Plasma Membrane: Definition, Structure & Function (with Diagram)

••• Dana Chen | Sciencing

The plasma membrane is a protective barrier that surrounds the interior of the cell. Also called the cell membrane, this structure is semi-porous and allows certain molecules in and out of the cell. It serves as a boundary by keeping the cell's contents inside and preventing them from spilling out.

Both prokaryotic and eukaryotic cells have plasma membranes, but the membranes vary among different organisms. In general, plasma membranes consist of phospholipids and proteins.

Phospholipids and the Plasma Membrane

Phospholipids form the base of the plasma membrane. The basic structure of a phospholipid includes a hydrophobic (water-fearing) tail and a hydrophilic (water-loving) head. The phospholipid consists of a glycerol plus a negatively charged phosphate group, which both form the head, and two fatty acids that do not carry a charge.

Even though there are two fatty acids connected to the head, they are lumped together as one "tail." These hydrophilic and hydrophobic ends allow a bilayer to form in the plasma membrane. The bilayer has two layers of phospholipids arranged with their tails on the inside and their heads on the outside.

Plasma Membrane Structure: Lipids and Plasma Membrane Fluidity

The fluid mosaic model explains the function and structure of a cell membrane.

First, the membrane looks like a mosaic because it has different molecules inside like phospholipids and proteins. Second, the membrane is fluid because the molecules can move. The entire model shows that the membrane is not rigid and is capable of changing.

The cell membrane is dynamic, and its molecules can move rapidly. Cells can control the fluidity of their membranes by increasing or decreasing the numbers of molecules of certain substances.

Saturated and Unsaturated Fatty Acids

It is important to note that different fatty acids can make up phospholipids. The two main types are saturated and unsaturated fatty acids.

Saturated fatty acids do not have double bonds and instead have the maximum number of hydrogen bonds with carbon. The presence of only single bonds in saturated fatty acids makes it easy to pack phospholipids together tightly.

On the other hand, unsaturated fatty acids have some double bonds between carbons, so it is harder to pack them together. Their double bonds make kinks in the chains and affect the fluidity of the plasma membrane. The double bonds create more space between phospholipids in the membrane, so some molecules can pass through easier.

Saturated fats are more likely to be solid at room temperature, while unsaturated fatty acids are liquid at room temperature. A common example of a saturated fat you may have in the kitchen is butter.

An example of an unsaturated fat is liquid oil. Hydrogenation is a chemical reaction that can make liquid oil turn into a solid like margarine. Partial hydrogenation turns some of the oil molecules into saturated fats.

••• Dana Chen | Sciencing

Trans Fats

You can divide unsaturated fats into two more categories: cis-unsaturated fats and trans-unsaturated fats. Cis-unsaturated fats have two hydrogens on the same side of a double bond.

However, trans-unsaturated fats have two hydrogens on opposite sides of a double bond. This has a big impact on the shape of the molecule. Cis-unsaturated fats and saturated fats occur naturally, but trans-unsaturated fats are created in the lab.

You may have heard about health concerns related to eating trans fats in recent years. Also called trans-unsaturated fats, food manufacturers create trans fats through partial hydrogenation. Research has not shown that people have the enzymes necessary to metabolize trans fats, so eating them can increase the risk of developing cardiovascular diseases and diabetes.

Cholesterol and the Plasma Membrane

Cholesterol is another important molecule that affects fluidity in the plasma membrane.

Cholesterol is a steroid that occurs naturally in the membrane. It has four linked carbon rings and a short tail, and it is spread out randomly throughout the plasma membrane. The main function of this molecule is to help hold the phospholipids together so that they do not travel too far away from each other.

At the same time, cholesterol provides some necessary spacing between phospholipids and prevents them from becoming so tightly packed that important gases cannot get through. Essentially, cholesterol can help regulate what leaves and enters the cell.

Essential Fatty Acids

Essential fatty acids, such as omega-3s, make up part of the plasma membrane and can affect fluidity as well. Found in foods like fatty fish, omega-3 fatty acids are an essential part of your diet. After you eat them, your body can add omega-3s to the cell membrane by incorporating them into the phospholipid bilayer.

Omega-3 fatty acids can influence the protein activity in the membrane and modify gene expression.

Proteins and the Plasma Membrane

The plasma membrane has different types of proteins. Some are on the surface of this barrier, while others are embedded inside. Proteins can act as channels or receptors for the cell.

Integral membrane proteins are located inside the phospholipid bilayer. Most of them are transmembrane proteins, which means parts of them are visible on both sides of the bilayer because they stick out.

In general, integral proteins help transport larger molecules such as glucose. Other integral proteins act as channels for ions.

These proteins have polar and nonpolar regions similar to the ones found in phospholipids. On the other hand, peripheral proteins are located on the surface of the phospholipid bilayer. Sometimes they are attached to integral proteins.

Cytoskeleton and Proteins

Cells have networks of filaments called the cytoskeleton that provide structure. The cytoskeleton usually exists right under the cell membrane and interacts with it. There are also proteins in the cytoskeleton that support the plasma membrane.

