A cells is the smallest living organism and the basic unit of life on earth. Together trillions of cells make up the human body. Cells have three parts: the membrane, the nucleus, and the cytoplasm.

People can think of cells as tiny packages that contain minute factories, warehouses, transport systems, and power plants. They function on their own, creating their own energy and self-replicating — the cell is the smallest unit of life that can replicate. Cells are the basic units of life.

The body contains around 50—100 trillionTrusted Source cells, and they vary widely in size, number, structure, and use.

Cells also communicate with each other. Whether in plants, humans, or animals, they connect to create a solid, well formed organism. In humans, cells build tissues, tissues form organs, and organs work together to keep the body alive.

Robert Hooke first discovered cells in the 1600s. He gave them their name because they resembled the “cella,” the Latin term for “small rooms” where monks lived in monasteries.

Inside the cell
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Experts estimate that there are around 200Trusted Source cell types in the human body.

Cell types can look different, and carry out distinct roles within the body.

For instance, a sperm cell resembles a tadpole, a female egg cell is spherical, and nerve cells are essentially thin tubes.

Despite their differences, cells often share certain structures. These are known as organelles or mini-organs. Below are some of the most important:

Nucleus
The nucleus represents the cell’s headquarters. There is typically one nucleus per cell. However, this is not always the case. Skeletal muscle cells, for instance, have two.

The nucleus contains the majority of the cell’s DNA, and the mitochondria house a small amount. The nucleus sends out messages to tell the cell to grow, divide, or die.

A membrane called the nuclear envelope separates the nucleus from the rest of the cell. Nuclear pores within the membrane allow small molecules and ions to cross back and forth, while larger molecules need to transport proteins to help them through.

Plasma membrane
To ensure each cell remains separate from its neighbor, a special membrane, known as the plasma membrane, envelops the cell. Phospholipids make most of this membrane and prevent water-based substances from entering the cell. The plasma membrane contains a range of receptors, which carry out a number of tasks, including being:

Gatekeepers: Some receptors allow certain molecules through and stop others.
Markers: These receptors act as name badges, informing the immune system that they are part of the organism and not foreign invaders.
Communicators: Some receptors help the cell communicate with other cells and the environment.
Fasteners: Some receptors help bind the cell to its neighbors.
Cytoplasm
The cytoplasm is the interior of the cell that surrounds the nucleus. It includes the organelles and a jelly-like fluid called the cytosol. Many of the important reactions that take place in the cell occur in the cytoplasm.

Cytoskeleton
The cytoskeleton forms the scaffolding within the cytoplasm of the human cell. It helps the cell maintain the correct shape. However, unlike regular scaffolding, the cytoskeleton is flexible. It plays a role in cell division and cell motility — the ability of some cells to move, such as sperm cells, for instance.

The cytoskeleton also helps with cell signaling through the uptake of material from the endocytosis, or the area outside the cell, and moving materials within the cell.