Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

Self and the Beginning of Life 19

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

Table 3.2. Types of Compounds in Living Matter

1. Amino acids: Building blocks of proteins; the latter comprises structural proteins,
   membrane components, enzymes, hormones, signal transducers, transcription
   factors, growth factors, antibodies, etc. Proteins are built up of 20 different kinds
   of amino acids in various combinations and permutations, all arranged in a linear
   sequence.


2. Nucleobases: Two types of purines (adenine, guanine) and three types of pyrimidines
   (uracil, thymine, cytosine); as components of DNA and RNA, as coenzymes, as high-
   energy phosphate compounds. A nucleobase and a sugar make up a nucleoside; a
   nucleoside plus a phosphate make up a nucleotide. In DNA the sugar is deoxyribose
   and the bases are adenine (A), guanine (G), thymine (T) and cytosine (C). In RNA
   the sugar is ribose and the bases are adenine (A), guanine (G), uracil (U) and cytosine
   (C). In DNA the polymers of nucleotides form two complementary chains (double
   helix), with the nucleobases from one chain paired with those of the other chain in the
   following manner: A-T; G-C. In RNA, T is replaced by U. Because the RNA chains
   are flexible, the three dimensional structure of RNA is much more complex than
   double-stranded DNA; depending on the base sequence, RNA may form various
   degrees of intra-chain complementary pairing.


3. Carbohydrates: As substrates of metabolism; as a form of energy storage; as protein/
   lipid modifier; as components of connective tissue, cell wall, and nucleic acids; as
   structural materials in plants.


4. Lipids: As backbones of cell membranes and cell wall; as a form of energy storage; as
   hormones and signal transducers.


5. Vitamins and coenzymes: As small molecules that assist enzyme action.


6. Others: Water, electrolytes (sodium, potassium, calcium, magnesium, chloride),
   phosphoric acid and its derivatives (including the “high-energy” ATP), carbon dioxide,
   hormones, neurotransmitters, second messengers, minerals and trace metals, waste
   products, and other minor components.
   Note: Even the simplest bacterium today contains over 5,000 different organic compounds.

or atoms), and to serve as sensors of outside environment (receptors).
In bacteria and plants, the plasma membrane is protected outside by
a cell wall, made of lipids and carbohydrates. Inside the cell there is a
long thread of DNA (deoxyribonucleic acid), the carrier of the genetic
message (Fig. 3.2). In plants and animals, DNA is bound to a group
of small proteins called histones and neatly packaged into a compact
“ball” (chromatin) inside a membranous enclosure — the nucleus.
When a cell is ready to divide, the chromatin is repackaged into rod-