Introduction to Lipids
The cell is composed of two distinctive environments: the hydrophilic aqueous cytoplasm and the hydrophobic lipid membranes. The lipid environment is defined by the family of molecules that are characterized by their hydrophobic nature and their common metabolic origin. Three members of the lipid family of molecules will be discussed in this course: fats (triacylglcerol), phospholipids, and steroids.
The Structure of Lipids
Lipid molecules are slightly soluble to insoluble in water. Lipids are hydrophobic because the molecules consist of long, 16-18 carbon, hydrocarbon backbones with only a small amount of oxygen containing groups. Lipids serve many functions in organisms. They are the major components of waxes, pigments, steroid hormones, and cell membranes. Fats, steroids, and phospholipids are very important to the functioning of membranes in cells and will be the focus of this tutorial.
Fats (triacylglycerols, triglycerides)
Fats are synthesized from two different classes of molecules: fatty acids attached to the alcohol glycerol. The fatty acids are generally, 16-22 carbons long, unbranched hydrocarbons that terminate with a single carboxyl functional group. The fatty acid can be of two types: saturated and unsaturated. Saturated fatty acids have no carbon-carbon double bonds (they are saturated with hydrogen) while the unsaturated fatty acids have one to three double bonds along the backbone carbon chain. These double bonds introduce "kinks" in the carbon chain which have important consequences on the fluid nature of lipid membranes. Unsaturated fatty acids have lower melting points than saturated fatty acids.
To construct a fat, or triacylglycerol, three fatty acid molecules are attached to the glycerol through an ester bond between the carboxyl group of the fatty acids and the three alcohol groups of a glycerol molecule. This is another example of a condensation reaction that results in formation of an ester in this case and the release of a water molecule. A fat molecule can be composed of one, two, or three different types of fatty acids each of which can be saturated or unsaturated.
An unsaturated fat has at least one unsaturated fatty acid whereas a saturated fat has none. Because the double bonds of the unsaturated fatty acids introduce kinks in the hydrocarbon backbone, unsaturated fats will not pack into a regular structure and thus remain fluid at lower temperatures. A saturated fat will pack well and be a solid a low temperatures.
Fats are mainly energy storage and insulating molecules. Per gram, fats contain twice as much energy as carbohydrates. Layers of fat also surround the vital organs of animals to cushion them, and layers of fat under the skin of animals provide insulation.
Phospholipids contain only two fatty acids attached to a glycerol head. This occurs by a condensation reaction similar to the one discussed above. The third alcohol of the glycerol forms an ester bond through reaction with phosphoric acid. This is another example of a condensation reaction between an acid and an alcohol with the release of water. As a triprotic acid (i.e it has three acidic functions on the phorphorus atom) the phosphate group attached to the glycerol has the potential to form ester links with a variety of other molecules such as carbohydrates, choline, inositol and amino acids. The phosphate group along with the glycerol group make the head of the phospholipid hydrophilic, whereas the fatty acid tail is hydrophobic. Thus phospholipids are amphipathic:a molecule with a polar end and a hydrophobic end. When phospholipids are in an aqueous solution they will self assemble into micelles or bilayers, structures that exclude water molecules from the hydrophobic tails while keeping the hydrophilic head in contact with the aqueous solution. View the animation that demonstrates the formation of micelles and bilayers.
Phospholipids serve a major function in the cells of all organisms: they form the phospholipid membranes that surround the cell and intracellular organelles such as the mitochondria. The cell membrane is a fluid, semi-permeable bilayer that separates the cell's contents from the environment, see animation below. The membrane is fluid at physiological temperatures and allows cells to change shape due to physical constraints or changing cellular volumes. The phospholipid membrane allows free diffusion of some small molecules such as oxygen, carbon dioxide, and small hydrocarbons, but not charged ions, polar molecules or other larger molecules such as glucose. This semi-permeable nature of the membrane allows the cell to maintain the composition of the cytoplasm independent of the external environment.
The steroids are a family of lipids based on a molecule with four fused carbon rings. This family includes many hormones and cholesterol. Cholesterol is a component of the cell membrane in animals and functions to moderate membrane fluidity because it restricts the motion of the fatty acid tails.
Review of Lipids
Use the following animation to review the discussion of lipids.
The Structures of the Cell Membrane
Fluid Quality of Membranes
The cell membrane must be a dynamic structure if the cell is to grow and respond to environmental changes. To keep the membrane fluid at physiological temperatures the cell alters the composition of the phospholipids. The right 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. 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. The fluidity of the membrane is demonstrated in the following animation. The lipids in the membrane are in random bulk flow moving about 22 µm (micrometers) per second. Phospholipids freely move in the same layer of the membrane and rarely flip to the other layer. Flipping of phospholipids from one layer to the other rarely occurs because flipping requires the hydrophilic head to pass through the hydrophobic region of the bilayer.
The Mosaic Quality of Membranes
Because the cell membrane is only semipermeable, the cell needs a way to communicate with other cells and exchange nutrients with the extracellular space. These roles are primarily filled by proteins. Membrane proteins are classified into two major categories, integral proteins and peripheral proteins. Integral membrane proteins are those proteins that are embedded in the lipid bilayer and are generally characterized by their solubility in non-polar, hydrophobic solvents. Transmembrane proteins are examples of integral proteins with hydrophobic regions that completely span the hydrophobic interior of the membrane. The parts of the protein exposed to the interior and exterior of the cell are hydrophilic. Integral proteins can serve as pores that selectively allow ions or nutrients into the cell. They also transmit signals into and out of the cell. Unlike integral proteins that span the membrane, peripheral proteins reside on only one side of the membrane and are often attached to integral proteins. Some peripheral proteins serve as anchor points for the cytoskeleton or extracellular fibers. Proteins are much larger than lipids and move more slowly. Some move in seemingly directed manner while others drift.
The extracellular surface of the cell membrane is decorated with carbohydrate groups attached to lipids and proteins. Carbohydrates are added to lipids and proteins by a process called glycosylation, and are called glycolipids or glycoproteins. These short carbohydrates, or oligosaccharides, are usually chains of 15 or fewer sugar molecules. Oligosaccharides give a cell identity (i.e., distinguishing self from non-self) and are the distinguishing factor in human blood types and transplant rejection.
Membranes are Asymmetric
As discussed above and seen in the picture, the cell membrane is asymmetric. The extracellular face of the membrane is in contact with the extracellular matrix. The extracellular side of the membrane contains oligosaccharides that distinguish the cell as self. It also contains the end of integral proteins that interact with signals from other cells and sense the extracellular environment. The inner membrane is in contact the contents of the cell. This side of the membrane anchors to the cytoskeleton and contains the end of integral proteins that relay signals received on the external side.
Summary: Membranes as Mosaics of Structure and Function
The biological membrane is a collage of many different proteins embedded in the fluid matrix of the lipid bilayer. The lipid bilayer is the main fabric of the membrane, and its structure creates a semi-permeable membrane. The hydrophobic core impedes the diffusion of hydrophilic structures, such as ions and polar molecules but allows hydrophobic molecules, which can dissolve in the membrane, to cross it with ease. 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.