Chemical Evolution
Chemical Evolution
Both Haldane and Oparin proposed that earth’s primitive atmosphere consisted of simple compounds such as water, carbon dioxide, molecular hydrogen, methane, and ammonia, but lacked oxygen. The nature of the primeval atmosphere is critical for understanding life’s origin. The organic compounds that compose living organisms are neither synthesized outside cells nor stable in the presence of molecular oxygen, which is abundant in the atmosphere today. The best evidence indicates, however, that the primitive atmosphere contained not more than a trace of molecular oxygen, most of which had reacted with hydrogen to form the water present on the earth’s surface. The primeval atmosphere therefore was a reducing one, consisting primarily of molecules in which hydrogen exceeds oxygen; methane (CH4) and ammonia (NH3), for example, constitute fully reduced compounds. During this time, the earth was bombarded by large (100 km diameter) comets and meteorites, generating heat that repeatedly vaporized the oceans.
This reducing atmosphere was conducive to the prebiotic synthesis that led to life’s beginnings, although totally unsuited for the organisms that exist today. Haldane and Oparin proposed that when such a gas mixture was exposed to ultraviolet radiation, many organic substances such as sugars and amino acids could be formed. Haldane believed that the early organic molecules accumulated in the primitive oceans to form a “hot dilute soup.” In this primordial broth, carbohydrates, fats, proteins, and nucleic acids could have assembled to form the earliest structures capable of guiding their own replication.
If the simple gaseous compounds present in the early atmosphere are mixed with methane and ammonia in a closed glass system and kept at room temperature, they never react chemically with each other. To produce a chemical reaction, a continuous source of free energy sufficient to overcome reaction-activation barriers must be supplied. Ultraviolet light from the sun must have been intense on the primitive earth before the accumulation of atmospheric oxygen; ozone, a threeatom form of oxygen located high in the atmosphere, now blocks much of the ultraviolet radiation from reaching the earth’s surface. Electrical discharges could have provided further energy for chemical evolution. Although the total amount of electrical energy released by lightning is small compared with solar energy, nearly all of the energy of lightning is effective in synthesizing organic compounds in a reducing atmosphere. A single flash of lightning through a reducing atmosphere generates a large amount of organic matter. Thunderstorms may have been one of the most important sources of energy for organic synthesis.
Widespread volcanic activity on the primitive earth is another possible source of energy. One hypothesis maintains, for example, that life did not originate on the surface of the earth, but deep beneath the sea in or around hydrothermal vents. Hydrothermal vents are submarine hot springs, in which seawater seeps through cracks in the bottom until the water comes close to hot magma. The water is superheated and expelled forcibly, carrying a variety of dissolved molecules from the superheated rocks. These molecules include hydrogen sulfide, methane, iron ions, and sulfide ions. Hydrothermal vents have been discovered in several locations beneath the deep sea, and they would have been much more widely prevalent on the early earth. Interestingly, many heat- and sulfur-loving bacteria grow in hot springs today.
Water and Life
The origin and maintenance of life on earth depends critically upon water. Water is the most abundant of all compounds in cells, comprising 60% to 90% of most living organisms. Water has several extraordinary properties that explain its essential role in living systems and their origin. These properties result largely from hydrogen bonds that form between its molecules.
Water has a high specific heat
capacity: 1 calorie is required to elevate
the temperature of 1 g of water
1° C, a higher thermal capacity than any
other liquid except ammonia. Much of
this heat energy is used to rupture some
hydrogen bonds in addition to increasing
the kinetic energy (molecular movement),
and thus the temperature, of the
water. Water’s high thermal capacity
greatly moderates environmental temperature
changes, thereby protecting living
organisms from extreme thermal fluctuation.
Water also has a high heat of
vaporization, requiring more than 500
calories to convert 1 g of liquid water to
water vapor. All hydrogen bonds
between a water molecule and its neighbors
must be ruptured before that water
molecule can escape the surface and
enter the air. For terrestrial animals (and
plants), cooling produced by evaporation
of water is important for expelling
excess heat.
Another property of water important
for life is its unique density behavior during changes of temperature. Most liquids
become denser with decreasing
temperature. Water, however, reaches its
maximum density at 4° C while still a liquid,
then becomes less dense with further
cooling. Therefore, ice floats rather
than forming on the bottoms of lakes
and ponds. If ice were denser than liquid
water, bodies of water would freeze
solid from the bottom upward in winter
and would not necessarily melt completely
in summer. Such conditions
would severely limit aquatic life. In ice,
water molecules form an extensive,
open, crystal-like network supported by
hydrogen bonds that connect all molecules.
The molecules in this lattice are
farther apart, and thus less dense, than in
liquid water at 4° C.
Water has high surface tension, exceeding that of any other liquid but mercury. Hydrogen bonding among water molecules produces a cohesiveness that is important for maintaining protoplasmic form and movement. The resulting surface tension creates an ecological niche for insects, such as water striders and whirligig beetles, that skate on the surfaces of ponds. Despite its high surface tension, water has low viscosity, permitting movement of blood through minute capillaries and of cytoplasm inside cellular boundaries.
