The Cell as the Basic Unit of Life
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Like cement sets the foundation for a house, the cell theory sets the foundation for biology. Schleiden and Schwann proposed the cell theory, which states that the cell is the basic unit of life and that new cells arise only from the reproduction of already existing cells via cell cycle, in 1839.
Since the cell is the structural and functional unit of life as the house is for a community, without the house the community would be irritated and would not be stable, the community would therefore cease to exist. Just as there are requirements for a house to be classified as safe, secure and livable, there are 10 requirements for an organism to be classified as living. These 10 characteristics include:
The ability to move~ either by using motion or locomotion
Ability to respond to short-term environmental changes~ with irritability when detrimental or responsiveness when positive.
Ability to adapt to long-term environmental changes~ by making necessary changes to survive. I.e.: evolution
Ability to reproduce~ asexual vs. sexual
Reaches a point of maturity~ at which point successful reproduction is possible
Able to acquire basic nutrients for energy~ through photosynthesis or absorption following a digestive process.
Able to convert basic nutrients to a more useable form~ through cellular respiration or synthesis
Able to eliminate waste~ the products from the energy transformations
Has a limited life span~ will eventually cease to exist and die.
Is made up of one or more cells~ and the product of those cells (uni-cellular vs. multi-cellular)
The absence of any single characteristic would mean the organism has a non-living status.
The majority of these characteristics aid in the balance of homeostasis and the need for control. As with the second characteristic, there is the need for a reaction to a short-term change in the surrounding environment to aid in maintaining the homeostatic balance. Take for instance the heater breaks in the middle of winter; your body will adjust its internal temperature by mobilizing energy reserves to conserve body heat and to keep you alive, as well as sending out messages to your brain for you to put on extra clothes and involuntarily start to shiver. Without this internal control your organs would freeze up and shut down. In order to be able to respond you must be able to move, which brings in characteristic one: the ability to move, which also is needed in order to acquire basic nutrients, which aids in homeostatic balance. The ability to eliminate the waste products of the energy transformations will also assist in the balancing of homeostasis.
The macromolecules of life include lipids, carbohydrates, proteins and nucleic acids. These are of high importance to the cell as they contribute the needed energy to fuel cellular processes. Not only this, they also serve as structural components of the cell. They must first be ingested in the polymer form and through the digestive process are mechanically and chemically degraded to their monomer form, which are then transported to the necessary cell. Carbohydrates are the most abundant organic molecules synthesized by photosynthesis in autotrophs. They take the form of either a monosaccharide, being their monomer form as glucose, galactose or fructose, oligliosaccharide, or a polysaccharide, which is their polymer form. They function as the initial fuel molecule of catabolic processes, synthetic building block of cells and cell components and as cell surface markers or receptors used in cell-to-cell communication and identification. The lipids function in energy storage and as building blocks for the cell. The phospholipids are the key element composing cellular membranes; this bi-layer is composed with a polar head that is hydrophilic, pointing itself into the inner and outer membrane space, and the non-polar tails that are hydrophobic and point in towards each other. Proteins are highly organized polymers of amino acid monomers coded for by an organisms DNA. Proteins comprise 50% of the dry mass of the cell. The general functions of proteins include structural building blocks like keratin, collagen and fibrin, functional building blocks like enzymes, antibodies