A cell is the smallest unit that can carry out the basic processes of life. Bacteria, plants, animals, fungi, and many other organisms are made of cells, yet cells themselves are remarkably complex. They take in materials, obtain and use energy, maintain their internal conditions, respond to changes, grow, repair themselves, and reproduce.
What makes a cell alive is not one particular structure or molecule. Life emerges from a coordinated system of processes that allows a cell to maintain itself and, ultimately, produce new cells. Understanding those processes explains why a living cell is fundamentally different from a collection of chemicals, even though cells are made entirely of chemicals.
Cells have an organized boundary
Every living cell is enclosed by a plasma membrane, a thin, flexible boundary made primarily of lipids and proteins. The membrane separates the cell’s internal environment from its surroundings while allowing controlled exchange between them.
This boundary is essential because life requires a degree of internal organization. A cell must maintain particular concentrations of ions, nutrients, water, proteins, and other substances even when conditions outside the cell change.
The membrane is selectively permeable, meaning that some substances cross it readily while others require specialized proteins or are prevented from crossing. Small molecules such as oxygen and carbon dioxide can often cross the lipid portion of the membrane, while ions and many larger or charged molecules typically require transport proteins.
The membrane therefore does more than contain the cell. It helps the cell regulate what enters and leaves, communicate with its environment, and maintain the conditions needed for its internal chemistry.
Cells maintain an internal state
Living cells constantly work to keep their internal conditions within ranges compatible with life. This ability is called homeostasis.
Homeostasis does not mean that the inside of a cell remains perfectly unchanged. Instead, cells continuously adjust to disturbances. For example, if the concentration of a particular ion changes, membrane proteins can alter its movement across the membrane. If energy supplies change, metabolic pathways can be adjusted.
Maintaining these conditions requires energy. A cell that could not regulate its internal environment would eventually lose the organization necessary for its biochemical reactions.
This is one reason a virus is generally not considered a living cell: outside a host cell, a virus does not independently maintain the kind of regulated internal environment characteristic of cellular life.
Cells use energy to sustain themselves
Life requires a continuous supply of usable energy. Cells use energy to build molecules, transport substances, maintain ion gradients, move cellular structures, repair damage, and carry out many other forms of work.
The network of chemical reactions through which cells acquire and use energy is called metabolism. Some metabolic reactions break molecules apart and release energy; others use energy to construct larger molecules from smaller components.
A central energy-transfer molecule in cells is ATP (adenosine triphosphate). Cells can use the chemical energy associated with ATP to drive processes that would not occur efficiently on their own.
Different organisms obtain energy in different ways. Animals and fungi obtain chemical energy from food, while plants and certain microorganisms can capture energy from sunlight or obtain it through chemical reactions involving inorganic substances. Despite these differences, all cells need mechanisms for converting available energy into forms that can power cellular work.
Cells carry information in DNA
Living cells contain genetic information, usually in the form of DNA (deoxyribonucleic acid). DNA stores instructions for producing RNA and proteins and for regulating many aspects of cellular activity.
A gene is a segment of DNA that contributes to a functional product, such as a protein or a functional RNA molecule. The information in genes influences the cell’s structures, chemical reactions, responses, and ability to reproduce.
DNA is more than a storage system. Cells must be able to copy their genetic material, use particular genes at appropriate times, and pass genetic information to new cells. This creates a connection between information and cellular activity: genes help specify cellular machinery, while that machinery is responsible for copying and expressing the genes.
The ability to store, use, and transmit biological information is a defining feature of cellular life.
Cells build and maintain complex molecules
A living cell is constantly making and breaking molecules. Proteins, lipids, carbohydrates, nucleic acids, and many smaller compounds must be produced, modified, transported, and eventually broken down.
Proteins are especially important because they perform a huge range of cellular functions. Enzymes, for example, are proteins that accelerate specific chemical reactions. Other proteins form structures, transport substances, receive signals, generate movement, or regulate cellular processes.
Cells also continually replace damaged or worn-out components. This ongoing maintenance distinguishes living systems from passive collections of molecules. A cell is not simply assembled once and left unchanged; it continually uses energy and information to preserve and rebuild its organization.
