Summary
What is a Cell?
The cell is the fundamental unit of life. All living organisms are composed of one or more cells, and all cells arise from pre-existing cells. This principle, known as the cell theory, forms the foundation of modern biology.
Prokaryotic vs. Eukaryotic Cells
Key differences:
- Prokaryotes lack a membrane-bound nucleus; DNA floats freely in the cytoplasm
- Eukaryotes have a defined nucleus enclosed by a nuclear envelope
- Eukaryotic cells are generally 10–100 µm; prokaryotes 1–10 µm
- Only eukaryotes contain membrane-bound organelles like mitochondria and ER
The Plasma Membrane
The plasma membrane is a phospholipid bilayer with embedded proteins that controls what enters and exits the cell. The fluid mosaic model describes the membrane as a dynamic structure where proteins float within or on the lipid bilayer.
Key Organelles and Their Functions
| Organelle | Found In | Primary Function |
|---|---|---|
| Nucleus | Eukaryotes | Houses DNA, controls gene expression |
| Mitochondria | Eukaryotes | ATP production via cellular respiration |
| Ribosome | All cells | Protein synthesis |
| Endoplasmic Reticulum | Eukaryotes | Protein & lipid synthesis, transport |
| Golgi Apparatus | Eukaryotes | Sorting, modifying, shipping proteins |
Cell Transport
Substances move across the plasma membrane by passive transport (no energy required — diffusion, osmosis, facilitated diffusion) or active transport (requires ATP — pumps, endocytosis, exocytosis). The direction of movement depends on concentration gradients and electrochemical potential.