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Cell Theory
Cells and Immunity (Grade 8)
The basic units of all living things are called cells. Through centuries of observations
scientists have identified two main ideas which together are called cell theory:
 All living things are composed of one ore more cells.
 All new cells originate from cells which already exist.
Cell Structures Visible with a Light Microscope
The Nucleus: The control center of the cell is called the nucleus. All
of the cells activities are directed by the nucleus. For both plant and
animal cells, the nucleus is surrounded by a membrane.
Chromosomes:
Chromosomes, which contain DNA or genetic material, are contained
within the nucleus. DNA provides the “blue prints” or “construction
plans” for all the pieces of the cell. Before being passed on to other identical cells, the
genetic information of DNA must be duplicated.
The Cell Membrane: The cell membrane holds all of the
cell material in place; as well as, acting like a
gatekeeper, controlling the movements of materials, like
nutrients, and wastes in and out of the cell.
The Cytoplasm:
A watery fluid called cytoplasm holds everything inside
the cell membrane but outside of the nucleus. Much of
the cells chemical activity takes place within cytoplasm. Its fluid allows materials to
steadily pass between different cell structures. As well, cytoplasm
can store waste material until it can be disposed.
The Vacuole: A vacuole has an appearance like bubble. It acts as a
fluid filled storage area. Vacuoles can be used to store water and
nutrients like glucose (sugar) and minerals. Excess cell waste may
also be stored in a vacuole.
Tiny Structures (organelles) of a Cell which can only be
seen with an Electron Microscope
Within the working of a cell, or cytoplasm, are tiny
structures, which can only be seen with an electron
microscope, called organelles.
Mitochondria; Energy production:
Mitochondria are the cells power source. They
convert glucose (sugar) into energy by combining it
with oxygen to produce carbon dioxide and water.
This process is called cellular respiration.
vacuole
ribosomes
Ribosomes: Protein Manufacturing:
Ribosome’s use the information provided by the
nucleus to build protein molecules. Different proteins
are needed for the growth, repair and reproduction.
Endoplasmic Reticulum: Material Transport:
Endoplasmic reticulum may be “rough” if it has
ribosomes attached or “smooth” if it has none. Both
types of endoplasmic reticulum carry materials, like
glucose, through the cell. “Rough” endoplasmic
reticulums have more responsibility for moving
amino acids (the small particles from which
ribosomes manufacture proteins); whereas, “smooth”
endoplasmic reticulums have more responsibility for moving fatty acids (for the
manufacture of fats {lipids}).
The Golgi Apparatus: Protein Storage:
The Golgi apparatus (or Gogli body) is an organelle which stores proteins and releases
them to the surface of the cell in packages called vesicles.
Lysomes: Recycling: Lysomes are manufactured by the
Golgi apparatus (or Gogli body) and sent out to clean
and patrol the cytoplasm. They contain proteins, called
enzymes, which chemically change large molecules into
smaller molecules that can be used by the cell. Lysomes
also play a role in destroying harmful substances (like
alcohols) and invading bacteria.
The difference between Plant and Animal Cells
Two cell structures, the cell wall and chloroplasts, carry out functions in plant cell that
are not needed by animal cells. The cell wall is tough material which surrounds the cell
membrane giving the plant cell a box-like shape. This rigid outer wall, allows plant cells
to support each other structurally as the plant grows.
Plants contain green fluids called chlorophyll which capture sunlight to convert water and
carbon dioxide into sugar (food) and oxygen in the process called photosynthesis.
Chloroplasts are the organelles which house chlorophyll.
Also note, the vacuole is much larger on a plant cell then an animal cell.
Diffusion, Osmosis and Turgor Pressure.
Did you know? Concentration usually refers to the amount of dissolved solid (like salt)
packed into a liquid (usually water).
