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Chapter 1 –
Cell Structure
Introduction
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Robert Hooke examined slices of cork under a
microscope and decided to call the ‘pore-like’
structures cells.
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About 200 years later, a general cell theory was proposed using the
works of German scientists Schleiden (botanist, 1838) and Schwannn
(zoologist, 1939).
The cell theory states:
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Robert Hooke, through his observations, had discovered
and described in his 1665 book the fundamental unit of
all living things.
The basic unit of structure and function of all living organisms is the cell.
All cells arise from pre-existing cells by cell division. (Virchow, 1855)
Cells a very similar to a bag in which the chemistry of life is allowed to
happen. This is permitted because the cell is partially separated from its
external environment by a thin, partially permeable membrane.
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The cell’s membrane is a very effective barrier, but also allows for certain
materials to move in and out of the cell. This partial permeability allows for the
cell to maintain a stable environment for optimal function. (homeostasis)
Cell Microscopy
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The branch of biology that studies cells is known as cell biology
(normal cellular anatomy). Although cells can be studied using
various methods, scientists began by “looking” at the cells using
different types of microscopes.
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Nowadays, we use 2 fundamentally different types of
microscopes. Both of the microscopes use a form of radiation in
order to create am image of the specimen that is being
examined.
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The light microscope uses light as its source of radiation.
The electron microscope uses electrons.
Early in the nineteenth century, dramatic improvements were
made in the quality of glass lenses which allowed for rapid
progress in microscope design and in preparing material for
examination with microscopes.
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Cytology is the branch of biology that deals with the study of cells in
terms of structure, function, and chemistry.
Light Microscopy
Light Microscopy
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This is how the light
microscope works.
Photomicrographs
Plant Cell
Cheek Cells
Guard Cells
Plant vs. Animal Cell
There are many similarities and differences between animal and plant
cells. In order to see and identify certain cell structures under a
microscope the specimens need to stained.
Animal vs. Plant Cells – Differences
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A centriole, a small structure near the nucleus involved
in cell division, is only found in animal cells. These
are found as a pair and lie at right angles to
each other in the region known as the centrosome.
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Plant cells are usually much larger than animals cells, so they are easier
to see under a light microscope.
Plant cells are also surrounded by a cell wall, a rigid membrane that
surrounds the plasma (cell) membrane. The cell wall is freely permeable
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and allows the free movement of molecules and ions through to the cell
membrane.
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The cell wall gives the cell a definite shape and prevents it from bursting
when water enters the cell, increasing the internal pressure. This is
because the cell wall is made of
cellulose and can sometimes even be
reinforced with lignin to provide extra
strength.
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Neighboring plant cells are linked by fine
strands of cytoplasm called
plasmodesmata.
Animal vs. Plant Cells – Differences
Plant cells possess a large central vacuole
surrounded by the tonoplast. (Unlike animal
cells, the plant vacuole is large and
permanent)
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The central vacuole is composed of a
solution of mineral salts, sugars, O2, CO2,
pigments, enzymes, and other organic
compounds including waste products.
The tonoplast controls any exchange
between the cytoplasm and the vacuole.
One of the main purposes of the vacuole is to regulate the osmotic
properties of cells.
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Chloroplasts are large green organelles found
mainly in the leaves of plants used in
photosynthesis.
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They contain chlorophyll, the green pigment
that absorbs light that is needed for the light
dependent reactions of photosynthesis.
Animal vs. Plant Cells – Similarities
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A thin, partially permeable cell surface
(plasma) membrane that surrounds the cell.
A relatively large nucleus containing chromatin,
a mass of loosely coiled threads.
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Chromatin condenses to form the visible
separated chromosomes used during
cell division.
DNA, a molecule which contains the
instructions that control the activities of
the cell.
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Loops of DNA form the nucleolus
within the nucleus.
Animal vs. Plant Cells – Similarities
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The cytoplasm, an aqueous material that is in between that plasma
membrane and the nucleus. This can be liquid or jelly-like.
Mitochondria is the most abundant organelle
seen with a microscope responsible for aerobic
respiration.
