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Cells
The following article was provide by encarta.com Cell
(biology) I.
INTRODUCTION ,
basic unit of life. Cells are the smallest structures capable of basic life
processes, such as taking in nutrients, expelling waste, and reproducing. All
living things are composed of cells. Some microscopic organisms, such as
bacteria and protozoa, are unicellular, meaning they consist of a single cell.
Plants, animals, and fungi are multicellular; that is, they are composed of a
great many cells working in concert. But whether it makes up an entire bacterium
or is just one of millions in a human being, the cell is a marvel of design and
efficiency. Cells carry out thousands of biochemical reactions each minute and
reproduce new cells that perpetuate life. Cells
vary considerably in size. The smallest cell, a type of bacterium known as a
mycoplasma, measures 0.0001 mm (0.000004 in) in diameter; 10,000 mycoplasmas in
a row are only as wide as the diameter of a human hair. Among the largest cells
are the nerve cells that run down a giraffe's neck; these cells can exceed 3 m
(9.7 ft) in length. Human cells also display a variety of sizes, from small red
blood cells that measure 0.00076 mm (0.00003 in) to liver cells that may be ten
times larger. About 10,000 average-sized human cells can fit on the head of a
pin. Along
with their differences in size, cells present an array of shapes. Some, such as
the bacterium Escherichia coli, resemble rods. The paramecium,
a type of protozoan, is slipper shaped; and the amoeba,
another protozoan, has an irregular form that changes shape as it moves around.
Plant cells typically resemble boxes or cubes. In humans, the outermost layers
of skin
cells are flat, while muscle
cells are long and thin. Some nerve cells, with their elongated, tentacle-like
extensions, suggest an octopus. In
multicellular organisms, shape is typically tailored to the cell's job. For
example, flat skin cells pack tightly into a layer that protects the underlying
tissues from invasion by bacteria. Long, thin muscle cells contract readily to
move bones. The numerous extensions from a nerve cell enable it to connect to
several other nerve cells in order to send and receive messages rapidly and
efficiently. By
itself, each cell is a model of independence and self-containment. Like some
miniature, walled city in perpetual rush hour, the cell constantly bustles with
traffic, shuttling essential molecules from place to place to carry out the
business of living. Despite their individuality, however, cells also display a
remarkable ability to join, communicate, and coordinate with other cells. The
human body, for example, consists of an estimated 20 to 30 trillion cells.
Dozens of different kinds of cells are organized into specialized groups called tissues.
Tendons and bones, for example, are composed of connective tissue, whereas skin
and mucous membranes are built from epithelial tissue. Different tissue types
are assembled into organs, which are structures specialized to perform
particular functions. Examples of organ include the heart, stomach, and brain.
Organs, in turn, are organized into systems such as the circulatory, digestive,
or nervous systems. All together, these assembled organ systems form the human
body. The
components of cells are molecules,
nonliving structures formed by the union of atoms. Small molecules serve as
building blocks for larger molecules. Proteins,
nucleic
acids, carbohydrates,
and lipids,
which include fats and oils, are the four major molecules that underlie cell
structure and also participate in cell functions. For example, a tightly
organized arrangement of lipids, proteins, and protein-sugar compounds forms the
plasma membrane, or outer boundary, of certain cells. The organelles,
membrane-bound compartments in cells, are built largely from proteins.
Biochemical reactions in cells are guided by enzymes, specialized proteins that
speed up chemical reactions. The nucleic acid deoxyribonucleic
acid (DNA) contains the hereditary information for cells, and another
nucleic acid, ribonucleic
acid(RNA), works with DNA to build the thousands of proteins the cell needs. II. CELL
STRUCTURE Cells
fall into one of two categories: prokaryotic or eukaryotic (see Prokaryote).
