Modify Material Characteristics
The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.
Modify Size/Shape/Mass/Volume
Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.
Physically Assemble Structure
Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.
Chemically Assemble Organic Compounds
Part of the reason that synthesis reactions (chemical assembly) can occur under such mild conditions as ambient temperature and pressure in water is because most often, they occur in a stepwise, enzyme-mediated fashion, sipping or releasing small amounts of energy at each step. For example, the synthesis of glucose from carbon dioxide in the Calvin cycle is a 15-step process, each step regulated by a different enzyme.
Chemically Assemble on Demand
The vast majority of biochemical assembly and break down processes––even by the most complex organisms––occur within cells. In fact, cells are able to perform hundreds, even thousands, of chemical transformations at the same time under life-friendly conditions (ambient temperature and pressure in an aqueous environment). For example, venomous snakes store precursor molecules to instantly synthesize a suite of toxins via enzyme-mediated cascades.