Cytoplasm: Composition, Functions, and How It Differs from Cytosol and Protoplasm

Almost everything a cell does happens in its cytoplasm. It is the crowded, gel-like interior where organelles sit, where thousands of chemical reactions run at once, and where the machinery that builds proteins, generates energy, and moves materials is suspended. Understanding the cytoplasm clearly is not just a fact to memorize for one exam question. It is the setting in which nearly every later topic in cell biology takes place, from metabolism and protein synthesis to how a bacterium differs from a human cell. Getting this foundation right makes the bigger topics much easier.

This article explains what the cytoplasm is, what it is made of, what it does, and the distinctions that students most often get wrong: cytoplasm versus cytosol, and cytoplasm versus protoplasm.

What is the cytoplasm?

The cytoplasm is the entire contents of a cell enclosed by the cell membrane, excluding the nucleus. It is a semi-fluid, gel-like material in which the cell's organelles are suspended, along with dissolved molecules and a supporting protein scaffold.

Two points in that definition matter for everything that follows. First, the cytoplasm is the contents of the cell, not a structure in its own right, which is why the cytoplasm itself is not an organelle (more on this below). Second, in a eukaryotic cell the cytoplasm stops at the nuclear envelope: the material inside the nucleus is called nucleoplasm, not cytoplasm.

The word cytoplasm was coined in 1863 by the Swiss biologist Rudolf von Kölliker. He first used it as a synonym for protoplasm, but the meaning narrowed over time to what we mean today: the cell contents outside the nucleus. (This corrects a common textbook error: Robert Hooke, in 1665, described the walls of dead cork cells, not the living cytoplasm.)

Composition of the cytoplasm

The cytoplasm has four components: the cytosol, the organelles, the cytoskeleton, and inclusions. Learning these four is the key to the whole topic, because most exam questions and most later concepts hang off this list.

1. Cytosol

The cytosol is the liquid part of the cytoplasm, also called the cytoplasmic matrix. It is the fluid in which everything else is suspended. About 70% of the cytosol is water, with dissolved ions and organic molecules making up the rest. The main ions in a mammalian cytosol are potassium (K⁺), chloride (Cl⁻), magnesium (Mg²⁺), calcium (Ca²⁺), and bicarbonate (HCO₃⁻), and the dissolved organic molecules include amino acids, sugars, and small proteins.

The cytosol is not just filler. It is where a large share of the cell's chemistry happens. In eukaryotic cells, many metabolic reactions (such as glycolysis) occur in the cytosol; in prokaryotic cells, which have no membrane-bound organelles, almost all metabolic reactions take place there. Its dissolved ions also make it central to osmoregulation and cell signaling, and to generating the electrical changes (action potentials) that nerve and muscle cells depend on.

2. Organelles

Organelles are the specialized internal structures that carry out specific jobs, suspended within the cytoplasm like organs within a body. Membrane-bound organelles are found in eukaryotic cells; prokaryotic cells lack them. The major organelles and their one-line roles:

• Mitochondria produce most of the cell's energy (ATP).
• Ribosomes build proteins.
• Endoplasmic reticulum synthesizes lipids, processes proteins, and transports materials between organelles.
• Golgi apparatus modifies, packages, and sorts molecules for secretion.
• Lysosomes digest waste and defend the cell.
• Chloroplasts carry out photosynthesis (in plant cells only).
• Vacuoles store materials and, in plants, maintain internal pressure.

Each of these has its own detailed biology. The point at this level is to see them as residents of the cytoplasm, held in place and connected by the cytosol and cytoskeleton around them.

3. Cytoskeleton

The cytoskeleton is a network of protein fibers running through the cytoplasm that gives the cell its shape, holds organelles in position, and drives movement and division. It has three classes of fiber, differing in size:

• Microfilaments (actin), the thinnest, involved in shape and cell movement.
• Intermediate filaments, which give mechanical strength.
• Microtubules, the thickest, which act as tracks for transport and form the spindle during cell division.

The cytoskeleton is why the cytoplasm is organized rather than a random soup, and why organelles do not simply drift and pile up.

4. Inclusions

Inclusions are non-membrane-bound substances stored in the cytoplasm, essentially the cell's stored reserves rather than working machinery. They include lipid droplets, and stored carbohydrate: starch in plant cells and glycogen in animal cells. Both are energy stores; the difference in which polymer is used (starch versus glycogen) is a classic plant-versus-animal exam point.

