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18.3 Molecules of Life

The nature of life on Earth is amazingly complex and diverse. Yet, all life on Earth is composed of the same types of molecules. For instance, all living organisms utilize the same information containing molecules, DNA, to pass on genetic information from generation to generation. So the question of what is the nature of life on Earth can just as easily be posed as, what is the nature of life's molecules on Earth? 

We can think of life as being comprised of four main types of bimolecular: carbohydrates, lipids, proteins, and nucleic acids. You may recognize two of these molecules as being listed on the back label of common, everyday grocery store products. This makes sense; after all, you are what you eat! Although we are careful about the amount of carbohydrates and proteins that we eat, we will begin our look at the bimolecular of life with nucleic acids. 

The two nucleic acids with which we are most familiar are ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). However, many people get confused about what these molecules actually are and how they function. In our cells, the genetic information that defines who we are is contained in chromosomes. Chromosomes are just tightly wound molecules consisting of DNA. Therefore, all the information that dictates how living organisms function is encoded within molecules of DNA. A single molecule of DNA is comprised of several nucleic acids. Each nucleic~acid is a building block of a larger molecule and can be referred to as a monomer. Several monomers linked together make a polymer. Each of these monomers contains a sugar ring, a phosphate unit, and a nucleotide base. The monomers differ depending on which of four different nucleotide bases they contain: adenine (A), guanine (G), cytosine (C), or thymine (T). These monomers are bonded together to make long linear molecules. The sequence in which the monomers are bonded is what contains our genetic information. Most often in living organisms, two strands of DNA are bonded together to make a double helix. RNA molecules are made in much the same way as DNA molecules. The main difference is that RNA makes use of a different sugar ring and instead of using T as one of its nucleotide bases; RNA uses uracil (U). 

DNA plays a vital role in living cells. It contains instructions for the assemblage of proteins, which in turn organize the synthesis and breakdown of other molecules that form parts of the cell, and therefore living organisms. DNA and RNA work together to string together a different set of monomers that comprise proteins. These monomers are called amino acids. On Earth, there are 20 amino acids that are used to produce proteins. The order in which these amino acids are bonded together into a linear molecule is contained in the molecules of DNA. In the DNA molecule, a sequence of 3 nucleotides in a row specifies a particular amino acid. As one "reads" down a strand of DNA, each set of 3 nucleotides would encode a single amino acid. These individual amino acids are bonded together to make a linear molecule known as a peptide. Peptides do not stay linear for long. Within the aquatic environment of the cell, peptides fold up into three-dimensional structures. A protein can be made of a single peptide or several peptides. The key to protein function is their three dimensional structure. Proteins perform many pivotal functions in the cell. They can span cell membranes and control the import and export of molecules into and out of cells, they can catalyze the reactions necessary for life, and they can serve as important structural elements within the cell. 

Just as nucleic acids and proteins are important to life, so are carbohydrates and lipids. Because of the roles they play in all forms of life, carbohydrates make up most of the organic matter on Earth. For instance, carbohydrates are important as fuel for living organisms, but also serve as a mechanism to store energy. They are also important constituents of larger molecules; sugar rings are an important element of nucleic acids like DNA. Finally, carbohydrates also function as structural elements within the cell, most commonly for cell walls. Just as proteins are large molecules made up of smaller molecules (amino acids), carbohydrates themselves can be large molecules as well. However, carbohydrates are made of monosaccharides, small monomers that are molecules comprised of three to nine carbon atoms. An example of a large carbohydrate molecule is starch. Starch is made up of several monosaccharides (glucose) linked together and serves as a way for plant cells to store energy. When we eat starch, we break the bonds linking the individual glucose molecules together and use this sugar as fuel. Another example is cellulose. This is one of the most abundant organic compounds in the entire biosphere! Cellulose is also made up of glucose molecules linked together. However, many organisms cannot digest cellulose without the assistance of special proteins. Consequently, cellulose is used as a structural element to provide the rigidity needed for cellular components like cell walls. 

Besides cell walls, cellular membranes provide a mechanism to separate what's inside a cell from what's outside. Lipids are a key component, along with proteins, of the cellular membranes that define cells. Without cellular membranes, there would be no way to keep molecules created in a cell from leaking out and to prevent unwanted molecules from getting in. In eukaryotic cells, lipids also form the internal membranes that define cellular organelles such as the mitochondria or chloroplasts. Lipids are unusual molecules in that they have to components with different characteristics. Part of a lipid molecule is hydrophilic, which means it is attracted to molecules such as water. The other part is hydrophobic, which means it is repelled by water. This part of the lipid is made up of fatty acids which have long chains of carbon atoms. A very famous lipid is cholesterol. We mostly hear about cholesterol as a bad molecule, one that clogs our arteries and shortens the lives of humans. However, cholesterol is an important molecule for life. Cholesterol is one of three main types of lipids often found in cellular membranes. Without it, many membranes would lose their fluidity, which would be detrimental to living cells. 

When looking at the amazing diversity of life on our planet, it may be difficult to believe that it could arise from just four types of bio molecules. What is even more amazing is that the similarities between living organisms on Earth are what may enable us to recognize the similarities of life beyond Earth. A central question for astro biology is whether or not the chemical "toolkit" and architecture of life elsewhere is the same as it is on Earth?