Cell Biology Basics: Structure and Organelles

Here's a walkthrough of the basics of cell biology — the structures every living thing is built from.

Cell theory
Cell theory holds that every living thing is made of one or more cells, that the cell is the basic structural and functional unit of life, and that new cells can only arise from existing cells that divide. It was first proposed in 1838-1839 by Germany's Schleiden and Schwann, and later completed when Virchow added the principle that 'every cell comes from a cell.'
Prokaryotic vs. eukaryotic cells
A prokaryotic cell, like a bacterium, has no distinct nucleus bound by a nuclear membrane, so its genetic material is scattered through the cytoplasm. A eukaryotic cell — found in animals, plants, fungi, and more — has a membrane-bound nucleus along with various membrane-bound organelles like mitochondria.
What the cell membrane does
The cell membrane is made of a phospholipid bilayer, two layers of phospholipid molecules arranged side by side. It marks the boundary between the inside and outside of the cell while also acting as a semi-permeable membrane, letting only the substances the cell needs pass through selectively.
What the nucleus does
The nucleus is the cell's control center, storing the genetic information encoded in DNA. Enclosed by a nuclear membrane, it keeps the genetic information safely protected while regulating when and which genes are used, directing the cell's overall activity.
What mitochondria do
Mitochondria produce ATP — the energy source needed for life's activities — through cellular respiration, using glucose and oxygen, which is why they're often called the 'powerhouse of the cell.' A distinctive feature is that they carry their own separate DNA.
The endoplasmic reticulum and Golgi apparatus
The endoplasmic reticulum acts as a pathway for synthesizing and transporting proteins and lipids, while the Golgi apparatus processes and packages material passed on from the endoplasmic reticulum and sends it wherever it's needed, inside or outside the cell — the two organelles work together like a cellular logistics system.
The difference between animal and plant cells
The biggest difference is that plant cells have a rigid cell wall made of cellulose, chloroplasts for photosynthesis, and a large vacuole for storing water and nutrients, none of which are present in animal cells.

Every living thing is made of cells

The cell is the smallest structural and functional unit that makes up a living thing — some organisms, like bacteria, are made of just a single cell, while others, like humans, are made of tens of trillions of cells working together. Understanding a cell's basic structure and what each organelle does lays the groundwork for understanding life itself.

See how mitochondria make energy, too

Among a cell's organelles, mitochondria are especially central — they generate the energy a cell needs to stay alive. If you're curious about the specific process by which mitochondria turn glucose and oxygen into energy, that's what cellular respiration covers.

Frequently Asked Questions

Can you see a cell with the naked eye?

Most cells are far too small to see with the naked eye and require a microscope to observe. There are exceptions, though — an ostrich egg cell, for instance, is exceptionally large and can be seen without any magnification.

How many cells does the human body have?

The human body is estimated to be made up of roughly 37 trillion cells, each specialized for a different function — muscle, nerve, skin, and many others.