In a prokaryotic cell, found only in bacteria
and archaebacteria,
all the components, including the DNA, mingle freely in the cell's interior, a
single compartment. Eukaryotic cells, which make up plants, animals, fungi, and
all other life forms, contain numerous compartments, or organelles, within each
cell. The DNA in eukaryotic cells is enclosed in a special organelle called the nucleus,
which serves as the cell's command center and information library. The term prokaryote
comes from Greek words that mean "before, or pre-, nucleus," while eukaryote
means "true nucleus." A. Prokaryotic Cells
Prokaryotic
cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003
mm (0.000004 to 0.0001 in) in diameter. About a hundred typical prokaryotic
cells lined up in a row would match the thickness of a book page. These cells,
which can be rodlike, spherical, or spiral in shape, are surrounded by a
protective cell wall. Like most cells, prokaryotic cells live in a watery
environment, whether it is soil moisture, a pond, or the fluid surrounding cells
in the human body. Tiny pores in the cell wall enable water and the substances
dissolved in it, such as oxygen, to flow into the cell; these pores also allow
wastes to flow out. Pushed
up against the inner surface of the prokaryotic cell wall is a thin membrane
called the plasma membrane. The plasma membrane, composed of two layers of
flexible lipid molecules and interspersed with durable proteins, is both supple
and strong. Unlike the cell wall, whose open pores allow the unregulated traffic
of materials in and out of the cell, the plasma membrane is selectively
permeable, meaning it allows only certain substances to pass through. Thus, the
plasma membrane actively separates the cell's contents from its surrounding
fluids. While
small molecules such as water, oxygen, and carbon dioxide diffuse freely across
the plasma membrane, the passage of many larger molecules, including amino acids
(the building blocks of proteins) and sugars, is carefully regulated.
Specialized transport proteins accomplish this task. The transport proteins span
the plasma membrane, forming an intricate system of pumps and channels through
which traffic is conducted. Some substances swirling in the fluid around the
cell can enter it only if they bind to and are escorted in by specific transport
proteins. In this way, the cell fine-tunes its internal environment. The
plasma membrane encloses the cytoplasm, the semifluid that fills the cell.
Composed of about 65 percent water, the cytoplasm is packed with up to a billion
molecules per cell, a rich storehouse that includes enzymes and dissolved
nutrients, such as sugars and amino acids. The water provides a favorable
environment for the thousands of biochemical reactions that take place in the
cell. Within
the cytoplasm of all prokaryotes is deoxyribonucleic acid (DNA), a complex
molecule in the form of a double helix, a shape similar to a spiral staircase.
The DNA is about 1000 times the length of the cell, and to fit inside, it
repeatedly twists and folds to form a compact structure called a chromosome.
The chromosome in prokaryotes is circular, and is located in a region of the
cell called the nucleoid. Often, smaller chromosomes called plasmids are located
in the cytoplasm. The DNA is divided into units called genes, just like a long
train is divided into separate cars. Depending on the species, the DNA contains
several hundred or even thousands of genes. Typically, one gene contains coded
instructions for building all or part of a single protein. Enzymes, which are
specialized proteins, determine virtually all the biochemical reactions that
support and sustain the cell. Also
immersed in the cytoplasm are the only organelles in prokaryotic cells—tiny
bead-like structures called ribosomes.
These are the cell's protein factories. Following the instructions encoded in
the DNA, ribosomes churn out proteins by the hundreds every minute, providing
needed enzymes, the replacements for worn-out transport proteins, or other
proteins required by the cell. While
relatively simple in construction, prokaryotic cells display extremely complex
activity. They have a greater range of biochemical reactions than those found in
their larger relatives, the eukaryotic cells. The extraordinary biochemical
diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of
the archaebacteria and the bacteria, whose habitats include polar ice, deserts,
and hydrothermal
vents—deep regions of the ocean under great pressure where hot water
geysers erupt from cracks in the ocean floor. B. Eukaryotic Animal Cells Eukaryotic
cells are typically about ten times larger than prokaryotic cells. In animal
cells, the plasma membrane, rather than a cell wall, forms the cell's outer
boundary. With a design similar to the plasma membrane of prokaryotic cells, it
separates the cell from its surroundings and regulates the traffic across the
membrane. The
eukaryotic cell cytoplasm is similar to that of the prokaryote cell except for
one major difference: Eukaryotic cells house a nucleus and numerous other
membrane-enclosed organelles. Like separate rooms of a house, these organelles
enable specialized functions to be carried out efficiently. The building of
proteins and lipids, for example, takes place in separate organelles where
specialized enzymes geared for each job are located. The
nucleus is the largest organelle in an animal cell. It contains numerous strands
of DNA, the length of each strand being many times the diameter of the cell.
