Unicellular vs Multicellular Organisms, Size, and Shape

Unicellular organisms consist of a single cell that carries out all life processes, while multicellular organisms are made of many cells that specialize and work together. In brief: bacteria, amoebae and yeast are unicellular, and plants, animals and most fungi are multicellular. Cells are small because a cell must exchange materials across its surface, and the surface area-to-volume ratio falls as a cell grows larger.

Cell size and shape are not random. They are closely linked to what the cell does and to the physics of diffusion. This guide compares the two kinds of organisms, explains why cells are small, shows how shape relates to function, works through a surface area-to-volume calculation and corrects common mistakes.

What are unicellular organisms?

A unicellular organism is a complete living thing made of one cell. This one cell must perform every function: obtaining energy, getting rid of waste, responding to the environment, growing and reproducing.

Examples are bacteria, archaea, protists such as Amoeba and Paramecium, and yeasts, which are unicellular fungi. Unicellular organisms live in nearly every environment, from soil and oceans to the bodies of animals. They usually reproduce asexually by dividing into two, and they can multiply very fast under good conditions.

Some unicellular organisms live together in groups called colonies, in which each cell is still independent. Volvox, an alga, forms spherical colonies with some division of labor, and it is often used to illustrate how multicellularity might have begun.

What are multicellular organisms?

A multicellular organism is made of many cells that are organized and depend on each other. The cells arise from a single fertilized egg or spore by cell division, and they differentiate into different types.

This brings advantages. Specialization lets cells do their jobs more efficiently, and an organism can grow large, which can help in escaping predators or reaching resources. It also allows more complex bodies, and the loss of a few cells does not kill the whole organism. A disadvantage is that cells depend on each other and need transport systems to reach cells deep inside the body.

Unicellular and multicellular organisms compared

Why are cells so small?

Most cells are between 1 and 100 micrometers (µm) across. A micrometer is one thousandth of a millimeter. The main reason is the way materials enter and leave a cell.

Oxygen, nutrients and wastes move across the cell membrane, mostly by diffusion. The rate of exchange depends on the surface area of the membrane, but the need for materials depends on the volume of the cell. When a cell grows, its volume increases faster than its surface area. The surface area-to-volume ratio therefore decreases, and a large cell may not be able to supply its interior fast enough.

Diffusion is also slow over large distances, so materials take too long to travel through a big cell. Cells solve the problem by staying small, by being flat or elongated, or by having folded membranes that increase surface area. Some cells are exceptions: an ostrich egg is a single large cell, but most of it is yolk, which is storage, and the active part is small.

Worked example: surface area-to-volume ratio

Model cells as cubes to see the effect of size. For a cube with side length L, the surface area is 6L² and the volume is L³. The ratio is 6L²/L³ = 6/L.

1. For L = 2 µm: surface area = 6 × 2² = 24, volume = 2³ = 8, ratio = 24 ÷ 8 = 3.
2. For L = 4 µm: surface area = 6 × 16 = 96, volume = 64, ratio = 1.5.
3. For L = 10 µm: surface area = 6 × 100 = 600, volume = 1000, ratio = 0.6.

Each doubling of the side length halves the ratio. A cell of side 10 µm has only one tenth of the ratio of a cell of side 1 µm. This is why a small cell can supply itself much more easily. In real cells, the shape is not a cube, but the principle is the same.

How are cell shape and function related?

Cells have many shapes, and in most cases the shape suits the job.

• Red blood cells are biconcave discs, which gives a large surface for gas exchange and flexibility to pass through narrow capillaries.
• Nerve cells have long extensions that carry signals over distances, in humans up to about a meter.
• Muscle cells are long fibers that can contract along their length.
• Root hair cells have a long thin extension that increases the surface for absorbing water.
• Sperm cells have a tail for swimming and a streamlined head.
• Bacteria are commonly spheres (cocci), rods (bacilli) or spirals, and shape affects how they move and attach.

Typical cell sizes

How did scientists learn about cell size?

Early microscope users could only estimate sizes by comparing cells to familiar objects, such as grains of sand. Leeuwenhoek compared the size of microorganisms with grains of sand and with the width of a hair. Calibrated measuring scales fitted into the eyepiece of microscopes made accurate measurement possible in the nineteenth century. Electron microscopes in the twentieth century revealed structures that are much smaller than a micrometer.

Which mistakes are common?

Mistake 1: Saying that larger organisms have larger cells

An elephant has far more cells than a mouse, but its cells are not much bigger. Organisms grow mainly by adding cells.

Mistake 2: Saying cells are small because they are crowded

The limit comes from surface area-to-volume ratio and diffusion distance, not crowding.

Mistake 3: Thinking unicellular means simple

One cell must carry out all functions, and many protists have complex internal structures. They are small but not simple.

Mistake 4: Mixing up units

One millimeter equals 1000 micrometers. Reading a cell size in the wrong unit changes the answer by a factor of 1000.