Download 2nd lecture Cell Biology Classification of cells: Prokaryotic cells

2nd lecture
Cell Biology
 Classification of cells:
1) All living organisms (bacteria, blue green algae, plants and animals)
have cellular organization and may contain one or many cells.
2) The organisms with only one cell in their body are called unicellular
organisms (bacteria, blue green algae, some algae, Protozoa, etc.).
3) The organisms having many cells in their body are called multicellular
organisms (fungi, most plants and animals).
4) Any living organism may contain only one type of cell either:
a) Prokaryotic cells (pro means "before"; karyote means "nucleus).
b) Eukaryotic cells (Eu means "true"; karyote means "nucleus).
5) The terms prokaryotic and eukaryotic were suggested by Hans Ris in
the 1960’s. This classification is based on their complexity.
6) Further based on the kingdom into which they may fall for example,
the plant or the animal kingdom, plant and animal cells bear many
differences.
 Prokaryotic cells
1) Prokaryote means before nucleus in Greek. They include all cells,
which lack nucleus, and other membrane bound organelles.
2) Indeed, bacteria can be considered as a typically prokaryotic cell,
which contain essentially no organelles not even a nucleus to hold its
DNA.
3) Most prokaryotes range between 1 µm to 10 µm, but they can vary in
size from 0.2 µm to 750 µm.
4) They belong to two taxonomic domains, which are the bacteria and
the archaea.
5) The
terms
“bacterium”
and
“prokaryote”
are
often
used
interchangeably, although that the category of prokaryotes also
includes another class of cells, the archaea (singular archaeon).
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
6) Prokaryotes are membrane bound mostly unicellular organisms
lacking any internal membrane bound organelles.
7) A typical prokaryotic cell is schematically illustrated in Figure 1.
8) Prokaryotes lack cell organelles they harbor few internal structures,
such as the cytoskeletons, ribosomes, which translate mRNA to
proteins.
9) Membranous organelles are known in some groups of prokaryotes,
such as vacuoles or membrane systems devoted to special metabolic
properties, for example, photosynthesis or chemolithotrophy.
10) In addition, some species also contain protein-enclosed microcompartments, which have distinct physiological roles (carboxysomes
or gas vacuoles).
Figure 1: Schematic diagram of a Prokaryotic cell
 Morphology of prokaryotic cells
Prokaryotic cells have various shapes; the four basic shapes are
(Figure 2):
1) Cocci - spherical
2) Bacilli - rod-shaped
3) Spirochetes - spiral-shaped
4) Vibrio - comma-shaped
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
Figure 2: Morphology of Prokaryotic cells
 Similarities and differences between bacteria and archaea
Archaea and bacteria have a few similarities in which they share:
1) They are both prokaryotes, without any complex cell structure like
eukaryotes.
2) Archaea and bacteria have cell walls on the outside, providing
structural support, and lets certain elements/substances pass through
for cellular work.
3) Both distinguish almost identical looks when looking through a
microscope, since they are both prokaryotes.
4) Bacteria and archaea both reproduce using binary fission, and move
around using flagella.
Archaea and bacteria have many differences with each other.
1) Bacteria have cell wall made of peptidoglycan, whereas archaea do
not.
2) Both have different lipid composition. Archaeal lipids do not have any
fatty acids, which are found in the other 2 domains (bacteria and
eukaryote).
3) Archaea have side chains (polymers) made up of units of isoprene
(chemical compound composed of C5H8).
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
4) Archaea and bacteria similarly have 70S ribosomes, but archaea has a
different shape. Archaea also have a complex RNA polymerases,
bacteria has a more simple RNA polymerases.
5) Archaea and bacteria are metabolically different from each other.
Archaea do not use the process glycolysis to break down glucose.
Most types of archaea do not have Kreb's Cycle pathways, but few
types do.
 The eukaryotic cell
 Introduction
1) Eukaryotic cells, in general, are bigger and more elaborate than
bacteria and archaea.
