Cells can be divided into two main types: prokaryotic and eukaryotic. Prokaryotic cells lack a nucleus and other organelles, while eukaryotic cells have a nucleus enclosed within membranes. It is believed that eukaryotic cells evolved from prokaryotic cells when ancient bacteria were engulfed but not destroyed by their hosts, eventually evolving into the mitochondria and chloroplasts found in modern eukaryotic cells. This endosymbiotic theory explains the origin of the more complex eukaryotic cell structure.

1. Prepared by group 5

2.  Cells are divided into two main classes, initially defined by
whether they contain a nucleus. Prokaryotic cells (bacteria)
lack a nuclear envelope; eukaryotic cells have a nucleus in
which the genetic material is separated from the
cytoplasm. Prokaryotic cells are generally smaller and
simpler than eukaryotic cells; in addition to the absence of
a nucleus, their genomes are less complex and they do not
contain cytoplasmic organelles or a cytoskeleton (Table 1.1).
In spite of these differences, the same basic molecular
mechanisms govern the lives of both prokaryotes and
eukaryotes, indicating that all present-day cells are
descended from a single primordial ancestor.

3.  Table 1.1 Prokaryotic and Eukaryotic Cells
 Characteristic
 Prokaryote
 Eukaryote
 Nucleus
 Absent
 Present
 Diameter of a typical cell
 ≈1μm
 10–100 μm
 Cytoskeleton
 Absent
 Present
 Cytoplasmic organelles
 Absent
 Present
 DNA content (base pairs)
 1 × 106 to 5 × 106
 1.5 × 107 to 5 × 109
 Chromosomes
 Single circular DNA molecule
 Multiple linear DNA molecules


4.  Two Types of Cells: (Prokaryotic Cells and Eukaryotic
Cells)


5. Prokaryotic Cells
 Prokaryote, relatively simple unicellular organism lacking a nucleus and other
features found in the more complex cells of all other organisms, called eukaryotes.
In 1938 American biologist Herbert Copeland proposed that unicellular organisms
lacking nuclei be classified in their own kingdom, Monera, also called Kingdom
Prokaryotae. All bacteria were categorized in this newly established kingdom. This
scheme was the first to establish separate kingdoms for prokaryotes (organisms
without nuclei) and eukaryotes (organisms with nuclei).
 In 1990 American microbiologist Carl Woese proposed that bacteria be divided
into two groups, the archaea, or archaebacteria, and bacteria, based on their
structural and physiological differences. In some classification systems, the
archaea are considered prokaryotes; in others, they are classified in their own
domain, the archaea. Archaebacteria consist of a small group of
primitive anaerobes (organisms that do not require oxygen). They are found in
a narrow range of habitats–often in extreme environments with
high temperature, high salt, or high acidity. In contrast, bacteria live in a wide
range of environments with or without oxygen, at various temperatures, and at
various levels of acidity.

6. Structure of Prokaryotic Cells

7. Prokaryotic cells are relatively small, ranging in size from 0.0001 to 0.003 mm
(0.000004 to 0.0001 in) in diameter. With the exception of a few species,
prokaryotic cells are surrounded by a protective cell wall.
The cell walls of archaebacteria and bacteria contain forms of peptidoglycan, a
protein-sugar molecule not present in the cell walls of fungi, plants, and certain
other eukaryotes.
The archaebacteria cell wall has a more diverse chemical composition than the
cell wall of bacteria just inside the cell wall of prokaryotes is the plasma
membrane, a thin structure that is both flexible and strong. In both prokaryotes
and eukaryotes, the plasma membrane is composed of two layers of phospholipid
molecules interspersed with proteins, and regulates the traffic that flows in and
out of the cell.
The prokaryotic plasma membrane, however, carries out additional functions. It
participates in replication of deoxyribonucleic acid (DNA) for cell division and
synthesis of adenosine triphosphate (ATP), an energy molecule. In some
prokaryotes, the plasma membrane is essential for photosynthesis, the process
that uses light energy to convert carbon dioxide and water to glucose.

8.  In the interior of the prokaryotic cell is the cytoplasm, a watery fluid that is
rich in dissolved salts, nutrients, enzymes, and other molecules. The great
majority of the cell's biochemical reactions, which number in the thousands,
take place within the cytoplasm.
 Ribosomes, tiny bead-like structures that manufacture proteins, are also
located in the cytoplasm. The ribonucleic acid (RNA) in the ribosomes differs
significantly between the archaebacteria and bacteria. With the exception of
the ribosomes, prokaryotes lack organelles (specialized structures such as the
nucleus, chloroplasts, mitochondria, lysosomes, and Golgi apparatus), which
are present in eukaryotes (see Cell). Some photosynthetic archaebacteria and
bacteria have internal membranes, extensions of the plasma membrane known
as chromatophores or thylakoids, which contain the pigments for
photosynthesis.
 Some species of prokaryotes form endospores, thick-walled, dehydrated
structures that can resist extreme dryness and very high temperatures for long
periods of time. Anthrax, tetanus, and botulism are diseases caused by
endospore-forming bacteria.

9.  Certain prokaryotes move independently by using flagella, long structures that
rotate in a propeller-like fashion. Prokaryotic flagella consist of intertwined
fibrils (small fibers) of the protein flagellin. A prokaryote may have a single
flagellum, a group of flagella at one or both poles of the cell, or may be covered
with flagella. Many species of prokaryotes also have pili (singular, pilus)–
slender, hairlike extensions used for attachment to soil, rocks, teeth, or other
structures.

