Cellular Respiration Overview and Stages

Cellular Respiration Overview

• Cellular respiration is a set of catabolic pathways that break down nutrients into usable chemical energy (ATP)
• It includes both aerobic and anaerobic respiration.
• Mitochondrion is the site for cellular respiration

Stages of Cellular Respiration

• Glycolysis: Breaks down glucose into two pyruvate molecules in the cytosol
  • Two stages: energy investment, energy harvest
  • Net gain: two NADHs, two ATPs per glucose molecule
• Pyruvate Oxidation: Pyruvate moves into the mitochondrial matrix and is further oxidized to Acetyl CoA
  • A molecule of CO2 is removed and NAD+ is reduced to NADH during this step
  • Acetyl CoA enters the Krebs cycle
• Krebs Cycle (Citric Acid Cycle):
  • Oxidizes organic fuel derived from pyruvate generating 1 ATP, 3 NADH, and 1 FADH2 per turn
  • Produces 2 CO2 as a waste product (total of 3 CO2, including one from pyruvate oxidation.)
  • The cycle runs twice per glucose molecule consumed.
  • Has eight steps, each catalyzed by a specific enzyme
  • Acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate
  • The next seven steps decompose citrate, making the process a cycle.
  • NADH and FADH2 carry electrons to the electron transport chain.
• Oxidative Phosphorylation:
  • Electrons drop in free energy as they are transferred down the chain.
  • It finally passes to O2 to form H₂O
  • Electron transport chain breaks the large free-energy drop from glucose to O2 into smaller steps.
  • This releases energy in manageable amounts.
  • No ATP is produced directly by the chain
  • Chemiosmosis is the energy coupling mechanism in this stage.
    • Electrons are passed down the electron transport chain. H+ is pumped from the mitochondrial matrix to the intermembrane space.
    • H+ moves down its concentration gradient back across the membrane, passing through the protein complex ATP synthase.
    • This causes ATP synthase to spin, catalyzing the phosphorylation of ADP to ATP.
    • The H⁺ gradient is referred to as a proton-motive force.
    • Certain electron carriers accept and release H⁺ along with the electrons.
• About 34% of the energy in glucose is converted into ATP (30 or 32) during cellular respiration. The rest is lost as heat

Anaerobic Respiration (Fermentation):

• Fermentation is an extension of glycolysis that oxidizes NADH by transferring electrons to pyruvate
• Two common types:
  • Alcohol fermentation (used in brewing, winemaking, and baking)
  • Lactic acid fermentation (used in cheese and yogurt production; muscle cells use this during strenuous exercise when oxygen is scarce)