Chapter 4 A Tour of the Cell

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Chapter 4A Tour of the Cell
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4.1- Microscopy. The microscopes you are most
familiar with are called light microscopes or
brightfield microscopes. Figure A, page 52,
shows the path of light in the working
microscope. The light source is white light
which is focused on the specimen on the slide by
the condenser lens. Light passes through the
specimen and into the objective lens where the
image is magnified.
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Most microscopes today have multiple objectives
used for different purposes. The 4x or 10x
objective is used to scan the slide for the
specimen. The 40x objective is referred to as
high-dry and is powerful enough to see most
eukaryotic cells. Some light microscopes have a
100x objective called the oil immersion lens.
The opening to the objective is so small that oil
must be used to refract more light into the lens
to see anything. The light next passes through
the ocular (eye piece) lens where it is magnified
again. To get total magnification you must
multiply the power of the ocular and object. So,
if you have a 10x ocular with a 40x objective,
the total magnification is 400x.
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Why not just grind better lens and place more
lens in the sequence to increase the total
magnification? The limiting factor on our
ability to observe an image is not the total
magnification, it is the resolving power of the
microscope. Resolving power is the ability to
see independent images as independent.
Ultimately, the limiting factor on resolving
power is the wavelength of light used to
illuminate the specimen. The resolving power of
light microscopes is about 0.2 micrometers.Rober
t Hooke used some of the earliest microscopes to
observe tissues. He saw the compartmental nature
of living tissue and called the compartments
"cells". He developed the cell theory that
states that all living things are made up of
these cells.
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There are several other types of microscopes.
Your text discusses two types of electron
microscopes. These scopes use electron beams to
illuminate the specimen. The Scanning electron
microscope has a resolving power of 0.2 nm or
about 1000 times that of the best light
microscopes. The scanning scopes bounces a beam
of electrons off the surface of the specimen
after it has been coated with a heavy metal
stain. This allows us to observe surface detail
very well. The Transmission electron microscope
is used to observe internal cell structure after
the tissue has been sliced into thin slices using
an ultramicrotome, and stained with heavy metal
stains. Organelles can be observed easily with
TEM.
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Know the difference between a Photomicrograph and
a Microphotograph!
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4.2- Cell size. The figure on page 54 shows the
sizes of a variety of organisms, cells,
organelles, and subcellular particles. Spend
some time getting a feel for these but don't try
to memorize them. Note, however, that the scale
on is logarithmic. The prefixes on the chart are
more important and you should memorize the
following

• deci- 1/10 10-1
• centi- 1/100 10-2
• milli- 1/1000 10-3
• micro- 1/1,000,000 10-6
• nano- 1/1,000,000,000 10-9
• pico- 1/1,000,000,000,000 10-12

