Photosynthesis: Capturing Light Energy

Principles of Energy Capture: Photosynthesis and Light Reactions

• Fundamental Role of Photosynthesis:

  • Photosynthesis converts light energy into stable chemical energy stored in organic molecules.

  • The general redox equation representing oxygenic photosynthesis is:     $6\,\text{CO}_2 + 12\,\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 + 6\,\text{H}_2\text{O}$

  • In this process, water ($\text{H}_2\text{O}$) undergoes oxidation to yield molecular oxygen ($\text{O}_2$), providing electrons and protons, whereas carbon dioxide ($\text{CO}_2$) undergoes reduction to synthesize carbohydrates ($\text{C}_6\text{H}_{12}\text{O}_6$).

• Properties of Light and Photons:

  • Light behaves simultaneously as continuous electromagnetic waves and as discrete particles or packets of energy known as photons.

  • The energy of an individual photon is inversely proportional to its wavelength ($\lambda$):

  • Shorter-wavelength radiation contains greater energy per photon.

  • Longer-wavelength radiation contains lower energy per photon.

  • The electromagnetic spectrum ranges from high-energy, short-wavelength gamma rays ($\gamma$-rays), X-rays, and ultraviolet (UV) radiation, to lower-energy, long-wavelength infrared (IR), microwaves, and radio/television waves.

  • Visible light spans approximately $380\,nm$ to $760\,nm$:

  • Violet light corresponds to the shortest visible wavelengths ($\sim 380\text{--}400\,nm$) and the highest energy.

  • Blue light spans approximately $400\text{--}500\,nm$.

  • Green light spans approximately $500\text{--}550\,nm$.

  • Yellow light spans approximately $550\text{--}600\,nm$.

  • Orange light spans approximately $600\text{--}650\,nm$.

  • Red light spans approximately $650\text{--}760\,nm$ and represents the lowest energy visible photons.

• Mechanisms of Electron Excitation:

  • When a pigment absorbs a photon of suitable energy, an excitable electron transitions from a low-energy ground-state orbital to a higher-energy orbital.

  • The excited electron encounters one of two primary fates:

  • De-excitation to Ground State: The electron falls back to its ground state level, releasing the absorbed energy either as thermal dissipation (heat) or as a longer-wavelength, less energetic photon (fluorescence).

  • Electron Transfer to an Acceptor: The energized electron is captured by a neighboring electron acceptor molecule, initiating photochemical oxidation-reduction reactions.

Leaf and Chloroplast Architecture

• Internal Structure of the Leaf:

  • Leaves serve as the primary photosynthetic organs in terrestrial vascular plants.

  • Epidermis: Covered by a waxy cuticle that minimizes transpirational water loss. Specialized microscopic pores called stomata (singular: stoma) punctuate the epidermis to permit bidirectional gas exchange ($\text{CO}_2$ influx and $\text{O}_2$ / $\text{H}_2\text{O}$ vapor efflux).

  • Mesophyll: The interior photosynthetic ground tissue located between the upper and lower epidermis, comprised of:

  • Palisade mesophyll: Columnar, tightly packed cells positioned directly below the upper epidermis, optimized for capturing incoming direct light.

  • Spongy mesophyll: Loosely organized cells with extensive intercellular air spaces that facilitate the rapid diffusion of $\text{CO}_2$ throughout the leaf tissue.

  • Veins (Vascular Bundles): Contain xylem (transporting water and minerals from roots) and phloem (exporting synthesized sucrose and other assimilates to non-photosynthetic plant tissues).

• Compartmental Anatomy of the Chloroplast:

  • Chloroplasts are double-membrane-bound organelles concentrated within the cytoplasm of mesophyll cells (often 20 to 100 chloroplasts per cell).

  • Outer Membrane: Smooth, porous membrane containing porins that allow passive diffusion of small solutes.

  • Intermembrane Space: Thin compartment situated between the outer and inner membranes.

  • Inner Membrane: Highly selective barrier regulating solute movement into the internal fluid compartment.

  • Stroma: Gel-like, enzyme-rich aqueous matrix enclosed by the inner membrane; contains soluble enzymes required for carbon fixation (the Calvin cycle), alongside chloroplast DNA (cpDNA) and ribosomes.

  • Thylakoids: Interconnected system of flattened, disc-shaped membranous sacs suspended within the stroma.

  • Granum (plural: grana): Tightly stacked thylakoid membranes functioning in light absorption and photochemical reactions.

  • Thylakoid Lumen: Continuous interior aqueous space enclosed by the thylakoid membrane, across which a proton ($\text{H}^+$) gradient develops during light-driven electron transport.

Photosynthetic Pigments and Photochemistry

• Structure of Chlorophyll:

  • Porphyrin Ring: Flat, light-absorbing head comprised of four pyrrole rings coordinated around a central magnesium ion ($\text{Mg}^{2+}$).

  • The system of alternating conjugated double and single bonds allows delocalized electrons to circulate across the ring, stabilizing transient excited states.

  • The central $\text{Mg}^{2+}$ ion carries formal positive charges that help attract and hold electrons within the delocalized orbitals.

  • Hydrocarbon Tail: Long, nonpolar phytol side chain ($\text{C}_{20}\text{H}_{39}$) attached via an ester linkage that firmly anchors the chlorophyll molecule into the hydrophobic core of the thylakoid lipid bilayer.

  • Structural Distinction Between Chlorophyll a and Chlorophyll b:

  • Chlorophyll a: Possesses a methyl group ($-\text{CH}_3$) at carbon-3 of the porphyrin ring; acts as the primary reaction center pigment that directly initiates the photochemical reactions (appears bright blue-green).

  • Chlorophyll b: Possesses a formyl/aldehyde group ($-\text{CHO}$) at carbon-3; serves as an accessory antenna pigment that transfers captured excitation energy to chlorophyll a (appears yellow-green).

• Accessory Pigments:

  • Carotenoids: Lipid-soluble yellow and orange accessory pigments (including $\beta$-carotene and lutein).

  • Expand the range of photosynthetically active radiation by absorbing photons in the blue-green spectrum ($400\text{--}500\,nm$) where chlorophylls absorb less efficiently.

