Table of Contents
Overview: Two Basic Ways of Being Alive
All organisms are made of cells, but they are organized in two fundamentally different ways:
- Unicellular organisms: one single cell is the whole organism.
- Multicellular organisms: an individual consists of many cells that work together.
This chapter focuses on how these two life strategies differ, what each can and cannot do easily, and why multicellularity was a major transition in evolution.
Unicellular Organisms
What “unicellular” means in practice
A unicellular organism (single‑celled organism) is an individual made of just one cell. This one cell must perform all tasks needed for life:
- taking up nutrients and water
- metabolism and energy production
- growth and repair
- movement (if mobile)
- sensing the environment
- responding to stimuli
- reproduction
Common examples (details are covered elsewhere):
- Many bacteria and archaea (prokaryotes)
- Some protists (e.g. amoebae, paramecia, many algae)
- Some unicellular fungi (e.g. yeasts)
Size, shape, and complexity
Even though unicellular organisms consist of one cell, they can be quite complex:
- Often very small (typical bacteria ~1–5 µm), but some protists can be visible to the naked eye.
- Show a variety of shapes: rods, spheres, spirals, irregular amoeboid forms, flagellated swimming forms, etc.
- Many unicellular eukaryotes contain numerous organelles and can display surprisingly sophisticated behavior (e.g. hunting, escape responses).
Advantages of being unicellular
- Rapid reproduction
- Many unicellular organisms divide quickly (sometimes in minutes to hours).
- This allows fast population growth and quick adaptation to changing environments.
- Simplicity and low resource needs
- One cell needs relatively few resources compared to a large multicellular body.
- No need to maintain complex tissues or organs.
- Direct contact with the environment
- Every part of the cell is close to the outside.
- Diffusion is usually sufficient for gas exchange, nutrient uptake, and waste removal.
- No need for circulatory or transport systems.
- Flexibility and independence
- The single cell can often change shape, move freely, or enter resting stages (e.g. cysts, spores) when conditions get bad.
- Many can survive extreme environments.
Limitations of unicellularity
Despite their success, unicellular organisms face some constraints:
- Size limitations
- Growth beyond a certain size is difficult because:
- Volume (and thus metabolic demand) grows faster than surface area.
- Diffusion becomes too slow to supply the interior with nutrients and oxygen.
- This often restricts unicellular organisms to microscopic sizes, with few exceptions.
- Limited division of labor
- One cell must carry out all functions.
- Although internal compartmentalization (organelles) exists in eukaryotes, there is no specialization of different cell types within one individual.
- Simple body plans
- Complex body structures (e.g. large nervous systems, skeletons, leaves, roots) cannot form from just one cell.
- This limits the range of possible life strategies and habitats.
- Reproductive strategies and vulnerability
- Many unicellular organisms reproduce mainly asexually.
- Individual cells are easily killed by local hazards (heat, toxins, predators), though high numbers often compensate for this.
Multicellular Organisms
What “multicellular” means in practice
A multicellular organism consists of many cells that stay physically connected and function as a coordinated whole. These cells usually:
- arise by repeated divisions of a single starting cell (often a fertilized egg)
- remain attached to each other
- communicate and cooperate
- specialize for different tasks (cell differentiation)
Examples include:
- Most plants
- Most fungi
- All animals (including humans)
- Large multicellular algae and some other protists
From many identical cells to division of labor
In a multicellular organism, not all cells are identical. Over development, groups of cells:
- receive different signals
- switch different genes on or off
- become specialized cell types (e.g. muscle, nerve, leaf mesophyll, root hair cells)
This division of labor makes new functions possible:
- Some cells focus on movement.
- Others process information (nervous systems).
- Others specialize in photosynthesis, storage, protection, or reproduction.
The result is the formation of tissues, organs, and organ systems (covered in other chapters).
Advantages of being multicellular
- Larger body size
- Many cells together can form a much larger organism.
- Larger size provides benefits such as:
- protection from many small predators
- more stable internal environment
- access to new food sources and habitats (e.g. deep soil, tall canopy, open water)
- Division of labor and specialization
- Different cell types perform different functions more efficiently.
- Complex organs (e.g. brain, heart, leaves, roots, gills) can evolve.
- Specialized reproductive structures can increase survival of offspring (e.g. seeds, eggs, flowers).
- Protection and homeostasis
- Outer cell layers (e.g. skin, bark) can protect inner cells.
- Internal environments (temperature, pH, ion concentrations) can be regulated more tightly than in the external environment.
- Damage to some cells does not necessarily kill the entire organism.
- Complex behavior and information processing
- Networks of specialized cells allow:
- fast signal transmission (nervous systems in animals)
- coordinated movements (muscles)
- sophisticated behavior (predation, social behavior, communication)
Challenges and costs of multicellularity
Multicellularity also creates new problems that must be solved:
- Coordination and communication
- Cells must coordinate growth, division, and activity.
