Cellular Respiration

Overview

  • Cellular respiration: process by which living cells obtain energy from organic molecules
  • Primary aim to make ATP and NADH
  • Aerobic respiration uses oxygen
    • O2 consumed and CO2 released
  • Primarily use glucose but other organic molecules also used

Oxidative Phosphorylation

  • Protons are translocated outside of a membrane as a consequence of a flow of electrons through membrane carriers 
  • Establishes a PROTON MOTIVE FORCE
  • The establishment of a PMF is then coupled to ATP synthesis

4 Metabolic Pathways

  1. Glycolysis
  2. Breakdown of pyruvate
  3. Citric Acid Cycle
  4. Oxidative phosphorylation

Glycolysis

  • Stage 1 of cellular respiration

  • Glycolysis can occur with or without oxygen

  • Steps in glycolysis nearly identical in all living species

  • Ten steps in three phases:

    1. Energy Investment
    2. Cleavage
    3. Energy Liberation

3 Phases of Glycolysis

  1. Energy investment: 2 ATP hydrolyzed to create fructose-1,6 bisphosphate
  2. Cleavage: 6 carbon molecules broken into two 3 carbon molecules of glyceraldehyde-3-phosphate
  3. Energy liberation: two glyceraldehyde-3-phosphate molecules broken down into two pyruvate molecules – produces 2 NADH and 4 ATP

Net yield = 2 ATP

Breakdown of Pyruvate

  • Stage 2 of cellular respiration
  • In eukaryotes, pyruvate is transported into the mitochondrial matrix
  • Broken down by pyruvate dehydrogenase 
  • Molecule of CO₂ removed from each pyruvate
  • Remaining acetyl group attached to CoA to make acetyl CoA
  • Yield = 1 NADH for each pyruvate

Citric Acid Cycle

  • Stage 3 of cellular respiration
  • Metabolic cycle
    • Some molecules enter while others leave
    • Series of organic molecules regenerated in each cycle in order to keep the cycle going
  • Acetyl is removed from Acetyl CoA and attached to oxaloacetate to form citrate (aka citric acid)
  • Series of steps releases 2 CO2, 1 ATP, 3 NADH, and \n 1 FADH2
  • Oxaloacetate is regenerated to start the cycle again

Oxidative Phosphorylation

  • Stage 4 of cellular respiration
  • High energy electrons removed from NADH and FADH2 to make ATP
  • Typically requires oxygen
  • Oxidative process involves electron transport chain
  • Phosphorylation occurs by ATP synthase

Oxidation by Electron Transport Chain

  • Protein complexes and small organic molecules embedded in the inner mitochondrial membrane
  • Accept and donate electrons in a linear manner in a series of redox reactions
  • Movement of electrons generates an H⁺ electrochemical gradient (proton-motive force)
  • This provides energy for the next step –synthesizing ATP

NADH

  • NADH oxidation makes most of the cell’s ATP
  • NADH oxidation creates the H+ electrochemical gradient used to synthesize ATP
    • Yield = up to 30-34 ATP molecules / glucose
  • But rarely achieve maximal amount because:
    • NADH also used in anabolic pathways
    • H+ gradient used for other purposes

Phosphorylation by ATP Synthase

  • Lipid bilayer of inner mitochondrial membrane is relatively impermeable to H⁺
  • Protons can only pass through ATP synthase
  • Harnesses free energy to synthesize ATP from ADP
  • Chemiosmosis: chemical synthesis of ATP as a result of pushing H + across a membrane

ATP Synthase

  • ATP synthase captures free energy as H+ ions flow through
  • The enzyme converts energy from the proton motive force of the H+ gradient to chemical bond energy in ATP
  • Racker and Stoeckenius confirmed ATP uses an H+ electrochemical gradient
  • ATP synthase is a rotary machine that makes ATP as it spins

Connections Among Carbohydrates, Protein and Fat Metabolism

  • Besides glucose, other molecules also used for energy: carbohydrates, proteins, fats
  • Enter into glycolysis or citric acid cycle at different points
  • Utilizing the same pathways for breakdown increases efficiency
  • Metabolism can also be used to make molecules (anabolism)

Protein Metabolism

  • Proteins can be metabolized for energy
  • They are first hydrolyzed into individual amino acids by proteolytic enzymes
  • Amino acids are deaminated (remove the amino group)
  • These molecules enter glycolysis, fermentation or the Kreb’s cycle

Fat Metabolism

  • Most microorganisms, like most animals, can obtain energy from lipids
  1. Fats are hydrolyzed to glycerol and three fatty acids
  2. Glycerol is metabolized by glycolysis
  3. The fatty acids are broken down into 2-carbon pieces by beta oxidation

Anaerobic Respiration and Fermentation

  • For environments that lack oxygen or during oxygen deficient times
  • Two strategies:
    • Use substance other than O2 as final electron acceptor in electron transport chain
    • Produce ATP only via substrate-level phosphorylation

Fermentation

  • Fermentation: the breakdown of organic molecules without net oxidation
  • Many organisms can only use O2 as final electron acceptor, so under anaerobic conditions, they need a different way to produce ATP, like using glycolysis
  • But glycolysis uses up NAD⁺ and makes too much NADH under anaerobic conditions (dangerous situation)
  • Muscle cells solve problem by reducing pyruvate into lactate
  • Yeast solves the problem by making ethanol
  • Fermentation produces far less ATP than oxidative phosphorylation