  • Provide photoprotection by quenching excess radiant energy, de-exciting reactive triplet chlorophyll states, and scavenging reactive oxygen species.

• Absorption Spectrum vs. Action Spectrum:

  • Absorption Spectrum: A spectrophotometric plot of the percentage or fraction of light absorbed by a pure pigment solution across varied wavelengths.

  • Chlorophyll a exhibits absorption peaks in the violet-blue ($\sim 430\,nm$) and red ($\sim 660\,nm$) regions.

  • Chlorophyll b exhibits shifted absorption peaks in the blue ($\sim 450\,nm$) and orange-red ($\sim 640\,nm$) regions.

  • Carotenoids show broad absorption in the blue-green range ($400\text{--}500\,nm$).

  • Action Spectrum: A biological plot measuring the relative rate of photosynthesis (e.g., rate of $\text{O}_2$ production) as a function of wavelength in an intact organism.

  • Correlates closely with the composite absorption spectra of all thylakoid pigments acting collectively, displaying distinct maximal peaks in the blue and red wavelengths and a broad minimum in the green region ($\sim 500\text{--}560\,nm$).

• T. W. Engelmann's Classical Experiment (1883):

  • Illuminating a filament of the green alga Spirogyra with light dispersed by a glass prism exposed different segments of the algal strand to distinct spectral wavelengths.

  • Aerotactic bacteria, which move toward regions of higher dissolved oxygen concentration, were introduced into the medium.

  • The bacteria clustered densely around segments of Spirogyra illuminated by blue ($\sim 400\text{--}450\,nm$) and red ($\sim 650\text{--}680\,nm$) light, demonstrating experimentally that these wavelengths drive the highest rates of photosynthetic oxygen evolution.

Light-Dependent Reactions: Noncyclic and Cyclic Photophosphorylation

• Structural Organization of Photosystems:

  • Photosystems are multiprotein pigment complexes embedded within the thylakoid membrane.

  • Antenna Complex (Light-Harvesting Complex):

  • Houses approximately 200 to 300 pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) bound to specific organizing proteins.

  • Photons absorbed anywhere within the antenna funnel excitation energy via inductive resonance transfer downward to the reaction center.

  • Reaction Center:

  • Contains a specialized pair of chlorophyll a molecules situated adjacent to a primary electron acceptor.

  • Photosystem II (PS II): Contains a reaction center chlorophyll a dimer designated $\text{P}_{680}$, with peak absorption at $680\,nm$.

  • Photosystem I (PS I): Contains a reaction center chlorophyll a dimer designated $\text{P}_{700}$, with peak absorption at $700\,nm$.

• Noncyclic Electron Transport (The Z-Scheme):

  • Represents the standard linear pathway utilizing both PS II and PS I in series to generate ATP, NADPH, and molecular oxygen ($\text{O}_2$).

  • Step-by-Step Flow:

  1. Excitation of PS II: Photons absorbed by PS II excite $\text{P}_{680}$ to an elevated oxidation-reduction potential of approximately $-0.8\,\text{volts}$, causing the ejection of two energized electrons ($2\,e^-$) to the primary electron acceptor (pheophytin).

  2. Photolysis of Water: The resulting strongly oxidizing $\text{P}_{680}^+$ is reduced by electrons extracted from water by the oxygen-evolving complex located on the lumenal face of PS II:        $2\,\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\,\text{H}^+ + 4\,e^-$

     • Protons ($\text{H}^+$) accumulate directly within the thylakoid lumen, and $\text{O}_2$ is released into the atmosphere.

  3. Electron Transport Chain Between PS II and PS I: Electrons move from the primary acceptor through plastoquinone ($\text{PQ}$), through the cytochrome $b_6f$ complex, and onto plastocyanin ($\text{PC}$, a copper-containing peripheral membrane protein).

     • As electrons pass through the cytochrome complex, exergonic redox energy drives the translocation of protons from the stroma into the thylakoid lumen.

  4. Excitation of PS I: Photons absorbed by PS I excite $\text{P}_{700}$ to an oxidation-reduction potential exceeding $-1.0\,\text{volts}$, ejecting $2\,e^-$ to primary acceptor $A_0$ (a specialized chlorophyll monomer), then through $A_1$ (phylloquinone), and iron-sulfur centers ($\text{FeS}_X$, $\text{FeS}_B$, $\text{FeS}_A$).

     • The electron deficit in $\text{P}_{700}^+$ is replenished by incoming electrons delivered from reduced plastocyanin ($\text{PC}$.

  5. Reduction of $\text{NADP}^+$: Electrons transfer from the iron-sulfur centers to ferredoxin ($\text{Fd}$). Ferredoxin-$\text{NADP}^+$ reductase ($\text{FNR}$) catalyzes the terminal reduction of stromal $\text{NADP}^+$ to form NADPH:        $\text{NADP}^+ + 2\,\text{H}^+ + 2\,e^- \rightarrow \text{NADPH} + \text{H}^+$

• Cyclic Photophosphorylation:

  • Alternate pathway occurring when chloroplast ATP demands exceed NADPH requirements or when $\text{NADP}^+$ pools are depleted.

  • Operates exclusively via Photosystem I ($\text{P}_{700}$):

  • Energized electrons ejected from $\text{P}_{700}$ pass through ferredoxin ($\text{Fd}$).

  • Instead of moving to $\text{NADP}^+$, electrons cycle back to plastoquinone ($\text{PQ}$).

  • Plastoquinone delivers the electrons through the cytochrome $b_6f$ complex, pumping additional protons ($\text{H}^+$) into the thylakoid lumen.

  • Electrons return via plastocyanin ($\text{PC}$) back to ground-state $\text{P}_{700}$.

  • Generates ATP through chemiosmosis without evolving $\text{O}_2$ or producing NADPH.

  • Present in photosynthetic bacteria and primitive plants, and actively utilized by higher plants when the Calvin cycle has ample carbohydrate but requires supplementary ATP to avoid Calvin cycle stall.

• Comparison Between Electron Transport Pathways:

  • Electron Source:

  • Noncyclic: Exogenous $\text{H}_2\text{O}$.

  • Cyclic: None (electrons cycle continuously within the closed loop).

