Cellular Respiration Overview

Cellular Respiration

• Key process in metabolism
• Breaks down molecules for energy (catabolism)
• Uses energy from glucose to make ATP
• Releases energy in glucose's chemical bonds making it accessible to cells
• Glucose has more energy per molecule than needed
• ATP is a more convenient energy source with smaller amounts of energy per molecule

Big Picture

• Cellular respiration consists of three major processes: glycolysis, Krebs cycle, and electron transport chain

Glycolysis

• Occurs in the cytoplasm
• Begins breaking down glucose
• Breaks glucose into two pyruvate molecules
• Net yield: 2 ATP, 2 NADH

Pre-Krebs - Pyruvate Oxidation

• Molecules move into mitochondria
• Preps pyruvate for the next step
• 2 pyruvate (pyruvic acid) enter mitochondria
• 2 NADH enter mitochondria (uses 2 ATP)
• Converts pyruvate to acetyl CoA
• Net yield: -2 ATP, 2 NADH

Krebs Cycle (Citric Acid Cycle)

• Occurs in the mitochondria
• Stores energy from glucose in carrier molecules
• Produces NADH and FADH2
• Molecules are regenerated and re-enter the cycle
• Net yield: 2 ATP, 6 NADH, 2 FADH2 and 4 CO2
• By the end of the Krebs cycle, glucose is completely broken down, all carbons are released as CO2 and all hydrogens are on carriers

Electron Transport Chain

• Occurs in mitochondria
• NADH in the matrix enters the first protein complex
• NADH releases electrons (and H+) becoming NAD+
• Electrons are passed from one protein complex to another, losing energy with each step
• Energy lost is used to pump H+ into the intermembrane space
• Oxygen is the final electron acceptor
• Electrons combine with 2H+ and 1/2 O2 to form water
• H+ buildup creates a concentration gradient in the intermembrane space

Chemiosmosis

• H+ flows back into the matrix through ATP synthase
• Energy from the H+ flow turns ADP back into ATP
• Each NADH produces 3 ATP
• Each FADH2 produces 2 ATP (starts farther along the chain, at lower energy)
• Electron transport and chemiosmosis produce 34 ATP (10 NADH x 3 ATP + 2 FADH2 x 2 ATP)
• Final overall tally is 36 ATP for each glucose molecule
• Oxygen is required for this process due to its high electronegativity

Photosynthesis Equation:

• CO2 + H2O -> glucose + O2

Cellular Respiration Equation:

• glucose + O2 -> CO2 + H2O