- Requires signal molecules, receptors, and often specialized communication systems (nervous and hormonal).
- Transport and supply
- Inner cells are far from the environment.
- Organisms need transport systems:
- Plants: conductive tissues (xylem, phloem)
- Animals: circulatory systems, respiratory organs
- Nutrients, gases, and wastes must be moved over long distances within the body.
- Development and pattern formation
- Cells must adopt the correct identity in the correct position.
- Complex programs control how an organism’s body is built during development.
- Risk of internal “cheaters”
- Cells that divide uncontrollably and ignore cooperative rules can form tumors and cancers.
- Multicellular organisms require mechanisms to control or eliminate such rogue cells.
- Slower reproduction
- Often larger and more complex life cycles.
- Developing a multicellular body from a single cell can take time and resources.
- Many multicellular organisms have lower reproductive rates than fast‑dividing unicellular microbes.
Transitions and Intermediate Forms
The distinction between unicellular and multicellular is not always sharp. There are intermediate stages and special organizational forms.
Colonial organisms
A colony is a group of cells (often genetically identical) that live together:
- Cells may be loosely attached or embedded in a common matrix.
- Each cell often can survive on its own if separated.
- There is usually little or no permanent division of labor.
Examples:
- Some algae forming spherical or filamentous colonies
- Chains or clusters of bacteria
- Some colonial protists
Colonies show how cells can start living together without fully committing to one integrated organism.
Simple multicellularity vs. complex multicellularity
Biologists sometimes distinguish:
- Simple multicellularity:
- Cells stick together in a group.
- Little differentiation; most cells look and function similarly.
- Communication and transport between cells are limited.
- Complex multicellularity:
- Clear cell differentiation into many types.
- Organized tissues and organs.
- Extensive intercellular communication and internal transport systems.
- Developmental programs build a body plan.
Large animals, flowering plants, and many fungi exhibit complex multicellularity.
Life cycles that alternate between forms
Some organisms switch between unicellular and multicellular stages within one life cycle:
- Certain algae and slime molds:
- Live mainly as independent unicellular organisms.
- Under certain conditions (e.g. lack of food), cells aggregate to form a multicellular structure for reproduction and dispersal.
- Many fungi:
- Have unicellular stages (e.g. spores, yeasts) and multicellular stages (mycelium).
These cases illustrate that multicellularity can be reversible or conditional in some lineages.
Ecological and Evolutionary Consequences
Ecological roles of unicellular organisms
Unicellular organisms are essential to ecosystems:
- Major primary producers in oceans and freshwater (many unicellular algae and cyanobacteria fix carbon by photosynthesis).
- Key players in decomposition and nutrient cycling (bacteria and unicellular fungi break down organic matter).
- Important symbionts, pathogens, and commensals in and on multicellular hosts.
Their small size and rapid reproduction make them especially important in global biogeochemical cycles.
Ecological roles of multicellular organisms
Multicellular organisms shape environments on large scales:
- Plants form forests, grasslands, and other vegetation types, influencing climate, soil, and water cycles.
- Animals act as herbivores, predators, and ecosystem engineers, modifying habitats (e.g. beavers, corals, earthworms).
- Fungi form extensive mycelial networks in soil, interacting with plants and decomposing organic material.
Their size and complexity allow them to create and occupy three-dimensional habitats (e.g. tree canopies, coral reefs).
Evolutionary perspectives
Over evolutionary time:
- Multicellularity has evolved multiple times independently (in animals, plants, fungi, some algae, and other protists).
- Each origin involved:
- Cells that remained attached after division
- Mechanisms for communication and cooperation
- Emergence of some level of differentiation
The repeated evolution of multicellularity shows that, under certain conditions, cooperation among cells is a successful and favored strategy, complementing the equally successful but different strategy of independent unicellular life.
Summary of Key Differences
- Number of cells
- Unicellular: one cell per organism.
- Multicellular: many cells forming one integrated organism.
- Division of labor
- Unicellular: all functions in one cell.
- Multicellular: different cell types, tissues, and organs with specialized tasks.
- Size and structure
- Unicellular: usually microscopic, limited structural complexity.
- Multicellular: often macroscopic, capable of complex body plans.
- Transport and communication
- Unicellular: mainly diffusion with the environment.
- Multicellular: internal transport systems and cell‑cell signaling.
- Reproduction
- Unicellular: often fast, mainly asexual (with many exceptions).
- Multicellular: often slower, with complex life cycles and frequent sexual reproduction.
Both strategies—being unicellular and being multicellular—are highly successful, but they shape how organisms live, grow, and interact with their environments in fundamentally different ways.