  • Oxygen Release:

  • Noncyclic: Yes (derived from the photolysis of $\text{H}_2\text{O}$).

  • Cyclic: No.

  • Terminal Electron Acceptor:

  • Noncyclic: $\text{NADP}^+$.

  • Cyclic: None (electrons return to the $\text{P}_{700}$ reaction center).

  • Energy Form Captured:

  • Noncyclic: ATP (via chemiosmosis) and NADPH.

  • Cyclic: ATP only (via chemiosmosis).

  • Photosystems Required:

  • Noncyclic: Both Photosystem II ($\text{P}_{680}$) and Photosystem I ($\text{P}_{700}$).

  • Cyclic: Photosystem I ($\text{P}_{700}$) only.

Photophosphorylation and Chemiosmosis in Thylakoids

• Chemiosmotic Coupling Hypothesis (Peter Mitchell Mechanism):

  • Light-driven electron transport establishes a steep electrochemical proton gradient ($\Delta\mu_{\text{H}^+}$) across the thylakoid membrane.

  • Proton Accumulation Mechanisms:

  1. Protons deposited into the thylakoid lumen during the photolytic splitting of $\text{H}_2\text{O}$ at PS II.

  2. Protons pumped actively from the stroma into the lumen by the coupled redox action of plastoquinone and the cytochrome $b_6f$ complex.

  3. Removal of stromal protons during the reduction of $\text{NADP}^+$ to NADPH by ferredoxin-$\text{NADP}^+$ reductase.

  • As a result, the thylakoid lumen becomes acidic ($\text{pH} \approx 5$), while the surrounding stroma becomes relatively alkaline ($\text{pH} \approx 8$), establishing a magnitude gradient of up to 3 pH units.

• ATP Synthase Structure and Function:

  • The thylakoid membrane is impermeable to hydrophilic protons except through specialized transmembrane channels formed by the $CF_0\text{-}CF_1$ ATP synthase complex.

  • Proton Motive Force: Protons flow down their electrochemical gradient from the thylakoid lumen back into the stroma through the $CF_0$ rotor channel.

  • Catalytic Synthesis: The kinetic energy of proton flow induces conformational rotations in the catalytic head ($CF_1$), coupling proton dissipation to the phosphorylation of ADP:     $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

  • The noncyclic light reactions yield the stoichiometry:     $12\,\text{H}_2\text{O} + 12\,\text{NADP}^+ + 18\,\text{ADP} + 18\,\text{P}_i \rightarrow 6\,\text{O}_2 + 12\,\text{NADPH} + 18\,\text{ATP}$

Carbon Fixation: The Calvin Cycle (C3 Pathway)

• Operational Localization and Energy Coupling:

  • Occurs entirely within the aqueous stroma of the chloroplast.

  • Consumes the ATP and NADPH synthesized during the thylakoid light-dependent reactions to reduce inorganic $\text{CO}_2$ into three-carbon sugars.

  • Often termed the $C_3$ pathway because the first stable intermediate formed is a three-carbon compound (3-phosphoglycerate).

• Three Enzymatic Phases of the Calvin Cycle:

  • Phase 1: Carbon Dioxide Uptake (Fixation Phase):

  • Enzyme: Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco).

  • Reaction: Atmospheric $\text{CO}_2$ combines with the 5-carbon phosphorylated acceptor molecule ribulose 1,5-bisphosphate (RuBP).

  • Product: Generates an unstable 6-carbon intermediate that immediately hydrolyzes into two molecules of 3-phosphoglycerate (PGA or 3-PGA).

  • Stoichiometry for 1 net glucose: $6\,\text{CO}_2 + 6\,\text{RuBP} (30\text{C}) \rightarrow 12\,\text{PGA} (36\text{C})$.

  • Phase 2: Carbon Reduction Phase:

  • Step A (Phosphorylation): 12 molecules of 3-PGA are phosphorylated by 12 ATP (yielding 12 ADP) to form 12 molecules of 1,3-bisphosphoglycerate (1,3-BPG).

  • Step B (Reduction): 12 molecules of 1,3-BPG are reduced by 12 NADPH (yielding 12 $\text{NADP}^+$ and 12 $\text{P}_i$) via glyceraldehyde-3-phosphate dehydrogenase to produce 12 molecules of glyceraldehyde-3-phosphate (G3P).

  • Net Output: For every $6\,\text{CO}_2$ fixed, 12 molecules of G3P are formed:

    • 2 molecules of G3P (3C each = 6C) exit the cycle as net carbohydrate product used for the synthesis of glucose, fructose, and starch.

    • 10 molecules of G3P (30C total) remain inside the cycle to regenerate RuBP.

  • Phase 3: RuBP Regeneration Phase:

  • Through a complex sequence of sugar-phosphate rearrangements involving transketolases and aldolases, 10 molecules of G3P (3C) are reassembled into 6 molecules of ribulose-5-phosphate (RP, 5C).

  • Phosphorylation of 6 RP molecules by 6 ATP (yielding 6 ADP) regenerates the initial 6 molecules of ribulose 1,5-bisphosphate (RuBP, 5C), resetting the cycle.

• Overall Stoichiometric Balance of the Calvin Cycle:   $6\,\text{CO}_2 + 12\,\text{NADPH} + 18\,\text{ATP} + 12\,\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 12\,\text{NADP}^+ + 18\,\text{ADP} + 18\,\text{P}_i$

• Metabolic Fates of Glyceraldehyde-3-Phosphate (G3P):

  • G3P functions as the central chemical precursor for total plant anabolism:

  • Glucose and Starch Synthesis: Converted inside chloroplasts to glucose-1-phosphate, polymerizing into insoluble starch granules for short-term energy storage.

  • Cytosolic Sucrose Synthesis: Exported via phosphate translocators into the cytosol and joined with fructose to form the non-reducing disaccharide sucrose, the primary mobile sugar translocated via phloem.

  • Biosynthetic Precursor: Converted via secondary pathways into fatty acids, glycerol, amino acids, and nucleotides.

Photorespiration and Alternative Carbon Fixation Pathways (C4 and CAM)

• The Problem of Photorespiration:

  • Rubisco possesses dual catalytic activities: carboxylase activity (binding $\text{CO}_2$) and oxygenase activity (binding $\text{O}_2$).

