Cells are the foundation of life, and understanding them is essential for studying more complex systems like ecosystems.
This article introduces classical cell theory, the difference between prokaryotic and eukaryotic cells, and the structures that make up a typical cell.
What is a Cell
A cell is considered the most basic unit of life.
Cells were discovered in 1665 by Robert Hooke who named them after cella, meaning ‘a small room’.
Cell Theory
Cell theory refers to the proposition that all living organisms are made up of cells.
Classical cell theory has three main postulates:
- All living things are composed of one or more cells
- The cell is the most basic unit of life
- New cells are made by existing cells
The first two were proposed in 1839 by Theodor Schwann and Matthias Schleiden after comparing their observations of animal and plant cells.
Rudolf Virchow is credited with the third postulate (‘Omnis cellula e cellula’ – all cells come from other cells), though there is some debate about whether he plagiarised the work of Robert Remak.
Some modern sources include additional principles, which can be summarised as:
- An organism’s structure and functions stem from the activities of its cells.
- Cells of all species share fundamental similarities in the composition and metabolic mechanisms
- The cells of related species often look and function in similar ways at the structural and chemical level.
- In all known living organisms, genetic information is stored in DNA.
- Energy flow occurs within cells
Prokaryotic & Eukaryotic Cells
There are two types of cells that make up living organisms: prokaryotic and eukaryotic. There are three general distinctions that separate the two:
| Prokaryotes | Eukaryotes |
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Animals, plants, fungi, and protists are all eukaryotic organisms. Bacteria and archaea are prokaryotic.
Cell Structures
Cells contain various structures called organelles. The types of organelles present depend on whether the cell is prokaryotic or eukaryotic.
Organelles common to all cells, including prokaryotes:
- Cytoplasm
- Ribosomes
- Cell Membrane
Organelles unique to eukaryotic cells:
- Nucleus
- Nucleolus
- Cytoskeleton
- Rough & Smooth Endoplasmic Reticulum
- Golgi apparatus
- Mitochondria
- Lysosome
- Peroxisome
- Vesicles
- Vacuoles
Plant-specific organelles:
- Cell wall (also in fungi & some protists)
- Large central vacuole
- Chloroplasts