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The lives of all living things would cease to exist if not for four very important kinds of molecules, called organic molecules or bio-molecules.
Carbohydrates are the first, and least important, kind of bio-molecule. The base compound of a carbohydrate is the monosaccharide. It is important to remember that when dealing with any kind of bio-molecule that breaking an atomic bond releases energy, and creating an atomic bond uses energy.
When many saccharides combine, they form a polysaccharide. This can be a chain of thousands of carbon atoms that takes hours for the body to break. Carbohydrates are the things an organism digests first.
Lipids are the next-highest kind of bio-molecule. They store large amounts of energy due to their double carbon bonds and look like a branched ladder. Lipids are used primarily as stored energy, as it is easier and more cost-effective to use carbohydrates first.
There is perhaps, no more important bio-molecule than the protein. Not only does it effectively repair the body and facilitate transport through cells, it is responsible for breaking things down and building them up. Proteins at their most important come in the form of enzymes that cause chemical reactions to happen faster or slower. It is enzymes (proteins) that are responsible for food digestion. Without enzymes, it would take 1,000 years to successfully digest your lunch.
These bio-molecules form the very backbone of our existence -- DNA and RNA. Nucleic acids in DNA come in the form of four kinds of chemical bases, called nucleotides (adenine, guanine, cytosine, thymine), that pair in the following ways: A-T or C-G. The different combinations of these bases are endless when there are thousands of them on a single strand of DNA. RNA is much the same, except that it has the base uracil instead of thymine. It bonds with adenine.
Cells are "the building block of life," the smallest part of a living thing that can still be considered "alive." In the average human adult, there are an estimated 6 trillion cells of varying shapes and functions. It is in cells that energy production takes place and where wastes are disposed of.
"Organelles" is the term applied to the sub-cellular structures inside the cell that perform the cell's duties. Here is a list of organelles, with a brief description of each:
These small, spherical organelles produce proteins.
The nucleus lies roughly at the center of the cell, and is the storage facility for the DNA. It is referred to as the "control center" of the cell.
The "highway" of the cell, where monosaccharides are transported around the cell.
These terms refer to the organelle that packages nutrients for storage or for shipping to another cell. It is the UPS of the cell.
The powerhouse, processing molecules to make energy (the process of making energy is a very complex one).
Also called a plasma membrane, this is the layer that protects the cell, holds in the cytoplasm, and allows transport (next lesson). Proteins are embedded in the cell membrane, and serve as doormen, allowing only certain molecules in, and helping molecules that need assistance.
The Jell-O like fluid in which resides the organelles. The cytoplasm stretches from the membrane to the nucleus.
Its root word sounds like "Lysol," the cleanser, and that's exactly what this organelle does -- it removes waste products from the cell.
A vacuole is just a large storage container. In plant cells, vacuoles are huge and filled with water.
This structure is found only in plants and adds an extra layer of rigidity to the cell. The bark of trees is nothing more than trillions of cell walls.
This structure is also only found in plant cells. The chloroplast is responsible for photosynthesis, changing sunlight to chlorophyll. It is believed that the chloroplast and mitochondria evolved along somewhat the same lines.
In order for the cell to utilize nutrients, they must first be moved into the cell. Small molecules (monosaccharides) can move across the semi-permeable cell membrane in the process of diffusion. When one of the proteins in the membrane helps a molecule across the membrane, it is called facilitated diffusion. Water moves across the membrane in the process of osmosis. Both these kinds of transport are passive transport systems, meaning that no energy is required to move the molecules across the membrane, since the molecules are moving from a high to low concentration (like going downhill -- no energy needed). Active transport refers to any kind of movement across the cell that requires energy. Energy is needed because the concentration of molecules goes from low to high (like going uphill -- oops, energy is needed).
It is generally accepted that things are living if:
They have the ability to grow and develop.
They have the ability to reproduce.
They have the ability to use energy.
They have cells.
They have the ability to adapt.
In order for things to be considered alive, they must meet all of the above characteristics. It is also understood, though, that as new lifeforms are encountered, they may not meet all of the above criteria. In this case, it is up to the science officer to determine whether or not something is living.