  • Environmental Triggers: Under hot, arid conditions, terrestrial plants close their stomata to conserve water vapor through transpiration.

  • Internal stomatal closure leads to rapid depletion of leaf $\text{CO}_2$ while photosynthetic light reactions cause $\text{O}_2$ to accumulate inside chloroplasts.

  • Oxygenase Reaction: Rubisco binds $\text{O}_2$ instead of $\text{CO}_2$, cleaving RuBP into one molecule of 3-phosphoglycerate (3C) and one molecule of phosphoglycolate (2C).

  • Phosphoglycolate is enzymatically metabolized through the chloroplast, peroxisome, and mitochondrion, releasing $\text{CO}_2$ and $\text{H}_2\text{O}$.

  • Energetic Cost: Photorespiration wastes metabolic energy, consumes ATP and NADPH, and reverses carbon fixation by up to $50\%$ without generating ATP or sugar.

• The C4 Pathway (Spatial Separation of Carbon Fixation):

  • Utilized by plants adapted to high light intensities and elevated temperatures (e.g., corn, sugarcane, crabgrass).

  • Kranz Anatomy: Distinct structural arrangement where photosynthetic bundle sheath cells tightly ring leaf vascular veins, enveloped externally by radially arranged mesophyll cells.

  • In $C_3$ plants, the Calvin cycle operates throughout the mesophyll, while bundle sheath cells are nonphotosynthetic.

  • In $C_4$ plants, initial carbon capture occurs in mesophyll cells, whereas the Calvin cycle is confined strictly to bundle sheath cells.

  • Step-by-Step Pathway:

  1. Mesophyll Fixation: Atmospheric $\text{CO}_2$ is converted to bicarbonate ($\text{HCO}_3^-$) and captured by phosphoenolpyruvate (PEP) carboxylase, which combines PEP (3C) with $\text{CO}_2$ to form oxaloacetate (4C).

     • PEP carboxylase has an exceptionally high affinity for $\text{HCO}_3^-$ and exhibits no oxygenase activity, functioning efficiently even at very low $\text{CO}_2$ levels.

  2. Organic Acid Reduction: Oxaloacetate (4C) is reduced to malate (4C) by NADPH:        $\text{oxaloacetate} + \text{NADPH} + \text{H}^+ \rightarrow \text{malate} + \text{NADP}^+$

  3. Translocation: Malate is transported through cytoplasmic plasmodesmata into bundle sheath cells.

  4. Decarboxylation: Malate is decarboxylated by malic enzyme to produce pyruvate (3C), releasing a high concentration of $\text{CO}_2$ and generating NADPH:        $\text{malate} + \text{NADP}^+ \rightarrow \text{pyruvate} + \text{CO}_2 + \text{NADPH}$

  5. Calvin Cycle Operation: Concentrated $\text{CO}_2$ overwhelms Rubisco within bundle sheath cells, eliminating photorespiration and maximizing carbohydrate synthesis.

  6. Regeneration of PEP: Pyruvate (3C) diffuses back into mesophyll cells, where pyruvate phosphate dikinase consumes ATP to phosphorylate pyruvate back to PEP (3C).

• The CAM Pathway (Crassulacean Acid Metabolism - Temporal Separation):

  • Evolutionary adaptation found in succulent arid plants (e.g., cactuses, pineapples, agaves, lilies, orchids).

  • Temporal Coordination Within a Single Cell:

  • Nocturnal Phase (Night): Stomata open during cooler, humid night hours to minimize transpirational water loss. Atmospheric $\text{CO}_2$ enters the cell, where PEP carboxylase combines it with PEP to form oxaloacetate (4C). Oxaloacetate is reduced to malate (4C) and sequestered within large central vacuoles as malic acid.

  • Diurnal Phase (Day): Stomata remain tightly closed during daylight hours to conserve water. Malic acid is exported from vacuoles back into the cytoplasm, where decarboxylation releases high concentrations of $\text{CO}_2$.

  • Light-dependent reactions produce ATP and NADPH during the day, driving the Calvin cycle to fix the released $\text{CO}_2$ without risking transpirational desiccation.

• Comparative Summary of Photosynthetic Strategies:

  • $C_3$ Plants:

  • Initial enzyme: Rubisco.

  • Initial product: 3-phosphoglycerate (3C).

  • Site of Calvin cycle: Mesophyll cells.

  • Vulnerability: Significant photorespiration under hot, dry conditions.

  • $C_4$ Plants:

  • Initial enzyme: PEP carboxylase.

  • Initial product: Oxaloacetate (4C).

  • Spatial separation: Carbon fixed in mesophyll; Calvin cycle in bundle sheath cells.

  • Advantage: Suppresses photorespiration in hot, bright climates.

  • CAM Plants:

  • Initial enzyme: PEP carboxylase.

  • Initial product: Oxaloacetate (4C) stored as malate.

  • Temporal separation: Carbon fixed at night; Calvin cycle operates during the day within the same mesophyll cell.

  • Advantage: Maximizes water-use efficiency in arid environments.

Metabolic Diversity in Carbon and Energy Utilization

• Classification by Carbon and Energy Source:

  • Photoautotrophs:

  • Energy Source: Radiant light energy.

  • Carbon Source: Inorganic carbon dioxide ($\text{CO}_2$).

  • Examples: Land plants, eukaryotic algae, cyanobacteria.

  • Significance: Produce virtually all primary organic matter sustaining terrestrial and aquatic biotas; photolysis of water generates atmospheric $\text{O}_2$, which subsequently forms the protective stratospheric ozone ($\text{O}_3$) layer.

  • Photoheterotrophs:

  • Energy Source: Radiant light energy.

  • Carbon Source: Preformed organic compounds (cannot fix $\text{CO}_2$).

  • Examples: Nonsulfur purple and green photosynthetic bacteria.

  • Chemoautotrophs:

  • Energy Source: Oxidation of reduced inorganic chemical compounds (e.g., hydrogen sulfide [$\text{H}_2\text{S}$, nitrite [$\text{NO}_2^-$, ammonia [$\text{NH}_3$, or ferrous iron [$\text{Fe}^{2+}$]).

