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Cellular Respiration Detailed Notes

Cellular Respiration

C2: Cellular Respiration – I Can Statements

  • Explain how glucose is oxidized during glycolysis and the Krebs cycle to produce reducing power in the form of NADH and FADH.
  • Explain how chemiosmosis converts the reducing power of NADH and FADH to store chemical potential energy in the form of ATP.
  • Describe where in the mitochondrion these processes occur.
  • Distinguish between aerobic and anaerobic respiration and fermentation.
  • Summarize and explain the role of ATP in cellular metabolism.

Introduction

  • Cellular respiration occurs in the body during exercise.

Recall: Metabolic Pathways

  • Photosynthesis: 6CO2 + 6H2O + energy (light) \rightarrow C6H{12}O6 + 6O2
    • Anabolism: Builds bigger molecules.
  • Cellular Respiration: C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + energy (heat)
    • Catabolism: Breaks down into smaller molecules.

Redox Reaction

  • During cellular respiration, glucose is oxidized by losing electrons, while oxygen is reduced by gaining electrons which leads to high reducing power.

Two Ways to Release Energy

  • Aerobic cellular respiration
    • Requires oxygen to produce ATP.
    • Occurs in organisms that live in oxygen-rich environments.
  • Anaerobic cellular respiration
    • Does not require oxygen to produce ATP.
    • Occurs in organisms living in anoxic environments.
    • Can be carried out by aerobic organisms; yeast, bacteria, muscle cells of mammals (fermentation).

Ways Cells Harvest Energy From Food (Glucose)

  • Anaerobic Cell Respiration
    • Glucose metabolism in the absence of oxygen that produces 2 ATP molecules and either ethanol or lactic acid.
    • Involves 2 stages:
      • Glycolysis
      • Fermentation
  • Aerobic Cellular Respiration
    • Glucose metabolism with oxygen that produces 36 ATP molecules, CO2 and H2O.
    • Involves 4 stages:
      • Glycolysis
      • Pyruvate oxidation
      • Krebs cycle
      • Electron transport and chemiosmosis

Releasing Energy: Aerobic Respiration

  • Aerobic respiration is a series of enzyme-catalyzed reactions whereby electrons are transferred from glucose to oxygen to generate ATP.
  • Four main steps produce ATP:
    • Glycolysis (anaerobic process)
    • Pyruvate Oxidation (Pre-Krebs)
    • Krebs cycle
    • Electron transport system

Anaerobic Respiration: Overview

  • Glycolysis occurs in the cytosol, producing ATP via substrate-level phosphorylation and pyruvate. Electrons are carried via NADH.
  • The Krebs cycle occurs in the mitochondrion, producing ATP via substrate-level phosphorylation. Electrons are carried via NADH and FADH2.
  • The electron transport chain and oxidative phosphorylation occur in the mitochondrion, with ATP synthase producing ATP via oxidative phosphorylation.

Releasing Energy: Anaerobic Respiration

  • Anaerobic respiration is a series of reactions that do not use O_2 as an electron acceptor.
  • Produces 2 ATP molecules and either ethanol or lactic acid.
  • Two main steps produce ATP:
    • Glycolysis
    • Fermentation

Anaerobic Respiration

  • Glycolysis: Occurs in the cytoplasm.
  • Fermentation:
    • NAD+ regeneration is a key step.
    • Two types:
      • Alcoholic fermentation: produces ethyl alcohol + CO_2
      • Lactic acid fermentation: produces lactate.

Glycolysis

  • Anaerobic process that occurs in all living cells outside the mitochondria (in cytosol/cytoplasm).
  • The role of glycolysis is to split glucose into two molecules of pyruvate (3-carbon molecule).
  • Several more reactions result in the reduction of NAD^+ (nicotinamide adenine dinucleotide) to produce NADH.
  • Uses 2 ATP to initiate glycolysis, but 4 ATP are produced, resulting in a net gain of 2 ATP.

Glycolysis: Step 2

  • NAD+ is a necessary electron carrier for glycolysis to occur.
  • You only have a limited supply of NAD+ in your cells, so NADH must be converted back to NAD+ later (recycled) so it can undergo this conversion again – otherwise glycolysis stops!

Glycolysis Summary

Phase 1: Energy Investment

  • Uses 2 ATP
    Phase 2: Energy Payoff
  • Produces 4 ATP
  • Net ATP = 2 ATP

Overall equation:
Glucose + 2 NAD^+ + 2 ADP + 2 P --> 2 Pyruvate + 2 H_2O + 2 ATP + 2 NADH + 2 H^+

Glycolysis in Summary

  • Products of Glycolysis:
    • 2 ATP
    • 2 NADH
    • 2 pyruvate

Fate of Pyruvate

  • In the presence of oxygen, pyruvate enters the Krebs cycle and electron transport chain, producing up to 36 ATP.
  • In the absence of oxygen, pyruvate undergoes fermentation, producing lactic acid or ethanol.

Fermentation

  • If oxygen is not available following glycolysis, pyruvate is further reduced via oxidation of NADH to NAD^+.
  • Occurs in the cytoplasm of the cell (not mitochondria).
  • Two common types include lactate (animal cells) and ethanol (in yeast) fermentation in eukaryotes.

Lactate Fermentation

  • Occurs during times of strenuous exercise when muscles require more ATP than aerobic respiration can provide.
  • NADH (reduced form) generated during glycolysis transfers its H^+ to pyruvate, changing pyruvate to lactic acid (lactate) and regenerating NAD^+ (oxidized form).
  • The liver can change lactic acid back to pyruvate, allowing aerobic respiration to continue when exercise has ceased.

Lactate Fermentation

  • Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate.
  • Lactate dehydrogenase converts pyruvate to lactate, regenerating NAD^+.

Ethanol Fermentation

  • After glycolysis, ethanol fermentation begins when a carbon dioxide molecule is removed from pyruvate, creating a 2-carbon molecule called acetaldehyde.
  • NADH passes their e- and H^+ to acetaldehyde creating NAD^+ and ethanol – ethanol is a waste product like carbon dioxide
  • This process supplies a small amount of energy (from glycolysis) and regenerates NAD^+ (returning to glycolysis)

Ethanol Fermentation

  • Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate.
  • Pyruvate is converted to acetaldehyde, releasing CO_2.
  • Acetaldehyde is reduced to ethanol, regenerating NAD^+.

Examples of Fermentation

  • Products of fermentation include:
    • wine
    • beer
    • soy sauce
    • kombucha
    • bread
    • carbonated beverages
    • Cheese
  • Lactic acid production (in excess/lactic acidosis) causes:
    • heart failure
    • Shock
    • Liver damage
    • Sepsis
    • Severe infection
  • Sore muscles are no longer believed to be a side effect of lactic acid buildup.