2.3: Cell Anatomy — Introduction

A cell is the smallest unit of a living thing and is the basic building block of all organisms.

Master this section and you'll be able to

• Describe the general characteristics shared by all cells.
• Compare and contrast prokaryotic and eukaryotic cells.
• Explain why cells have different shapes.
• Summarize the main principles of the cell theory.

Close your eyes and picture a brick wall. What is the basic building block of that wall? A single brick, of course. Like a brick wall, your body is composed of basic building blocks, and the building blocks of your body are cells.

Cells as Building Blocks

A cell is the smallest unit of a living thing. Any living thing, whether it consists of just one cell (like bacteria) or many cells (like a human), is called an organism. This means that cells are the basic building blocks of all organisms.

When many cells of the same type work together to perform a shared function, they form a tissue. Multiple tissues then combine to form an organ (such as the stomach, heart, or brain). A group of organs that work together make up an organ system (like the digestive, circulatory, or nervous system). Several systems working in coordination form an entire organism, such as a human being.

There are many different types of cells, but they all fall into one of two broad categories: prokaryotic or eukaryotic cells. Prokaryotic cells, found in organisms like bacteria, are simpler and lack a nucleus or membrane-bound organelles. In contrast, eukaryotic cells, found in animals, plants, fungi, and protists, are more complex, with a defined nucleus and various organelles that carry out specialized functions. Eukaryotic organisms can be unicellular or multicellular, while prokaryotic organisms are always unicellular. Below is a concise comparison of eukaryotes and prokaryotes.
 

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	❌ Absent	✅ Present
Organelles	❌ No membrane-bound organelles	✅ Membrane-bound organelles present
Cell Type	Always unicellular	Unicellular or multicellular
Cell Size	Small (1–10 µm)	Larger (10–100 µm)
DNA Structure	Circular DNA	Linear DNA in chromosomes
Examples	E. coli	Homo sapiens

 

From One Cell to Trillions

You began life as a single fertilized egg cell known as a zygote. From that one cell, you developed into the complex human you see in the mirror today, a body composed of trillions of cells. Estimates state that a human body is made up of approximately 37 trillion cells. The cells are organized into tissues, organs, and organ systems. This incredible transformation is made possible by two processes called cell division (mitosis) and cell differentiation.  

After fertilization, the zygote started to do something remarkable: it divided. First, it split into two identical cells. Then each of those two divided again, making four. Those four became eight, then sixteen, and so on. This pattern of exponential growth continued rapidly as the embryo (= early stage of development in a multicellular organism) started to grow. With each round of division, more and more cells were created. Eventually, this growing collection of cells began to organize, specialize (= differentiate), and form the tissues and organs of your body. But it all started with one tiny cell, using a process called mitosis to multiply again and again.

However, just multiplying cells is not enough. In the early stages of development, these new cells are unspecialized (= undifferentiated), meaning they do not yet have a specific job. Over time, they undergo a process called differentiation, during which they become specialized in both their structure and function. For example, some cells become muscle cells that contract, while others become nerve cells that carry electrical signals, or red blood cells that transport oxygen.

Differentiation is guided by the cell’s environment and by specific instructions in its DNA. Even though nearly all the cells in your body contain the same genetic material, different genes are turned “on” or “off” in each type of cell. This selective gene expression is what allows cells to take on unique roles, ultimately giving rise to the specialized tissues and organs that keep your body alive and functioning.
 

Why Cell Shape Matters

You will see this theme throughout your study of anatomy and physiology. For now, let us look at a simple comparison to illustrate the point:
a skin cell and a nerve cell. Each one is uniquely shaped to carry out its specialized role in the body.

 

Comparison of Two Specialized Cell Types

Cell Type	Shape	Function	Lifespan
Skin (Epithelial)	Flat, plate-like	Forms protective barriers	Short; constantly replaced
Nerve (Neuron)	Star-shaped, long processes	Sends fast signals across long distances	Long-lived; can last a lifetime

Just as a home is made from a variety of building materials, the human body is constructed from many cell types. In fact, there are over 200 different types of cells in the human body. To name just a few, these include familiar ones like:

• Epithelial cells, which protect the surface of the body and cover the organs and internal body cavities;

• Neurons, which transmit electrical and chemical signals to communicate information throughout the body;

• Muscle cells, which contract to produce movement and generate force; and

• Immune cells, which detect and respond to harmful invaders to protect the body from infection and disease.

But there are also less familiar (but equally fascinating) examples, such as the Merkel cell, a specialized mechanoreceptor found in the outer layer of the skin. Merkel cells play a crucial role in sensing touch and transmitting sensory information to the brain.

Each of these cell types plays a vital role during the growth, development, and day-to-day maintenance of the body.

The fact that cells in the body can have strikingly different shapes is not random. It reflects the specific job each cell is designed to do. In fact, the differences between cell shapes highlight a central concept in biology:

Form follows function — the shape and structure of a cell directly relate to the job it performs.

Cells do not work in isolation. They cooperate in tissues, organs, and organ systems — each one contributing to your survival. No matter the job, every cell plays a part in keeping your body balanced.

A Quick History of Cell Discovery

The first person to use the word “cell” was Robert Hooke in 1665. Looking at cork tissue under a microscope, he noticed tiny compartments that reminded him of cells, the rooms where monks lived.

A few years later, Antonie van Leeuwenhoek (1632 – 1723) invented the microscope and observed living cells for the first time. Over the following decades, scientists developed the cell theory, a fundamental concept in biology, which tells us:

• All living organisms are made of cells.
• The cell is the basic unit of life.
• All cells arise from pre-existing cells.

The illustration above depicts stages of cell division as understood by Eduard Strasburger in 1875 (Bonn, Germany).