Unlike the circular prokaryotic DNA, long sections of eukaryotic DNA pack into
the nucleus by wrapping around proteins. As a cell begins to divide, each DNA
strand folds over onto itself several times, forming a rod-shaped chromosome. The
nucleus is surrounded by a double-layered membrane that protects the DNA from
potentially damaging chemical reactions that occur in the cytoplasm. Messages
pass between the cytoplasm and the nucleus through nuclear pores, which are
holes in the membrane of the nucleus. In each nuclear pore, molecular signals
flash back and forth as often as ten times per second. For example, a signal to
activate a specific gene comes in to the nucleus and instructions for production
of the necessary protein go out to the cytoplasm. Attached
to the nuclear membrane is an elongated membranous sac called the endoplasmic
reticulum. This organelle tunnels through the cytoplasm, folding back and
forth on itself to form a series of membranous stacks. Endoplasmic reticulum
takes two forms: rough and smooth. Rough endoplasmic reticulum (RER) is so
called because it appears bumpy under a microscope. The bumps are actually
thousands of ribosomes attached to the membrane's surface. The ribosomes in
eukaryotic cells have the same function as those in prokaryotic cells—protein
synthesis—but they differ slightly in structure. Eukaryote ribosomes bound to
the endoplasmic reticulum help assemble proteins that typically are exported
from the cell. The ribosomes work with other molecules to link amino acids to
partially completed proteins. These incomplete proteins then travel to the inner
chamber of the endoplasmic reticulum, where chemical modifications, such as the
addition of a sugar, are carried out. Chemical modifications of lipids are also
carried out in the endoplasmic reticulum. The
endoplasmic reticulum and its bound ribosomes are particularly dense in cells
that produce many proteins for export, such as the white blood cells of the
immune system, which produce and secrete antibodies. Some ribosomes that
manufacture proteins are not attached to the endoplasmic reticulum. These
so-called free ribosomes are dispersed in the cytoplasm and typically make
proteins—many of them enzymes—that remain in the cell. The
second form of endoplasmic reticulum, the smooth endoplasmic reticulum (SER),
lacks ribosomes and has an even surface. Within the winding channels of the
smooth endoplasmic reticulum are the enzymes needed for the construction of
molecules such as carbohydrates and lipids. The smooth endoplasmic reticulum is
prominent in liver cells, where it also serves to detoxify substances such as
alcohol, drugs, and other poisons. Proteins
are transported from free and bound ribosomes to the Golgi
apparatus, an organelle that resembles a stack of deflated balloons. It is
packed with enzymes that complete the processing of proteins. These enzymes add
sulfur or phosphorous atoms to certain regions of the protein, for example, or
chop off tiny pieces from the ends of the proteins. The completed protein then
leaves the Golgi apparatus for its final destination inside or outside the cell.
