THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

 

16. Prokaryotes: The First Single-Celled Creatures

 

16.4. Comparing Prokaryotes to Eukaryotes

 

Prokaryotes differ from eukaryotes in many respects: The cytoplasm of prokaryotes has very little internal organization, prokaryotes are unicellular and much smaller than eukaryotes, the prokaryotic chromosome is a single circle of DNA, cell division and flagella are simple, and prokaryotes are far more metabolically diverse than eukaryotes. Differences between prokaryotes and eukaryotes are illustrated in table 16.1.

 

TABLE 16.1. PROKARYOTES COMPARED TO EUKARYOTES

 

Feature

Example

Internal compartmentalization.

Unlike eukaryotic cells, prokaryotic cells contain no internal compartments, no internal membrane system, and no cell nucleus.

Cell size. Most prokaryotic cells are only about 1 micrometer in diameter, whereas most eukaryotic cells are well over 10 times that size.

Unicellularity. All prokaryotes are fundamentally single celled. Even though some may adhere together in a matrix or form filaments, their cytoplasms are not directly interconnected, and their activities are not integrated and coordinated, as is the case in multicellular eukaryotes.

Chromosomes. Prokaryotes do not possess chromosomes in which proteins are complexed with the DNA, as eukaryotes do. Instead, their DNA exists as a single circle in the cytoplasm.

Cell division. Cell division in prokaryotes takes place by binary fission (see chapter 8). The cells simply pinch in two. In eukaryotes, microtubules pull chromosomes to opposite poles during the cell division process, called mitosis.

Flagella. Prokaryotic flagella are simple, composed of a single fiber of protein that is spun like a propeller. Eukaryotic flagella are more complex structures, with a 9 + 2 arrangement of microtubules, that whip back and forth rather than rotate.

Metabolic diversity. Prokaryotes possess many metabolic abilities that eukaryotes do not: Prokaryotes perform several different kinds of anaerobic and aerobic photosynthesis; prokaryotes can obtain their energy from oxidizing inorganic compounds (so-called chemoautotrophs); and prokaryotes can fix atmospheric nitrogen.

 

Prokaryotic Metabolism

Prokaryotes have evolved many more ways than eukaryotes to acquire the carbon atoms and energy necessary for growth and reproduction. Many are autotrophs, organisms that obtain their carbon from inorganic CO2. Autotrophs that obtain their energy from sunlight are called photoautotrophs, whereas those that harvest energy from inorganic chemicals are called chemo-autotrophs. Other prokaryotes are heterotrophs, organisms that obtain at least some of their carbon from organic molecules like glucose. Heterotrophs that obtain their energy from sunlight are called photoheterotrophs, whereas those that harvest energy from organic molecules are called chemoheterotrophs.

Photoautotrophs. Many prokaryotes carry out photosynthesis, using the energy of sunlight to build organic molecules from carbon dioxide. The cyanobacteria use chlorophyll a as the key light-capturing pigment and use H2O as an electron donor, leaving oxygen gas as a by-product. Other prokaryotes use bacteriochlorophyll as their pigment and H2S as an electron donor, leaving elemental sulfur as the by-product.

Chemoautotrophs. Some prokaryotes obtain their energy by oxidizing inorganic substances. Nitrifiers, for example, oxidize ammonia or nitrite to form the nitrate that is taken up by plants. Other prokaryotes oxidize sulfur, hydrogen gas, and other inorganic molecules. On the dark ocean floor at depths of 2,500 meters, entire ecosystems subsist on prokaryotes that oxidize hydrogen sulfide as it escapes from volcanic vents.

Photoheterotrophs. The so-called purple nonsulfur bacteria use light as their source of energy but obtain carbon from organic molecules such as carbohydrates or alcohols that have been produced by other organisms.

Chemoheterotrophs. Most prokaryotes obtain both carbon atoms and energy from organic molecules. These include decomposers and most pathogens (disease-causing bacteria).

 

Key Learning Outcome 16.4. Prokaryotes differ from eukaryotes in having no nucleus or other interior compartments, in being far more metabolically diverse, and in many other fundamental respects.