Which Cell Structure Is Thought To Have Risen Through An Endosymbiosis Animal Cell

Cells are divided into 2 principal classes, initially defined by whether they contain a nucleus. Prokaryotic cells (bacteria) lack a nuclear envelope; eukaryotic cells take a nucleus in which the genetic material is separated from the cytoplasm. Prokaryotic cells are more often than not smaller and simpler than eukaryotic cells; in addition to the absenteeism of a nucleus, their genomes are less complex and they do not comprise cytoplasmic organelles or a cytoskeleton (Tabular array ane.ane). In spite of these differences, the same basic molecular mechanisms govern the lives of both prokaryotes and eukaryotes, indicating that all present-twenty-four hours cells are descended from a single primordial antecedent. How did this first jail cell develop? And how did the complication and multifariousness exhibited past present-day cells evolve?

The First Cell

It appears that life first emerged at least iii.8 billion years agone, approximately 750 million years subsequently Globe was formed (Figure i.ane). How life originated and how the first cell came into being are matters of speculation, since these events cannot be reproduced in the laboratory. Nonetheless, several types of experiments provide important evidence begetting on some steps of the process.

Figure 1.1

Time scale of development. The scale indicates the guess times at which some of the major events in the evolution of cells are thought to have occurred.

It was get-go suggested in the 1920s that simple organic molecules could form and spontaneously polymerize into macromolecules under the conditions thought to be in primitive Earth's atmosphere. At the time life arose, the atmosphere of Earth is thought to take contained fiddling or no free oxygen, instead consisting principally of CO₂ and N_ii in addition to smaller amounts of gases such equally H_two, H_twoS, and CO. Such an atmosphere provides reducing conditions in which organic molecules, given a source of free energy such every bit sunlight or electrical discharge, can class spontaneously. The spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s, when Stanley Miller (then a graduate pupil) showed that the discharge of electric sparks into a mixture of H_two, CH_iv, and NH₃, in the presence of h2o, led to the formation of a variety of organic molecules, including several amino acids (Effigy i.2). Although Miller's experiments did not precisely reproduce the conditions of primitive Earth, they conspicuously demonstrated the plausibility of the spontaneous synthesis of organic molecules, providing the basic materials from which the first living organisms arose.

Figure ane.two

Spontaneous germination of organic molecules. Water vapor was refluxed through an atmosphere consisting of CH_iv, NH_iii, and H₂, into which electrical sparks were discharged. Analysis of the reaction products revealed the formation of a diversity of organic molecules, (more...)

The next step in development was the formation of macromolecules. The monomeric edifice blocks of macromolecules have been demonstrated to polymerize spontaneously under plausible prebiotic conditions. Heating dry out mixtures of amino acids, for example, results in their polymerization to form polypeptides. But the critical feature of the macromolecule from which life evolved must have been the power to replicate itself. But a macromolecule capable of directing the synthesis of new copies of itself would accept been capable of reproduction and further evolution.

Of the two major classes of informational macromolecules in present-twenty-four hours cells (nucleic acids and proteins), but the nucleic acids are capable of directing their own cocky-replication. Nucleic acids can serve as templates for their own synthesis as a result of specific base of operations pairing between complementary nucleotides (Effigy one.three). A critical step in understanding molecular development was thus reached in the early 1980s, when it was discovered in the laboratories of Sid Altman and Tom Cech that RNA is capable of catalyzing a number of chemic reactions, including the polymerization of nucleotides. RNA is thus uniquely able both to serve as a template for and to catalyze its own replication. Consequently, RNA is generally believed to have been the initial genetic organization, and an early stage of chemical development is thought to have been based on self-replicating RNA molecules—a menstruation of evolution known as the RNA world. Ordered interactions between RNA and amino acids then evolved into the present-twenty-four hours genetic code, and Dna eventually replaced RNA as the genetic material.

Figure 1.3

Self-replication of RNA. Complementary pairing between nucleotides (adenine [A] with uracil [U] and guanine [G] with cytosine [C]) allows i strand of RNA to serve as a template for the synthesis of a new strand with the complementary sequence.

