Boundaries, skins, membranes, peels, shells, coverings, surfaces, barriers.
Their names suggest their functions: keeping things in, keeping things out,
defining areas or volumes. Like parts of some buildings, they could serve as
fences, doors, windows, turnstiles, mailboxes, garages, or the walls
Some of these limits are transitory. Some are circumstantial, that is, they exist
because whatever is next to it exists. Some are very simple, monolithic,
singular in function - consider a soap membrane. Others are complex,
integrated, and multifunctional - the human skin, for example. There are also
invisible limits, unseen but well known - spaces around people whose limits are determined by the demands of protocol. Enormous efforts are sometimes made to erect and maintain some limits and boundaries - the Great Wall of China, l,500 miles of determined effort to keep out northern invaders.
The very first hint of a membrane may well have been made up of molecules held together by chemical bonds surrounding a concentration of larger molecules in coacervates, precursors to the first cells. The same need for a structure to hold together a vital concentration of complex molecules exists in all cells now just as it did with the primitive cells 5 billion years ago.
Membranes of many different kinds are found in all plants and animals. and even those that may seem to be neither! The membrane, and analogous structures, is an important part of a living body. Membranes cover individual cells, organized cells, compact organs, and, ultimately, entire organisms. What do these ubiquitous membranes do?
Their functions are varied, but their most obvious function hasn't changed from the days of the primitive soup from which emerged the earliest cells. Membranes separate what's inside from what's outside. In doing so, they enable what's inside to behave in a certain way, uninterrupted by the outside environment.
But, that is not all they do. Some membranes contain the means to actually control the composition of the inside and outside environments and drive the flow of substances back and forth. This capability is exploited in the use of gramicidin, an antibiotic that creates a pathway through bacterial membranes, enabling the passage of potassium and hydrogen ions that are lethal to the wee organisms.
Other examples can be seen in plants that are able to grow in salty environments. Membranes in such plants can take in water or exclude salt that may be held within bladders on the leaf surfaces. The flow of water or salts is driven by chemical pumps within the cells.
Something to do:
A simple experiment at home will illustrate one of the roles of
membranes around cells. Take a small piece of meat, absorb any extra fluids
with a paper towel, then freeze the meat in a leak-proof plastic bag for
several days. Thaw the meat slowly without taking it out of the bag. What do
you notice? What does this have to do with membranes? Muscle cells are
enclosed by a soft membrane that are damaged by the sharp ice crystals that
form when the cells are frozen. The damage results in leakage of cell fluids.
Hence, meat that has been frozen for a long time tends to become dry and
stringy as the fluids leak out and are sublimated in the freezer. Other
tissues from fish, vegetables and fruit show this well.
There are many different kinds of boundaries in larger organisms, or parts of them. And the boundaries themselves are sometimes quite complex. Citrus fruit skins contain special oil glands that contain volatile, flammable oils. Human skin is a complex organ that is an organized extension of several body systems: circulation, nervous, excretory, regulatory, muscular. We rely on the tannins found in red grape skins to provide astringency to red wines and flavor and color to the grapes themselves.
There are some coverings that we choose to do without even though they are an integral part of the things they cover. The coats of grain seeds, for example, have challenged people from early times. Teeth simply last longer, the grains taste better, and cook better, if the hard outer covering were gone. So technology in agriculture evolved from the smashing of the grains between rocks to complex machinery that would do just that. However, part of the coating contains the valuable fiber, oils, B vitamins, and about 25% of the grain's protein. Thus, it is prudent to retain or consume the bran, or edible part of the coverings and just remove the hard, outermost layer.
Something to do
Smell the peel of a citrus fruit. Gently squeeze a part of its peel and smell it now. What do you notice? Have you ever noticed the fine spray that is sometimes released when you peel an orange? The next time you do this, direct the spray toward a candle flame. What happens?
Living things are not the only things that are membrane-bound. But, for non-living things, let us change our image from the membrane to, say, a boundary. In fluid mechanics, a boundary layer appears and disappears circumstantially. It is not composed of different material. It merely appears when a flowing gas or liquid is interrupted in some way.
Something to do:
Boundary layers are easily seen in a flowing stream of water. This can work at home if you have a faucet or tap that permits the water to flow smoothly. Of course, it's more enjoyable in a pleasant flowing stream! In either case, place a smooth, flat object into the stream, parallel to the
direction of flow, say, a plastic card or a flat rock. What do you see? You
might notice that there is a change in the "thickness" or appearance of the
fluid at the interface between your object and the fluid. That is called the
boundary layer. This layer has a different velocity from the rest of the
fluid, has different thicknesses, and can become stable or unstable, from
laminar to turbulent. Try this with surfaces of different types, sizes,
thicknesses. Remove the object. What happens to the boundary layer?
There is a skin effect in electricity which is not visible in itself but is the tendency of currents to flow near the surface boundary. This accounts of the shiny look of fresh metal surfaces. Metals have high conductivity. Electromagnetic waves are reflected from metal surfaces. The reflection occurs through the skin effect, the generation of electric current in the surface, that re-radiates the light outward, thus, a shiny appearance.
The shiny surface of chrome on automobiles is not for beauty alone, although shiny has become synonymous with their desirability. The chromium is a good and deliberate choice as a coating surface to serve as a barrier between the primary, underlying metal and the oxidation and corrosion that would attack it. The chromium also protects against heat attack.
It is apparent by now that nearly everything on earth, and even beyond, has its structure or space defined by some kind of a boundary or membrane. Indeed, it is almost inconceivable that any entity has no boundary structure of some sort. Except perhaps the universe itself. Or, does it?