Comprehensive Biology Study Notes: Photosynthesis, Respiration, Fermentation, Cell Cycle, and Cell Junctions

Principles of Energy Flow and Autotrophic Nutrition

• Ecological Energy Dynamics:

  • Solar radiation serves as the ultimate energetic foundation for virtually all biological ecosystems on Earth.

  • Autotrophs ("self-feeders") synthesize organic macromolecules directly from inorganic inorganic substrates, harvesting solar or chemical energy.

  • Heterotrophs ("other-feeders") cannot manufacture their own organic food and survive by consuming autotrophs or other heterotrophic organisms, either directly or indirectly.

  • Photosynthesis converts solar radiant energy into stable chemical energy stored within covalent bonds of carbohydrates.

• Diversity of Photoautotrophic Organisms:

  • Photosynthesis is utilized across diverse evolutionary lineages, encompassing prokaryotic and eukaryotic taxa:

  • Plants: Terrestrial multicellular photoautotrophs containing complex tissues.

  • Multicellular Algae: Aquatic photosynthetic eukaryotes such as macroscopic brown, red, and green seaweeds.

  • Unicellular Eukaryotes: Aquatic protists including flagellates and diatoms.

  • Cyanobacteria: Oxygenic photosynthetic prokaryotes possessing internal thylakoid membranes.

  • Purple Sulfur Bacteria: Anoxygenic photosynthetic prokaryotes that utilize electron donors other than water.

• Contrasting Photosynthesis and Cellular Respiration:

  • Photosynthesis is an endergonic, anabolic process that consumes light energy to reduce carbon dioxide into carbohydrates:

  • Overall stoichiometric equation: $6\text{ CO}_2 + 6\text{ H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{ O}_2$

  • Cellular Respiration is an exergonic, catabolic process that oxidizes organic sugars into carbon dioxide, liberating usable energy:

  • Overall stoichiometric equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{ O}_2 \rightarrow 6\text{ CO}_2 + 6\text{ H}_2\text{O} + \text{ATP}$

Photosynthesis: Chloroplast Anatomy and Light Capture

• Chloroplast Ultrastructure:

  • In eukaryotic photosynthetic organisms, photosynthesis takes place within specialized double-membrane organelles called chloroplasts, located primarily inside the interior leaf mesophyll tissue.

  • Structural components include:

  • Outer Membrane and Inner Membrane: Separated by an intermembrane space, regulating the transport of metabolites.

  • Stroma: The semi-fluid interior compartment situated between the thylakoid system and the inner chloroplast membrane, containing soluble enzymes for carbon reduction.

  • Thylakoids: Flattened, interconnected membranous sacs where light reactions occur.

  • Grana (singular: granum): Dense, columnar stacks of individual thylakoid discs.

  • Thylakoid Lumen (Thylakoid Space): The continuous interior aqueous compartment enclosed by the thylakoid membrane.

• Stomatal Regulation and Gas Exchange:

  • Leaf surfaces are perforated with microscopic epidermal apertures termed stomata (singular: stoma).

  • Each stoma consists of a central microscopic pore flanked by two specialized epidermal guard cells.

  • Gaseous Diffusion:

  • Atmospheric $\text{CO}_2$ enters the leaf interior across stomata, diffusing through extracellular spaces to photosynthetic mesophyll cells.

  • Molecular oxygen ($\text{O}_2$) generated inside mesophyll cells diffuses outward through stomatal pores.

  • Osmotic Mechanism of Guard Cells:

  • Active influx of potassium ions ($\text{K}^+$) drives osmotic water movement into the guard cell vacuoles, increasing turgor pressure and bowing the guard cells outward to open the stoma.

  • Efflux of $\text{K}^+$ prompts osmotic water loss from the vacuoles, deflating guard cells and closing the stoma to conserve water.

  • Transpiration Cost:

  • For every single molecule of $\text{CO}_2$ gained through open stomata, approximately $400$ molecules of $\text{H}_2\text{O}$ are lost to the atmosphere via transpiration.

• The Dual Physical Nature of Light Energy:

  • Light represents a form of electromagnetic radiation, behaving simultaneously as continuous waves and discrete particles.

  • Photons: Discrete packets of energy lacking mass; each photon contains a fixed quantum of energy inversely proportional to its wavelength.

  • Wavelength ($\lambda$): The linear distance between successive wave crests.

