Cytoplasm and Cytosol

The cytoplasm is everything enclosed by the plasma membrane apart from the nucleus, comprising the cytosol together with the organelles, cytoskeleton, and inclusions suspended within it. The cytosol is the aqueous gel-like phase of the cytoplasm in which most intermediary metabolism occurs; it is densely packed with proteins and other macromolecules, a crowded environment that shapes the behaviour of the reactions taking place there.

Definition

The cytoplasm is the contents of the cell within the plasma membrane excluding the nucleus, and the cytosol is its aqueous, soluble phase, the medium in which organelles are suspended and in which many metabolic reactions occur.

Scope

This entry distinguishes cytoplasm from cytosol and describes their composition, physical properties such as macromolecular crowding, and the processes they host. It is a reference and educational topic in cell biology. Detailed treatment of individual organelles and of the cytoskeleton appears in related entries, and no clinical guidance is provided.

Core questions

• What is the difference between cytoplasm and cytosol?
• What is dissolved or suspended in the cytosol?
• How does the dense, crowded nature of the cytosol affect biochemistry?
• Which cellular processes take place in the cytosol versus in organelles?

Key concepts

• Cytoplasm versus cytosol
• Cytosol as an aqueous phase
• Macromolecular crowding
• Cytoplasmic inclusions (e.g. glycogen, lipid droplets)
• Intermediary metabolism in the cytosol
• Diffusion and viscosity of the cytoplasm
• Biomolecular condensates

Key theories

Macromolecular crowding

Zhou, Rivas, and Minton describe how the high total concentration of macromolecules in the cytosol occupies a large fraction of the available volume, thermodynamically favouring compact states, association reactions, and altered reaction rates relative to dilute solution.

Mechanisms

The cytosol is a concentrated aqueous solution of ions, metabolites, and a high density of proteins and RNA; this crowding means a substantial fraction of the volume is excluded, which thermodynamically favours macromolecular compaction and binding and influences reaction rates and equilibria compared with dilute conditions. Suspended within the cytoplasm are the membrane-bounded organelles, the cytoskeletal network, and storage inclusions such as glycogen granules and lipid droplets. Many core metabolic pathways, including glycolysis, occur in the cytosol, and membraneless biomolecular condensates can form by phase separation to organize particular reactions without a bounding membrane.

Clinical relevance

The cytoplasm is the site of much of the cell's metabolism and of the storage materials seen in tissue sections, so its composition is relevant to histology and pathology. This entry describes normal cytoplasmic biology for reference and education and does not offer diagnostic or treatment guidance.

Evidence & guidelines

The account here is grounded in biophysical reviews of macromolecular crowding and in standard cell biology textbooks; it is descriptive basic science rather than clinical guideline content.

History

Nineteenth-century microscopists distinguished the cell's protoplasm from the nucleus, and the term cytoplasm was introduced to name the non-nuclear contents. As biochemistry localized metabolic pathways and electron microscopy resolved the organelles, the soluble fraction came to be called the cytosol. Later biophysical work, reviewed by Zhou and colleagues, recognized that this fraction is far from a dilute solution and that macromolecular crowding strongly conditions cellular biochemistry, and recent studies have added membraneless condensates as a mode of cytoplasmic organization.

Key figures

• Allen P. Minton
• Huan-Xiang Zhou
• Germán Rivas

Seminal works

• zhou-2008
• alberts-2015

Frequently asked questions

Are cytoplasm and cytosol the same thing?

No. The cytoplasm is everything inside the plasma membrane except the nucleus, including organelles and the cytoskeleton; the cytosol is just the surrounding soluble aqueous phase, one component of the cytoplasm.

Why does crowding in the cytosol matter?

Because macromolecules occupy a large fraction of the volume, the effective concentrations and behaviour of molecules differ from dilute solution, favouring compact protein conformations and binding interactions, which affects how reactions proceed in the cell.