Water is an excellent solvent. Salts
dissolve more extensively in water than
in any other solvent. This property
results from the dipolar nature of water,
which causes it to orient around charged
particles dissolved in it. When, for example,
crystalline NaCl dissolves in water,
the Na+ and Cl− ions separate. The negative
zones of the water dipoles attract
the Na+ ions while the positive zones
attract the Cl− ions. This orientation
keeps the ions separated, promoting
their dissociation. Solvents lacking this
dipolar character are less effective at
keeping the ions separated. Binding of
water to dissolved protein molecules is
essential to the proper functioning of
many proteins.
Water also participates in many chemical reactions in living organisms. Many compounds are split into smaller pieces by the addition of a molecule of water, a process called hydrolysis. Likewise, larger compounds may be synthesized from smaller components by the reverse of hydrolysis, called condensation reactions.
Both Haldane and Oparin proposed that earth’s primitive atmosphere consisted of simple compounds such as water, carbon dioxide, molecular hydrogen, methane, and ammonia, but lacked oxygen. The nature of the primeval atmosphere is critical for understanding life’s origin. The organic compounds that compose living organisms are neither synthesized outside cells nor stable in the presence of molecular oxygen, which is abundant in the atmosphere today. The best evidence indicates, however, that the primitive atmosphere contained not more than a trace of molecular oxygen, most of which had reacted with hydrogen to form the water present on the earth’s surface. The primeval atmosphere therefore was a reducing one, consisting primarily of molecules in which hydrogen exceeds oxygen; methane (CH4) and ammonia (NH3), for example, constitute fully reduced compounds. During this time, the earth was bombarded by large (100 km diameter) comets and meteorites, generating heat that repeatedly vaporized the oceans.
This reducing atmosphere was conducive to the prebiotic synthesis that led to life’s beginnings, although totally unsuited for the organisms that exist today. Haldane and Oparin proposed that when such a gas mixture was exposed to ultraviolet radiation, many organic substances such as sugars and amino acids could be formed. Haldane believed that the early organic molecules accumulated in the primitive oceans to form a “hot dilute soup.” In this primordial broth, carbohydrates, fats, proteins, and nucleic acids could have assembled to form the earliest structures capable of guiding their own replication.
If the simple gaseous compounds present in the early atmosphere are mixed with methane and ammonia in a closed glass system and kept at room temperature, they never react chemically with each other. To produce a chemical reaction, a continuous source of free energy sufficient to overcome reaction-activation barriers must be supplied. Ultraviolet light from the sun must have been intense on the primitive earth before the accumulation of atmospheric oxygen; ozone, a threeatom form of oxygen located high in the atmosphere, now blocks much of the ultraviolet radiation from reaching the earth’s surface. Electrical discharges could have provided further energy for chemical evolution. Although the total amount of electrical energy released by lightning is small compared with solar energy, nearly all of the energy of lightning is effective in synthesizing organic compounds in a reducing atmosphere. A single flash of lightning through a reducing atmosphere generates a large amount of organic matter. Thunderstorms may have been one of the most important sources of energy for organic synthesis.
Widespread volcanic activity on the primitive earth is another possible source of energy. One hypothesis maintains, for example, that life did not originate on the surface of the earth, but deep beneath the sea in or around hydrothermal vents. Hydrothermal vents are submarine hot springs, in which seawater seeps through cracks in the bottom until the water comes close to hot magma. The water is superheated and expelled forcibly, carrying a variety of dissolved molecules from the superheated rocks. These molecules include hydrogen sulfide, methane, iron ions, and sulfide ions. Hydrothermal vents have been discovered in several locations beneath the deep sea, and they would have been much more widely prevalent on the early earth. Interestingly, many heat- and sulfur-loving bacteria grow in hot springs today.
 |
| Geometry of water molecules. Each water molecule is linked by hydrogen bonds (dashed lines) to four other molecules. If imaginary lines are used to connect the divergent oxygen atoms, a tetrahedron is obtained. |
The origin and maintenance of life on earth depends critically upon water. Water is the most abundant of all compounds in cells, comprising 60% to 90% of most living organisms. Water has several extraordinary properties that explain its essential role in living systems and their origin. These properties result largely from hydrogen bonds that form between its molecules.
 |
| When water freezes at 0° C, the four partial charges of each atom in the molecule interact with the opposite charges of atoms in other water molecules. The hydrogen bonds between all the molecules form a crystal-like lattice structure, and the molecules are farther apart (and thus less dense) than when some of the molecules have not formed hydrogen bonds at 4° C. |
 |
| Because of hydrogen bonds between water molecules at the water- air interface, the water molecules cling together and create a high surface tension. Thus some insects, such as this water strider, can literally walk on water. |
Water has high surface tension, exceeding that of any other liquid but mercury. Hydrogen bonding among water molecules produces a cohesiveness that is important for maintaining protoplasmic form and movement. The resulting surface tension creates an ecological niche for insects, such as water striders and whirligig beetles, that skate on the surfaces of ponds. Despite its high surface tension, water has low viscosity, permitting movement of blood through minute capillaries and of cytoplasm inside cellular boundaries.
 |
| When a crystal of sodium chloride dissolves in water, the negative ends of the dipolar molecules of water surround the Na+ ions, while the positive ends of water molecules face the Ci− ions. The ions are thus separated and do not reenter the salt lattice. |
Water also participates in many chemical reactions in living organisms. Many compounds are split into smaller pieces by the addition of a molecule of water, a process called hydrolysis. Likewise, larger compounds may be synthesized from smaller components by the reverse of hydrolysis, called condensation reactions.
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