Cells regulate chemical reactions
The chemical reactions necessary for life do not simply occur at useful rates by themselves. Cells control them through enzymes, regulatory molecules, changes in molecular concentrations, and the physical organization of cellular components.
Enzymes lower the activation energy required for particular chemical reactions, allowing those reactions to proceed rapidly under cellular conditions. Because different enzymes act on different molecules or reactions, cells can organize metabolism into interconnected pathways.
Regulation is crucial. Producing a substance is useful only when the cell needs it, and breaking down a molecule at the wrong time can be harmful. Cells therefore control metabolic pathways according to their internal state and environmental conditions.
This regulation allows thousands of chemical reactions to occur in a coordinated system rather than as an uncontrolled mixture.
Cells respond to their environment
Living cells detect and respond to changes around them. Their responses can involve movement, changes in metabolism, altered gene activity, or changes in the substances they take up or release.
Cells can respond to physical conditions such as temperature and chemical signals such as nutrients or hormones. In multicellular organisms, cells also communicate with one another through signaling molecules and specialized receptors.
A response does not necessarily involve movement. A cell might instead change which proteins it produces or alter the activity of an existing protein. What matters is that the cell can detect relevant changes and modify its behavior in ways that help maintain its functioning.
Cells grow and reproduce
Living cells can increase their mass and, under appropriate conditions, produce new cells. Growth requires the coordinated acquisition of materials and energy and the production of additional cellular components.
Reproduction occurs through cell division. Before dividing, a cell must generally duplicate its genetic material and distribute that information to its daughter cells. The details differ among organisms, but the underlying challenge is the same: biological information and cellular machinery must be passed on accurately enough to preserve life.
In multicellular organisms, cell division is also essential for growth, development, and tissue maintenance. In single-celled organisms, cell division can produce an entirely new individual.
Cells can evolve as populations
Individual cells do not evolve during their own lifetimes in the usual biological sense. Evolution occurs across populations over generations when heritable genetic differences affect survival or reproduction.
Cells nevertheless have the properties that make evolution possible. Their genetic information can be copied with occasional changes, and those changes can sometimes be inherited. If a genetic difference influences reproductive success, natural selection can alter its frequency in a population over generations.
This capacity for evolution is one of the most important characteristics of life. It explains how cellular organisms can become adapted to remarkably different environments and how the diversity of life can arise from populations of reproducing organisms.
What all living cells have in common
The enormous diversity of cells can make their common features easy to overlook. A bacterium and a human nerve cell differ dramatically in size, structure, and function, but both must solve the same fundamental problems.
They need a boundary that separates an internal environment from the outside world. They need chemical reactions that acquire and use energy. They need mechanisms for storing and using biological information. They need to regulate their internal conditions, build and repair cellular components, respond to their surroundings, and reproduce or contribute to reproduction.
There are two major cellular organizations. Prokaryotic cells, found in bacteria and archaea, generally lack a membrane-bound nucleus. Eukaryotic cells, found in animals, plants, fungi, and protists, contain a nucleus and other membrane-bound organelles. These differences are important, but both types are living cells because they perform the fundamental processes that characterize cellular life.
Why a cell is more than a collection of molecules
A cell contains the same basic kinds of matter found in the nonliving world. Proteins, lipids, nucleic acids, water, ions, and other chemicals are not alive individually.
What makes the cell alive is the organization and interaction of those components. Membranes create compartments and regulate exchange. DNA stores biological information. Proteins carry out and regulate chemical processes. Metabolic pathways transfer and use energy. Feedback systems coordinate activity and help maintain internal conditions.
These processes depend on one another. A cell needs energy to maintain its organization, but the machinery that captures and uses energy is itself produced and maintained by the cell. Genetic information guides the production of cellular components, while those components are required to copy and express genetic information.
Life therefore cannot be reduced to a single molecule or organelle. A living cell is a self-maintaining, information-based chemical system capable of using energy, regulating its internal state, interacting with its environment, and reproducing. Its essential feature is not simply that it contains certain ingredients, but that those ingredients operate together as an organized living system.
