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Life in a Cell
A little background, please.
In 1665, an English scientist named Robert Hooke
looked at thin slices of cork under a microscope. He saw
something similar to the picture to the right. Hooke saw
that cork had a lot of empty spaces. Hooke used the
word cells to describe the empty spaces. He called them
cells because they reminded him of the tiny rooms
monks lived in called cells. He was the first person to do
so. He used a new fangled instrument called a
microscope.
Today, biologists know that Hooke did not see
living cells. He saw the walls of cells that were once alive.
Anton Van Leeuwenhoek was the first person to see a living cell in the late 1600’s.
A cell is the basic unit of living organisms. The type of microscope Hooke and Van
Leeuwenhoek used is called a simple light microscope. It was not really anything
more than a fancy magnifying glass that light was directed through. By the
nineteenth century, microscopes had been improved. The science of producing
lenses and optics had improved so much the compound light microscope was
produced. Instead of a single lens like the simple microscope had, a compound
microscope has many lenses through with the image is magnified in several steps.
This gave the ability to see smaller objects in the cell. Scientists could see that the
cell had parts.
First Robert Brown discovered the central part of the cell, the nucleus. Then,
two German biologists Matthias Schleiden and Theodor Schwann, did experiments
to see what kinds of living things had cells. They formed hypotheses that all plants
and animals were made of cells.
The experiments of Schleiden, Schwann and other scientists lead to the
development of the cell theory. The major ideas of the cell theory are listed below.
1. All living things are made of one or more cells.
2. Cells are the basic units of structure and functions in living things (they’re
where the real action takes place)
3. All cells come from other cells.
Today, compound light microscopes are not only common (almost every high
school biology classroom has them) but they are really
powerful. The really good ones can magnify an image 1500
times. In the 1940’s, a new kind of microscope was invented.
It was called an electron microscope. The principle was
similar to the light microscope but instead of shining a light
through the extremely thin section, this microscope sent a
beam of electrons through the section. This was amazing not
only could you see the parts of the cell, but you could see the
parts that make up the parts. Today, there are two widely
used types of electron microscopes, transmission electron
microscope (TEM) and scanning electron microscope (SEM).
The TEM send a beam of electrons through the thin section.
SEM bounces them off the surface of the section so intricate
details of the surface can be seen. Both produce cool pictures
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as you can see in the pictures to the left.
With each advance in microscope technology came an equal advance in the
knowledge of the cell. What was once information reserved for scientists, now is
common knowledge in most high school biology courses. We have come a long way,
baby.
Cell parts and their jobs (or Parts are definitely not parts)
Cells are microscopic units that make up all living things. Cells are alive.
They do everything needed to stay alive. They carry on cellular respiration. They
grow and reproduce. A cell has many different parts to do all of these jobs. As you
study the parts of the cell look at the pictures on the next page so you can put a
name with a face (or organelle in this case). The top picture is an animal cell and the
one on the
bottom is
plant cell.
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Cell membrane
The cell membrane is sometimes called the plasma membrane. Both plant
and animal cells have a cell membrane. It gives the cell some sort of shape but
mostly it keeps things out that should be out and keeps the inside stuff inside. It
also controls what can go in and out of the cell.
The cell membrane is a double-layered membrane with lots of stuff sticking in
it and through it. A close up diagram of the cell membrane can be seen in the
picture below. Notice the two layers are called a lipid bilayer. That is because the
two rows are made out of weird lipids. The head end is hydrophilic or water loving
and the tail end is hydrophobic or water hating. This makes the hydrophilic heads
want to be next to the water inside and outside the cell. When this happens, the
hydrophobic tails get sandwiched between the water loving heads and get away from
the water.
Also notice the membrane has several big globby molecules that seem to go
right through it. These are used to move large substances through the membrane
and are called transporter proteins. In addition, cell membranes have little bumps
and branches sticking out of them that act as markers that cells identify each other
with. No two people have exactly the same cell markers with the exception of
identical twins. When you receive an organ transplant, if the cell markers are not a
close enough match to your own, your body will think it is some invading organism
and will start attacking it. This is what doctors call rejection of a transplant.
Cytoplasm (cell juice)
Inside the cell membrane is a whole host of little cell structures called
organelles. Organelle means little organ. These organelles have to have some way
to move substances from one to another. The cell often needs to move substances
that have been let into the cell to another area. The liquid inside the cell called the
cytoplasm accomplishes all this. All cells have cytoplasm. It is very important for
slow transport of substance and to keep everything nicely wet. The cell needs to be
wet inside because the reactions that keep the cell alive are all are water based.
Without the water, the right molecules may never bump into each other.
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The Nucleus (the big cheese of the cell)
The nucleus is the cell part that controls most of the cell’s activities. All cells
have a nucleus at some time during their lives. It is really important. It determines
how and when proteins will be made. Proteins, as you know, are complex
substances that can act as enzymes to make reactions go. Proteins can also be
structural like those that form some cell parts. The nucleus is also where all the
information is stored that will be passed onto the offspring. The picture below
shows a nucleus and some of its important parts.
