Cells are the building blocks of life. You need to understand cell theory and how cells are classified to grasp the basics of life science. Use this resource to start your journey into the world of biology.
Cell theory is a key idea in biology that changed how we view living things. It helps us understand that cells are essential building blocks for all life.
Cell theory states that:
The cell is the most basic unit of life.
All living organisms are composed of one or more cells.
All cells arise only from pre-existing cells.
German physician and physiologist Theodor Schwann (1810–1882) and botanist Matthias Jakob Schleiden (1804–1881) are credited as the founders of cell theory, but many scientists from the 1800s helped develop our current understanding of cells.
At the most basic level, cells are classified into two types, based on their structure and function: prokaryotic or eukaryotic.
Prokaryotic cells
Basic representation of a prokaryotic cellProkaryotic cells lack a cell nucleus and membrane-bound organelles. They contain genetic material that gives them instructions on how to function, grow and develop. However, this genetic material is found in the central area of the cell and is not surrounded by any membrane.
Prokaryotic cells are very small and simple, typically existing as single-celled (or unicellular) organisms. This means that one cell makes up the entire organism. Imagine you were made of just one cell!
These cells reproduce quickly and from a single parent through a process called binary fission.
Prokaryotic cells include organisms from the bacteria and archaea domains, such as:
E. coli bacteria
cyanobacteria (blue-green algae)
methanogenic (methane-generating) archaea
thermophiles (heat-loving archaea).
Bacterial colonies grown on an agar plate, by Adrian Lange via Unsplash
The biological domains are archaea, bacteria and eukarya. This is the first level of classifying life.
Eukaryotic cells
Basic representation of a eukaryotic cellEukaryotic cells are bigger and more complex than prokaryotic cells. They have a well-defined cell nucleus. This means that their genetic material is inside a nuclear membrane.
Eukaryotic cells also have membrane-bound organelles. This allows different cellular processes to happen separately and at the same time. This allows eukaryotic cells to specialise, giving organisms more complex functions.
Unlike prokaryotic cells, eukaryotic cells can form both unicellular and multicellular organisms. These organisms usually need two parents to reproduce, but can also reproduce with one parent.
Some examples of organisms formed from eukaryotic cells are:
In life sciences, many terms come from Latin or Greek roots. Knowing these can help you remember complex biological words.
For example, "pro–" and "eu–" come from Ancient Greek, meaning "before" and "true", respectively. "Karyon" means "nut" or "seed", referring to the cell nucleus. So:
Prokaryotic cells are "before the nucleus", meaning they lack a nucleus.
Eukaryotic cells have a "true nucleus", meaning they have a defined cell nucleus.
The difference between prokaryotic and eukaryotic cells is summarised in the table.
Feature
Prokaryotic cells
Eukaryotic cells
Nucleus
No defined nucleus; genetic material found in central area
Well-defined nucleus; genetic material enclosed in nuclear membrane
Organelles
No membrane-bound organelles
Presence of membrane-bound organelles
Size
\(0.1\) to \(0.5\,\mu\textrm{m}\)
\(10\) to \(100\,\mu\textrm{m}\)
Reproduction
Asexually through binary fission
Sexually through meiosis or asexually through mitosis