  • Electromagnetic Spectrum:

  • Spans from high-energy, short wavelengths like gamma rays ($10^{-5}\,\text{nm}$) and X-rays ($10^{-3}\,\text{nm}$), through ultraviolet radiation ($10^{-1}\,\text{nm}$ to $10^1\,\text{nm}$), to visible light ($400\,\text{nm}$ to $710\,\text{nm}$), infrared ($10^3\,\text{nm}$ to $10^5\,\text{nm}$), microwaves ($10^7\,\text{nm}$ to $10^9\,\text{nm}$), and radio waves ($10^{11}\,\text{nm}$ to $10^{13}\,\text{nm}$).

  • Shorter wavelengths possess greater energy per photon than longer wavelengths.

• Photosynthetic Pigments and Spectra:

  • When photons strike matter, they can be absorbed, transmitted, or reflected; pigments are chemical compounds that absorb only specific wavelengths of visible light.

  • Primary Classes of Pigments:

  • Chlorophylls (chlorophyll $a$ and chlorophyll $b$): The dominant photosynthetic pigments; absorb violet-blue and red light while transmitting/reflecting green wavelengths.

  • Carotenoids: Accessory pigments that absorb blue and green wavelengths while transmitting yellow, orange, or red light; include $\beta\text{-carotene}$, lutein, zeaxanthin, and lycopene.

  • Chemical Structure of Chlorophyll:

  • Porphyrin Head: A planar, light-absorbing ring system coordinated around a central magnesium atom ($\text{Mg}$); chlorophyll $a$ possesses a methyl group ($-\text{CH}_3$), whereas chlorophyll $b$ features a formyl group ($-\text{CHO}$).

  • Hydrophobic Tail: An extended lipid tail composed of repeating isoprene subunits anchoring the pigment within thylakoid membranes.

  • Absorption vs. Action Spectrum:

  • An absorption spectrum plots light absorbance across wavelengths ($400\,\text{nm}$ to $700\,\text{nm}$) for purified pigment extracts.

  • An action spectrum plots the operational rate of photosynthesis (measured as relative $\text{O}_2$ output) as a function of incident wavelength.

The Light-Dependent Reactions and Photophosphorylation

• Photon Absorption and Electron Energetics:

  • When a chlorophyll molecule absorbs a photon, energy transfers to an electron within the porphyrin head, elevating it to an orbital of higher potential energy:

  • Red Photons: Contain lower energy, promoting electrons to excitation state 1.

  • Blue Photons: Contain higher energy, elevating electrons to a higher excitation state 2.

  • Green Photons: Possess intermediate energy levels not readily absorbed by chlorophyll, causing them to be reflected or transmitted.

• Alternative Pathways for Excited Electron Energy:

  • Fluorescence / Thermal Loss: The electron drops back to ground state, emitting lower-energy light (fluorescence) and heat.

  • Resonance Energy Transfer: Excitation energy passes non-radiatively between adjacent antenna pigment molecules.

  • Reduction-Oxidation (Redox): The excited electron transfers permanently to an electron acceptor in the reaction center.

• Photosystem Architecture and Organization:

  • Pigments organize within the thylakoid membrane into multi-protein supercomplexes termed photosystems:

  • Antenna Complex: Surrounding cluster of chlorophyll and accessory pigment molecules that gather photons and channel energy inward via resonance transfer.

  • Reaction Center: Central pair of specialized chlorophyll molecules that undergo photo-oxidation, transferring an excited high-energy electron to a primary electron acceptor.

  • Two Functional Photosystems:

  • Photosystem II (PSII): Contains reaction-center chlorophyll with an absorption maximum at $680\,\text{nm}$ (designated P680).

  • Photosystem I (PSI): Contains reaction-center chlorophyll with an absorption maximum at $700\,\text{nm}$ (designated P700).

  • Spatial Segregation in Chloroplasts:

  • Photosystem II concentrates in the interior, appressed membrane domains of stacked grana lamellae.

  • Photosystem I and ATP synthase complexes localize in non-appressed, unstacked stroma lamellae facing the stroma.

• Photosystem II Mechanism and Photolysis:

  • Photon absorption triggers P680 photo-oxidation, donating an electron to the primary electron acceptor pheophytin.

  • Reduced pheophytin transfers the electron to plastoquinone (PQ), a mobile, lipid-soluble quinone carrier.

  • Proton Translocation:

  • As PQ accepts two electrons, it picks up two protons ($\text{H}^+$) from the stroma, diffuses across the thylakoid membrane, and delivers electrons to the cytochrome complex.

  • The protons are released directly into the thylakoid lumen, generating a strong trans-thylakoid proton gradient.

  • Photolysis of Water:

  • The oxidized ${\text{P680}}^+$ radical becomes electronegative enough to extract electrons from water.

