Energy, Cell Respiration, and Photosynthesis Quiz Preparation

Quiz Preparation and Core Study Requirements

• Quiz Focus Areas:

  • Energy fundamentals and biochemical principles.
  • Detailed biochemical and chemistry vocabulary.
  • Exact physical locations and cellular compartmentalization where metabolic reactions occur.
  • Stoichiometric inputs and outputs for all metabolic stages of photosynthesis, cellular respiration, and fermentation.

• Target Biological Pathways:

  • Photosynthesis (Light-Dependent Reactions and the Light-Independent Calvin Cycle).
  • Cellular Respiration (Glycolysis, Pyruvate Oxidation / Link Reaction, Krebs / Citric Acid Cycle, and Oxidative Phosphorylation).
  • Anaerobic fermentation pathways (Lactic Acid Fermentation and Alcoholic Fermentation).

Energy and Chemistry Vocabulary

• Forms of Energy and Fundamental Principles:

  • Energy: The capacity to cause change or perform biological work.
  • Kinetic Energy: The energy of motion; includes thermal energy, which is associated with the random movement of atoms or molecules.
  • Potential Energy: Stored energy resulting from physical location or chemical structure.
  • Chemical Energy: Potential energy available for release in a chemical reaction; stored within the covalent chemical bonds of organic molecules such as glucose and $ATP$.
  • First Law of Thermodynamics (Law of Conservation of Energy): Energy can be transferred or transformed, but it cannot be created or destroyed.
  • Second Law of Thermodynamics: Every energy transfer or transformation increases the entropy ($S$), or disorder, of the universe; in biological systems, usable energy is continuously dissipated as metabolic heat.
  • Free Energy ($G$): The portion of a biological system's energy that can perform work under conditions of constant temperature and pressure. The change in free energy is expressed as:     $\Delta G = \Delta H - T\,\Delta S$

• Reaction Energetics and Thermodynamic Classifications:

  • Exergonic Reaction: A spontaneous chemical reaction accompanied by the net release of free energy ($\Delta G < 0$), exemplified by catabolic breakdown pathways such as cellular respiration.
  • Endergonic Reaction: A non-spontaneous chemical reaction that absorbs free energy from its surroundings ($\Delta G > 0$), exemplified by anabolic biosynthetic pathways such as photosynthesis.
  • Exothermic Reaction: A chemical process that releases thermal energy ($\Delta H < 0$) into the surrounding environment.
  • Endothermic Reaction: A chemical process that absorbs thermal energy ($\Delta H > 0$) from the surrounding environment.
  • Anabolism: Metabolic pathways that assemble complex polymers from simpler precursors, requiring an input of free energy ($ATP$).
  • Catabolism: Metabolic pathways that hydrolyze and degrade complex organic molecules into simpler compounds, yielding chemical energy.
  • Metabolism: The totality of all interrelated biochemical reactions and pathways occurring within a living organism.

• Enzyme Kinetics and Catalysis:

  • Activation Energy ($E_a$): The minimum kinetic energy required to contort reactant bonds into an unstable transition state, initiating a chemical reaction.
  • Catalyst: Any substance that selectively increases the rate of a chemical reaction without undergoing permanent chemical alteration or being consumed.
  • Enzyme: A macromolecular biological catalyst (primarily a globular protein) that accelerates biological reactions by lowering the activation energy ($E_a$).
  • Substrate: The specific reactant molecule or molecules bound and chemically transformed by an enzyme.
  • Active Site: The specialized geometric pocket or cleft on an enzyme where the substrate binds via induced fit.
  • Enzyme-Substrate Complex: The transient intermediate formed when an enzyme and its substrate are bound together during catalysis.

• Oxidation-Reduction (Redox) Concepts:

  • Redox Reaction: A chemical reaction involving the transfer of one or more electrons ($e^-$) from an electron donor to an electron acceptor.
  • Oxidation: The loss of electrons, loss of hydrogen atoms, or gain of oxygen by a chemical species.
  • Reduction: The gain of electrons, gain of hydrogen atoms, or loss of oxygen by a chemical species.
  • OIL RIG Mnemonic: Oxidation Is Loss; Reduction Is Gain of electrons.
  • Reducing Agent: The electron donor that undergoes oxidation in a redox reaction.
  • Oxidizing Agent: The electron acceptor that undergoes reduction in a redox reaction.

