CHAPTER 2 The Origin and Chemistry of Life

Belgian plant nutritionist Jean Baptistevan Helmont (1648).

“If you press a piece of underwear soiled with sweat together with some wheat in an open jar, after about 21 days the odor changes and the ferment …changes the wheat into mice. But what is more remarkable is that the mice which came out of the wheat and underwear were not small mice, not even miniature adults or aborted mice, but adult mice emerge!”

The great French scientist Louis Pasteur convinced scientists that living organisms do not arise spontaneously from nonliving matter.

All living organisms share a common ancestor; most likely, a population of microorganisms that lived almost 4 billion years ago was the last universal common ancestor (LUCA) of life on earth. This common ancestor was itself the product of a long period of prebiotic assembly of nonliving matter, including organic molecules and water, to form self-replicating units. All living organisms retain a fundamental chemical composition inherited from their ancient common ancestor.

British biologist J. B. S. Haldane independently proposed that life originated on earth after an inconceivably long period of “abiogenic molecular evolution.”

2.1 Water and Life

• Water has a high specific heat capacity.

• Water also has a high heat of vaporization.

• unique density behavior

• Water has high surface tension.

• water has low viscosity.

• Water is an excellent solvent.

• Water also participates in many chemical reactions.

• Many compounds are split into smaller pieces by the addition of a molecule of water, a process called hydrolysis.

pH of Water Solutions

The pH scale quantifies the degree to which a solution is acidic or basic. The scale ranges from 0 to 14 and represents the additive inverse of the logarithm (base 10) of the H+ concentration (in moles/ liter) of the solution.

A buffer is a dissolved substance (solute) that causes a solution to resist changes in pH because the buffer can remove added H+ and OH− ions from the solution by binding them into compounds. larger compounds may be synthesized from smaller components by the reverse of hydrolysis, called condensation reactions.

2.2 MACROMOLECULES

The term “organic” refers broadly to compounds that contain carbon.

Carbohydrates: Nature’s Most Abundant Organic Substance

Carbohydrates are compounds of carbon, hydrogen, and oxygen. These elements usually occur in the ratio of 1 C: 2 H: 1 O and are grouped as H—C—OH. Carbohydrates function in protoplasm mainly as structural elements and as a source of chemical energy

Glucose is the most important of these energy-storing carbohydrates.

Carbohydrates are synthesized by green plants from water and carbon dioxide, with the aid of solar energy. This process, called photosynthesis, is a reaction upon which all life depends, for it is the starting point in the formation of food.

Carbohydrates are usually grouped into the following three classes:

(1) monosaccharides, or simple sugars.

(2) disaccharides, or double sugars; and

(3) polysaccharides, or complex sugars.

Chitin is an important structural polysaccharide in the exoskeletons of insects and other arthropods.

Glycogen, a multibranched polymer of glucose, is an important polymer for storing sugar in animals.

Another polymer is cellulose, the principal structural carbohydrate of plants.

Lipids: Fuel Storage and Building Material

Lipids are fats and fatlike substances. They are molecules of low polarity; consequently, they are virtually insoluble in water but are soluble in organic solvents, such as acetone and ether.

1. Triglycerides

   The triglycerides or “true” fats are major fuels of animals. Stored fat is derived either directly from dietary fat or indirectly from dietary carbohydrates that the body has converted to fat for storage.

2. Phospholipids

   Unlike fats that fuel and serve no structural roles in the cell, phospholipids are important components of the molecular organization of tissues, especially membranes. They resemble triglycerides in structure, except that one of the three fatty acids is replaced by phosphoric acid and an organic base.

3. Steroids

   Steroids are complex alcohols. Although they are structurally unlike fats, they have fatlike properties, including low polarity.

Amino Acids and Proteins

The amino acids are linked by peptide bonds to form long, chainlike polymers.

A protein is not just a long string of amino acids; it is a highly organized molecule.

The primary structure of a protein is the sequence of amino acids composing the polypeptide chain.

The secondary structure, such as the alpha-helix, which makes helical turns in a clockwise direction like a screw.

Many proteins function as enzymes, the biological catalysts required for almost every reaction in the body. Enzymes lower the activation energy required for specific reactions and enable life processes to proceed at moderate temperatures rather than requiring high temperatures.

A prion is an infectious protein particle in which a protein of the host organism is contorted into an abnormal three-dimensional structure. Upon infection, the prion causes its host’s normal copies of the protein to be refolded into the abnormal form, with pathological results.

Nucleic Acids

Nucleic acids are complex polymeric molecules whose sequence of nitrogenous bases encodes the genetic information necessary for biological inheritance.

The two kinds of nucleic acids in cells are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They are polymers of repeated units called nucleotides, each of which contains a sugar, a nitrogenous base, and a phosphate group.