Just like a cell, the nucleus has a membrane around it. It is called the nuclear
membrane or nuclear envelope. It does
most of the same things a cell membrane
does. It keeps the inside, in and the
outside, out. It also controls what can go in
and out. The nuclear membrane has little
holes in it called pores that let stuff pass
through without too much trouble. It also
is not made of one lipid bilayer like the cell
membrane; it is a double lipid bilayer, one
on top of the other.
The information to control all of
things that have to go on in a cell is stored
in the cell’s DNA. Big wads of this DNA are called chromosomes. They are found
inside the nucleus and usually stain darker colored. Chromosomes are usually all
tangled up and spread out during most of the
cell’s life. When they are spread out, they are
not really visible. When the cell is going to
divide, the chromosomes condense and they
look like big roundy X’s. A picture of a
chromosome can be seen to the right.
Sitting in the nucleus is a smaller
organelle called the nucleolus. The nucleolus is the cell part that makes ribosomes.
You will read about ribosomes later but just remember that the nucleolus makes
them for now.
Just like a cell, inside the nucleus is a water solution. Now you might expect
this since this container has lots of holes in it and it’s sitting in a container of water.
The cytoplasm would naturally go into the nucleus through the holes. But since the
nucleus is considered the most important organelle in a cell, its water can’t be called
cytoplasm. Nooooo! It must be more fancy, dancy than that. It is called
nucleoplasm. Big deal. Nucleoplasm is nothing but cytoplasm with a better
address.
Ribosomes (Time to make the protein)
As mentioned earlier, the nucleolus has the job of making ribosomes.
Ribosomes are tiny (I mean really tiny) organelles that make protein. Ribosomes are
made out of a special kind of ribonucleic acid called ribosomal RNA (rRNA). Wow,
what rocket scientist named that one? Two big pieces of rRNA are put together to
make the ribosome. The protein is made between the two pieces. You can see a
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picture of a ribosome to the right. Some ribosomes are
found floating freely in the cytoplasm but more are found
attached to another organelle, the endoplasmic reticulum
(ER).
Endoplasmic reticulum (FedEx of the cell)
The cytoplasm is riddled with a network of canals
called the endoplasmic reticulum. This organelle is in
both plant and animal cell. The canals serve as a highspeed delivery system for the cell. Since the cytoplasm
takes its own sweet time moving stuff from one place
to another, the cell must have a special speedy delivery
system. This ensures that materials are where they
need to be, when they need to be there. The
endoplasmic reticulum has a couple of other jobs as
well. As mentioned before, it is a place where
ribosomes attach. Pretty handy to make a protein in
the ribosome, then whisk it to the place its needed in
the ER. ER also makes and stores lipids in the cell. A
picture of ER can be seen to the left.
Golgi Apparatus (Gift wrap department of the cell)
When the ER makes something, it
usually puts it in a temporary package then
sends it to the Golgi apparatus. The Golgi
apparatus is a stack of flattened sacks that
kind of look like bumpy pancakes. The
Golgi sorts the substances in the incoming
vesicles (sometimes called vacuoles) and
groups them together. It then repackages
them to travel around the cell. You can see
a drawing of a Golgi to the right.
Vacuole (Tupperware for the cell)
Sometimes the cell doesn’t need what the
organelles have produced right away. If you have extra
food for a meal, sometimes it gets put in a plastic
container and waits in your fridge until someone wants
a snack. The cell also has containers for things it isn’t
going to use immediately. They are called vacuoles or
vesicles. They are just globs of some substance with a
membrane around them. Plant cells have really large
vacuoles that they use for storing water to maintain
shape. Animal cells also have vacuoles but they are
small compared to plant vacuoles. You can see how
large the vacuole is in the plant cell above and to the left.
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Lysosomes (Deadly vacuoles)
Now, organelles don’t last forever, or even
as long as a cell lives. Sometimes the cell takes
in stuff that is dangerous to it. All these used up
organelles and nasty poisonous stuff need to be
broken down. That is the job of the lysosomes.
Lysosomes are vacuoles with attitude. Like all
vacuoles, the Golgi apparatus packages them.
They contain digestive enzymes (chemicals) that
break stuff down. The enzymes have to be kept
inside a membrane or the chemicals would start
eating perfectly healthy organelles and things the
cell needs. When the chemicals are needed, the membrane fuses with the thing it is
trying to destroy. This lets the enzymes to do their duty without damaging any of
the valuable cell parts. The resulting smaller pieces are either used by the cell or
passed out through the membrane. Both plants and animal cells have lysosomes.
You can see a drawing of a lysosome being formed by the Golgi apparatus to the left.
Centrioles (what do they do? Not really sure.)