During its assembly on the ribosome, each protein has acquired a group of from 4
to 100 amino acids called a signal. The signal works as a molecular shipping
label to direct the protein to its proper location. Lysosomes
are small, often spherical organelles that function as the cell's recycling
center and garbage disposal. Powerful digestive enzymes concentrated in the
lysosome break down worn-out organelles and ship their building blocks to the
cytoplasm where they are used to construct new organelles. Lysosomes also
dismantle and recycle proteins, lipids, and other molecules. The
mitochondria
are the powerhouses of the cell. Within these long, slender organelles, which
can appear oval or bean shaped under the electron microscope, enzymes convert
the sugar glucose and other nutrients into adenosine
triphosphate (ATP). This molecule, in turn, serves as an energy battery for
countless cellular processes, including the shuttling of substances across the
plasma membrane, the building and transport of proteins and lipids, the
recycling of molecules and organelles, and the dividing of cells. Muscle and
liver cells are particularly active and require dozens and sometimes up to a
hundred mitochondria per cell to meet their energy needs. Mitochondria are
unusual in that they contain their own DNA in the form of a prokaryote-like
circular chromosome; have their own ribosomes, which resemble prokaryotic
ribosomes; and divide independently of the cell. Unlike
the tiny prokaryotic cell, the relatively large eukaryotic cell requires
structural support. The cytoskeleton, a dynamic network of protein tubes,
filaments, and fibers, crisscrosses the cytoplasm, anchoring the organelles in
place and providing shape and structure to the cell. Many components of the
cytoskeleton are assembled and disassembled by the cell as needed. During cell
division, for example, a special structure called a spindle is built to move
chromosomes around. After cell division, the spindle, no longer needed, is
dismantled. Some components of the cytoskeleton serve as microscopic tracks
along which proteins and other molecules travel like miniature trains. Recent
research suggests that the cytoskeleton also may be a mechanical communication
structure that converses with the nucleus to help organize events in the cell. C. Eukaryotic Plant Cells Plant
cells have all the components of animal cells and boast several added features,
including chloroplasts,
a central vacuole, and a cell wall. Chloroplasts convert light
energy—typically from the sun—into the sugar glucose, a form of chemical
energy, in a process known as photosynthesis. Chloroplasts, like mitochondria,
possess a circular chromosome and prokaryote-like ribosomes, which manufacture
the proteins that the chloroplasts typically need. The
central vacuole of a mature plant cell typically takes up most of the room in
the cell. The vacuole, a membranous bag, crowds the cytoplasm and organelles to
the edges of the cell. The central vacuole stores water, salts, sugars,
proteins, and other nutrients. In addition, it stores the blue, red, and purple
pigments that give certain flowers their colors. The central vacuole also
contains plant wastes that taste bitter to certain insects, thus discouraging
the insects from feasting on the plant. In
plant cells, a sturdy cell wall surrounds and protects the plasma membrane. Its
pores enable materials to pass freely into and out of the cell. The strength of
the wall also enables a cell to absorb water into the central vacuole and swell
without bursting. The resulting pressure in the cells provides plants with
rigidity and support for stems, leaves, and flowers. Without sufficient water
pressure, the cells collapse and the plant wilts. III. CELL
FUNCTIONS To
stay alive, cells must be able to carry out a variety of functions. Some cells
must be able to move, and most, divide. All cells must maintain the right
concentration of chemicals in their cytoplasm, ingest food and use it for
energy, recycle molecules, expel wastes, and construct proteins. Cells must also
be able to respond to changes in their environment. A. Movement Many
unicellular organisms swim, glide, thrash, or crawl to search for food and
escape enemies. Swimming organisms often move by means of a flagellum, a long
tail-like structure made of protein. Many bacteria, for example, have one, two,
or many flagella that rotate like propellers to drive the organism along. Some
single-celled eukaryotic organisms, such as euglena, also have a flagellum, but
it is longer and thicker than the prokaryotic flagellum. The eukaryotic
