All Cells Are Prokaryotic or Eukaryotic

The biological universe consists of two types of cells— prokaryotic and eukaryotic. Prokaryotic cells consist of a sin-

gle closed compartment that is surrounded by the plasma membrane, lacks a defined nucleus, and has a relatively simple internal organization (Figure 1-2a). All prokaryotes have cells of this type. Bacteria, the most numerous prokaryotes, are single-celled organisms; the cyanobacteria, or blue-green algae, can be unicellular or filamentous chains of cells. Although bacterial cells do not have membrane-bounded compartments, many proteins are precisely localized in their aqueous interior, or cytosol, indicating the presence of internal organization. A single Escherichia coli bacterium has a dry weight of about

(a) Prokaryotic cell

(b) Eukaryotic cell

Periplasmic space and cell wall

(a) Prokaryotic cell

Periplasmic space and cell wall

Outer membrane Inner (plasma) membrane

Nucleoid

Outer membrane Inner (plasma) membrane

Nucleoid

Nucleoid

Nucleoid

Inner (plasma) membrane Cell wall

Periplasmic space Outer membrane

Inner (plasma) membrane Cell wall

Periplasmic space Outer membrane

Nucleus

Golgi vesicles

(b) Eukaryotic cell

Nucleus

Golgi vesicles

Lysosome

Mitochondrion

Endoplasmic reticulum

Lysosome

Endoplasmic reticulum

Mitochondrion

Nuclear membrane

Plasma membrane

Secretory vesicle

Rough endoplasmic reticulum

Nuclear membrane

Plasma membrane

Golgi vesicles Mitochondrion Peroxisome Lysosome

Rough endoplasmic reticulum

Secretory vesicle

▲ FIGURE 1-2 Prokaryotic cells have a simpler internal organization than eukaryotic cells. (a) Electron micrograph of a thin section of Escherichia coli, a common intestinal bacterium. The nucleoid, consisting of the bacterial DNA, is not enclosed within a membrane. E. coli and some other bacteria are surrounded by two membranes separated by the periplasmic space. The thin cell wall is adjacent to the inner membrane. (b) Electron micrograph of a plasma cell, a type of white blood cell that secretes antibodies. Only a single membrane (the plasma membrane) surrounds the cell, but the interior contains many membrane-limited compartments, or organelles. The defining

25 X 10~14 g. Bacteria account for an estimated 1-1.5 kg of the average human's weight. The estimated number of bacteria on earth is 5 X 1030, weighing a total of about 1012 kg. Prokaryotic cells have been found 7 miles deep in the ocean and 40 miles up in the atmosphere; they are quite adaptable! The carbon stored in bacteria is nearly as much as the carbon stored in plants.

Eukaryotic cells, unlike prokaryotic cells, contain a defined membrane-bound nucleus and extensive internal mem-

characteristic of eukaryotic cells is segregation of the cellular DNA within a defined nucleus, which is bounded by a double membrane. The outer nuclear membrane is continuous with the rough endoplasmic reticulum, a factory for assembling proteins. Golgi vesicles process and modify proteins, mitochondria generate energy, lysosomes digest cell materials to recycle them, peroxisomes process molecules using oxygen, and secretory vesicles carry cell materials to the surface to release them. [Part (a) courtesy of I. D. J. Burdett and R. G. E. Murray. Part (b) from P C. Cross and K. L. Mercer, 1993, Cell and Tissue Ultrastructure: A Functional Perspective, W. H. Freeman and Company.]

branes that enclose other compartments, the organelles (Figure 1-2b). The region of the cell lying between the plasma membrane and the nucleus is the cytoplasm, comprising the cytosol (aqueous phase) and the organelles. Eukaryotes comprise all members of the plant and animal kingdoms, including the fungi, which exist in both multicellular forms (molds) and unicellular forms (yeasts), and the protozoans (proto, primitive; zoan, animal), which are exclusively unicellular. Eukaryotic cells are commonly about 10-100 ^m across,

Animals Plants

Animals Plants

■ Presumed common progenitor of all extant organisms

■ Presumed common progenitor of archaebacteria and eukaryotes

▲ FIGURE 1-3 All organisms from simple bacteria to complex mammals probably evolved from a common, single-celled progenitor. This family tree depicts the evolutionary relations among the three major lineages of organisms. The structure of the tree was initially ascertained from morphological criteria: Creatures that look alike were put close together. More recently the sequences of DNA and proteins have been examined as a more information-rich criterion for assigning relationships. The greater the similarities in these macromolecular sequences, the more closely related organisms are thought to be. The trees based on morphological comparisons and the fossil record generally agree well with those those based on molecular data. Although all organisms in the eubacterial and archaean lineages are prokaryotes, archaea are more similar to eukaryotes than to eubacteria ("true" bacteria) in some respects. For instance, archaean and eukaryotic genomes encode homologous histone proteins, which associate with DNA; in contrast, bacteria lack histones. Likewise, the RNA and protein components of archaean ribosomes are more like those in eukaryotes than those in bacteria.

generally much larger than bacteria. A typical human fibroblast, a connective tissue cell, might be about 15 ^m across with a volume and dry weight some thousands of times those of an E. coli bacterial cell. An amoeba, a single-celled protozoan, can be more than 0.5 mm long. An ostrich egg begins as a single cell that is even larger and easily visible to the naked eye.

All cells are thought to have evolved from a common progenitor because the structures and molecules in all cells have so many similarities. In recent years, detailed analysis of the DNA sequences from a variety of prokaryotic organisms has revealed two distinct types: the so-called "true" bacteria, or eubacteria, and archaea (also called archaebacteria or archaeans). Working on the assumption that organisms with more similar genes evolved from a common progenitor more recently than those with more dissimilar genes, researchers have developed the evolutionary lineage tree shown in Figure 1-3. According to this tree, the archaea and the eukaryotes diverged from the true bacteria before they diverged from each other.

Many archaeans grow in unusual, often extreme, environments that may resemble ancient conditions when life first appeared on earth. For instance, halophiles ("salt loving") require high concentrations of salt to survive, and thermoacidophiles ("heat and acid loving") grow in hot (80o C) sulfur springs, where a pH of less than 2 is common. Still other archaeans live in oxygen-free milieus and generate methane (CH4) by combining water with carbon dioxide.

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