Comprehensive Study Notes on Photosynthesis, Cell-Matrix Interactions, and Mitochondrial Bioenergetics

Evolutionary Stages of Photosynthesis

• First organisms were heterotrophs that survived on organic molecules formed abiotically in the environment.

• Chemoautotrophs evolved to manufacture organic nutrients using energy from inorganic molecules ($\text{CO}_2$ and $\text{H}_2\text{S}$).

• Photoautotrophs initially used $\text{H}_2\text{S}$ as an electron source (e.g., photosynthetic green sulfur bacteria):   $\text{CO}_2 + 2\text{H}_2\text{S} \xrightarrow{\text{light}} (\text{CH}_2\text{O}) + 2\text{S}$

• Oxygenic photoautotrophs (cyanobacteria) evolved to split $\text{H}_2\text{O}$ for electrons, releasing oxygen:   $\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{light}} (\text{CH}_2\text{O}) + \text{O}_2$

• Endosymbiotic theory posits that ancient cyanobacteria were engulfed by a mitochondria-containing proeukaryote, leading to plant and algal cells.

Chloroplast Structure and Pigments

• Outer membrane contains porins and is permeable; thylakoid membrane contains pigments, electron carriers, and ATP-synthesizing enzymes.

• Thylakoid membranes fold into flattened sacs arranged in stacks called grana.

• Stroma contains Calvin cycle enzymes, double-stranded circular DNA, and ribosomes.

• Lumen maintains a high proton concentration.

• Chlorophyll molecules consist of a light-absorbing porphyrin ring with a central magnesium atom and a hydrophobic phytol tail anchored in the thylakoid membrane.

• Secondary pigments include carotenoids (e.g., $\beta\text{-carotene}$); chromoplasts accumulate pigments such as carotene and lycopene to color plant tissues.

• Antenna pigments in photosynthetic units absorb light and pass energy to a central reaction-center chlorophyll.

Light-Dependent Reactions and Photosystems

• Photosystem II (PSII / P680) absorbs light via Light-Harvesting Complex II (LHCII); an oxygen-evolving complex performs photolysis to split water:   $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + \text{O}_2 + 4e^-$

• Plastoquinone (PQ) accepts two electrons and two protons to form plastoquinol ($\text{PQH}_2$), which diffuses through the membrane.

• Cytochrome $b_6f$ receives electrons from $\text{PQH}_2$, engages in a Q cycle translocating 4 $\text{H}^+$ per electron pair into the lumen, and passes electrons to plastocyanin.

• Photosystem I (PSI / P700) receives electrons from plastocyanin, boosts them above the energy level of $\text{NADP}^+$, and passes them through ferredoxin to ferredoxin $\text{NADP}^+$ reductase to produce NADPH.

• Overall non-cyclic electron transport equation:   $2\text{H}_2\text{O} + 2\text{NADP}^+ \rightarrow \text{O}_2 + 2\text{NADPH}$

• Cyclic photophosphorylation redirects high-energy electrons from PSI back to cytochrome $b_6f$ to pump protons without forming NADPH.

• Herbicide actions: Atrazine blocks plastoquinone at PSII; Paraquat competes for PSI electrons at ferredoxin, generating cell-damaging oxygen radicals.

Calvin Cycle and Carbon Fixation Pathways

• $C_3$ Pathway: Rubisco condenses $\text{CO}_2$ with ribulose 1,5-bisphosphate (RuBP) into an unstable 6-carbon intermediate that splits into two 3-phosphoglycerate (PGA) molecules.

• Glyceraldehyde phosphate (GAP): Fixation of six $\text{CO}_2$ produces 12 GAP; GAP is exported to the cytosol for sucrose synthesis or retained in the chloroplast for starch synthesis.

• Rubisco efficiency: Highly abundant enzyme with a low turnover rate; binds $\text{O}_2$ instead of $\text{CO}_2$ during photorespiration, forming glycolate and causing up to a 50% loss of fixed carbon.

• $C_4$ Pathway: Uses PEP carboxylase in mesophyll cells to fix $\text{CO}_2$ into a 4-carbon compound (malate); malate is transported to bundle sheath cells where $\text{CO}_2$ is released directly to Rubisco to prevent photorespiration.

Extracellular Matrix Components and Associated Disorders

• Extracellular Matrix (ECM): Organized extracellular network that provides physical support, determines cell shape, and regulates cellular activity.

• Collagens: Fiber-forming trimers representing 25% of human body protein.

  • Scurvy: Caused by Vitamin C deficiency, an essential coenzyme for collagen synthesis.

  • Ehlers-Danlos syndromes: Caused by defects in collagen structure, leading to hyperflexible joints and extensible skin.

  • Osteogenesis imperfecta: Fragile bones and thin skin resulting from type I collagen mutations.

  • Cartilage deformities/dwarfism: Associated with type II collagen mutations.