flagellum works by waving up and down like a whip. In higher animals, the sperm
cell uses a flagellum to swim toward the female egg for fertilization. Movement
in eukaryotes is also accomplished with cilia, short, hairlike proteins built by
centrioles, which are barrel-shaped structures located in the cytoplasm that
assemble and break down protein filaments. Typically, thousands of cilia extend
through the plasma membrane and cover the surface of the cell, giving it a
dense, hairy appearance. By beating its cilia as if they were oars, an organism
such as the paramecium propels itself through its watery environment. In cells
that do not move, cilia are used for other purposes. In the respiratory tract of
humans, for example, millions of ciliated cells prevent inhaled dust, smog, and
microorganisms from entering the lungs by sweeping them up on a current of mucus
into the throat, where they are swallowed. Eukaryotic flagella and cilia are
formed from basal bodies, small protein structures located just inside the
plasma membrane. Basal bodies also help to anchor flagella and cilia. Still
other eukaryotic cells, such as amoebas and white blood cells, move by amoeboid
motion, or crawling. They extrude their cytoplasm to form temporary pseudopodia,
or false feet, which actually are placed in front of the cell, rather like
extended arms. They then drag the trailing end of their cytoplasm up to the
pseudopodia. A cell using amoeboid motion would lose a race to a euglena or
paramecium. But while it is slow, amoeboid motion is strong enough to move cells
against a current, enabling water-dwelling organisms to pursue and devour prey,
for example, or white blood cells roaming the blood stream to stalk and engulf a
bacterium or virus. B. Nutrition All
cells require nutrients for energy, and they display a variety of methods for
ingesting them. Simple nutrients dissolved in pond water, for example, can be
carried through the plasma membrane of pond-dwelling organisms via a series of
molecular pumps. In humans, the cavity of the small intestine contains the
nutrients from digested food, and cells that form the walls of the intestine use
similar pumps to pull amino acids and other nutrients from the cavity into the
bloodstream. Certain unicellular organisms, such as amoebas, are also capable of
reaching out and grabbing food. They use a process known as endocytosis, in
which the plasma membrane surrounds and engulfs the food particle, enclosing it
in a sac, called a vesicle, that is within the amoeba's interior. C. Energy Cells
require energy for a variety of functions, including moving, building up and
breaking down molecules, and transporting substances across the plasma membrane.
Nutrients contains energy, but cells must convert the energy locked in nutrients
to another form—specifically, the ATP molecule, the cell's energy
battery—before it is useful. In single-celled eukaryotic organisms, such as
the paramecium, and in multicellular eukaryotic organisms, such as plants,
animals, and fungi, mitochondria are responsible for this task. The interior of
each mitochondrion consists of an inner membrane that is folded into a mazelike
arrangement of separate compartments called cristae. Within the cristae, enzymes
form an assembly line where the energy in glucose and other energy-rich
nutrients is harnessed to build ATP; thousands of ATP molecules are constructed
each second in a typical cell. In most eukaryotic cells, this process requires
oxygen and is known as aerobic respiration. Some
prokaryotic organisms also carry out aerobic respiration. They lack
mitochondria, however, and carry out aerobic respiration in the cytoplasm with
the help of enzymes sequestered there. Many prokaryote species live in
environments where there is little or no oxygen, environments such as mud,
stagnant ponds, or within the intestines of animals. Some of these organisms
produce ATP without oxygen in a process known as anaerobic respiration, where
sulfur or other substances take the place of oxygen. Still other prokaryotes,
and yeast, a single-celled eukaryote, build ATP without oxygen in a process
known as fermentation. Almost
all organisms rely on the sugar glucose to produce ATP. Glucose is made by the
process of photosynthesis, in which light energy is transformed to the chemical
energy of glucose. Animals and fungi cannot carry out photosynthesis and depend
on plants and other photosynthetic organisms for this task. In plants, as we
have seen, photosynthesis takes place in organelles called chloroplasts.
