Skip to content

Choose a Chapter below or view the Sitemap

Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 4.  These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What do scientists mean when they refer to “blueberries” on the surface of Mars?
  2. What distinguishes an element’s atomic number from its atomic mass number?
  3. What type of chemical bond results from the time-sharing of electrons between two atoms?
  4. What are the differences between a solution, a solute, and a solvent?
  5. Why are non-polar molecules considered to be hydrophobic?
  6. What biological role do buffers play in regulating the pH of a solution?
  7. Aside from a type of food you might purchase at your local grocery store, what does the term organic mean?
  8. Walking on water is no small task: however, the water-strider does it with ease.  How does this small insect accomplish this feat?
  9. Consumption of too many saturated fats can result in a variety of diseases.  What is the primary difference between saturated and unsaturated fats?

A Guide to the Reading

When exploring the content in Chapter 4 for the first time, the following concepts typically give students the most difficulty.  For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas. 


Each element is characterized by its atomic number, which is the number of protons found in the atom’s nucleus.  Recall that the nucleus also contains a certain number of neutrons as well.  The number of neutrons usually equals the number of protons for a particular element, although the number of neutrons may vary.  An isotope is an element with the same number of protons, but a different number of neutrons.  The example described in the chapter is carbon, which can exist as C-12 (6 protons + 6 neutrons) or C-14 (6 protons + 8 neutrons).  Isotopes such as C-14 may in fact be unstable as a result of having more than the normal number of neutrons.  Unstable isotopes tend to give off energy (in the form of radiation) through the process of radioactive decay.  As a result, these unstable isotopes are often referred to as radioisotopes.  High energy radioisotopes can cause damage to DNA and result in cancer.  Lower energy radioisotopes can be useful for medical diagnostics as they can be used to trace molecules as they proceed through a biological process, serving as a chemical “tracer”.

For more information on this concept, be sure to focus on:

  • In Section 4.1, Some elements can exist as isotopes with different atomic mass numbers
  • Figure 4.2, Isotopes in Medicine

Nonpolar molecules

As discussed in the chapter, molecules can be held together through the process of sharing electrons.  When electrons are not shared equally between bonded atoms, this results in the uneven distribution of electrical charge in the resulting molecule – one half of the molecule will have a slight positive charge; the other half will carry a slight negative charge.  Such molecules are called polar.  A good example of a polar molecule would be water.  Polar molecules tend to be attracted to one another through these partial electrical charges.  The electrical interaction which holds these molecules together are called hydrogen bonds.  Polar molecules as a class will be soluble in water since they can interact with the partial electrical charges of the water molecules.  Molecules that have an affinity for interacting with water like this are called hydrophilic, or “water-loving”.  In contrast, molecules that are held together by the equal sharing of electrons tend to have no partial electrical charge and are called nonpolar.  With no charges available to interact with partially charged water molecules, nonpolar molecules tend to be insoluble in water.  A good example of a nonpolar molecule would be a fat molecule, or lipid.  These molecules are called hydrophobic or “water-fearing”.

For more information on this concept, be sure to focus on:

  • In Section 4.3, Hydrogen bonds are important temporary bonds
  • Figure 4.4, Hydrogen Bonds Determine the Properties of Water and How Other Compounds Interact with Water

pH Scale

The acidity of a solution is a measure of the solution’s concentration of free hydrogen ions.  The higher the concentration of free hydrogen ions in a solution the more acidic that solution will be.  The lower the concentration of free hydrogen ions, the less acidic (and more basic) the solution will be.  The pH scale was developed to describe how acidic a solution is by measuring the concentration of free hydrogen ions in a solution.  The scale ranges from 0-14, with a lower pH value indicating a higher concentration of hydrogen ions in solution and a more acidic solution.  At the other end of the scale, higher pH values indicate a lower concentration of hydrogen ions and a more basic solution.  Most biological systems maintain a pH around 7, which is considered neutral.  Maintaining proper pH is vital to the survival of biological systems.  To assist in this process, special compounds called buffers are utilized which help to absorb excess hydrogen ions when a solution becomes too acidic or donate hydrogen ions when a solution becomes too basic.  The key to understanding the relationship of pH and acidity is realizing that the pH scale is actually inverted in terms of hydrogen ion concentration (i.e. low pH = high acidity; high pH = low acidity).

For more information on this concept, be sure to focus on:

  • In Section 4.4, Acids and bases affect the pH of water,
  • In Section 4.4, Buffers prevent large changes in pH
  • Figure 4.5, The pH Scale

Saturated vs. Unsaturated Fats

Fatty acids are molecules which typically consist of long chains of carbon and hydrogen atoms.  These long chains can take one of two forms.  When each carbon atom in a chain is attached to carbon atoms on either side and two hydrogen atoms, the chain is said to be “saturated” or “full” of hydrogen atoms.  Saturated fatty acid chains tend to be straight, which enables them to pack together tightly.  Fat molecules consisting of mostly saturated fatty acids tend to be solid at room temperature.  In contrast, fatty acid chains in which two or more of the carbon atoms are held together with double covalent bonds will have fewer than the maximum number of two hydrogen atoms per carbon.  As a result, unsaturated fatty acid chains will have small bends, or “kinks” in their chain.  These kinks tend to prohibit the molecules from packing together tightly.  Fats consisting of mostly unsaturated fatty acids will tend to be liquid at room temperature.  The key to understanding the difference between saturated and unsaturated fatty acids is understanding that saturated fats have carbon chains that are full or “saturated” with hydrogen atoms while the carbon chains of unsaturated fats will have fewer than the maximum.

For more information on this concept, be sure to focus on:

  • In Section 4.5, Fatty acids store energy and form membranes
  • Figure 4.14, Saturated and Unsaturated Fatty Acids

Tying it all together

Several concepts presented in this chapter may be revisited and discussed in greater detail in subsequent chapters, including:

Biological Membranes

  • Chapter 5 – Section 5.2, The Plasma Membrane: Separating Cells from the Environment


  • Chapter 7 – Section 7.1, The Role of Energy in Living Systems

Macromolecules and Nutrition

  • Chapter 7 – Section 7.2, Using Energy from the Controlled Burning of Food
  • Chapter 21 – Nutrition and Digestion

Chemical Reactions

  • Chapter 7 – in Section 7.2, Chemical reactions are governed by simple energy laws
  • Chapter 7 – Section 7.3, How Cells Speed Up Chemical Reactions

Radiation in Science

  • Chapter 12 – in Section 12.1, The genetic instructions of viruses are contained in its DNA


  • Chapter 12 –Section 12.2, The Three-Dimensional Structure of DNA

Building Biological Molecules

  • Chapter 12 – Section 12.3, How DNA Is Replicated
  • Chapter 13 – Section 13.5, Translation: Information Flow from mRNA to Protein

Chapter Menu

Other Resources

Norton Gradebook

Instructors now have an easy way to collect students’ online quizzes with the Norton Gradebook without flooding their inboxes with e-mails.

Students can track their online quiz scores by setting up their own Student Gradebook.