  • Enzymes in PSII catalyze water oxidation, releasing protons into the lumen and evolving diatomic oxygen:       $2\text{ H}_2\text{O} \rightarrow 4\text{ H}^+ + 4\text{ e}^- + \text{O}_2$

• Photophosphorylation via Chemiosmosis:

  • Proton accumulation from water splitting and PQ pumping establishes an electrochemical proton-motive force, creating a low-pH interior thylakoid lumen relative to the stroma.

  • Protons flow down this electrochemical gradient through ATP synthase, driving the photophosphorylation of adenosine diphosphate ($\text{ADP}$) and inorganic phosphate ($\text{P}_i$) into adenosine triphosphate ($\text{ATP}$) on the stromal side.

• Photosystem I Mechanism and NADPH Production:

  • Low-energy electrons exiting the cytochrome complex transfer via plastocyanin (Pc) to reduce the oxidized reaction center of Photosystem I (P700).

  • Incident photons excite P700, passing electrons through an internal electron transport chain to the iron-sulfur protein ferredoxin (Fd).

  • The enzyme NADP+ reductase transfers electrons from ferredoxin to terminal electron acceptor $\text{NADP}^+$, producing stromal $\text{NADPH}$:     $\text{NADP}^+ + \text{H}^+ + 2\text{ e}^- \rightarrow \text{NADPH}$

  • Summary of Products:

  • PSII supplies the proton-motive force for ATP synthesis.

  • PSI supplies high-potential reducing power in the form of NADPH.

The Calvin Cycle and Carbon Fixation Mechanisms

• Calvin Cycle Overview:

  • Occurs within the chloroplast stroma independently of direct illumination, utilizing the ATP and NADPH synthesized during light reactions to reduce atmospheric $\text{CO}_2$ into carbohydrates.

  • Three Functional Phases:

  1. Phase 1: Carbon Fixation:

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

     • The reaction produces a transient six-carbon intermediate that immediately cleaves into two molecules of three-carbon 3-phosphoglycerate (3-PGA).

     • Net stoichiometry for three turns:          $3\text{ RuBP} + 3\text{ CO}_2 \rightarrow 6\text{ 3-PGA}$

  2. Phase 2: Carbon Reduction:

     • Each 3-PGA molecule is phosphorylated by ATP to yield 1,3-bisphosphoglycerate (consuming $6\text{ ATP}$ across six molecules of 3-PGA).

     • 1,3-bisphosphoglycerate is reduced by NADPH, releasing inorganic phosphate to produce glyceraldehyde 3-phosphate (G3P) (consuming $6\text{ NADPH}$).

     • Of the $6\text{ G3P}$ produced, $1\text{ G3P}$ represents net product exported toward hexose synthesis, while $5\text{ G3P}$ remain in the cycle.

  3. Phase 3: Regeneration of RuBP:

     • The remaining five molecules of G3P undergo complex enzymatic rearrangements requiring three additional molecules of ATP to regenerate three molecules of RuBP:          $5\text{ G3P} + 3\text{ ATP} \rightarrow 3\text{ RuBP} + 3\text{ ADP} + 2\text{ P}_i$

• Enzymology and Inefficiency of Rubisco:

  • Carbon fixation is catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco), present in all Calvin cycle photoautotrophs.

  • Structural Characteristics: Composed of $16$ polypeptide subunits containing a total of $8$ catalytic active sites.

  • Catalytic Competition and Photorespiration:

  • Rubisco active sites can bind both $\text{CO}_2$ and $\text{O}_2$.

  • Carboxylation (Photosynthesis): Adds $\text{CO}_2$ to RuBP, generating two molecules of 3-PGA for the Calvin cycle.

  • Oxygenation (Photorespiration): Adds $\text{O}_2$ to RuBP, yielding one molecule of 3-PGA and one molecule of toxic two-carbon 2-phosphoglycolate:       $\text{RuBP} + \text{O}_2 \rightarrow \text{3-PGA} + \text{2-phosphoglycolate}$

  • Salvaging 2-phosphoglycolate consumes additional ATP and releases previously fixed $\text{CO}_2$, significantly diminishing photosynthetic yield.

• Calvin Cycle Stoichiometric Requirements:

  • Fixing three molecules of $\text{CO}_2$ requires $3$ turns of the Calvin cycle, synthesizing $1$ net molecule of G3P.

  • Total energetic cost per net G3P exported: $9\text{ ATP}$ and $6\text{ NADPH}$.

  • Synthesizing a single six-carbon glucose molecule ($\text{C}_6\text{H}_{12}\text{O}_6$) requires $6$ turns of the cycle ($6\text{ CO}_2$ fixed), utilizing $18\text{ ATP}$ and $12\text{ NADPH}$.

