Cells are divided into two main classes, initially defined by whether they contain a nucleus. Prokaryotic cells (bacteria) lack a nuclear envelope; eukaryotic cells have a nucleus in which the genetic material is separated from the cytoplasm. Prokaryotic cells are generally smaller and simpler than eukaryotic cells; in addition to the absence of a nucleus, their genomes are less complex and they do not contain cytoplasmic organelles or a cytoskeleton (Table 1.1). In spite of these differences, the same basic molecular mechanisms govern the lives of both prokaryotes and eukaryotes, indicating that all present-day cells are descended from a single primordial ancestor.
Table 1.1 Prokaryotic and Eukaryotic Cells Characteristic Prokaryote Eukaryote Nucleus Absent Present Diameter of a typical cell ≈1μm 10–100 μm Cytoskeleton Absent Present Cytoplasmic organelles Absent Present DNA content (base pairs) 1 × 106 to 5 × 106 1.5 × 107 to 5 × 109 Chromosomes Single circular DNA molecule Multiple linear DNA molecules
Two Types of Cells: (Prokaryotic Cells and Eukaryotic Cells)
Prokaryotic Cells Prokaryote, relatively simple unicellular organism lacking a nucleus and other features found in the more complex cells of all other organisms, called eukaryotes. In 1938 American biologist Herbert Copeland proposed that unicellular organisms lacking nuclei be classified in their own kingdom, Monera, also called Kingdom Prokaryotae. All bacteria were categorized in this newly established kingdom. This scheme was the first to establish separate kingdoms for prokaryotes (organisms without nuclei) and eukaryotes (organisms with nuclei). In 1990 American microbiologist Carl Woese proposed that bacteria be divided into two groups, the archaea, or archaebacteria, and bacteria, based on their structural and physiological differences. In some classification systems, the archaea are considered prokaryotes; in others, they are classified in their own domain, the archaea. Archaebacteria consist of a small group of primitive anaerobes (organisms that do not require oxygen). They are found in a narrow range of habitats–often in extreme environments with high temperature, high salt, or high acidity. In contrast, bacteria live in a wide range of environments with or without oxygen, at various temperatures, and at various levels of acidity.
Prokaryotic cells are relatively small, ranging in size from 0.0001 to 0.003 mm(0.000004 to 0.0001 in) in diameter. With the exception of a few species,prokaryotic cells are surrounded by a protective cell wall.The cell walls of archaebacteria and bacteria contain forms of peptidoglycan, aprotein-sugar molecule not present in the cell walls of fungi, plants, and certainother eukaryotes.The archaebacteria cell wall has a more diverse chemical composition than thecell wall of bacteria just inside the cell wall of prokaryotes is the plasmamembrane, a thin structure that is both flexible and strong. In both prokaryotesand eukaryotes, the plasma membrane is composed of two layers of phospholipidmolecules interspersed with proteins, and regulates the traffic that flows in andout of the cell.The prokaryotic plasma membrane, however, carries out additional functions. Itparticipates in replication of deoxyribonucleic acid (DNA) for cell division andsynthesis of adenosine triphosphate (ATP), an energy molecule. In someprokaryotes, the plasma membrane is essential for photosynthesis, the processthat uses light energy to convert carbon dioxide and water to glucose.
In the interior of the prokaryotic cell is the cytoplasm, a watery fluid that is rich in dissolved salts, nutrients, enzymes, and other molecules. The great majority of the cells biochemical reactions, which number in the thousands, take place within the cytoplasm. Ribosomes, tiny bead-like structures that manufacture proteins, are also located in the cytoplasm. The ribonucleic acid (RNA) in the ribosomes differs significantly between the archaebacteria and bacteria. With the exception of the ribosomes, prokaryotes lack organelles (specialized structures such as the nucleus, chloroplasts, mitochondria, lysosomes, and Golgi apparatus), which are present in eukaryotes (see Cell). Some photosynthetic archaebacteria and bacteria have internal membranes, extensions of the plasma membrane known as chromatophores or thylakoids, which contain the pigments for photosynthesis. Some species of prokaryotes form endospores, thick-walled, dehydrated structures that can resist extreme dryness and very high temperatures for long periods of time. Anthrax, tetanus, and botulism are diseases caused by endospore-forming bacteria.
Certain prokaryotes move independently by using flagella, long structures that rotate in a propeller-like fashion. Prokaryotic flagella consist of intertwined fibrils (small fibers) of the protein flagellin. A prokaryote may have a single flagellum, a group of flagella at one or both poles of the cell, or may be covered with flagella. Many species of prokaryotes also have pili (singular, pilus)– slender, hairlike extensions used for attachment to soil, rocks, teeth, or other structures.
