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Basic Unit of Life

Cell Origins

Evolution of Life on Earth

All cells descended from an early ancestral cell.

The formation of living things required the presence of three types of molecules: genetic molecules such as DNA and RNA, proteins to synthesize other molecules, and lipids to form the membranes to enclose cells. How these complex molecules first formed is a topic of ongoing scientific research. Scientists look to the conditions of planet Earth when it was first formed. The early Earth was hot and water condensed on the planet as it cooled. Energy for chemical reactions was available from the ultraviolet (UV) radiation in unfiltered sunlight and from electrical storms. The atmosphere of the early Earth is thought to have contained hydrogen cyanide (HCN) from comets and hydrogen sulfide (H2S) from volcanic eruptions, and these simpler molecules have been shown to be possible building blocks for more-complex organic molecules. Scientific research suggests that these basic components could possibly have formed molecules that were in turn the precursors to nucleic acids, amino acids, and lipids. However, formation of nucleic acids, amino acids, monosaccharides, and fatty acids produced by these processes would have created racemic mixtures. Such racemic mixtures differ in their optical and some chemical properties, making formation of more-complex molecules in the harsh environment caused by intense UV radiation and electrical storms extremely difficult. Ribonucleic acid (RNA), once formed, can catalyze the formation of additional RNA molecules. It can also promote the polymerization of amino acid chains, forming proteins.

Evidence suggests that the first cell likely arose when these reactions were enclosed in a membrane made of phospholipid molecules. These phospholipids had a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. By forming a phospholipid bilayer, a membrane made of these molecules could have possibly enclosed an aqueous interior. This eventually became the cytoplasm of cells, containing proteins and nucleic acids. A genetic code developed, in which a different sequence of three RNA nucleotides (the individual molecules of the RNA polymer) corresponded to a specific amino acid. RNA could thus orchestrate the synthesis of proteins according to this genetic code. All modern cells use essentially the same code for protein synthesis. The similarity across different organisms of such a fundamental function is a line of evidence indicating that all organisms trace their heritage back to a single ancestral cell line. For example, humans and chimpanzees share 99% of their genetic code.

As cells evolved and became more complex, DNA replaced RNA as the molecule for storing hereditary information. DNA is more stable than RNA because of slight chemical differences and because of its double-stranded helical structure. In all modern cells, DNA is the repository for the genetic information. DNA directs the formation of RNA, which in turn directs protein synthesis.

Origins of Prokaryotic and Eukaryotic Cells

According to the endosymbiotic theory, some ancient prokaryotes took up residence inside other cells, giving rise to the first eukaryotic cells millions of years ago.

The first cells, ancestral prokaryotes lacking a nucleus, gave rise to organisms of the domains Archaea and Bacteria. The earliest fossils of prokaryotic cells include stromatolites, fossilized mats of cyanobacteria, a group of bacteria capable of photosynthesis, that are between 3.5 and 3.7 billion years old. Scientists have found even older fossils, from nearly 3.8 billion years ago, that may indicate early prokaryotes also existed in the ocean, near ancient deep-sea vents. The structure of the prokaryote is relatively simple. It has a cell membrane but no membrane-bound organelles. Its DNA, occupying a region called the nucleoid, is in contact with the cytoplasm.

Prokaryotic cells developed a variety of different metabolic pathways, enabling them to succeed in diverse habitats. Glycolysis, in which glucose is broken down in the absence of oxygen to produce ATP, is thought to be among the earliest metabolic processes. This pathway is common to all modern living cells. Some ancient bacteria evolved the ability to produce organic molecules using nitrogen gas (N2) and carbon dioxide gas (CO2), through the process of photosynthesis. These cyanobacteria are named for the color that their photosynthetic pigment gives them. This process formed oxygen as a byproduct, which slowly oxygenated Earth's atmosphere. As the concentration of oxygen in the atmosphere increased, it became advantageous for some bacteria to use this reactive chemical to enhance their metabolic pathways. The process of aerobic cellular respiration, breaking down glucose in the presence of oxygen, provides the cell with more energy per glucose molecule. While glycolysis provides the cell with 2 ATP for every glucose molecule, aerobic cellular respiration yields 36 ATP per glucose molecule.

The first eukaryote, an organism characterized by membrane-bound organelles, such as the nucleus, evolved more recently. There are few fossils of early eukaryotes, but scientists think they arose around 2 billion years ago. The cell membrane is fluid and flexible. Parts of the membrane can pinch off and move into the cell to be used for transport. The first cell nucleus is thought to have been formed in a similar manner, when a cell membrane infolding enclosed the genetic material. Other membrane-bound organelles, such as the endoplasmic reticulum, are also thought to have been formed by additional infoldings. The endoplasmic reticulum (ER) is a network of membranes that helps process molecules in a cell and transports cell materials. The formation of the first eukaryotes is thought to be the result of these cell membrane infoldings.

According to the endosymbiotic theory, other organelles were originally separate organisms that were engulfed by early eukaryotic cells. Some prokaryotic cells had evolved to function in an anaerobic (non-oxygen-containing) environment and were at a disadvantage as the atmosphere became more oxygenated. Such cells are thought to have entered into a symbiotic relationship, one in which both organisms benefit from the other, with aerobic (oxygen-using) prokaryotes, giving rise to the first eukaryotes.

