Lecture 1: Does Life Have Defining Properties? PDF

Does Life Have Defining Properties?  what is life? - Although many attempts have been made to define life, simple definitions are doomed to failure. When we try to give life a simple definition, we look for fixed properties maintained throughout life’s history. Howeve...

Does Life Have Defining Properties?  what is life? - Although many attempts have been made to define life, simple definitions are doomed to failure. When we try to give life a simple definition, we look for fixed properties maintained throughout life’s history. However, the properties that life exhibits today are very different from those present at its origin. The history of life shows extensive and ongoing change, which we call evolution.  As the genealogy of life progressed and branched from the earliest living form to the millions of species alive today, new properties evolved and passed from parents to their offspring. Through this process, living systems have generated many rare and spectacular features that have no counterparts in the nonliving world.  General Properties of Living Systems The most outstanding general features in life’s history include: 1- Chemical uniqueness 2-Complexity and hierarchical organization 3- Reproduction (heredity and variation) 4-Possession of a genetic program 5-Metabolism 6-Development 7-Environmental interaction 8-Movement  1-Chemical uniqueness Living systems demonstrate a unique and complex molecular organization. Living systems assemble large molecules, known as macromolecules, that are far more complex than the small molecules of nonliving matter. These macromolecules are composed of the same kinds of atoms and chemical bonds that occur in nonliving matter and they obey all fundamental laws of chemistry; it is only the complex organizational structure of these macromolecules that makes them unique. We recognize four major categories of biological macromolecules: nucleic acids, proteins, carbohydrates, and lipids.  These categories differ in the structures of their component parts, the kinds of chemical bonds that link their subunits together, and their functions in living systems. The general structures of these macromolecules evolved and stabilized early in the history of life. With some modifications, these same general structures are found in every form of life today. Proteins, for example, contain about 20 specific kinds of amino acid subunits linked together by peptide bonds in a linear sequence (Figure 1.2)  Additional bonds occurring between amino acids that are not adjacent to each other in the protein chain give the protein a complex three-dimensional structure. A typical protein contains several hundred amino acid subunits. Despite the stability of this basic protein structure, the ordering of the different amino acids in the protein molecule is subject to enormous variation. This variation underlies much of the diversity that we observe among different kinds of living forms. The nucleic acids, carbohydrates, and lipids likewise contain characteristic bonds that link variable subunits This organization gives living systems both a biochemical unity and great potential diversity.  2-Complexity and hierarchical organization ▪ Living systems demonstrate a unique and complex hierarchical organization ▪ Nonliving matter is organized at least into atoms and molecules and often has a higher degree of organization as well. However, atoms and molecules are combined into patterns in the living world that do not exist in the nonliving world. ▪ In living systems, we find a hierarchy of levels that includes, in ascending order of complexity, macromolecules, cells, organisms, populations, and species ▪ Each level builds on the level below it and has its own internal structure, which is also often hierarchical. Within the cell, for example, macromolecules are compounded into structures such as ribosomes, chromosomes, and membranes, and these are likewise combined in various ways to form even more complex subcellular structures called organelles, such as mitochondria. ▪ The organismal level also has a hierarchical substructure; cells combine to form tissues, which combine to form organs, which likewise combine to form organ systems. Cells are the smallest units of the biological hierarchy that are semiautonomous in their ability to conduct basic functions, including reproduction. ▪ Replication of molecules and subcellular components occurs only within a cellular context, not independently. ▪ Cells are therefore considered the basic units of living systems. We can isolate cells from an organism and cause them to grow and to multiply under laboratory.  3- Reproduction ▪ Living systems can reproduce themselves. ▪ Life does not arise spontaneously but comes only from prior life, through reproduction. ▪ Although life certainly originated from nonliving matter at least once, this origin featured enormously long periods of time and conditions very different from the current biosphere. ▪ At each level of the biological hierarchy, living forms reproduce to generate others like themselves.  Genes are replicated to produce new genes. Cells divide to produce new cells. Organisms reproduce, sexually or asexually, to produce new organisms. Populations may become fragmented to produce new populations, and species may split to produce new species through a process called speciation. Reproduction at any hierarchical level usually features an increase in numbers. Individual genes, cells, organisms, populations, or species may fail to reproduce themselves, but reproduction is nonetheless an expected property of these individuals. Reproduction at each of these levels shows the phenomena of heredity and variation  Heredity :is the faithful transmission of traits from parents to offspring, usually (but not necessarily) observed at the organismal level. Variation: is the production of differences among the traits of different individuals. In a reproductive process, properties of descendants resemble those of their parents to varying degrees but usually are not identical to them. Replication of deoxyribonucleic acid (DNA) occurs with high fidelity, but errors occur at repeatable rates.  