⚗️Biological Chemistry II Unit 9 – Photosynthesis & Carbon Fixation

Key Concepts and Overview

• Photosynthesis converts light energy into chemical energy stored in glucose or other sugars
• Occurs in plants, algae, and some bacteria (cyanobacteria)
• Consists of light-dependent reactions and light-independent reactions (Calvin cycle)
• Light-dependent reactions capture energy from sunlight to produce ATP and NADPH
• Calvin cycle uses ATP and NADPH to convert CO2 into organic compounds (glucose)
• Oxygen is released as a byproduct of light-dependent reactions
• Photosynthesis is essential for maintaining Earth's oxygen levels and providing energy for most living organisms

Photosynthesis Basics

• Overall reaction: $6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$
• Photosynthetic organisms contain light-absorbing pigments (chlorophyll, carotenoids) that capture light energy
• Photosystems, protein complexes containing pigments, are responsible for light absorption and electron transport
• Two types of photosystems: Photosystem I (PSI) and Photosystem II (PSII)
• Light-dependent reactions occur in the thylakoid membranes of chloroplasts
• Calvin cycle takes place in the stroma of chloroplasts
• Photosynthesis efficiency is affected by factors such as light intensity, temperature, and CO2 concentration

Light-Dependent Reactions

• Begin with light absorption by photosystem II (PSII)
• Light energy excites electrons in PSII, which are then transferred to an electron transport chain
• Electron transport chain creates a proton gradient across the thylakoid membrane
• ATP synthase uses the proton gradient to generate ATP (chemiosmosis)
• Photosystem I (PSI) absorbs light and excites electrons, reducing NADP+ to NADPH
• Electron from PSI is replaced by splitting water molecules (photolysis), releasing oxygen as a byproduct
• Products of light-dependent reactions (ATP and NADPH) are used in the Calvin cycle

Calvin Cycle (Light-Independent Reactions)

• Occurs in the stroma of chloroplasts and does not directly require light
• Consists of three main stages: carbon fixation, reduction, and regeneration
• Carbon fixation: Enzyme RuBisCO incorporates CO2 into a 5-carbon sugar (ribulose bisphosphate) to form two 3-carbon molecules (3-phosphoglycerate)
• Reduction: ATP and NADPH from light-dependent reactions are used to convert 3-phosphoglycerate into glyceraldehyde 3-phosphate (G3P)
• Regeneration: Some G3P is used to regenerate ribulose bisphosphate to continue the cycle, while the rest is used to synthesize glucose and other organic compounds
• The Calvin cycle is the primary pathway for carbon fixation in C3 plants

Carbon Fixation Mechanisms

• C3 carbon fixation: Primary pathway in most plants, where the first stable product is a 3-carbon compound (3-phosphoglycerate)
• C4 carbon fixation: Adaptation in some plants (maize, sugarcane) to minimize photorespiration
  • CO2 is initially fixed into a 4-carbon compound (oxaloacetate) in mesophyll cells
  • CO2 is then released in bundle sheath cells, where it enters the Calvin cycle
• Crassulacean Acid Metabolism (CAM): Adaptation in plants (cacti, succulents) to conserve water
  • Stomata open at night to take in CO2, which is stored as malic acid
  • During the day, stored CO2 is released and enters the Calvin cycle, allowing stomata to remain closed and reduce water loss

Cellular Structures Involved

• Chloroplasts: Organelles in plant and algal cells where photosynthesis occurs
  • Thylakoid membranes: Site of light-dependent reactions, contain photosystems and electron transport chain components
  • Stroma: Aqueous space surrounding thylakoids, site of the Calvin cycle
• Chlorophyll: Primary pigment responsible for light absorption
  • Chlorophyll a: Found in all photosynthetic organisms, primary pigment in photosystems
  • Chlorophyll b: Accessory pigment in plants and green algae, extends the range of light absorption
• Carotenoids: Accessory pigments that absorb light in the blue and green regions of the spectrum and transfer energy to chlorophyll

Biochemical Pathways and Enzymes

• Light-dependent reactions:
  • Photosystem II (PSII): Absorbs light and initiates electron transport
  • Cytochrome b6f complex: Transfers electrons from PSII to PSI and contributes to proton gradient formation
  • Photosystem I (PSI): Absorbs light and reduces NADP+ to NADPH
  • ATP synthase: Generates ATP using the proton gradient (chemiosmosis)
• Calvin cycle enzymes:
  • RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase): Catalyzes carbon fixation by incorporating CO2 into ribulose bisphosphate
  • Phosphoglycerate kinase and glyceraldehyde-3-phosphate dehydrogenase: Reduce 3-phosphoglycerate to glyceraldehyde 3-phosphate using ATP and NADPH
  • Triose phosphate isomerase, aldolase, and fructose-1,6-bisphosphatase: Convert glyceraldehyde 3-phosphate into fructose 6-phosphate
  • Phosphoribulokinase: Regenerates ribulose bisphosphate from ribulose 5-phosphate

Environmental Factors and Adaptations

• Light intensity: Higher light intensity generally increases photosynthesis rate until a saturation point is reached
• Temperature: Optimal temperature range for photosynthesis varies among species; enzymes like RuBisCO are sensitive to temperature changes
• CO2 concentration: Higher CO2 levels can increase photosynthesis rate, particularly in C3 plants
• Water availability: Water stress can limit photosynthesis by causing stomata to close, reducing CO2 uptake
• Photorespiration: Process in C3 plants where RuBisCO incorporates oxygen instead of CO2, reducing photosynthetic efficiency
  • C4 and CAM photosynthesis are adaptations to minimize photorespiration
• Shade adaptation: Plants adapted to low-light environments (shade plants) have higher chlorophyll content and more efficient light capture compared to sun-adapted plants
• Nitrogen availability: Nitrogen is a key component of chlorophyll and enzymes; low nitrogen can limit photosynthesis