10. Origin of Prokaryotes
 In 1862, Pasteur disproved the spontaneous-generation theory but left open a
question: How did life begin? Miller's synthesis is a possible answer, or it may
be the seeding of organic molecules by meteorites from outer space, or a God
event that started life. It is generally held that the first organisms were formed
around four billion years ago, with the earliest forms being simple molecular
groupings that somehow gained the ability to metabolize and reproduce. It is
also held that these simple molecular arrangements formed from existing
inorganic substances—life from nonlife!

11. Evolution of Prokaryotes
 Prokaryotes are mostly bacteria, and their advancements led to more complex
living organisms. It has been suggested that the diverse nature of bacteria and
archaebacteria resulted from this evolution. As bacteria modified structures to
expand their territory and tolerance, they changed into newer species of
bacteria with diverse structures and functions. Due to their uniqueness,
bacteria are classified in their own kingdom!
 Advancements in the structure and function of prokaryotes continued to the
juncture where two separate types are now identifiable: bacteria and archaea.

12. Examples of Prokaryotic Cells

13. Eukaryotic Cells
 A eukaryote is an organism whose cells contain complex structures enclosed
within membranes. Eukaryotes may more formally be referred to as the taxon
Eukarya or Eukaryota. The defining membrane-bound structure that sets
eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear
envelope, within which the genetic material is carried. The presence of a
nucleus gives eukaryotes their name, which comes from the Greek (eu, "good")
and κάρυον (karyon, "nut" or "kernel"). Most eukaryotic cells also contain other
membrane-bound organelles such as mitochondria, chloroplasts and the Golgi
apparatus. All large complex organisms are eukaryotes, including
animals, plants and fungi. The group also includes many unicellular organisms.
 Eukaryotes appear to be monophyletic, and so make up one of the three
domains of life. The two other domains, Bacteria and Archaea, are prokaryotes
and have none of the above features. Eukaryotes represent a tiny minority of all
living things; even in a human body there are 10 times more microbes than
human cells. However, due to their much larger size their collective worldwide
biomass is estimated at about equal to that of prokaryotes.

14. Structure of Eukaryotic Cells

15.  Eukaryotic cells are complex structures that make up animal and
human tissue. Eukaryotic cells are different from prokaryotes, which is
the term given to bacterial cells. Eukaryotes are distinct from
prokaryotes in that they have membrane bound organelles and DNA is
contained within a nucleus. A eukaryote cell has several structures that
help the cell maintain homeostasis, and provide energy and the
mechanisms for protein synthesis.

16. Origin of Eukaryotes
 The origin of the eukaryotic cell is considered a milestone in the evolution of
life, since they include all complex cells and almost all multicellular organisms.
The timing of this series of events is hard to determine; Knoll (2006) suggests
they developed approximately 1.6–2.1 billion years ago. Some acritarchs are
known from at least 1650 million years ago, and the possible alga Grypania has
been found as far back as 2100 million years ago.

17. Evolution of Eukaryotes
 Fossil records indicate that eukaryotes evolved from prokaryotes
somewhere between 1.5 to 2 billion years ago. Two proposed pathways
describe the invasion of prokaryote cells by two smaller prokaryote
cells. They subsequently became successfully included as part of a now
much larger cell with additional structures and capable of additional
functions.
 • Endosymbiosis
 • Membrane infolding

18. Endosymbiosis
 Research conducted by Lynn Margulis at the University of Massachusetts
supports the hypothesis that two separate mutually beneficial invasions of
a prokaryote cell produced the modern-day mitochondria and chloroplast
as eukaryotic organelles. In this model, ancestral mitochondria were small
heterotrophs capable of using oxygen to perform cellular respiration and
thereby create useful energy. They became part of a large cell either by
direct invasion as an internal parasite or as an indigestible food source.
Later, a second invasion brought ancestral chloroplasts, which are thought
to be small, photosynthetic cyanobacteria. Modern-day supporting
evidence for endosymbiosis shows that both the mitochondria and
chloroplasts have their own genes, circular DNA and RNA, and reproduce
by binary fission independent of the host's cell cycle. They therefore
appear to be more similar to prokaryotes than eukaryotes.

19. Membrane Infolding
 The invasions of the host prokaryote cell probably were successful
because the host cell membrane infolded to surround both invading
prokaryote cells and thereby help transport them into the cell. The
membrane did not dissolve but remained intact, and thereby created a
second membrane around the protomitochondria and
protochloroplast. It is also known that in modern-day eukaryotes the
inner membrane of both the mitochondria and chloroplast contain
structures more similar to prokaryotes than eukaryotes, whereas the
outer membrane retains eukaryote characteristics! It is also suggested
that continued membrane infolding created the endomembrane
system. It can be said that possibly the first eukaryotic cell type was
miraculously born from prokaryotic, symbiotic, multicellular
interactions.

20. Examples of Eukaryotic Cell

21. From prokaryotes to eukaryotes
 Living things have evolved into three large clusters of closely related
organisms, called "domains": Archaea, Bacteria, and Eukaryota.
Archaea and Bacteria are small, relatively simple cells surrounded by a
membrane and a cell wall, with a circular strand of DNA containing
their genes. They are called prokaryotes.

22.  Virtually all the life we see each day — including plants and animals —
belongs to the third domain, Eukaryota. Eukaryotic cells are more
complex than prokaryotes, and the DNA is linear and found within a
nucleus. Eukaryotic cells boast their own personal "power
plants", calledmitochondria. These tiny organelles in the cell not only
produce chemical energy, but also hold the key to understanding the
evolution of the eukaryotic cell.