CEL - Plasma Membrane [LESSON]

Plasma Membrane

The cell’s plasma membrane is a complex barrier of phospholipids, cholesterol, proteins, and carbohydrates that separates the cell from its external environment. This selectively permeable membrane regulates the movement of ions, organic molecules, and other compounds in and out of the cell. The plasma membrane is also important to cellular function by providing structure and assisting in cell communication. The permeability of the cell membrane depends on the characteristics of the lipid bilayer and the characteristics of the molecules moving across the membrane.

The plasma membrane not only defines the borders of the cell but also allows the cell to interact with its environment in a controlled way. Cells must be able to exclude, take in, and excrete various substances, all in specific amounts. In addition, they must be able to communicate with other cells, identify themselves and share information. To perform these roles, the plasma membrane needs lipids, which make a semi-permeable barrier between the cell and its environment. It also needs proteins, which are involved in cross-membrane transport and cell communication, and carbohydrates (sugars and sugar chains), which decorate both the proteins and lipids and help cells recognize each other.

Phospholipid

First, let’s recall the properties of lipids from Module 1. Lipids are formed from a lot of very nonpolar C-H bonds. The lipid specific to the membrane is the phospholipid.  Look at the picture below and note that the fatty acid (hydrophobic) tails are composed of only C-H bonds. The polar heads have a phosphate group attached which makes them polar and hydrophilic. Thus, the phospholipids orient themselves as a bilayer in the cell membrane with the hydrophobic tails pointing towards each other and the hydrophilic heads pointing towards the aqueous solution on either side.

Proteins

Proteins are the second major component of plasma membranes. There are two main categories of membrane proteins: integral and peripheral.  Integral proteins span the membrane and peripheral proteins are located on one side or the other.

Watch the following video to investigate each type of protein in the membrane.

Cholesterol

Cholesterol is a steroid lipid (composed of four fused rings) that is located amongst the phospholipids. Cholesterol helps to regulate membrane fluidity by controlling for the effects of temperature.

Carbohydrates

Carbohydrates are found on the outside surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). Along with membrane proteins, these carbohydrates form distinctive cellular markers, sort of like molecular ID badges, that allow cells to recognize each other. For example, these markers are very important in the immune system, allowing immune cells to differentiate between body cells.

To investigate the structure of the plasma membrane, hover over each number to see what the molecule is.

Fluid Mosaic Model

The currently accepted model for the structure of the plasma membrane, called the fluid mosaic model, was first proposed in 1972. This model has evolved over time, but it still provides a good basic description of the structure and behavior of membranes in many cells. According to the fluid mosaic model, the plasma membrane is a mosaic of components—primarily, phospholipids, cholesterol, and proteins—that move freely and fluidly in the plane of the membrane.

Watch the following video Fluid Mosaic to look at the fluid mosaic model of the plasma membrane and how this can determine the function. We will learn more about transport in the next module.

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