Structure and function of the plasma membrane and cytoplasm of cells. Also covers the phospholipid bilayer and microvilli.

Introduction

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.
Phospholipid, showing hydrophilic head with phosphate group and hydrophobic fatty acid tails
Image credit: "Lipids: Figure 8" by OpenStax College, Biology, CC BY 3.0
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.
An image of plasma membrane shows the phospholipid bilayer, embedded proteins, and cholesterol molecules. The membrane separates the extracellular space, outside of the cell, from the cytosol inside the cell.
Image credit: modified from OpenStax Biology
The plasma membrane is the border between the interior and exterior of a cell. As such, it controls passage of various molecules—including sugars, amino acids, ions, and water—into and out of the cell. How easily these molecules can cross the membrane depends on their size and polarity. Some small, nonpolar molecules, such as oxygen, can pass directly through the phospholipid portion of the membrane. Larger and more polar, hydrophilic, molecules, such as amino acids, must instead cross the membrane by way of protein channels, a process that is often regulated by the cell. You can learn more about cellular transport in the membranes and transport section.
The surface area of the plasma membrane limits the exchange of materials between a cell and its environment. Some cells are specialized in the exchange of wastes or nutrients and have modifications to increase the area of the plasma membrane. For instance, the membranes of some nutrient-absorbing cells are folded into fingerlike projections called microvilli, singular, microvillus. Cells with microvilli cover the inside surface of the small intestine, the organ that absorbs nutrients from digested food. The microvilli help intestinal cells maximize their absorption of nutrients from food by increasing plasma membrane surface area.
People with celiac disease have an immune response to gluten—a protein found in wheat, barley, and rye. This immune response damages the microvilli of the small intestine. Because of the damage, intestinal cells cannot absorb nutrients normally, leading to malnutrition, cramping, and diarrhea. Fortunately, a gluten-free diet prevents the immune response from taking place, allowing intestinal cells to remain healthy and structurally intact.
Diagram and micrograph of intestinal cells, showing the protruding "fingers" of plasma membrane—called microvilli—that contact the fluid inside the small intestine.
Image credit: OpenStax Biology. Micrograph is a modification of work by Louisa Howard.

The cytoplasm

The part of the cell referred to as cytoplasm is slightly different in eukaryotes and prokaryotes. In eukaryotic cells, which have a nucleus, the cytoplasm is everything between the plasma membrane and the nuclear envelope. In prokaryotes, which lack a nucleus, cytoplasm simply means everything found inside the plasma membrane.
One major component of the cytoplasm in both prokaryotes and eukaryotes is the gel-like cytosol, a water-based solution that contains ions, small molecules, and macromolecules. In eukaryotes, the cytoplasm also includes membrane-bound organelles, which are suspended in the cytosol. The cytoskeleton, a network of fibers that supports the cell and gives it shape, is also part of the cytoplasm and helps to organize cellular components.
Even though the cytosol is mostly water, it has a semi-solid, Jello-like consistency because of the many proteins suspended in it. The cytosol contains a rich broth of macromolecules and smaller organic molecules, including glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, and fatty acids. Ions of sodium, potassium, calcium, and other elements are also found in the cytosol. Many metabolic reactions, including protein synthesis, take place in this part of the cell.

Attribution

This article is a modified derivative of “Eukaryotic cells” by OpenStax College, Biology, CC BY 3.0. Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:18/Biology.
The modified article is licensed under a CC BY-NC-SA 4.0 license.

Additional references

Cooper, G. M. "Structure of the plasma membrane." In The cell: A molecular approach. 2nd ed. Sunderland, MA: Sinauer Associates, 2000. http://www.ncbi.nlm.nih.gov/books/NBK9898/.
Kaiser, G. E. "The Prokaryotic Cell: Bacteria." BIOL 230 Lecture on the Web. March 2014. http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit1/prostruct/procyto.html.
OpenStax College, Biology. "Components and Structure." OpenStax CNX. Last modified May 27, 2016. http://cnx.org/contents/GFy_h8cu@10.53:QOGUelqL@9/Components-and-Structure.
Raven, P. H., G. B. Johnson, K. A. Mason, J. B. Losos, and S. R. Singer. "All cells exhibit basic structural similarities." In Biology, 62-63. 10th ed. AP ed. New York, NY: McGraw-Hill, 2014.
Reece, J. B., L. A. Urry, M. L. Cain, S. A. Wasserman, P. V. Minorsky, and R. B. Jackson, R. B. "A tour of the cell." In Campbell Biology, 92-123. 10th ed. San Francisco, CA: Pearson, 2011.
Tobin, A. J. and J. Dusheck. "The cytosol is the cytoplasm that lies outside the organelles." In Asking about life, 71. 3rd ed. Belmont, CA: Thomson Learning. 2005.