The Building Blocks of Life
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Life on Earth is organically based (i.e. Earth life is predominately made up of carbon). Various molecules are synthesized in cells for structural, functional, energetic, or informational purposes. These include polysaccharides (sugars), lipids (fats and oils), polypeptides (proteins), and polynucleotides (DNA and RNA). What you should at least know is the following. Polymers are long chains of some repeating unit. For example, polysaccharides are made up of repeating units of simpler sugar molecules -- monomers such as glucose, fructose, or galactose. Similarly, amino acids are monomers that make up proteins, and nucleic acids make up DNA and RNA. Polysaccharides (sugars) can be used for either structural purposes or for energy storage. For example, cellulose is a structural polysaccharide (composed of glucose monomers) which makes up plant cell walls, whereas glycogen in animal cells and starch in plant cells are used to store energy. Lipids can form either micelles or lipid bilayers, depending on the type of lipid it is (e.g. if the lipids are triangular shaped, they form micelles; if they are more cylindrical shaped, they form bilayers). A cell membrane is usually made of a lipid bilayer that also has proteins and sugars embedded in it, and it has been described by the "fluid-mosaic" model (ref. 1). That is, the cell membrane is made up of several different molecules (a mosaic of different molecules) and it is "fluid", because the molecules that make it up can move around within the membrane. Not only are lipids used in membranes (i.e. a structural purpose), they are also used as energy sources (i.e. fats), hormones (e.g. steroids) and vitamins. In another section, I will provide diagrams of lipids and show how they can form membranes. The genetic code (which has the function of storage and transfer of information) is composed of nucleic acids, however, nucleic acids can also be used for an energy source (e.g. ATP and GTP), for catalysis (e.g. ribozymes), as co-enzymes (e.g. CoA), and for signal transduction (e.g. via transfer of phosphate groups). DNA and/or RNA make up the genetic code. DNA and RNA are similar in some ways, and different in others. DNA uses the sugar deoxyribose and it has the base thymine, whereas RNA uses ribose and uracil. Cells typically have a DNA based genome which is transcribed into RNA, while viruses have either a DNA or RNA genome (but not both -- however, some viruses are retroviruses and can convert their RNA into DNA and back to RNA once they infect a cell). The genetic code is made of just four letters: adenine (A), guanine (G), cytosine (C), and either thymine (T) in DNA or uracil (U) in RNA. These "letters" are called bases and each base is attached to a sugar molecule (i.e. ribose in RNA and deoxyribose in DNA). The nucleotides can form chains by making phosphodiester bonds. A phosphate group is present on the 3'-carbon of the sugar, and this attaches to the 5'-carbon atom in the sugar component on an adjacent nucleotide. In effect a chain is made up of alternating phosphate groups and sugar groups. The bases pair up by forming hydrogen bonds in specific ways due to molecular geometry, and this is referred to as "complementarity". In particular, Adenine pairs with Thymine in DNA or with Uracil in RNA and they form two Hydrogen bonds. Guanine pairs with Cytosine and they form three Hydrogen bonds. These are called Watson-Crick base pairs (after Watson and Crick who elucidated the structure of the DNA helix in 1953): A binds with T (or U) and G pairs with C. However, guanine can also pair up with adenine, uracil, or with another guanine, and these are non-watson-crick base pairs. The DNA double helix is made up of two DNA strands that are anti-parallel (i.e. they run in opposite directions), with the bases on one strand complementary to the bases on the opposite strand -- complementarity keeps the two strands together. Most RNA molecules are single stranded, but can form localized double strands due to base complementarity (e.g. as in tRNA and rRNA). Proteins can be structural or functional. Structural proteins are those proteins that do not catalyze any reaction -- rather, they serve the purpose of maintaining cellular shape (i.e. they have a structural role in the cell). Functional proteins are called enzymes. Enzymes play an important role in metabolic processes, as they catalyse reactions. A catalyst is any atom or molecule that increases the rate of a reaction by lowering the initial minimal energy requirements (i.e. it lowers the "activation energy" -- the amount of energy required to initiate the reaction). Catalysts are reused once the reaction is done: once the reaction is completed, the catalyst returns to its original state it was in before the reaction and it can be reused in another reaction. Catalysts can be used in chemical processes to increase the rate of reaction, but in non-biological reactions (those reactions that are not done by enzymes), there can be several by-products from any particular reaction (if you take an "Organic Chemistry" course, you will learn all about this). Enzymes are like catalysts: they speed up the rate of the reaction, but they are much more specific and more efficient than inorganic catalysts. Enzymes catalyse specific steps in bio-chemical pathways. Some enzymes may been used in more than one type of reaction. For example, in plants, Rubisco (or ribulose-1,5-bisphosphate carboxylase oxygenase) is involved in CO2 fixation (i.e. it grabs a CO2 gas molecule and puts it onto another molecule called RuBP or Ribulose-1,5-bisphosphate). Rubisco can also oxygenate RuBP by grabbing an O2 gas molecule and placing it on RuBP. CO2 and O2 compete with one another in the plant cell for Rubisco, which determines the rate of reaction of CO2 fixation. In summary: sugars are used as energy sources or as structural molecules; lipids are used in membranes or as hormones or vitamins; proteins are used for structural or functional purposes; and nucleic acids are used for signal transduction, catalysis, or for information storage. References:
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