Cellular Respiration

Catabolic pathways release energy from food to make ATP.
1. There are 2 main catabolic pathways that organisms use:
  1. Anaerobic cellular respiration doesn’t use oxygen and an example is fermentation.
  2. Aerobic cellular respiration uses oxygen & is often just called cellular respiration.
2. Aerobic cellular respiration uses redox reactions which is the transferring of electrons from one molecule to another.
  1. Oxidation is the loss of electrons and the molecule that gets oxidized in the reducing agent.
  2. Reduction is the gain of elections and the molecule that gets reduced is the oxidizing agent.
3. Two things that makes aerobic cellular respiration more efficient
  1. Electron carriers transfer electrons from one molecule to another with a small release of free energy.
    1. NAD+ is the main electron carrier, & when it gains electrons to become NADH, it stores energy with a ΔG of +53 kcal/mol.
  2. The electron transport chain couples the transfer of electrons from NADH to NAD+, creating a membrane potential.
    1. At the bottom of the electron transport chain, oxygen takes the depleted electrons & forms H2O.
Aerobic cellular respiration requires oxygen and is a slower but more efficient catabolic pathway. Its overall formula is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
1. Glycolysis is the first step of aerobic cellular respiration and takes place in the cytoplasm. It has two main phases:
  1. The energy investment phase uses 2 ATPs to convert 1 glucose molecule into 2 molecules of glyceraldehyde 3-phosphate (G3P).
  2. The energy payoff phase produces 4 ATPs & 2 NADHs, while converting 2 G3P molecules into 2 pyruvates.
    1. The ATPs produced in glycolysis are generated through substrate-level phosphorylation.
  3. The net results of glycolysis per glucose in energy order from highest to lowest: 2 pyruvates, 2 NADH, 2 ATP
2. Pyruvate oxidation takes place in the mitochondrial matrix & further strips energy from pyruvate.
  1. Pyruvate loses CO₂ in an exergonic reaction, & NAD+ is reduced to NADH in an endergonic reaction during pyruvate oxidation.
  2. Coenzyme A joins with the 2-carbon fragment from pyruvate to form acetyl-CoA.
  3. For each glucose, pyruvate oxidation results in 2 acetyl-CoA, 2 NADH, 2 CO2.
3. The Citric Acid Cycle fully oxidizes the acetyl-CoA from pyruvate oxidation, & it also takes place in the mitochondria.
  1. The acetyl-CoA is converted to citrate, giving the cycle its name.
  2. Each acetyl-CoA when fully oxidized will create 3 NADH, 1 FADH2, 1 ATP, 2 CO2
  3. For each glucose, citric acid cycle happens twice with net results of 6 NADH, 2 FADH2, 2 ATP, 4 CO2
    1. ATP is produced through the citric acid cycle via substrate-level phosphorylation.
  4. Oxaloacetate is regenerated each time to accept the next acetyl-coA
4. Oxidative phosphorylation produces most of the ATP during aerobic cellular respiration.
  1. The electron transport chain, located in the inner mitochondrial membrane, is the first stage of oxidative phosphorylation. It uses energy from NADH and FADH₂ to pump H⁺ ions from the mitochondrial matrix to the intermembrane space, creating a proton-motive force.
    1. NADH provides enough energy to generate approximately 2.5 ATP. Its electrons follow a specific path: NADH → NADH-Q reductase → ubiquinone → cytochrome c reductase → cytochrome c → cytochrome c oxidase → O₂, which is reduced to form H₂O.
    2. FADH₂ provides enough energy to generate approximately 1.5 ATP. Its electrons follow this pathway: FADH₂ → succinate dehydrogenase → ubiquinone → cytochrome c reductase → cytochrome c → cytochrome c oxidase → O₂, which is reduced to form H₂O.
  2. Chemiosmosis is the process of generating ATP through ATP synthase. It occurs when H⁺ ions move from the intermembrane space back to the matrix via facilitated diffusion. This movement couples kinetic (mechanical) energy with the conversion of chemical potential energy into ATP.
    1. ATP synthase consists of several components that work together to produce ATP: the stator anchors the enzyme to the inner membrane; the rotor binds H⁺ ions, causing it to spin; the internal rod connects the rotor to the catalytic knobs, which catalyze the conversion of ADP to ATP.
5. Accounting for ATP synthesis in aerobic cellular respiration (ACR):
  1. Glycolysis: 2 NADH, 2 ATP
  2. Pyruvate oxidation: 2 NADH
  3. Citrate Acid Cycle: 6 NADH, 2 FADH2, 2 ATP
  4. Total 10 NADH (times) 2.5 = 25 ATPS, 2 FADH2 (times) 1.5 = 3 ATPS, 4 ATPs (from glycolysis and pyruvate oxidation) TOTALING to 32 ATPS which is the maximum amount & most efficient
    1. The 2 NADH produced during glycolysis can transfer their electrons via two shuttles: the efficient NADH → NADH shuttle, resulting in a total of 32 ATP, or the NADH → FADH₂ shuttle, yielding a total of 30 ATP.
    2. The production of 32 ATP molecules, each providing ~7.3 kcal/mol, yields approximately 34% efficiency when divided by the 686 kcal/mol of energy available per glucose molecule, with the remainder lost as heat.
Anaerobic respiration is the process of generating ATP from food without the use of oxygen.
1. Fermentation is a type of anaerobic respiration that is fast but very inefficient, producing only 2 ATP per glucose in the cytoplasm. It consists of two main steps: glycolysis, which generates ATP, and NAD+ regeneration, which allows glycolysis to continue.
  1. Alcoholic fermentation can be carried out by yeast and some bacteria.
    1. The pyruvates produced after glycolysis are converted into acetaldehyde, releasing CO₂. NAD+ is regenerated in the process, & acetaldehyde is then reduced to ethanol.
  2. Lactic acid fermentation can be carried out by muscle cells when oxygen is limited. It results in the production of lactic acid, which causes the "burn" sensation during intense exercise.
    1. The pyruvates from glycolysis are converted to lactate through NAD+ regeneration, allowing glycolysis to continue producing ATP in the absence of oxygen.
Food is our source of energy but only 3 macromolecules are used for energy:
1. Each macromolecule is used in different ways to provide energy
  1. Carbohydrates provide approximately 4 kcal/gram.
    1. Glucose-based carbohydrates enter glycolysis at the beginning.
    2. Others, like fructose, enter near the beginning of the pathway.
  2. Proteins also provide approximately 4 kcal/gram. The amino acids are broken down, & the nitrogen (NH₃) is released as waste.
    1. Amino acid fragments are converted into pyruvate, acetyl CoA, or intermediates of the citric acid cycle, depending on the resulting carbon chain.
  3. Fats are kcal/gram.
    1. Fatty acids carry the most energy, with each of the three converting into two-carbon fragments that form acetyl CoA.
    2. Glycerol →Glyceraldehyde 3-phosphate
2. If ATP supplies are high, ATP (and Citrate) perform feedback inhibition in phosphofructokinase to stop or flow aerobic cellular respiration.
  1. Instead of breaking down food for energy, the body undergoes biosynthesis to support growth or build new structures.