C1.2.4 Cell respiration Notes

Cell Respiration Produces ATP

1. Cell respiration is the controlled release of energy from organic compounds to produce ATP, the energy currency of cells.
2. Cells break down carbon compounds (like glucose and fatty acids) and extract energy stored in their chemical bonds.
3. This energy is released through oxidation and captured in ATP molecules.

Main Substrates for Cell Respiration

While cells can use a wide range of organic compounds, glucose and fatty acids are the principal substrates.

1. Glucose
   1. Glucose is a 6-carbon sugar and the most commonly used substrate.
   2. It's broken down during glycolysis into two molecules of pyruvate (3C).
   3. Pyruvate can then enter the mitochondria for complete oxidation in aerobic respiration, or be converted to other products in anaerobic respiration
2. Fatty Acids
   1. Fatty acids are long carbon chains that are highly energy-dense.
   2. They're broken down through beta-oxidation into 2-carbon acetyl groups, which form acetyl-CoA.Acetyl-CoA enters the Krebs cycle, just like the acetyl-CoA produced from glucose.
   3. Fatty acids yield more ATP per gram than glucose because they contain more carbon-hydrogen bonds.Amino Acids
3. Other Carbon Compounds
   1. Cells can also use amino acids (when glucose and fats are scarce), glycerol (from fat breakdown), and other organic molecules.
   2. These compounds are converted into intermediates that can enter glycolysis, the link reaction, or the Krebs cycle at various points.

Overview: How Cell Respiration Works

1. Cell respiration involves multiple stages that progressively extract energy from substrates.
2. Each stage will be explored more in depth in the succeeding articles (if you're an HL student), but here's an overview:

1. Stage 1: Glycolysis (Cytoplasm)
   1. Glucose (6C) is broken down into two pyruvate (3C) molecules.
   2. Produces 2 ATP (net) and 2 NADH.
2. Stage 2: Link Reaction (Mitochondrial Matrix)
   1. Each pyruvate (3C) is converted into acetyl-CoA (2C).
   2. Produces 2 NADH and 2 CO₂ per glucose.
3. Stage 3: Krebs Cycle (Mitochondrial Matrix)
   1. Acetyl-CoA enters a cycle of reactions that completely oxidizes the carbon atoms.
   2. Produces 6 NADH, 2 FADH₂, 2 ATP, and 4 CO₂ per glucose.
4. Stage 4: Electron Transport Chain and Oxidative Phosphorylation (Inner Mitochondrial Membrane)
   1. NADH and FADH₂ donate electrons to the electron transport chain.
   2. Energy from electron flow creates a proton gradient that drives ATP synthesis via chemiosmosis.
   3. Produces approximately 34 ATP per glucose.
   4. Oxygen acts as the final electron acceptor, forming water (H₂O).

Distinguishing Cell Respiration from Gas Exchange

Students often confuse these two processes, but they're distinct: