Scientists trace the evolution of life from simple cells to complex organisms by studying fossils. The evolution of cells remains an open question that requires expertise from multiple sciences. Early Earth's atmosphere contained small molecules that collided due to radiation and storms, forming amino acids and nucleotides - the building blocks of proteins and DNA. Cells are the basic units of life that take in nutrients, expel waste, and reproduce, with some consisting of just one cell while others contain many coordinated cells. Cell shape and components are tailored to their functions within multicellular organisms.

1. Theory/ introduction
Scientists study fossils to trace the evolution of life from simple cells to more complex organisms.
Shown here are fossils of trilobites, primitive arthropods that once dominated the seas but became
extinct about 250 million years ago.
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.
A Cell, is a 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
trillions 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 in diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a human
hair. Along with their differences in size, cells present an array of shapes. Some, such as the

2. 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. 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 organs 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. Biochemica l
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.

3. Animal Cell
An animal cell typically contains several types of membrane-bound organs, or organelles. The
nucleus directs activities of the cell and carries genetic information from generation to generation.
The mitochondria generate energy for the cell. Proteins are manufactured by ribosomes, which are
bound to the rough endoplasmic reticulum or float free in the cytoplasm. The Golgi apparatus
modifies, packages, and distributes proteins while lysosomes store enzymes for digesting food.
The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out
of the cytoplasm.
Cell components
Plasma Membrane, thin molecular layer that surrounds all living cells. The plasma membrane
separates the cell from its surroundings, protects it from changes in the chemical and physical
environment, and regulates the traffic of molecules into and out of the cell. Although flexible and
exceedingly thin. The plasma membrane is very strong. In the cells of plants, bacteria, fungi, and
most algae, the plasma membrane is surrounded by a cell wall, a rigid structure that helps support
the cell and prevent it from drying out. The plasma membrane is composed primarily of two types
of molecules—lipids, which are fatty or oily molecules, and proteins. The basic structural
framework of the plasma membrane is formed by two sheets of lipids, each sheet a single molecule
thick. Within this double layer, or bilayer, of lipids, the protein molecules are embedded. Proteins
are responsible for a host of functions, including transporting substances across the membrane,
aiding communication between cells, and carrying out chemical reactions. in most cells, the plasma
membrane is about 40 percent lipid and 60 percent protein, but these proportions vary greatly,
from as little as 20 percent to as much as 75 percent protein depending on the type of cell

4. Mitochondria, small cellular structures, or organelles, found in the cytoplasm of eukaryotic cells
(cells with a nucleus). Mitochondria are responsible for converting nutrients into the energy-yielding
molecule adenosine triphosphate (ATP) to fuel the cell's activities. This function, known
as aerobic respiration, is the reason mitochondria are frequently referred to as the powerhouse of
the cell. Mitochondria are unusual organelles in that they contain deoxyribonucleic acid (DNA),
typically found in the cell’s nucleus, and ribosomes, protein-producing organelles abundant in the
cytoplasm. Within the mitochondria, the DNA directs the ribosomes to produce proteins, many of
which function as enzymes, or biological catalysts, in ATP production. The number of
mitochondria in a cell depends on the cell's function. Cells with particularly heavy energy
demands, such as muscle cells, have more mitochondria than other cells.
Golgi apparatus, also Golgi body or Golgi complex, network of stacked sacs found within
nucleated cells that store, package, and distribute the proteins and lipids made in the endoplasmic
reticulum. Proteins and lipids manufactured in the endoplasmic reticulum bud off in tiny, hollow
structures, or vesicles, and fuse with the cis cisterna of the Golgi apparatus. The proteins and lipids
move progressively through the stack of cisternae until they reach the trans cisterna. There they
may be modified by the attachment of lipids or carbohydrates. The proteins and lipids are enclosed
in a membrane to form a vesicle so that they do not affect the rest of the cell. The vesicles are then
sorted and their destination is determined. Proteins that are meant to return to the endoplasmic
reticulum carry a distinctive tag. The Golgi apparatus recognizes the tag and transports the proteins
back to the endoplasmic reticulum. Some proteins and lipids are sent to the surface of the cell to
be released into the external environment. Others are transferred to the small structures that hold
digestive enzymes, called lysosomes. The Golgi apparatus also manufactures long-chained sugars
called polysaccharides that cells secrete into their external environments. Examples include
cellulose and pectin used to construct plant cell walls, and the polysaccharides in the mucus of
animal cells.
Lysosome, membrane-bound sac found in nucleated cells that contains digestive enzymes that
break down complex molecules in the body. Lysosomes are numerous in disease-fighting cells,
such as white blood cells, that destroy harmful invaders or cell debris. Lysosomes vary greatly in
size, typically ranging from 0.05 to 0.5 micrometers in diameter. Each lysosome is surrounded by
a membrane that protects the cell from the lysosome’s digestive enzymes—if the lysosome breaks
open, the enzymes would destroy the cell. Proteins embedded in the lysosome membrane protect
the activity of the enzymes by maintaining the proper internal acidity. Membrane proteins also
transport digested products out of the lysosome. Lysosome enzymes are manufactured in the rough
endoplasmic reticulum and processed in the Golgi apparatus. They are delivered by sacs known as
transport vesicles to fuse with three types of membrane-bound structures: endosomes,
phagosomes, and autophagosomes. Endosomes form when the cell membrane surrounds
nutritional molecules like polysaccharides, complex lipids, nucleic acids, or proteins. In a process
called endocytosis, these molecules are broken down for reuse. Phagosomes form when the cell
membrane engulfs large objects, like debris from sites of injury or inflammation or disease-causing
bacteria, in a process called phagocytosis. Autophagosomes form when the endoplasmic reticulum
wraps around spent cell structures, such as mitochondria, that are destined for recycling. In all
cases the digestive enzymes supplied by the lysosomes digest the membrane-bound objects into

