Cellular Respiration and Metabolic Pathways

• Cellular respiration is a process where living cells obtain energy from organic molecules.
• The primary goal of cellular respiration is to produce ATP and NADH.
• Aerobic respiration uses oxygen: O2 is consumed and CO2 is released.
• Glucose is primarily used, but other organic molecules can also be used.
• The overall equation for aerobic respiration is C6H12O6 + 6O2 yields 6CO2 + 6H2O + energy (36 or 38 ATP).
• Glucose is oxidized, and oxygen is reduced during aerobic respiration.

Metabolic Pathways

• Glycolysis.
• Breakdown of pyruvate to an acetyl group.
• Citric acid cycle.
• Oxidative phosphorylation.

Glycolysis

• Glycolysis, the first stage of cellular respiration, occurs within the cytoplasm (outside the mitochondria).
• Glucose is broken down into 2 pyruvate molecules.
• This process does not require oxygen.
• Glycolysis involves 10 steps, divided into 3 phases.
• The steps in glycolysis are nearly identical in all living species.
• The three phases are: energy investment, cleavage, and energy liberation.
• Two ATP molecules are hydrolyzed to create fructose-1,6 bisphosphate.
• A 6-carbon molecule breaks down into two 3-carbon molecules of glyceraldehyde-3 phosphate.
• Two glyceraldehyde-3-phosphate molecules are broken down into two pyruvate molecules, producing 2 NADH and 4 ATP.
• The net ATP yield in glycolysis is 2.

Breakdown of Pyruvate to Acetyl Group

• In eukaryotes, pyruvate is transported to the mitochondrial matrix.
• Pyruvate dehydrogenase breaks down pyruvate.
• A molecule of CO2 is removed from each pyruvate.
• The remaining acetyl group attaches to CoA (coenzyme A) to make acetyl CoA.
• One NADH is produced for each pyruvate molecule.

Citric Acid Cycle

• Acetyl CoA is formed from acetyl and removes it to form citrate, starting the citric acid cycle.
• The process releases 2 CO2, 1 ATP, 3 NADH, and 1 FADH2
• Oxaloacetate is regenerated to start the cycle again.
• The cycle begins when the acetyl group from acetyl CoA attaches to oxaloacetate to form citrate.
• Citrate is rearranged to isocitrate
• Isocitrate is oxidized to α-ketoglutarate and CO2 is released, NADH is formed
• α-Ketoglutarate is oxidized as it combines with CoA to form succinyl CoA, once again, CO2 is released and NADH, is formed.
• Succinyl CoA is broken down to CoA and succinate.
• Succinate can be oxidized to fumarate and FADH2 is made
• Fumarate then combines with water to make malate
• Malate is oxidized to oxaloacetate and NADH is made,

Oxidative Phosphorylation

• High-energy electrons from NADH and FADH2 used to make ATP.
• This process typically requires oxygen.
• The oxidative process involves the electron transport chain (ETC).
• Phosphorylation occurs by ATP synthase.
• FADH2 produces ~2 ATPs per molecule.
• NADH produces ~3 ATPs per molecule.

Electron Transport Chain

• Both photosynthesis and respiration use an electron transport system with membrane-bound carriers.
• These carriers pass electrons from one to another.
• High-energy electrons enter the system, and low-energy electrons exit.
• Each transfer of electrons releases energy, ultimately producing ATP.
• Protein complexes and small organic molecules are embedded in the inner mitochondrial membrane
• Molecules acceot and donate electrons in a linear manner in a series of redox reactions.
• Movement of electrons generates a H+ electrochemical gradient/proton-motive force with excess positive charge outside of the matrix.

Other Organic Molecules

• Besides glucose, other molecules like carbohydrates, proteins, and fats can be used for energy.
• These molecules enter glycolysis or the citric acid cycle at different points.
• Using the same pathways for breakdown increases efficiency.
• Metabolism can also be used to create molecules (anabolism).

Anaerobic Metabolism

• Anaerobic metabolism does not require oxygen.
• Anaerobic bacteria (like those in cheese) use fermentation.
• Yeast uses this pathway to produce alcohol and CO2 (which makes bread rise).
• Glycolysis is followed by the reduction of pyruvate by NADH to either lactate or alcohol and carbon dioxide.
• Animal cells (e.g., humans) use fermentation.
• This is common in muscle cells.
• Fermentation provides a rapid burst of ATP.
• Lactate produces and blood carries away
• Lactate buildup causes muscles fatigue
• When stop running and heavy breathing to restore