Photosynthesis: Overview and Major Stages

Introduction to Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing carbohydrates and oxygen from carbon dioxide and water. This process occurs in the chloroplasts and is essential for life on Earth.

• Two Major Stages:

  1. Energy Transduction (Light Reactions): Conversion of light energy into ATP and NADPH.

  2. Carbon Assimilation (Calvin Cycle): Use of ATP and NADPH to fix CO2 into carbohydrates.

Energy Transduction: The Light Reactions

Purpose and Location

• Purpose: Convert light energy into chemical energy (ATP and NADPH).

• Location: Thylakoid membranes of chloroplasts.

Main Steps of the Light Reactions

1. Light Harvesting: Chlorophyll absorbs sunlight, exciting electrons.

2. Electron Transport Chain (ETC): Excited electrons move through a series of carriers, similar to the mitochondrial ETC.

3. Proton Pumping: Electron movement pumps H+ ions into the thylakoid lumen, creating a proton gradient.

4. ATP Synthesis: Proton gradient drives ATP synthase, producing ATP via photophosphorylation.

Photophosphorylation vs. Oxidative Phosphorylation

NADPH Formation (Photoreduction)

• Water donates electrons, which are transferred to NADP+ to form NADPH.

• Oxygen is released as a byproduct.

Overall reaction: Water → electrons → NADP+ → NADPH (O2 released)

Oxygenic vs. Anoxygenic Photosynthesis

Plants use oxygenic photosynthesis.

Chloroplast Structure and Function

Chloroplasts and Plastids

• Chloroplasts: Organelles where photosynthesis occurs; typically 5–10 µm long; 20–100 per plant cell.

• Plastids: Group of organelles with specialized functions.

Chloroplast Membrane Systems

• Outer membrane: Contains porins; permeable to small molecules.

• Inner membrane: Controls metabolite movement; forms a transport barrier.

• Thylakoid membranes: Site of light reactions; contain chlorophyll, electron carriers, and enzymes.

Thylakoid Structure

• Thylakoids: Flattened membrane sacs.

• Grana: Stacks of thylakoids.

• Stroma thylakoids: Connect grana stacks.

• Thylakoid lumen: Internal space where protons accumulate during light reactions.

Comparison with Mitochondria

Photosynthesis in Bacteria and Endosymbiotic Theory

• Photosynthetic bacteria lack chloroplasts; their plasma membrane folds inward to form thylakoid-like structures.

• Endosymbiotic theory: Chloroplasts evolved from cyanobacteria engulfed by early eukaryotic cells.

Light Capture and Photosystems

Light Energy and Photons

• Light behaves as both waves and particles (photons).

• Photon energy is inversely related to wavelength.

Visible light range: 380 nm – 750 nm

Photoexcitation

• When a pigment absorbs a photon, an electron is excited to a higher energy orbital (photoexcitation).

• Fates of the excited electron:

  1. Return to ground state (releasing heat or fluorescence).

  2. Resonance energy transfer to another pigment.

  3. Electron transfer (photochemical reduction) to another molecule, starting the electron transport chain.

Chlorophyll and Accessory Pigments

• Chlorophyll: Main pigment; has a porphyrin ring (with Mg2+) and a hydrophobic phytol tail.

• Chlorophyll a: Main pigment; absorbs at ~420 nm (blue) and ~660 nm (red).

• Chlorophyll b: Accessory pigment; absorbs slightly different wavelengths due to a formyl group.

• Accessory pigments: Carotenoids (absorb blue, appear yellow/orange) and phycobilins (found in red algae and cyanobacteria).

Plants appear green because chlorophyll reflects green light.

Photosystems and Light Harvesting Complexes

• Photosystems: Complexes of pigments and proteins in the thylakoid membrane.

• Antenna pigments: Collect and transfer light energy to the reaction center via resonance energy transfer.

• Reaction center: Special pair of chlorophyll molecules where light energy is converted to chemical energy.

• Light Harvesting Complexes (LHCs): Capture additional light and transfer energy to photosystems.

Two Photosystems in Plants

Order of operation: PSII acts first, then PSI. Each electron is excited twice.

Emerson Enhancement Effect: Photosynthesis is most efficient when two wavelengths are used, proving the cooperation of two photosystems.

Electron Transport and NADPH Formation

Overview of Electron Flow

• Excited electrons move through an electron transport chain (ETC) to produce NADPH (photoreduction).

• Common ETC components: cytochromes, iron-sulfur proteins, quinones.

Photosystem II (PSII)

1. Light excites P680, which releases an electron.

2. Electron moves through carriers: pheophytin → QA plastoquinone → QB plastoquinone (becomes plastoquinol, PQH2).

3. Water splitting (photolysis) by the oxygen-evolving complex (OEC):

   • Oxygen is released, electrons replace those lost by P680, and protons are added to the lumen.

Cytochrome b6f Complex and Plastocyanin

• PQH2 transfers electrons to cytochrome b6f, which pumps protons into the lumen and passes electrons to plastocyanin (PC).

• Plastocyanin is a mobile carrier, shuttling electrons to PSI.

