Cells

The cell is the smallest unit that is genuinely alive. An adult human is made of roughly 30–40 trillion of them.

The parts of a cell. Select any label to open that structure’s article.

What a cell is

A cell is a tiny, self-contained compartment wrapped in a membrane, filled with fluid and specialized structures called organelles, each doing a job.

Parts of a cell

• Cell membrane — the selective boundary enclosing the cell.
• Cytoplasm — the internal fluid and working space.
• Nucleus — holds the DNA and controls the cell.
• Mitochondria — convert fuel into usable energy.
• Ribosomes — build proteins.
• Endoplasmic reticulum — makes and processes proteins and lipids.
• Golgi apparatus — modifies, packages, and ships molecules.
• Lysosomes — digest waste and recycle materials.
• Cytoskeleton — the internal scaffold for shape, support, and movement.

Cells are specialized

Although nearly every cell carries the same DNA, cells switch different genes on and off, which makes a nerve cell, a muscle cell, and a skin cell look and behave completely differently. This differentiation is how one fertilized egg becomes a body of hundreds of cell types.

Why it matters

All physiology is, at bottom, cells doing their jobs. Organs work because their cells work; disease often begins when cells malfunction.