Cell Theory & the Cell Membrane
Cell theory unifies all living things. It has three core ideas:
- All living organisms are made of one or more cells (and their products).
- The cell is the structural and functional unit of life — the chemistry of life happens inside cells.
- Cells arise only from the division of pre-existing cells, inheriting the genes that instruct growth and function.
The cell membrane
The membrane separates the cytoplasm from the outside and controls what enters and leaves — it's semi-permeable. The accepted model is the fluid mosaic model (Singer & Nicolson, 1972): a phospholipid bilayer with proteins scattered through it like tiles in a mosaic, and "fluid" because the molecules can move.
Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. The tails tuck inward, away from the water on both sides.
Fluid mosaic model: a phospholipid bilayer (heads out, tails in) studded with proteins.