What Is Life?
Living Creatures Are Made of Cells
Essential Properties of a Living Cell
Prokaryotic Cells Lack a Nucleus
Eubacteria and Archaebacteria Are Genetically Distinct
Bacteria Were Used for Fundamental Studies of Cell Function
Escherichia coli (E. coli) Is a Model Bacterium
Where Are Bacteria Found in Nature?
Some Bacteria Cause Infectious Disease, but Most Are Beneficial
Eukaryotic Cells Are Sub-Divided into Compartments
The Diversity of Eukaryotes
Eukaryotes Possess Two Basic Cell Lineages
Some Widely Studied Organisms Serve as Models Yeast Is a Widely Studied Single-Celled Eukaryote A Roundworm and a Fly are Model Multicellular Animals Zebrafish are used to Study Vertebrate Development Mouse and Man
Arabidopsis Serves as a Model for Plants Haploidy, Diploidy, and the Eukaryotic Cell Cycle Viruses Are Not Living Cells Bacterial Viruses Infect Bacteria Human Viral Diseases Are Common A Variety of Subcellular Genetic Entities Exist
No satisfactory technical definition of life exists. Despite this we understand what life entails. In particular, life involves a dynamic balance between duplication and alteration.
What Is Life?
Although there is no definition of life that suits all people, everyone has an idea of what being alive means. Generally, it is accepted that something is alive if it can grow and reproduce, at least during some stage of its existence. Thus, we still regard adults who are no longer growing and those individuals beyond reproductive age as being alive. We also regard sterile individuals, such as mules or worker bees as being alive, even though they lack the ability to reproduce. Part of the difficulty in defining life is the complication introduced by multicellular organisms. Although a multicellular organism as a whole may not grow or reproduce some of its cells may still retain these abilities.
Perhaps the key factor that characterizes life is the ability to self-replicate. This includes both the replication of the genetic information (the genome) and of the structure carrying and protecting it (the cell). Growth and reproduction need both information and energy in order to process raw materials into new living matter, and ultimately to create new organisms identical or, at any rate very similar, to the original organism. This brings us to another characteristic of life, which is that it evolves. Descendents are not identical to their ancestors but gradually accumulate changes in their genetic information over time. Both accurate replication and occasional evolutionary change are due to the properties of the nucleic acid molecules, DNA and RNA, which carry the genetic information. Furthermore, life forms do not merely grow and divide they also respond to stimuli from the environment. Some responses involve such complex structures as the nervous system of higher animals. However, many responses operate at the genetic level and are therefore included in this book. The basic ingredients needed to sustain life include the following:
Genetic information Biological information is carried by the nucleic acid molecules, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The units of genetic information are known as genes and each consists physically of a segment of a nucleic acid molecule. DNA is used for long-term storage of large amounts of genetic information (except by some viruses—see Ch. 17).Whenever genetic information is actually used, working copies of the genes are carried on RNA. The total genetic information possessed by an organism is known as its genome.Whenever an organism reproduces, the DNA molecules carrying the genome must be replicated so that the descendents may receive a complete copy of the genetic information.
Mechanism for energy generation By itself, information is useless. Energy is needed to put the genetic information to use. Living creatures must all obtain energy for growth and reproduction. Metabolism is the set of processes in which energy is acquired, liberated and used for biosynthesis of cell components. Machinery for making more living matter Synthesis of new cell components requires chemical machinery. In particular, the ribosomes are needed for making proteins, the macromolecules that make up the bulk of all living tissue.
