Having taken a general survey of the building materials of living nature, microscopic unit masses of protoplasm, termed cells, we are now in a position to consider in some detail the structure of a typical cell. With the diversity of gross structure of animals and plants in mind,one is not surprised that there are considerable, even great, variations in their component elements. In fact the characteristics of an organism or part of an organism are determined by those of its cells. But there are certain generalized cell characters which are common to all cells by virtue of which they are cells and it is important to emphasize these.
In its simplest form a cell is a small spherical mass of protoplasm. Such are the eggs of various animals and the complete body of some of the lowest plants and animals.
Cells forming the units of multicellular organisms, however, frequently exhibit more or less hexagonal surfaces on account of stresses and strains incident to their position among other cells, while specializations and differentiations, for one purpose or another, produce forms which are characteristic of different parts of the organism, as, for example, the long spindle shaped cells of smooth muscle, and the widely branching cells of parts of the brains of animals. Broadly speaking, the greater diversity of cell form is found in animals, while in plants, owing to the more general presence of rigid cell walls about the protoplasm, the units more frequently present symmetrical, angular outlines.
The term cell is a relic of the time when the cell wall was regarded as the more important part, and its protoplasmic contents, if observed at all, were considered as only of secondary importance, if not a waste product. Now we recognize many cells which are merely naked masses of protoplasm, such as certain types of blood cells. In other words, the protoplasm is the essential living part the cell wall frequently being a non-living accessory which more or less sharply delineates one unit mass of protoplasm from another and lends rigidity and form to the group of cells as a whole.
With regard to the origin of life on the Earth absolutely nothing is known. But all the evidence at hand tends to show that, at the present time at least, living matter never arises except under the influence of preexisting living matter.
That is, protoplasm grows cells grow and, having attained a certain size, reproduce by dividing into two more or less equal parts. The process of cell division involves the division of both cytoplasm and nucleus, and therefore we must enlarge our conception of a cell as a small mass of protoplasm differentiated into cytoplasm and nucleus, by adding that both cytoplasm and nucleus arise through the division of the corresponding elements of a preexisting cell.
We shall later have occasion to make a study of the details of cell division, known as Mitosis, but from what has been said it must occur to the reader that, since cells arise only by division, those of the present day, whether complete free living organisms or units composing the bodies of higher animals and plants, are actually lineal descendants in unbroken series from the beginning of life on the Earth. The bond of discontinuity between parent and offspring is typically a single cell division.
The cell is the structural and functional unit of all livingorganisms, and is sometimes called the "building block of life."[1] Some organisms, such as bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an ostrichegg.
The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
The word cell comes from the Latincellula, a small room. The name was chosen by Robert Hooke when he compared the cork cells he saw to the small rooms monks lived in.[2]
Overview
Properties of cells
Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.
Metabolism, including taking in raw materials, building cell components, converting energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is derived from metabolic pathways.
Response to external and internal stimuli such as changes in temperature, pH or nutrient levels.
There are two Types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually singletons, while eukaryotic cells are usually found in multicellular organisms.
Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack most of the intracellular organelles and structures that are characteristic of eukaryotic cells (an important exception is the ribosomes, which are present in both prokaryotic and eukaryotic cells). Most functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili — proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:
The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
Most prokaryotes have a cell wall (some exceptions are Mycoplasma (a bacterium) and Thermoplasma (an archaeon)). It consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" (cytolysis) from osmotic pressure against a hypotonic environment. A cell wall is also present in some eukaryotes like fungi, but has a different chemical composition.
A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Even without a real nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.
Eukaryotic cells are about 10 times the size of a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true nucleus". Other differences include:
The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles also contain some DNA.
Eukaryotes can move using cilia or flagella. The flagella are more complex than those of prokaryotes.
Table 1: Comparison of features of prokaryotic and eukaryotic cells
The cells of eukaryotes (left) and prokaryotes (right).
All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a saltycytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article will list these primary components of the cell, then briefly describe their function.
The cytoplasm of a cell is surrounded by a plasma membrane. The plasma membrane in plants and prokaryotes is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (hydrophobic fat-like molecules) and hydrophilicphosphorus molecules. Hence, the layer is called a phospholipid bilayer. It may also be called a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signalling molecules such as hormones.
