Cells
Grade 8 science study
By Edward King-Grey
Cells
Unit 1. The differences between animal and plant cells
Animal Cells
Irregular shape
Lysosome
Centrosome
Centriole
Plant Cells
Nucleus
Cytoplasm
Mitochondria
Vacuole
Cell Wall
Chlorophyll
Photosynthesising
Chloroplasts
Definite shape
The quintessential differences between plant and animal cells are as follows:
 Plant cells have a tough cell wall made out of a substance called cellulose. This gives
the plant cells a cube-like appearance. Animal cells lack this cell wall giving them a
more liquid appearance.
 Plant cells have chlorophyll in them. Chlorophyll is what gives plant cells, and thus
the plant itself, its distinct green colour. This enables the plant to photosynthesise
which is the process of making glucose (a type of sugar) out of sunlight, carbon
dioxide and water. Glucose is what feeds the plant thus keeping it alive. Chlorophyll
is contained in a part of the cell called chloroplasts. Animal cells do not have
chloroplast or chlorophyll and can thus not photosynthesise.
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Both plant and animal cells have vacuoles. These are often described as a water or
liquid filled cavity surrounded by a membrane. It is thought the purpose of vacuoles
is to store extra water, waste and toxins. However, animal cells usually have more
than one vacuole which are rather small. Plant cells however usually only have one
vacuole that can take up as much as 90% of the cells volume.

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Only animal cells have lysosomes. These are often described as the stomach of the
cell. They break down food particles, invading virus and bacterium and worn out bits
of the cell with their acidic juices and enzymes.
Animal cells have centrosomes. These make up the main part of a microtubule
organising centre (MOC). The purpose of the MOC is to produce microtubules which
are a quintessential part of the cell acting like building blocks. Microtubules bind
together to form centrioles which have the same roles as microtubules. Plant cells
have none of these components.
Unit 2. Investigation
Question: How do we get images such as these? (See Fig.1.1)
Fig.1.1
Cells are small. The average size of a human cell, though it varies extremely, is
around 10 to 100 micrometres, or about 0.1 to 0.01 millimetres which is far
too small for the human eye to see. The best way to observe cells is through an
electron microscope. Electron microscopes are the most powerful microscopes
in the world. Through them you can get clear pictures of tiny objects like cells,
bacteria and viruses and even atoms which are the smallest units of an
element! One of the first people to witness cells through a microscope was a
man called Antony van Leeuwenhoek in 1676. He saw millions of tiny creatures
in a sample of pond water he had collected and was one of the first people in
history to link these tiny creatures to disease.
Unit 3. Investigation
Question: What sort of creatures would you find lots of in a drop of
water?
The answer to this question is not very simple. It would depend on the source
and state of the water when the sample was collected. For example, water
collected from a rough sea would be different from that collected from a calm
sea and the water collected from a stagnant pond would be different again.
But, however, there may be some creatures that most would contain:
Tardigrades (see Fig.1.2)
Also known as water bears or moss piglets, these miniscule creatures live
anywhere there is water, the bottom of the ocean to the highest peaks in the
world. They are thought to be the hardiest creatures on the Earth and can
withstand extreme temperatures, pressures and radiation doses without ill
effects; they can also survive for up to 100 years without water.
Fig.1.2
Bacteria (see Fig.1.3)
Bacteria live absolutely everywhere. Every corner of the earth is infested with
them; they live inside every satellite and spaceship and have even been found
on different planets! There are millions of species of them coming in all
different shapes and sizes.
Fig.1.3
Amoebas (see Fig.1.4)
Amoebas are probably one of the most famous micro-organisms. They are one
celled animals and are vicious hunters. Their sizes vary quite a bit with the
biggest getting up to one centimetre. They are often transparent and have an
irregular shape due to their lack of a cell wall.
Fig.1.4
Phytoplankton (see Fig.1.5) and Pollen (see Fig.1.6)
Phytoplankton’s are tiny animal and plant species that are the base of the
entire oceans ecosystem. If removed the entire ocean would suffer as this food
supply was cut off. There can be thousands of them in one drop of ocean
water and they usually have only one cell. Pollen is the reproductive cells of
many plants species and is the way plants reproduce. Pollen is a one-celled
organism and can survive for up to 1000 years.
Fig.1.5
Fig.1.6
Unit 4. Investigation
Question: What do humans and plants have in common?
Humans and plants are very unalike in many different ways, but despite these
differences, they have some similarities. Both humans and plants are
composed of cells which provide them with energy. Both have immune
systems which protect them from disease and infection. Both require the same
basic needs: energy in the form of glucose, water and gases, oxygen for
humans and carbon dioxide for plants in order to survive.
Unit 5. Investigation
Question: Are mushrooms a plant?
No, mushrooms are fungi and although fungi and plants share the same
domain (Eukarya) they are of different kingdoms. The main differences that
mushrooms (fungi) and plants have are as follows:
 Fungi have no leaves or roots and therefore cannot photosynthesise
 Plant cell walls are mainly composed of cellulose while fungi cell walls
are mainly made of a substance called chitin
 Plants produce food via photosynthesis; fungi produce food through
parasitic means. This means they absorb food through a source, such as
a plant.
 Most plants reproduce via pollen, mushrooms reproduce via spores
Unit 6: Investigation
Question: Why don’t we (usually) catch chickenpox twice?
Chickenpox is a disease caused by the varicella zoster virus (see Fig.2.1). It is a
very common highly contagious non-fatal disease. When a varicella virus
enters your body your body’s immune system kicks in. This is made up of many
different cells doing many different jobs all to kill the invader. Your immune
system is mainly made up of white blood cells (see Fig.2.2) which use special
cells called antibodies to identify what is a disease causing microbe and what is
one of the body’s own cells. When a white blood cell finds a varicella virus it
kills and eats it with the help of other white blood cells. When this has been
done the white blood cell produces an antibody that is specially designed to
find any other varicella viruses. So, after you have had chickenpox these
varicella antibodies notify white blood cells whenever they come across the
varicella virus. This is why you only get chickenpox once. But, this doesn’t work
for the common cold as there are hundreds of different cold-causing viruses
and ones you already have had can mutate themselves just enough to sneak
past the antibodies designed to find them.
Fig.2.1
Fig.2.2
Unit 7: Investigation
Question: why do we get injections called vaccinations?
Vaccinations are the easiest, safest and most efficient way to prepare the body
for an infectious disease. During a vaccination an inactive, harmless form of or
the outer skin of a virus is introduced to the body’s immune system. When a
white blood cell discovers this harmless form of the disease it treats it like the
proper thing. It disposes of (eats) the virus and produces an antibody (see Unit
6) for it so when a dangerous form of the virus intrudes the body the immune
system is prepared for it and can easily dispose of it. Vaccinations have been
made for a large range of infectious diseases such as HPV (Human Papilloma
Virus), Varicella zoster virus (chickenpox) and the influenza A H3N2,influenza A
H1N1 and influenza B mixed vaccine(the seasonal flu vaccination).
Unit 8: Conclusion
Cells are a quintessential part of life. Without them life could not exist. They
make us who we are and, although small, they pack a huge punch. They make
us sick and then better again, they shape the way we look and think and play
out our lives. They are a very interesting and vital part of life.
Information and images sourced from the following websites:
http://www.microscopyu.com/galleries/fluorescence/cells.html
http://www.google.com.au/imghp?hl=en&tab=wi
http://en.wikipedia.org/wiki/
http://www.google.com.au/webhp?hl=en