Cell Metabolism: Key Concepts, Pathways, and Calculations

Cell Metabolism

Key Terms in Cell Metabolism

Understanding cell metabolism requires familiarity with several foundational terms. Below is a table defining each key term relevant to metabolic processes in cells.

Term	Definition
Metabolism	The sum of all chemical reactions that occur within a living organism to maintain life, including both energy-producing and energy-consuming processes.
Anabolism	The set of metabolic pathways that construct molecules from smaller units, typically requiring energy input (e.g., protein synthesis).
Catabolism	The set of metabolic pathways that break down molecules into smaller units, releasing energy (e.g., glycolysis, cellular respiration).
Metabolic Pathway	A series of interconnected biochemical reactions that convert a substrate molecule through a series of metabolic intermediates, ultimately yielding a final product.
Intermediate	A compound that forms in the middle steps of a metabolic pathway, between the initial substrate and the final product.
Phosphorylation	The addition of a phosphate group (PO43−) to a molecule, often regulating activity or function, especially in proteins and metabolic intermediates.
Dephosphorylation	The removal of a phosphate group from a molecule, often reversing the effects of phosphorylation.
Oxidation-reduction (Redox)	Chemical reactions involving the transfer of electrons between molecules; oxidation is the loss of electrons, and reduction is the gain of electrons.
Oxidation	The process by which a molecule loses electrons, often associated with the release of energy.
Substrate-level phosphorylation	The direct transfer of a phosphate group to ADP from a phosphorylated intermediate, producing ATP during glycolysis and the Krebs cycle.
Oxidative phosphorylation	The production of ATP using energy derived from the transfer of electrons in the electron transport chain, coupled to the movement of protons across a membrane.
Chemiosmotic coupling	The mechanism by which the energy stored as a proton gradient across a membrane is used to drive cellular work such as ATP synthesis.

Structure and Function of the Mitochondrion

The mitochondrion is the primary site of ATP production in eukaryotic cells. Its structure is specialized for efficient energy conversion.

• Intermembrane space: The region between the inner and outer mitochondrial membranes; site of proton accumulation during electron transport.

• Mitochondrial matrix: The innermost compartment containing enzymes for the Krebs cycle and mitochondrial DNA.

• H+ pumps (I, III, IV): Protein complexes in the inner mitochondrial membrane that actively transport protons (H+) from the matrix to the intermembrane space, creating a proton gradient.

• ATP Synthase: An enzyme complex that synthesizes ATP as protons flow back into the matrix down their concentration gradient.

• Area of greater H+ concentration: Intermembrane space.

• Area of lesser H+ concentration: Mitochondrial matrix.

Additional info: The electron transport chain (ETC) is embedded in the inner mitochondrial membrane and is responsible for establishing the proton gradient used in chemiosmotic coupling.

Glycolysis: Products and Calculations

Glycolysis is the first stage of glucose metabolism, occurring in the cytoplasm. It breaks down one molecule of glucose into two molecules of pyruvate, generating ATP and NADH.

• Products per glucose molecule:

  • Pyruvate: 2

  • ATP: 2 (net gain)

  • NADH: 2

  • H+: 2 (from NADH formation)

Krebs Cycle (Citric Acid Cycle): Products and Calculations

The Krebs cycle occurs in the mitochondrial matrix and completes the oxidation of glucose derivatives, producing electron carriers and CO2.

• Products per glucose molecule (2 cycles):

  • NADH: 6

  • FADH2: 2

  • ATP (via substrate-level phosphorylation): 2

  • CO2: 4

  • Acetyl CoA: 2 (input, not product)

ATP Synthesis: Substrate-Level and Oxidative Phosphorylation

ATP can be synthesized by two main mechanisms:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated substrate.

• Oxidative phosphorylation: ATP synthesis powered by the transfer of electrons through the electron transport chain and the resulting proton gradient.

ATP Yield Calculations

ATP yield from NADH and FADH2 during oxidative phosphorylation:

Coenzyme	Total Coenzymes produced	Multiply by	Total ATP per coenzyme type
NADH	See tallies above	2.5	Calculated value
FADH2	See tallies above	1.5	Calculated value
Total ATP by Oxidative Phosphorylation			Sum of above

Additional info: The actual ATP yield may vary depending on cell type and conditions, but the theoretical maximum is often cited as 32-34 ATP per glucose molecule.

Phosphorylation vs. Dephosphorylation Reactions

Phosphorylation and dephosphorylation are key regulatory mechanisms in metabolism.

• Phosphorylation: Addition of a phosphate group, often activating or deactivating enzymes.

• Dephosphorylation: Removal of a phosphate group, reversing the effect of phosphorylation.

Example reactions:

• $\text{ADP} + P_i \rightarrow \text{ATP}$ (Phosphorylation)

• $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + P_i + \text{energy}$ (Dephosphorylation)

Stages of Glucose Oxidation and Cellular Locations

Glucose oxidation occurs in three main stages, each in a specific cellular location:

Phase	Location
Glycolysis	Cytoplasm
Krebs Cycle (Citric Acid Cycle)	Mitochondrial matrix
Electron Transport Chain & Oxidative Phosphorylation	Inner mitochondrial membrane

Alternative Molecules for Cellular Respiration

Besides glucose, other molecules can be used for cellular respiration:

• Fatty acids (via beta-oxidation)

• Amino acids (after deamination)

• Other carbohydrates (e.g., fructose, galactose)

Additional info: These alternative fuels enter the metabolic pathways at various points, such as acetyl CoA or intermediates of the Krebs cycle.