  • Carbon Source: Inorganic carbon dioxide ($\text{CO}_2$).

  • Examples: Nitrifying bacteria, deep-sea hydrothermal vent sulfur-oxidizing bacteria.

  • Chemoheterotrophs:

  • Energy Source: Oxidation of chemical bonds within organic molecules.

  • Carbon Source: Preformed organic molecules synthesized by other organisms.

  • Examples: Animals, fungi, protozoa, and the vast majority of non-photosynthetic bacteria.

• Cross-Regulation Between Respiration and Photosynthesis in Plants:

  • Illumination drives proton transport into thylakoids, increasing stromal pH ($\sim 8.0$), which selectively activates Calvin cycle regulatory enzymes (including Rubisco and sedoheptulose-1,7-bisphosphatase).

  • Light directly inhibits cytosolic glycolytic enzymes, avoiding futile cycles of simultaneous hexose production and degradation.

  • High light intensities accompanied by high $\text{O}_2$ levels and low $\text{CO}_2$ concentrations accelerate photorespiration.

Cellular Respiration: Catabolic Release of Energy and Overview

• Overview of Aerobic Respiration:

  • Catabolic pathway wherein chemical energy stored within nutrient bonds is systematically extracted and transferred to synthesize ATP.

  • Complete oxidation of glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) using molecular oxygen ($\text{O}_2$) as the terminal electron acceptor:     $\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 + 6\,\text{H}_2\text{O} \rightarrow 6\,\text{CO}_2 + 12\,\text{H}_2\text{O} + \text{energy (ATP + heat)}$

  • The reaction is a biological redox process: glucose is oxidized to $\text{CO}_2$, and oxygen is reduced to $\text{H}_2\text{O}$.

• Stepwise Energy Extraction vs. Uncontrolled Combustion:

  • A boulder falling from a cliff to the ground in a single free fall dissipates its entire potential energy ($E$) at once as uncontrolled heat and kinetic destruction.

  • If the same boulder rolls down a staircase, energy is released in discrete, manageable increments ($e_1 + e_2 + e_3 + e_4 + e_5 = E$).

  • Cellular respiration releases free energy incrementally via enzyme-catalyzed transfers, enabling the cell to harvest energy efficiently into ATP rather than losing it all as damaging heat.

• Fundamental Classes of Respiration Reactions:

  • Dehydrogenations: Redox reactions where two hydrogen atoms (equivalent to $2\,\text{H}^+$ and $2\,e^-$) are transferred from an organic substrate to coenzymes such as $\text{NAD}^+$ or FAD.

  • Decarboxylations: Reactions where a carboxyl group ($-\text{COOH}$) is removed from a carbon backbone and released as molecular carbon dioxide ($\text{CO}_2$).

  • "Make-Ready" Reactions: Isomerizations, additions, or removals of water molecules that structurally rearrange a substrate to facilitate subsequent dehydrogenation or decarboxylation steps.

• Mechanisms of ATP Generation:

  • Substrate-Level Phosphorylation: Direct enzymatic transfer of a high-energy phosphate group from a phosphorylated metabolic intermediate directly to ADP, forming ATP.

  • Chemiosmotic Coupling (Oxidative Phosphorylation): Electron transfer through an electron transport chain drives active proton pumping across a membrane, generating a proton gradient that drives ATP synthesis through ATP synthase.

• The Four Stages of Aerobic Respiration:

  1. Glycolysis (occurs in the cytosol).

  2. Formation of Acetyl Coenzyme A / Pyruvate Oxidation (occurs in the mitochondrial matrix).

  3. Citric Acid Cycle / Krebs Cycle (occurs in the mitochondrial matrix).

  4. Electron Transport Chain and Chemiosmosis (occurs across the inner mitochondrial membrane).

Glycolysis: Ten-Step Cytosolic Pathway

• General Characteristics of Glycolysis:

  • Ancient, universal pathway operating in the cytosol under both aerobic and anaerobic conditions.

  • Converts one 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

  • Divided into two distinct operational phases:

  • Energy investment phase (steps 1–5): Consumes 2 ATP.

  • Energy capture/payback phase (steps 6–10): Produces 4 ATP and 2 NADH.

  • Net Yield per glucose: 2 Pyruvate + 2 ATP (net) + 2 NADH.

• Phase 1: Energy Investment and Splitting of Glucose:

  • Step 1: Phosphorylation of Glucose:

  • Reactants: Glucose + ATP.

  • Enzyme: Hexokinase.

  • Products: Glucose-6-phosphate (G6P) + ADP.

  • Mechanism: Hexokinase consumes 1 ATP to attach a phosphate to carbon-6 of glucose. Phosphorylation traps glucose within the cell and makes it chemically more reactive.

  • Step 2: Isomerization of Glucose-6-Phosphate:

  • Reactant: Glucose-6-phosphate.

  • Enzyme: Phosphoglucoisomerase.

  • Product: Fructose-6-phosphate (F6P).

  • Mechanism: Preparation reaction rearranging the pyranose ring of G6P into the furanose isomer F6P.

  • Step 3: Phosphorylation of Fructose-6-Phosphate:

  • Reactants: Fructose-6-phosphate + ATP.

  • Enzyme: Phosphofructokinase (PFK).

  • Products: Fructose-1,6-bisphosphate (F1,6BP) + ADP.

  • Mechanism: PFK consumes a second ATP to attach a phosphate group to carbon-1. This is a critical commitment step and primary regulatory control point of glycolysis.

  • Step 4: Cleavage of Fructose-1,6-Bisphosphate:

  • Reactant: Fructose-1,6-bisphosphate.

  • Enzyme: Aldolase.

  • Products: Glyceraldehyde-3-phosphate (G3P) + Dihydroxyacetone phosphate (DHAP).

  • Mechanism: The 6-carbon bisphosphorylated sugar is cleaved into two distinct 3-carbon phosphorylated sugars.

  • Step 5: Isomerization of Dihydroxyacetone Phosphate:

  • Reactant: Dihydroxyacetone phosphate (DHAP).

  • Enzyme: Triose phosphate isomerase (Isomerase).

  • Product: Glyceraldehyde-3-phosphate (G3P).

