Cellular Composition: What Are Cells Made Of?

Cells are made mostly of water, with the remainder consisting of four major classes of large molecules: proteins, lipids, nucleic acids, and carbohydrates, plus a scattering of ions and small organic compounds. Water accounts for roughly 55 to 92 percent of a cell’s mass, depending on cell type, while macromolecules fill the rest of the space in a surprisingly dense, gel-like interior. That basic recipe is shared by virtually every living cell on Earth, but the proportions shift dramatically from one cell type to another, and even within the same cell as its conditions change.

Water Is the Overwhelming Majority

If you could weigh a single human cell and sort out what’s inside, water would dominate. Estimates put the mass percentage of water in a typical cell somewhere between about 55 and 92 percent, with macromolecules making up the remaining 7.5 to 45 percent.¹ That wide range reflects genuine diversity. A fat-storing adipocyte is packed with lipid droplets and has less free water relative to its mass. A freshly divided immune cell circulating in your blood is comparatively lean and watery. But in every case, water is the medium in which virtually all of a cell’s chemistry takes place. It dissolves salts and sugars, shuttles molecules around, and participates directly in many chemical reactions. Removing water from a cell doesn’t just shrink it; it stops essentially every process the cell performs.

Proteins Run the Show

After water, proteins are generally the most abundant macromolecule by mass in most animal cells. They serve as enzymes that speed up chemical reactions, structural scaffolds that give cells shape, molecular motors that move cargo along internal tracks, and receptors that sense signals from the outside world. A typical human cell expresses thousands of different protein types simultaneously, and the total number of individual protein molecules in a single cell can run into the billions.

Red blood cells offer an unusually clear window into how protein-centric a cell can be. During maturation, these cells eject their nucleus and break down nearly all their internal organelles, leaving behind a simplified package built around roughly a thousand distinct proteins that control shape, flexibility, and gas transport.² The most famous of those proteins is hemoglobin, which fills the cell interior so densely that it accounts for about a third of the cell’s total mass. That level of specialization is extreme, but it illustrates the general principle: proteins are the workhorse molecules, and their identity determines what a given cell actually does.

Lipids Build Barriers and Store Energy

Every cell is wrapped in a lipid membrane, a thin double layer of fatty molecules that separates the cell’s interior from the outside world. This membrane isn’t just a passive barrier. It’s studded with proteins and carbohydrates that regulate what enters and exits, and its composition affects how flexible or rigid the cell surface is. Cholesterol, for instance, sits between the fatty acid chains of the membrane and adjusts its fluidity. In the membranes of macrophages, the immune cells that clean up debris in your blood vessels, cholesterol transport between the cell surface and circulating lipoproteins plays a central role in the development of atherosclerosis.³

Beyond membranes, lipids also serve as long-term energy storage. A gram of fat stores more than twice the energy of a gram of carbohydrate, which is why your body preferentially stores surplus calories as lipid droplets inside fat cells. Lipids also act as signaling molecules: steroid hormones like estrogen and testosterone are built from cholesterol, and certain lipids released from damaged cell membranes trigger inflammation.

The proportion of lipid in a cell varies wildly depending on function. Myelin-producing cells in the nervous system are a striking example. The myelin sheath that insulates nerve fibers contains an unusually high content of lipids, and building it requires intense fatty acid and lipid synthesis along with uptake of fatty acids from the surrounding environment.⁴ Myelin is roughly 70 to 80 percent lipid by dry weight, which is the reverse of most cell membranes. That lipid-rich insulation allows nerve signals to travel at high speed, and when myelin breaks down in diseases like multiple sclerosis, the result is a dramatic slowing or loss of signal transmission.

Nucleic Acids and Carbohydrates

DNA and RNA together make up a smaller fraction of a cell’s total mass than proteins or lipids, but their importance is disproportionate. DNA in the nucleus stores the instructions for building every protein the cell can produce, while various forms of RNA carry those instructions to the protein-building machinery and help regulate which genes are active at any given time. In rapidly dividing cells, RNA levels spike because the cell needs to churn out proteins quickly to furnish two daughter cells.