Functions of the cytoplasm

The cytoplasm's overall role is to host and support the life of the cell. More specifically:

• Site of metabolism. A large share of the cell's chemical reactions, including the breakdown of complex molecules into simpler ones, happens in the cytosol. In bacteria, nearly all of it does.
• Support and shape. The cytoskeleton maintains the cell's shape and holds organelles in place, and the cytosol fills the spaces between structures.
• Transport. Materials, organelles, and inclusions are moved around the cell, partly by a process called cytoplasmic streaming (the bulk flowing movement of cytoplasm within the cell). This also helps clear waste.
• Storage. The cytoplasm stores enzymes, energy reserves (starch or glycogen), lipids, and the ions and small molecules the cell needs on hand.
• Protection and stability. By holding organelles steady in position and cushioning them, the cytoplasm reduces damage from mechanical stress.

Cytoplasmic streaming

Cytoplasmic streaming (also called cyclosis) is the directed flowing movement of the cytoplasm within a living cell. It is easiest to see in large plant cells, where it carries chloroplasts and other structures in a steady circulation, and it is one of the clearest ways to demonstrate under a microscope that the cytoplasm is a moving, living material and not an inert filling.

Cytoplasm vs cytosol vs protoplasm

These three terms are constantly confused, and keeping them straight is one of the most useful things you can take from this topic, because the distinction recurs throughout cell biology.

Cytoplasm is everything inside the cell membrane except the nucleus: the cytosol plus the organelles, cytoskeleton, and inclusions suspended in it.

Cytosol is only the liquid part, the fluid matrix, without the organelles. So the cytosol is a part of the cytoplasm, not a synonym for it. Cytoplasm = cytosol + organelles + cytoskeleton + inclusions.

Protoplasm is the entire living content of the cell, which means the cytoplasm plus the nucleus. So protoplasm is the larger term: protoplasm = cytoplasm + nucleus.

The history behind these names is worth a line, because it explains the overlap. Purkinje coined "protoplasm" in 1839 for the living substance of the cell; von Mohl applied it to plant cells in 1846; and Kölliker coined "cytoplasm" in 1863, at first as a synonym before its narrower modern meaning settled in.

Cytoplasm in plant and animal cells

The cytoplasm of plant and animal cells is largely the same, but a few differences follow from the organelles and inclusions each contains:

• Plastids, including chloroplasts, are present in the plant cell cytoplasm and absent in animal cells.
• A large central vacuole dominates the cytoplasm of a mature plant cell, taking up much of its volume; animal cells have small vacuoles or none.
• Stored carbohydrate differs: plant cells store starch, animal cells store glycogen.
• Phragmoplast, a structure formed during the late stage of cell division, appears in plant cells only.

How to Remember

Cytoplasm is the room; organelles are the furniture; cytosol is the air. The cytoplasm is the whole interior of the cell (minus the nucleus). The organelles are the working structures placed in it. The cytosol is the fluid filling the space around them. The cytoplasm is not a single thing, it is the room and everything in it.

Three nested terms, smallest to largest: cytosol, cytoplasm, protoplasm. Cytosol is just the fluid. Add the organelles and you get cytoplasm. Add the nucleus and you get protoplasm. Each term contains the one before it.

Four parts of cytoplasm: "SOCI." CytoSol, Organelles, Cytoskeleton, Inclusions. Or simply: the fluid, the machines, the scaffold, and the stores.

Starch for plants, glycogen for animals. Both are stored in the cytoplasm as inclusions; plants store starch, animals store glycogen.

Key exam facts

Where Students Get Confused

Cytoplasm and cytosol are the same thing. They are not. The cytosol is only the liquid matrix. The cytoplasm is the cytosol plus the organelles, cytoskeleton, and inclusions suspended in it. Cytosol is a part of the cytoplasm.

Cytoplasm includes the nucleus. It does not. The cytoplasm is everything inside the cell membrane except the nucleus. The term that includes the nucleus is protoplasm (protoplasm = cytoplasm + nucleus).

The cytoplasm is an organelle. No. Organelles are structures that sit within the cytoplasm; the cytoplasm is the surrounding material they sit in. Calling the cytoplasm an organelle is like calling a room a piece of furniture.

Robert Hooke discovered the cytoplasm. No. Hooke (1665) described the walls of dead cork cells and could not have seen living cytoplasm. The living cell substance was first observed by Dujardin (1835) and named protoplasm by Purkinje (1839); the term cytoplasm came from Kölliker (1863).

Prokaryotes have no cytoplasm because they have no organelles. They do have cytoplasm. Bacteria have cytoplasm (mostly cytosol, with ribosomes and the nucleoid), and because they lack membrane-bound organelles, nearly all their metabolism happens right there in the cytoplasm. This is a key foundation for understanding bacterial cells.

References

1. Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell. 7th ed. New York: W. W. Norton; 2022.
2. Iwasa J, Marshall W. Karp's Cell and Molecular Biology. 8th ed. Hoboken: Wiley; 2016.
3. Reece JB, Urry LA, Cain ML, et al. Campbell Biology. 12th ed. New York: Pearson; 2021.