  • Mechanism: DHAP is converted to G3P. Because G3P is continuously removed by step 6, mass action rapidly shifts DHAP into G3P. Hence, 1 glucose yields 2 G3P molecules.

• Phase 2: Energy Capture (Payback) Phase (All reactions occur twice per original glucose molecule):

  • Step 6: Oxidation and Phosphorylation of G3P:

  • Reactants: 2 G3P + 2 $\text{NAD}^+$ + 2 $\text{P}_i$ (inorganic phosphate).

  • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase.

  • Products: 2 1,3-Bisphosphoglycerate (1,3-BPG) + 2 NADH + 2 \text{H}^+$.\n - Mechanism: Exergonic dehydrogenation removes hydrogen atoms to reduce \text{NAD}^+ to NADH, while cytosolic inorganic phosphate is attached to carbon-1 via an unstable high-energy bond.\n - Step 7: Substrate-Level Phosphorylation of ADP:\n - Reactants: 2 1,3-Bisphosphoglycerate + 2 ADP.\n - Enzyme: Phosphoglycerokinase.\n - Products: 2 3-Phosphoglycerate (3-PGA) + 2 ATP.\n - Mechanism: Direct enzymatic transfer of the high-energy phosphate from carbon-1 to ADP, forming ATP. This step recovers the 2 ATP invested in Phase 1.\n - Step 8: Isomerization of 3-Phosphoglycerate:\n - Reactant: 2 3-Phosphoglycerate.\n - Enzyme: Phosphoglyceromutase.\n - Product: 2 2-Phosphoglycerate (2-PGA).\n - Mechanism: Shift of the remaining ester phosphate group from carbon-3 to carbon-2.\n - Step 9: Dehydration of 2-Phosphoglycerate:\n - Reactant: 2 2-Phosphoglycerate.\n - Enzyme: Enolase.\n - Products: 2 Phosphoenolpyruvate (PEP) + 2 \text{H}_2\text{O}.\n - Mechanism: Removal of a water molecule creates a high-energy enol-phosphate bond (indicated by a wavy line).\n - Step 10: Substrate-Level Phosphorylation by Pyruvate Kinase:\n - Reactants: 2 Phosphoenolpyruvate (PEP) + 2 ADP.\n - Enzyme: Pyruvate kinase.\n - Products: 2 Pyruvate + 2 ATP.\n - Mechanism: Pyruvate kinase transfers the phosphate group from PEP directly to ADP, producing 2 ATP and two final 3-carbon pyruvate molecules.\n\n\n# Pyruvate Oxidation and the Formation of Acetyl Coenzyme A\n\n- Transition from Cytosol to Mitochondrion:\n - In eukaryotic cells under aerobic conditions, pyruvate produced during glycolysis in the cytosol crosses the outer mitochondrial membrane and is actively transported across the inner mitochondrial membrane into the mitochondrial matrix.\n\n- Pyruvate Dehydrogenase Multienzyme Complex:\n - The oxidative decarboxylation of pyruvate is catalyzed in the matrix by the multienzyme pyruvate dehydrogenase complex.\n - Three Sequential Steps:\n 1. Decarboxylation: The carboxyl group (-\text{COO}^-$) of pyruvate (3C) is cleaved off and released as gaseous$\text{CO}_2.\n 2. Dehydrogenation (Oxidation): The remaining two-carbon hydroxyethyl group is oxidized to an acetyl group (\text{CH}_3\text{CO}-$), transferring electrons and protons to$\text{NAD}^+ to form NADH:\n       \text{NAD}^+ + 2\,\text{H}^+ + 2\,e^- \rightarrow \text{NADH} + \text{H}^+\n 3. Attachment of Coenzyme A: The acetyl fragment is attached to the sulfur atom of Coenzyme A (CoA-SH) via an unstable, high-energy thioester bond (\sim\text{S}), producing Acetyl Coenzyme A (Acetyl CoA).\n - Stoichiometry per Glucose (2 Pyruvate):\n    2\,\text{Pyruvate} + 2\,\text{CoA} + 2\,\text{NAD}^+ \rightarrow 2\,\text{Acetyl CoA} + 2\,\text{CO}_2 + 2\,\text{NADH} + 2\,\text{H}^+\n\n\n\n\n# The Citric Acid Cycle (Krebs Cycle / TCA Cycle)\n\n- Matrix Localization and Cyclical Nature:\n - Occurs entirely within the mitochondrial matrix.\n - Consists of eight enzyme-catalyzed steps forming a closed cycle, where oxaloacetate (4C) is continuously consumed in the first reaction and regenerated in the final reaction.\n - Requires two complete turns of the cycle to fully catabolize the 2 acetyl CoA molecules derived from one glucose molecule.\n\n- The Eight Enzymatic Reactions of the Citric Acid Cycle:\n - Step 1: Formation of Citrate:\n - Reactants: Acetyl CoA (2C) + Oxaloacetate (4C) + \text{H}_2\text{O}.\n - Enzyme: Citrate synthase.\n - Products: Citrate (6C) + CoA-SH.\n - Mechanism: Cleavage of the unstable thioester bond provides free energy to condense the acetyl group with oxaloacetate, forming citrate (a tricarboxylic acid).\n - Step 2: Isomerization to Isocitrate:\n - Reactant: Citrate.\n - Enzyme: Aconitase.\n - Product: Isocitrate (6C).\n - Mechanism: Dehydration removes a molecule of water to create an intermediate (cis-aconitate), followed immediately by rehydration to form isocitrate.\n - Step 3: First Oxidative Decarboxylation:\n - Reactants: Isocitrate + \text{NAD}^+$.

  • Enzyme: Isocitrate dehydrogenase.

  • Products: $\alpha$-Ketoglutarate (5C) + $\text{CO}_2$ + NADH + \text{H}^+$.\n - Mechanism: Isocitrate is dehydrogenated and decarboxylated to produce the 5-carbon dicarboxylic acid \alpha-ketoglutarate.\n - Step 4: Second Oxidative Decarboxylation:\n - Reactants: \alpha$-Ketoglutarate + CoA-SH +$\text{NAD}^+$.

