1. Cell membrane: This acts as a barrier, controlling what enters and exits the cell, and maintaining its internal environment.

2. Cytoplasm: This gel-like substance fills the cell and houses the organelles, providing a medium for chemical reactions.

3. DNA (Deoxyribonucleic Acid): This genetic material carries the instructions for building and maintaining the cell.

4. Ribosomes: These tiny structures are responsible for protein synthesis, which is essential for carrying out all cellular functions.

These are the fundamental components that make up all cells, regardless of whether they are simple prokaryotes or complex eukaryotes.