High School Biology/Cellular Respiration and Glycolysis

Cellular Energy

  • Your cells transfer the energy in organic compounds, especially glucose, to ATP through a process called cellular respiration.
  • Oxygen makes the production of ATP more efficient, although some ATP is made without oxygen.
  • Metabolic processes that  require oxygen are called aerobic.
  • Metabolic processes that do not require oxygen are called anaerobic.

The Stages of Cellular Respiration

  • Stage 1: Glucose is converted to pyruvate, producing a small amount of ATP and NADH.
  • Stage 2: When oxygen is present, pyruvate and NADH are used to make a large amount of ATP. This process is called aerobic respiration. Aerobic respiration occurs in mitochondria of eukaryotic cells and in the cell membrane of prokaryotic cells. When oxygen is not present, pyruvate is converted to either lactate or ethanol and carbon dioxide.
  • Aerobic respiration produces most of the ATP made by cells.
  • Intermediate products of aerobic respiration form the organic compounds that help build and maintain cells.

Stage One: Breakdown of Glucose

  • The primary fuel for cellular respiration is glucose, which is formed when carbohydrates such as starch and sucrose are broken down.
  • If too few carbohydrates are available to meet an organism’s glucose needs, other molecules, such as fats, can be broken down to make ATP.
  • Proteins and nucleic acids can also be used to make ATP, but they are usually used for building important cell parts.


  • In the first stage of cellular respiration, glucose is broken down in the cytoplasm during a process called glycolysis.
  • Glycolysis is an enzyme-assisted anaerobic process that breaks down one six-carbon molecule of glucose to three-carbon pyruvate ions.
  • Pyruvate is the ion of a three-carbon organic acid called pyruvic acid.
  • The pyruvate produced during glycolysis still contains some energy that was stored in the glucose molecule.
  • As glucose is broken down, some of its hydrogen atoms are transferred to an electron acceptor called NAD+.
  • This forms an electron carrier called NADH.
  • For cellular respiration to continue, the electrons carried by NADH are eventually donated to other organic compounds.
  • This recycles NAD+, making it available to accept more electrons.
  • Step 1: In a series of three reactions, phosphate groups from two ATP molecules are transferred to a glucose molecule.
  • Step 2: In two reactions, the resulting six-carbon compound is broken down to two three-carbon compounds, each with a phosphate group.
  • Step 3: Two NADH molecules are produced, and one more phosphate group is transferred to each three-carbon compound.
  • Step 4: In a series of four reactions, each three-carbon compound is converted to a three-carbon pyruvate, producing four ATP molecules in the process.
  • Glycolysis uses two ATP molecules but produces four ATP molecules, yielding a net gain of two ATP molecules.
  • Glycolysis is followed by another set of reactions that use the energy temporarily stored in NADH to make more ATP.

Stage Two: Production of ATP

  • When oxygen is present, pyruvate produced during glycolysis enters a mitochondrion and is converted to a two-carbon compound.
  • This reaction produces one carbon dioxide molecule, one NADH molecule, and one two-carbon acetyl group.
  • The acetyl group is attached to a molecule called coenzyme A (CoA), forming a compound called acetyl-CoA.

Krebs Cycle

  • Acetyl-CoA enters a series of enzyme-assisted reactions called the Krebs cycle.
  • Step 1: Acetyl-CoA combines with a four-carbon compound, forming a six-carbon compound release coenzyme A.
  • Step 2: Carbon dioxide, Co2, is released from the six-carbon compound, forming a five-carbon compound. Electrons are transferred to NAD+, making a molecule of NADH.
  • Step 3: Carbon dioxide is released from the five-carbon compound, resulting in a four-carbon compound. A molecule of ATP is made, and a molecule of NADH is also produced.
  • Step 4: The existing four-carbon compound is converted to a new four-carbon compound. Electrons are transferred to an electron acceptor called FAD, making a molecule of FADH2. FADH2 is another type of electron carrier.
  • Step 5: The new four-carbon compound is then converted to the four-carbon compound that began the cycle. Another molecule of NADH is produced.
  • After the Krebs cycle, NADH and FADH2 now contain much of the energy that was previously stored in glucose and pyruvate. When the Krebs cycle is completed, the four-carbon compound that began the cycle has been recycled, and acetyl-CoA can enter the cycle again.

Electron Transport Chain

  • In aerobic respiration, electrons donated by NADH and FADH2 pass through an electron transport chain. In eukaryotic cells, the electron transport chain is located in the inner membranes of the mitochondria.
  • The energy of these electrons is used to pump hydrogen ions out of the inner mitochondrial compartment.
  • Hydrogen ions accumulate in the outer compartment, producing a concentration gradient across the inner membrane.
  • Hydrogen ions diffuse back into the inner compartment through a carrier protein that adds a phosphate group to ADP, making ATP.
  • AT the end of the electron transport chain, hydrogen ions and spent electrons combine with oxygen molecules, O2, forming water molecules, H2O.