Comprehensive Study Guide on Cellular Respiration and Photosynthesis

Overview of Cellular Respiration

• Organismal respiration versus cellular respiration:

  • Organismal respiration involves inhaling oxygen ($O_2$) and exhaling carbon dioxide ($CO_2$) and water vapor ($H_2O$).
  • Cellular respiration carries out this identical chemical exchange at the microscopic level within individual cells.

• Chemical equation and substrates:

  • Input substrates: Glucose ($C_6H_{12}O_6$) and oxygen ($O_2$).
  • Output products: Carbon dioxide ($CO_2$), water ($H_2O$), and adenosine triphosphate ($\text{ATP}$).

• Molecular structure of glucose:

  • Glucose exists as a six-carbon molecule ($C_6H_{12}O_6$) arranged in a hexagonal ring structure.
  • Chemical energy is stored within the carbon-carbon ($C-C$), carbon-hydrogen ($C-H$), and carbon-oxygen ($C-O$) chemical bonds connecting the six carbons.

• Mechanism of energy release:

  • Chemical bonds holding the carbons together are systematically broken or snapped apart.
  • Cleaving these bonds converts the carbon chain into individual molecules of $CO_2$ and $H_2O$.
  • The energy released from breaking these chemical bonds is captured to synthesize $\text{ATP}$.

• Cellular uses of ATP:

  • Transports energy to drive endergonic chemical reactions throughout the cell.
  • Powers active transport mechanisms, such as proton pumps moving molecules up their concentration gradients.

• Primary stages of cellular respiration:

  • Glycolysis
  • The Krebs Cycle (Citric Acid Cycle)
  • The Electron Transport System (Electron Transport Chain)

Glycolysis and Energy Investment

• Structural transformation during glycolysis:

  • Starts with a single six-carbon glucose molecule.
  • The very first stage of cellular respiration snaps the six-carbon ring/chain directly in half.

• Energy investment phase:

  • Glycolysis requires an initial input of energy to proceed.
  • Spending energy upfront enables greater energy production later.
  • Two molecules of $\text{ATP}$ ($2\,\text{ATP}$) are spent to transfer phosphate groups onto the ends of the glucose molecule.
  • Phosphorylation destabilizes the carbon chain, making it significantly easier to split in half.

• Formation of G3P and Pyruvate:

  • The phosphorylated six-carbon glucose is split into two three-carbon molecules known as glyceraldehyde 3-phosphate ($G3P$), each carrying an attached phosphate group.
  • The two $G3P$ molecules are subsequently modified into two three-carbon molecules called pyruvate.

• Energy payoff phase and net yield:

  • Reduces electron carriers to produce two charged molecules of $NADH$ ($2\,\text{NADH}$).
  • Yields a total gross output of four $\text{ATP}$ molecules ($4\,\text{ATP}$).
  • Subtracting the initial investment of $2\,\text{ATP}$ results in a net gain of $2\,\text{ATP}$ from glycolysis.

• Anaerobic nature:

  • Glycolysis does not require oxygen ($O_2$) at any point and occurs entirely in the cellular cytoplasm.

Pyruvate Oxidation and the Krebs Cycle

• Pyruvate transport:

  • Pyruvate produced in glycolysis moves into the mitochondrion, crossing both the outer membrane and inner membrane into the mitochondrial matrix.

• Preparatory phase (Pyruvate Oxidation):

  • Each three-carbon pyruvate releases one carbon atom in the form of carbon dioxide ($CO_2$).
  • This decarboxylation reaction charges another electron carrier, forming $NADH$.
  • Coenzyme A attaches to the remaining two-carbon compound to form acetyl-coenzyme A ($\text{acetyl-CoA}$).

• Krebs cycle operations:

  • Acetyl-CoA enters the Krebs cycle within the mitochondrial matrix.
  • The remaining carbon bonds are broken, releasing further $CO_2$ gas.
  • Direct release of additional $\text{ATP}$ occurs during these cyclic reactions.
  • The cycle charges high volumes of mobile electron carriers, converting low-energy state carriers into high-energy electron transport batteries: $NADH$ and $FADH_2$.

• Total energy carrier tally prior to the Electron Transport Chain:

  • Gross $\text{ATP}$ count: $4\,\text{ATP}$ synthesized so far (via substrate-level phosphorylation in glycolysis and the Krebs cycle).
  • $NADH$ count: $10\,\text{NADH}$ charged batteries accumulated.
  • $FADH_2$ count: $2\,\text{FADH}_2$ charged batteries accumulated.

The Electron Transport Chain and Oxidative Phosphorylation

• Mobile electron carriers as biological batteries:

  • Glycolysis and the Krebs cycle serve primarily to charge the electron carrier batteries $NADH$ and $FADH_2$.
  • The third stage of cellular respiration releases the stored chemical energy from these batteries to produce high quantities of bite-sized $\text{ATP}$ units.

