THE ORIGIN AND EVOLUTION OF CELLS
All present-day cells, both prokaryotes and eukaryotes, are descended from a single ancestor. The first cell is thought to have arisen at least 3.8 billion years ago as a result of enclosure of self-replicating RNA in a phospholipid membrane.
The earliest reactions for the generation of metabolic energy were a form of anaerobic glycolysis. Photosynthesis then evolved, followed by oxidative metabolism.
Present-day prokaryotes are divided into two groups, the archaebacteria and the eubacteria, which diverged early in evolution.
Eukaryotic cells, which are larger and more complex than prokaryotic cells, contain a nucleus, cytoplasmic organelles, and a cytoskeleton. They are thought to have evolved from symbiotic associations of prokaryotes.
The simplest eukaryotes are unicellular organisms, such as yeasts and amoebas. Multicellular organisms evolved from associations between such unicellular eukaryotes, and division of labor led to the development of the many kinds of specialized cells that make up present-day plants and animals.
KEY TERMS: yeast, Saccharomyces cerevisiae, pseudopodium, parenchyma cell, collenchyma cell, schlerenchyma cell, epidermal cell, epithelial cell, fibroblast, erythrocyte, granulocyte, monocyte, macrophage, lymphocyte, neuron
The unicellular eukaryote Dictyostelium is widely used for experimental analysis of cell movement.
KEY TERMS:Dictyostelium discoideum
The nematode C. elegans is a simple multicellular organism that serves as an important model in developmental biology.
KEY TERMS:Caenorhabditis elegans, somatic cell, germ cell
Because of extensive genetic analysis, studies of the fruit fly Drosophila have led to major advances in understanding animal development.
KEY TERMS:Drosophila melanogaster
The small flowering plant Arabidopsis is widely used as a model for studies of plant molecular biology and development.
KEY TERMS:Arabidopsis thaliana
Many kinds of vertebrate cells can be grown in culture, where they can be studied under controlled laboratory conditions. Specialized cell types, such as neurons and muscle cells, provide useful models for investigating particular aspects of cell biology. The frog Xenopus laevis and zebrafish are important models for studies of early vertebrate development, and the mouse is a mammalian species suitable for genetic analysis.
A variety of methods are used to visualize cells and subcellular structures and to determine the intracellular localization of specific molecules using the light microscope.
KEY TERMS: resolution, bright-field microscopy, phase-contrast microscopy, differential interference-contrast microscopy, video-enhanced differential interference-contrast microscopy, fluorescence microscopy, green fluorescent protein (GFP), confocal microscopy, two-photon excitation microscopy
Electron microscopy, with a resolution that is approximately a hundredfold greater than that of light microscopy, is used to analyze details of cell structure.
The organelles of eukaryotic cells can be isolated for biochemical analysis by differential centrifugation.
KEY TERMS: differential centrifugation, ultracentrifuge, density-gradient centrifugation, velocity centrifugation, equilibrium centrifugation
The propagation of animal cells in culture has allowed studies of the mechanisms that control cell growth and differentiation.
Cultured plant cells can differentiate to form specialized cell types and, in some cases, can regenerate entire plants.
Viruses provide simple models for studies of cell function.