Just outside the nucleus in animal cell are two strange looking cylinders that
hang out at right angles to each other. These little cylinders are called the
centrioles or centrosomes. Just before the cell is ready to divide, the centrioles
move to opposite sides of the nucleus. That’s what scientists do know for sure.
Here’s where they aren’t quite so sure. When the chromosomes in the nucleus
condense, little fibers called spindle fibers grow out of
the centrioles and attach themselves to the
chromosomes. The centrioles, they think, act as little
anchors for the chromosomes to pull against to get to the
opposite sides of the cell. When they get to the opposite
sides the cell, the cell can divide. Why do animal cells
have them and plant cells don’t? They don’t know.
Centrioles are so tiny they were not even seen before the
electron microscope was invented. They have only known about them for a short
time so there is bound to be lots to learn. You can see a drawing of centrioles in the
picture to the right.
Cell wall (plants only, please)
Plant cells, fungi cells, and some bacteria have a special outer coating that
covers the cell membrane. This overcoat is called a cell wall seen to the left. It is
thicker than a cell membrane and gives the cell lots of shape
and protection. In plants its made out of cellulose, a
carbohydrate that is indigestible for humans. When people
eat lots of plants (fruits and veggies) the cellulose usually
goes right through their digestive tract, pretty much cleaning
out any thing in its path. People who don’t eat lots of fruits
and veggies run the risk of being constipated. Yuck. In
fungi, it is made out of another kind of carbohydrate called chitin. Strangely
enough, lobsters and crabs use chitin to make their shells. Bacteria have cell walls
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made out of other carbohydrates. Although, the cell wall is the outermost boundary,
it does not decide what gets in or out of the cell. That is still the job of the cell
membrane.
Mitochondria (nuclear power plants for the cell)
All of the activities of the cell take lots and lots of energy. Simple sugars can
come in but they need to be broken down to have the energy released. The releasing
energy job is done by an organelle called a mitochondrion (pl. mitochondria). You
can see a mitochondrion in the picture to the
right. Mitochondria look like little cocktail
wieners. They have an outer membrane and
an inner membrane that is folded. It is on the
inner membrane where most of the energy is
released. The mitochondria takes the released
energy and packages it in small, portable,
molecules called ATP. ATP can be used as an
energy source in any reaction in the cell. What is more ATP from plant mitochondria
can be used in animal cell reactions. In other words, ATP is a universal compound.
If a cell is really active like a heart muscle cell, it will have more mitochondria than a
cell that is less active like a skin cell. This makes sense because more activity
requires more energy; to get more energy, you need more mitochondria to supply it.
Chloroplast (energy transformers)
Chloroplasts are only found in animal cells and other cells that carry on
photosynthesis. You may remember photosynthesis changes light energy from the
sun into chemical energy like sugar (glucose). There
isn’t a cell that exists that can use light energy directly
without changing it to chemical energy first.
Chloroplasts look a great deal like green jellybeans.
The have an outer membrane and an inner membrane
that holds stacks of disks that contain chlorophyll.
The chlorophyll is the thing that captures the light so
it can be changed into chemical energy. For this to
happen, the cell needs carbon dioxide and water to
build the glucose molecule. Building the molecule requires energy that has to be put
in. When the molecules are broken down in the mitochondria, the energy is
released. You can see a picture of a chloroplast to the left.
Cytoskeleton (dem bones, dem bones?)
The cell membrane and cell wall in plants are
not the only organelles that contribute to a cell
shape. The cytoskeleton gives both plant and
animal cell their shape. The cytoskeleton is a
network of little protein tubes found in the
cytoplasm. Using these tubules, the cell is able to
change maintain its shape or change it if the need
arises. Cells that move a great deal have more
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substantial cytoskeletons as seen in the picture to the right. Cells that do not have
to move much will have less substantial cytoskeletons.
Flagella (whip it, whip it good)
Some cells are not happy just floating along in the
current of liquid they are sitting in. Some cells like to take
charge of their destiny and are responsible for their own
movement. One of the ways cells can move is by using an
organelle called a flagellum (pl. flagella). It is a long taillike appendage that pokes out of the cell membrane and/or
cell wall. Cells can have one or more flagella. Flagella are
found in both plant and animal cells but not all plant and
animal cells have them. Flagella help the cell to move by
whipping back and forth. This usually does not make for a
very smooth movement, but it gets them to go where they
want. You can see a picture of flagella to the right.
Cilia (row, row, row your boat)
Other cells that move around do not have long flagella. Instead they have lots
of little short hair-like appendages called cilia. The cilia
usually cover the cell. Cilia are found mainly on animal cells
but can occasionally be found on plant cells. Cilia move the
cell along by beating in unison like men rowing a Viking ship.
The cilia also move substances along the cell surface in
stationary cells. Cells in your respiratory tract have cilia on
them to move dirt and mucus to where it can be swallowed or
coughed up. This is a very important function. If the dirt
stays in your lungs it could cause infection and illness. So,
coughing up a little phlegm is not such a bad thing. You can see a picture of cilia to
the left.