  • Enzyme: $\alpha$-Ketoglutarate dehydrogenase multienzyme complex.

  • Products: Succinyl CoA (4C) + $\text{CO}_2$ + NADH + \text{H}^+$.\n - Mechanism: Dehydrogenation and decarboxylation analogous to the pyruvate dehydrogenase reaction, forming an unstable thioester bond with CoA.\n - Step 5: Substrate-Level Phosphorylation:\n - Reactants: Succinyl CoA + GDP + \text{P}_i.\n - Enzyme: Succinyl-CoA synthetase.\n - Products: Succinate (4C) + GTP + CoA-SH.\n - Mechanism: Hydrolysis of the high-energy thioester bond powers phosphorylation of GDP to GTP. GTP readily transfers its terminal phosphate to ADP to generate ATP:\n      \text{GTP} + \text{ADP} \rightleftharpoons \text{GDP} + \text{ATP}\n - Step 6: Dehydrogenation of Succinate:\n - Reactants: Succinate + FAD.\n - Enzyme: Succinate dehydrogenase (embedded in the inner mitochondrial membrane as Complex II).\n - Products: Fumarate (4C) + \text{FADH}_2.\n - Mechanism: Two hydrogens are transferred directly to enzyme-bound FAD, reducing it to \text{FADH}_2.\n - Step 7: Hydration of Fumarate:\n - Reactants: Fumarate + \text{H}_2\text{O}.\n - Enzyme: Fumarase.\n - Product: Malate (4C).\n - Mechanism: Addition of water across the carbon-carbon double bond of fumarate forms malate.\n - Step 8: Dehydrogenation of Malate:\n - Reactants: Malate + \text{NAD}^+$.

  • Enzyme: Malate dehydrogenase.