Chloroplasts contain numerous internal compartments called thylakoids where
enzymes aid in the energy conversion process. A single leaf cell contains 40 to
50 chloroplasts. With sufficient sunlight, one large tree is capable of
producing upwards of two tons of sugar in a single day. Photosynthesis in
prokaryotic organisms—typically aquatic bacteria—is carried out with enzymes
clustered in plasma membrane folds called chromatophores. Aquatic bacteria
produce the food consumed by tiny organisms living in ponds, rivers, lakes, and
seas. D. Protein Synthesis
A
typical cell must have on hand about 30,000 proteins at any one time. Many of
these proteins are enzymes needed to construct the major molecules used by
cells—carbohydrates, lipids, proteins, and nucleic acids—or to aid in the
breakdown of such molecules after they have worn out. Other proteins are part of
the cell's structure—the plasma membrane and ribosomes, for example. In
animals, proteins also function as hormones and antibodies, and they function
like delivery trucks to transport other molecules around the body. Hemoglobin,
for example, is a protein that transports oxygen in red blood cells. The cell's
demand for proteins never ceases. Before
a protein can be made, however, the molecular directions to build it must be
extracted from one or more genes. In humans, for example, one gene holds the
information for the protein insulin, the hormone that cells need to import
glucose from the bloodstream, while at least two genes hold the information for
collagen, the protein that imparts strength to skin, tendons, and ligaments. The
process of building proteins begins when enzymes, in response to a signal from
the cell, bind to the gene that carries the code for the required protein, or
part of the protein. The enzymes transfer the code to a new molecule called
messenger RNA, which carries the code from the nucleus to the cytoplasm. This
enables the original genetic code to remain safe in the nucleus, with messenger
RNA delivering small bits and pieces of information from the DNA to the
cytoplasm as needed. Depending on the cell type, hundreds or even thousands of
molecules of messenger RNA are produced each minute. Once
in the cytoplasm, the messenger RNA molecule links up with a ribosome. The
ribosome moves along the messenger RNA like a monorail car along a track,
stimulating another form of RNA—transfer RNA—to gather and link the
necessary amino acids, pooled in the cytoplasm, to form the specific protein, or
section of protein. The protein is modified as necessary by the endoplasmic
reticulum and Golgi apparatus before embarking on its mission. Cells teem with
activity as they forge the numerous, diverse proteins that are indispensable for
life. For a more detailed discussion about protein synthesis, see Genetics:
The Genetic Code. E. Cell Division Most
cells divide at some time during their life cycle, and some divide dozens of
times before they die. Organisms rely on cell division for reproduction, growth,
and repair and replacement of damaged or worn out cells. Three types of cell
division occur: binary fission, mitosis,
and meiosis.
Binary fission, the method used by prokaryotes, produces two identical cells
from one cell. The more complex process of mitosis, which also produces two
genetically identical cells from a single cell, is used by many unicellular
eukaryotic organisms for reproduction. Multicellular organisms use mitosis for
growth, cell repair, and cell replacement. In the human body, for example, an
estimated 25 million mitotic cell divisions occur every second in order to
replace cells that have completed their normal life cycles. Cells of the liver,
intestine, and skin may be replaced every few days. Recent research indicates
that even brain cells, once thought to be incapable of mitosis, undergo cell
division in the part of the brain associated with memory. The
type of cell division required for sexual reproduction
is meiosis. Sexually reproducing organisms include seaweeds, fungi, plants, and
animals—including, of course, human beings. Meiosis differs from mitosis in
that cell division begins with a cell that has a full complement of chromosomes
and ends with gamete cells, such as sperm and eggs, that have only half the
complement of chromosomes. When a sperm and egg unite during fertilization,
the cell resulting from the union, called a zygote, contains the full number of
chromosomes. IV. ORIGIN
OF CELLS The
story of how cells evolved remains an open and actively investigated question in
science (see Life).
The combined expertise of physicists, geologists, chemists, and evolutionary
biologists has been required to shed light on the evolution of cells from the
nonliving matter of early Earth. The planet formed about 4.5 billion years ago,
and for millions of years, violent volcanic eruptions blasted substances such as
carbon dioxide, nitrogen, water, and other small molecules into the air. These
small molecules, bombarded by ultraviolet radiation and lightning from intense
storms, collided to form the stable chemical bonds of larger molecules, such as
amino acids and nucleotides—the building blocks of proteins and nucleic acids.