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4.3- The limits to cell size. The take home
message to this section is that volume to surface
area ratio is very important to a cells ability
to take in sufficient nutrients and expel waste
products. It does, on the other hand, have to be
large enough to carry on its function, and house
and replicate DNA and organelles. Read this
section, but don't worry about the math for an
exam in this class.
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4.4- Prokayotic cells. Prokaryotes do not have
a nucleus. They also do not have any membrane
bound organelles. The typical bacterial cell is
depicted in the figure on page 55. Be sure you
know the structures called flagella, plasma
membrane, cell wall, ribosomes, and pili. Also,
be sure you know their function. I would point
out that in many cases it is the membrane of an
organelle where the activity of that organelle
takes place. I prokayotes those activities take
place in the plasma membrane.
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4.5- Eukaryotic cells. Page 56 shows a figure
of a typical animal cell, whatever that means.
The next few sections are going to cover the
organelles, one at a time. The figure on page 57
shows a plant cell. Note the three main
differences in these figures are the presence of
cell walls, central vacuole, and chloroplasts in
plant cells. These are lacking in animal cells.
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4.6- The nucleus. I beg to differ with the
description of the nucleus as being a control
center. You can argue that the rest of the cell
and even the environment controls what happens in
the nucleus. The nucleus houses chromatin which
is a mass of DNA and protein. During cell
division the chromatin coils up into recognizable
chromosomes. The nuclear envelope is a double
membrane perforated with pores that allow
transport of material back and forth to the
cyotplasm. The nucleus is the site of DNA
replication and RNA synthesis (transcription).
It is the site of the control of gene expression.
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4.7- Endoplasmic reticulum (ER). This section
tells you that the rough ER and smooth ER are
connected and are continuous with the nuclear
envelope.
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4.8- The rough ER. It is rough because imbedded
in the membrane are ribosomes, the site of the
synthesis of secretory proteins. The rough ER is
also the site for the synthesis of membrane.
Enzymes synthesize phospholipid that forms all
the membranes of the cell. Ribosomes in the
rough ER synthesize protein that then are
converted to glycoprotein and packaged in
transport vesicles for secretion.
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4.9- The smooth ER. The smooth ER is the site
for the synthesis of lipids, phospholipids, and
steriods. Note that the production of steriod
hormones is tissue specific. For example, it is
the smooth ER of the cells of the ovaries and
testes that synthesize the sex hormones. The
smooth ER of the liver has several additonal
functions. Enzymes in the smooth ER regulate the
release of sugar into the bloodstream. Other
enzymes break down toxic chemicals. As the liver
is exposed to additional doses of a drug the
liver increases the amount of smooth ER to handle
it. It then takes more drug to get past the
detoxifiying ability of the liver. We become
more tolerant of the drug.
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Finally the smooth ER functions to store calcium
ions. Ca ions are required for muscle
contraction as we will discuss later.
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4.10- The Golgi apparatus. The Golgi apparatus,
like the ER, is a series of folded membranes. It
functions in processing enzymes and other
products of the ER to a finished product. It is
the source of the production of lysosomes (see
the figure on page 61).
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4.11- Lysosomes. These are membrane bound
vesicles that harbor digestive enzymes. The
membrane of a lysosome will fuse with the
membrane of vacuoles releases these digestive
enzymes to the interior of the vacuole to digest
the material inside the vacuole.
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4.12- Tay-Sachs disease is one of a group of
diseases causes by genes that result in abnormal
lysosomes. In this case the lysosomes lack a
lipid digesting enzyme that results in brain
cells that accumulate excess lipids. The disease
is fatal. There is a DNA test to determine the
likelyhood that a couple will produce children
with the disease.
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4.13- Vacuoles. These are membrane bound sacs
that have many different functions. The central
vacuole of a plant cell serves as a large
lysosome. It may also function in absorbing
water. The central vacuoles of flower petal
cells may hold the pigments that give the flower
its color. The contractile vacuoles of protists
collect and excrete water.
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4.14- This section reviews the endomembrane
system which encludes the nuclear envelope, the
rough and smooth ER, the Golgi apparatus,
lysosomes and vacuoles. Make sure you read and
review this section.
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4.15- Chloroplasts. Chloroplasts are double
membrane bound organelles which are the sites of
photosynthesis. The inner membrane forms a
series of stacked plates called granum. These
contain the pigments that absorb light energy.
The stroma is the thick fluid in chloroplasts.
Further discussion of chloroplast structure and
function is reserved for chapter 7.
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4.16- Mitochondria. These organelles are the
sites of respiration and convert the chemical
energy of sugars and other organic compounds into
the high-energy phosphate bonds of an ATP
molecule. These are also bound by a double
membrane. The inner membrane is the folded (the
folds are called cristae) and is the site of the
electron transport system. Further discussion of
the structure and function of mitochondria
(plural, mitochondrion is singular) is held for
chapter 6.
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4.17- Cytoskeleton. Eukaryotic cells has a
meshwork of tiny fibers that support the
structure. This network is the cytoskeleton.
Three types of fibers exist. Microfilaments are
solid helical rods composed of the protein actin.
There is a twist double chain of actin molecules
that make up microfilaments. These are found in
cells that must contract such as muscle cells.
Intermediate filaments are variable but in
general are ropelike structures made of twisted
filaments of fibrous proteins. These function in
bearing tension and anchoring organelles.
Microtubles are straight, hollow tubes composed
of proteins called tubulins. These anchor
organelles and provide tract along which
organelles may move. They also make up flagella
and cilia.
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4.18- Cilia and flagella. These are found on
cells, such as protists, that are motile. Cilia
are short and numerous. Longer less numerous
appendages are flagella. These are composed of a
core of microtubules wrapped in an extension of
the plasma membrane. It is sufficient to know
that Energy is required to move the cilia or
flagella in a whiplike motion to propel the cell.
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4.19- Cell surfaces. Cells are held tightly
together is higher organisms. There is also a
considerble amount of cell communication for lack
of a better term. Cell junctions are structures
that hold cells together. There are three types.
Tight junctions bind cells together forming a
leakproof sheet. Anchoring junctions attach
adjacent cells or cells to an extracellular
matrix (the substance in which tissues cells are
embedded. These are leaky compared to tight
junctions. Communicating junctions are channels
between similar cells. Plasmodesmata are
passages between adjacent plant cells that allow
material to go from one cell to the next.
Communication junctions fulfill the same role
between animal cells.
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4.20- The four catagories of organelle function.
There is a chart on page 67 that reviews these
catagories and which organelles fall into each
category. Study the chart.