18 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”It is interesting to note that the most abundant elements in our
body (Table 3.1) are readily found not only on Earth, but also in the
stars and interstellar space, evidence that a chemical continuum exists
between the animate and the inanimate worlds.
3.2 How Chemistry Shapes Life
Molecules in living things are much more complex than those in the
inanimate world. Life on Earth is based on carbons connected in long
chains. Carbon forms relatively stable covalent bonds not only with
another carbon (C-C), but also with hydrogen (C-H), oxygen (C-O),
nitrogen (C-N) and, less frequently, sulfur (C-S). Phosphorus is fre-
quently found in biological molecules, but it is never directly connected
to carbon; it does this through oxygen in the form of phosphoric acid.
Molecules with two or more phosphates connected in tandem (called
pyrophosphates) contain high levels of energy, such as ATP, an abbrevia-
tion for adenosine triphosphate. In fact, ATP is the energy currency that
is used to synthesize almost all the components inside a cell. It makes
possible the incorporation of molecules into the living system. Table 3.2
lists the major types of compounds found in the living system.
What, then, is in the chemistry of life? Let me start with a cell
(Fig. 3.1). First and foremost, each cell is enclosed with a membrane,
called plasma membrane, made up of two layers of phospholipids
stacked back to back, with specialized proteins studded here and there
to control the entry and exit of molecules and ions (charged molecules
Table 3.1. Major Elements in the Human Body
Element Atomic number % Total body weight % Total body atoms
Hydrogen 1 10 63.0
Oxygen 8 65 23.5
Carbon 6 18 9.5
Nitrogen 7 3.0 1.4
Sum 96 97.4