Table of Contents
Why the Cell Is Considered the Basic Unit of Life
All known living organisms are made of cells. This simple statement has profound consequences: it means that, at some level, every function we associate with “being alive” is carried out by cells or by structures built from them.
When we call the cell the “basic unit of life,” we mean several related things:
- It is the smallest structure that can perform all essential life functions on its own.
- Larger structures (tissues, organs, whole organisms) are built from cells or their products.
- Every new cell comes from a pre-existing cell.
These ideas are summarized in the cell theory.
The Core Ideas of Cell Theory
Modern cell theory can be expressed in a few central statements:
- All living organisms are composed of one or more cells.
- A unicellular organism consists of a single cell that must do everything (e.g., many bacteria, many protists).
- A multicellular organism consists of many cells that often specialize and cooperate (e.g., plants, animals, fungi).
- The cell is the basic structural and functional unit of life.
Nothing smaller than a complete cell—no organelle, no molecule—can carry out the full set of functions that define life (such as regulated metabolism, growth, reproduction, and response to the environment). - All cells arise from pre-existing cells by division.
Spontaneous generation (life suddenly appearing fully formed from nonliving matter) is not observed under present-day conditions on Earth. - Cells contain hereditary information (DNA) that is passed on during cell division.
This links the continuity of life directly to the continuity of cells. - Cells share fundamental chemical and structural features.
For example, all cells use similar kinds of macromolecules and similar genetic code and are bounded by a membrane.
These are general principles; details of chemical building blocks and types of cells are discussed in other chapters.
What Makes a Cell “Alive”?
To call something a cell, biologists look for a combination of features, not just one property. A cell typically:
- Is separated from its surroundings by a boundary
Usually a flexible cell membrane that controls what enters and leaves. - Maintains an internal environment
Conditions inside (such as ion concentrations and pH) are regulated and often differ from the outside environment—this is called homeostasis at the cellular level. - Carries out metabolism
It takes in nutrients, converts them through chemical reactions (metabolic pathways), and uses the energy and materials obtained to: - build and repair cellular structures,
- drive processes such as movement or active transport.
- Grows and can reproduce
A healthy cell can increase in size or complexity and then give rise to new cells (e.g., by splitting into two). - Processes and uses information
It stores genetic information (in DNA) and uses it to control which proteins are made and when. Cells also sense signals from their environment and respond to them. - Can evolve as part of a population
Over generations, changes in cellular DNA (mutations and recombinations) can lead to evolution of cell lineages.
A droplet of oil or a crystal might grow or change shape, but because they lack this integrated set of properties—especially metabolism and genetic information—they are not considered cells.
Basic Structural Features Shared by All Cells
Despite enormous diversity in size, shape, and lifestyle, all cells share some key features:
- Plasma membrane (cell membrane)
- A thin, flexible boundary enclosing the cell contents.
- Composed mainly of lipids and proteins.
- Selectively permeable: some substances pass easily, others are actively transported, and some are blocked.
- Cytoplasm
- The internal, water-rich environment inside the cell membrane.
- Contains dissolved ions and molecules and a network of structural components.
- Many metabolic reactions take place here.
- Genetic material
- Typically DNA, organized in different ways in different kinds of cells.
- Contains instructions for building and maintaining the cell.
- Ribosomes
- Small, complex structures where proteins are built from amino acids according to genetic instructions.
- Present in all known cells.
These features form a minimal toolkit for cellular life; additional components (such as a nucleus, mitochondria, chloroplasts) are found only in some types of cells and are discussed elsewhere.
Cells, Size, and Surface–Volume Relationships
Cells come in a wide range of sizes, but most are microscopic. A crucial physical reason for this is the relationship between a cell’s surface area and its volume.
- The surface area (mainly the membrane) is where:
- nutrients and gases enter,
- waste products and signals leave.
- The volume represents:
- the amount of cytoplasm that needs resources,
- the amount of waste that must be removed.
As a cell gets larger:
- Its volume increases faster than surface area.
- This can cause transport limitations: not enough membrane area to supply and clear the interior efficiently.
Mathematically, for a roughly spherical cell:
$$$$
\text{Surface area} \propto r^2, \quad \text{Volume} \propto r^3
$$$$
So:
- Doubling the radius increases surface area by a factor of 4, but volume by a factor of 8.
- The surface area–to–volume ratio ($\text{SA}/\text{V}$) therefore decreases as size increases.
Consequences for cells:
- Most cells remain small (often around a few micrometers, $\mu\text{m}$) to keep a favorable $\text{SA}/\text{V}$ ratio.
- Some cells overcome this limit by:
- developing elongated or flattened shapes (e.g., nerve cells, leaf cells),
- having folded membranes that increase surface area,
- forming internal transport systems (e.g., in large eukaryotic cells).
Understanding surface–volume relationships helps explain both the small size of many cells and the complex internal organization of larger ones.
Cells as Building Blocks of Multicellular Organisms
In multicellular organisms, cells rarely live in isolation. Instead, they:
- Differentiate
Genetically identical cells can become specialized types (e.g., muscle cells, nerve cells, leaf mesophyll cells) by using different parts of their genetic information. - Form tissues and organs
Groups of similar or cooperating cells form tissues; different tissues combine into organs with specific functions. - Communicate and coordinate
Cells send and receive signals (chemical messengers, electrical impulses, contact signals) to coordinate: - growth and development,
- metabolism,
- responses to the environment.
Even in a large animal or plant, all complex processes ultimately depend on what happens inside and between individual cells.
Why Viruses Are Not Considered Cells
Viruses will be examined in detail elsewhere, but in the context of this chapter it is important to note:
- Viruses lack a cellular structure: they have no plasma membrane enclosing cytoplasm with ribosomes.
- They cannot carry out metabolism on their own and must use the machinery of a host cell to reproduce.
- For these reasons, viruses are not classified as cells and are often described as “acellular particles” or “obligate intracellular parasites.”
This underlines the central idea: the cell, not the virus, is the minimal system that independently carries out all key functions of life as we define them in biology.
Cells as the Common Thread of Life
From bacteria in hot springs to human brain cells, every living organism is connected by this shared cellular basis. The cell:
- embodies the chemical and structural organization required for life,
- links inheritance (DNA) with function (metabolism, structure, behavior),
- and provides the level at which evolution and ecology ultimately operate.
Understanding cells is therefore essential for understanding any higher-level topic in biology, because all biological phenomena—development, behavior, health and disease, evolution, and ecosystems—are rooted in what cells are and what they do.