5. simple compounds that are delivered to the cytoplasm as new cell-building materials. Lysosome
enzyme disorders can cause disease. Infants born with Tay-Sachs disease lack an enzyme that
breaks down a complex lipid called ganglioside. When this lipid accumulates in the body, it
damages the central nervous system, causes mental retardation, and results in death by age five.
The inflammation and pain associated with rheumatoid arthritis and gout are related to the escape
of lysosome enzymes. Some scientists classify plant vacuoles as a type of lysosome. These
membrane-bound structures are much larger than other lysosomes, measuring up to 20
micrometers in diameter. Vacuoles maintain water pressure within plant cells, called turgor,
preventing wilting. Vacuoles may also provide long-term storage of polysaccharides, lipids,
proteins, pigments, and harmful materials such as rubber or opium that may deter predators.
Centriole is a rod-shaped structure in cell in an animal cell, a two-part rod-shaped structure with the
parts lying at right angles to each other, located in pairs near the nucleus. During cell division,
centrioles move to opposite ends of the cell and form the poles of the spindle fibers that pull the
chromosomes apart.
Endoplasmic Reticulum (ER), an extensive network of tubes that manufacture, process, and
transport materials within nucleated cells. The ER consists of a continuous membrane in the form
of branching tubules and flattened sacs that extend throughout the cytoplasm (the cell’s contents
outside of the nucleus) and connect to the double membrane that surrounds the nucleus. There are
two types of ER: rough and smooth. The outer surface of rough ER is covered with tiny structures
called ribosomes, where protein synthesis occurs. Proteins are created as long polypeptide chains,
some of which require modification. These proteins are transported into the rough ER, where
enzymes fold and link them into the three dimensional shape that completes their structure. The
rough ER also transports proteins either to regions of the cell where they are needed or to the Golgi
apparatus, from which they may be exported from the cell. Rough ER is particularly dense in cells
that manufacture proteins for export. White blood cells, for example, which produce and secrete
antibodies, contain abundant rough ER. Smooth ER lacks ribosomes and so has a smooth
appearance. It is involved in the synthesis of most of the lipids that make up the cell membrane, as
well as membranes surrounding other cell structures like mitochondria. It also manufacture s
carbohydrates, stores carbohydrates and lipids, and detoxifies alcohol and drugs such as morphine
and phenobarbital. Cells that specialize in lipid and carbohydrate metabolism, such as brain and
muscle cells, or those that carry out detoxification, such as liver cells, tend to have smoother ER.
Smooth ER also is involved in the uptake and release of calcium to mediate some types of cellula r
activity. In skeletal muscle cells, for example, the release of calcium from the smooth ER triggers.
Nucleus, membrane-bound structure of a cell that plays two crucial roles. The nucleus carries the
cell’s genetic information that determines if the organism will develop, for instance, into a tree or a
human; and it directs most cell activities including growth, metabolism, and reproduction by
regulating protein synthesis (the manufacture of long chains of amino acids). The presence of a
nucleus distinguishes the more complex eukaryotic cells of plants and animals from the simpler
prokaryotic cells of bacteria and cyanobacteria that lack a nucleus. The nucleus is the most
prominent structure in the cell. It is typically round and occupies about 10 percent of the cell’s total
volume. The nucleus is wrapped in a double-layered membrane called the nuclear envelope. The

6. space between the nuclear envelope layers is called perinuclear space. The nuclear envelope is
attached to a network of membrane-enclosed tubules that extends throughout the cell called the
endoplasmic reticulum. The nuclear envelope is perforated by many holes, called nuclear pores,
which permit the movement of selected molecules between the nucleus and the rest of the cell, while
blocking the passage of other molecules. The nucleus contains the nucleolus, which manufacture s
protein-producing structures called ribosomes. Genetic information in the form of deoxyribonucle ic
acid (DNA) is stored in threadlike, tangled structures called chromatin within the nucleus. During
the process of cell division known as mitosis, in which the nucleus divides, the chromatin condense
into several distinct structures called chromosomes. Each time the cell divides, the heredity
information carried in the chromosomes is passed to the two newly formed cells. The DNA in the
nucleus also contains the instructions for regulating the amount and types of proteins made by the
cell. These instructions are copied, or transcribed, into a type of ribonucleic acid (RNA) called
messenger RNA (mRNA). The mRNA is transported from the nucleus to ribosomes, where proteins
are assembled.
Gap junction is the space between cells: a passage through the membranes of adjacent cells that
allows the transfer of small molecules or ions between cells.
Macular adherens/Desmosome is a patch that welds cells together: a small patch of interlocking
fibers between the outer membranes of adjacent cells that helps to hold cells together in tissues such
as skin.
References
 Thomson A.D & cotton R.D, (1983), Lecture notes on pathology, 3rd edition, Blackwell
scientific publications, oxford, London.
 Norman F. & Cheville D.V.M. (1976), cell pathology, 1st edition, the IOWA state univers it y
press/ AMES, USA.