cytosol
Summary
The cytosol is the water-based fluid inside the plasma membrane but outside the organelles, holding enzymes, ions and metabolites. Its most testable point is pathway location: glycolysis and fatty acid synthesis occur here, while the citric acid cycle and β-oxidation occur in the mitochondria.
Detail
Meaning: The cytosol is the water-based fluid inside the plasma membrane but outside the membrane-bound organelles. It holds dissolved ions, metabolites and enzymes, and the cytoskeleton lies within it. Many pathways run here, including glycolysis, fatty acid synthesis, the pentose phosphate pathway, and glycogen synthesis and breakdown. Free ribosomes in the cytosol make proteins that function in the cytosol or are imported into organelles such as the nucleus and mitochondria. Ribosomes bound to the rough ER make secreted and membrane proteins.
On the MCAT: Expect questions on where a pathway occurs. Glycolysis is cytosolic, while pyruvate dehydrogenase and the citric acid cycle work in the mitochondrial matrix. Fatty acid synthesis (cytosol) is kept apart from β-oxidation (matrix). Gluconeogenesis and the urea cycle span the cytosol and mitochondria. NADH made in glycolysis cannot cross the inner mitochondrial membrane directly, so shuttles such as the malate-aspartate shuttle and the glycerol-3-phosphate shuttle carry its electrons in. A passage that disables a mitochondrial transporter may ask which cytosolic metabolite builds up.
Common confusion: Cytosol is not a synonym for cytoplasm. Cytoplasm is the cytosol plus the organelles suspended in it. Prokaryotes lack mitochondria, but their glycolysis is still cytosolic.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.