Cell - The Unit of Life
Syllabus Unit: Cell Structure and Function
Why it matters
Chapter overview
Every living organism, from a single bacterium to a blue whale, is built from cells — the smallest unit that can independently carry out life's functions. This topic starts with the Cell Theory and the simpler prokaryotic cell, moves into the membrane-bound organelles that make eukaryotic cells so much more compartmentalised, zooms into the nucleus-ribosome-endomembrane information-and-processing system, and finishes by contrasting plant and animal cells — the structural foundation NEET builds all of physiology on.
1Cell Theory & Prokaryotic Cells
The Cell Theory establishes the cell as life's basic unit, and prokaryotic cells show the simplest version of that unit — no nucleus, no membrane-bound organelles.
Learning Objectives
- State the three tenets of the Cell Theory and name its founders (Schleiden, Schwann, Virchow).
- List the defining features of a prokaryotic cell: nucleoid, 70S ribosomes, no membrane-bound organelles.
- Describe the bacterial cell envelope: cell wall, plasma membrane, and glycocalyx (capsule/slime layer).
- Explain the function of mesosomes and plasmids in a bacterial cell.
Key Terms
**Matthias Schleiden** (plants, 1838) and **Theodor Schwann** (animals, 1839) proposed that all living organisms are made of cells; **Rudolf Virchow** (1855) added that all cells arise only from pre-existing cells (Omnis cellula-e cellula) — together these three statements form the **Cell Theory**.
The prokaryotic cell: the simplest living unit
- No true nucleus — the genetic material (one circular, double-stranded DNA) lies free in the cytoplasm in a region called the nucleoid, not bound by a nuclear membrane.
- No membrane-bound organelles — no mitochondria, ER, Golgi or plastids; only 70S ribosomes (50S + 30S subunits) are present, scattered in the cytoplasm.
- Cell envelope: a rigid cell wall (peptidoglycan in bacteria) surrounds the plasma membrane; some bacteria have an outer, loosely-bound glycocalyx as a slime layer or a well-organised capsule.
- Mesosomes — plasma membrane infoldings that help in cell wall formation, DNA replication and act as respiratory sites, effectively increasing membrane surface area without true organelles.