12.2: The Chemistry of Life and the Primitive Sea
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The first making of life’s chemicals seems to have required few ingredients. In that time—the Precambrian Era—it’s thought that the planet’s atmosphere was rich in such gases as methane, ammonia, and carbon dioxide. These gases dissolve in water, allowing them to react with one another.
The reactions that combine these gases require intense energy. In that era, the seas would have been exposed to the energy from such sources as lightning and cosmic rays, which were much more powerful than they are today. So it’s thought that such a combination of water, primitive gases, and intense energy brought forth the first of life’s chemicals.
Decades ago, scientists began to test this hypothesis of life’s origins. They reproduced what was thought to be the ancient chemical soup of life. Into water, they dissolved the simple gases methane (CH4), ammonia (NH3), and carbon dioxide (CO2).
These gases, along with water (H2O), hold the key atomic elements of life—carbon, hydrogen, oxygen, and nitrogen. From the simplest plant to the complex brain, living substances are composed mainly of these four elements.
The scientists then exposed this chemical soup to a steady bombardment of electrical energy. And day by day, new chemicals emerged. First, the solution began to yield such new molecules as acetic acid (vinegar) and formaldehyde. Then, as the bombardment continued, these chemicals became richer in the solution, and the simplest amino acids appeared. Gradually, other more complex amino acids formed. And finally, these amino acids began to link together to make protein—one of life’s most complex structures.
The Dual Role of Water
What part did water play in these formative reactions? First, there was the obvious role of solvent—dissolving. Once dissolved in water, the gases had a greater potential for combination. The components of the gases—such as the carbon and the oxygen of carbon dioxide—were then ready to separate and recombine.
We see many household examples of this common fact of chemistry, that many chemicals must dissolve before they can react. Consider the baking of a cake. None of the chemistry of cake-making occurs as long as the dry and liquid ingredients are kept separate. If this weren’t so, cake mixes wouldn’t keep on the supermarket shelf.
The liquids of a cake recipe—whether milk, applesauce, eggs, or any other fluid substance except oil—are mainly water. When these fluids are combined with the dry flour, baking powder, sugar, etc., the reactions begin. Dough is made, and it goes into action. Gases, mainly carbon dioxide, begin to form, and the dough rises. The relatively inert solids have suddenly become active. They change; they interact.
There’s a message for us here. It’s that the chemicals of life, like the chemical ingredients of a cake, change once they are dissolved. Water makes their reactions possible.
But water is far more than a holding place and meeting ground for the chemicals of life. It’s far more than a transporter of chemicals through the body—whether in blood or cellular fluid. Water itself is an active participant in the chemistry of life.
In many biochemical reactions, a molecule of water is added or subtracted. We see this in the formation of protein, starch, and triglycerides, and in their digestion: A molecule of water is removed when an amino acid is joined to another amino acid to make protein (see Fig.12-1). The same is true when glucose is joined to another glucose to make starch, or when fatty acids are joined to glycerol to form a triglyceride. The reverse happens as well: water molecules are added back in the digestive reactions that release amino acids from proteins, glucose from starch, and fatty acids from triglycerides.
To make the role of water as a participant in life’s chemical reactions even clearer, recall that it’s by combining carbon dioxide and water that plants are able to make carbohydrates and store the energy of the sun. Recall further, that as animal life uses these plant sources of carbohydrates for energy, the metabolic reactions yield carbon dioxide and water.
Clearly, water is central to the chemistry of life.