Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Biochemistry: The Molecular Logic of Living Organisms
Living matter is characterized by several distinctive features.

About 20 billion years ago, a mega-explosion occurred, filling all space with incandescent, ultra-high-energy subatomic particles. This is how the Universe was born. Gradually, as the Universe cooled, these elementary particles formed positively charged nuclei, which began attracting negatively charged electrons. In this way, about a hundred or slightly more chemical elements were formed. All atoms present in the Universe today, including those making up living organisms, originated from this "Big Bang," meaning that we humans, and indeed all living creatures, are made of stardust.

The simple Organic compounds that build all organisms are unique to the living world and, under modern terrestrial conditions, are products exclusively of biological activity. These compounds, known as Biomolecules, serve as the building blocks for biological structures; they were selected during biological evolution for their suitability to perform strictly defined Functions within living Cells. These compounds are identical across all organisms. Biomolecules are interconnected and interact According to the rules of the "molecular game"—the molecular logic of the living state. The size, shape, and Chemical properties of biomolecules enable them not only to act as building blocks in complex cellular structures, but also to participate in continuous processes of energy and matter transformation. Biomolecules should therefore be viewed from two Perspectives: chemical and biological. Biochemistry is super-chemistry, that is, the Chemistry of the most highly organized matter.

Interstellar gas and dust in the constellation Sagittarius. Interstellar space contains A large number of simple organic compounds that are thought to act as precursors to the biomolecules existing on Earth and, who knows, perhaps elsewhere in the Universe.

Living systems are composed of "lifeless" molecules. When these molecules are isolated and studied individually, they are found to obey all the laws of physics and chemistry that describe The behavior of inanimate matter. Nevertheless, living organisms possess extraordinary properties that are absent in collections of inanimate molecules. A closer look at these special properties helps clarify the fundamental questions that Biochemistry seeks to answer.

One of the most remarkable features of living organisms is their complexity and high degree of Organization. They feature an intricate internal Structure and contain a vast array of complex molecules. Living organisms comprise millions of diverse species, whereas the surrounding inanimate matter—clay, sand, stones, Water—consists of disordered mixtures of relatively simple chemical compounds.

A second characteristic of living organisms is that every component of an Organism has a specific purpose and performs a strictly defined function. This applies not only to macroscopic structures and Organs, such as The Heart, Lungs, or Brain, but also to microscopic intracellular structures, such as the Cell Nucleus. Even individual chemical compounds within The Cell, such as Proteins or Lipids, are endowed with specialized functions. Therefore, it is entirely valid to ask what purpose a particular molecule or chemical reaction serves in a living organism, whereas asking about the function of various chemical compounds in inanimate matter is completely meaningless.

A third characteristic of living matter, which brings us closer to The Essence of life processes, is the ability of living organisms to extract, transform, and utilize energy from their environment—either in the form of organic nutrients or solar radiation. This energy enables organisms to build their own energy-rich, complex structures and maintain their integrity. In addition, organisms use this energy to perform mechanical work during locomotion and to drive The Active Transport of various substances across membranes. Living organisms are never in a state of equilibrium—neither in the processes occurring within them nor in their interactions with the environment. Inanimate matter, by contrast, is incapable of purposefully utilizing energy to maintain its structure and perform work. Left to itself, it gradually degrades and eventually reaches a disordered state, establishing equilibrium with its surroundings.

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Fig. 1-1. Some Characteristic Features of living matter—"Signs of Life." A. Cytology/practical/72.html">Cross section of a photosynthesizing cell, revealing its delicate and complex structure: the dark structures are METABOLISM/14.html">Chloroplasts containing thousands of chlorophyll molecules oriented to capture solar energy. B. The long proboscis of a hummingbird hawk-moth, adapted through long biological evolution to extract nectar from deep-throated flowers. C. Dolphins, which feed on small fish, convert the chemical energy of food into powerful pulses of muscular energy. D. Biological self-Replication occurs with near-perfect fidelity.

Yet the most striking feature of living organisms is their capacity for precise self-replication—a property that can truly be considered the quintessence of the living state. The known mixtures of substances that make up inanimate objects show no capacity for GROWTH AND REPRODUCTION that would preserve the identical form, mass, and Internal Structure of these objects from generation to generation.