Cell Respiration and Glycolysis

C1.2.7: Role of NAD (HL)

• Cell Respiration Overview:

  • Cell respiration is a complex and efficient process, facilitated by key molecules.

  • This section focuses on breaking down this process and exploring the function of a key molecule: Nicotinamide adenine dinucleotide (NAD).

• The Role of NAD:

  • Nicotinamide adenine dinucleotide (NAD) is a crucial molecule in cell respiration (Figure 1).

  • It acts as a coenzyme, essential for enzyme function.

  • NAD's ability to be reduced and oxidized allows it to function as a hydrogen carrier.

• Learning Outcomes:

  • Explain the role of NAD as a hydrogen carrier in cell respiration.

  • Describe the conversion of glucose to pyruvate through glycolysis.

Reduction-Oxidation (Redox) Reactions:

• Redox Reactions:

  • Chemical reactions involving the exchange of electrons between molecules.

  • Oxidation: Loss of electrons from a molecule, atom, or ion.

  • Reduction: Gain of electrons by a molecule, atom, or ion.

  • Whenever a substance is oxidized, another substance is reduced simultaneously.

• NAD as an Oxidizer:

  • NAD is an oxidizer because it oxidizes other molecules by accepting electrons, thereby becoming reduced itself.

  • In cell respiration, this involves the transfer of hydrogen (containing electrons).

  • The molecule being oxidized loses a hydrogen atom (dehydrogenation).

• NAD and NADH Interconversion:

  • NAD is reduced to NADH (reduced NAD) (Figure 2).

  • This process is vital in cell respiration steps where a substrate needs to be oxidized.

  • NADH carries electrons and hydrogen ions (protons) which drive oxidative phosphorylation, generating the majority of ATP.

Glycolysis

• Glycolysis - The First Step:

  • The initial step of cell respiration, occurring in the cytoplasm, regardless of aerobic or anaerobic conditions.

  • Involves splitting a single molecule of glucose (6-carbon compound) into two molecules of pyruvate (3-carbon compound).

• Process of Glycolysis:

  • Glycolysis consists of a complex sequence of 10 steps, each catalyzed by its own enzyme (Figure 3).

• Four Main Phases of Glycolysis:

  • Phosphorylation:

    • Requires the use of two molecules of ATP.

    • ATP phosphorylates the glucose molecule, making it more unstable.

  • Lysis:

    • The unstable, phosphorylated glucose molecule is split into two.

    • Forms two glyceraldehyde 3-phosphate (G3P) molecules.

  • Dehydrogenation and Oxidation:

    • Each G3P molecule undergoes dehydrogenation and oxidation.

    • Reduces two molecules of NAD to NADH (reduced NAD).

  • ATP Formation:

    • Two ATP molecules are generated by substrate-level phosphorylation from each G3P.

    • The final product of pyruvate is formed.

• Substrate-Level Phosphorylation:

  • The process of generating ATP using ADP and a phosphate taken from another molecule (the substrate).

• Summary of Glycolysis:

  • Involves the investment of two ATP to produce four ATP, resulting in a net yield of two ATP.

  • Two molecules of NADH (reduced NAD) are formed.

  • Two molecules of pyruvate are produced.

Note: You are not required to memorize the intermediates formed during glycolysis or the enzymes involved. You should know that glycolysis involves phosphorylation, lysis, oxidation and ATP formation and that each step in the pathway is catalysed by a different enzyme.