Cell division is exceptionally precise, especially about the nuclear material, but chromosomal changes occur nonetheless at measurable rates. Organismal reproduction likewise demonstrates both heredity and variation.  4- Possession of a genetic program. A genetic program provides fidelity of inheritance (Figure 1.6). Structures of the protein molecules needed for organismal development and functioning are encoded in nucleic acids. For animals and most other organisms, genetic information is contained in DNA. DNA is a very long, linear chain of subunits called nucleotides, each of which contains a sugar phosphate (deoxyribose phosphate) and one of four nitrogenous bases (adenine, cytosine, guanine, or thymine, abbreviated A, C, G, and T, respectively).  The sequence of nucleotide bases contains a code for the order of amino acids in the protein specified by the DNA molecule. The correspondence between the sequence of bases in DNA and the sequence of amino acids in a protein is called the genetic code. The genetic code has undergone very little evolutionary change since its origin because an alteration would disrupt the structure of nearly every protein, which would in turn severely disrupt cellular functions that require very specific protein structures.  Evolutionary change in the genetic code has occurred in the DNA contained in animal mitochondria, the organelles that regulate cellular energy. The genetic code in animal mitochondrial DNA therefore is slightly different from the standard code of nuclear and bacterial DNA.  5-Metabolism. ▪ Living organisms maintain themselves by acquiring nutrients from their environments. ▪ The nutrients are used to obtain chemical energy and molecular components for building and maintaining the living system. We call these essential chemical processes metabolism.  They include digestion, acquisition of energy (respiration), and synthesis of molecules and structures. Metabolism is often viewed as an interaction of destructive (catabolic) and constructive (anabolic) reactions. The most fundamental anabolic and catabolic chemical processes used by living systems arose early in the evolutionary history of life, and all living forms share them. These reactions include synthesis of carbohydrates, lipids, nucleic acids, and proteins and their constituent parts and cleavage of chemical bonds to recover energy stored in them.  In animals, many fundamental metabolic reactions occur at the cellular level, often in specific organelles found throughout the animal kingdom. Cellular respiration occurs, for example, in mitochondria. Cellular and nuclear membranes regulate metabolism by controlling the movement of molecules across the cellular and nuclear boundaries, respectively. The study of complex metabolic functions is called physiology  6-Development All organisms pass through a characteristic life cycle. Development describes the characteristic changes that an organism undergoes from its origin (usually the fertilization of an egg by sperm) to its final adult form. Development usually features changes in size and shape, and differentiation of structures within an organism. Even the simplest one-celled organisms grow and replicate their component parts until they divide into two or more cells  Multicellular organisms undergo more dramatic changes during their lives. Different developmental stages of some multicellular forms are so dissimilar that they are hardly recognizable as belonging to the same species. Embryos are distinctly different from juvenile and adult forms into which they develop. Even postembryonic development of some organisms includes stages dramatically different from each other. The transformation that occurs from one stage to another is called metamorphosis.  7-Environmental interaction All animals interact with their environments. The study of organismal interaction with an environment is called ecology. The science of ecology reveals how an organism perceives environmental stimuli and responds in appropriate ways by adjusting its metabolism and physiology. All organisms respond to environmental stimuli, a property called irritability. The stimulus and response may be simple, such as a unicellular organism moving from or toward a light source or away from a noxious substance, or it may be quite complex, such as a bird responding to a complicated series of signals in a mating ritual.  8-Movement Living systems and their parts show precise and controlled movements arising from within the system. The energy that living systems extract from their environments permits them to initiate controlled movements. Such movements at the cellular level are essential for reproduction, growth, and many responses to stimuli in all living forms and for development in multicellular ones. Autonomous movement reaches great diversity in animals,  On a larger scale, entire populations or species may disperse from one geographic location to another one over time through their powers of movement. Movement characteristic of nonliving matter, such as that of particles in solution, radioactive decay of nuclei, and eruption of volcanoes is not precisely controlled by the moving objects themselves and often involves forces entirely external to them. The adaptive and often purposeful movements initiated by living systems are absent from the nonliving world.