Origin of Prokaryotes In 1862, Pasteur disproved the spontaneous-generation theory but left open a question: How did life begin? Millers synthesis is a possible answer, or it may be the seeding of organic molecules by meteorites from outer space, or a God event that started life. It is generally held that the first organisms were formed around four billion years ago, with the earliest forms being simple molecular groupings that somehow gained the ability to metabolize and reproduce. It is also held that these simple molecular arrangements formed from existing inorganic substances—life from nonlife!
Evolution of Prokaryotes Prokaryotes are mostly bacteria, and their advancements led to more complex living organisms. It has been suggested that the diverse nature of bacteria and archaebacteria resulted from this evolution. As bacteria modified structures to expand their territory and tolerance, they changed into newer species of bacteria with diverse structures and functions. Due to their uniqueness, bacteria are classified in their own kingdom! Advancements in the structure and function of prokaryotes continued to the juncture where two separate types are now identifiable: bacteria and archaea.
Eukaryotic Cells A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the Greek (eu, "good") and κάρυον (karyon, "nut" or "kernel"). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplasts and the Golgi apparatus. All large complex organisms are eukaryotes, including animals, plants and fungi. The group also includes many unicellular organisms. Eukaryotes appear to be monophyletic, and so make up one of the three domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things; even in a human body there are 10 times more microbes than human cells. However, due to their much larger size their collective worldwide biomass is estimated at about equal to that of prokaryotes.
Eukaryotic cells are complex structures that make up animal and human tissue. Eukaryotic cells are different from prokaryotes, which is the term given to bacterial cells. Eukaryotes are distinct from prokaryotes in that they have membrane bound organelles and DNA is contained within a nucleus. A eukaryote cell has several structures that help the cell maintain homeostasis, and provide energy and the mechanisms for protein synthesis.
Origin of Eukaryotes The origin of the eukaryotic cell is considered a milestone in the evolution of life, since they include all complex cells and almost all multicellular organisms. The timing of this series of events is hard to determine; Knoll (2006) suggests they developed approximately 1.6–2.1 billion years ago. Some acritarchs are known from at least 1650 million years ago, and the possible alga Grypania has been found as far back as 2100 million years ago.
Evolution of Eukaryotes Fossil records indicate that eukaryotes evolved from prokaryotes somewhere between 1.5 to 2 billion years ago. Two proposed pathways describe the invasion of prokaryote cells by two smaller prokaryote cells. They subsequently became successfully included as part of a now much larger cell with additional structures and capable of additional functions. • Endosymbiosis • Membrane infolding
Endosymbiosis Research conducted by Lynn Margulis at the University of Massachusetts supports the hypothesis that two separate mutually beneficial invasions of a prokaryote cell produced the modern-day mitochondria and chloroplast as eukaryotic organelles. In this model, ancestral mitochondria were small heterotrophs capable of using oxygen to perform cellular respiration and thereby create useful energy. They became part of a large cell either by direct invasion as an internal parasite or as an indigestible food source. Later, a second invasion brought ancestral chloroplasts, which are thought to be small, photosynthetic cyanobacteria. Modern-day supporting evidence for endosymbiosis shows that both the mitochondria and chloroplasts have their own genes, circular DNA and RNA, and reproduce by binary fission independent of the hosts cell cycle. They therefore appear to be more similar to prokaryotes than eukaryotes.
Membrane Infolding The invasions of the host prokaryote cell probably were successful because the host cell membrane infolded to surround both invading prokaryote cells and thereby help transport them into the cell. The membrane did not dissolve but remained intact, and thereby created a second membrane around the protomitochondria and protochloroplast. It is also known that in modern-day eukaryotes the inner membrane of both the mitochondria and chloroplast contain structures more similar to prokaryotes than eukaryotes, whereas the outer membrane retains eukaryote characteristics! It is also suggested that continued membrane infolding created the endomembrane system. It can be said that possibly the first eukaryotic cell type was miraculously born from prokaryotic, symbiotic, multicellular interactions.
From prokaryotes to eukaryotes Living things have evolved into three large clusters of closely related organisms, called "domains": Archaea, Bacteria, and Eukaryota. Archaea and Bacteria are small, relatively simple cells surrounded by a membrane and a cell wall, with a circular strand of DNA containing their genes. They are called prokaryotes.
Virtually all the life we see each day — including plants and animals — belongs to the third domain, Eukaryota. Eukaryotic cells are more complex than prokaryotes, and the DNA is linear and found within a nucleus. Eukaryotic cells boast their own personal "power plants", calledmitochondria. These tiny organelles in the cell not only produce chemical energy, but also hold the key to understanding the evolution of the eukaryotic cell.