Nick Lane --The Vital Question, Post #8 -- June 12, 2019

Nick Lane raises interesting questions about cell membranes in the last three pages of chapter 2.

These three pages need to be read again, and again, and then note the questions that he raises, and then read how he answers those questions in the next  chapter, that beings Part II, "The Origin of Life."

He begins with this question: what was the membrane like for the last common ancestor to bacteria and archaea.

Lane notes that bacterial membranes are very, very different from the membranes of archaea. A key ingredient for both is glycerol but one uses "right-handed" glycerol; the other uses "left-handed" glycerol.

See this link.

The most striking chemical differences between Archaea and other living things lie in their cell membrane. Their are four fundamental differences between the archaeal membrane and those of all other cells: (1) chirality of glycerol, (2) ether linkage, (3) isoprenoid chains, and (4) branching of side chains. These may sound like complex differences, but a little explanation will make the differences understandable.

Then this:

(1) Chirality of glycerol : The basic unit from which cell membranes are built is the phospholipid. This is a molecule of glycerol which has a phosphate added to one end, and two side chains attached at the other end. When the cell membrane is put together, the glycerol and phosphate end of the molecules hang out at the surface of the membrane, with the long side chains sandwiched in the middle (see illustration above). This layering creates an effective chemical barrier around the cell and helps maintain chemical equilibrium.

The glycerol used to make archaeal phospholipids is a stereoisomer of the glycerol used to build bacterial and eukaryotic membranes. Two molecules that are stereoisomers are mirror-images of each other. Put your hands out in front of you, palms up. Both hands are oriented with fingers pointing away from you, wrists toward you, and with palms upwards. However, your thumbs are pointing different directions because each hand is a mirror image of the other. If you turn one hand so that both thumbs point the same way, that one will no longer be palm-up.

This is the same situation as the stereoisomers of glycerol. There are two possible forms of the molecule that are mirror images of each other. It is not possible to turn one into the other simply by rotating it around.

While bacteria and eukaryotes have D-glycerol in their membranes, archaeans have L-glycerol in theirs.

This is more than a geometric difference. Chemical components of the cell have to be built by enzymes, and the "handedness" (chirality) of the molecule is determined by the shape of those enzymes. A cell that builds one form will not be able to build the other form.

 From The Smithsonian:

One of the strangest aspects of life on Earth—and possibly of life elsewhere in the cosmos—is a feature that puzzles chemists, biologists and theoretical physicists alike.

Each of life’s molecular building blocks (amino acids and sugars) has a twin—not an identical one, but a mirror image. Just like your right hand mirrors your left but will never fit comfortably into a left-handed glove, amino acids and sugars come in both right and left versions. 

This phenomenon of biological shape selection is called “chirality”—from the Greek for handedness.

On Earth, the amino acids characteristic of life are all “left-handed” in shape, and cannot be exchanged for their right-handed doppelgänger. Meanwhile, all sugars characteristic of life on Earth are “right-handed.” The opposite hands for both amino acids and sugars exist in the universe, but they just aren’t utilized by any known biological life form. (Some bacteria can actually convert right-handed amino acids into the left-handed version, but they can’t use the right-handed ones as is.) In other words, both sugars and amino acids on Earth are homochiral: one-handed.