Lipids

 Lipids have 3 major roles in cells.

• They provide an important form of energy storage.
• They are the major components of cell membranes.
• They play important roles in cell signaling, both as steroid hormones (e.g. testosterone and estrogen) and as messenger molecules that convey signals from cell surface receptors to targets within the cell.

Fatty acids consist of long hydrocarbon chains, most frequently containing 16 or 18 carbon atoms, with a carboxyl group at one end.

 UNSATURATED fatty acids contain one or more double bonds between carbon atoms (Fig. 1)

 In SATURATED fatty acids all of the carbon atoms are bonded to the maximum number of hydrogen atoms.(Fig. 1)

 The long hydrocarbon chains of fatty acids contain only nonpolar C-H bonds, which are unable to interact with water. The hydrophobic nature of these fatty acid chains is responsible for much of the behavior of complex lipids, particularly in the formation of biological membranes.  The hydrophillic heads (carboxyl group) gives fatty acids a dual nature referred to as amphiphatic.

 Fatty acids are stored in the form of TRIACYLGLYCEROLS (Fig.4), or fats, which consist of three fatty acids linked to a glycerol molecule (Fig. 2). These molecules are insoluble in water and therefore accumulate as fat droplets in the cytoplasm. When required, they can be broken down for use in energy-yielding reactions.

Fats are actually a more efficient form of energy than carbohydrates, yielding more than twice as much energy per weight of material broken down.

Well over 100 different kinds of fatty acids have been isolated from various lipids of animals, plants, and microorganisms. Bacteria contain fewer and simpler types of fatty acids than higher organisms -- fatty acids with more than one double bond have not been found in bacteria.
 

Phospholipids are the principle components of cell membranes, consisting of two fatty acids joined by a polar head group (Fig. 3). In the glycerol phospholipids, the 2 fatty acids are bound to carbon atoms in glycerol but the third C is bound to a phosphate group, which in turn is often attached to another small polar molecule, such as choline, serine, inositol, or ethanolamine (Fig. 5).

ALL phospholipids have POLAR head groups and NON-POLAR fatty acid tails. This amphiphatic nature is the driving force for the formation of membrane structures.

 SPHINGOMYELIN

Sphingomyelin is a nonglycerol phospholipid in cell membranes, contains two hydrocarbon chains linked to a polar head group formed from serine rather than glycerol (Fig. 6).

Many cell membranes also contain GLYCOLIPIDS (Fig. 8).
Glycolipids consist of two hydrocarbon chains linked to polar head groups (like serine) that contain carbohydrates.

Cholesterol is also found in and in contrast to phospho- and glyco-lipids, consists of four hydrocarbon rings rather than linear hydrocarbon chains (Fig. 7).  Cholesterol tends to make membranes more rigid because of the inflexible ring structures.

 The hydrocarbon rings are strongly hydrophobic, but the hydroxyl group attached to one end of cholesterol is weakly hydrophilic, so cholesterol is amphipathic.

 Steroid hormones, such as testosterone, are derivatives of cholesterol (Fig. 7).

 These hormones are a diverse group of chemical messengers, all of which contain four hydrocarbon rings to which distinct functional groups are attached.

When fatty acids are mixed into water the fatty acids will coat the surface of the water with the hydrophillic polar heads in contact with the water and the hydrophobic tails pointed skyward (Fig. 9).

Fatty acids that get swept under the surface of the water, they will aggregate into spherical micelles with the hydrophillic polar heads in contact with the water and the hydrophobic tails pointed inward (Fig. 9).

When phospholipids get swept under the surface of the water, they will form into spherical liposomes or membrane bound vessicles (Fig. 10).   The membrane consists of two layers of phospholipids, called a lipid bilayer, with the hydrophobic regions in contact with the water and the hydrophobic tails in association with each other.  Membranes are about 4 nm thick (Fig. 10).

Membrane structures are found surrounding the cell and in eukaryotes there are also membrane bound organelles inside the cell (Fig. 12). Membranes are permeable to small hydrophobic molecules and small uncharged polar molecules.  They are not permeable to large uncharged polar molecules and ions (Fig. 11).

Imbedded into membranes are a variety of proteins (Fig. 13) that provide regulated transport for external signals and molecules that are not nomally  membrane permeable (Fig. 11).

Membranes serve as attachment sites for enzymes that need to be localized near signal transduction centers (Fig. 11).

Some membranes proteins serve as anchors for structural attachments that maintain the integrity of the cell when it is placed under stress (Fig. 11).  For example, some of the linker molecules include distrophin.  Lack of functional linkers such as distrophin are the basis for diseases such as muscular distrophy.

 Quiz