2) Some live independent lives as single-celled organisms, such as
amoebae and yeasts others live in multicellular assemblies.
3) All of the more complex multicellular organisms—including plants,
animals, and fungi—are formed from eukaryotic cells.
4) By definition, all eukaryotic cells have a nucleus.
5) However, possession of a nucleus goes hand-in-hand with possession
of a variety of other organelles, most of which are membrane-enclosed
and common to all eukaryotic organisms.
 The origin of eukaryotic cell
1) It seems highly likely that eukaryotes evolved from prokaryotes.
2) The most likely explanation of this process is the endosymbiotic
theory. The basis of this hypothesis is that some eukaryotic organelles
originated as free-living prokaryotes that were engulfed by larger cells
in which they established a mutually beneficial relationship.
3) For example, mitochondria would have originated as free-living
aerobic bacteria and chloroplasts as Cyanobacteria, photosynthetic
prokaryotes formerly known as blue-green algae.
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
4) The endosymbiotic theory provides an attractive explanation for the
fact that both mitochondria and chloroplasts contain DNA and
ribosomes of the prokaryotic type.
5) The case for the origin of other eukaryotic organelles is less
persuasive. Most biologists are now prepared to accept that the
endosymbiotic theory provides at least a partial explanation for the
evolution of the eukaryotic cell from a prokaryotic ancestor.
6) Unfortunately,
living
forms
having
a
cellular
organization
intermediate between prokaryotes and eukaryotes are rare.
7) Some primitive protists possess a nucleus and prokaryotic type of
ribosomes but lack mitochondria and other typical eukaryotic
organelles.
Figure 3: The origin of eukaryotic cell
 History of eukaryotic cell
Year
Event
1833
Brown publishes his microscopic observations of orchids, clearly describing the cell
nucleus.
1857
Kolliker describes mitochondria in muscle cells.
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
1898
Golgi first sees and describes the Golgi apparatus by staining cells with silver nitrate.
1905
Russian botanist Konstantin Mereschkowsky was the first person who discover the
chloroplast
1974
Lazarides and Weber use fluorescent antibodies to stain the cytoskeleton.
1974
Albert Claude discovered the Endoplasmic Reticulum
 Types of eukaryotic cell (cell specialization)
1) All the body cells that comprise a single organism share the same set
of genetic instructions in their nuclei.
2) However, the cells are not all identical.
3) Therefore, plants and animals are composed of different tissues,
groups of cells that are specialized to carry out a common function.
4) This specialization occurs because different cell types read out
different parts of the DNA blueprint and therefore make different
proteins.
5) The process of cell specialization called differentiation (process led to
the evolution of multicellular organisms in which different activities
are conducted by different types of specialized cells).
6) In animals, there are four major tissue (cell) types: epithelium,
connective tissue, nervous tissue, and muscle.
7) In plant, the basic organization of a shoot or root is into an outer
protective layer, or epidermis, a vascular tissue that provides support
and transport, and a cortex that fills the space between the two.
 Model organisms
1) They are non-human species that are used in the laboratory to help
scientists for understanding biological processes.
2) They are usually organisms that are easy to maintain and breed in a
laboratory setting.
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
3) For example, they may have particularly robust embryos that are
easily studied and manipulated in the LAB, this is useful for scientists
studying development.
4) They may occupy a pivotal position in the evolutionary tree, this is
useful for scientists studying evolution.
 Why are model organisms useful in genetics research?
1) Many model organisms can breed in large numbers.
2) Some have a very short generation time.
3) Mutants in theses model organisms allow scientists to study certain
characteristics or diseases.
4) Some model organisms have similar genes or similar-sized
genomes to humans.
5) Model organisms can be used to create highly detailed genetic
maps.
 Examples of model organisms
A. Budding yeast, Saccharomyces cerevisiae
1) S. cerevisiae is a small, single-celled fungus that is at least as
closely related to animals as it is to plants.
2) It is useful in genetic and biochemical studies specially the division
of cell cycle.