Experiments indicate that these larger molecules form spontaneously under
laboratory conditions that simulate the probable early environment of Earth. Scientists
speculate that rain may have carried these molecules into lakes to create a
primordial soup—a breeding ground for the assembly of proteins, the nucleic
acid RNA, and lipids. Some scientists postulate that these more complex
molecules formed in hydrothermal vents rather than in lakes. Other scientists
propose that these key substances may have reached Earth on meteorites from
outer space. Regardless of the origin or environment, however, scientists do
agree that proteins, nucleic acids, and lipids provided the raw materials for
the first cells. In the laboratory, scientists have observed lipid molecules
joining to form spheres that resemble a cell's plasma membrane. As a result of
these observations, scientists postulate that millions of years of molecular
collisions resulted in lipid spheres enclosing RNA, the simplest molecule
capable of self-replication. These primitive aggregations would have been the
ancestors of the first prokaryotic cells. The
identity of the first cells, and the pathway by which different cells evolved,
is explained by several competing hypotheses. There is substantial evidence that
the oldest fossils, which are at least 3.5 billion years old, are those of
archaebacteria, which may have evolved in the hot waters of hydrothermal vents
or hot springs. In the environment of the early earth, there was no free oxygen
in the atmosphere or water, and the archaebacteria probably relied on
fermentation to synthesize ATP. The chemicals in their watery home constantly
changed as gasses from frequent volcanic eruptions settled into the water. The
archaebacteria further changed their environment by their own metabolic
activity, which many have paved the way for evolution of bacteria. Cyanobacteria,
bacteria capable of photosynthesis, were among the earliest bacteria to evolve
an estimated 2.5 to 3.4 billion years ago. Like the archaebacteria, they
probably used fermentation to metabolize glucose and generate ATP. Over the
millennia, photosynthesis, which produces oxygen, resulted in the gradual
accumulation of oxygen in the atmosphere. The presence of oxygen set the stage
for the evolution of bacteria that used it in aerobic respiration, a more
efficient process than fermentation. Eukaryotic
cells may have evolved from primitive prokaryotes about 2 billion years ago. One
hypothesis suggests that some prokaryotic cells lost their cell walls,
permitting the cell's plasma membrane to expand and fold. These folds,
ultimately, may have given rise to separate compartments within the cell—the
forerunners of the nucleus and other organelles now found in eukaryotic cells.
Another key hypothesis is known as endosymbiosis. Molecular studies of the
bacteria-like DNA and ribosomes in mitochondria and chloroplasts indicate that
mitochondrion and chloroplast ancestors were once free-living bacteria.