• Cellular Energy Currencies and Electron Carriers:

  • Adenosine Triphosphate (ATP): The universal intracellular energy carrier, composed of adenine, ribose, and three negatively charged phosphate groups.
  • Adenosine Diphosphate (ADP): The lower-energy molecule generated when the terminal phosphoanhydride bond of $ATP$ is hydrolyzed:     $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$
  • Inorganic Phosphate ($P_i$): The unattached phosphate ion ($HPO_4^{2-}$ or $H_2PO_4^-$) released or attached during phosphorylation reactions.
  • Phosphorylation: The covalent addition of an inorganic phosphate group to an organic acceptor molecule, altering its conformation and reactivity.
  • Substrate-Level Phosphorylation: The direct enzymatic transfer of a phosphate group from a phosphorylated metabolic intermediate to $ADP$, producing $ATP$.
  • Oxidative Phosphorylation: The production of $ATP$ coupled to the movement of electrons down the respiratory electron transport chain to molecular oxygen.
  • Photophosphorylation: The light-driven synthesis of $ATP$ from $ADP$ and $P_i$ occurring in chloroplasts.
  • Nicotinamide Adenine Dinucleotide ($NAD^+$ / $NADH$): A soluble dinucleotide coenzyme functioning as an electron carrier in cellular respiration:     $NAD^+ + 2\,e^- + 2\,H^+ \rightarrow NADH + H^+$
  • Flavin Adenine Dinucleotide ($FAD$ / $FADH_2$): A prosthetic redox cofactor in the citric acid cycle that accepts two electrons and two protons:     $FAD + 2\,e^- + 2\,H^+ \rightarrow FADH_2$
  • Nicotinamide Adenine Dinucleotide Phosphate ($NADP^+$ / $NADPH$): The phosphorylated electron carrier utilized as a reducing agent in photosynthesis:     $NADP^+ + 2\,e^- + 2\,H^+ \rightarrow NADPH + H^+$

• Trophic Classifications:

  • Autotroph: An organism that produces complex organic compounds from simple inorganic substances ($CO_2$) utilizing external energy sources.
  • Heterotroph: An organism that cannot fix carbon and relies on ingesting or absorbing organic carbon produced by other organisms.

Photosynthesis: Compartments, Pathways, and Stoichiometry

• Overall Purpose and Net Chemical Equation:

  • Photosynthesis transforms solar electromagnetic radiation into chemical potential energy stored in covalent carbohydrate bonds.
  • Overall Balanced Equation:     $6\,CO_2 + 6\,H_2O \rightarrow C_6H_{12}O_6 + 6\,O_2$

• Where Photosynthesis Takes Place (Chloroplast Structure):

  • Mesophyll Tissue: Leaf layer where photosynthetic cells containing abundant chloroplasts reside.
  • Outer and Inner Chloroplast Membranes: Phospholipid bilayers that envelope the organelle.
  • Intermembrane Space: The thin aqueous compartment between the outer and inner chloroplast membranes.
  • Stroma: The semi-gelatinous, alkaline fluid filling the interior of the chloroplast; contains RuBisCO, metabolic enzymes, chloroplast DNA, and ribosomes. Site of the Calvin Cycle (Light-Independent Reactions).
  • Thylakoids: Flat, disc-like membranous sacs suspended within the stroma; contain chlorophyll pigments, photosystems, and the photosynthetic electron transport chain. Site of the Light-Dependent Reactions.
  • Thylakoid Lumen: The continuous internal acidic aqueous space enclosed by the thylakoid membrane where protons accumulate during photolysis and electron transport.
  • Granum (plural: Grana): Densely stacked columns of thylakoid discs.

• Stage 1: Light-Dependent Reactions:

  • Location: Embedded in the thylakoid membrane and within the thylakoid lumen.
  • Inputs:
  • Sunlight (photons absorbed primarily at $680\,nm$ and $700\,nm$)
  • Water ($H_2O$)
  • $NADP^+$ (oxidized electron acceptor)
  • $ADP$ and inorganic phosphate ($P_i$)
  • Outputs:
  • Molecular Oxygen ($O_2$) (byproduct released to the atmosphere via stomata)
  • $NADPH$ (reduced high-energy electron carrier)
  • $ATP$ (chemical energy for carbon fixation)
  • Step-by-Step Mechanism:
  • Photolysis of Water: The oxygen-evolving complex of Photosystem II splits water on the luminal face:       $2\,H_2O \rightarrow 4\,H^+ + 4\,e^- + O_2$
  • Photosystem II ($P680$): Light excitation of reaction-center chlorophylls ejects electrons into the primary electron acceptor, which are replaced by electrons from photolyzed water.
  • Electron Transport Chain: Electrons pass sequentially through plastoquinone ($Pq$), the cytochrome $b_6f$ complex, and plastocyanin ($Pc$). The cytochrome $b_6f$ complex translocates $H^+$ from the stroma into the thylakoid lumen, establishing a steep electrochemical proton gradient.
  • Photosystem I ($P700$): Photons re-energize electrons in $P700$, transferring them via ferredoxin ($Fd$) to $NADP^+$ reductase, forming $NADPH$ in the stroma.
  • Photophosphorylation: Protons diffuse down their electrochemical gradient from the thylakoid lumen into the stroma via the catalytic rotor of ATP synthase, driving the phosphorylation of $ADP$ to $ATP$.

