The outside boundary that separates a cell without a cell wall from its environment is the cell membrane, also known as the plasma membrane.

Here's a bit more about it:

* Structure: The cell membrane is a thin, flexible barrier composed of a phospholipid bilayer. This means it has two layers of phospholipids, with their hydrophilic (water-loving) heads facing outwards and their hydrophobic (water-fearing) tails facing inwards.

* Function: The cell membrane plays a crucial role in regulating what enters and exits the cell. It acts as a selective barrier, allowing some substances to pass through while preventing others. This is essential for maintaining the cell's internal environment and carrying out its functions.

* Other components: The cell membrane also contains proteins embedded within it. These proteins have various roles, including:

* Transport: They help move molecules across the membrane.

* Signaling: They receive signals from the environment and transmit them inside the cell.

* Attachment: They provide points of attachment for other molecules.

Cells without cell walls:

While plants, bacteria, fungi, and some protists have cell walls, animal cells and some protists do not. Instead, they rely on the cell membrane to define their external boundary.