🌱Plant Physiology Unit 3 – Photosynthesis: Light & Carbon Fixation

What's This Unit About?

• Photosynthesis converts light energy into chemical energy stored in sugars or other organic compounds
• Consists of two stages: the light reactions and the dark reactions (Calvin cycle)
• In the light reactions, light energy is captured and used to produce ATP and NADPH
  • Occurs in the thylakoid membranes of chloroplasts
• The dark reactions use the ATP and NADPH from the light reactions to produce glucose from carbon dioxide
  • Takes place in the stroma of chloroplasts
• Photosynthesis is vital for life on Earth as it provides energy for plants and oxygen for many organisms
• Understanding photosynthesis is crucial for improving crop yields and developing sustainable energy solutions (artificial photosynthesis)

Key Concepts and Definitions

• Photosynthesis: the process by which plants, algae, and some bacteria convert light energy into chemical energy
• Light reactions: the stage of photosynthesis where light energy is captured and converted into ATP and NADPH
• Dark reactions (Calvin cycle): the stage of photosynthesis where ATP and NADPH are used to convert CO2 into glucose
• Chloroplasts: organelles in plant cells where photosynthesis occurs
  • Contain thylakoid membranes (site of light reactions) and stroma (site of dark reactions)
• Chlorophyll: the primary pigment responsible for absorbing light energy in photosynthesis
• Photosystems: protein complexes in thylakoid membranes that capture light energy and initiate electron transport
• ATP synthase: an enzyme that generates ATP using the proton gradient created during the light reactions
• Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase): the enzyme responsible for carbon fixation in the Calvin cycle

Light Reactions: The Basics

• Light reactions occur in the thylakoid membranes of chloroplasts
• Light energy is absorbed by chlorophyll and other pigments in photosystems (PS I and PS II)
• Electrons are excited and transferred through an electron transport chain
  • Electron transport chain consists of a series of redox reactions
• Protons (H+) are pumped into the thylakoid lumen, creating a proton gradient
• ATP synthase uses the proton gradient to generate ATP (chemiosmosis)
• NADP+ is reduced to NADPH by accepting electrons at the end of the electron transport chain
• Oxygen is released as a byproduct of the light reactions when water is split (photolysis)

Dark Reactions: Calvin Cycle Explained

• The Calvin cycle occurs in the stroma of chloroplasts
• ATP and NADPH from the light reactions are used to convert CO2 into glucose
• The cycle has three main stages: carbon fixation, reduction, and regeneration
• Carbon fixation: CO2 is combined with a 5-carbon sugar (ribulose bisphosphate) by the enzyme Rubisco to form two 3-carbon molecules (3-phosphoglycerate)
• Reduction: ATP and NADPH are used to convert 3-phosphoglycerate into glyceraldehyde 3-phosphate (G3P)
  • Some G3P is used to make glucose, while some is used to regenerate ribulose bisphosphate
• Regeneration: ATP is used to convert G3P back into ribulose bisphosphate, allowing the cycle to continue
• The Calvin cycle is regulated by the availability of CO2, ATP, and NADPH

Important Molecules and Structures

• Chlorophyll a and b: the primary pigments responsible for absorbing light energy in photosynthesis
• Carotenoids: accessory pigments that absorb light energy and protect chlorophyll from damage
• Photosystem I (PS I) and Photosystem II (PS II): protein complexes in thylakoid membranes that capture light energy and initiate electron transport
• Cytochrome b6f complex: a protein complex in the electron transport chain that facilitates electron transfer and proton pumping
• Ferredoxin: an iron-sulfur protein that accepts electrons from PS I and reduces NADP+ to NADPH
• Ribulose-1,5-bisphosphate (RuBP): the 5-carbon sugar that accepts CO2 in the Calvin cycle
• 3-Phosphoglycerate (3-PGA): the 3-carbon molecule formed after carbon fixation in the Calvin cycle
• Glyceraldehyde 3-phosphate (G3P): the 3-carbon sugar produced during the reduction phase of the Calvin cycle

Step-by-Step Process Breakdown

1. Light absorption: Chlorophyll and other pigments in photosystems absorb light energy
2. Electron excitation: Light energy excites electrons in chlorophyll, causing them to move to a higher energy level
3. Electron transport: Excited electrons are transferred through the electron transport chain (PS II → cytochrome b6f complex → PS I)
4. Proton pumping: As electrons move through the electron transport chain, protons are pumped into the thylakoid lumen, creating a proton gradient
5. ATP synthesis: ATP synthase uses the proton gradient to generate ATP (chemiosmosis)
6. NADPH production: Electrons from PS I are accepted by ferredoxin, which reduces NADP+ to NADPH
7. Carbon fixation: In the Calvin cycle, Rubisco combines CO2 with RuBP to form 3-PGA
8. Reduction: ATP and NADPH from the light reactions are used to convert 3-PGA into G3P
9. Regeneration: Some G3P is used to regenerate RuBP, allowing the Calvin cycle to continue
10. Glucose synthesis: The remaining G3P is used to synthesize glucose and other organic compounds

Real-World Applications

• Improving crop yields: Understanding photosynthesis can help develop strategies to increase crop productivity (genetically modified crops with enhanced photosynthetic efficiency)
• Biofuels: Algae and other photosynthetic organisms can be used to produce renewable biofuels (biodiesel, bioethanol)
• Carbon sequestration: Photosynthesis plays a crucial role in removing CO2 from the atmosphere, helping to mitigate climate change (reforestation, algal cultivation)
• Artificial photosynthesis: Researchers are working on developing artificial systems that mimic photosynthesis to produce clean energy (solar fuels, hydrogen production)
• Space exploration: Understanding photosynthesis is essential for developing life support systems for long-term space missions (growing plants in space for food and oxygen production)

Common Misconceptions and FAQs

• Misconception: Plants photosynthesize only during the day
  • Fact: While light reactions require light, the dark reactions (Calvin cycle) can occur both day and night
• Misconception: Photosynthesis and respiration are opposite processes
  • Fact: While photosynthesis and respiration are related, they are not opposite processes; both involve electron transport and ATP synthesis
• FAQ: Do all plants photosynthesize?
  • Answer: Most plants photosynthesize, but some parasitic plants (dodder) have lost the ability to photosynthesize and rely on their host for nutrients
• FAQ: Can photosynthesis occur without chlorophyll?
  • Answer: While chlorophyll is the primary pigment for photosynthesis, other pigments (carotenoids, phycobilins) can also absorb light energy and contribute to photosynthesis
• FAQ: How does temperature affect photosynthesis?
  • Answer: Photosynthesis is sensitive to temperature; the optimal temperature range for most plants is between 20-35°C, and extreme temperatures can inhibit photosynthesis