Diffusion refers to the movement of molecules from an area of high concentration to low
concentration. A drop of dye is actually mixture of dye molecules and water. In the dye,
the concentration of dye molecules is high (the dye molecules are packed tightly
together), so once a dye drop hits the water, the dye molecules will try to move away
from each other - putting more and more water molecules between themselves and other
dye molecules. As the dye spreads through the water, the concentration of dye in any one
area is lowered. A similar thing will happen with sugar water. If you add a cup (250 ml)
of sugar water to a liter of water, the sugar molecules will spread out in the water,
lowering the overall concentration of sugar. Adding sugar at this point will increase the
concentration of sugar; whereas, adding water will lower the concentration. Adding water
always weakens or dilutes a solution so that certain properties like the “sweet taste” are
less noticeable.
Diffusion is one way in which molecules move in and out of cells. Oxygen, which a cell
uses up continually, will be of low concentration in the cell and of high concentration
outside the cell. Substances like oxygen which are of high concentration outside the cell
will diffuse into the cell through the cell membrane. On the other hand, a waste product,
like carbon dioxide, will be at higher concentration inside the cell than outside, so they
will diffuse out of the cell. Will glucose levels be higher inside the cell? Glucose is also
known as (AKA) blood sugar because it is continually being added to the bloodstream as
we digest carbohydrates. Inside the cell glucose is continually used up to supply energy.
Hence, glucose levels are usually lower inside the cell than out.
Did You Know? Carbohydrates are starchy foods like bread, cereals, potatoes, pastas
and vegetables. Starch molecules must be cut into maltose and then into glucose before
the reach the blood stream.
Did You Know? A selectively permeable membrane is a material which allows some
particles to pass through, but others are kept out. The lining of the small intestine is a
selectively permeable membrane because - for example – glucose can pass through but
similar sugars like maltose cannot. The cell membrane is also a semi-permeable
membrane. This is especially important for skin cells, because we do not want everything
we touch in the environment into absorb into our skin (especially bacteria).
Osmosis refers specifically to the diffusion of water across a selectively permeable
membrane (i.e. a cell membrane). If you were to surround a cell with salt water, the
water in the cell would be of higher concentration, than the water surrounding the cell.
(Wait a minute – Isn’t the salt water
of higher concentration? It is if we
Osmosis Example 1: Outward Flow
were speaking in terms of salt, but in
terms of water, fresh water has more
water molecules in a volume then the
salt water does). A cell in salt water
H2O
will send the water inside itself,
diffusing out into the salt water. This
H2O
outward diffusion of water, or
osmosis, is an attempt to dilute the
salt water outside cell or increase its
= water molecule
water concentration. This is usually
= salt particle
not a good thing for a cell because the
water moving out of the cell dehydrates the cell.
Did You Know? There is good reason to salt a wound. Bacteria are everywhere and
many are harmful. Any time you cut yourself bacteria can enter. All bacteria are singlecelled creatures. If you surround them with salt, the water on their insides will undergo
outward osmosis until the bacteria dies of dehydration.
Water can also diffuse into a cell (inward flow). A cell usually contains water with other
molecules like glucose (sugar), so if a cell is surrounded by very pure water (of high
water concentration), that water will attempt to move into the cell. This is fine if the cell
was dehydrated to begin with, but if not, the inflow of water may overly swell up the cell.
You can see this with certain fruits or vegetables like grapes, cherries or tomatoes. Overwatering tomatoes or cherries will
Osmosis Example 2: Inward Flow
cause them to split open as the cells
within over expand. Wilted grapes on
the other hand, can be rejuvenated to
a plump state by sitting them in water
for a while before eating them.
H2O
H2O
Turgor Pressure: Water will enter a
plant cell by osmosis if the water
outside the plant cell is of higher
concentration than inside the plant
= water molecule
cell. (Remember: The diffusion of
= glucose (sugar) molecule
water is called osmosis). A
dehydrated plant is easy to spot, it will look wilted (picture a dried out piece of celery). In
this situation the cytoplasm within the cells will shrink away from the cell wall. When
fresh water reaches the cell, the cytoplasm and vacuoles will fill back up and once again
push outward against the cell wall. This outward pressure is called turgor pressure.
