Plant Evolution

Now this is where I want to start my story about plant evolution because the important feature of virtually all plant cells except for some specialized reproductive ones, is that each plant cell is encapsulated within a crystalline exoskeleton called the cell wall. This is a marvel of biological architecture because at the ultra-structural level, shown by this ultra-thin highly magnified section with an electron microscope, you see that the cell wall is composed of microfibrils. A fibrillar kind of organization. Each of these microfibrils is composed of cellulose.

Let me just show you this architecture diagrammatically. Here is a small portion of the cell wall. You see what looks like a coaxial cable to a computer, which is actually called a macrofibril, visible under the light microscope. This macrofibril can be dissected at the ultra-structural level into what are called microfibrils. At the molecular level these in turn are composed of cellulose, which of course is polymer of sugars linked together in a more or less paracrystalline organization.

Now what is not fully appreciated (and this is idle cocktail chatter amongst biomechanists) is that cellulose is one of the strongest and stiffest materials known. This is strength of material plotted against stiffness. Forget that fancy phrase, it's just stiffness versus strength. There you see coral, bone, wood, tendon, collagen, concrete, shells, cast iron, aluminum, steel and there's cellulose. Notice that cellulose competes well with all these engineered materials in terms of strength and stiffness.

There is a dimension that this slide isn't showing you. You must imagine an additional axis coming from the projector towards you. That axis represents density. You'll find that of all these materials, the least dense is cellulose. Now, why is that important? Well, ask this of your students. Why is it important that in addition to being strong and stiff, you also use light weight materials? The reason is that if you're an architect or engineer and you're designing a structure that has to be strong and stiff, you also have to be worried about the weight that material contributes to the structure per unit volume. So ideally, you are looking for a light weight but yet strong and stiff material. That's why aluminum is so terribly important in the manufacture of engineered structures. It's fairly strong, fairly stiff yet low in its density. This is the idle chatter that you'd get at a cocktail party of biomechanicists - for its density, cellulose is the strongest and stiffest naturally occurring material known. There is nothing better than cellulose.

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