Cells: Building Blocks of Life

The human body is a jigsaw made up of around 75 trillion microscopic pieces called cells. Every part of your body is constructed from these tiny building blocks, from your eyelashes to your toenails.

Individual cells are too small to see with the naked eye. The average cell is less than half as wide as a human hair, but some are so tiny that 30,000 could fit inside a period. Cells work as independent units, absorbing food, oxygen, and other basic chemicals from their surroundings, and manufacturing the complex organic compounds they need to grow and function.

Some cells, such as blood cells, travel around the body singly. Others are fixed together in sheets to form tissues such as skin or muscle.

Cell membrane

The outer part of a cell is called the membrane. It consists of two layers of special molecules (phospholipids) that together form an oily film around the cell’s watery contents, preventing substances from leaking in or out. Large protein molecules are embedded in the membrane. These act as gates, allowing only certain molecules to enter or leave the cell.

Inside a cell

Despite their tiny size, cells are immensely complicated inside. Each one is like a factory, packed with machines that carry out particular jobs. The machines inside a cell are called organelles, which means "tiny organs." The most important of these is the nucleus, which controls the rest of the organelles by sending out chemical instructions. Other organelles release energy, manufacture chemicals, and transport substances through the cell.


The nucleus is the cell’s control center. It contains all the instructions needed to operate the cell, stored as code in DNA molecules.

Golgi body

Chemicals manufactured by a cell are packaged by the Golgi body before they leave the cell.


Some cells have tiny, fingerlike extensions to help them contact their surroundings and absorb chemicals from outside the cell.

Secretory vesicle

These tiny bubbles eject substances out of the cell by bursting as they join with the cell membrane.


These tiny molecular machines construct large protein molecules by joining small units called amino acids into chains. They use genes as templates to get the order of amino acids correct.


Fine strands of protein form the cell’s inner framework.


Cells get their power from mitochondria, which break down sugar molecules to release stored chemical energy.


This organelle makes powerful chemicals that attack and break down unwanted substances and worn-out cell parts.


A jellylike fluid made mostly of water fills the space between the cell’s organelles.

Endoplasmic reticulum

Folded membranes form a network of separate compartments in which various chemicals are manufactured, broken down, or stored.

Types of cell

There are more than 200 different types of cells in the human body, each type specialized to do a particular job. Some, such as skin cells and blood cells, last just a few weeks before dying and are continually replaced. Others, such as brain cells, can last a lifetime.

Red blood cell

These disc-shaped cells carry oxygen around the body. They are packed with a red pigment called hemoglobin.

Epithelial cell

The inside of the mouth and intestines are lined with these cells. Special toughened epithelial cells form the skin.

Fat cell

Body fat is stored in fat cells, which expand like balloons. As well as storing energy, fat cells help keep you warm.

Egg cell

The largest cells in the human body are egg cells. These develop into babies if they are fertilized by sperm cells.

Genes and DNA

The nucleus of most cells contains a complete set of your genes, stored as a chemical code in DNA. A single cell contains about 8 ft (2 m) of DNA. When not in use, this DNA is wound up tightly in packages called chromosomes. There are 46 of these in every nucleus.

Making new cells

The human body starts out as a single cell. By dividing in two again and again, it multiplies until there are billions of cells. The most common form of cell division is called mitosis. First, the chromosomes are copied, forming double strands. Next, the chromosome strands are pulled apart, and finally, the rest of the cell divides.