When the cell is full of water, and the turgor pressure highest, the cell wall begins to
resist the push. The squeezing of the cell wall against the turgor pressure acts like a shut
off valve, preventing more water from entering the cell.
The Invaders and Immunity
The Invaders
There are a number of invaders which can enter our system to cause infections and
illness. These include bacteria, viruses, protists, fungi, and parasitic worms.
Bacteria
There are over 5000 different types of bacteria (single bacterium) on the Earth inhabiting almost every conceivable environment, from hot springs to the undersides of
glaciers. Bacteria are single-celled creatures or unicellular organisms – they may also be
called microbes or microorganisms because they can only be seen under a microscope.
They also have no nucleus, so they are prokaryotic cells.
Did you know? Cells with a nuclear membrane are called eukaryotic cells. If the nucleus
of a cell is not surrounded by a membrane, as is the case with bacteria, then the cells are
called prokaryotic cells.
Bacteria can wreak havoc on your system, but not all bacteria are bad. Some good
bacteria are used to make cheese and yogurt. Healthy bacteria in the digestive system
help to break down waste and release minerals.
Bacteria can be harmful because they are little machines that consume other cells. If your
body cannot fight them off (and your illness becomes too severe), you may need to take
antibiotics. “Superbugs” are bacteria that are resistant to antibiotics, and as such, can
cause extreme illness, even death.
Did you know? Some harmful bacteria include salmonella which causes food poisoning,
and E.Coli which causes diarrhea. Necrotizing fasciitis is a rare bacteria which causes
flesh eating disease.(Note: Flesh eating disease may also be called Necrotizing Fasciitis)
Protists unlike bacteria are eukaryotic unicellular organisms – the single cell that makes
up their being contains a nucleus. Like a bacteria, protists are neither animal nor plant.
One type of protist that is close to a plant is called a diatom. These one-celled organisms
contain chlorophyll and can produce food by photosynthesis. A protist that is both plantlike and animal-like is the euglena which propels itself with a whip-like tail called a
flagellum. The euglena can use chlorophyll to make food if sunlight is available (plantlike) or consume feed on smaller cells if necessary (animal-like). Two of the most
common animal-like protists are the amoeba and the paramecia which both feed on
smaller cells and bacteria. The amoeba is a blob-like, shape changing, unicellualar
oraganism that moves itself by stretching out and anchoring its pseudopod (false foot).
The paramecium (plural is paramecia), moves with use of hair-like oars called cilia.
Did you know? Some single-celled animals will have a flagellum or whip-like tail which
they use to propel themselves with. Others will be covered with cilia or tiny hairs which
work together to move surrounding fluid.
Fungi
Bread moulds, mushrooms, puff balls and yeasts are all fungi (singular is fungus). Some
fungi like mushrooms are multicellular, while others, like yeast, are unicellular. Fungi
like yeast will grow on any piece of decaying plant or animal matter. Some harmful fungi
include those that cause ring worm (named for the shape the fungi grows in), and
athelete’s foot
Did you know? Fruit flies circle fruit because they eat the yeast that grows on fruit.
Viruses
If bacteria are the most primitive organisms – existing as prokaryotic cells (without a
nuclear membrane) – then viruses are one stage below. Viruses are not considered living
because they have no cell; they are just strands of DNA (chromosomes) which puncture
through cells of living things. When virus invades a cell, it takes over the nucleus,
reprogramming it to replicate (or clone) the virus over and over until the cell explodes.
The most deadly viruses destroy organs; less deadly ones produces skin pustules like
those of chicken pox or cold sores; while others irritate breathing tracts and mucus
membranes too mimic the common cold.
Did you know? Deadly viruses include the avian flu virus which originally wiped out
flocks of birds, but has made its way into humans, and Ebola which quickly destroys
internal organs.
The Immune System
“Germs” or disease causing invaders like bacteria and viruses are called pathogens.
Pathogens never stop attacking your body. To prevent infections your immune system
has powerful defenses against pathogens.