B. Flowering plant, Arabidopsis thaliana
1) They can be grown indoors in large numbers: one plant can
produce thousands of offspring within 8–10 weeks.
2) The genes found in Arabidopsis have counterparts in agricultural
species
3) Thus its useful in the studying of development and physiology of
the crop plants upon which our lives depend.
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
C. Fruit fly, Drosophila melanogaster
1) The genetic analysis of fruit fly genes proof that genes (the units of
heredity) are carried on chromosomes.
2) The genetic instructions encoded in its DNA molecules direct the
development of a fertilized egg cell (or zygote) into an adult
multicellular organism.
3) The genes responsible for the development of Drosophila have
turned out to be amazingly similar to those of humans.
4) Thus, the fly serves as a valuable model for studying human
development and disease.
D. Nematode, Caenorhabditis elegans
1) Caenorhabditis elegans is a harmless relative of the eelworms that
attack the roots of crops.
2) Some 70% of human genes have some counterpart in the worm.
3) It is useful in molecular understanding of apoptosis, a form of
programmed cell death.
 The Sizes of Cells and Their Components
1) Two units of linear measure are most commonly used to describe
structures within a cell: the micrometer (µm) and the nanometer (nm).
One µm is equal to 10-6 meters, and one nm is equal to 10-9 meters.
2) Cells and their organelles are more easily defined in micrometers.
3) Nuclei, for example, are approximately 5–10 µm in diameter, and
mitochondria are approximately 2 µm in length.
4) Prokaryotic cells typically range in length from about 1 to 5 µm, while
eukaryotic cells from about 10 to 30 µm.
5) There are a number of reasons most cells are so small. Consider the
following.
a) Most eukaryotic cells possess a single nucleus that contains only
two copies of most genes.
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
b) As a cell increases in size, the surface area/volume ratio decreases.
c) A cell depends to a large degree on the random movement of
molecules (diffusion).
 Comparison of Prokaryotic and Eukaryotic Cells
 Features held in common by the two types of cells:
1) Plasma membrane of similar construction.
2) Genetic information encoded in DNA using identical genetic code.
3) Similar mechanisms for transcription and translation of genetic
information, including similar ribosomes.
4) Shared metabolic pathways (e.g., glycolysis and TCA cycle).
5) Similar apparatus for conservation of chemical energy as ATP
(located in the plasma membrane of prokaryotes and the mitochondrial
membrane of eukaryotes).
6) Similar mechanism of photosynthesis (between Cyanobacteria and
green plants).
7) Similar mechanism for synthesizing and inserting membrane proteins.
8) Proteasomes (protein digesting structures) of similar construction
(between archaebacteria and eukaryotes).
 Features of eukaryotic cells not found in prokaryotes:
1) Division of cells into nucleus and cytoplasm, separated by a nuclear
envelope containing complex pore structures.
2) Complex chromosomes composed of DNA and associated proteins
that are capable of compacting into mitotic structures.
3) Complex membranous cytoplasmic organelles (includes endoplasmic
reticulum, Golgi complex, lysosomes, endosomes, peroxisomes, and
glyoxisomes).
4) Specialized
cytoplasmic
organelles
for
aerobic
respiration
(mitochondria) and photosynthesis (chloroplasts).
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Dr. Mohanad J.K. Al-Dawah
2nd lecture
Cell Biology
5) Complex cytoskeletal system (including microfilaments, intermediate
filaments, and microtubules) and associated motor proteins.
6) Complex flagella and cilia.
7) Ability to ingest fluid and particulate material by enclosure within
plasma membrane vesicles (endocytosis and phagocytosis).
8) Cellulose-containing cell walls (in plants)
9) Cell division using a microtubule-containing mitotic spindle that
separates chromosomes.
10) Presence of two copies of genes per cell (diploidy), one from each
parent.
11) Presence of three different RNA synthesizing enzymes (RNA
polymerases).
12) Sexual reproduction requiring meiosis and fertilization.
Figure 4: Organization of prokaryotic and eukaryotic cells.
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