Anatomy & Physiology 02 Chemical Organization


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Human dna

This figure shows a double helix.
Human DNA is described as a double helix that resembles a molecular spiral staircase. In humans the DNA is organized into 46 chromosomes.

Chapter objectives

After studying this chapter, you will be able to:

  • Describe the fundamental composition of matter
  • Identify the three subatomic particles
  • Identify the four most abundant elements in the body
  • Explain the relationship between an atom’s number of electrons and its relative stability
  • Distinguish between ionic bonds, covalent bonds, and hydrogen bonds
  • Explain how energy is invested, stored, and released via chemical reactions, particularly those reactions that are critical to life
  • Explain the importance of the inorganic compounds that contribute to life, such as water, salts, acids, and bases
  • Compare and contrast the four important classes of organic (carbon-based) compounds—proteins, carbohydrates, lipids and nucleic acids—according to their composition and functional importance to human life

The smallest, most fundamental material components of the human body are basic chemical elements. In fact, chemicals called nucleotide bases are the foundation of the genetic code with the instructions on how to build and maintain the human body from conception through old age. There are about three billion of these base pairs in human DNA.

Human chemistry includes organic molecules (carbon-based) and biochemicals (those produced by the body). Human chemistry also includes elements. In fact, life cannot exist without many of the elements that are part of the earth. All of the elements that contribute to chemical reactions, to the transformation of energy, and to electrical activity and muscle contraction—elements that include phosphorus, carbon, sodium, and calcium, to name a few—originated in stars.

These elements, in turn, can form both the inorganic and organic chemical compounds important to life, including, for example, water, glucose, and proteins. This chapter begins by examining elements and how the structures of atoms, the basic units of matter, determine the characteristics of elements by the number of protons, neutrons, and electrons in the atoms. The chapter then builds the framework of life from there.

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Anatomy & Physiology 02 Chemical Organization
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Sample Questions from the Anatomy & Physiology 02 Chemical Organization Flashcards

Question: Magnesium is an important element in the human body, especially in bones. Magnesium's atomic number is 12. Is it stable or reactive? Why? If it were to react with another atom, would it be more likely to accept or to donate one or more electrons?


Magnesium's 12 electrons are distributed as follows: two in the first shell, eight in the second shell, and two in its valence shell. According to the octet rule, magnesium is unstable (reactive) because its valence shell has just two electrons. It is therefore likely to participate in chemical reactions in which it donates two electrons.

Question: The most abundant elements in the foods and beverages you consume are oxygen, carbon, hydrogen, and nitrogen. Why might having these elements in consumables be useful?


These four elements-oxygen, carbon, hydrogen, and nitrogen-together make up more than 95 percent of the mass of the human body, and the body cannot make elements, so it is helpful to have them in consumables.

Question: Oxygen, whose atomic number is eight, has three stable isotopes: 16O, 17O, and 18O. Explain what this means in terms of the number of protons and neutrons.


Oxygen has eight protons. In its most abundant stable form, it has eight neutrons, too, for a mass number of 16. In contrast, 17O has nine neutrons, and 18O has 10 neutrons.

Question: AB + CD -> AD + BE Is this a legitimate example of an exchange reaction? Why or why not?


It is not. An exchange reaction might be AB + CD -> AC + BD or AB + CD -> AD + BC . In all chemical reactions, including exchange reactions, the components of the reactants are identical to the components of the products. A component present among the reactants cannot disappear, nor can a component not present in the reactants suddenly appear in the products.

Question: Could two atoms of oxygen engage in ionic bonding? Why or why not?


Identical atoms have identical electronegativity and cannot form ionic bonds. Oxygen, for example, has six electrons in its valence shell. Neither donating nor accepting the valence shell electrons of the other will result in the oxygen atoms completing their valence shells. Two atoms of the same element always form covalent bonds.

Question: Visit this website ( to view the periodic table. In the periodic table of the elements, elements in a single column have the same number of electrons that can participate in a chemical reaction. These electrons are known as "valence electrons." For example, the elements in the first column all have a single valence electron-an electron that can be "donated" in a chemical reaction with another atom. What is the meaning of a mass number shown in parentheses?


The mass number is the total number of protons and neutrons in the nucleus of an atom.

Question: Visit this website ( to learn about electrical energy and the attraction/repulsion of charges. What happens to the charged electroscope when a conductor is moved between its plastic sheets, and why?


The plastic sheets jump to the nail (the conductor), because the conductor takes on electrons from the electroscope, reducing the repellant force of the two sheets.

Question: When you do a load of laundry, why do you not just drop a bar of soap into the washing machine? In other words, why is laundry detergent sold as a liquid or powder?


Recall that the greater the surface area of the reactants, the more quickly and easily they will interact. It takes energy to separate particles of a substance. Powder and liquid laundry detergents, with relatively more surface area per unit, can quickly dissolve into their reactive components when added to the water.

Question: In a hurry one day, you merely rinse your lunch dishes with water. As you are drying your salad bowl, you notice that it still has an oily film. Why was the water alone not effective in cleaning the bowl?


Water is a polar molecule. It has a region of weakly positive charge and a region of weakly negative charge. These regions are attracted to ions as well as to other polar molecules. Oils are nonpolar, and are repelled by water.

Question: Watch this video ( to observe the formation of a disaccharide. What happens when water encounters a glycosidic bond?


The water hydrolyses, or breaks, the glycosidic bond, forming two monosaccharides.

Question: Explain why CH4 is one of the most common molecules found in nature. Are the bonds between the atoms ionic or covalent?


A carbon atom has four electrons in its valence shell. According to the octet rule, it will readily participate in chemical reactions that result in its valence shell having eight electrons. Hydrogen, with one electron, will complete its valence shell with two. Electron sharing between an atom of carbon and four atoms of hydrogen meets the requirements of all atoms. The bonds are covalent because the electrons are shared: although hydrogen often participates in ionic bonds, carbon does not because it is highly unlikely to donate or accept four electrons.

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This course does NOT provide the education or experience needed for the diagnosing or treating any medical condition, all site contents are provided as general information only and should not be taken as a medical advice.

Source:  OpenStax College. Anatomy & Physiology, OpenStax-CNX Web site., Jun 11, 2014
Subramanian Divya
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