Cellular Respiration

• Cellular respiration refers to the series of chemical reactions (some requiring oxygen, some not) that produce much of a cell's energy.
• All cells, and thus all living things, must respire to produce energy for normal cellular processes.

Oxidation and Reduction in Cell Respiration

• Oxidation involves the chemical mechanisms for oxidation and reduction in cell respiration, specifically as it relates to the electron carriers.
• Glycolysis includes two major phases, the net production of which has been summarized
• Anaerobic respiration occurs in eukaryotic and prokaryotic organisms.

Purpose and Net Reaction of Cellular Respiration

• Electron carriers are molecules such as NADH and FADH₂.
• Oxidation and reduction reactions also play a role

Anaerobic Respiration

• Types of anaerobic respiration include:
  • Glycolysis
  • Lactic Acid Fermentation
  • Alcohol Fermentation

Cellular Respiration

• Series of chemical reactions collectively produces cellular energy.
• Reactions require or don't require oxygen.
• All cells and living things use cellular respiration.
• Cellular respiration produces necessary energy for cell processes.

Cell Respiration Overview (Net Reaction)

• Common in aerobic and anaerobic respiration:

  • Glycolysis

• Happens only in aerobic respiration:

  • Pyruvate oxidation
  • Krebs cycle
  • Oxidative phosphorylation

Electron Carriers

• Electron carriers are produced in the process of synthesizing ATP through anaerobic respiration and aerobic respiration.
• They "carry" electrons in the form of hydrogen atoms to the electron transport chain to provide the electromotive force for oxidative phosphorylation.
• NADH is derived from niacin (Vitamin B3) and oxidized to become NAD⁺
• FADH₂ is derived from riboflavin (Vitamin B₂) and oxidized to become FADH and FAD

Oxidation and Reduction ("Redox")

• Oxidation-reduction reactions transfer electrons between molecules.
• This is not the same as forming ionic bonds.
• An atom/molecule is oxidized when it loses an electron.
• An atom/molecule is reduced when it gains an electron.
• Atoms/molecules don't necessarily need a formal charge.
• Exchange of hydrogen atoms indicates oxidized or reduced molecules during cellular respiration.

Glycolysis

• Glycolysis, a form of anaerobic respiration in all cells (specifically the cytosol), converts a single molecule of glucose (6-carbon sugar) into two molecules of pyruvate (pyruvic acid) a 3-carbon sugar.
• The process produces:
  • Two NADH molecules to be used in oxidative phosphorylation
  • Four ATP molecules, however, net result is two ATP molecules

Irreversible Reactions

• Reactions are irreversible in glycolysis:
  • Reaction 1 (enzyme hexokinase):
    • Glucose + ATP -> glucose-6-phosphate + ADP
  • Reaction 3 (enzyme phosphofructokinase-1):
    • Fructose-6-phosphate + ATP -> fructose-1,6-bisphosphate + ADP
  • Reaction 10 (enzyme pyruvate kinase):
    • Phosphoenolpyruvate + ADP -> pyruvate+ ATP

The "Investment" Phase

• Reactions 1-5 make up the energy "investment" phase of glycolysis, which uses two ATP molecules and produces no ATP molecules:
• By this phase's end, two glyceraldehyde-3-phosphate (G3P) molecules are formed, doubling the reactions in the subsequent phase.

The "Payoff" Phase

• Reactions 6-10 make up the energy "payoff" phase of glycolysis.
• Four ATP molecules produce a net gain of two ATP per original glucose molecule.
• By the end of the payoff phase, two pyruvate (pyruvic acid) molecules are produced.
• The fate of these pyruvate molecules depends upon oxygen presence:
  • Oxygen present: pyruvate converted to Acetyl-CoA and shuttled to the mitochondria for aerobic respiration.
  • Oxygen absent: pyruvate used to produce lactate (lactic acid) or ethanol (alcohol), depending on the type of cell and enzymes.

Shunting the Glycolysis Pathways

• Cells can skip reactions in glycolysis (and potentially produce more net ATP) if downstream carbohydrates are present.
• The glycolysis pathway can be shunted, or interrupted, at:
  • Glucose-6-phosphate can be used as a substrate for glycogen synthesis or gluconeogenesis, as well as in the pentose phosphate pathway.
  • Glyceraldehyde-3-phosphate: can be used to synthesize glycolipids.
  • Pyruvate: can be used to synthesize fatty acids.

Pyruvate Oxidation

• Assuming oxygen is present in a eukaryotic cell, pyruvate molecules from glycolysis are transported into the mitochondria's innermost compartment, called the matrix, to convert to Acetyl-CoA (the necessary substrate for the Krebs Cycle).
• Acetyl-CoA is a two-carbon molecule with the large coenzyme A attached.
• Carbon Dioxide (CO₂) is produced in the process of removing a carbon from pyruvate and is eventually removed via exhalation.
• An additional NADH molecule is produced during the reaction.

Lactic Acid Fermentation

• Pyruvate is converted to lactate (lactic acid) through lactic acid fermentation when oxygen is not present and/or in the case of many prokaryotic cells.
• Lactic acid fermentation takes place in highly active muscle tissue.
• NADH is recycled to NAD+ during the process, allowing the cell to undergo glycolysis.

Alcohol (Ethanol) Fermentation

• Alcohol (ethanol) fermentation is the process by which pyruvate (in the absence of oxygen) is converted first to acetaldehyde, and then to alcohol (ethanol).
• Carbon Dioxide (CO₂) is produced, which makes bread rise. and bubbles form during fermentation.
• NADH is recycled to NAD⁺.