The first line of defense is a physical barrier of skin, organ linings, mucus, tiny hairs, and
acidic fluids. In addition to its texture – which is difficult to for pathogens to penetrate –
skin is covered by a slightly acidic oil which traps and kills off pathogens. Of course
many invaders can get around the skin by entering your body through your mouth.
Luckily stomach acid is an effective killer of most pathogens. To trap air borne pathogens
your nose and lungs are lined with mucus and cilia (small hairs).
Let’s say a pathogen like a bacteria does get past the bodies physical barriers. All foreign
organisms release a chemical signal which identifies
white blood cell
them as invaders to our own defense cells, the white
blood cells. Once the chemical signals from an
invader are detected, an increased blood flow to the pathogen
infected area will bring in white blood cells. The
white blood cells capture, engulf and destroy the
invading pathogens.
Did you know? You can detect an invading organism
in your system by how your body responds. When the increase in blood flow reaches the
infected area, swelling, and redness will occur – this known as inflammation. In addition
to inflammation you may also experience a fever. You can also recognize when you white
blood cells are effectively destroying the invader by the presence of pus. Pus is the waste
product of destroyed pathogens and dead white blood cells.
Sometimes an invader is so aggressive that your white blood cells cannot stop it before
its starts entering tissue and organ cells, this is when your bodies last line of defense will
take over. The last line of defense is called the acquired immune system. The key
defenders in the immune system are chemical agents called antibodies. When an invader,
like bacteria , breaches our physical defenses its chemical signals identify it as an
invading cell. The chemicals signals given off by an invading organism are called
antigens. Antigen is short for “antibody generator”, because, as the name suggests, these
chemicals cause the body to start to produce or “generate” antibodies.
Essentially, the work of antibody is simple; it finds the chemical signal and attaches
itself, chemically, to site of that signal. This binds up the organism so that it can no
longer attack the body’s cells. The problem for the immune system is that every invader
has a different chemical signal - a different antigen – so your body must have antibody
ready, or make new one for every invader. Worse, the invaders, especially viruses and
bacteria are always evolving and changing, so our immune system must continually
change to stop the onslaught of invaders.
antibodies
antigens
(chemical
signals)
"bound
up"
pathogen
pathogen
Did you know? Non-living things like toxins (harmful chemicals compounds and metals),
and pollens (and other allergens), also give off chemical signals. Antibodies also bind to
the sight of these signals, preventing them from entering cells. Allergens like pollens are
actually harmless substances which your body mistakes for harmful invaders. The body’s
response to allergens has symptoms like an infection – itchy redness and inflammation –
which can make an allergy sufferer very uncomfortable.
As you get older, you are exposed to more and more pathogens. Every time your body
builds a new antibody to destroy a new pathogen, it keeps some of the antibody around in
case the same pathogen returns. This is why you only get the chicken pox once – you will
be exposed to the chicken pox virus many times in your life, but once you have the
chicken pox antibody, your immune system easily fights it off future invasions. The
accumulation of antibodies is called an acquired immune response – it is called such
because you acquire the immunity by building antibodies as you go.
Did you know? The cells that keep your antibody memory roam freely in the blood and
are called B-cells. They also manufacture antibodies when needed.
A vaccination is an injection of blood plasma containing dead or weakened pathogens.
The chemical signals (antigens) carried by these “compromised” pathogens stimulate the
immune system to develop antibodies.
Did you know? A “flu” shot is a vaccine which targets the various strains of the influenza
virus. Influenza causes symptoms of the common cold, but with severe body and head
aches, extreme tiredness and fever. A breakout of influenza in 1918, killed more people
than World War I.
Before vaccinations were invented the only way to acquire antibodies, was to get a
disease, get really sick, and let your system make the antibodies while you suffer. With a
vaccination, you get to acquire the antibodies, but you do not need to get really sick in the
process.