Cellular Respiration: Overview and Glycolytic Pathway

• Cellular Respiration and Bioenergetics:

  • Biological energy capture occurs through ordered catabolic sequences that gradually oxidize organic fuels into metabolic water and carbon dioxide.

  • Terminology:

  • Metabolism: From Greek metabolē ("to change"); the sum total of all controlled biochemical reactions in an organism.

  • Oxidation: The removal of one or more electrons from an atom or chemical group.

  • Hydrolysis: Cleaving chemical covalent bonds via the stoichiometric addition of a water molecule ($\text{H}_2\text{O}$).

  • Phosphorylation: The covalent attachment of an inorganic phosphate group ($\text{PO}_4^{3-}$) to a target molecule.

  • Cellular Respiration vs. Breathing:

  • Cellular respiration refers strictly to cellular ATP generation from nutrient oxidation.

  • Breathing represents macroscopic organismal ventilation and gas exchange across respiratory surfaces.

• Adenosine Triphosphate (ATP) Structure:

  • Consists of the purine base adenine, a ribose pentose sugar, and a chain of three covalently linked phosphate groups.

  • Free energy release occurs via hydrolysis of the phosphoanhydride (pyrophosphate) bond bridging the second ($\beta$) and third ($\gamma$) phosphate groups:     $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

• The Four Stages of Aerobic Respiration:

  1. Glycolysis (cytosol)

  2. Pyruvate oxidation / breakdown to an acetyl group (mitochondrial matrix)

  3. Citric acid cycle (mitochondrial matrix)

  4. Oxidative phosphorylation (inner mitochondrial membrane)

• Glycolysis (Embden-Meyerhof-Parnas Pathway):

  • From glyco ("sugar") and lysis ("breaking"); a sequence of ten enzyme-catalyzed reactions localized exclusively in the cytosol.

  • Operates under both aerobic and anaerobic conditions without requiring cellular organelles.

  • Phase 1: Energy Investment Phase (Steps 1–3):

  • Hexose activation requires investing two molecules of ATP.

  • Glucose is phosphorylated to glucose-6-phosphate, isomerized to fructose-6-phosphate, and phosphorylated to fructose-1,6-bisphosphate.

  • Phase 2: Cleavage Phase (Steps 4–5):

  • Fructose-1,6-bisphosphate ($6\text{C}$) is cleaved enzymatically into two interconvertible three-carbon molecules of glyceraldehyde-3-phosphate (G3P).

  • Phase 3: Energy Liberation / Harvest Phase (Steps 6–10):

  • Each G3P molecule is oxidized and phosphorylated by $\text{NAD}^+$ and inorganic phosphate, yielding NADH and high-energy acyl phosphates.

  • Substrate-level phosphorylation synthesizes four total ATP molecules ($2\text{ ATP}$ per three-carbon unit).

  • Converts the carbon skeleton into two molecules of pyruvate ($3\text{C}$).

  • Net Balance Sheet per Glucose:

  • Inputs: $1\text{ Glucose}$, $2\text{ ATP}$, $2\text{ NAD}^+$, $4\text{ ADP}$, $2\text{ P}_i$.

  • Outputs: $2\text{ Pyruvate}$, $2\text{ Net ATP}$ ($4\text{ synthesized} - 2\text{ invested}$), $2\text{ NADH}$, $2\text{ H}^+$, $2\text{ H}_2\text{O}$.

Pyruvate Oxidation and the Citric Acid Cycle

• Pyruvate Translocation and Oxidation:

  • Pyruvate produced via glycolysis is imported from the cytosol across the mitochondrial outer and inner membranes into the mitochondrial matrix.

  • Translocation occurs via an outer membrane channel and a specialized $\text{H}^+/\text{pyruvate}$ symporter on the inner mitochondrial membrane.

  • Pyruvate Dehydrogenase Multienzyme Complex:

  • Catalyzes oxidative decarboxylation of pyruvate ($3\text{C}$) into an acetyl group ($2\text{C}$) and $\text{CO}_2$.

  • The acetyl moiety attaches to Coenzyme A (CoA) via a high-energy thioester bond, forming Acetyl CoA ($\text{CH}_3\text{CO-S-CoA}$).

  • High-energy electrons reduce $\text{NAD}^+$ to NADH.

  • Net equation per pyruvate:       $\text{Pyruvate} + \text{CoA-SH} + \text{NAD}^+ \rightarrow \text{Acetyl CoA} + \text{CO}_2 + \text{NADH}$

• The Citric Acid Cycle (Krebs Cycle / Tricarboxylic Acid Cycle):

  • An eight-step circular metabolic pathway in the mitochondrial matrix that oxidizes acetyl groups derived from carbohydrates, fats, and proteins.

