Lipids are a structurally heterogeneous group of compounds.   They are characterized by the presence of distinct hydrophilic and hydrophobic domains.

For example stearic acid has a "tail" of 17 carbon atoms and their associated hydrogens attached to a carboxylic acid (COOH) group.

The hydrocarbon tail is extremely hydrophobic and so is insoluble in water.

In contrast, the carboxylic acid group is extremely hydrophilic.

Molecules with both hydrophobic and hydrophilic groups are termed amphipathic.  

a bilayer

In aqueous solution, lipids tend to self-assemble into micelles, bilayers and other complex structures.

In these structures, the lipid's hydrophobic tail is sequestered away from contact with water, while at the same time, its hydrophilic head is immersed in water.

The cell boundary:  The common feature of all living cells is a distinct, lipid-rich barrier, the plasma membrane.

This membrane defines the boundary between the outside and the inside of the cell.

The difference between the two is profound.  Outside is mostly water, with few complex molecules.

Inside is a highly concentrated solution of proteins, nucleic acids, and smaller molecules - known collectively as cytoplasm.

This bounded system, or cell, has the properties of life. It can reproduce itself by using energy taken from beyond the boundary.

Cells are capable of directed movement and complex behaviors.  They can import energy and matter and use them to build copies of themselves. They are also homeostatic - assuming that a perturbation is not too severe, they can recover and return to their normal state.  They are also adaptive, they can change their behavior in response to signals from their environment.

In multicellular organisms, the membrane is also the site of cell-cell coordination and communication.

• Lipid molecules self assemble into micelles in water.   Micelles are more ordered than individual lipid molecules.  Why does that not violate the second law of thermodynamics?
• Does the process of micelle formation require energy?  
• How is a bilayer like a micelle?  How would you minimize the effects at the edges of a lipid bilayer?
• What types of molecules might be able to go through the plasma membrane?
• In the light of the cell theory, what can we say about the history of cytoplasm and the surrounding plasma membrane? 

Building a plasma membrane:  The plasma membrane is built on a foundation of lipids.  All earthly organisms use lipids built on glycerol.

Glycerol has three hydroxyl (-OH) groups. 

In the phospholipids, one of these groups is linked to a phosphate group.

Phosphate is derived from phosphoric acid. A phosphate group is added to one of glycerol's terminal hydroxyl groups through a condensation reaction.

Phosphoglycerol is even more hydrophilic than glycerol.

In the bacteria and eukarya, glycerol's two other -OH groups are coupled to unbranched fatty acid chains through condensation reactions. 

This forms an an ester linkage between the fatty acid and the glycerol moieties.

In the Archaea, branched isoprene chains (rather than fatty acids) are attached to glycerol via ether linkages.

In water, both types of lipid molecules organize themselves to minimize the exposure of their hydrophobic groups to water.

This leads to the formation of a bilayer membrane.

• Are the membranes of bacteria and archaea homologous or analogous? 
• Why do these different lipids form a similar bilayer structure?
• What is the effect of the phosphate attached to the glycerol group?
• What is a moiety?

As temperature increases, the membrane melts. The lipid tails become increasingly disordered. 

Proteins embedded in a fluid membrane can move around within the plane of the membrane.

Membrane fluidity is critical for the correct functioning of many membrane proteins.

Cells control membrane fluidity by regulating the lipid composition of the membrane.  In particular, lipids can have hydrocarbon chains that are saturated or unsaturated.

In saturated hydrocarbon chains, the carbons are linked by single bonds. The hydrocarbon chain is flexible, but more or less straight.

In unsaturated hydrocarbon chains, some of the carbons are linked by double-bonds.

When a lipid is dehydrogenated, hydrogens are removed and –C=C– bonds are formed.

The presence of a –C=C– bond leads to a kink in the hydrocarbon chain.  Kinked chains cannot pack together as regularly as can straight (saturated) hydrocarbon chains.

Compare the saturated fatty acid stearic acid to the unsaturated fatty acid oleic acid. Both have the same number of carbons in their hydrocarbon chains.

While stearic acid melts at 69°C oleic acid melts at 13°C.

• Why do membrane lipids solidify at low temperature? How are van der Waals interactions involved?  
• Why does the presence of unsaturated bonds in the lipid chain alter the temperature at which the membrane solidifies?
• Predict the effect of changing the position of a double bond in a hydrocarbon chain on the temperature of solidification?
• Would a membrane be more permeable to small molecules at high or low temperature and why?