This process of endosymbiosis, a symbiotic relationship in which one organism lives inside another, is thought to be the origin of the chloroplast and the mitochondrion. The mitochondrion (plural, mitochondria) is an organelle that changes energy from food into energy a cell can use. Each of these organelles has its own DNA, has a double membrane, and reproduces independently. This evidence supports the idea that they were once independent organisms. Chloroplasts are similar in size and appearance to modern photosynthetic bacteria. Likewise, mitochondria are similar in size and appearance to modern aerobic bacteria. These two eukaryotic organelles provide energy to eukaryotic cells. The chloroplast, a light-capturing organelle, stores energy in sugars through photosynthesis, and the mitochondrion converts sugars to cellular energy through respiration. Mitochondria and chloroplasts are thought to be the result of primary endosymbiosis, in which one cell engulfs another and the two cells become interdependent. The endosymbiosis of proteobacteria (a type of gram-negative bacteria) is proposed to have resulted in ancestral eukaryotic cells; over time, the endosymbiont proteobacteria became mitochondria. The presence of mitochondria in all eukaryotes, including plants, suggests that this endosymbiosis was established first. A separate endosymbiosis of cyanobacteria is thought to have resulted in the development of chloroplasts. Chloroplasts are present only in plants, suggesting that this event took place later in the evolution of eukaryotes. Furthermore, some cells that have engulfed and incorporated other cells are later engulfed themselves. This process is called secondary endosymbiosis.

Mechanism of Serial Endosymbiosis

Origins of eukaryotic cells from prokaryotic cells suggest two pathways. The development of eukaryotes from prokaryotic ancestors involved the infolding of various membranes to form the nucleus and endomembrane system. The endosymbiotic theory suggests the mitochondrion and the chloroplast were originally independent prokaryotes that entered an early eukaryotic cell and became symbiotic with it.

Comparing Prokaryotic and Eukaryotic Cells

Cells of prokaryotes and eukaryotes have fundamental structural differences.

Prokaryotic and eukaryotic cells have several structures and functions in common. All cells include a cell membrane, cytoplasm, ribosomes, and genetic material. There are also important structural differences. Prokaryotic cells lack a nucleus, which is a membrane-bound organelle that contains most of the genetic material (DNA). This structure directs a cell's growth, division, and death. They also lack other membrane systems, such as the endoplasmic reticulum and membrane-bound organelles such as mitochondria. Furthermore, they are much smaller and simpler than eukaryotic cells. Prokaryotic organisms are all single-celled, and most have a cell wall surrounding their cell membrane. A cell wall is a rigid carbohydrate structure that provides overall support and protection for the cell. This cell wall is made from a carbohydrate/protein complex called peptidoglycan or, in Archaea, pseudopeptidoglycan. Prokaryotes carry their genes on a single, central, circular chromosome (the nucleoid), unprotected in the jellylike cytoplasm filling the interior of the cell. This nucleoid contains DNA, the genetic material that is passed from one generation to the next. Some prokaryotes also carry some genetic material in plasmids. A plasmid is a small, circular piece of bacterial DNA that replicates on its own and can be transferred between cells. As single-celled organisms, prokaryotes can reproduce asexually via simple cell division. Prokaryotic cell division consists of replication of the single chromosome followed by a simple split of the cell, a process known as binary fission. Some prokaryotes can reproduce by transferring plasmids from one to another via a process called conjugation.

Eukaryotic cells are more complex than prokaryotic cells. They are characterized by membrane-bound structures, called organelles, that perform specific tasks. Eukaryotes have a nucleus, a double membrane–bound organelle that contains most of the genetic material (DNA). Eukaryotic DNA molecules are linear and are organized on multiple chromosomes. The mitochondria provide cellular energy and (in plants) chloroplasts build sugars in eukaryotic cells. Other organelles include the endoplasmic reticulum and Golgi apparatus, which attaches chemical markers to molecules produced in the endoplasmic reticulum in order to transport the molecules. Other organelles, such as lysosomes and peroxisomes, specialize in breaking down materials. A lysosome is an organelle that digests bacteria that enter a cell, eliminates toxins, and recycles worn cell materials. A peroxisome is a structure that transforms fatty acids into sugars and aids chloroplasts in oxidizing plant sugars.

Eukaryotic organisms may be single-celled or multicellular, and they include plants, animals, fungi, and protists. Eukaryotic cells of the plant and fungi kingdoms (and some in kingdom Protista) have cell walls, but those of the animal kingdom do not. There are two different types of cell division in eukaryotes. Mitosis produces cells that are identical to the parent cell, and meiosis produces cells with half the number of chromosomes, for sexual reproduction.

Structural Differences between Prokaryotes and Eukaryotes
Prokaryotes Eukaryotes
Domain Bacteria and Archaea Eukarya
Nucleus No, nucleoid only Yes, membrane-bound nucleus
DNA Circular, on single chromosomal DNA and plasmids Linear, on multiple chromosomes, organized on histone proteins
Reproduction Binary fission, conjugation Mitosis (asexual), meiosis (sexual)
Cell membrane Lipid bilayer, with associated proteins Lipid bilayer, with associated proteins
Cell wall Bacteria: peptidoglycan Archaea: pseudopeptidoglycan Plants: cellulose
Fungi: chitin
Animals: no cell wall
Protists: most have no cell wall
Organelles None Nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria; may include lysosomes, peroxisomes, chloroplasts
Cell size range 0.1–5.0 microns 10–100 microns
Examples Bacteria: Vibrio cholerae, which causes the disease cholera in humans
Archaea: Methanobacterium, which lives in the rumen of cows and produces methane gas
Plant: Elodea, a popular aquarium plant
Fungi: Saccharomyces cerevisiae, a single-celled fungal cell known as yeast and used in baking bread
Animal: human skin cell, an epithelial cell
Protist: Paramecium, one of the first cells seen under a microscope

Prokaryotic and eukaryotic cells have several structural differences. Generally, eukaryotic cells are larger and more complex. They contain organelles, which are absent from prokaryotes.