A characteristic outward physical form Living creatures all have a material body that is characteristic for each type of life form. This structure contains all the metabolic and biosynthetic machinery for generating energy and making new living matter. It also contains the DNA molecules that carry the genome. Identity or self All living organisms have what one might call an identity. The term self-replication implies that an organism knows to make a copy of itself—
deoxyribonucleic acid (DNA) The nucleic acid polymer of which the genes are made gene A unit of genetic information genome The entire genetic information from an individual macromolecule Large polymeric molecule; in living cells especially DNA, RNA, protein or polysaccharide metabolism The processes by which nutrient molecules are transported and transformed within the cell to release energy and to provide new cell material nucleic acid Polymer made of nucleotides that carries genetic information replication Duplication of DNA prior to cell division ribonucleic acid (RNA) Nucleic acid that differs from DNA in having ribose in place of deoxyribose and having uracil in place of thymine ribosome The cell's machinery for making proteins not merely to assemble random organic material. Living organisms use raw material from the environment to make more of their own selves, ultimately to make complete copies of themselves. This concept of self versus non-self reaches its most sophisticated expression in the immune systems that protect higher animals against disease. But even primitive creatures attempt to preserve their own existence.
Matter is divided into atoms. Genetic information is divided into genes. Living organisms are divided into cells.
Looking around at the living creatures that inhabit this planet, one is first struck by their immense variety: squids, seagulls, sequoias, sharks, sloths, snakes, snails, spiders, strawberries, soybeans, and so forth. Although highly diverse to the eye, the biodiversity represented by these creatures is actually somewhat superficial. The most fascinating thing about life is not its superficial diversity but its fundamental unity. All of these creatures, together with microscopic organisms too small to see with the naked eye, are made up of cells, structural units or compartments that have more or less the same components.
The idea that living cells are the structural units of life was first proposed by Schleiden and Schwann in the 1830s. Cells are microscopic structures that vary considerably in shape. Many are spherical, cylindrical or roughly cuboidal but many other shapes are found, such as the long branched filaments of nerve cells. Many microscopic life forms consist of a single cell, whereas creatures large enough to see usually contain thousands of millions. Each cell is enclosed by a cell membrane composed of proteins and phospholipids and contains a complete copy of the genome (at least at the start of its life). Living cells possess the machinery to carry out metabolic reactions and generate energy and are usually able to grow and divide. Moreover, living cells always result from the division of pre-existing cells; they are never assembled from their component parts. This implies that living organisms too can only arise from pre-existing organisms. In the 1860s, Louis Pasteur confirmed experimentally that life cannot arise spontaneously from organic matter. Sterilized nutrient broth did not "spoil" or "go bad" unless it was exposed to microorganisms in the air.
In most multicellular organisms, the cells are specialized in a variety of ways (Fig. 2.01). The development of specialized roles by particular cells or whole tissues is referred to as differentiation. For example, the red blood cells of mammals lose their nucleus and the enclosed DNA during development. Once these cells are fully differentiated, they can perform only their specialized role as oxygen carriers and can no longer grow and divide. Some specialized cells remain functional for the life span of the individual organism, whereas others have limited life spans, sometimes lasting only a few days or hours. For multicellular organisms to grow and reproduce, some cells clearly need to keep a complete copy of the genome and retain the ability to grow and divide. In single-celled organisms, such as bacteria or protozoa, each individual cell has a complete genome and can grow and reproduce; hence, the complications of having multiple types of cell are largely absent.
At least in the case of unicellular organisms, each cell must possess the characteristics of life as discussed above (Fig. 2.02). Each living cell must generate its own energy and bacteria Primitive, relatively simple, single-celled organisms that lack a cell nucleus cell The cell is the basic unit of life. Each cell is surrounded by a membrane and usually has a full set of genes that provide it with the genetic information necessary to operate differentiation Progressive changes in the structure and gene expression of cells belonging to a single organism that leads to the formation of different types of cell phospholipid A hydrophobic molecule found making up cell membranes and consisting of a soluble head group and two fatty acids both linked to glycerol phosphate protein Polymer made from amino acids that does most of the work in the cell
In multicellular organisms, cells differentiate from unspecialized precursor cells. Differentiation allows cells to specialize functionally. Their form is related to their function.
Membrane transport of nutrients energy generation transport of nutrients energy generation
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