Did You Know? You can acquire immunity to toxins or even poisons. Antibodies will
develop to bind up toxins and poisons (provided they do not kill you first). At the turn of
the 19th Century, Russian politicians tried to kill the “Tsar influencing Monk” Rasputin
with poison. Rasputin did not die from the poison, much to his adversary’s dismay.
Historians believe that Rasputin had the antibody for the poison in his system because he
had taken small amounts throughout his life, thus, he had acquired immunity to it.
Disease Carriers
If a person with HIV (the virus that causes AIDS) is bitten by a mosquito it is unlikely
that the virus will be transferred to another human. For one thing, a drop of HIV infected
blood contains (maybe) one virus and that virus is easily destroyed by the mosquito’s
digestive systems. (Recall, that a virus is not a cell, but a strand of DNA protected only
by a thin layer of protein). The single-celled parasite that causes malaria is, on the other
hand, difficult to kill. The mosquito acts like a hypodermic needle injecting a half billion
people with the malaria parasite each year causing over one million deaths.
Did you know? The Black Plague, or Black Death, or Bubonic Plague occurred in
Europe the late 1340s when biting fleas (living on rats) carried the disease to humans.
The “Black Death” was known as a “pandemic” which essentially means severe
“epidemic” or severe breakout in disease.
The “Hygiene Hypothesis”
Many scientists believe that North American’s are just too clean. By not exposing
children to enough “uncleanly” microbes, the immune system which is supposed to be
busily attacking invaders does not have enough to do. As overcompensation harmless
allergens are attacked instead, causing unnecessary allergy symptoms well into
adulthood. This is known currently as the “hygiene hypothesis”. Notice that the word
“hypothesis” is used here instead of “theory”. To be accepted as a scientific theory, a
notion like the “hygiene hypothesis” must tested and re-tested over and over while the
world’s brightest scientific minds examine every aspect of the hypothesis in detail. In
other words, a scientific theory is the most thoroughly examined statement of explanation
we humans offer to explain our observations. An extension of the “hygiene hypothesis” is
the idea that it’s okay for young children to eat dirt once in a while. In fact this would be
good thing, because the microbes (microscopic pathogens like bacteria) in dirt will give
the immune system something worthy to attack. (A Mr. G Note: If “hygiene hypothesis”
does become a scientific theory some day, then I suggest they call it the “Happy Kids Eat
Dirt Theory”.).
Superbugs
In the fight against infectious disease the term antibiotic means any substance that can
kill bacteria (singular: bacterium). Penicillin (discovered by Alexander Fleming in 1928)
was the first antibiotic used by doctors. There are several variations of penicillin –
including tetracycline – which are used as bacteria killers today.
Antibiotics are not the only killers of bacteria. Disinfectants like soap and bleach will kill
bacteria. Bacteria will also kill themselves off – a feat they accomplish by developing and
releasing toxins. White blood cells and the production of antibodies by the immune
systems of animals also kill bacteria. To survive all these threats, bacteria must mutate –
this means their genes or DNA programs must change. Thankfully – for the sake of
bacteria – their cell design makes it easy for a mutation to occur.
A bacterium does not have a nucleus to house its chromosomes; instead a thin layer of
protein protects a tangle of chromosomes – the group of which is called a plasmid. With
no nucleus to interfere, gene segments are easily exchanged with other bacteria. In fact,
bacteria are always exchanging genes with each other in search of a life-saving
advantage.
Sometimes, a bacterium acquires a gene which allows it to survive an encounter with an
antibiotic. If this happens, the new gene segment provides the bacterium with instructions
for building an enzyme. The enzyme is an energized protein which acts chemically to
chop the antibiotic molecule into smaller pieces, effectively destroying it.
A bacterium that is able to destroy an antibiotic without dying off in the process is called
resistant. This type of bacterium has an advantage over other bacteria when exposed to
antibiotics.
Antibiotics are not selective – they destroy all different types of bacteria. Only resistant
bacteria can survive in the presence of antibiotics. This is where the trouble begins; if the
gene mutation allows a bacterium to survive an antibiotic attack; then the resistant
bacterium will go on to multiply (by the billons) and thrive.
