The modern cell membrane is composed of a number of different types of lipids. Those lipids with one or more hydrophobic “tails” have tails that typically range from 16 to 20 carbons in length. The earliest membranes, however, were likely to have been composed of similar, but simpler molecules with shorter hydrophobic chains. Based on the properties of lipids, we can map out a plausible sequence for the appearance of membranes. Lipids with very short hydrophobic chains, from 2 to 4 carbons in length, can dissolve in water (can you explain why?) As the lengths of the hydrophobic chains increases, the molecules begin to self-assemble into micelles. By the time the hydrophobic chains reach ~10 carbons in length, it becomes increasingly more difficult to fit the hydrocarbon chains into the interior of the micelle without making larger and larger spaces between the hydrophilic heads. Water molecules can begin to move through these spaces and interact with the hydrocarbon tails. At this point, the hydrocarbon-chain lipid molecules begin to associate into semi-stable bilayers. One interesting feature of these bilayers is that the length of the hydrocarbon chain is no longer limiting in the same way that it was limiting in a micelle. One problem, though, are the edges of the bilayer, where the hydrocarbon region of the lipid would come in contact with water, a thermodynamically unfavorable situation. This problem is avoided by linking edges of the bilayer to one another, forming a balloon-like structure. Such bilayers can capture regions of solvent, that is water and any solutes dissolved within it.
Bilayer stability increases further as hydrophobic chain length increases. At the same time, membrane permeability decreases. It is a reasonable assumption that the earliest biological systems used shorter chain lipids to build their "proto-membranes" and that these membranes were relatively leaky171. The appearance of more complex lipids, capable of forming more impermeable membranes, must therefore have depended upon the appearance of mechanisms that enabled hydrophilic molecules to pass through membranes. The process of interdependence of change is known as co-evolution. Co-evolutionary processes were apparently common enough to make the establishment of living systems possible. We will consider the ways through a membrane in detail below.
Questions to answer & to ponder:
- Is the universe at equilibrium? If not when will it get to equilibrium?
- Draw diagrams to show how increasing the length of a lipid's hydrocarbon chains affects the structures that it can form.
- How are the effects at the hydrophobic edges of a lipid bilayer minimized?
- What types of molecules might be able to go through the plasma membrane on their own?
- Draw what “double-headed” lipids look like in the context of a bilayer membrane.
- In the light of the cell theory, what can we say about the history of cytoplasm and the plasma membrane?
- Why do fatty acid and isoprene lipids form similar bilayer structures?
- Speculate on why it is common to see phosphate and other highly hydrophilic groups attached to the glycerol groups of lipids?
- Are the membranes of bacteria and archaea homologous or analogous? What type of data would help you decide?
- Why is the movement of materials through the membrane essential for life?
- Why do membrane lipids solidify at low temperature? How are van der Waals interactions involved? Are H-bond type electrostatic interactions involved?
- Predict (and justify) the effect of changing the position of a double bond in a hydrocarbon chain on the temperature of membrane solidification.
- Would a membrane be more permeable to small molecules at high or low temperatures and why?