Wednesday, February 02, 2005

The Tree's Knees

Knees Are Like Trees, Except...

Bend down and take a handful of that dark, rich soil. (I am thinking Iowa here, not desert Idaho.) There are yards of humus-laden soil beneath your feet. The dark color and nutrient-absorbing properties are provided by the second most abundant biological macromolecule on Earth, lignin. Lignin is part of the secondary walls of plants and is produced by free radical polymerization. So, unlike other macromolecules, proteins, polysaccharides and nucleic acids, lignin cannot be hydrolyzed and thus accumulates in soil. The only reason that it is number two in abundance is that the first place goes to cellulose, which is not degraded because it is present in dehydrated fibers that are not readily attacked by microbe enzymes.
Lignins make trees stiff and resistant to compression. That is why leaves can suck on xylem vessels and the vessels don’t collapse. The lignin molecules, which can reach the dimensions of a Sequoia are rigid and non-compressible. But since they are so stiff and neither extend nor shrink, plants can’t jump.
If you want to jump, you need cartilage that can stretch and bounce back when compressed. Articular cartilage is made up of hyaluronan, a polysaccharide with uronic acid residues related in structure to those of the pectin (polygalacturonic acid) in plant walls; collagen fibers, similar in rigidity to the cellulose fibers of plant walls; and proteogylcans, similar to the hemi-celluloses in plant walls. The bounceable difference is that joint cartilage has sulfated polysaccharides (heparan sulfate, chondroitin sulfate, dermatan sulfate) attached to the protein backbone (aggrecan) of its proteoglycans. The sulfates make all the difference -- no sulfate, no bounce. Sulfates have a net negative charge and lots of oxygens for hydrogen bonding. So, instead of just having a simple hydroxyl (OH-) on the sugars, there is a sulfate that forms a huge, multilayered shell of water molecules. In fact, the articular cartilage doesn’t have a lot of free water, so that if you squeeze it, it is like closed-cell foam and not a water-saturated sponge.
Sulfation does for knee cartilage what lignin does for trees.