A place that uses a lot of antibiotics is a hospital. Due to overexposure to antibiotics,
hospitals tend to have more resistant bacteria then other places. This makes sense because
the bacteria in hospitals will fight for survival by mutating (exchanging genes);
eventually some will become antibiotic resistant. Most bacteria do not cause lifethreatening diseases (most in fact are harmless), but if the resistant gene is passed to a
disease-causing bacterium then a “superbug” occurs.
The term superbug is synonymous with the terms “super germ” or “super pathogen” and
“super bacteria”. A “super” bacterium is “super” because it causes disease and it can
survive antibiotics. In other words, a superbug is an infectious, resistant, bacterium. What
is the bad news? The only disease fighting defense we have against resistant infectious
bacteria is the immune system. Recall, that the immune system relies on white blood cells
and the production of antibodies to kill invaders like our “superbug”. Without antibiotics
to help, how fast our immune system reacts to a “superbug” will be the only factor which
will determine of how sick we get or whether we will survive at all.
The H1N1 Flu Vaccine
A virus is an infectious biological agent. Many are simply strands of DNA with no cell
housing them. Often described a “pre-life form” or an “organism on the edge of life”, a
virus cannot replicate (reproduce or clone itself) without taking over the cells of another
organism. The process of replicating within a host cell usually results in the death of that
cell.
The influenza or “flu” virus is well known for causing disease in humans. Symptoms of a
flu infection include chills, fever, muscle pains, headache, and a sore throat. The modern
“flu” is a relative of the Spanish Flu that killed over 50 million people in the 1918
pandemic. (The term pandemic refers to the spread of disease on a world-wide scale).
With no cell to contain their DNA strands a virus can easily evolve or mutate by
exchanging DNA with another virus (or simply changing the links in its own DNA
strands). Hence, the flu we catch today is often similar to the Spanish flu.
Anytime you catch the flu and survive, your B-Cells develop an antibody that fights off
any further attack by that particular virus. However, since a virus will strive to live on,
the “flu” you catch next year will likely be a slightly different version (a mutation) of the
one you caught in years prior. If the flu virus is only slightly different than those
previously encountered, your B-Cells will easily manufacture new antibodies to ward of
the infection. It is when the flu virus is significantly different from previous ones that
trouble arrives in the form of a pandemic.
A virus is usually specific to a species. Stated differently, a flu virus that infects humans
will generally not harm pigs and vice versa. However, if a flu virus does make the jump
from pigs to humans there is a good chance that the virus will be significantly different
from other human flu viruses. This is the case with H1N1 or “swine flu”.
Samples of circulating influenza viruses are routinely sent to the World Heath
Organization. The virus sample for H1N1 was identified as having pandemic potential
because it differed significantly from other samples (this was not to surprising, since it
jumped form pigs to humans).
A vaccine is actually a “cheat” in the infectious process – it allows you to develop
antibodies without suffering the full effects of the disease (which may include death).
Manufacturing a vaccine flu virus is complicated because the vaccine virus is most easily
grown in chicken eggs.
To make sure the vaccine virus is safe and easy to mass produce the H1N1 virus is first
mixed with other standard lab viruses. Any mix of two influenza viruses will eventually
grow together to produce a hybrid virus. The most important task is to find a hybrid virus
that is both similar to the real H1N1 virus and able to grow quickly in a hen’s egg.
Once isolated the hybrid virus is injected into thousands of eggs, where it multiplies. The
harvested virus, now called the vaccine virus is killed – a process which breaks the
vaccine virus into proteins. These proteins give off the same chemical signals or antigens
as the live virus. Once purified, the virus proteins are ready for injection.
Upon injection into humans, the virus proteins, or antigens, will signal the B-cells to
begin constructing new antibodies. These antibodies will bind to the antigen site
effectively tying up the virus protein. Full immunity takes about 8 days to develop. After
the vaccine injection, B-Cells loaded with many of the correct antibodies will easily bind
up any invasion by the real H1N1 virus.