  • Products: Oxaloacetate (4C) + NADH + \text{H}^+$.\n - Mechanism: Malate is oxidized to regenerate oxaloacetate, which is then ready to react with another incoming acetyl CoA.\n\n- Cumulative Output of the Citric Acid Cycle per Glucose (Two Turns):\n - 4 molecules of \text{CO}2.\n - 6 molecules of NADH.\n - 2 molecules of \text{FADH}_2.\n - 2 molecules of ATP (via substrate-level phosphorylation).\n\n\n\n\n# The Mitochondrial Electron Transport Chain and Chemiosmosis\n\n- Components of the Inner Mitochondrial Membrane ETC:\n - Complex I (NADH-ubiquinone oxidoreductase):\n - Accepts electrons from matrix NADH, transferring them via flavin mononucleotide (FMN) and iron-sulfur (\text{Fe-S}$) centers to ubiquinone ($\text{Q}).\n - Pumps 4 protons (\text{H}^+) from the matrix into the intermembrane space per electron pair.\n - Complex II (Succinate-ubiquinone reductase):\n - Accepts electrons from the oxidation of succinate via \text{FADH}_2$, passing them through$\text{Fe-S}$centers directly to ubiquinone ($\text{Q}).\n - Does not pump protons across the membrane because of the low free-energy drop.\n - Ubiquinone (Coenzyme Q):\n - Small, nonpolar, highly mobile lipid carrier residing in the hydrocarbon core of the inner membrane; shuttles electrons from Complex I and Complex II to Complex III.\n - Complex III (Ubiquinone-cytochrome *c* oxidoreductase):\n - Accepts electrons from reduced ubiquinol (\text{QH}_2$) and transfers them via cytochrome *b*, an$\text{Fe-S}$protein, and cytochrome *$c_1* to mobile cytochrome *c*.\n - Pumps 4 protons (\text{H}^+) into the intermembrane space per electron pair.\n - Cytochrome *c*:\n - Soluble peripheral hemeprotein associated with the outer surface of the inner membrane; transfers electrons individually from Complex III to Complex IV.\n - Complex IV (Cytochrome *c* oxidase):\n - Accepts electrons from cytochrome *c*, transferring them through cytochrome *a* and cytochrome *a_3$* copper centers to molecular oxygen ($\text{O}_2).\n - Reduces oxygen to water:\n      4\,\text{cyt } c^{2+} + 4\,\text{H}^+ + \text{O}_2 \rightarrow 4\,\text{cyt } c^{3+} + 2\,\text{H}_2\text{O}\n - Pumps 2 protons (\text{H}^+) into the intermembrane space per electron pair.\n\n\n\n- The Proton Gradient and Chemiosmosis:\n - As electrons pass exergonically down the redox chain (from high negative redox potentials toward \text{O}_2$at$+0.82\,\text{volts}), Complexes I, III, and IV pump protons from the matrix into the intermembrane space.\n - Creates a steep electrochemical gradient (\Delta\mu{\text{H}^+}):\n - Intermembrane space exhibits a lower pH (high [\text{H}^+]) and net positive electrical charge.\n - Mitochondrial matrix exhibits a higher pH (lower [\text{H}^+]) and net negative charge.\n - Complex V (ATP Synthase):\n - Protons flow down their electrochemical gradient from the intermembrane space back into the matrix only through the central channel of ATP synthase.\n - The proton-motive force causes rotation of the F_0$ring and attached central axle ($\gamma$subunit), inducing cyclical catalytic conformational changes in the$F_1$head ($\alpha_3\beta_3$) that synthesize ATP from ADP and$\text{P}i.\n\n\n\n\n# Overall Energy Balance Sheet and Shuttle Systems\n\n- The Mitochondrial Membrane Shuttle Systems:\n - The inner mitochondrial membrane is completely impermeable to cytoplasmic NADH produced during glycolysis.\n - Shuttle mechanisms transfer reducing equivalents (electron pairs) into the matrix:\n - Malate-Aspartate Shuttle:\n - Predominates in liver, kidney, and heart cells.\n - Transfers electrons from cytosolic NADH through the membrane onto mitochondrial \text{NAD}^+.\n - Matrix NADH enters Complex I, yielding approximately 3 ATP per pair of electrons (yielding 6 ATP from the 2 cytosolic NADH).\n - Glycerol Phosphate Shuttle:\n - Operates in skeletal muscle, brain, and other animal tissues.\n - Transfers electrons onto dihydroxyacetone phosphate to form glycerol-3-phosphate, which transfers electrons directly to FAD/ubiquinone inside the inner membrane.\n - Because electrons bypass Complex I and enter at a lower energy state (Complex II/ubiquinone level), each cytosolic NADH yields only 2 ATP (yielding 4 ATP from the 2 cytosolic NADH).\n\n- Quantitative Balance Sheet for Aerobic Respiration of One Glucose Molecule:\n - Substrate-Level Phosphorylation:\n - Glycolysis: 2 ATP (net).\n - Citric Acid Cycle: 2 ATP.\n - Subtotal Substrate-Level: 4 ATP.\n - Oxidative Phosphorylation (via Electron Transport & Chemiosmosis):\n - Glycolytic 2 NADH: Yields 4 to 6 ATP (depending on the tissue-specific shuttle mechanism).\n - Pyruvate Oxidation 2 NADH: Yields 6 ATP (2 \times 3\,\text{ATP}).\n - Citric Acid Cycle 6 NADH: Yields 18 ATP (6 \times 3\,\text{ATP}).\n - Citric Acid Cycle 2 \text{FADH}_2$: Yields 4 ATP ($2 \times 2\,\text{ATP}).\n - Subtotal Oxidative Phosphorylation: 32 to 34 ATP.\n - Total Theoretical ATP Yield:\n - With glycerol phosphate shuttle: 36 ATP per glucose (4 + 32 = 36).\n - With malate-aspartate shuttle: 38 ATP per glucose (4 + 34 = 38).\n\n\n\n\n# Catabolism of Non-Glucose Nutrients and Metabolic Regulation\n\n- Convergence of Diverse Nutrients into Central Pathways:\n - Carbohydrates:\n - Polysaccharides (glycogen, starch) and disaccharides are hydrolyzed to glucose or other monosaccharides, entering glycolysis at glucose or fructose-6-phosphate.\n - Fats (Lipids):\n - Triacylglycerols are hydrolyzed into glycerol and fatty acids.\n - Glycerol is phosphorylated and converted to G3P, directly entering glycolysis.\n - Fatty acids undergo \beta$-oxidation in the mitochondrial matrix, where successive two-carbon fragments are cleaved off to form acetyl CoA, which enters the Citric Acid Cycle, and$\text{H}_2\text{O} is formed at the ETC.\n - Proteins:\n - Hydrolyzed into individual amino acids.\n - Deamination: Amino groups (-\text{NH}_2$) are removed, yielding toxic ammonia ($\text{NH}_3), which is excreted or converted to urea.\n - The resulting carbon skeletons (keto acids) enter cellular respiration at various strategic points depending on their structure: as pyruvate, acetyl CoA, \alpha-ketoglutarate, succinyl CoA, fumarate, or oxaloacetate.\n\n\n\n- Allosteric Regulation of Cellular Respiration:\n - Phosphofructokinase (PFK) acts as the primary homeostatic pacemaker and master control enzyme of glycolysis.\n - Negative Feedback Inhibition:\n - High levels of ATP allosterically bind to an inhibitory regulatory site on PFK, lowering its affinity for fructose-6-phosphate and slowing down glycolysis when cellular energy is abundant.\n - Citrate (the first product of the Citric Acid Cycle) also allosterically inhibits PFK; accumulation of citrate signals that downstream mitochondrial pathways are saturated, backing up and inhibiting glycolysis.\n - Allosteric Activation (Stimulation):\n - High levels of AMP (adenosine monophosphate, an indicator that ATP has been depleted) bind to PFK, relieving ATP inhibition and strongly activating glycolytic flux to restore cellular energy stores.\n\n\n# Anaerobic Pathways: Anaerobic Respiration and Fermentation\n\n- Anaerobic Respiration:\n - Used by specialized prokaryotes living in environments depleted of oxygen (e.g., waterlogged soils, intestines, stagnant ponds, deep-sea vents).\n - Utilizes a functional electron transport chain and chemiosmosis, but employs an inorganic compound other than \text{O}_2 as the terminal electron acceptor.\n - Inorganic Terminal Acceptors:\n - Nitrate (\text{NO}_3^-$): Reduced to nitrite ($\text{NO}_2^-$) or nitrogen gas ($\text{N}_2).\n - Example reaction: \text{C}_6\text{H}{12}\text{O}_6 + 12\,\text{KNO}_3 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + 12\,\text{KNO}_2 + \text{ATP}.\n - Sulfate (\text{SO}_4^{2-}$): Reduced to hydrogen sulfide ($\text{H}_2\text{S}).\n - Mechanism of ATP generation involves both substrate-level phosphorylation and chemiosmotic oxidative phosphorylation.\n\n- Fermentation Pathways:\n - Anaerobic process occurring in the absence of an electron transport chain and oxygen.\n - Relies solely on substrate-level phosphorylation in glycolysis, yielding a net of only 2 ATP per glucose molecule.\n - Crucial Function: Solves the coenzyme availability crisis by transferring electrons from cytosolic NADH back onto an organic intermediate derived from pyruvate, regenerating \text{NAD}^+ so glycolysis can continue running.\n - Alcohol (Ethanolic) Fermentation:\n - Found in yeasts and certain plant tissues under anaerobic conditions.\n - Step 1: Pyruvate (3C) undergoes decarboxylation by pyruvate decarboxylase, releasing \text{CO}_2 to form acetaldehyde (2C).\n - Step 2: Acetaldehyde is reduced by NADH via alcohol dehydrogenase, forming ethyl alcohol (ethanol, 2C) and regenerating \text{NAD}^+.\n - Net Reaction: \text{Glucose} + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Ethanol} + 2\,\text{CO}_2 + 2\,\text{ATP}.\n - Lactate (Lactic Acid) Fermentation:\n - Occurs in active skeletal muscle cells under strenuous anaerobic exertion and in lactic acid bacteria (e.g., *Lactobacillus* in yogurt production).\n - Reaction: Pyruvate is reduced directly by NADH via lactate dehydrogenase, producing 2-hydroxypropanoic acid (lactate, 3C) and regenerating \text{NAD}^+$without releasing$\text{CO}_2.\n - Net Reaction: \text{Glucose} + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Lactate} + 2\,\text{ATP}$$.