The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. Eukaryotic cytoskeleton is composed of microfilaments, intermediate filaments and microtubules. There is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments.
Genetic material
Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long-term information storage, but some viruses (e.g., retroviruses) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA) in organisms that use DNA for the genetic code itself.
Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).
A human cell has genetic material in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some important proteins.
Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is.
The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs," called organelles, that are adapted and/or specialized for carrying out one or more vital functions. Membrane-bound organelles are found only in eukaryotes.
Cell nucleus (a cell's information center)
The cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis occur. The nucleus is spheroid in shape and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm.
Mitochondria and Chloroplasts (the power generators)
Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell, they play a critical role in generating energy in the eukaryotic cell, organelles that are modified chloroplasts; they are broadly called plastids, and are often involved in storage.
Endoplasmic reticulum and Golgi apparatus (macromolecule managers)
The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface, and the smooth ER, which lacks them.
The ER contains many Ribosomes (the protein production machine)
The ribosome is a large complex composed of many molecules, in prokaryotes only exist floating freely in the cytosol, whereas in eukaryotes they can be either free or bound to membranes.
Lysosomes and Peroxisomes (of the eukaryotic cell)
The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.
Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba have contractile vacuoles, which are able to pump water out of the cell if there is too much water.
Between successive cell divisions, cells grow through the functioning of cellular metabolism.
Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.
Complex sugars consumed by the organism can be broken down into a less chemically-complex sugar molecule called glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form of energy, via two different pathways.
The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy.
The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.
An overview of protein synthesis. Within the nucleus of the cell (light blue), genes (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly-synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.
Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.
DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.
Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.
Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.
The origin of cells has to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.
Origin of the first cell
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If life is viewed from the point of view of replicators, that is DNA molecules in the organism, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. The latter is fundamental for the evolution of biological complexity. If freely-floating DNA molecules that code for enzymes are not enclosed into cells, the enzymes that benefit a given DNA molecule (for example, by producing nucleotides) will automatically benefit the neighbouring DNA molecules. This might be viewed as "parasitism by default." Therefore the selection pressure on DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: All molecules in a given neighbourhood are almost equally advantaged.
If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules won't benefit from a positive mutation in a neighbouring molecule: this in turn means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it.
(Note. This is more a metaphor given for simplicity than complete accuracy since the earliest molecules of life, probably up to the stage of cellular life, were most likely RNA molecules that acted as both replicators and enzymes: see RNA world hypothesis. However, the core of the reasoning is the same.)
Biochemically, cell-like spheroids formed by proteinoids are observed by heating amino acids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth.
Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water tend to burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The Phospholipid is a good example of a common oily compound prevalent in the prebiotic seas. Phospholipids can be constructed in one's mind as a hydrophilic head on one end, and a hydrophobic tail on the other. Phospholipids also possess an important characteristic, that is having the function to link together to form a bilayer membrane. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble [1] can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multi-cellular organisms could be achieved. This theory is expanded upon in the book, The Cell: Evolution of the First Organism by Joseph Panno Ph. D.
Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell – a theory termed the endosymbiotic theory.
There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or viceversa: see the hydrogen hypothesis for the origin of eukaryotic cells.
1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.[2]
1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
The belief that life forms are able to occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822–1895) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
↑ 2.02.1 "... I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular [..] these pores, or cells, [..] were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this. . ." – Hooke describing his observations on a thin slice of cork. Robert Hooke
↑ A. Rose, S. J. Schraegle, E. A. Stahlberg and I. Meier (2005) "Coiled-coil protein composition of 22 proteomes--differences and common themes in subcellular infrastructure and traffic control" in BMC evolutionary biology Vulume 5 article 66. Template:Entrez Pubmed Rose et al. suggest that coiled-coil alpha helical vesicle transport proteins are only found in eukaryotic organisms.
Landmark Papers in Cell Biology Gall JG, McIntosh JR, eds. Landmark Papers in Cell Biology. Bethesda, MD and Cold Spring Harbor, NY: The American Society for Cell Biology and Cold Spring Harbor Laboratory Press; 2001. Commentaries and links to original research papers published in the ASCB Image & Video Library Landmark Papers Collection in September 2006.