Carbohydrates play several roles. Inside the cell, sugars like glucose are the immediate fuel for energy production. But cells also decorate their outer surfaces with complex sugar chains called glycans, which act like molecular name tags. The endothelial glycocalyx, a mesh of sugar-rich molecules lining the inner surface of blood vessels, is made up of membrane-bound proteoglycans and glycoproteins, along with soluble molecules from both the cell and the blood plasma that integrate into the network.⁵ This sugar coat helps regulate blood flow, controls which immune cells can stick to the vessel wall, and acts as a physical barrier against pathogens. When the glycocalyx is damaged, as happens in severe infections or diabetes, blood vessels become leaky and more prone to clotting.

Ions and Small Molecules

Scattered among the large macromolecules are inorganic ions and small organic compounds that together account for a modest percentage of cell mass but are critical for almost everything the cell does. Potassium is the dominant positive ion inside cells, while sodium dominates outside. This imbalance is actively maintained by pumps in the cell membrane, and the resulting difference in electrical charge across the membrane is what allows nerve and muscle cells to fire. Calcium ions, kept at extremely low concentrations inside the cell under normal conditions, serve as a rapid on-off switch for processes like muscle contraction and hormone release.

When a cell’s energy supply is disrupted, these ion gradients collapse in a predictable sequence. Sodium floods in, potassium leaks out, the membrane loses its electrical charge, and eventually calcium surges to toxic levels.⁶ That calcium surge is one of the main triggers of cell death during a stroke. The fact that a cell can die not from running out of building material but from losing control of a few charged atoms underscores how central these small ions are to staying alive.

Other important small molecules include ATP (the cell’s main energy currency), amino acids waiting to be assembled into proteins, and metabolic intermediates like pyruvate and citrate that serve as stepping stones in energy production. Trace elements such as iron, zinc, copper, and manganese are present in tiny amounts but are embedded in the active sites of enzymes where they catalyze essential reactions. Iron, for example, sits at the core of each hemoglobin molecule and is what actually binds oxygen in your red blood cells.

A Surprisingly Crowded Interior

One of the most counterintuitive things about the inside of a cell is how packed it is. Textbook diagrams often depict organelles floating in a spacious, watery cytoplasm, but the reality is more like a subway car at rush hour. Macromolecules, ribosomes, filaments, and membrane-bound compartments fill the space so densely that the environment behaves less like a liquid and more like a gel or a porous medium.

This crowding has real physical consequences. Large molecules can’t simply diffuse freely through the cytoplasm the way they would in a beaker of water. Recent work modeling the cytoplasm as a hierarchical porous medium with obstacles at both nanometer and micrometer scales found that both tortuous pathways and drag from nearby obstacles significantly slow down diffusion.⁷ In practical terms, a protein released at one end of a cell takes substantially longer to reach the other end than it would in pure water.

That slowdown isn’t necessarily a problem. Crowding also pushes molecules closer together, which can speed up certain reactions by effectively raising the local concentration of the participants. The physical properties of a crowded cellular environment influence everything from how fast enzymes work to how proteins fold to whether two binding partners can find each other.⁸ This is one reason why reactions that work perfectly in a test tube sometimes behave differently inside a living cell. The chemical ingredients are the same, but the physical environment is not.

How Plant and Fungal Cells Differ

The basic recipe of water, proteins, lipids, nucleic acids, carbohydrates, and ions applies across all life. But plant and fungal cells add a structural feature that animal cells lack: a rigid cell wall outside the membrane. The composition of that wall varies in ways that matter both for the organism and, sometimes, for medicine.

Plant cell walls are built primarily from cellulose and pectin, two types of carbohydrate polymer. The ratio between them affects how the plant interacts with water. Species whose epidermal cell walls are rich in cellulose absorb more water overall but do so slowly, while species with more pectin in their walls absorb water at a faster rate, because pectin creates greater porosity.⁹ This distinction has ecological significance: plants in foggy environments that depend on absorbing water through their leaves have evolved different wall compositions to match their particular water-availability patterns.

Fungal cell walls are chemically distinct. Their structural backbone is chitin, a carbohydrate polymer that is not found in vertebrates at all.¹⁰ That biochemical difference is medically useful: antifungal drugs can target chitin synthesis without harming human cells, which have no equivalent pathway. It’s the same logic behind antibiotics that attack bacterial cell walls. If the target molecule doesn’t exist in the patient, the drug can be selective.

