Unit 3 Study Guide: Biological Molecules

3a. List the characteristics of water that make it important to life as we know it.

Water is so indispensable for life that our primary method for searching for life outside of earth is to search for evidence of water.

  • What is special about water?
  • What is the electrical charge distribution on a water molecule?
  • Why does the polarity of water make it well suited to its functions in biology?

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:

  • Water molecules can form hydrogen bonds with other polar molecules, including other water molecules.
  • Water is less dense in the frozen state than in the liquid state. Because of this, bodies of water freeze from the top down, and thaw during seasonal warming.
  • Water has a high specific heat capacity, requiring more energy than most substances to change its temperature. This stabilizes the temperature of bodies of water more than land masses.
  • Water has a high cohesion, allowing for capillary attraction that can lift water through vessels to the tops of the tallest trees.
  • Water is an excellent solvent, and the chemistry of life is mostly aqueous solution chemistry.
  • Water has high surface tension, allowing small organisms to walk on the surface of water.
  • Water has a high latent heat of vaporization, requiring much energy to change its state from liquid to gas. This allows for effective evaporative cooling by sweating.
  • Water exists in all three states (solid, liquid, and gas) within a comparative narrow range of temperatures that organisms can tolerate.

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.


3b. Explain the characteristics that are necessary to make something alive.

There are three types of objects in the universe:

  • Living things
  • Dead things
  • Non-living things

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.

  • What makes a dead thing fundamentally different from a living thing?

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.


3c. Recognize the structure of the four major biological macromolecules.

  • What are the four classes of biological macromolecules?
  • What are the structural difference between the different classes of biological macromolecules?

All organisms have four major classes of large biological molecules, or macromolecules:

  • Lipids comprise a diverse set of hydrocarbon molecules (containing hydrogen and carbon). This makes them largely partially non-polar, because the covalent bonds in hydrocarbons (between two carbon atoms or between a carbon atom and a hydrogen atom) feature equal sharing of electrons.
  • Polysaccharides are complex carbohydrates made up of carbon, hydrogen, and oxygen in a 1:2:1 ratio, giving them an empirical formula generalized as (CH2O)n.
  • Proteins are enormously diverse in structure and function, yet they all feature the substructure of amino acids. Each amino acid features a central carbon atom simultaneously connected to a hydrogen atom, an amino group, a carboxyl group, and a variable R group.
  • Nucleic acids are informational molecules with a basic structure in which each subunit includes a five-carbon sugar (either ribose or deoxyribose) attached to a phosphate group and a nitrogenous base.

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.


3d. Describe the functions of the four major biological macromolecules.

  • What are the major functions of the four classes of biological macromolecules?

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.

As you review the major functions of lipids, polysaccharides, proteins, and nucleic acids, try to make sense of why each macromolecule’s basic structure is well suited to its particular function.


3e. Indicate the monomers and polymers of carbohydrates, proteins, and nucleic acids.

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.

  • What are the monomers that make up polysaccharides?
  • What are the monomers that make up proteins?
  • What are the monomers that make up nucleic acids?

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.


3f. Describe the four levels of protein structure.

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:

  • Primary structure refers the particular sequence of amino acids (both the number and the order) making up a single polypeptide. A polypeptide is one continuous strand made up of some number of amino acids connected end-to-end in a particular order. If a protein consists of just one polypeptide, then that "polypeptide" is itself a "protein".
  • Secondary structure refers to repeating patterns found within a polypeptide. The repeating patterns include alpha helices (a singular helix) and beta pleated sheets.
  • Tertiary structure refers to the particular three-dimensional structure of a single polypeptide. In other words, the particular shape that a single polypeptide assumes when it bends and folds is called its tertiary structure.

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.

  • Quaternary structure refers to the particular way in which multiple individual polypeptides (each with their own primary, secondary, and tertiary structures) come together to form the overall shape of a multimeric protein. In this case, "polypeptide" and "protein" are not interchangeable terms. The polypeptides are just parts of the overall protein.

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.


Unit 3 Vocabulary

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.

  • Amino Acid
  • Amino Group
  • Carbohydrate
  • Carboxyl Group
  • Catalyst
  • Cohesion
  • Deoxyribose
  • Dna
  • Enzyme
  • Hydrogen Bond
  • Latent Heat Of Vaporization
  • Lipid
  • Metabolism
  • Monomer
  • Monosaccharide
  • Multimeric
  • Nitrogenous Base
  • Non-Polar
  • Nucleic Acid
  • Nucleotide
  • Phospholipid
  • Polar
  • Polymer
  • Polynucleotide
  • Polypeptide
  • Polysaccharide
  • Primary Structure
  • Protein
  • Quaternary Structure
  • Ribose
  • Rna
  • Secondary Structure
  • Solvent
  • Specific Heat Capacity
  • Surface Tension
  • Tertiary Structure
Last modified: Wednesday, July 17, 2019, 5:48 PM