For example, animal cells have actin filaments that act as a network. These filaments are attached to the plasma membrane through connector proteins. Cells need the cytoskeleton for structural support and to prevent damage.

Similar to phospholipids, proteins have hydrophilic and hydrophobic regions that predict their placement in the cell membrane.

For instance, transmembrane proteins have parts that are hydrophilic and hydrophobic, so the hydrophobic parts can pass through the membrane and interact with the hydrophobic tails of the phospholipids.

Carbohydrates in the Plasma Membrane

The plasma membrane has some carbohydrates. Glycoproteins, which are a type of protein with a carbohydrate attached, exist in the membrane. Usually, glycoproteins are integral membrane proteins. The carbohydrates on glycoproteins help with cell recognition.

Glycolipids are lipids (fats) with attached carbohydrates, and they are also part of the plasma membrane. They have hydrophobic lipid tails and hydrophilic carbohydrate heads. This allows them to interact with and bind to the phospholipid bilayer.

In general, they help stabilize the membrane and can help with cell communication by acting as receptors or regulators.

Cell Identification and Carbohydrates

One of the important features of these carbohydrates is that they act like identification tags on the cell membrane, and this plays a role in immunity. The carbohydrates from glycoproteins and glycolipids form the glycocalyx around the cell that is important for the immune system. The glycocalyx, also called the pericellular matrix, is a coating that has a fuzzy appearance.

Many cells, including human and bacterial cells, have this type of coating. In humans, the glycocalyx is unique in each person because of genes, so the immune system can use the coating as an identification system. Your immune cells can recognize the coating that belongs to you and will not attack your own cells.

Other Properties of the Plasma Membrane

The plasma membrane has other roles such as helping the transportation of molecules and cell-to-cell communication. The membrane allows sugars, ions, amino acids, water, gases and other molecules to enter or leave the cell. Not only does it control the passage of these substances, but it also determines how many can move.

The polarity of the molecules helps determine if they can enter or leave the cell.

For example, nonpolar molecules can go through the phospholipid bilayer directly, but polar ones must use the protein channels to pass. Oxygen, which is nonpolar, can move through the bilayer, while sugars must use the channels. This creates selective transport of materials into and out of the cell.

The selective permeability of plasma membranes gives cells more control. The movement of molecules across this barrier is divided into two categories: passive transport and active transport. Passive transport does not require the cell to use any energy to move molecules, but active transport uses energy from adenosine triphosphate (ATP).

Passive Transport

Diffusion and osmosis are examples of passive transport. In facilitated diffusion, proteins in the plasma membrane help the molecules move. Generally, passive transport involves the movement of substances from a high concentration to a low concentration.

For instance, if a cell is surrounded by a high concentration of oxygen, then the oxygen can move freely through the bilayer to a lower concentration inside the cell.

Active Transport

Active transport happens across the cell membrane and usually involves the proteins embedded in this layer. This type of transport allows cells to work against the concentration gradient, which means they can move things from a low concentration to a high concentration.

It requires energy in the form of ATP.

Communication and the Plasma Membrane

The plasma membrane also helps cell-to-cell communication. This can involve the carbohydrates in the membrane that stick out on the surface. They have binding sites that allow for cell signaling. The carbohydrates of one cell's membrane can interact with the carbohydrates on another cell.

The plasma membrane's proteins can also help with communication. Transmembrane proteins act as receptors and can bind to signaling molecules.

Since the signaling molecules tend to be too large to enter the cell, their interactions with the proteins help create a pathway of responses. This happens when the protein changes because of interactions with the signal molecule and starts a chain of reactions.

Health and Plasma Membrane Receptors

In some cases, the membrane receptors on a cell are used against the organism to infect it. For instance, human immunodeficiency virus (HIV) can use the cell's own receptors to enter and infect the cell.

HIV has glycoprotein projections on its exterior that fit the receptors on cell surfaces. The virus can bind to these receptors and get inside.

Another example of the importance of marker proteins on cell surfaces is seen in human red blood cells. They help determine whether you have the A, B, AB or O blood type. These markers are called antigens and help your body recognize its own blood cells.

The Importance of the Plasma Membrane

Eukaryotes do not have cell walls, so the plasma membrane is the only thing that prevents substances from entering or leaving the cell. However, prokaryotes and plants have both cell walls and plasma membranes. The presence of only a plasma membrane allows eukaryotic cells to be more flexible.

The plasma membrane or cell membrane acts as a protective coating for the cell in eukaryotes and prokaryotes. This barrier has pores, so some molecules can enter or exit the cells. The phospholipid bilayer plays an important role as the base of the cell membrane. You can also find cholesterol and proteins in the membrane. Carbohydrates tend to be attached to proteins or lipids, but they play a crucial role in immunity and cell communication.

The cell membrane is a fluid structure that moves and changes. It looks like a mosaic because of the different embedded molecules. The plasma membrane offers support for the cell while helping with cell signaling and transportation.

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