The starting time prison cell is presumed to have arisen by the enclosure of self-replicating RNA in a membrane composed of phospholipids (Effigy i.four). Every bit discussed in item in the next affiliate, phospholipids are the bones components of all present-twenty-four hours biological membranes, including the plasma membranes of both prokaryotic and eukaryotic cells. The key characteristic of the phospholipids that form membranes is that they are amphipathic molecules, pregnant that one portion of the molecule is soluble in water and another portion is not. Phospholipids take long, water-insoluble (hydrophobic) hydrocarbon bondage joined to water-soluble (hydrophilic) caput groups that comprise phosphate. When placed in water, phospholipids spontaneously aggregate into a bilayer with their phosphate-containing head groups on the outside in contact with water and their hydrocarbon tails in the interior in contact with each other. Such a phospholipid bilayer forms a stable barrier betwixt two aqueous compartments—for case, separating the interior of the cell from its external environment.

Figure i.4

Enclosure of self-replicating RNA in a phospholipid membrane. The offset cell is thought to take arisen by the enclosure of self-replicating RNA and associated molecules in a membrane composed of phospholipids. Each phospholipid molecule has ii long hydrophobic (more...)

The enclosure of self-replicating RNA and associated molecules in a phospholipid membrane would thus take maintained them every bit a unit, capable of self-reproduction and further evolution. RNA-directed poly peptide synthesis may already have evolved past this time, in which case the commencement cell would have consisted of cocky-replicating RNA and its encoded proteins.

The Evolution of Metabolism

Because cells originated in a sea of organic molecules, they were able to obtain food and energy directly from their environment. But such a situation is cocky-limiting, so cells needed to evolve their own mechanisms for generating energy and synthesizing the molecules necessary for their replication. The generation and controlled utilization of metabolic energy is central to all jail cell activities, and the principal pathways of energy metabolism (discussed in item in Chapter 2) are highly conserved in present-day cells. All cells use adenosine 5′-triphosphate (ATP) equally their source of metabolic energy to drive the synthesis of cell constituents and deport out other free energy-requiring activities, such as move (e.chiliad., musculus contraction). The mechanisms used by cells for the generation of ATP are thought to have evolved in three stages, corresponding to the evolution of glycolysis, photosynthesis, and oxidative metabolism (Figure ane.5). The development of these metabolic pathways changed Earth's atmosphere, thereby altering the course of farther evolution.

Figure i.five

Generation of metabolic energy. Glycolysis is the anaerobic breakdown of glucose to lactic acid. Photosynthesis utilizes energy from sunlight to drive the synthesis of glucose from CO_two and H₂O, with the release of O₂ every bit a by-production. The O_two released by (more...)

In the initially anaerobic temper of Globe, the beginning energy-generating reactions presumably involved the breakdown of organic molecules in the absence of oxygen. These reactions are likely to have been a form of nowadays-mean solar day glycolysis—the anaerobic breakdown of glucose to lactic acrid, with the net energy gain of 2 molecules of ATP. In addition to using ATP as their source of intracellular chemic energy, all present-day cells carry out glycolysis, consistent with the notion that these reactions arose very early in evolution.

Glycolysis provided a mechanism by which the energy in preformed organic molecules (e.g., glucose) could be converted to ATP, which could then be used every bit a source of energy to drive other metabolic reactions. The development of photosynthesis is generally idea to accept been the next major evolutionary step, which allowed the cell to harness energy from sunlight and provided independence from the utilization of preformed organic molecules. The first photosynthetic bacteria, which evolved more than iii billion years ago, probably utilized H₂S to catechumen CO₂ to organic molecules—a pathway of photosynthesis nevertheless used by some bacteria. The employ of H_iiO as a donor of electrons and hydrogen for the conversion of CO₂ to organic compounds evolved after and had the of import consequence of changing Globe'southward temper. The use of H₂O in photosynthetic reactions produces the by-production free O₂; this machinery is thought to take been responsible for making O₂ arable in Earth'due south atmosphere.

The release of O₂ equally a event of photosynthesis changed the environment in which cells evolved and is commonly thought to have led to the development of oxidative metabolism. Alternatively, oxidative metabolism may have evolved before photosynthesis, with the increment in atmospheric O₂ and so providing a stiff selective advantage for organisms capable of using O_two in energy-producing reactions. In either case, O_ii is a highly reactive molecule, and oxidative metabolism, utilizing this reactivity, has provided a mechanism for generating energy from organic molecules that is much more than efficient than anaerobic glycolysis. For instance, the complete oxidative breakdown of glucose to CO₂ and H₂O yields energy equivalent to that of 36 to 38 molecules of ATP, in contrast to the 2 ATP molecules formed past anaerobic glycolysis. With few exceptions, present-day cells use oxidative reactions equally their principal source of free energy.

