Description and Function of the Cell Membrane Study Guide

Description and Function of the Cell Membrane Study Guide
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The description and function of the cell membrane is something we all have to learn eventually. Cells are the basic building blocks of life and without a cell membrane they could not survive. The cell membrane is a complex structure with a variety of components and functions essential to life.

What is the Cell Membrane?

The cell membrane is a structure responsible for being the barrier between the outside environment and the inside components of a cell. In addition to creating a wall between the outside of a cell and the inside of a cell, the membrane must also be the threshold in which certain molecules can enter and exit the cell when necessary.

In general, the cell membrane keeps the cell components separated from organisms and cells on the outside and it defines the cell.

What is the Lipid Bilayer?

This lipid bilayer is a part of all cell membranes. The barrier that determines a cell’s boundary is the role of this layer. This layer is actually two layers of fat cells that are organized as two sheets.

Every lipid molecule contains a hydrophobic, or nonpolar tail region, and a hydrophilic or polar head region.

The hydrophobic layer is repelled from aqueous water conditions while the hydrophilic region is attracted to them. Since lipid molecules contain both nonpolar and polar regions, they are referred to as amphipathic molecules. The most numerous class of this molecule within cell membranes is in the phospholipids.

Only gases and water can pass easily though the bilayer. Small polar molecules and large molecules cannot pass through without other structures assisting them. It also has fluidity that influences membrane transport. The lipid bilayer is structurally asymmetrical.

What Structures are Responsible for Membrane Transport?

While a cell membrane must be selective in what it allows to pass through it, certain large polar molecules must be able to be moved across their membranes, such as amino acids, nucleotides, and sugars. Because of this, this membrane must have specific structures that allow certain molecules to be transported. Understanding this is crucial in fully understanding the description and function of the cell membrane.

Passive transport involves diffusion and osmosis. Diffusion is related to nonpolar molecules and osmosis is related to water molecules. Neither of these processes requires energy to be expended.

Active transport involves a molecule being pumped by a cell across its membrane, dictated by osmosis, diffusion, or polarity, against the natural direction. This process does require energy.

Transporting proteins involve transmembrane proteins that act as transporters. The two primary transport protein classes include channel proteins and carrier proteins. These proteins are crucial to cell interactions and cell life. They make it possible for molecules and ions to be properly distributed in multicellular organisms. They are also involved in protein synthesis, facilitate communication between cells, and help maintain the proper extra-cellular and intra-cellular pH levels.


Kimball’s Biology Pages. (2003). Cell Membrane. Retrieved on April 20, 2011 from Kimball’s Biology Pages:

University of Arizona. (2002). Cell Membranes Tutorial. Retrieved on April 20, 2011 from the University of Arizona:

Image Credits

Cell Membrane: Bibi Saint-Pol – Wikimedia Commons