Red Blood Cells and Neurons Show How Composition Reflects Function

Knowing the general ingredients of a cell is useful, but the proportions matter at least as much. Two of the most dramatically specialized cells in the human body illustrate this point.

Red blood cells are stripped-down delivery vehicles. They have lost their nucleus, mitochondria, and most other organelles during maturation, leaving behind a cell that is essentially a membrane bag packed with hemoglobin.¹¹ This extreme simplification maximizes the space available for oxygen-carrying protein. It also means red blood cells cannot repair themselves or divide. They are disposable, replaced by the bone marrow at a rate of about two million per second. Their composition is so protein-heavy and organelle-poor that they barely resemble most other human cells under a microscope.

Neurons sit at the opposite end of the complexity spectrum. They are among the longest-lived cells in the body, can extend axons up to a meter long, and depend on a lipid-rich myelin sheath produced by neighboring glial cells to conduct electrical signals efficiently.⁴ Where a red blood cell is almost all protein and water, a myelinated nerve fiber has a composition skewed heavily toward lipid. Brain tissue overall is one of the most lipid-rich organs in the body, and that lipid isn’t inert insulation; it participates in signaling, helps form synapses, and its breakdown products activate immune responses when damage occurs.

Cell Composition Changes with Conditions

A cell’s composition is not fixed. It shifts as the cell grows, divides, ages, or encounters stress. Using microscopy techniques that can map protein, lipid, and water concentrations in individual living cells, researchers have found that these concentrations are maintained within a tight range under stable conditions but change measurably when the cell enters a different phase of its life cycle, attaches to a surface of different stiffness, or enters a dormant state called senescence.¹²

This matters because diseases often alter the composition of affected cells in ways that can be detected. Cancer cells, for instance, often show elevated levels of certain lipids and altered protein profiles compared to their healthy neighbors. Senescent cells, the ones that have stopped dividing but refuse to die, accumulate lipids and change their protein output in characteristic patterns. These compositional fingerprints are being explored as potential diagnostic markers, with the goal of catching disease-related changes earlier than current methods allow.

Even something as ordinary as the mechanical stiffness of the tissue a cell lives in can shift its internal makeup. Cells growing on a stiff surface tend to produce more cytoskeletal proteins (the filaments that give the cell shape and allow it to pull on its surroundings), while cells on soft surfaces redistribute their resources differently. Your body already exploits this: cells in bone are biochemically different from cells in your liver in part because they experience vastly different mechanical environments, and those environments actively shape what the cell produces.

Measuring What’s Inside a Single Cell

For most of the history of cell biology, knowing what cells were made of required grinding up millions of them and measuring the resulting mixture. That approach gives good average values but hides the cell-to-cell variation that turns out to be biologically meaningful. Two genetically identical cells sitting side by side can have measurably different protein or lipid concentrations, and those differences can determine whether one cell divides and the other stays quiet.

Modern single-cell analysis techniques have changed the picture dramatically. Methods like mass spectrometry adapted for individual cells, Raman spectroscopy that maps molecular vibrations without destroying the sample, and fluorescent labeling that tracks specific molecules in real time all allow researchers to profile the composition of one cell at a time.¹³ The challenge is sensitivity: a single human cell weighs roughly a nanogram, and some important molecules are present in only a few thousand copies per cell. Detecting those low-abundance molecules against the background of billions of water molecules and millions of proteins requires instruments operating near the edge of their capabilities.

Cryo-imaging approaches that flash-freeze cells and then analyze thin slices can preserve the spatial arrangement of elements and molecules, showing not just what is present but where it sits within the cell. Work using these methods has revealed, for example, that the water and element content inside a cell changes in response to stress on the nucleolus, the structure inside the nucleus where ribosomes are assembled.¹ Findings like these would be invisible with bulk measurements, because the signal from a few stressed cells would be drowned out by the thousands of unstressed cells in the same sample. The ability to look at one cell at a time is what makes such discoveries possible and is steadily rewriting the textbook understanding of what cells are actually made of, one cell at a time.