Introduction
As far as we know, as far as we are taught, there are three forms of stuff in the world: animals, vegetables, and minerals. Using this broad categorization rocks and soil are certainly mineral. In the broader categorization as one of the three types of things, all of the elements on the periodic table, and all compounds made from them would be considered mineral. Thus, even the bodies of animals and vegetables are composed of is minerals, albeit with something extra. On a macro level, we think it is relatively easy to distinguish among them. However, when we look more closely, it becomes difficult. A sponge is an animal, so is coral. However, sponges, coral, and even sea anemonies sit in one place, a mobile, and catch and eat food that passes by. And what about the Venus fly trap? It has chlorophyll, it turns toward the sun, and it converts minerals it receives through its roots into energy for its growth. That’s all very plantlike. However, the Venus fly trap also catches and eats insects! The fine distinctions between plant and animal can be very deceptive. In fact, one could say that the distinction was purely artificial. As far as we know, everything is composed of some form of mineral, by the above definition.
Modern taxonomy.
A taxonomy is a method of organizing objects or ideas. For example if we have a bag of cubes and balls, we could organize the stuff in the bag by putting the bags content on the table and segregating it into two groups by shape, a taxonomy of shapes. We could also have a bag of cubes and balls in several different materials such as Styrofoam, leather, wood and plastic. Both the cubes and the balls could come in all of those materials. We can organize them by material of manufacture, a materials taxonomy. It is much the same with living in nonliving things. Scientists over the years have attempted to categorize living in nonliving things.
Historically, we learned in school that there were plants animals and minerals, a taxonomy with three categories. Scientists today have decided that the original three categories were insufficient. The new categories are:
• Minerals -- nonliving things which compose all matter, including living matter. One distinguishing feature between living things and minerals is that all living things that we know of are made up of a structure containing minerals. Living things are characterized by cellular structures. Minerals do not form cellular structures. Also, one mineral does not take into fuel in the form of light and organic compounds in order to produce exact replicas of itself.
• Viruses and prions -- things that reproduce by taking over or forcing the other living things to construct copies of themselves, often at the distructions of the original host. These objects are literally bundles of DNA with few structures. Since they do not conduct any of their own metabolic processes, scientists are reluctant to classify them as living things. However, unlike a chemical process, they do reproduce through other organisms and use the energy and other byproducts of the host organism's metabolic processes.
• Prokaryots -- primitive, living things that do not have a cell nucleus, and exist as simple, one celled creatures, or as an organization of several cells that are not differentiated. This is a very primitive form of life, possibly from which modern life sprang. Their physiology necessarily limits their size.
• Eukaryots -- living things that have a cell nucleus and may exist as single celled or multicelled organisms.
o Protists -- simple organisms that may have only one celled, or may exist as multicellular creatures that have no cellular differentiation.
o Fungi -- a class of living things such as mushrooms which does not photosynthesize. Like plants, fungi cell have a cell wall, however, the cell wall is composed of chitin, a substance used by animals to produce shells, rather than cellulose, which is unique to plants. According to modern taxonomists, structurally, fungi are closer to animals than plants!
o Plans -- multi-celled organisms with both the cellular membrane and a cell wall.
o Animals -- multicellular organisms without a cell wall.
As we have previously observed, it can be very difficult to tell a floral still filled animals such as a sponge from a carnivorous plant such as the Venus fly trap.
Again, we reiterate, originally, things could be divided between living in nonliving. Then they were divided into living plants versus animals, and nonliving. Now, as we can see, living things consist of more than plants and animals, realizing that some previously known entities such as viruses, prions, protests and fungi are clearly members of neither class.
How to tell living from nonliving.
This may seem like a silly question, however it is not so simple. One might say living things grow and nonliving things do not grow. But don't crystals grow? Can I not grow as salt crystals in water? Do we natural world is in other minerals as the crystals used in lasers? Do not stalagtites and stalagmites grow in caves, getting larger as the centuries pass? So growth, as in going from smaller to larger, is obviously not the determining factor.
We could say that all living things are composed of cells. However, what would that do to viruses and prions? They use the functions of living cells to reproduce copies of themselves. Of course, we have the question, are viruses and prions a stage between living in nonliving.
It would appear that all living things make use of the cellular structure. All in nonliving things, in the sense that they would never living, do not. So it could be said that all living things contained in, or make use of, a cellular structure, we produce things that have the same lifecycle, and multiply.
Living things.
How do we tell if something is a plant or an animal? We could say all plants have chlorophyll and conduct photosynthesis. But there's a problem. Sponges are animals. Some sponges have chlorophyll, or structures known as chloroplasts, in their makeup. If we classify sponges as animals, that means that all things containing chlorophyll cannot be plants. I could say plants are immobile, and draw nutrients through their roots and usually also perform photosynthesize. But what about the Western Tumbleweed? It just blows around with the wind! It's not fixed anywhere.
The most reliable definition is one that tells us that both plants and animals have a cell membrane, that which holds in their fluids and other structures that make up the cell. However, only plans have a cell wall, made of cellulose, outside of the cell membrane, and only fungi have a cell wall made of chitin, which allows them to have a rigid form without a skeleton.
So we can have plants that eat meat and animals that use sunlight to create energy. But we can always tell a plant from an animal by its cellular structure.
Continued ...
https://docs.google.com/Doc?docid=0AZ1nweotz-8MZGNiNHB6YzhfMjdjZDV6d2JkYw&hl=en
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