Organelles, small and distinct functional and
structural parts of the cell.
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Each organelle is separated from the cytoplasm by its own membrane.
(compartmentalisation)
This allows the cell to show division of labor, in which the work to reach
the cell’s ultimate function is shared among
the different specialized organelles.
The Golgi apparatus, a part of a complex
internal sorting and distribution system found
within the cell.
Measuring in Cell Studies
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When measuring objects in the microscopic world, small units of
measurement should be used. The basic unit of length using the
International System of Units (SI units) is the meter (m).
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However, in order to measure some of these microscopic
objects we must sometimes use units even smaller than the
millimeter (mm).
Fraction of a meter
Unit
Symbol
millimeter
mm
One millionth = 0.000 001 = 1/1 000 000 = 10-6
micrometer
μm
One thousand millionth = 0.000 000 001 =
1/1 000 000 000 = 10-9
nanometer
nm
One thousandth = 0.001 = 1/1000 = 10-3
Magnification and Resolution
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Magnification is the number of times larger an image is
compared with the real size of the object.
Magnification =
observed size of image
actual size of specimen
M= I
A
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NOTE: Make sure that when calculating magnification ALL units
are the SAME!!!
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Resolution is the ability to distinguish between two separate
points. If 2 objects are closer together than the resolution of the
apparatus used, then the objects cannot be distinguished as
separate.
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Resolution is the amount of detail that can be
seen.
An increase in magnification is not necessarily
accompanied by an increase in resolution.
Light microscope maximum resolution is 200 nm,
while the electron microscope resolution is
0.5 nm.
Measuring Cells
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An eye-piece graticule can be used when measuring cells
and organelles under a microscope. The eye-piece graticule
is a transparent scale which is placed in the microscope
eyepiece. This allows the object to be measured while it is
being observed.
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However, before you can determine the correct size of a
specimen, the eyepiece
graticule must be calibrated.
This is done by placing a
miniature transparent ruler
(stage micrometer scale) on
the stage of the microscope
and focusing it .
Measuring Cells
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Once the scales are superimposed, you can find the value of
each eyepiece graticule division by:
Stage micrometer scale = Eyepiece graticule division
Eyepiece graticule scale
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Once the measurement for each division is found, then
observe how many divisions the specimen measures and you
can find the actual diameter of your specimen.
Number of
divisions
X
Value (measurement) = Actual diameter of
of each division
specimen
The Electron Microscope
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In order to observe objects smaller than 200nm, scientists needed to
use radiation with a shorter wavelength than visible light. The best
solution was the use of electrons.
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When electrons gain too much energy, they escape from their orbits
and behave much like electromagnetic radiation. Therefore, since
they are high energy, they have shorter wavelengths.
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Electrons are a great form of radiation for microscopy because
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Their wavelength is extremely short.
They are negatively charged, so they can be focused using
electromagnets.
Electron Microscopy
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This is how an electron
microscope works.
Transmission vs. Scanning Electron
Microscope
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There are two types of electron microscopes used today.
 The transmission electron microscope (TEM) has the electron
beam passing through the specimen before it is viewed.
Unfortunately, the only portion of the specimen that can be seen
is the parts where the electrons have actually passed through. This
allows us to see thin sections of specimens. The resolution of a TEM
is between 3 nm and 20 nm.
 The scanning electron microscope (SEM) uses the electron beam
to scan the surfaces of the specimen and only the reflected
beam is observed. This allows for surface structures to be seen as
well a greater depth of field to be obtained, which allows the
specimen to be in better focus. However, the SEM cannot
achieve the same resolution as the TEM.
Transmission vs. Scanning Electron
Microscope
Electron Micrographs
TEM Micrograph
SEM Micrograph
The Electromagnetic Spectrum
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Light is capable of traveling in waves. However, the length of the
waves of light varies and can be distinguished by the human eye
and changed into specific colors by the brain.
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The range of the variation of wavelengths is the electromagnetic
spectrum.
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The longer the waves, the lower the frequency.
