1. Plasma Membrane: This is the outer boundary of the cell, acting as a barrier and controlling what enters and exits the cell. It's made of a phospholipid bilayer with embedded proteins.

2. Cytoplasm: This is the gel-like substance that fills the cell, where many of the cell's metabolic reactions take place. It contains various organelles and molecules.

3. DNA: The genetic material of the cell, containing the instructions for building and maintaining the organism. In prokaryotes, DNA is located in a region called the nucleoid. In eukaryotes, it's contained within the nucleus.

4. Ribosomes: These are tiny structures responsible for protein synthesis. They are found in the cytoplasm of all cells, and some are attached to the endoplasmic reticulum in eukaryotic cells.

These four structures are essential for the basic functions of all cells, regardless of their complexity or type.