(The images below are shown for educational purposes only. They are taken from the web, with links to the original sites given in the caption of each image)
A movie summarizing the main steps of cells‘ evolution on Earth is given here.
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aaaaaaaaaaHow Large Are Cells?
aaaaaa Cells are about a millionth of our body size
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aaaaaaaaaaaa Relative sizes of cells and viruses
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aaaaaaaaThe Three kingdoms of Life
aaaaaaaaaaaaaaaaaaaEvolutionary division of organisms
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AAAaaaaAA The Prokaryotic Cell
1. Eubacteria
I. General Shape
aaaaaa Bacillus (rod shape)
aaaaaaaaaa Cocci (ball-shaped)
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II.Structure
aaaaaaaaaaaaaaaaaaaaaaaa E. coli
Gram positive & Gram negative bacteria have different structures
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aaaaaaaaaaaa Cell wall of Gram negative bacteria
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aaaaaaaaaaa Cell wall of Gram positive bacteria
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III. Division
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2. Archaea
I. Shape
aaaaaaaaaaaaa Different shapes of Archaeal cells
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II. Distinctive Features
- Usually reside in niches under extreme conditions (thermophiles, halophiles, acidophiles).
- Resemble gram-positive bacteria structurally: most have cell wall & flagella; lack internal organization.
- Membrane built from ether lipids with branched chains (unlike both bacteria & eukaryotes).
- Resemble eukaryotes in some biochemical and genetic mechanisms (Ref).
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aaaaaaaaaaaaaThe Eukaryotic Cell
1. Plant Cells
I. General Shape
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II. Structure
aaaaaaaaaaaaaaa A typical plant cell
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2. Animal Cells
I. Structure
The structure of a typical animal cell
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II. Shapes
A. Blood
aaaaaaa Red (RBC) and white (WBC) blood cells
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aaaaaaaaaaaaaaa Human T-lymphocyte
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Macrophage (white) attacking a bacterium (green)
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B. Nervous System
aaaaaaaa Nerve cells (yellow) and astrocyte (green)
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C. Liver
aaaaaaaaa Hepatocytes (liver cells)
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D. Bone
aaaaaaaaaaaaaaa Osteoblasts (bone cells)
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E. Reproductive System
Egg fertilization by sperm cells
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aaaaaaaaaaa 6-day old human embryo in the womb
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F. Cancer cells
aaaaaaaaaaaaaaaaaa Lung cancer cells
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Cervical cancer cell (scanning electron micrograph x5000) (source)
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3. Sub-cellular organelles
A. The Cytoplasm
The filling of the cell, including small organ-shaped bodies (organelles) surrounded by a dense water-like fluid. The latter includes most of the cell’s water-soluble molecules, including proteins.
Each organelle specifies in a distinct cellular role, and concentrate within them all the molecules needed to that role.
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(Source)
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B. The Plasma Membrane
Diagram:
(Source)
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Scanning electron micrograph:
(Source)
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Atomic force micrograph (showing surface topology)
Proteins protruding from the surface of the plasma membrane (source)
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What is it?
A flat body engulfing the entire cell. It is made of two attached layers of lipid molecules, within which proteins and carbohydrates are embedded.
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Functions
- Giving the cell its shape.
- Engulfing and protecting intra-cellular components
- Determining the chemical composition of the cell by controlling what goes in and out (channels/transporters)
- Relaying signals into the cell (cell-surface receptors)
- Mediating cell-cell and cell-matrix interactions (cell-surface proteins and sugars)
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C. The Cytoskeleton
aaaaaaaaaaaaaaaaaaaaaa The cytoskeleton of two fibroblasts (source)
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aaaaaaaaaaaaaaaaaaawaaaaWhat is it?
aaaaaaaaaaaaaaaeeeeeeeaaFunctions
- Provides mechanical support
- Involved in targeting of transport vesicles
- Allows the cell to change its shape (in movement and phagocytosis)
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D. The nucleus
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aaaaaaaaaaaaaaaaaaaaaaaaFunctions
- Storage & utilization of the genome.
- Synthesis of ribosomal subunits.
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E. The Endoplasmic Reticulum (ER)
Diagram:
(Source)
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Transmission electron micrograph:
ER sacks (left) emanating from the cell’s nucleus (lower right)
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aaaaaaaaaaaaaaaaaeeeaaaWhat is it?
aaaaaaaaaaaaaaaaaaaaaaaaFunctions
- Synthesis and modification of membrane proteins and also water-soluble proteins, most of which are destined for secretion, integration within the cell membrane, or for other organelles
- Lipids synthesis
- Storage of Ca2+ ions
F. The Golgi Network
Diagram:
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Transmission electron micrograph:
aaaaaaaaaa Transmission electron micrograph
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aaaaaaaaaaaaaaaaaaaaaaaaaaWhat is it?
Membrane-bound stacks located peripherally to the ER.
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aaaaaaaaaaa Functions
G. Mitochondria
Diagram:
a (Source)
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Scanning electron micrograph:
(Source)
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aaaaaaaaaaaaaaaaaaaaWhat is it?
