The **cell membrane**, or plasma membrane, is found in all cells. The cell membrane acts as a semi-permeable barrier that separates the interior of the cell from the external environment.

The cell membrane also plays a role in the transport of substances in and out of the cell. It only allows certain substances to enter and exit the cell via passive or active transport systems.

On the other hand, the **cell wall** is typically found in plant and fungal cells, as well as certain prokaryotes and protists. Similar to the cell membrane, the cell wall also acts as a barrier that separates the interior of the cell from the external environment.

However, the cell wall provides rigid, structural support unlike the more fluid cell membrane. The cell wall also defines the shape of the cell. It also helps prevent the cell from being lysed due to osmotic pressure that is exerted by fluids (i.e. water) within the cell.

**Passive transport** is a type of cell transport that t does not need the energy to move and carry substances across cell membranes.

One example of passive transport is **diffusion**. **Diffusion** is where the substances move from a higher concentration area to a lower concentration in a given set-up.

**Osmosis** is the travel of water through a selectively permeable membrane from more to a less concentrated area.

This travel of water results in the equalization of solutes in both sides of the area.

The cell membrane is composed of a lipid bilayer or two layers of lipids that contain proteins and carbohydrate chains. This structure allows the cell membranes to have a flexible and strong protection from the external environment.

Phagocytosis is the cell’s way of engulfing or eating a particle. An example of this is the amoeba’s method of taking in food and the white blood cell’s way of ingesting a bacteria. On the other hand, pinocytosis is the cell’s process of taking up liquid from its surrounding environment. An example of this method is the way our body cells absorb enzyme and hormones.

Both active transport and passive transport (diffusion) carry ions or molecules across cell membranes. However, they are different in some ways. Active transport moves ions or molecules from low to high concentration using transport proteins. This process requires energy from adenosine triphosphate (ATP). On the contrary, diffusion moves ions or molecules from high to low concentration and it does not require energy.