Formation of Glucose and Other Sugars: G3P can be processed into glucose and other carbohydrates for energy storage and growth; out of 6 G3P molecules, only 1 G3P exits the cycle, requiring the cycle to run twice to make one glucose molecule (C₆H₁₂O₆); the remaining 5 G3P regenerate RuBP
• Regeneration of RuBP: Five G3P molecules regenerate 3 RuBP at the cost of 3 ATP molecules and is crucial because it restarts the cycle by binding with more CO2

C4 vs CAM Pathway in Plants

• Both C4 and CAM plants evolved to reduce photorespiration and conserve water in hot and dry environments

• C4 Pathway: Plants capture CO₂ in mesophyll cells using PEP carboxylase, forming a 4-carbon compound (oxaloacetate), then CO₂ is released for the Calvin Cycle in bundle sheath cells
• CAM Pathway: Plants open stomata at night to capture CO₂ and store it as a 4-carbon compound (malate) in vacuoles, then during the day, CO₂ is released for the Calvin Cycle