  • Enzymatic Progression:

  • Step 1 (Citrate Synthase): The two-carbon acetyl group of Acetyl CoA combines with four-carbon oxaloacetate ($4\text{C}$) to yield six-carbon citrate ($6\text{C}$) and liberate free CoA-SH.

  • Step 2 (Aconitase): Citrate undergoes a two-step isomerization through dehydration and rehydration to form its isomer isocitrate ($6\text{C}$).

  • Step 3 (Isocitrate Dehydrogenase): Isocitrate is oxidized to $\alpha$-ketoglutarate ($5\text{C}$), releasing one molecule of $\text{CO}_2$ and reducing $\text{NAD}^+$ to NADH.

  • Step 4 ($\alpha$-Ketoglutarate Dehydrogenase): $\alpha$-ketoglutarate combines with CoA-SH and oxidizes to succinyl CoA ($4\text{C}$), liberating a second $\text{CO}_2$ and producing NADH.

  • Step 5 (Succinyl-CoA Synthetase): The exergonic cleavage of succinyl CoA into succinate ($4\text{C}$) drives substrate-level phosphorylation of guanosine diphosphate (GDP) to guanosine triphosphate (GTP); GTP transfers its terminal phosphate to ADP to yield ATP.

  • Step 6 (Succinate Dehydrogenase): Succinate is oxidized to fumarate ($4\text{C}$), while flavin adenine dinucleotide (FAD) acts as electron acceptor, reducing to $\text{FADH}_2$.

  • Step 7 (Fumarase): Fumarate combines with a molecule of water ($\text{H}_2\text{O}$) in a stereospecific hydration reaction to produce malate ($4\text{C}$).

  • Step 8 (Malate Dehydrogenase): Malate is oxidized to regenerate oxaloacetate ($4\text{C}$), reducing a final $\text{NAD}^+$ to NADH, enabling the cycle to repeat.

• Summary Yield of the Citric Acid Cycle:

  • Per Acetyl CoA entering the cycle: $3\text{ NADH}$, $1\text{ FADH}_2$, $1\text{ ATP}$ (via GTP), $2\text{ CO}_2$.

  • Per starting glucose molecule ($2\text{ Acetyl CoA}$): $6\text{ NADH}$, $2\text{ FADH}_2$, $2\text{ ATP}$, $4\text{ CO}_2$.

Oxidative Phosphorylation, Electron Transport, and Chemiosmosis

• Components of the Mitochondrial Electron Transport Chain (ETC):

  • Integral protein complexes localized within the inner mitochondrial membrane execute sequential redox reactions, passing electrons from reduced coenzymes to oxygen.

  • Complex I (NADH Dehydrogenase): Oxidizes matrix NADH to $\text{NAD}^+$, releasing protons and passing electrons to the hydrophobic mobile lipid carrier ubiquinone (coenzyme Q); pumps protons ($\text{H}^+$) from the matrix into the intermembrane space.

  • Complex II (Succinate Reductase / Dehydrogenase): Oxidizes $\text{FADH}_2$ to FAD; transfers electrons directly to ubiquinone without pumping protons across the membrane.

  • Complex III (Cytochrome $b\text{-}c_1$ Complex): Accepts electrons from reduced ubiquinone, passes them to peripheral membrane protein cytochrome $c$, and pumps protons ($\text{H}^+$) into the intermembrane space.

  • Complex IV (Cytochrome Oxidase): Accepts electrons from cytochrome $c$, pumps protons ($\text{H}^+$) into the intermembrane space, and transfers electrons to terminal acceptor $\text{O}_2$:     $2\text{ H}^+ + \frac{1}{2}\text{O}_2 + 2\text{ e}^- \rightarrow \text{H}_2\text{O}$

• Chemiosmosis and ATP Synthase:

  • Protons pumped by Complexes I, III, and IV establish a significant electrochemical proton-motive force across the inner mitochondrial membrane, characterized by a high $\text{H}^+$ concentration (lower pH) in the intermembrane space relative to the matrix.

  • ATP Synthase Structure and Operation:

  • Protons flow down their electrochemical gradient back into the matrix through the central channel of ATP synthase.

  • This proton movement drives rotary conformational changes that catalyze the phosphorylation of ADP and $\text{P}_i$ into ATP.

• Overall Net Energetics of Aerobic Respiration:

  • Oxidation of one mole of glucose yields:

  • $6\text{ CO}_2$ ($2$ from pyruvate oxidation, $4$ from the citric acid cycle)

  • $6\text{ H}_2\text{O}$ (formed at Complex IV)

  • $\sim 30\text{ to }34\text{ ATP}$ (combining substrate-level phosphorylation and oxidative phosphorylation)

Anaerobic Energy Harvesting: Fermentation Pathways

• Evolutionary Significance of Glycolysis:

  • Glycolysis is considered the universal, most primitive metabolic pathway for biological energy harvesting.