Scientists propose that these free-living bacteria were engulfed and maintained
by other prokaryotic cells for their ability to produce ATP efficiently and to
provide a steady supply of glucose. Over generations, eukaryotic cells complete
with mitochondria—the ancestors of animals—or with both mitochondria and
chloroplasts—the ancestors of plants—evolved (see Evolution). V. THE
DISCOVERY AND STUDY OF CELLS The
first observations of cells were made in 1665 by the English scientist Robert
Hooke, who used a crude microscope of his own invention to examine a variety
of objects, including a thin piece of cork. Noting the rows of tiny boxes that
made up the dead wood's tissue, Hooke coined the term cell because the
boxes reminded him of the small cells occupied by monks in a monastery. While
Hooke was the first to observe and describe cells, he did not comprehend their
significance. At about the same time, the Dutch maker of microscopes Antoni
van Leeuwenhoek pioneered the invention of one of the best microscopes of
the time. Using his invention, Leeuwenhoek was the first to observe, draw, and
describe a variety of living organisms, including bacteria gliding in saliva,
one-celled organisms cavorting in pond water, and sperm swimming in semen. Two
centuries passed, however, before scientists grasped the true importance of
cells. Modern
ideas about cells appeared in the 1800s, when improved light microscopes enabled
scientists to observe more details of cells. Working together, the German
botanist Matthias
Jakob Schleiden and the German zoologist Theodor
Schwann recognized the fundamental similarities between plant and animal
cells. In 1839 they proposed the revolutionary idea that all living things are
made up of cells. Their theory gave rise to modern biology: a whole new way of
seeing and investigating the natural world. By
the late 1800s, as light microscopes improved still further, scientists were
able to observe chromosomes within the cell. Their research was aided by new
techniques for staining parts of the cell, which made possible the first
detailed observations of cell division, including observations of the
differences between mitosis and meiosis in the 1880s. In the first few decades
of the 20th century, many scientists focused on the behavior of chromosomes
during cell division. At that time, it was generally held that mitochondria
transmitted the hereditary information. By 1920, however, scientists determined
that chromosomes carry genes and that genes transmit hereditary information from
generation to generation. During
the same period, scientists began to understand some of the chemical processes
in cells. In the 1920s, the ultracentrifuge was developed. The ultracentrifuge
is an instrument that spins cells or other substances in test tubes at high
speeds, which causes the heavier parts of the substance to fall to the bottom of
the test tube. This instrument enabled scientists to separate the relatively
abundant and heavy mitochondria from the rest of the cell and study their
chemical reactions. By the late 1940s, scientists were able to explain the role
of mitochondria in the cell. Using refined techniques with the ultracentrifuge,
scientists subsequently isolated the smaller organelles and gained an
understanding of their functions. While
some scientists were studying the functions of cells, others were examining
details of their structure. They were aided by a crucial technological
development in the 1940s: the invention of the electron microscope, which uses
high-energy electrons instead of light waves to view specimens. New generations
of electron microscopes have provided resolution, or the differentiation of
separate objects, thousands of times more powerful than that available in light
microscopes. This powerful resolution revealed organelles such as the
endoplasmic reticulum, lysosomes, the Golgi apparatus, and the cytoskeleton. The
scientific fields of cell structure and function continue to complement each
other as scientists explore the enormous complexity of cells. The
discovery of the structure of DNA in 1953 by James
D. Watson and Francis
H. C. Crick ushered in the era of molecular biology. Today, investigation
inside the world of cells—of genes and proteins at the molecular
level—constitutes one of the largest and fastest moving areas in all of
science. One particularly active field in recent years has been the
investigation of cell signalling, the process by which molecular messages find
their way into the cell via a series of complex protein pathways in the cell. Another
busy area in cell biology concerns programmed cell death, or apoptosis. Millions
of times per second in the human body, cells commit suicide as an essential part
of the normal cycle of cellular replacement. This also seems to be a check
against disease: when mutations build up within a cell, the cell will usually
self-destruct. If this fails to occur, the cell may divide and give rise to
mutated daughter cells, which continue to divide and spread, gradually forming a
growth called a tumor. This unregulated growth by rogue cells can be benign, or
harmless, or cancerous, which may threaten healthy tissue. The study of
apoptosis is one avenue that scientists explore in an effort to understand how
cells become cancerous. Scientists
are also discovering exciting aspects of the physical forces within cells. Cells
employ a form of architecture called tensegrity, which enables them to withstand
battering by a variety of mechanical stresses, such as the pressure of blood
flowing around cells or the movement of organelles within the cell. Tensegrity
stabilizes cells by evenly distributing mechanical stresses to the cytoskeleton
and other cell components. Tensegrity also may explain how a change in the
cytoskeleton, where certain enzymes are anchored, initiates biochemical
reactions within the cell, and can even influence the action of genes. The
mechanical rules of tensegrity may also account for the assembly of molecules
into the first cells. Such new insights—made some 300 years after the tiny
universe of cells was first glimpsed—show that cells continue to yield
fascinating new worlds of discovery. |
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