Photosynthesis

Introduction

Photosynthesis is the process by which photoautotrophic organisms, such as plants, algae, and some bacteria, convert light energy into chemical energy stored in glucose. This process is fundamental to life on Earth, providing both energy and organic matter for nearly all living organisms. Photosynthesis occurs in two main stages: the light reactions and the Calvin cycle.

Key Vocabulary Terms

• Absorption spectrum: The range of wavelengths of light absorbed by a pigment.

• Action spectrum: The rate of photosynthesis plotted against wavelength; shows which wavelengths are most effective.

• ADP (Adenosine diphosphate): A molecule that, when phosphorylated, becomes ATP.

• ATP (Adenosine triphosphate): The main energy currency of the cell, produced during the light reactions.

• ATP synthase: An enzyme complex that synthesizes ATP from ADP and inorganic phosphate, powered by a proton gradient.

• Autotroph: An organism capable of producing its own food from inorganic substances.

• Carbon fixation: The process of incorporating inorganic CO2 into organic molecules during the Calvin cycle.

• Carotenoids: Accessory pigments that absorb light and protect chlorophyll from damage.

• Chemiosmosis: The movement of ions across a semipermeable membrane, generating ATP.

• Chlorophyll: The main pigment involved in capturing light energy for photosynthesis.

• Chloroplast: The organelle in plant cells where photosynthesis occurs.

• Cyclic electron flow: An alternative pathway for electrons during the light reactions that produces ATP but not NADPH.

• Electron transport chain: A series of proteins that transfer electrons, releasing energy used to pump protons and generate ATP.

• Excitation: The process by which a pigment molecule absorbs a photon and an electron is raised to a higher energy level.

• Glyceraldehyde-3-phosphate (G3P): A three-carbon sugar produced in the Calvin cycle; precursor to glucose.

• Heterotroph: An organism that obtains energy by consuming other organisms.

• Inner membrane: The inner boundary of the chloroplast envelope.

• Linear electron flow: The primary pathway of electron movement in the light reactions, producing both ATP and NADPH.

• Lumen: The interior space of the thylakoid where protons accumulate during the light reactions.

• NADP+/NADPH: Electron carrier molecules; NADP+ is reduced to NADPH during the light reactions.

• NADP+ reductase: The enzyme that catalyzes the reduction of NADP+ to NADPH.

• Outer membrane: The outer boundary of the chloroplast envelope.

• Oxidation: The loss of electrons from a molecule.

• Photoautotroph: An organism that uses light energy to synthesize organic compounds from CO2.

• Photon: A particle of light energy.

• Photosystem II (PSII): The first protein complex in the light reactions that absorbs light and splits water molecules.

• Photosystem I (PSI): The second protein complex in the light reactions that absorbs light and facilitates NADPH production.

• Reduction: The gain of electrons by a molecule.

• Regeneration: The phase of the Calvin cycle where RuBP is regenerated to continue carbon fixation.

• Resonance energy transfer: The transfer of energy from one pigment molecule to another within a photosystem.

• Rubisco: The enzyme that catalyzes the first step of carbon fixation in the Calvin cycle.

• Stroma: The fluid-filled space surrounding the thylakoids in the chloroplast.

• Thylakoid membrane: The membrane system within the chloroplast where the light reactions occur.

• Thylakoid space (lumen): The internal compartment of the thylakoid where protons accumulate.

• Visible light spectrum: The range of light wavelengths visible to the human eye (approximately 400–700 nm).

Overall Photosynthesis Equation

The general equation for photosynthesis is:

• CO2: Used in the Calvin cycle for carbon fixation.

• H2O: Split during the light reactions to provide electrons and protons; oxygen is released as a byproduct.

• O2: Produced during the light reactions from the splitting of water.

• Glucose (C6H12O6): The main product of the Calvin cycle, used for energy and as a building block for other organic molecules.

Light Reactions

Overview

The light reactions occur in the thylakoid membranes and convert solar energy into chemical energy in the form of ATP and NADPH. These products are then used in the Calvin cycle.

• Pigments: Chlorophyll a, chlorophyll b, and carotenoids absorb light energy. Pigments are organized into photosystems (PSII and PSI) to maximize energy capture and transfer.

• Energy Transfer: Light excites electrons in chlorophyll, which are transferred through the electron transport chain.

• Electron Flow:

  • Linear electron flow: Electrons move from water through PSII, the cytochrome complex, PSI, and finally to NADP+ reductase, producing ATP and NADPH.

  • Cyclic electron flow: Electrons cycle back from ferredoxin to the cytochrome complex and PSI, producing ATP but not NADPH.

• Order of Components in Linear Electron Flow:

  1. Photosystem II (PSII)

  2. Cytochrome complex

  3. Photosystem I (PSI)

  4. Ferredoxin (Fd)

  5. NADP+ reductase

• Proton Gradient:

  • Protons are pumped into the thylakoid lumen as electrons move through the electron transport chain.

  • Water splitting at PSII releases protons into the lumen.

  • Reduction of NADP+ in the stroma removes protons from the stroma.

  • The resulting proton gradient drives ATP synthesis via ATP synthase.

ATP and NADPH Production

• ATP: Produced by chemiosmosis as protons flow through ATP synthase from the thylakoid lumen to the stroma.

• NADPH: Produced by the reduction of NADP+ at the end of the electron transport chain.

The Calvin Cycle

Overview

The Calvin cycle occurs in the stroma of the chloroplast and uses ATP and NADPH from the light reactions to fix carbon dioxide and produce G3P, a precursor to glucose.

• Three Phases of the Calvin Cycle:

  1. Carbon fixation: CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme Rubisco, forming 3-phosphoglycerate (3-PGA).

  2. Reduction: ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).

  3. Regeneration: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.

• Major Product: G3P, which can be used to form glucose and other carbohydrates.

• ATP and NADPH Consumption: For each turn of the cycle (fixing one CO2), 3 ATP and 2 NADPH are consumed. To produce one glucose (from two G3P), 6 CO2, 18 ATP, and 12 NADPH are required.

• CO2 and O2 Stoichiometry: Six CO2 molecules are fixed to produce one glucose; six O2 molecules are released as a byproduct of water splitting in the light reactions.

Tracing Molecules Through Photosynthesis

• High-Energy Electrons: Start in water, energized by light in PSII, move through the electron transport chain, and end up reducing NADP+ to NADPH.

• Protons: Accumulate in the thylakoid lumen, creating a gradient used to synthesize ATP.

• Carbon: Enters as CO2, is fixed in the Calvin cycle, and exits as G3P (eventually forming glucose).

Table: Comparison of Light Reactions and Calvin Cycle

Example: Photosynthesis in a Leaf

In a typical plant leaf, chloroplasts in mesophyll cells capture sunlight. Water absorbed by roots is split in the thylakoid membranes, releasing O2. The ATP and NADPH produced are used in the stroma to fix atmospheric CO2 into G3P, which is then used to form glucose and other organic molecules.

Additional info: The study guide suggests using concept maps and flashcards to reinforce understanding of these terms and processes. It is also important to be able to trace the movement of electrons, protons, and carbon atoms through the entire process of photosynthesis for a comprehensive understanding.