• Nature of hydrogen ions / protons:

  • Hydrogen ions ($H^+$) stripped from electron carriers are isolated protons.
  • A proton may exist with or without an attached neutron.

• Proton pump mechanism and chemiosmosis:

  • High-energy electrons delivered by $NADH$ and $FADH_2$ power active transport proton pumps located along the inner mitochondrial membrane.
  • Proton pumps actively push protons ($H^+$) across the membrane, concentrating them in the tiny intermembrane space.
  • Protons trapped in the intermembrane space build a strong electrochemical and concentration gradient.
  • Driven to move down their concentration gradient, the crowded protons escape through a specialized transmembrane enzyme called $\text{ATP synthase}$.
  • The flow of protons through $\text{ATP synthase}$ rotates the molecular turbine, driving the phosphorylation of ADP into $\text{ATP}$.

• Terminal electron acceptor and water formation:

  • At the end of the electron transport chain, oxygen ($O_2$) acts as the final electron acceptor.
  • Oxygen binds with the discharged hydrogen ions ($H^+$) and electrons to form water ($H_2O$).

• Summary of maximum ATP yield per single glucose molecule ($C_6H_{12}O_6$):

  • Glycolysis yield: $2\,\text{ATP}$
  • Krebs cycle yield: $2\,\text{ATP}$
  • Electron transport system yield: $32\,\text{ATP}$
  • Maximum total yield under ideal aerobic conditions: $36\,\text{ATP}$

Anaerobic Respiration and Fermentation

• Characteristics of fermentation:

  • Occurs when oxygen ($O_2$) is absent or unavailable.
  • Operates anaerobically without the electron transport chain.
  • Does not produce the large $\text{ATP}$ yields of aerobic respiration.

• Common fermentation products and organisms:

  • Produces metabolic byproducts such as ethanol and carbon dioxide gas ($CO_2$).
  • Responsible for producing alcoholic beverages, including beer and whiskey.
  • Utilized by microorganisms to produce fermented foods such as sauerkraut, kombucha, and kimchi.

• Safety and physical phenomena of fermentation:

  • Fermentation continuously generates gas byproducts.
  • When fermenting sauerkraut, the fermentation vessel must remain ventilated.
  • Fully sealing a jar of fermenting sauerkraut causes gas pressure to build until it results in an exploding jar.

Photosynthesis: Light-Dependent Reactions

• Complementary nature of photosynthesis and cellular respiration:

  • Photosynthesis performs cellular respiration processes essentially in reverse.
  • Cellular respiration breaks down glucose to release energy; photosynthesis uses energy to construct sugar molecules.
  • Instead of ending with glucose, photosynthesis specifically produces $G3P$, enabling plants to custom-build various types of carbohydrates.

• Location and light absorption:

  • Takes place inside organelle structures called chloroplasts, specifically within membrane-bound sacs called thylakoids.
  • Uses the green pigment chlorophyll alongside accessory pigments to capture radiant energy.
  • Pigment variety appears visually as distinct orange, light green, and dark green pigments working in tandem.
  • Darker colors absorb a broader spectrum of light wavelengths and higher total energy (analogous to wearing dark clothing in the sun to absorb heat energy).

• Light reaction process:

  • Requires direct exposure to radiant energy (sunlight or indoor ambient lighting).
  • Absorbed light energy breaks water molecules ($H_2O$) apart.
  • Splitting water releases oxygen gas ($O_2$) as a byproduct.
  • Bouncing high-energy electrons generates chemical energy carriers: $\text{ATP}$ and $NADPH$.

Photosynthesis: The Calvin Cycle (Light-Independent Reactions)

• Light independence:

  • Also referred to as the dark reactions or light-independent reactions.
  • Does not directly require light energy to operate; utilizes the $\text{ATP}$ and $NADPH$ produced by the light-dependent reactions.

• Carbon fixation mechanism:

  • Carbon dioxide ($CO_2$) gas is taken in from the air.
  • Carbon from atmospheric $CO_2$ is attached/fixed onto an existing organic acceptor molecule called ribulose 1,5-bisphosphate ($RuBP$).

• G3P synthesis and carbohydrate assembly:

  • Energy stored in $\text{ATP}$ and $NADPH$ drives the conversion of fixed carbon into glyceraldehyde 3-phosphate ($G3P$).
  • A complete set of Calvin cycle turns generates six molecules of $G3P$ ($6\,\text{G3P}$).
  • Five of these $G3P$ molecules ($5\,\text{G3P}$) remain in the cycle to regenerate the initial $RuBP$ acceptor molecules.
  • The sixth $G3P$ molecule ($1\,\text{G3P}$) is booted or kicked out of the cycle as net product.
  • Running the Calvin cycle through a second iteration produces a second net $G3P$ molecule.
  • Joining two $G3P$ molecules together synthesizes a six-carbon sugar, such as glucose ($C_6H_{12}O_6$).
  • Plants utilize this chemical capability to capture light energy from any source and convert it into stable organic chemical bonds.