Cellular Respiration: Glycolysis & Krebs Cycle

• Cellular respiration extracts energy from food in a controlled manner.

Glycolysis

• Glycolysis is the initial stage of cellular respiration.
• During glycolysis, one 6-carbon molecule of glucose is transformed into two 3-carbon molecules of pyruvic acid.
• ATP and NADH are produced during glycolysis.
• The cell "deposits" 2 ATP molecules to initiate glycolysis.
• Glycolysis produces 4 ATP molecules, resulting in a net gain of 2 ATP for each glucose molecule entering glycolysis.
• The electron carrier NAD+ (nicotinamide adenine dinucleotide) accepts a pair of high-energy electrons, becoming NADH.
• NADH carries high-energy electrons to the electron transport chain for further ATP production.
• Two NADH molecules are produced per glucose molecule during glycolysis.
• Glycolysis occurs rapidly, providing an advantage when energy demands suddenly increase.
• Glycolysis functions without oxygen, enabling quick energy production.

Krebs Cycle

• Pyruvic acid is broken down into carbon dioxide during the Krebs cycle via energy-extracting reactions.
• The Krebs cycle occurs in the mitochondrial matrix, the innermost compartment of the mitochondrion, and is the second stage of cellular respiration..
• The Krebs cycle is also called the citric acid cycle because citric acid forms first in this series of reactions.
• Pyruvic acid from glycolysis enters the matrix, the innermost compartment of the mitochondrion.
• Once pyruvic acid is in the mitochondrial matrix, NAD+ accepts 2 high-energy electrons to form NADH, also producing one molecule of CO2.
• The two remaining carbon atoms then react to form acetyl-CoA.
• Acetyl-CoA combines with a 4-carbon molecule to produce citric acid.
• Citric acid is broken down into a 5-carbon then a 4-carbon compound and two molecules of CO2 are released.
• The 4-carbon compound is then combined with acetyl-CoA, restarting the cycle.
• Energy released during carbon bond breaking and rearranging is captured in the forms of ATP, NADH, and FADH2.
• For each cycle turn, one ADP molecule is converted into ATP.
• Electron carriers NAD+ and FAD accept pairs of high-energy electrons to form NADH and FADH2, which are then used in the electron transport chain to generate ATP.
• Per glucose molecule, the Krebs cycle produces 6 CO2 molecules, 2 ATP molecules, 8 NADH molecules, and 2 FADH2 molecules.
• The Krebs cycle occurs twice per glucose molecule.

Electron Transport and ATP Synthesis

• The electron transport chain leverages high-energy electrons from glycolysis and the Krebs cycle to convert ADP into ATP.
• NADH and FADH2 pass high-energy electrons to electron carrier proteins in the electron transport chain.
• At the end of the electron transport chain, electrons combine with H+ ions and oxygen to form water.
• The electron transport chain uses energy to move H+ ions across the inner mitochondrial membrane and into the intermembrane space against a concentration gradient.
• H+ ions pass back across the mitochondrial membrane via ATP synthase causing the molecule to spin, and with each rotation ATP synthase attaches a phosphate to ADP to produce ATP.

Energy Totals via Cellular Repsiration

• Glycolysis, Krebs cycle, and the electron transport chain together release approximately 36 ATP molecules per glucose molecule.
• About 36 percent of total glucose energy is used and remaining 64 percent is released as heat.
• Cells can generates ATP from other sources complex carbohydrates are broken down into simple sugars, similar to how lipids and proteins can be broken down into molecules at various points in either the Krebs cycle or glycolysis.