Lecture 21
Cell Discovery and Theory
Ozgur Unal
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Observe the cork given to you and try to describe what you
see.
Using the microscope, look at the cork and describe what
you see.
A cell is the basic structural and functional unit of all living
things.
Cells are the building blocks of organisms.
How small do you think cells are?
How many cells do you think you have in your body?
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In 1590, Hans and Zacharias Janssen invented the first
compound microscope.
In 1655, Robert Hooke observed cork and
named the tiny chambers that he saw cells.
In 1683, Dutch biologist Anton
van Leeuwenhoek discovered
single-celled organisms.
In 1830-1855, scientists
discovered that plants and
animals have cells and all cells are produced from the
division of existing cells.
Figure 7.1
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All these discoveries lead to the cell theory with three
principles:
All living organisms are composed of one or more cells.
Cells are the basic unit of structure and organization of all
living organisms.
Cells arise only from previously existing cells, with cells
passing copies of their genetic material on to their daughter
cells.
How were scientists able to observe cells in the past?
What tools do they use to study cells today?
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The discovery of cells and the development of the cell
theory would not be possible without microscopes.
Miscroscopes help us see very small objects.
There are two important characteristics of microscopes:
Magnification
Resolution
Higher magnification helps you see smaller objects.
Higher resolution helps you see two objects very close to
each other (help us to see the details)
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Compound light microscopes:
Consists of a series of lenses and
uses visible light.
Contains an eyepiece and an
objective lens.
Maximum magnification around 1000x.
Electron microscopes:
Use electrons to produce magnified images.
Transmission Electron microscopes (TEM) magnify up to
500,000x.
Scanning electron microscopes (SEM) produce 3-D images.
TEM and SEM can be used only for dead cells.
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Scanning Tunneling Electron
Microscopes (STM) can be used
for live specimens.
STMs produce 3-D images for
very small objects.
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You have probably realized that the cells that make up
organisms must have different types.
For example, the cells in a plant are different than the ones
in a animal.
Scientist have grouped cells into two broad category:
Prokaryotic cells and Eukaryotic cells.
You see images of two
different cells. List the
similarities and differences
between each.
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Both types of cell have a boundary.
A plasma membrane is a special boundary that helps
control what enters and leaves the cell.
Plasma membrane exists
in all cells.
All cells contain genetic
material that provides
instructions.
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Eukaryotic Cells:
These cells have internal structures called organelles.
Each organelle carries out specific cell functions.
Nucleus is a central organelle that contains cell’s genetic
material in the form of DNA.
Most organisms are made up
of eukaryotic cells and are
called eukaryotes.
There are some unicellular
organisms that are eukaryotes.
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Prokaryotic Cells:
These cells are defined as cells without a nucleus or other
membrane-bound organelles.
Many scientists think that prokaryotes are similar to the
first organsism on Earth.
Up to 100 times smaller
than eukaryotic cells.
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Why are there two basic types of cells?
Eukaryotic cells probably evolved from prokaryotic cells
millions of years ago.
According to endosymbiont theory, a symbiotic mutual
relationship involved one prokaryotic cell living inside
another. (This will be discussed in Chapter 14)
Eukaryotic cells are larger and have distinct organelles.
These cells have developed specific functions and this lead
to the cell diversity, such as nerve cells, bone cells, leaf cells,
stem cells etc.
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Lecture 22
Plasma Membrane
Ozgur Unal
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Look at the filter paper as I pour muddy water on it.
Answer the following questions.
What did you just observe?
What is the purpose of the filter here?
What type of particles are selected by the filter paper
to pass?
What type of particles are not selected by the filter
paper to pass?
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How about this fishnet?
What is the purpose of this fishnet?
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Do you think cells need such a filtering system?
What controls what enters and leaves a cell?
Remember that plasma membrane is a special boundary
that helps control what
enters and leaves a cell.
Plasma membrane is a
thin and and flexible
boundary between a cell
and its environment.
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Homeostasis is important for cells.
Plasma membrane allows nutrients into the cell and allows
waste and other products leave the cell.
A key property of the plasma membrane is selective
permeability.
By selective permeability, a membrane allows some
substances to pass through while keeping others out.
The structure of plasma membrane controls how, when
and how much of these substances enter and leave a cell.
http://education.uoit.ca/lordec/ID_LORDEC/diffusion_o
smosis/garib_diffusion_osmosis.swf
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Most of the molecules in the plasma membrane are lipids.
Plasma membrane is composed of phospholipid bilayer, in
which two layers of phospholipids are arranged tail to tail.
Figure 7.6
Phospholipids have a polar
head and a non-polar tail.
What is the advantage of
having non-polar tails in a
plasma membrane?
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Plasma membrane animations:
http://telstar.ote.cmu.edu/biology/downloads/me
mbranes/
http://www.yellowtang.org/animations/bilayer.swf
Do you think vitamin C can enter cells?
No, although vitamin C is water-soluble it acnnot diffuse
through the plasma membrane because it is a charged, polar
molecule.
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Moving with and among the phospholipids in the plasma
membrane are cholesterol, proteins and carbohydrates.
Proteins at the inner surface anchor the plasma membrane
to the cell’s internal structure, giving the cell its shape.
Transport proteins move needed substances or wastes
through the plasma membrane,
contributing to the selective
permeability.
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Nonpolar cholesterol is repelled by water and it is
positioned among phospholipids.
Cholesterol prevents phospholipids bilayer tails from
sticking together. This contributes to the fluidity of the
plasma membrane.
Carbohydrates in the membrane are attached to proteins
adn they stick out to define cell’s characteristics and help
identify chemical signals.
For example, carbohydrates in the membrane help disease
fighting cells recognize and attack potentially harmful cells.
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Phospholipids bilayer in the plasma membrane create a
“sea” in which other molecules can float.
Phospholipids can move sideways in the membrane.
Components of the plasma membrane are in constant
motion, sliding past each other.
Fluid-mosaic model of the plasma membrane explains this
characteristics.
http://telstar.ote.cmu.edu/biology/downloads/membranes/
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