Present-Day Prokaryotes

Present-solar day prokaryotes, which include all the various types of bacteria, are divided into two groups—the archaebacteria and the eubacteria—which diverged early in evolution. Some archaebacteria live in extreme environments, which are unusual today merely may have been prevalent in primitive Earth. For example, thermoacidophiles alive in hot sulfur springs with temperatures as high as 80°C and pH values as low as 2. The eubacteria include the common forms of present-solar day bacteria—a big group of organisms that alive in a broad range of environments, including soil, h2o, and other organisms (east.thousand., human pathogens).

Most bacterial cells are spherical, rod-shaped, or screw, with diameters of i to ten μm. Their DNA contents range from most 0.6 million to 5 one thousand thousand base pairs, an amount sufficient to encode virtually 5000 different proteins. The largest and most complex prokaryotes are the cyanobacteria, bacteria in which photosynthesis evolved.

The structure of a typical prokaryotic cell is illustrated by Escherichia coli (E. coli), a mutual inhabitant of the human abdominal tract (Figure i.half-dozen). The cell is rod-shaped, virtually 1 μm in diameter and about 2 μm long. Like most other prokaryotes, East. coli is surrounded by a rigid cell wall composed of polysaccharides and peptides. Inside the jail cell wall is the plasma membrane, which is a bilayer of phospholipids and associated proteins. Whereas the cell wall is porous and readily penetrated by a diverseness of molecules, the plasma membrane provides the functional separation betwixt the inside of the cell and its external environment. The Dna of Eastward. coli is a unmarried circular molecule in the nucleoid, which, in dissimilarity to the nucleus of eukaryotes, is not surrounded by a membrane separating it from the cytoplasm. The cytoplasm contains approximately 30,000 ribosomes (the sites of protein synthesis), which business relationship for its granular advent.

Figure one.6

Electron micrograph of E. coli. The cell is surrounded by a cell wall, inside which is the plasma membrane. DNA is located in the nucleoid. (Menge and Wurtz/Biozentrum, University of Basel/Science Photo Library/Photo Researchers, Inc.)

Eukaryotic Cells

Like prokaryotic cells, all eukaryotic cells are surrounded past plasma membranes and contain ribosomes. All the same, eukaryotic cells are much more complex and incorporate a nucleus, a diversity of cytoplasmic organelles, and a cytoskeleton (Figure ane.7). The largest and most prominent organelle of eukaryotic cells is the nucleus, with a diameter of approximately 5 μm. The nucleus contains the genetic information of the cell, which in eukaryotes is organized every bit linear rather than circular DNA molecules. The nucleus is the site of Deoxyribonucleic acid replication and of RNA synthesis; the translation of RNA into proteins takes place on ribosomes in the cytoplasm.

Figure 1.vii

Structures of animal and constitute cells. Both animal and plant cells are surrounded by a plasma membrane and contain a nucleus, a cytoskeleton, and many cytoplasmic organelles in mutual. Institute cells are also surrounded past a cell wall and contain chloroplasts (more than...)

In addition to a nucleus, eukaryotic cells contain a variety of membrane-enclosed organelles within their cytoplasm. These organelles provide compartments in which dissimilar metabolic activities are localized. Eukaryotic cells are generally much larger than prokaryotic cells, frequently having a cell volume at least a thousandfold greater. The compartmentalization provided by cytoplasmic organelles is what allows eukaryotic cells to function efficiently. 2 of these organelles, mitochondria and chloroplasts, play critical roles in energy metabolism. Mitochondria, which are found in almost all eukaryotic cells, are the sites of oxidative metabolism and are thus responsible for generating nigh of the ATP derived from the breakdown of organic molecules. Chloroplasts are the sites of photosynthesis and are establish only in the cells of plants and green algae. Lysosomes and peroxisomes likewise provide specialized metabolic compartments for the digestion of macromolecules and for diverse oxidative reactions, respectively. In addition, most establish cells comprise large vacuoles that perform a multifariousness of functions, including the digestion of macromolecules and the storage of both waste product products and nutrients.

Because of the size and complexity of eukaryotic cells, the transport of proteins to their correct destinations within the cell is a formidable task. Two cytoplasmic organelles, the endoplasmic reticulum and the Golgi apparatus, are specifically devoted to the sorting and send of proteins destined for secretion, incorporation into the plasma membrane, and incorporation into lysosomes. The endoplasmic reticulum is an extensive network of intracellular membranes, extending from the nuclear membrane throughout the cytoplasm. It functions not only in the processing and transport of proteins, but also in the synthesis of lipids. From the endoplasmic reticulum, proteins are transported within small-scale membrane vesicles to the Golgi apparatus, where they are farther candy and sorted for transport to their final destinations. In addition to this role in protein transport, the Golgi apparatus serves as a site of lipid synthesis and (in institute cells) as the site of synthesis of some of the polysaccharides that compose the cell wall.