  • Its ancient origins are supported by two primary features:

  • Its presence in all three domains of biological life (Archaea, Bacteria, Eukarya).

  • Its cytosolic localization, utilizing soluble enzymes without requiring oxygen or membrane-bound organelles.

• Principles of Fermentation:

  • Under anaerobic conditions, lack of a terminal electron acceptor ($\text{O}_2$) stalls the mitochondrial electron transport chain, blocking matrix NADH re-oxidation.

  • Without $\text{NAD}^+$, glycolysis arrests at step 6, halting cellular ATP synthesis.

  • Fermentation provides an alternative cytosolic mechanism to oxidize NADH back to $\text{NAD}^+$, allowing glycolysis to continue generating $2\text{ ATP}$ per glucose via substrate-level phosphorylation.

• Lactic Acid Fermentation:

  • Utilized by animal skeletal muscle cells during intense exertion and by various bacteria used in dairy production (e.g., yogurt, cheese).

  • Reaction Mechanism:

  • Pyruvate is reduced directly by NADH, forming lactate without releasing carbon dioxide:       $\text{Pyruvate} + \text{NADH} + \text{H}^+ \rightleftharpoons \text{Lactate} + \text{NAD}^+$

  • Physiological Recovery: Circulating lactate is transported through the bloodstream to the liver, where it is converted back into pyruvate and subsequently oxidized in mitochondria when oxygen becomes available.

• Alcohol Fermentation:

  • Utilized by yeasts and specific microorganisms for thousands of years in brewing and baking.

  • Reaction Mechanism:

  • Pyruvate ($3\text{C}$) undergoes decarboxylation to produce acetaldehyde ($2\text{C}$), liberating gaseous $\text{CO}_2$.

  • Acetaldehyde is reduced by NADH to form ethanol ($2\text{C}$), regenerating $\text{NAD}^+$:       $\text{Pyruvate} \rightarrow \text{Acetaldehyde} + \text{CO}_2$       $\text{Acetaldehyde} + \text{NADH} + \text{H}^+ \rightarrow \text{Ethanol} + \text{NAD}^+$

• Organismal Adaptations to Oxygen Levels:

  • Obligate Anaerobes: Microorganisms that survive exclusively in oxygen-free environments and are poisoned by molecular oxygen; inhabit deep soils, stagnant ponds, and the animal gut lumen.

  • Facultative Anaerobes: Organisms that can synthesize ATP via aerobic respiration when oxygen is present, but switch to fermentation under anaerobic conditions; include domestic yeasts and many bacterial species.

Fuel Versatility: Catabolism of Alternative Macromolecules

• Macromolecular Substrate Convergence:

  • Although glucose is the primary metabolic reference fuel, cellular respiration oxidizes diverse dietary macromolecules by funneling their catabolic intermediates into glycolysis and the citric acid cycle:

  • Carbohydrates: Starch, glycogen, and disaccharides are hydrolyzed into simple monosaccharides (e.g., glucose, fructose), entering at the start of glycolysis.

  • Fats (Lipids):

    • Triglycerides are hydrolyzed into glycerol and free fatty acids.

    • Glycerol is phosphorylated and converted into glyceraldehyde 3-phosphate (G3P), entering the midpoint of glycolysis.

    • Fatty acids undergo mitochondrial $\beta$-oxidation, splitting into two-carbon units converted directly into Acetyl CoA.

    • Fats contain extensive hydrocarbon chains rich in hydrogen atoms and high-energy electrons, yielding significantly more ATP per gram than carbohydrates.

  • Proteins:

    • Hydrolyzed into individual amino acids.

    • Excess amino acids undergo deamination (removal of their amino groups, released as nitrogenous waste).

    • The remaining carbon skeletons enter cellular respiration as pyruvate, Acetyl CoA, or intermediate acids within the citric acid cycle.

    • Proteins are utilized as fuel as a secondary reserve after available carbohydrates and lipid stores are depleted.

The Eukaryotic Cell Cycle and Molecular Control System

• Biological Purpose and Medical Relevance:

  • Eukaryotic cell division produces two genetically identical daughter cells by duplicating and distributing chromosomes with high fidelity.

  • Decoding the molecular mechanisms regulating cell cycle timing is central to understanding oncology, as cancer cells bypass normal cell cycle controls to divide unrestrainedly.

