protoplasma

Protoplasma is the living content of a cell, enclosed by the plasma membrane. In many texts the term covers the entire contents inside the cell, including the cytoplasm and the nucleus; in others it is reserved for the living part of the cytoplasm. In modern biology the preferred terms are cytoplasm (the contents outside the nucleus) and nucleoplasm (the contents within the nucleus), while protoplasm is used mainly in historical or botanical contexts.

Composition: Protoplasm is a complex, aqueous mixture of water, salts, organic molecules, and macromolecules. Cytoplasm comprises cytosol—the gel-like fluid—and organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, and, in plant cells, plastids. The nucleus contains chromatin and nucleolus. The protoplasm also contains cytoskeletal elements.

Functions: It is the site of most cellular metabolic processes, protein synthesis, energy production, and, in many cells, movement and growth. In plant and algal cells, chloroplasts introduce photosynthetic activity into the protoplasm.

History: The term protoplasm entered scientific vocabulary in the 19th century to denote the living substance within cells, contrasting with the nonliving cell wall. As microscopy and biochemistry advanced, the term became less precise and is less used in modern cellular biology, though it remains common in some textbooks and in descriptions of plant protoplasts.

See also: Protoplast.