• Stage 2: Light-Independent Reactions (The Calvin Cycle):

  • Location: The stroma of the chloroplast.
  • Inputs (for net synthesis of one 3-carbon sugar, $G3P$):
  • $3\,CO_2$ (fixed from atmospheric air)
  • $9\,ATP$
  • $6\,NADPH$
  • Outputs (for net synthesis of one $G3P$):
  • $1\,\text{Glyceraldehyde-3-phosphate}\,(G3P)$ (precursor used to form glucose and other sugars)
  • $9\,ADP$
  • $8\,P_i$
  • $6\,NADP^+$ (recycled to the light-dependent reactions)
  • Stoichiometry for One Glucose ($C_6H_{12}O_6$):
  • Requires 2 net molecules of $G3P$.
  • Requires total inputs of $6\,CO_2$, $18\,ATP$, and $12\,NADPH$.
  • Three Phases of the Calvin Cycle:
  • Phase 1: Carbon Fixation: The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the carboxylation of the 5-carbon acceptor ribulose-1,5-bisphosphate ($RuBP$) with $CO_2$, producing an unstable 6-carbon intermediate that immediately cleaves into two molecules of 3-phosphoglycerate ($3\text{-}PGA$).
  • Phase 2: Reduction: Each $3\text{-}PGA$ is phosphorylated by $ATP$ to form 1,3-bisphosphoglycerate, which is then reduced by electrons from $NADPH$ to form glyceraldehyde-3-phosphate ($G3P$).
  • Phase 3: Regeneration of RuBP: Out of every 6 molecules of $G3P$ generated, 1 molecule exits as net product, while the remaining 5 molecules are enzymatically rearranged using $3\,ATP$ to regenerate 3 molecules of $RuBP$.

Cell Respiration: Pathways, Compartments, and Energy Yield

• Overall Purpose and Net Chemical Equation:

  • Cellular respiration breaks down organic fuels to harness released free energy into $ATP$.
  • Overall Balanced Equation for Aerobic Cellular Respiration:     $C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O + \text{energy}\,(30\text{ to }32\,ATP)$

• Where Cellular Respiration Takes Place (Mitochondrial and Cytoplasmic Compartments):

  • Cytoplasm (Cytosol): Aqueous intracellular fluid outside organelles; site of Glycolysis.
  • Outer Mitochondrial Membrane: Permeable lipid bilayer containing transport channels (porins).
  • Intermembrane Space: Aqueous region between the outer and inner mitochondrial membranes where protons ($H^+$) are concentrated during oxidative phosphorylation.
  • Inner Mitochondrial Membrane: Impermeable folded membrane containing cristae, respiratory complexes (Complexes I, II, III, IV), and ATP synthase.
  • Mitochondrial Matrix: The internal space enclosed by the inner membrane; contains pyruvate dehydrogenase, Krebs cycle enzymes, mitochondrial DNA, and ribosomes.

• Stage 1: Glycolysis:

  • Location: Cytoplasm (cytosol).
  • Oxygen Requirement: Strictly anaerobic; does not require oxygen ($O_2$).
  • Inputs (per 1 molecule of glucose):
  • $1\,\text{Glucose}\,(C_6H_{12}O_6)$
  • $2\,ATP$ (invested during the energy investment phase)
  • $2\,NAD^+$ (oxidized electron acceptor)
  • $4\,ADP + 4\,P_i$
  • Outputs (per 1 molecule of glucose):
  • $2\,\text{Pyruvate}\,(C_3H_4O_3)$ ($3\text{-carbon}$ keto-acids)
  • $4\,ATP$ total ($2\,ATP$ net yield via substrate-level phosphorylation)
  • $2\,NADH$
  • $2\,H_2O$
  • $2\,H^+$

• Stage 2: Pyruvate Oxidation (The Link Reaction / Transition Step):

