Cellular Respiration & Glycolysis

• Cells break down glucose to create energy (ATP), releasing carbon dioxide and water in the process.
• The overall chemical reaction for cellular respiration is C6H12O6 + 6O2 → 6CO2 + 6H2O + 36ATP.
• Cellular respiration releases energy stored in glucose and it is an exergonic and catabolic reaction.
• Aerobic cellular respiration (with oxygen) produces 36 ATP, while anaerobic (without oxygen) produces only 2 ATP.
• Activation energy is required to start a reaction, and cellular respiration uses ATP to initiate its reactions.
• Cellular respiration occurs in several steps to break glucose into smaller pieces, which requires less activation energy.

Aerobic Cell Respiration Steps

• Glycolysis
• Krebs Cycle preparation & Pyruvate Oxidation
• Krebs Cycle
• Electron Transport Chain

Glycolysis

• Glycolysis means "breaks sugar."
• It occurs in the cytoplasm of a cell.
• C6H12O6 accepts 2 ATP (activation energy).
• Produces 4 ATP.
• Two NAD+ molecules are reduced to 2 NADH.
• Two pyruvate molecules (3 carbons) are created.

Pyruvate Oxidation (Krebs Cycle Prep)

• Takes place on the inner membrane of the mitochondria.
• Pyruvate (3 carbon) loses a molecule of CO2 into the atmosphere.
• NAD+ is reduced to NADH (electron carrier).
• CoEnzyme A (vitamin) is added.
• The product is Acetyl CoA (2 carbon).

Krebs Cycle

• Occurs in the matrix.
• Acetyl CoA combines with a 4-carbon molecule to create a 6-carbon molecule.
• A molecule of CO2 is released, and an NAD+ is reduced to NADH, creating a 5-carbon molecule.
• A molecule of CO2 is released, and an NAD+ is reduced to NADH, creating a 4-carbon molecule.
• The 4-carbon molecule is recycled to the original 4-carbon molecule and creates NADH, FADH2 (electron carriers), and one ATP molecule.

Important Items Summary

• Glycolysis uses an activation energy of 2 ATP and creates 2 ATP and 2 NADH in the cytoplasm.
• Pyruvate Oxidation creates 0 ATP and 2 NADH in the inner membrane.
• Krebs Cycle creates 2 ATP, 6 NADH, and 2 FADH2 in the matrix.

Further Considerations

• The 2 ATP produced represent only 2% of the energy in glucose.
• 95% of the energy is trapped in pyruvate and NADH, with 3% lost as heat.
• What happens to pyruvate depends on the presence of oxygen.
• NAD+ must be regenerated because it is needed in glycolysis.
• If no oxygen is present, the process will create lactic acid.

Electron Transport Chain

• Takes place in the cristae.
• Very similar to the electron transport chain of photosynthesis.
• Includes 2 ETCs and ATP synthase.

Electron Transport Chain Steps

• NADH is oxidized to NAD+ when it drops off an electron to a protein of the ETC.
• Electrons move through the ETC, and their energy is used to pump H+ ions into the intermembrane space.
• FADH2 is oxidized to FAD+, and the electron energy pumps more H+ ions into the intermembrane space.
• Oxygen is the final electron acceptor and is reduced, binding to H+ to create water.
• The H+ concentration is high in the intermembrane space and low in the matrix.
• The cristae membrane is impermeable to H+ ions.
• H+ ions move through ATP Synthase (from high to low concentration), and ATP is created via chemiosmosis.

Electron Carriers

• NADH has more energy, containing 3 ATP.
• FADH2 contains slightly less energy, containing 2 ATP.
• NADH from glycolysis loses energy as it travels from the cytoplasm to the matrix, becoming FADH2, so the 2 NADH molecules from glycolysis become 2 FADH2.

ATP Count in Aerobic Cellular Respiration

• Glycolysis: 2 ATP, 2 FADH2 (from NADH)
• Pyruvate Oxidation: 0 ATP, 2 NADH
• Krebs Cycle: 2 ATP, 6 NADH, 2 FADH2
• Electron Transport Chain: 8 ATP from FADH2 (4 x 2 ATP), 24 ATP from NADH (8 x 3 ATP)
• Total ATP Created: 36 ATP (2 from Glycolysis, 2 from Krebs, 8 from FADH2, 24 from NADH)

Anaerobic Respiration

• Happens without oxygen.
• Produces energy quickly via only 2 ATP.
• It occurs in the cytoplasm (NOT the mitochondria).
• Types include Lactic Acid Fermentation (multicellular organisms) and Alcohol Fermentation (unicellular organisms).

Lactic Acid Fermentation

• Glycolysis occurs.
• No oxygen is present, so pyruvate cannot go through pyruvate oxidation.
• Pyruvate is reduced by NADH, regenerating NAD+ to return to glycolysis for more ATP creation.
• Lactic acid fermentation stores pyruvate until oxygen is present.
• Lactic acid is stored in muscles and the liver until O2 is present.
• Lactic acid changes back to pyruvate to undergo aerobic cellular respiration.

Alcohol Fermentation

• Glycolysis occurs.
• Pyruvate loses carbon dioxide and is reduced by NADH to create NAD+ to return to glycolysis to accept another electron.
• 2 ATP are created.
• Ethanol is formed.

Key Points for Anaerobic Respiration

• Anaerobic respiration occurs in the cytoplasm.
• It is fast and produces 2 ATP.
• Lactic acid fermentation occurs in multicellular organisms.
• Alcohol fermentation occurs in unicellular organisms and produces CO2.
• The whole point of anaerobic respiration is to regenerate NAD+.