• Phases of the Cell Cycle:

  • Interphase (roughly $90\%$ of cell lifespan; characterized by cell growth, organelle replication, and metabolic activity):

  1. $\text{G}_1$ Phase ("First Gap"): Intense cellular growth, biosynthesis of structural proteins, and duplication of cytoplasmic organelles.

  2. $\text{S}$ Phase ("Synthesis"): Replication of chromosomal nuclear DNA.

  3. $\text{G}_2$ Phase ("Second Gap"): Final preparations for division, synthesis of mitotic proteins, and duplication of centrosomes.

  • Mitotic (M) Phase:

  • Mitosis: The nuclear division process that separates duplicated chromosomes into two distinct nuclei.

  • Cytokinesis: The physical division of cytoplasm, organelles, and plasma membrane into two distinct daughter cells.

• Chromosome Mechanics and Terminology:

  • Chromosome: A continuous double-stranded DNA molecule packaged with histone proteins.

  • Centromere: The constricted chromosomal region containing specialized repetitive DNA sequences where sister chromatids link most tightly.

  • Sister Chromatids: Duplicated copies of an individual chromosome generated during $\text{S}$ phase, joined along their lengths by cohesin protein complexes.

  • Chromosome Number Invariance:

  • During $\text{S}$ phase DNA replication, the total amount of DNA doubles, but total chromosome count remains constant.

  • Chromatid units are designated as individual "chromosomes" only after sister chromatids physically separate during anaphase.

  • Historical Discovery: In 1882, German anatomist Walther Flemming developed specialized synthetic dyes to visualize chromosomal movements during mitosis and cytokinesis.

• Cell Cycle Checkpoint Control System:

  • Progression through the cell cycle is directed by a molecular control system regulated by internal and external signals.

  • $\text{G}_1$ Checkpoint ("Restriction Point"):

  • The primary decision point in the cell cycle.

  • If a cell receives a molecular go-ahead signal, it completes $\text{G}_1$, enters $\text{S}$ phase, and proceeds through division.

  • In the absence of a go-ahead signal, the cell exits the active cycle and enters $\text{G}_0$, a quiescent non-dividing state (typical of mature neurons and muscle cells).

  • $\text{G}_2$ Checkpoint: Assesses completion and accuracy of DNA replication before authorizing mitotic entry.

  • $\text{M}$ Checkpoint (Spindle Assembly Checkpoint):

  • Operates during prometaphase and metaphase.

  • Halts cycle progression if any chromosome kinetochores remain unattached to spindle microtubules, preventing aneuploidy resulting from nondisjunction.

Stages and Mechanics of Mitotic Division

• The Mitotic Spindle Apparatus:

  • A dynamic macromolecular machine composed of microtubules and associated motor proteins that coordinates chromosome movements during mitosis.

  • Centrosome: The primary microtubule-organizing center (MTOC) of animal cells, containing a pair of centrioles.

  • Asters: Radial arrays of shorter microtubules extending outward from each centrosome toward the cell cortex.

  • Kinetochores: Specialized multiprotein complexes assembled onto centromeric DNA regions; each sister chromatid possesses an outwardly facing kinetochore.

  • Microtubule populations comprise:

  • Kinetochore Microtubules: Spindle fibers that attach directly to chromatid kinetochores, exerting mechanical tension to align and separate them.

  • Nonkinetochore (Polar) Microtubules: Overlapping fibers that push against opposite poles via motor proteins to elongate the dividing cell.

• Sequential Stages of Mitosis:

  1. $\text{G}_2$ of Interphase:

  • Intact nuclear envelope enclosing one or more prominent nucleoli.

  • Duplicated centrosomes situated adjacent to the nucleus.

  • Chromosomes duplicated during $\text{S}$ phase remain uncoiled, uncondensed chromatin threads.

  1. Prophase:

  • Chromatin fibers condense into microscopically visible chromosomes composed of paired sister chromatids.

  • Nucleoli disappear from view.

  • Mitotic spindle begins assembling as centrosomes propel away from each other along polymerizing microtubule tracks.

  1. Prometaphase:

  • Nuclear envelope fragments into vesicular structures.

  • Spindle microtubules invade the nuclear region.

  • Kinetochore microtubules attach to chromatid kinetochores, initiating rapid oscillatory chromosome movements.

  • Nonkinetochore microtubules interact with fibers originating from the opposite spindle pole.

  1. Metaphase:

  • Centrosomes occupy opposite cell poles.

  • Chromosomes align their centromeres along the metaphase plate, an imaginary planar structure equidistant between the spindle poles.

  • Microtubules attached to sister kinetochores connect to opposite spindle poles, generating balanced mechanical tension.

  1. Anaphase:

  • Initiated when cohesin proteins are cleaved, freeing sister chromatids to separate into individual daughter chromosomes.