Eukaryotic cells have another level of internal organization: the cytoskeleton, a network of poly peptide filaments extending throughout the cytoplasm. The cytoskeleton provides the structural framework of the cell, determining jail cell shape and the general organization of the cytoplasm. In add-on, the cytoskeleton is responsible for the movements of entire cells (e.g., the contraction of musculus cells) and for the intracellular transport and positioning of organelles and other structures, including the movements of chromosomes during jail cell partitioning.

The eukaryotes adult at least 2.7 billion years ago, following some 1 to 1.5 billion years of prokaryotic evolution. Studies of their DNA sequences betoken that the archaebacteria and eubacteria are as different from each other as either is from present-twenty-four hour period eukaryotes. Therefore, a very early event in evolution appears to have been the departure of three lines of descent from a common ancestor, giving rise to present-twenty-four hour period archaebacteria, eubacteria, and eukaryotes. Interestingly, many archaebacterial genes are more similar to those of eukaryotes than to those of eubacteria, indicating that the archaebacteria and eukaryotes share a common line of evolutionary descent and are more than closely related to each other than either is to the eubacteria (Figure 1.8).

Figure 1.8

Evolution of cells. Nowadays-day cells evolved from a common prokaryotic ancestor along 3 lines of descent, giving ascent to archaebacteria, eubacteria, and eukaryotes. Mitochondria and chloroplasts originated from the endosymbiotic association of aerobic (more...)

A critical step in the evolution of eukaryotic cells was the conquering of membrane-enclosed subcellular organelles, assuasive the evolution of the complexity characteristic of these cells. The organelles are idea to accept been caused as a result of the association of prokaryotic cells with the ancestor of eukaryotes.

The hypothesis that eukaryotic cells evolved from a symbiotic association of prokaryotes—endosymbiosis—is peculiarly well supported by studies of mitochondria and chloroplasts, which are idea to have evolved from leaner living in large cells. Both mitochondria and chloroplasts are similar to bacteria in size, and like leaner, they reproduce past dividing in ii. Most important, both mitochondria and chloroplasts contain their ain DNA, which encodes some of their components. The mitochondrial and chloroplast DNAs are replicated each fourth dimension the organelle divides, and the genes they encode are transcribed inside the organelle and translated on organelle ribosomes. Mitochondria and chloroplasts thus contain their own genetic systems, which are singled-out from the nuclear genome of the cell. Furthermore, the ribosomes and ribosomal RNAs of these organelles are more than closely related to those of bacteria than to those encoded by the nuclear genomes of eukaryotes.

An endosymbiotic origin for these organelles is now generally accepted, with mitochondria thought to have evolved from aerobic leaner and chloroplasts from photosynthetic bacteria, such as the cyanobacteria. The acquisition of aerobic bacteria would have provided an anaerobic prison cell with the power to conduct out oxidative metabolism. The acquisition of photosynthetic bacteria would accept provided the nutritional independence afforded by the power to perform photosynthesis. Thus, these endosymbiotic associations were highly advantageous to their partners and were selected for in the grade of evolution. Through time, near of the genes originally present in these leaner apparently became incorporated into the nuclear genome of the cell, so simply a few components of mitochondria and chloroplasts are still encoded by the organelle genomes.

The Evolution of Multicellular Organisms

Many eukaryotes are unicellular organisms that, like bacteria, consist of only single cells capable of self-replication. The simplest eukaryotes are the yeasts. Yeasts are more than complex than bacteria, just much smaller and simpler than the cells of animals or plants. For instance, the commonly studied yeast Saccharomyces cerevisiae is about 6 μm in diameter and contains 12 1000000 base pairs of Deoxyribonucleic acid (Figure 1.ix). Other unicellular eukaryotes, however, are far more than complex cells, some containing every bit much DNA as human cells have (Table 1.ii). They include organisms specialized to perform a diversity of tasks, including photosynthesis, movement, and the capture and ingestion of other organisms as nutrient. Amoeba proteus, for instance, is a large, circuitous cell. Its volume is more than than 100,000 times that of E. coli, and its length can exceed 1 mm when the prison cell is fully extended (Effigy 1.10). Amoebas are highly mobile organisms that employ cytoplasmic extensions, called pseudopodia, to movement and to engulf other organisms, including leaner and yeasts, as food. Other unicellular eukaryotes (the green algae) contain chloroplasts and are able to carry out photosynthesis.

