Lecture Notes: Cytosol

Overview

• Cytosol: Semi-fluid component of the cytoplasm, located outside the nucleus and within the cell membrane in eukaryotic cells.
• Also known as intracellular fluid or cytoplasmic matrix.
• Forms the liquid matrix around organelles, excluding the cytoplasm within organelles.
• Term introduced by H. A. Hardy in 1965.

Difference Between Cytosol and Cytoplasm

• Cytoplasm:
  • Gelatin-like, semi-transparent.
  • Comprises cytosol, cell organelles, and inclusions.
  • Made up of 80% water, nucleic acids, enzymes, lipids, carbohydrates, amino acids, inorganic ions, and other low molecular weight compounds.
  • Contains dissolved salts and nutrients for water absorption.
  • Divided into endoplasm and ectoplasm.
• Cytosol:
  • Mostly water, dissolved ions, proteins, and other soluble molecules.
  • Different ion concentrations compared to extracellular fluid, crucial for osmoregulation, cell signaling, and action potentials.

Metabolic Pathways

• Eukaryotes: Many pathways within organelles, some in cytosol.
• Prokaryotes: Almost all metabolic pathways in cytosol.

Composition of Cytosol

• Forms bulk of cell types; less in plant cells due to large vacuole.
• Main components: water, proteins, dissolved ions, small molecules.
• Major ions: potassium, sodium, bicarbonate, chloride, calcium, magnesium, amino acids.
• Ions create gradients for cell communication (e.g., nerve cells).
• Includes cytoplasmic inclusions and non-membrane bound structures.

Cytosolic Water

• Makes up 70% of cytosol volume.
• pH: 7.4, higher if cell is growing.
• Viscosity similar to pure water but diffusion is 4x slower.
• 5% water is strongly bound (water of solvation).

Ion Concentration

• Cytosol has higher potassium, lower sodium compared to extracellular fluid.
• Sodium-potassium ATPase pumps potassium in, expels sodium.
• Calcium ions lower, function in signaling.
• Cellular osmoprotectants regulate osmotic changes.

Macromolecules in Cytosol

• Proteins: 20-30% of cytosol volume, many bound to cytoskeleton.
• Prokaryotic genome in nucleoid within cytosol.
• Macromolecular crowding affects reaction rates and equilibrium.

Protein Complexes and Compartments

• Proteins form complexes, compartments in cytosol.
• Example: proteasomes for protein degradation, carboxysomes in bacteria.

Functions of Cytosol

• Site for metabolic processes in eukaryotes and prokaryotes:
  • Protein synthesis, glycolysis, pentose phosphate pathway, gluconeogenesis, cytokinesis.
• Intracellular transport of molecules.
• Site for signal transduction.
• Maintains water balance and action potential.

Conclusion

• Cytosol is crucial for metabolic reactions in prokaryotes.
• In eukaryotes, supports activities like glycolysis, cell division, and other pathways.