  • Location: Mitochondrial matrix (pyruvate is imported across the inner membrane by active transport via a pyruvate-proton symporter).
  • Inputs (per 1 glucose molecule, which produces 2 pyruvates):
  • $2\,\text{Pyruvate}$
  • $2\,\text{Coenzyme A}\,(CoA\text{-}SH)$
  • $2\,NAD^+$
  • Outputs (per 1 glucose molecule):
  • $2\,\text{Acetyl-CoA}$ ($2\text{-carbon}$ acetyl moiety attached to CoA)
  • $2\,CO_2$ (decarboxylation waste product)
  • $2\,NADH$
  • $2\,H^+$

• Stage 3: Citric Acid Cycle (Krebs Cycle):

  • Location: Mitochondrial matrix.
  • Inputs (per 1 glucose molecule / 2 turns of the cycle):
  • $2\,\text{Acetyl-CoA}$
  • $6\,NAD^+$
  • $2\,FAD$
  • $2\,ADP + 2\,P_i$ (or $2\,GDP + 2\,P_i$)
  • $4\,H_2O$
  • Outputs (per 1 glucose molecule / 2 turns of the cycle):
  • $4\,CO_2$ (complete oxidation of remaining carbons)
  • $6\,NADH$
  • $2\,FADH_2$
  • $2\,ATP$ (or $GTP$ via substrate-level phosphorylation)
  • $2\,\text{Coenzyme A}\,(CoA\text{-}SH)$
  • Key Intermediate Reaction: Acetyl-CoA ($2\text{ carbons}$) condenses with oxaloacetate ($4\text{ carbons}$) via citrate synthase to form citrate ($6\text{ carbons}$). Subsequent isomerizations, oxidations, and decarboxylations regenerate oxaloacetate.

• Stage 4: Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis):

  • Location: Inner mitochondrial membrane (cristae) and the intermembrane space.
  • Inputs:
  • $10\,NADH$ ($2$ from glycolysis, $2$ from pyruvate oxidation, $6$ from Krebs cycle)
  • $2\,FADH_2$ ($2$ from Krebs cycle)
  • $6\,O_2$ (molecular oxygen acting as final electron acceptor)
  • $26\text{ to }28\,ADP + P_i$
  • Outputs:
  • $10\,NAD^+$ (regenerated oxidized carriers)
  • $2\,FAD$
  • $6\,H_2O$
  • $26\text{ to }28\,ATP$
  • Mechanisms:
  • Electron Transport Chain Complexes: Electrons from $NADH$ enter at Complex I (NADH dehydrogenase); electrons from $FADH_2$ enter at Complex II (succinate dehydrogenase). Electrons flow to ubiquinone ($Q$), Complex III (cytochrome $bc_1$), cytochrome $c$, and Complex IV (cytochrome $c$ oxidase).
  • Proton Pumping: Complexes I, III, and IV pump protons ($H^+$) from the matrix across the inner membrane into the intermembrane space, generating an electrical and chemical proton-motive force.
  • Terminal Electron Acceptor: Oxygen binds four electrons and four protons at Complex IV, forming metabolic water:       $O_2 + 4\,e^- + 4\,H^+ \rightarrow 2\,H_2O$
  • Chemiosmosis via ATP Synthase: Protons flow down their electrochemical gradient back into the matrix through the $F_0$ rotor channel of ATP synthase, driving mechanical rotation of the $F_1$ catalytic head that condenses $ADP$ and $P_i$ into $ATP$.

Anaerobic Respiration and Fermentation Pathways

• Physiological Purpose of Fermentation:

  • In the absence of oxygen ($O_2$), the electron transport chain shuts down, preventing the oxidation of $NADH$ back to $NAD^+$.
  • Without an oxidized pool of $NAD^+$, glycolysis cannot proceed because glyceraldehyde-3-phosphate dehydrogenase requires $NAD^+$ as an electron acceptor.
  • Fermentation does not generate any additional $ATP$ beyond the $2\,ATP$ net produced by glycolysis; its biological function is solely to re-oxidize $NADH$ into $NAD^+$.

