Copy of Cell Membrane Str Reading (5/28/2026)

Last updated about 3 hours ago

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Copy of Cell Membrane Str Reading (5/28/2026)

Last updated about 3 hours ago

0 questions

Phospholipid Bilayer
All cells have a cell membrane. This membrane surrounds the cell. So what is its role?

Can molecules enter and leave the cell? Yes. Can anything or everything enter or leave? No. So, what determines what can go in or out? Is it the nucleus? The DNA? Or the cell membrane?

The Cell Membrane

The cell membrane (also known as the plasma membrane) forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It protects and supports the cell and also controls everything that enters and leaves the cell. It allows only certain substances to pass through, while keeping others in or out. The ability to allow only certain molecules in or out of the cell is referred to as selective permeability or semipermeability. To understand how the cell membrane controls what crosses into or out of the cell, you need to know its composition.

A Phospholipid Bilayer

The cell membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the cell membrane are arranged in two layers, called a phospholipid bilayer. Each phospholipid molecule has a head and two tails. The head "loves" water (hydrophilic) and the tails "hate" water (hydrophobic). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell.

Molecules that are hydrophobic can easily pass through the cell membrane, if they are small enough, because they are water-hating like the interior of the membrane. Small lipids and steroids are hydrophobic and can readily cross the membrane. Molecules that are hydrophilic, on the other hand, cannot pass through the cell membrane—at least not without help—because they are water-loving like the exterior of the membrane, and are therefore excluded from the interior of the membrane. Hydrophilic molecules such as glucose and ions like Na+ and K+ need the help of special proteins to cross the membrane.

Review the images below and read the brief description of each above each.

The first image shows a single phospholipid displaying the hydrophilic head and the hydrophobic tails.
The second image shows the phospholipid's place within one layer of the lipid bilayer.
The third image displays the lipid bilayer, how the phospholipids are organized in the lipid bilayer, and the positioning of the hydrophilic heads and the hydrophobic tails of each phospholipid.

Membrane Proteins
Can anything or everything move in or out of the cell?

No. It is the semipermeable cell membrane that determines what can enter and leave the cell. So, if not everything can cross the membrane, how do certain things get across?

Membrane Proteins

The cell membrane contains molecules other than phospholipids, primarily other lipids and proteins. The yellow molecules between the phospholipid tails in the Figure below, for example, are the lipid cholesterol. Molecules of cholesterol help the cell membrane keep its shape. Many of the proteins in the cell membrane assist other substances in crossing the membrane.

The cell membranes also contain certain types of proteins. A membrane protein is a protein molecule that is attached to, or associated with, the membrane of a cell or an organelle. Membrane proteins can be put into two groups based on how the protein is associated with the membrane.

Integral membrane proteins are permanently embedded within the cell membrane. They have a range of important functions. Such functions include channeling or transporting molecules across the membrane. Other integral proteins act as cell receptors. Integral membrane proteins can be classified according to their relationship with the bilayer:

Transmembrane proteins span the entire cell membrane. Transmembrane proteins are found in all types of biological membranes.
Integral monotopic proteins are permanently attached to the membrane from only one side.
Some integral membrane proteins are responsible for cell adhesion (sticking of a cell to another cell or surface). On the outside of cell membranes and attached to some of the proteins are carbohydrate chains that act as labels that identify the cell type. Shown in the Figure below are two different types of membrane proteins and associated molecules.

Peripheral membrane proteins are proteins that are only temporarily associated with the membrane. They can be easily removed, which allows them to be involved in cell signaling. Peripheral proteins can also be attached to integral membrane proteins, or they can stick into a small portion of the lipid bilayer by themselves. Peripheral membrane proteins are often associated with ion channels and transmembrane receptors. Most peripheral membrane proteins are hydrophilic.

The Fluid Mosaic Model

In 1972, S.J. Singer and G.L. Nicolson proposed the now widely accepted Fluid Mosaic Model of the structure of cell membranes. The model proposes that integral membrane proteins are embedded in the phospholipid bilayer, as seen in the Figure above. Some of these proteins extend all the way through the bilayer, and some only partially across it. These membrane proteins act as transport proteins and receptors proteins.

Their model also proposed that the membrane behaves like a fluid, rather than a solid. The proteins and lipids of the membrane move around the membrane, much like buoys in water. Such movement causes a constant change in the "mosaic pattern" of the cell membrane.