What is responsible for membrane fluidity?

The cell membrane is a dynamic structure composed of lipids, proteins, and carbohydrates. It protects the cell by preventing materials from leaking out, controls what can enter or leave through the membrane, provides a binding site for hormones and other chemicals, and serves as an identification card for the immune system to distinguish between “self” and “non-self” cells. We will first investigate the anatomy of the cell membrane and then continue on to study the physiology of membrane transport.

The phospholipid bilayer is the main fabric of the membrane. The bilayer’s structure causes the membrane to be semi-permeable. Remember that phospholipid molecules are amphiphilic, which means that they contain both a nonpolar and polar region. Phospholipids have a polar head (it contains a charged phosphate group) with two nonpolar hydrophobic fatty acid tails. The tails of the phospholipids face each other in the core of the membrane while each polar head lies on the outside and inside of the cell. Having the polar heads oriented toward the external and internal sides of the membrane attracts other polar molecules to the cell membrane. The hydrophobic core blocks the diffusion of hydrophilic ions and polar molecules. Small hydrophobic molecules and gases, which can dissolve in the membrane’s core, cross it with ease.

Other molecules require proteins to transport them across the membrane. Proteins determine most of the membrane’s specific functions. The plasma membrane and the membranes of the various organelles each have unique collections of proteins. For example, to date more than 50 kinds of proteins have been found in the plasma membrane of red blood cells.

Importance of Phospholipid Membrane Structure

What is important about the structure of a phospholipid membrane? First, it is fluid. This allows cells to change shape, permitting growth and movement. The fluidity of the membrane is regulated by the types of phospholipids and the presence of cholesterol. Second, the phospholipid membrane is selectively permeable.

The ability of a molecule to pass through the membrane depends on its polarity and to some extent its size. Many non-polar molecules such as oxygen, carbon dioxide, and small hydrocarbons can flow easily through cell membranes. This feature of membranes is very important because hemoglobin, the protein that carries oxygen in our blood, is contained within red blood cells. Oxygen must be able to freely cross the membrane so that hemoglobin can get fully loaded with oxygen in our lungs, and deliver it effectively to our tissues. Most polar substances are stopped by a cell membrane, except perhaps for small polar compounds like the one carbon alcohol, methanol. Glucose is too large to pass through the membrane unassisted and a special transporter protein ferries it across. One type of diabetes is caused by misregulation of the glucose transporter. This decreases the ability of glucose to enter the cell and results in high blood glucose levels. Charged ions, such as sodium (Na+) or potassium (K+) ions seldom go through a membrane, consequently they also need special transporter molecules to pass through the membrane. The inability of Na+ and K+ to pass through the membrane allows the cell to regulate the concentrations of these ions on the inside or outside of the cell. The conduction of electrical signals in your neurons is based on the ability of cells to control Na+ and K+ levels.

Selectively permeable membranes allow cells to keep the chemistry of the cytoplasm different from that of the external environment. It also allows them to maintain chemically unique conditions inside their organelles.

Fluidity of Cell Membranes

The cell membrane is not a static structure. It is a dynamic structure that allows the movement of phospholipids and proteins. Fluidity is a term used to describe the ease of movement of molecules in the membrane and is an important characteristic for cell function. Fluidity is dependent on the temperature (increased temperatures it more fluid and decreased temperatures make it more solid), saturated fatty acids and unsaturated fatty acids. Saturated fatty acids make the membrane less fluid while unsaturated fatty acids make it more fluid. The correct ratio of saturated to unsaturated fatty acids keeps the membrane fluid at any temperature conducive to life. For example, winter wheat responds to decreasing temperatures by increasing the amount of unsaturated fatty acids in cell membranes to prevent the cell membrane from becoming too solid in the cold. In animal cells, cholesterol helps to prevent the packing of fatty acid tails and thus lowers the requirement of unsaturated fatty acids. This helps maintain the fluid nature of the cell membrane without it becoming too liquid at body temperature.

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All cells are surrounded by a cell membrane that forms a barrier between the cell and its surroundings. This membrane is often referred to as the phospholipid bilayer. As you can probably tell from the name, a phospholipid bilayer is made up of two layers of lipids. The fluidity of this membrane must be maintained within a certain range for the cell to function properly.  There are a number of factors that help influence membrane fluidity. Before we review those factors, let's start with a quick review of the structure of the bilayer.

What is the phospholipid bilayer?

The phospholipid bilayer is composed of two layers of lipids. Each lipid contains a hydrophobic (water repelling) tail and a hydrophilic (water attracting) head.  The lipids form into a bilayer with the hydrophobic tails facing the interior of the bilayer forming a hydrophobic region held together, in part, by intermolecular forces between the tails. The hydrophilic heads form a hydrophilic region on either side of the bilayer that can interact with the water rich environments on either side of the bilayer.

Now, let's take a look at the factors that influence membrane fluidity!

Factor #1: The length of the fatty acid tail

The length of the fatty acid tail impacts the fluidity of the membrane. This is because the intermolecular interactions between the phospholipid tails add rigidity to the membrane. As a result, the longer the phospholipid tails, the more interactions between the tails are possible and the less fluid the membrane will be.

Factor #2: Temperature

As temperature increases, so does phospholipid bilayer fluidity. At lower temperatures, phospholipids in the bilayer do not have as much kinetic energy and they cluster together more closely, increasing intermolecular interactions and decreasing membrane fluidity. At high temperatures the opposite process occurs, phospholipids have enough kinetic energy to overcome the intermolecular forces holding the membrane together, which increases membrane fluidity.

Factor #3: Cholesterol content of the bilayer

Cholesterol has a somewhat more complicated relationship with membrane fluidity. You can think of it is a buffer that helps keep membrane fluidity from getting too high or too low at high and low temperatures.

At low temperatures, phospholipids tend to cluster together, but steroids in the phospholipid bilayer fill in between the phospholipids, disrupting their intermolecular interactions and increasing fluidity.

At high temperatures, the phospholipids are further apart. In this case, cholesterol in the membrane has the opposite effect and pulls phospholipids together, increasing intermolecular forces and decreasing fluidity.

Factor #4: The degree of saturation of fatty acids tails

Phospholipid tails can be saturated or unsaturated. The terms saturated and unsaturated refer to whether or not double bonds are present between the carbons in the fatty acid tails. Saturated tails have no double bonds and as a result have straight, unkinked tails. Unsaturated tails have double bonds and, as a result, have crooked, kinked tails.

As you can see above, saturated fatty acids tails are arranged in a way that maximizes interactions between the tails. These interactions decrease bilayer fluidity. Unsaturated fatty acids, on the other hand, have more distance between the tails and thus fewer intermolecular interactions and more membrane fluidity.

In summary!