CorpusHuman Anatomy
Cells & TissuesOrganelle

Cytoplasm and cytosol

Cytoplasma; cytosolum

Also known as: cytosol, hyaloplasm, cytoplasmic matrix, intracellular fluid

Everything between the plasma membrane and the nuclear envelope; its fluid part, the cytosol, is a crowded protein-rich gel where glycolysis, free-ribosome protein synthesis and many signalling reactions take place.

Plasma Membrane — Fluid Mosaic Model The Eukaryotic Cell

Overview

Intracellular K⁺ / Na⁺≈140 mM / ≈10–15 mM
Free cytosolic Ca²⁺≈100 nM
Cytosolic pH≈7.2
Protein concentration≈200–300 mg/mL

The cytoplasm comprises the cytosol (the aqueous intracellular fluid), the organelles suspended in it, the cytoskeleton and inclusions. The cytosol is not a dilute solution but a highly crowded gel: proteins reach ≈200–300 mg/mL, so diffusion of large particles is slowed and many enzymes are organised into complexes or condensates.

The cytosol has a characteristic ionic composition maintained by membrane pumps: high K⁺ (≈140 mM), low Na⁺ (≈10–15 mM), very low free Ca²⁺ (≈0.1 µM, versus ≈1.2 mM ionised Ca²⁺ outside), pH ≈7.2, and negatively charged proteins and organic phosphates as the main anions. Intracellular fluid makes up about two-thirds of total body water.

Metabolic pathways in the cytosol include glycolysis, the pentose phosphate pathway, fatty-acid synthesis, glycogen synthesis and breakdown, and parts of gluconeogenesis, the urea cycle and haem synthesis (which are split between cytosol and mitochondria). Proteins without a signal sequence are made on free ribosomes here; misfolded or short-lived cytosolic proteins are destroyed by proteasomes.

Parts & subdivisions

  1. Cytosol
    Aqueous, protein-rich intracellular fluid.
  2. Organelles
    Membranous and non-membranous "little organs" with specific functions.
  3. Cytoskeleton
    Protein filament network.
  4. Inclusions
    Stored products such as glycogen, lipid droplets and pigments.

Anatomy

Histology

With H&E, cytoplasm rich in RER/free ribosomes is basophilic (blue-purple, RNA phosphate groups bind haematoxylin); cytoplasm rich in mitochondria or structural proteins is eosinophilic (pink). Lipid- or glycogen-rich cytoplasm looks pale or vacuolated because these are extracted during processing.

Function

  • Medium for metabolic reactions (glycolysis, pentose phosphate pathway, fatty-acid synthesis).
  • Site of synthesis of cytosolic, nuclear, mitochondrial and peroxisomal proteins on free ribosomes.
  • Intracellular signalling (kinase cascades, Ca²⁺ and cAMP signals).
  • Transport medium for diffusion and motor-driven traffic.

Clinical significance

Cytosolic Ca²⁺ overload in cell injury

Ischaemia depletes ATP, the Na⁺/K⁺-ATPase and Ca²⁺ pumps fail, Na⁺ and water enter (cellular swelling — the earliest light-microscopic change of reversible injury) and Ca²⁺ floods the cytosol, activating phospholipases, proteases and endonucleases that drive irreversible damage.

Mnemonics

Pathways in both cytosol and mitochondria

"HUGs take two" — Haem synthesis, Urea cycle and Gluconeogenesis use both compartments.

Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.