Comprehensive Study Notes on Photosynthesis: Light Reactions, Calvin Cycle, and Pathway Adaptations

Overview of Photosynthesis and Global Carbon Context

• General Chemical Reaction: The process of photosynthesis is represented by the following chemical equation:     $\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{Light}} (\text{CH}_2\text{O}) + \text{O}_2$
• Primary Site: The biological site of photosynthesis is the Chloroplast.
• Conceptual Dualism: Photosynthesis is functionally divided into two distinct parts:
  • Light Reactions: Transform light energy into chemical energy.
  • Dark Reactions (Calvin Cycle): Use chemical energy to reduce carbon dioxide into carbohydrates.
• Environmental Context: Global $\text{CO}_2$ concentrations (measured in ppm) have shown a steady increase over the decades:
  • 1960: Approximately $315\,\text{ppm}$
  • 1970: Approximately $325\,\text{ppm}$
  • 1980: Approximately $335\,\text{ppm}$
  • 1990: Approximately $355\,\text{ppm}$
  • 2000: Approximately $370\,\text{ppm}$

Structure of the Chloroplast

• Membrane Systems:
  • Outer Membrane: The exterior boundary of the organelle.
  • Inner Membrane: Located just beneath the outer membrane.
  • Intermembrane Space: The narrow region between the inner and outer membranes.
  • Thylakoid Membrane: The site of the light-dependent reactions where chlorophyll is localized.
• Internal Compartments:
  • Stroma: The fluid-filled space surrounding the thylakoids; the site of dark reactions (Calvin Cycle).
  • Thylakoid Space: The internal lumen of the thylakoid.
  • Stroma Lamellae: Connections between individual thylakoid stacks.

The Light Reactions: Energy Trapping and Conversion

• Fundamental Mechanism: The trapping of light energy is the central key to photosynthesis.
• Absorption and Excitation:
  1. The first step involves the absorption of light by a photoreceptor molecule.
  2. Chlorophyll a is identified as the principal photoreceptor in the chloroplasts of most green plants.
  3. Light energy excites an electron from its ground energy level to an excited energy level.
• Photoinduced Charge Separation:
  • If a suitable electron acceptor is nearby, the excited electron moves from the initial molecule to the acceptor.
  • This process creates a positive ($+$) charge on the initial molecule and a negative ($-$) charge on the acceptor.
  • The photosynthetic apparatus is highly organized to make this charge separation extremely efficient.
  • The extracted electron can reduce other species, effectively storing light energy in chemical forms.
• Electron Transport and Photosystems:
  • Chloroplasts convert light energy into chemical energy via two photosystems: Photosystem II and Photosystem I.
  • High-energy electrons are transported through these systems, culminating in the generation of reducing power (NADPH).
  • During this transit, a proton gradient is generated across the membrane.
  • ATP Synthesis: Protons return through ATP synthase to generate ATP from ADP and $P_i$. This process is analogous to mitochondrial ATP synthesis, but powered by light rather than mitochondrial electron transport from fuels.
• Light Reaction Reactants and Products:
  • Reactants: Light, $\text{H}_2\text{O}$, $\text{NADP}^+$, and ADP.
  • Products: $\text{O}_2$, NADPH, and ATP.

The Dark Reactions: The Calvin Cycle

• Function: The dark reactions constitute the Calvin Cycle, which involves the reduction of carbon dioxide into a more reduced state as a hexose sugar.
• Nomenclature: Referred to as "dark reactions" because they do not directly depend on the presence of light, though they use the products of the light reactions.
• The Three Stages of the Calvin Cycle:
  1. Fixation: $\text{CO}_2$ is fixed by ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate.
  2. Reduction: 3-phosphoglycerate is reduced to form hexose sugars.
  3. Regeneration: Ribulose 1,5-bisphosphate is regenerated to allow for continued $\text{CO}_2$ fixation.

Rubisco: The Key Enzyme of Carbon Fixation

• Full Name: Ribulose 1,5-bisphosphate carboxylase/oxygenase.
• Context: It is the most abundant protein in the biosphere and catalyzes the rate-determining step in hexose biosynthesis.
• Location: Found on the stromal surface of the thylakoid membranes.
• Structure:
  • Consists of 8 Large (L) subunits ($55\,\text{kd}$ each) and 8 small (S) subunits ($13\,\text{kd}$ each).
  • L chains: Contain both catalytic and regulatory sites.
  • S chains: Enhance the catalytic activity of the L subunits.
• Activation Requirements:
  • Requires a bound divalent metal ion, usually $\text{Mg}^{2+}$.
  • A $\text{CO}_2$ molecule is required to complete the assembly of the $\text{Mg}^{2+}$ binding site via carbamate formation.
  • This formation is facilitated by the enzyme rubisco activase.

