Water is so indispensable for life that our primary method for searching for life outside of earth is to search for evidence of water.
A water molecule is made up of one oxygen atom that is simultaneously bonded to two hydrogen atoms. The covalent bonds between the oxygen and each hydrogen are polar, because the sharing of electrons between oxygen and hydrogen is not equal. Because of this unequal sharing, the oxygen atom is partially negatively charged, and each hydrogen atom is partially positively charged, making the overall water molecule polar. This gives water several special characteristics:
Because of these special characteristics, it is no surprise that life evolved in the water of the ocean, and that every living cell can be viewed as a tiny bag of water and biological molecules. Keep these special properties in mind as you study biology, and be sure to review polarity and how it underlies these properties.
There are three types of objects in the universe:
It might seem as though "dead" and "non-living" are interchangeable, but they are not. A non-living thing has never been alive, whereas a dead thing used to be alive.
In Unit 1 you surveyed the various characteristics of living organisms. Those characteristics (growth, reproduction, evolutionary adaptation, etc.) are fundamental differences between living things and non-living things. The fundamental difference between a living thing and a dead thing is metabolism. Metabolism is the chemistry of life. It includes all of the chemical reactions occurring in all of the cells that make up an individual organism. An organism is living only for as long as it has a metabolism. When the chemical reactions of metabolism cease, then life ceases. The once-living organism becomes a dead organism. It is the same collection of materials, but the chemistry has stopped. Life is chemistry; life is metabolism.
All organisms have four major classes of large biological molecules, or macromolecules:
Knowing the chemical structure underlying these essential biomolecules not only allows you to recognize them. It allows you to understand how they are constructed within cells and how they chemically interact with each other in metabolism and to give rise to structural component of organisms. Be sure to review the sections of this chapter to test your understanding of the four classes.
Lipids are important for storing energy, for thermal insulation, and for providing protective padding. Phospholipids form the infrastructure of all cell membranes. Lipids also make up natural waxes and oils and many hormones.
Some polysaccharides (like cellulose and chitin) are important for their structural strength, whereas other polysaccharides (like starch and glycogen) are important for storing energy. Polysaccharides also serve as important identity markers on the surfaces of cells, so they play a role in immunity.
Proteins perform an impressively long list of biological functions. They function as enzymes, structural elements, chemical signals, transporters, and receptors. They also play important roles in cell-to-cell adhesion and immunity.
Nucleic acids include various DNA and RNA molecules. They serve informational purposes. DNA stores the genetic code, and various types of RNA help in the process of interpreting that code to build proteins. Certain RNA’s can also function as catalysts.
A polymer is a particular category of macromolecule that is built by connecting together many ("poly-" means "many") smaller subunits, called monomers. Of the four biological macromolecules that you have been studying, only three are polymers. Lipids are not polymers, but the others are.
Polysaccharides are macromolecular carbohydrates. Be careful with the words "polysaccharide" and "carbohydrate". Sometimes these words are used interchangeably, but they shouldn't be, as there is a distinction between them. Carbohydrates include both small molecules and large molecules (macromolecules). Put another way, all polysaccharides are carbohydrates, but not all carbohydrates are polysaccharides. As the name implies, polysaccharides are polymers made up of multiple monosaccharides. Monosaccharides are the monomers, and they can be connected in either a linear or branched arrangement.
Proteins are polymers that are made up of monomers called amino acids. Unlike polysaccharides, which may be branched, a protein must be a linear (or end-to-end) arrangement of amino acids. Organisms use twenty different kinds of amino acids (in an unlimited number of combinations and orders) to construct their proteins.
A nucleic acid can also be called a polynucleotide. That alternative name indicates that it is a polymer made up of many nucleotides. In the case of DNA, the monomers are nucleotides containing the pentose (five-carbon sugar) called deoxyribose. For RNA, the nucleotides contain ribose instead of deoxyribose. Although there are only four commonly used DNA nucleotides (and four commonly used RNA nucleotides), a typical DNA molecule contains millions of nucleotides, so there is an unlimited number of sequences of such nucleotides.
Be sure you can match each type of monomer to the type of polymer that can be made from such monomers. In addition to reviewing the various monomers, refresh your memory about how polymers are constructed using dehydration reactions and deconstructed using hydrolysis reactions.
As a polymer, a protein is a large and complex molecule, and proteins have the most complex and most variable shapes among the three classes of biological polymers. Any given protein has its particular function because of its particular shape (also called its conformation). This is why proteins have such diverse functions. Protein structure can be described at up to four different levels:
Every protein includes at least one polypeptide, so every protein features primary, secondary, and tertiary structure. Only some proteins (called multimeric proteins) are made up of two or more polypeptides. For multimeric proteins, there is a fourth level of structure in addition to the other three levels.
As you review these different levels of protein structure (look for the Protein Structure subsection), keep in mind that a protein cannot function properly unless it has the correct shape, regardless of its job in the cell.
This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.
Try to think of the reason why each term is included.