U01.01.079 Metabolism sites

Understanding the key metabolic pathways and their locations in the cell is crucial for USMLE Step 1. The following is a breakdown of major metabolic processes and their cellular locations.

1. Mitochondria

The mitochondria are the powerhouse of the cell, responsible for energy production and various metabolic processes.

Fatty Acid Oxidation (β-oxidation): Breakdown of fatty acids to acetyl-CoA for ATP production.
Acetyl-CoA Production: Key intermediate in cellular respiration and metabolic reactions.
TCA Cycle (Citric Acid Cycle): Central metabolic pathway involved in energy production.
Oxidative Phosphorylation: ATP production through electron transport and proton gradient.
Ketogenesis: Production of ketone bodies from fatty acids during fasting or low carbohydrate intake.

2. Cytoplasm

The cytoplasm is the site for various biosynthetic processes and intermediate metabolism.

Glycolysis: Breakdown of glucose to pyruvate, generating ATP.
HMP Shunt (Hexose Monophosphate Pathway): Involved in the production of NADPH and ribose-5-phosphate.
Synthesis of Cholesterol (Smooth Endoplasmic Reticulum, SER): Cholesterol biosynthesis occurs in the SER.
Synthesis of Proteins (Ribosomes, Rough Endoplasmic Reticulum, RER): Protein synthesis occurs on ribosomes attached to the RER.
Synthesis of Fatty Acids: Fatty acid synthesis takes place in the cytoplasm.
Synthesis of Nucleotides: Nucleotide biosynthesis is also cytoplasmic.

3. Both Mitochondria and Cytoplasm

Some metabolic processes occur in both the mitochondria and the cytoplasm.

Heme Synthesis: Begins in mitochondria, but the later steps occur in the cytoplasm.
Urea Cycle: Occurs partly in the mitochondria and partly in the cytoplasm.
Gluconeogenesis: Primarily occurs in the cytoplasm, but certain steps take place in the mitochondria.

Key Mnemonic

HUGs Take Two: Remember that Heme synthesis, Urea cycle, and Gluconeogenesis occur in both the mitochondria and cytoplasm.

Points to Remember:

Mitochondria: Key for energy production (β-oxidation, TCA cycle, oxidative phosphorylation) and ketogenesis.
Cytoplasm: Central for glycolysis, HMP shunt, protein, fatty acid, and nucleotide synthesis.
Both Locations: Heme synthesis, urea cycle, and gluconeogenesis take place in both the mitochondria and cytoplasm.

USMLE Step 1 Review Series

Curriculum