Which statement BEST describes the cell membrane in a typical bacterial cell? The membrane... a)has the same permeability to all substances moving in and out the cell.

b) selectively controls what enters and exits the cell.

c) is composed of two protein layers with lipids floating inside.

d) is composed of protein and carbohydrates only.

Explanation:What’s a cell? Well, on some level, it's a bag of goo. The plasma membrane—the outer boundary of the cell—is the bag, and the cytoplasm is the goo.

Of course, a cell is ever so much more than just a bag of goo. It's a complex, highly organized unit, the basic building block of all living things. And the plasma membrane and cytoplasm are actually pretty sophisticated.

The membrane is a delicate, two-layered structure of lipids and proteins, and it controls what can enter and exit the cell. Similarly, the cytoplasm of a eukaryotic cell consists not only of cytosol—a gel-like substance made up of water, ions, and macromolecules—but also of organelles and the structural proteins that make up the cytoskeleton, or "skeleton of the cell."

In this article, we’ll take a closer look at the plasma membrane and cytoplasm.

The plasma membrane

Both prokaryotic and eukaryotic cells have a plasma membrane, a double layer of lipids that separates the cell interior from the outside environment. This double layer consists largely of specialized lipids called phospholipids.

A phospholipid is made up of a hydrophilic, water-loving, phosphate head, along with two hydrophobic, water-fearing, fatty acid tails. Phospholipids spontaneously arrange themselves in a double-layered structure with their hydrophobic tails pointing inward and their hydrophilic heads facing outward. This energetically favorable two-layer structure, called a phospholipid bilayer, is found in many biological membranes. [Close-up of a phospholipid]

As shown below, proteins are also an important component of the plasma membrane. Some of them pass all the way through the membrane, serving as channels or signal receptors, while others are just attached at the edge. Different types of lipids, such as cholesterol, may also be found in the cell membrane and affect its fluidity.