Photosystem I (PSI) and NADPH Production

1. Light excites P700, which releases an electron.

2. Electron moves through carriers (chlorophyll A0, phylloquinone, iron-sulfur centers) to ferredoxin (Fd).

3. Ferredoxin transfers electrons to ferredoxin-NADP+ reductase (FNR):

Noncyclic Electron Flow

Electrons flow: Water → PSII → Plastoquinone → Cytochrome b6f → Plastocyanin → PSI → Ferredoxin → NADP+

• Products: NADPH, proton gradient (for ATP), O2

• For every 8 photons: 2 NADPH, 1 O2, and a proton gradient are produced.

ATP Synthesis and Photophosphorylation

Proton Gradient and ATP Synthase

• Protons accumulate in the thylakoid lumen, creating a strong proton gradient (pH difference: stroma ~8, lumen ~5).

• The energy stored is called the proton motive force (pmf), mainly from the pH gradient.

• ATP synthase (CF0 and CF1 subunits) uses this gradient to convert ADP + Pi into ATP.

• Approximate ratio: 4 protons → 1 ATP.

Cyclic vs. Noncyclic Electron Flow

• Cyclic electron flow allows plants to produce extra ATP without making NADPH or O2.

Summary of Light Reactions

• Major components: PSII, cytochrome b6f, PSI, ferredoxin–NADP+ reductase, ATP synthase.

• End products: ATP, NADPH, O2.

Calvin Cycle: Carbon Assimilation

Overview and Location

• The Calvin cycle (carbon fixation) converts CO2 into carbohydrates using ATP and NADPH from the light reactions.

• Occurs in the chloroplast stroma.

Three Stages of the Calvin Cycle

1. Carbon fixation: CO2 is attached to ribulose-1,5-bisphosphate (RuBP) by rubisco, forming two 3-phosphoglycerate (3-PGA) molecules.

2. Reduction: 3-PGA is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate (G3P).

3. Regeneration of RuBP: Most G3P is used to regenerate RuBP, using additional ATP.

Key Enzymes

• Rubisco: Fixes CO2 onto RuBP; most abundant protein on Earth.

• Phosphoglycerokinase: Uses ATP during reduction.

• Glyceraldehyde-3-phosphate dehydrogenase: Uses NADPH during reduction.

• Phosphoribulokinase (PRK): Regenerates RuBP using ATP.

ATP and NADPH Requirements

• For every 3 CO2 fixed: 9 ATP and 6 NADPH are used; 1 G3P is the net gain.

• Per CO2 fixed: 3 ATP + 2 NADPH required.

Summary of Carbon Flow

3 CO2 → 6 3-PGA → 6 G3P (1 leaves, 5 regenerate 3 RuBP)

Importance of G3P

• G3P is used to synthesize sucrose, starch, glucose, and other organic molecules.

Carbohydrate Synthesis After the Calvin Cycle

Transport of Triose Phosphates

• Triose phosphate/phosphate translocator exports G3P or DHAP from the stroma to the cytosol in exchange for inorganic phosphate (Pi).

Fates of Triose Phosphates

1. Exported to cytosol for sucrose synthesis.

2. Remain in chloroplast for starch synthesis.

Formation of Glucose-1-Phosphate

1. G3P + DHAP → fructose-1,6-bisphosphate

2. Fructose-1,6-bisphosphate → fructose-6-phosphate

3. Fructose-6-phosphate → glucose-6-phosphate

4. Glucose-6-phosphate → glucose-1-phosphate

These reactions occur in both the cytosol and chloroplast stroma (isoenzymes).

Sucrose Synthesis (Cytosol)

• Sucrose = glucose + fructose; main transport sugar in plants.

• Key steps: Glucose-1-phosphate + UTP → UDP-glucose; UDP-glucose + fructose-6-phosphate → sucrose-6-phosphate → sucrose.

• Sucrose is transported to non-photosynthetic tissues (roots, shoots, fruits).

Regulation of Sucrose Synthesis

• Key enzymes: Fructose-1,6-bisphosphatase, sucrose phosphate synthase.

• Sucrose synthesis increases with high energy/triose phosphate levels; decreases when sucrose is abundant.

Starch Synthesis (Chloroplast)

• Starch is the main storage carbohydrate; synthesized in the stroma.

• Key steps: Glucose-1-phosphate + ATP → ADP-glucose; ADP-glucose is added to starch by starch synthase.

• Starch accumulates as granules; broken down at night for energy.

Regulation of Starch Synthesis

• Key enzyme: ADP-glucose pyrophosphorylase (stimulated by G3P, inhibited by Pi).

• Ensures starch forms only when energy is abundant.

Other Products of Photosynthesis

• ATP and NADPH from light reactions also support synthesis of fatty acids, chlorophyll, carotenoids, amino acids, and nucleotides.

• Nitrogen assimilation: NO2- → NH3 (for amino acids, nucleotides).

• Sulfur assimilation: SO42- → S2- (for sulfur-containing amino acids).

Key Concepts and Exam Facts

• Light reactions occur in thylakoid membranes; produce ATP, NADPH, and O2.

• Calvin cycle occurs in the stroma; uses ATP and NADPH to fix CO2 into G3P.

• G3P is used to make sucrose (cytosol) and starch (chloroplast).

• Sucrose is the main transport sugar; starch is the main storage carbohydrate.

• Regulation ensures energy and carbon are allocated efficiently.

Overall photosynthesis reaction:

Additional info: The notes above integrate and expand on the provided material, ensuring all major concepts from the photosynthesis chapters are covered in a clear, academic format suitable for college-level cell biology students.