Catalytic Activities and Photorespiration

• Carboxylase Activity: The productive reaction resulting in two molecules of 3-phosphoglycerate.
  • Pathway: Ribulose 1,5-bisphosphate $\rightarrow$ Enediolate intermediate $\rightarrow$ 2-Carboxy-3-keto-D-arabinitol 1,5-bisphosphate $\rightarrow$ Hydrated intermediate $\rightarrow$ $2 \times$ 3-Phosphoglycerate.
• Oxygenase Activity (A Wasteful Side Reaction):
  • The reactive enediolate intermediate reacts with molecular $\text{O}_2$ instead of $\text{CO}_2$.
  • Produces one molecule of 3-phosphoglycerate and one molecule of phosphoglycolate.
• Comparison of Activities at $25^{\circ}\text{C}$:
  • Carboxylase rate is $4\times$ that of the oxygenase reaction.
  • Stromal $\text{CO}_2$ concentration is roughly $10\,\mu\text{M}$, while $\text{O}_2$ is $250\,\mu\text{M}$.
  • Oxygenase activity increases more rapidly with temperature than carboxylase activity.
• Photorespiratory Pathway:
  • Chloroplast: Phosphoglycolate is dephosphorylated to glycolate.
  • Peroxisome: Glycolate $\rightarrow$ Glyoxylate $\rightarrow$ Glycine.
  • Mitochondrion: Two molecules of Glycine convert into one Serine, losing one carbon as $\text{CO}_2$ and one ammonia ($\text{NH}_3$).
  • Ammonia is eventually salvaged back in the chloroplast.

Synthesis of Hexoses and Regeneration

• Hexose Monophosphate Pool: Includes Glucose 1-phosphate, Glucose 6-phosphate, and Fructose 6-phosphate.
• Intermediate Stages: 3-Phosphoglycerate $\rightarrow$ 1,3-Bisphosphoglycerate $\rightarrow$ Glyceraldehyde 3-phosphate (GAP) $\rightleftharpoons$ Dihydroxyacetone phosphate (DHAP).
• Regeneration Enzymes:
  • Transketolase: Transfers 2-carbon units between sugars.
  • Aldolase: Combines or splits 3-carbon and 4-carbon sugars.
  • Sedoheptulose 1,7-bisphosphate phosphatase.
  • Phosphopentose isomerase and Phosphopentose epimerase.
  • Phosphoribulose kinase: Uses ATP to convert Ribulose 5-phosphate to Ribulose 1,5-bisphosphate.
• Overall Calvin Cycle Reaction Stoichiometry:     $6\,\text{CO}_2 + 18\,\text{ATP} + 12\,\text{NADPH} + 12\,\text{H}_2\text{O} \rightarrow \text{C}_{6}\text{H}_{12}\text{O}_{6} + 18\,\text{ADP} + 18\,\text{P}_i + 12\,\text{NADP}^+ + 6\,\text{H}^+$
• Sucrose Synthesis: Sucrose 6-phosphate is formed by the reaction between Fructose 6-phosphate and the activated intermediate UDP-glucose, catalyzed by sucrose 6-phosphate synthase.

Environmental Regulation and Efficiency Adaptations

• Regulation: The Calvin cycle operates during the day. Light reactions induce stromal changes that activate enzymes:
  • Increase in pH (alkaline medium).
  • Increase in $\text{Mg}^{2+}$, NADPH, and reduced ferrodoxin levels.
  • Alkaline pH and high $\text{Mg}^{2+}$ favor the formation of the carbamate required for Rubisco activity.
• C4 Pathway (Hatch-Slack Pathway):
  • Used by plants in hot climates to maintain a high local concentration of $\text{CO}_2$ at the Calvin cycle site.
  • Process: $\text{CO}_2$ is fixed into 4-carbon compounds (Oxaloacetate and Malate) in mesophyll cells. These compounds are transported to bundle-sheath cells (the major site of photosynthesis) where they are decarboxylated.
  • Overall C4 + Calvin Stoichiometry:         $6\,\text{CO}_2 + 30\,\text{ATP} + 12\,\text{NADPH} + 12\,\text{H}_2\text{O} \rightarrow \text{C}_{6}\text{H}_{12}\text{O}_{6} + 30\,\text{ADP} + 30\,\text{P}_i + 12\,\text{NADP}^+ + 18\,\text{H}^+$
• Crassulacean Acid Metabolism (CAM):
  • Used by drought-resistant plants in warm places.
  • Strategy: Stomata stay closed during the hot day to prevent water loss. They open at night when it is cooler.
  • Night: $\text{CO}_2$ is fixed via the C4 pathway into malate and stored in vacuoles as malic acid.
  • Day: Malate is decarboxylated to provide $\text{CO}_2$ for the Calvin cycle during daylight when ATP and NADPH are available from light reactions.

Plant Classification Based on Dark Reactions

• C3 Plants: Standard Calvin cycle fixation (e.g., wheat, rice).
• C4 Plants: Spatial separation of $\text{CO}_2$ fixation and the Calvin cycle (e.g., corn, sugarcane).
• CAM Plants: Temporal separation of $\text{CO}_2$ fixation (night) and the Calvin cycle (day) (e.g., pineapple, cacti).