Chapter 3: Carbon Compounds in Cells
I.Organic Chemistry - Once believed to be synthesized by only living things, we now
know some organic compounds can be made by nonliving processes. We now
define organic chemistry as the branch of chemistry dealing with C-H bonds in
which carbon forms chains or rings.
A. Functional Groups When you look at organic compounds, much of the molecule is made of repeating monomers (usually carbon bound with hydrogen and oxygen). At some spots, unique chemical groups occur, and these are called functional groups. Figure 3.2 in the book lists some common functional groups. These are important to understand because they explain why certain molecules are polar or nonpolar and the shape of the proteins.
B. Reactions On page 37, five different reaction types are mentioned. The most important are the condensation reaction (modeled with glucose at lab tables) in which two smaller molecules are combined to form a larger molecule, leaving a water molecule as a byproduct. The second reaction is basically the reverse, known as a cleavage reaction, in which a larger molecule is split into two smaller molecules. A cleavage molecule requires a water molecule to be added.
II. Carbohydrates: The term comes from the fact that most (not all) carbohydrates have a
ratio of 1 carbon atom to 2 water molecules (e.g. C6H1206, glucose).
A. The monomers are monosaccharids, disaccharides and oligosaccharids, often known as sugars. The monomers include glucose (the main energy source for many organisms), fructose (common sugar in fruits), and galactose. The disaccharides (two monosaccharides linked together) include the following:
Glucose + Fructose = Sucrose (common table sugar)
Glucose + Galactose = Lactose (milk sugar)
Glucose + Glucose = Maltose
When two monosaccharides unite, water is released and the two form a covalent bond. This is called a condensation reaction (joining of two smaller units, usually accompanied by the release of water, thus condensation).
B. The polymers of the carbohydrates are polysaccharides. These include starches, cellulose, fiber and glycogen. Because of their large size, many polysaccharides are insoluble in water even though they have many OH groups.
The most common polysaccharides are:
Starch- Many glucose molecules covalently bonded together. Principal storage molecule for plants.
Glycogen Similar to starch in that it is many glucose molecules covalently bonded together, but the molecule has different side chains. This molecule is a principle energy storage molecule for animals.
Cellulose A long strand of glucose molecules, bound together in such a way that most animals are unable to digest this molecule. It is a structural molecule and gives plants their strength. Can be digested by cows due to the presence of bacteria in their stomachs.
Chitin Similar to cellulose, but nitrogen is added to each glucose unit. This is the primary molecule in the exoskeleton of insects and cell wall of fungi.
III.Lipids: This class of organic compounds includes oils, fats, waxes and sterols. The
basic building blocks of lipids are glycerols and fatty acids.
A. Fats: Consist of a glycerol linked with one, two or three fatty acid chains. Fatty acids are chains of carbon linked almost exclusively with hydrogen. These hydrocarbons are nonpolar and a great deal of energy is stored in carbon-hydrogen bonds. This accounts for the high-energy content of fats. A saturated fat contains only single bonds; an unsaturated fat contains at least one double bond between two carbons. A triglyceride is a glycerol combined with three fatty acids and is a common form of animal fat. Triglyceride levels are commonly measured in humans in health screenings.
B. Phospholipids: If a phosphate group is attached to the head of a lipid, it is called a phospholipid. These are important molecules because the phosphate end is hydrophilic, meaning it can connect with polar solvents such as water.
C. Sterols: A combination of four hydrocarbon rings makes a sterol molecule. Examples include cholesterol (necessary, but often overdone) and sex hormones.
IV. Proteins: There are 20 amino acids that serve as the building blocks (monomers) for proteins. Proteins serve a variety of functions, including enzymes (proteins that speed up reactions to make them possible), structural functions in organisms, transmembrane transport molecules, and muscles.
A. Amino Acid chains When two amino acids are joined together, they form a dipeptide. This is so named because the bond that links the amino group and the carboxyl group is a peptide bond. Many AAs together make a polypeptide chain, or a full protein. A chain of amino acids will have unique properties, depending upon the arrangement of amino acids in the chain. The order of amino acids (carbon/hydrogen bonds) will determine the proteins primary structure. The shape induced by the arrangement of amino acids is termed the primary structure.
B. Protein Structure If two amino acids interact but are not directly linked together with a peptide bond, this is known as secondary structure.
Usually, secondary structure is characterized by proteins that have the
shape of folded sheets or a coiled helix. Tertiary folding is characterized
by bending of the protein due to interaction of the functional groups.
Quaternary folding involves two or more peptide chains coming
together, such as the four chains that make up the globular protein,
hemoglobin (rounded in shape). Fibrous proteins have a more enlongated
orientation, such as keratin.
C. Denatured Proteins Because the function of a protein is dependent upon its shape and hydrogen bonds (and other weak bonds) play a major role in forming the shape, proteins are easily denatured. A denatured protein has lost its shape and therefore it functional role due to change in temperature or pH. You book gives the example of an egg white (rich in the protein albumin) and the results of cooking.
VI. ATP and Nucleic Acids We will cover this material in great detail in later chapters. Just know that ATP is the main energy store in living cells and nucleic acids are built around a sugar with a 5- or 6- carbon ring.