Energy changes wavelengths. The greater the energy, the shorter the
wavelength.
The limit of resolution is
about one half the
wavelength of the radiation
used to view the specimen.
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If the object is smaller than
half the wavelength of the
radiation used to see it, then
the object will not be
separated from nearby
objects.
Light vs. Electron Microscopes
Light vs. Electron Microscopes
Ultrastructure of Animal Cell
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The ultrastructure of a cell is the detailed structure that is revealed by
the electron microscope.
Nucleus
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Largest cell organelle that is surrounded by 2 membranes
(nuclear envelope).
A rounded structure enclosed in a membrane and
embedded in the cytoplasm. Its function is to control the type
and quantity of enzymes produced by the cytoplasm.
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This allows the nucleus to regulate the chemical changes
which take place within the cell and determines WHAT that cell
will be.
The nucleus also controls cell
division (reproduction). It
contains the thread-like
chromosomes that play an
important role in cell division
and inheritance.
Most cells contain one nucleus,
but some cells can have many
nuclei.
Endoplasmic Reticulum
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An extensive system of membranes that run through the
cytoplasm and can contain ribosomes. The membranes form
systems of flattened sacs (cisternae) that can go on to form the
Golgi Apparatus.
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Ribosomes are small organelles composed of 2 sub-units made up of
RNA and protein. These organelles manufacture proteins that are
then transported throughout the cell.
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The Rough ER contains ribosomes,
therefore it is responsible for
transporting the proteins that are
made by the ribosomes.
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The Smooth ER, in contrast, does
not contain ribosomes, so it is
responsible to make lipids and
steroids that can then be used by
the cell.
Golgi Body (Apparatus / Complex)
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A stack of flattened sacs that is constantly being formed at
one end from vesicles that bud off the ER.
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The Golgi apparatus is responsible for collecting, processing,
and sorting molecules that will be transported by the Golgi
vesicles to other parts of the
cell or out of the cell. This is
known as secretion.
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The enzymes of the Golgi body
can convert sugars into cell
wall components.
The Golgi vesicles can also
be used to make lysosomes.
Lysosomes
Spherical sacs surrounded by a single membrane with no internal
structure. They are usually 0.1-0.5 μm in diameter.
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Lysosomes contain hydrolytic enzymes that need to be kept
separate from the rest of the cell in order to prevent any damage.
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They are responsible to
digest unwanted
structures within the cell
or can be released
outside of the cell.
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They can break down
old organelles or whole
dead cells. In addition,
they can be used to
digest bacteria.
Mitochondrion
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Tiny spherical, rod-like, or elongated organelles surrounded by a
double membrane. They are usually about 1 μm in diameter.
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The inner membrane folds to form cristae that project into the matrix of the
organelle.
They are responsible for releasing energy from food substances
(cellular respiration). As a result, most of the energy is transferred to
molecules of ATP, which is needed
by the cell. In addition,
mitochondria can also be
involved in the synthesis of
lipids.
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These are found in large
amounts in areas of rapid
chemical activity.
Microtubules
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Microtubules are long, rigid, hollow tubes that are found throughout
the cytoplasm of the cell. They are about 25nm in diameter and
they make up the cytoskeleton of the cell.
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The microtubules are made up of tubulin (protein). When both forms of
tubulin (β and alpha) combine, they for a dimer. Thirteen protofilaments
will line up alongside each other in a ring to form a cylinder with a hollow
center. This is known as a microtubule.
Microtubule organising centres (MTOCs) are special locations within the
cells where the
assembly of
microtubules occur.
Centrioles
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Hollow cylinders that are formed from a ring of microtubules
that lie close to each other near the outside of the nucleus.
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The microtubules that form the centrioles are used to grow the
spindle fibers that are used for nuclear division.
Centrioles are bout 500 nm long and are formed from a ring of
short microtubules. Each centriole contains 9 triplets of
microtubules.
Cilia, Flagella and Microvilli
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Microvilli are finger-like extensions of the
surface membrane. Their function is to
increase cell surface area so that it can
maximize absorption.