  • Alport syndrome: Linked to mutations in type IV collagen genes.

• Fibronectin: Dimers with multiple binding domains that link ECM components to cell surface receptors and guide embryonic cell migration.

• Laminin: Trimeric glycoproteins that strengthen the basement membrane and guide neural outgrowth.

• Matrix Metalloproteinases (MMPs): Enzymes responsible for degrading ECM components during tissue remodeling, cell migration, wound healing, and angiogenesis.

Integrins and Cell-Matrix Interactions

• Integrin structure: Transmembrane heterodimers containing $\alpha$ and $\beta$ subunits that connect the cell interior to the ECM.

• Conformation: Inactive integrins maintain a bent structure; ligand binding or cytoplasmic signal induces an upright, active conformation linked to actin via talin.

• RGD Motif: Specific arginine-glycine-aspartic acid sequence present on ECM ligands recognized by integrin receptors.

• Inside-out signaling: Intracellular signals increase integrin extracellular affinity (e.g., platelet $\alpha_{IIb}\beta_3$ integrin binding fibrinogen during blood clotting).

• Outside-in signaling: Extracellular ligand binding induces cytoplasmic conformational shifts altering cell behavior.

• Antithrombotic therapeutics: Synthetic peptides like Aggrastat and Integrelin mimic RGD structure to prevent blood clotting.

Mitochondrial Structure and Metabolic Pathways

• Mitochondrial structure: Double-membrane organelle with a porous outer membrane, a folded inner membrane (cristae), an intermembrane space, and a matrix containing circular DNA, ribosomes, and TCA cycle enzymes.

• Pyruvate Oxidation: Pyruvate enters the matrix and is converted by pyruvate dehydrogenase:   $\text{Pyruvate} + \text{HS-CoA} + \text{NAD}^+ \rightarrow \text{acetyl-CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+$

• Tricarboxylic Acid (TCA) Cycle: Acetyl-CoA condenses with oxaloacetate to form citrate; each cycle turn yields 2 $\text{CO}_2$, 3 $\text{NADH}$, 1 $\text{FADH}_2$, and 1 GTP/ATP.

• Fast-twitch muscle fibers: Low mitochondrial content; generate ATP rapidly through anaerobic glycolysis; susceptible to fatigue from glycogen depletion and lactic acid buildup.

• Slow-twitch muscle fibers: Rich in mitochondria; generate ATP through sustained aerobic metabolism utilizing fatty acid oxidation.

Electron Transport Chain and Oxidative Phosphorylation

• Electron Carriers: Flavoproteins (FMN/FAD, transfer 2 $\text{H}^+$ and 2 $e^-$), Cytochromes (heme iron, transfer 1 $e^-$), Copper atoms (transfer 1 $e^-$), Ubiquinone (Coenzyme Q, lipid-soluble 2 $\text{H}^+$/2 $e^-$ carrier), and Iron-Sulfur centers (transfer 1 $e^-$).

• Complex I (NADH Dehydrogenase): Accepts electrons from NADH and pumps 4 $\text{H}^+$ into the intermembrane space.

• Complex II (Succinate Dehydrogenase): Direct TCA enzyme that transfers electrons from $\text{FADH}_2$ to ubiquinone without proton pumping.

• Complex III (Cytochrome $bc_1$): Translocates 4 $\text{H}^+$ via the Q cycle and transfers electrons to cytochrome c.

• Complex IV (Cytochrome c Oxidase): Reduces $\text{O}_2$ to $\text{H}_2\text{O}$, consuming 4 matrix protons and pumping 4 $\text{H}^+$ across the membrane; inhibited by cyanide.

• Proton-Motive Force: Generates an electrochemical gradient ($-180\,\text{mV}$) driving ATP synthesis via chemiosmosis.

• Uncouplers: 2,4-dinitrophenol (DNP) dissipates the proton gradient, collapsing ATP synthesis while dissipating energy as heat.

• ATP Synthase ($F_0F_1$): $\text{H}^+$ passage through the $F_0$ c-ring rotates the central $\gamma$ stalk, inducing sequential Open (O), Loose (L), and Tight (T) conformational changes in the $F_1$ $\alpha_3\beta_3$ catalytic head to produce ~30 ATP per glucose molecule.

Peroxisomes and Mitochondrial Pathologies

• Peroxisomes: Single-membraned organelles with crystalline enzyme cores; perform $\beta$-oxidation of long-chain fatty acids, synthesize plasmalogens, and decompose $\text{H}_2\text{O}_2$ using catalase.

• Mitochondrial DNA (mtDNA): Encodes 13 electron transport chain subunits; exhibits a 10-fold higher mutation rate than nuclear DNA due to limited repair systems and ROS exposure.

• Inheritance and Heteroplasmy: mtDNA is maternally inherited; cells contain a mix of wild-type and mutant mtDNA (heteroplasmy), producing variable clinical severity in mitochondrial disorders.