Cellular Respiration: Comprehensive Study
Guide
Cellular respiration is the multi-step metabolic process by which cells break down organic
molecules (usually glucose) to produce ATP (adenosine triphosphate), the primary energy
currency of the cell. This process is essential for biological restoration and the replenishment
of energy levels necessary for both automatic and controlled processing.
The Overall Chemical Equation
The aerobic respiration of glucose can be summarized by the following equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + Heat)
Phase 1: Glycolysis
Glycolysis occurs in the cytosol of the cell and does not require oxygen (anaerobic).
●​ Process: A single 6-carbon glucose molecule is split into two 3-carbon molecules
called pyruvate.
●​ Energy Investment: 2 ATP are consumed to prime the glucose molecule.
●​ Energy Payoff: 4 ATP and 2 NADH (electron carriers) are produced.
●​ Net Yield: 2 ATP, 2 NADH, and 2 Pyruvate.
Phase 2: Pyruvate Oxidation
This transition step occurs as pyruvate enters the mitochondrial matrix.
●​ Process: Each pyruvate is converted into a 2-carbon molecule called Acetyl CoA.
●​ Byproducts: One CO₂ is released per pyruvate (2 total per glucose), and 1 NADH is
produced per pyruvate (2 total per glucose).
Phase 3: The Citric Acid Cycle (Krebs Cycle)
The Citric Acid Cycle takes place in the mitochondrial matrix.
●​ Process: Acetyl CoA combines with oxaloacetate to form citrate. Through a series of
redox reactions, the carbon chain is broken down.
●​ Yield per Glucose: 2 ATP, 6 NADH, 2 FADH₂, and 4 CO₂.
●​ Significance: This stage completes the oxidation of organic fuel and generates a high
volume of electron carriers (NADH and FADH₂) for the final stage.
Phase 4: Oxidative Phosphorylation
This final stage occurs across the inner mitochondrial membrane (cristae) and produces the
vast majority of ATP.
The Electron Transport Chain (ETC)
●​ NADH and FADH₂ donate electrons to a series of protein complexes in the membrane.
●​ As electrons move down the chain, energy is released to pump H⁺ ions (protons) from
the matrix into the intermembrane space, creating an electrochemical gradient.
●​ Oxygen serves as the final electron acceptor, combining with electrons and H⁺ to
form water (H₂O).
Chemiosmosis
●​ The H⁺ ions flow back into the matrix through a protein called ATP Synthase.
●​ This flow of ions (the proton-motive force) powers the phosphorylation of ADP into
ATP.
Summary of Energy Yield
Stage
Location
Main Inputs
Net ATP Yield
Other Products
Glycolysis
Cytosol
Glucose, 2 ADP, 2 NAD⁺
2 ATP
2 Pyruvate, 2 NADH
Stage
Location
Main Inputs
Net ATP Yield
Other Products
Pyruvate Oxidation
Matrix
2 Pyruvate, 2 CoA
0 ATP
2 Acetyl CoA, 2 NADH, 2 CO₂
Citric Acid Cycle
Matrix
2 Acetyl CoA
2 ATP
6 NADH, 2 FADH₂, 4 CO₂
Oxidative Phosphorylation
Inner Membrane
NADH, FADH₂, O₂
~26–28 ATP
H₂O
Total Estimated Yield: Approximately 30–32 ATP per glucose molecule.
Anaerobic Respiration and Fermentation
When oxygen is absent, cells cannot perform oxidative phosphorylation. To keep glycolysis
running, they must recycle NAD⁺ through fermentation.
●​ Lactic Acid Fermentation: Pyruvate is reduced to lactate (common in muscle cells
during intense exercise).
●​ Alcohol Fermentation: Pyruvate is converted to ethanol and CO₂ (common in yeast).