Cilia and flagella are long, thin extensions
that move in a wave-like manner. They are covered by an extension of
the plasma membrane and contain microtubules that allow for their
movement. This movement allows substances around the cell to move in or
out, or, if the cell is not attached to anything, to move the cell itself.
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A small quantity of long
extensions are known as
flagella.
A larger quantity of shorter
extension are known as
cilia.
The Endosymbiont Theory
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Mitochondria and chloroplasts contain ribosomes that are smaller
than those that are found in the cytoplasm and are the same size as
the ones found in bacteria.
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Mitochondria and chloroplasts also contain small, circular DNA.
Later it was proved that mitochondria and chloroplasts are ancient
bacteria that now live inside larger cells.
Plant vs. Animal Cells
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Plant cells differ from animal cells in many ways. Plant cells
contain 3 important structures / organelles that are not present in
animal cells:
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Cell Wall – This is a tough wall made of cellulose or other compounds
that is tightly placed against the outside of the cell membrane. It is a
non-living structure that allows water and other dissolved substances
to pass through. IT IS FULLY permeable!!!
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Vacuole – This is a large, fluid-filled space that contains cell sap. Cell
sap is a watery solution made up of sugars, salts, and sometimes
pigments. Due to its size, the vacuole pushes the cytoplasm aside so
that it forms a thin lining inside the cell wall. This causes an outward
pressure that makes the plant cells and tissues extremely rigid.
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NOTE: Some animal cells can produce smaller vacuoles. However, these
vacuoles are only produced to carry out a particular job and are not
permanent.
Plastids – These are organelles that are only found in plant cells. If
they contain chlorophyll (green pigment), these plastid are known as
chloroplasts (photosynthesis). If they are colorless, they usually contain
starch, which is used as a food source.
Two Fundamentally Different Cell Types
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There are two fundamentally types of cells. The main difference is whether
the cell contains a nucleus or not.
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Prokaryotes are organisms that do NOT contain a nucleus. These are referred to
as bacteria. These are smaller and simpler in structure.
Eukaryotes are organisms that DO contain a nucleus. These contain animals,
plants, fungi, and protoctists.
Prokaryotes
Eukaryotes
Average diameter is 0.5 – 5 μm
Cell can measure up to 40 μm and have 1000-10 000 times the volume of
prokaryotic cells.
DNA is circular and lies free in the cytoplasm
DNA is not circular and is within a nucleus (surrounded by a double
membrane envelope)
DNA is naked
DNA is associated with proteins, forming chromosomes
Slightly smaller (70s) ribosomes (approx. 20 nm
in diameter)
Slightly larger (80s) ribosomes (Approx. 25 nm in diameter)
No ER present
ER present; Ribosomes can be attached
Very few organelles. No separate membranebound compartments unless formed by infolding
of the cell surface membrane
Many types of organelles present. Some are single membrane bound
(lysosomes, golgi bodies and vacuoles), some are double membrane bound
(nucleus, mitochondrion and chloroplasts), and some have no membrane
(ribosomes, centrioles and microtubules)
Cell wall present – wall contains murein, a
peptidoglycan (polysaccharide and amino acids)
Cell wall sometimes present (Not in animals at ALL!!!) in plants and fungi –
contains cellulose or lignin in plants and chitin (nitrogen-containing
polysaccharide similar to cellulose) in fungi
Prokaryotic Cell
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Pili – Used for the attachment to other cells or surfaces; involved
in sexual reproduction
Cell Wall – contains murein, a type of peptidoglycan
Flagellum – used for locomotion.
Plasmid – Small circle of DNA
Capsule – additional protection
Ribosomes – 70s
Viruses
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Viruses are tiny ‘organisms’ that are much smaller than bacteria
and are on the boundary between living and non-living.
Viruses do NOT have a cell structure. They mostly consist of:
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A self-replicating molecule of DNA or RNA which acts as its genetic
code.
A protective coat of protein molecules. (Capsid)
Viruses can range in size from 20-300 nm.
Viruses are parasitic since they can only
reproduce by infecting and taking over
other living cells.