  • Kinetochore microtubules depolymerize at their kinetochore ends, pulling daughter chromosomes toward opposite poles.

  • Nonkinetochore microtubules polymerize and slide past one another, elongating the cell along the polar axis.

  1. Telophase:

  • Two genetically identical daughter nuclei form within the cell.

  • Nuclear envelopes assemble around each set of separated chromosomes from membrane fragments and endomembrane systems.

  • Nucleoli reappear within each reforming nucleus.

  • Chromatin uncoils and decondenses; mitotic spindle depolymerizes completely.

Cytokinesis in Animal and Plant Systems

• Cytoskeletal Basis of Division:

  • Mitosis divides nuclear genetic content, whereas cytokinesis partitions the cytoplasm and organelles between two new cells.

  • Key Structural Cytoskeletal Elements:

  • Microtubules: Rigid tubular polymers constructed from heterodimers of the protein tubulin.

  • Microfilaments: Thin, flexible helical filaments constructed from globular subunits of the protein actin.

• Animal Cell Cytokinesis: Cleavage Furrow:

  • Occurs via a process known as cleavage.

  • During late anaphase and telophase, a shallow groove termed the cleavage furrow develops in the plasma membrane along the plane of the former metaphase plate.

  • A sub-membranous contractile ring of actin microfilaments associated with motor proteins (myosin) constricts the cell cortex.

  • As the ring tightens, the furrow deepens until it pinches the parent cell into two separate, membrane-enclosed daughter cells.

• Plant Cell Cytokinesis: Cell Plate Formation:

  • Rigid cell walls prevent plant cells from developing a constricting cleavage furrow.

  • During telophase, Golgi apparatus-derived vesicles carrying cell-wall precursor carbohydrates travel along microtubules to the center of the dividing cell.

  • Vesicles coalesce to assemble an expanding membrane-bound disc termed the cell plate within the phragmoplast structure.

  • The cell plate grows outward until its edges fuse with the lateral plasma membrane and parental cell wall, establishing two distinct cells.

Intercellular Junctions and Tissue Architecture

• Tissue Organization:

  • Tissues consist of coordinated assemblies of similar cells performing specialized functions.

  • Multicellular integrity relies on specialized structures that physically anchor adjacent cells, seal tissue layers, and facilitate intercellular communication.

• Tight Junctions (Zonula Occludens):

  • Multiprotein complexes embedded within the plasma membranes of adjacent animal cells that bind tightly together in continuous anastomosing ridges.

  • Physiological Role:

  • Forms a dynamic, watertight seal between epithelial cells, preventing intercellular leakage of fluids and solutes (e.g., across intestinal epithelial barriers).

  • Prevents lateral diffusion of integral membrane proteins between apical and basolateral membrane surfaces.

• Desmosomes (Anchoring Junctions):

  • Punctate mechanical anchoring junctions prevalent in tissues subjected to strong mechanical stress, such as cardiac muscle and stratified squamous epithelium.

  • Structural Components:

  • Transmembrane adhesion proteins belonging to the cadherin family link adjacent plasma membranes in a $\text{Ca}^{2+}$-dependent manner.

  • Intracellular attachment plaques link cadherins directly to tough, intermediate filaments (such as keratin) within the cytoskeleton, creating an interconnected mechanical network across cells.

• Gap Junctions (Communicating Junctions in Animal Cells):

  • Intercellular communication channels constructed from transmembrane protein assemblies termed connexons.

  • Functional Characteristics:

  • Align across adjacent animal cells to form continuous, aqueous channels bridging both cytoplasms.

  • Permit direct diffusion of ions, water, amino acids, and small signaling molecules ($\sim 1\,\text{kDa}$ or less) between neighboring cells without non-specific bulk cytoplasmic mixing.

  • Mediate electrical and metabolic coupling, such as coordinated contractions across cardiac muscle tissue.

• Plasmodesmata (Plant Intercellular Channels):

  • Narrow, membrane-lined cytoplasmic channels perforating rigid plant cell walls.

  • Connect the plasma membranes, cytoplasm, and smooth endoplasmic reticulum (forming an internal tubule called a desmotubule) of adjoining plant cells, establishing a continuous symplastic communication network.

• Extracellular Matrix (ECM) Integration and Adhesion Receptors:

  • Cadherins: Transmembrane glycoproteins that form dimers to link adjacent cells to one another; connected internally to actin microfilaments via specialized intracellular linker proteins.

  • Integrins: Heterodimeric transmembrane receptor proteins that link the intracellular cytoskeleton (actin microfilaments via linker proteins) to the extracellular matrix, binding exterior ligand proteins such as fibronectin, which attach to structural collagen fibers.