Chapter 3
The Molecules of Cells
Overview
In this chapter we will use the principles of the previous chapter to build molecules. A cell is the basic unit of life, and all
cells are made of compounds and other molecules. The central element in all living creatures is carbon, and carbon
compounds are called organic compounds. There are four classes of these organic compounds that we will examine in
this section:
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Carbohydrates
Fats and Lipids
Proteins
Nucleic acids
For each of these, we will examine how they are made and broken down, their structure, their roles in the cell (and
higher organisms) and finally their relationship to the other biomolecules.
Assigned Reading
Text, Pages 33-47
PowerPoint Presentation
Chapter Review, Page 48
Testing Your Knowledge, Page 49
Key Terms
 Isomer
 Hydrophobic, hydrophilic
 Polymer, monomer
 Dehydration synthesis
 Hydrolysis
 Carbohydrate, monosaccharide, disaccharide, polysaccharide
 Glucose, fructose, galactose
 Lactose, sucrose, maltose
 Glycogen, cellulose, starch, chitin
 Lipid, fatty acid, triglyceride, unsaturated, saturated
 Phospholipid, sterols, cholesterol
 Protein, amino acid, peptide bond, polypeptide
 Primary, secondary, tertiary, quaternary structures
 Pleated sheet, alpha helix
 Nucleic acid, nucleotide
 DNA, RNA, ATP
Introduction
This chapter can be very complex unless you approach it from an organized perspective. In the previous chapter we
examined the structure of an atom and the types of bonds that atoms may form. If you do not have a firm
understanding of these bonds, please review Chapter 2 before proceeding.
What we will be doing in this chapter is to move from single atoms to molecules of biological importance, which we will
call biomolecules. Biomolecules are molecules which are based on carbon. So first we will need to examine the role of
carbon as the backbone of these molecules.
Organic Chemistry
Organic chemistry is the chemistry of living organisms. The term organic means that is possesses carbon-carbon (C-C) or
carbon-hydrogen (C-H) bonds. Carbon is an interesting molecule (with silicon and a few others) in that it prefers to bind
with 4 other atoms. This trait allows carbon to form long chains and bend to form 3-D molecules. Carbon itself is not an
active part of the biomolecule, instead it forms a carbon backbone to which we will attach the working parts of the
molecule.
Think of carbon as being the basic blocks of a Lego set. You really can't build anything interesting using a bunch of
square blocks, but the blocks can give you a shape to which you can attach the working parts of your model (wheels,
cranes, etc.).
The working part of a biomolecule is the functional group. Functional groups give biomolecules their chemical reactivity.
Note that we have advanced one step. In the previous lesson, what gave atoms their chemical reactivity? Table 3.2
gives some examples of functional groups. Don't worry about learning them, but realize that these things are chemically
different, and thus will affect the chemistry of the molecules to which they are attached.
Preview to Chemical Reactions
There are two things to focus on in this area. The first is the different types of chemical reactions. We will discuss each
of these at some point in the course, but I would like to call your attention to two of these. Since the rest of this chapter
will focus on the building and breakdown of biomolecules, we will need to get a few terms down.
Examine the figure below. To build a biomolecule, two smaller molecules are combined and a new chemical bond is
formed between them. In this process, water is released and thus the process is called condensation. However, we may
also use a term called dehydration synthesis since in the process of dehydrating the molecules we have created a new
bond. I will tend to use this term in the course.
If we removed water to form a bond, then we should be able to add water to break the bond. This process by which
water (hydro) is used to break (lyse) a bond is called hydrolysis.
Carbohydrates
Carbohydrates (CHO) are important energy molecules. We will spend a significant amount of time examining the role of
CHOs as energy in the next module, but first we have to understand how they are manufactured and their primary roles.
All CHOs are built from monosaccharides, of which there are three. Notice that we make the bigger CHOs by
dehydration synthesis reactions. (What reaction would break these molecules down?)
The polysaccharides are a complex series of molecules. There are several types of these, and you need to take a look at
the PowerPoint presentation for more detail on each. Here is a quick review:
 Starches: plant energy storage
 Fibers (cellulose): plant structural
 Glycogen: animal energy
Please note that all complex CHO's are made from glucose only.
Fats and Lipids
In today's world fats and lipids often have a very negative meaning, but to biological organisms they are absolutely
essential to life. It is important at this level to distinguish between dietary fat and fat the biomolecule.
Fats are not organized the same as carbohydrates. For CHO, we built larger molecules from smaller ones. With fats,
there are simply different classes and one is not used to build the other. You should focus on the characteristics of each
and the function in a biological organism.
Triglycerides are the primary types of fats, and are large molecules with lots of C-C and C-H bonds (see Figures 3.8B and
3.8C). Fats are characterized by the degree of saturation of these long chains.
Proteins
Proteins are the working molecules of the cell. As will we see throughout this course, all cellular functions are typically
performed by proteins. The instructions for these proteins are contained within the DNA (we will explore this later in
the course). For now we will simply explore the general structure of these molecules.
Proteins are comprised of long chains of amino acids. All amino acids have the same structure, but vary in their
functional groups (R group). Since functional groups give biomolecules their chemical reactivity, each amino acid is
different. We use about 20 different types of amino acids in the manufacture of our proteins, but this may vary with
other organisms.
Pictures of amino acids
Pay special attention to the four levels of organization of proteins. What this section tells us is that the protein is threedimensional, which makes it the ideal working biomolecule.
Nucleic Acids
The fourth class of biomolecules is the nucleic acids. When we discuss nucleic acids we typically think of DNA and
hereditary material, but there are other functions to nucleic acids. We will spend a significant amount of time late in the
course discussing DNA and RNA. For now, examine the roles of nucleic acids and the differences between DNA and RNA.
Concepts
 Understand the special properties of carbon that allow it to bond in so many ways.
 Name and understand the four types of carbon (organic) compounds, and the polymer and monomer units of
each.
 Understand dehydration synthesis and hydrolysis, how they are different, how they occur what the function of
each is, and how water is involved in each.
 Understand the five different types of reaction categories.
 Understand the structure and functions of carbohydrates, lipids, proteins, and nucleic acids.
 Understand the four different types of complex carbohydrates.
 Understand the structure and function of triglycerides, phospholipids, waxes, and sterols.
 Understand how we determine if a fat is saturated or unsaturated, and how this affects the structure.
 Understand how the structures of amino acids are similar and different, and how many there are.
 Understand how the shape of a protein affects its function, and how a change in amino acids can affect the
function.
 Understand the three different types of nucleic acids.
Review Material
MyBiology.com—Study guides and resource for this text. Specifically look at MP3 Tutor and all Web Activities