Effigy 1.nine

Scanning electron micrograph of Saccharomyces cerevisiae. Artificial color has been added to the micrograph. (Andrew Syed/Science Photo Library/ Photograph Researchers, Inc.)

Figure i.x

Lite micrograph of Amoeba proteus. (1000. I. Walker/Photo Researchers, Inc.)

Multicellular organisms evolved from unicellular eukaryotes at least i.7 billion years ago. Some unicellular eukaryotes form multicellular aggregates that appear to represent an evolutionary transition from single cells to multicellular organisms. For example, the cells of many algae (e.1000., the greenish alga Volvox) acquaintance with each other to class multicellular colonies (Figure ane.11), which are thought to have been the evolutionary precursors of present-solar day plants. Increasing cell specialization then led to the transition from colonial aggregates to truly multicellular organisms. Standing cell specialization and sectionalization of labor among the cells of an organism have led to the complexity and multifariousness observed in the many types of cells that make up present-day plants and animals, including human beings.

Figure 1.11

Colonial green algae. Individual cells of Volvox class colonies consisting of hollow balls in which hundreds or thousands of cells are embedded in a gelatinous matrix. (Cabisco/Visuals Unlimited.)

Plants are equanimous of fewer prison cell types than are animals, merely each dissimilar kind of plant jail cell is specialized to perform specific tasks required by the organism every bit a whole (Figure one.12). The cells of plants are organized into three principal tissue systems: ground tissue, dermal tissue, and vascular tissue. The ground tissue contains parenchyma cells, which behave out most of the metabolic reactions of the institute, including photosynthesis. Basis tissue as well contains ii specialized jail cell types (collenchyma cells and sclerenchyma cells) that are characterized by thick cell walls and provide structural support to the plant. Dermal tissue covers the surface of the constitute and is equanimous of epidermal cells, which grade a protective coat and let the absorption of nutrients. Finally, several types of elongated cells form the vascular system (the xylem and phloem), which is responsible for the ship of water and nutrients throughout the plant.

Figure 1.12

Light micrographs of representative plant cells. (A) Parenchyma cells, which are responsible for photosynthesis and other metabolic reactions. (B) Collenchyma cells, which are specialized for support and have thickened cell walls. (C) Epidermal cells (more...)

The cells found in animals are considerably more various than those of plants. The human trunk, for case, is composed of more than 200 different kinds of cells, which are generally considered to be components of five main types of tissues: epithelial tissue, connective tissue, blood, nervous tissue, and muscle (Figure 1.13). Epithelial cells form sheets that cover the surface of the torso and line the internal organs. At that place are many different types of epithelial cells, each specialized for a specific function, including protection (the skin), absorption (e.g., the cells lining the small intestine), and secretion (e.chiliad., cells of the salivary gland). Connective tissues include bone, cartilage, and adipose tissue, each of which is formed past different types of cells (osteoblasts, chondrocytes, and adipocytes, respectively). The loose connective tissue that underlies epithelial layers and fills the spaces between organs and tissues in the trunk is formed by another cell blazon, the fibroblast. Blood contains several different types of cells, which function in oxygen transport (red claret cells, or erythrocytes), inflammatory reactions (granulocytes, monocytes, and macrophages), and the allowed response (lymphocytes). Nervous tissue is composed of nervus cells, or neurons, which are highly specialized to transmit signals throughout the body. Diverse types of sensory cells, such as cells of the eye and ear, are further specialized to receive external signals from the environment. Finally, several dissimilar types of muscle cells are responsible for the product of force and movement.

Figure 1.13

Light micrographs of representative animal cells. (A) Epithelial cells of the mouth (a thick, multilayered sheet), bile duct, and intestine. (B) Fibroblasts are connective tissue cells characterized by their elongated spindle shape. (C) Erythrocytes, (more...)

The evolution of animals clearly involved the development of considerable diversity and specialization at the cellular level. Agreement the mechanisms that control the growth and differentiation of such a circuitous array of specialized cells, starting from a single fertilized egg, is 1 of the major challenges facing contemporary cell and molecular biology.

Source: https://www.ncbi.nlm.nih.gov/books/NBK9841/

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