• Lactic Acid Fermentation:

  • Location: Cytoplasm (cytosol) of human skeletal muscle cells during anaerobic exertion, and certain bacteria (e.g., Lactobacillus).
  • Inputs:
  • $1\,\text{Glucose}$ (yielding $2\,\text{Pyruvate}$ via glycolysis)
  • $2\,NADH$
  • Outputs:
  • $2\,\text{Lactate}\,(C_3H_5O_3^-)$
  • $2\,NAD^+$ (recycled to sustain glycolysis)
  • Net yield of $2\,ATP$ (generated solely in glycolysis via substrate-level phosphorylation)
  • Biochemical Reaction: Lactate dehydrogenase transfers electrons and protons from $NADH$ directly to pyruvate, reducing it to lactate without releasing $CO_2$:     $\text{Pyruvate} + NADH + H^+ \rightleftharpoons \text{Lactate} + NAD^+$

• Alcohol (Ethanolic) Fermentation:

  • Location: Cytoplasm (cytosol) of yeast (e.g., Saccharomyces cerevisiae) and certain microorganisms.
  • Inputs:
  • $1\,\text{Glucose}$ (yielding $2\,\text{Pyruvate}$ via glycolysis)
  • $2\,NADH$
  • Outputs:
  • $2\,\text{Ethanol}\,(C_2H_5OH)$
  • $2\,CO_2$ (released as gas)
  • $2\,NAD^+$ (recycled to sustain glycolysis)
  • Net yield of $2\,ATP$ (generated solely in glycolysis)
  • Two-Step Biochemical Reaction:
  • Step 1 (Decarboxylation): Pyruvate decarboxylase removes a carboxyl group from pyruvate, producing acetaldehyde and releasing carbon dioxide:       $\text{Pyruvate} \rightarrow \text{Acetaldehyde} + CO_2$
  • Step 2 (Reduction): Alcohol dehydrogenase reduces acetaldehyde to ethanol using electrons from $NADH$, regenerating $NAD^+$:       $\text{Acetaldehyde} + NADH + H^+ \rightarrow \text{Ethanol} + NAD^+$

Comparative Analysis of Locations, Inputs, and Outputs

• Physical Locations of Metabolic Stages:

  • Light-Dependent Reactions: Thylakoid membranes and thylakoid lumen of chloroplasts.
  • Calvin Cycle: Stroma of chloroplasts.
  • Glycolysis: Cytoplasm (cytosol) of the cell.
  • Pyruvate Oxidation: Mitochondrial matrix.
  • Citric Acid Cycle (Krebs Cycle): Mitochondrial matrix.
  • Oxidative Phosphorylation: Inner mitochondrial membrane (cristae) and intermembrane space.
  • Fermentation (Lactic Acid and Alcoholic): Cytoplasm (cytosol) of the cell.

• Master Input and Output Matrix:

  • Light-Dependent Reactions:
  • Inputs: Light energy, $H_2O$, $NADP^+$, $ADP$, $P_i$
  • Outputs: $O_2$, $NADPH$, $ATP$
  • Calvin Cycle:
  • Inputs: $CO_2$, $ATP$, $NADPH$
  • Outputs: $G3P$ (carbohydrate), $ADP$, $P_i$, $NADP^+$
  • Glycolysis:
  • Inputs: Glucose, $2\,NAD^+$, $2\,ATP$, $4\,ADP + 4\,P_i$
  • Outputs: $2\,\text{Pyruvate}$, $2\,NADH$, $2\,ATP$ (net), $2\,H_2O$
  • Pyruvate Oxidation:
  • Inputs: $2\,\text{Pyruvate}$, $2\,CoA$, $2\,NAD^+$
  • Outputs: $2\,\text{Acetyl-CoA}$, $2\,CO_2$, $2\,NADH$
  • Krebs Cycle:
  • Inputs: $2\,\text{Acetyl-CoA}$, $6\,NAD^+$, $2\,FAD$, $2\,ADP + 2\,P_i$
  • Outputs: $4\,CO_2$, $6\,NADH$, $2\,FADH_2$, $2\,ATP$, $2\,CoA$
  • Oxidative Phosphorylation:
  • Inputs: $10\,NADH$, $2\,FADH_2$, $6\,O_2$, $26\text{ to }28\,ADP + P_i$
  • Outputs: $10\,NAD^+$, $2\,FAD$, $6\,H_2O$, $26\text{ to }28\,ATP$

• Interdependence and Energy Coupling:

  • The net outputs of photosynthesis ($C_6H_{12}O_6$ and $6\,O_2$) constitute the stoichiometric inputs required for aerobic cellular respiration.
  • The net outputs of cellular respiration ($6\,CO_2$ and $6\,H_2O$) constitute the raw material inputs necessary for photosynthesis.
  • Chemical matter cycles continuously between autotrophs and heterotrophs through these complementary reactions, while energy flows unidirectionally, entering as solar light, transforming into cellular chemical bonds, and eventually escaping as metabolic heat.