Organelles containing two membranes and a simple genome. They are thought to originate from certain bacteria that have been assimilated by a primordial eukaryotic cell.
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eeeeeeeeeeeeeeeeeeeeeeFunctions
-
Extraction of energy from foodstuff by oxidation
- Involved in programmed cell death and possibly anti-viral response
H. Lysosomes
Diagram:
(Source)
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Transmission electron micrograph:
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aaaaaaaaaaaaaaaaaaaaaaaaaaaWhat are they?
Spherical bodies that contain hydrolytic enzymes
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aaaaaaaaaaaaaaaaaaaaaaaaaaaaFunctions
I. Peroxisomes
Diagram:
(Source)
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Transmission electron micrograph:
(Source)
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aaaaaaaaaaaaaaaaaaaaaaaaaaaWhat are they?
Spherical bodies that contain redox (reduction-oxidation) enzymes
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aaaaaaaaaaaaaaaaaaaaaaaaaaaaFunctions
- Metabolism (e.g. lipid degradation)
- Detoxification of drugs and harmful substances
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J. Chloroplast (plant and algae only)
Diagram:
(Source)
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Transmission electron micrograph:
(Source)
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aaaaaaaaaaaaaaaaaaaaaaaaaaaWhat is it?
Organelle found in plants and green algae, and which contains two membranes and a small genome. They are thought to originate from certain bacteria that have been assimilated by a primordial eukaryotic cell.
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aaaaaaaaaaaaaaaaaaaaaaaaaaaaFunctions
K. Cell Wall (plants, bacteria, fungi, algae, some archaea)
Diagram:
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Micrograph:
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aaaaaaaaaaaaaaaaaaaaaaaaaaaWhat is it?
A tough barrier peripheral to the plasma membrane. In plants it is made primarily of cellulose
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aaaaaaaaaaaaaaaaaaaaaaaaaaaaFunctions
L. Vacuole
Micrograph:
(Source)
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aaaaaaaaaaaaaaaaaaaaaaaaaaaWhat is it?
A water-filled cavity
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aaaaaaaaaaaaaaaaaaaaaaaaaaaaFunctions
- Storing nutrients, waste products, and pigments
- Degradation
- Regulating cell size, pH, and turgor pressure
M. Vacuole
Micrographs:
Scanning electron micrograph of the extra-cellular matrix in neural tissue. Nerves and nerve bundles are in yellow, extra-cellular supporting matrix in red, and ganglion cells are in blue.
Source
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Collagen fibers in the extra-cellular matrix removed from a human knee.
Source
What is it?
A complex network of polysaccharides (such as glycosaminoglycans or cellulose) and proteins (collagen, elastin) secreted by cells. Prominent in connective tissues (e.g. bone, cartilage, nerve, blood).
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aaaaaaaaaaaaaaaa Function
Serves as a structural element and also influences their development and physiology.
Viruses
1. Animal Viruses
The human immunodeficiency virus (HIV)
(source)
2. Bacterial Viruses (Phages)
(source)
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Cell Physiology
1. Phagocytosis
Macrophages and poly-morpho-nuclear cells are white blood cells comprising an important part of the animal innate defense system. They destroy invading bacteria (or other pathogenic cells) by internalizing and digesting them chemically. They are therefore called ‘phagocytes‘, which means ‘cells that eat’ their prey.
Once the phagocyte tracks down the invading pathogen, it extends parts of its membrane towards the pathogen and engulfs it. The pathogen is then internalized into the phagocyte within a lysosome-like vesicle, which prevents it from harming the phagocyte. Shortly after, the pathogen is attacked by different chemicals produced by the phagocyte for exact purpose. For example, the phagocyte has enzymes that produce free oxygen radicals like superoxide and hypo-chlorite. These molecules are highly reactive and attack the pathogen’s membrane, DNA and proteins chemically. This leads to the destruction of the pathogen.
Phagocytosis of tuberculosis-causing bacteria (red rods) by a macrophage (source)
2. Apoptosis (programmed cell death)
There are some cases in nature, where cells are forced to commit suicide. This happens, for example, during embryonic development, where limbs form by creating spaces in the original tissue via the death of numerous cells. Apoptosis also occur when a virus-infested animal cell is attacked by a T-lymphocyte (a white blood cell that is part of the immune system). The T-cell perforates the membrane of the attacked cell and sends through the pore chemical messengers that re-program the attacked cell to commit suicide. This may be unfortunate to the attacked cell, but it saves the rest of the body by eliminating the virus along with cell it infested.
Apoptoses cells undergo multiple changes before they break down and phagocytosed by white blood cells. For example, their cytoskeleton breaks down, they become spherical, and vesicles form on their surface.
A scanning electron micrograph of a cell undergoing apoposis (source)
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FURTHER READING
* More details on cell biology can be found e.g. in Lodish’s textbook.
* Details on the biochemical activity of cells can be found e.g. in Lehninger’s textbook.
