A chemical context for introductory biology

How much chemistry should we teach in an introductory biology course for majors? Modern biology is integrative, and relies on understanding of chemistry, physics, geology, and other natural sciences. Therefore, all biology curricula require students to take multiple chemistry courses, up to two semesters of organic chemistry, as well as math and physics. The catch is that most biology students will take organic chemistry as sophomores or juniors, and begin their introductory biology sequence as freshmen with only high school chemistry.

I’ve recently reviewed chapters and content dealing with chemistry for both a leading traditional textbook and a new ebook, both for biology majors. I have reviewed hundreds of syllabi for introductory biology courses from all kinds of colleges and universities worldwide, in my role as gatekeeper of transfer credits for biology courses. Both the textbooks and the vast majority of introductory courses begin with chemistry basics, from atomic structure to chemical bonds, the properties of water, the chemistry of carbon, simple organic molecules, and then lipids and biological polymers. I think this approach is tedious to both students and instructors, and fails to integrate chemistry with biology.

Origin of life to introduce elements, atoms and chemical bonds

How did life on Earth get started? What experimental approaches are available to investigate this question? We can start with the following observations:

Q: Are living organisms made of the most abundant elements available in the environment?

Compare elemental (atomic) composition of living organisms: C, H, O, N, P, S, + trace minerals (Ca, Mg, Zn, Mn, Fe) with elemental composition of earth’s crust.

Q: What is special about the most abundant elements of living organisms (elements that comprise organic molecules)?

Elements react with each other to form molecules, held together by chemical bonds. The kinds of reactions and bonds that an element will form is determined by its valence electrons. The Periodic Table organizes elements by their valence electron configurations. All chemical bonds, and all chemical reactions, involve sharing or movement of valence electrons.

Metal atoms, such as those in group 1 and group 2, readily give up their electrons to atoms in group 17. The resulting ions are held together by strong electrostatic attraction, an ionic bond.

Different atoms have different avidities for electrons (electronegativity). Oxygen is one of the most electronegative elements. Carbon and hydrogen have much lower electronegativity. When carbon atoms and hydrogen atoms form chemical bonds with each other, they share their electrons almost equally, resulting in non-polar covalent bonds. When carbon or hydrogen atoms form bonds with oxygen, the oxygen nucleus hogs the shared electrons, such that the oxygen atom acquires a partial negative charge, and the hydrogen or carbon atom has a partial positive charge. Such unequal sharing of electron pairs results in polar covalent bonds.

Organic molecules consist of carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. The key element of life is carbon – life on earth is carbon-based. But not all molecules that contain carbon, or carbon, hydrogen and oxygen, are organic molecules. For example, carbon dioxide (CO2) and carbonic acid (H2CO3) are inorganic molecules. The distinction is that organic carbon is reduced.

I put together a few short screen capture videos to explain these concepts, and posted them on YouTube. These are revisions of my very first lecture video, which I created for my “flipped” class this semester. The original video was too long (28 min) and suffered from a bad lag between video and audio. The new videos:

Do you think this is enough for intro biology students, or am I missing key concepts? Some concepts, like redox, will be explained more fully when I get into energy metabolism, and are well explained already in a Khan academy video.

 

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About jchoigt

I'm an Associate Professor in the School of Biology at Georgia Tech, and Faculty Coordinator of the Professional MS Bioinformatics degree program.
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One Response to A chemical context for introductory biology

  1. Pingback: A chemical context for introductory biology | Jung's Biology Blog - University Lebanon | University Lebanon

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