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Hello, today we are going to talk about cells. First of all, the thing you need to know about
is the cell theory. The cell theory has three main components. First of all, it says that…
1. Cells are the fundamental unit of all life. 2. All living organisms are composed of cells.
3. All cells come from preexisting cells. They do not just miraculously appear.
Why are cells so small? Here are some pictures of various cells. Most cells are very, very,
very tiny. This is an ostrich egg. It is the world’s largest cell, but very few cells
are as large as an ostrich egg. It has to do with the surface area to volume ratio.
As the volume of a cell increases, the surface area decreases. This has important implications
for the cell. When the volume of the cell is very big, the surface is very small. How
big the cell is, the volume of the cell, determines how much metabolic activity can be carried
out inside the cell, but the surface area determines how those metabolic molecules either
enter or exit. If the surface area is small, then you are going to have an effect on how
much metabolic activity can occur no matter what the volume is. So there is this balance
that has to be created. As a result you are going to end up with very small cells. We
are going to watch a video here in a moment which will show you that.
Changes in surface area to volume ratio have important implications for limits on cell
sizes. In this experiment you will see a demonstration of how surface area to volume ratio changes
with the size of the cell and we will consider the impact this has on exchange of materials
between the cell and its environment. A cell is a metabolic compartment where many different
chemical reactions occur. In this demonstration the cell is represented by cubes of different
sizes--the larger the volume of the cell the larger the number of metabolic reactions that
take place. Here the pink phenolphthalein indicator is used to visualize the volume
of the cell. We are using three different sized cubes here to represent three different
size cells. The first has a side of 1 cm length, the second 2, and the third 3 cm length. Let’s
see how the surface area to volume varies when we increase the size of the cells. As
the size of the cell increases, so too does the surface area and volume. However, notice
that the volume of the cell increases faster than the surface area when a cell grows, therefore
decreasing it surface area to volume ratio. What this means is that the greater the size
of the cell, the less surface area it has relative to its volume. Here our beakers of
hydrochloric acid represent the raw materials which need to enter the cell from the outside
in order for its metabolic reactions to take place. Exchange of materials across the cell
membrane often occurs through the process of diffusion where dissolved molecules move
from areas of high concentration to areas of low concentration. When the cubes are placed
in the hydrochloric acid, the acid, representing the raw materials, will diffuse into the cubes
turning the indicator clear. Let’s see how this rate of diffusion varies. Raw materials
must be able to reach all parts of the cell’s volume quickly. Notice how the acid is diffusing
into each cube at the same rate. However, in a given time period, it is reaching a greater
proportion of the volume in the smaller cube as opposed to the bigger cube. Eventually
the acid reaches the center of the 1 cm cube, but in the same time period, it does not reach
the center of the 3 cm cube. So why is this important? Notice that where volume is too
large relative to surface area, or the surface area to volume ratio is too small as in the
3 cm cube, diffusion cannot occur at high enough rates to supply its raw materials to
the whole volume of the cell. At this point the cell cannot get any larger.
It is obvious from the activity that you can see how quickly the material moves in and
how it is going to have an effect on the ability of the cell to do its job.
Cell walls are materials that are found on the outside of the cell and they are made
from different materials. Plants have a cell wall that is always made from cellulose. Cellulose,
don’t forget, is a beta glucose molecule. So it is very difficult for you to digest
that. Fungi cell walls are made from usually chitin, occasionally cellulose. Bacteria cell
walls are made from a material called peptidoglycan which is a polysaccharide. Protists, if they
have a cell wall, it is going vary. It is either made from chitin or cellulose, never
peptidoglycan. Only bacteria have cell walls made from peptidoglycan
Inside a cell wall you will find the plasma membrane. Plasma membranes are composed of
a phospholipid bilayer with proteins that are embedded within it. There is a name for
the model they use to describe cell membranes. It is called the fluid mosaic model. When
we get to the next power point we are going to talk about fluid mosaic models. Basically
you have proteins embedded in this phospholipid bilayer and those proteins are not stationery.
They can move around. Some of them go all the way through the membrane. Some of them
are found on one side of the membrane. They can move around on the membrane; they are
not permanently attached, so it is a fluid moving membrane. A phospholipid bilayer, by
the way, is made from a phospholipid which remember has a body and two legs, one of which
is bent, and it is a bilayer which means it has two layers of them--one here and one here.
The legs are hydrophobic which means they hate water. The body ends love water. So they
line up in this way automatically. Let’s look at some of the differences between
cell walls and cell membranes. First of all the structure of them: they are
made from completely different molecules. the composition of them: whether it is one
molecule or several, different types of proteins and cholesterol, all types of things are found
in cell membranes. Cell walls are one type of molecule.
Prokaryotic and eukaryotic cells are what we are going to talk about now. As we go through
the rest of this power point, we are going to be primarily talking about eukaryotic cells.
Prokaryotic cells are cells that have their own DNA but it is not found inside a nucleus.
Usually prokaryotic DNA is circular and it is found in region called a nucleoid which
basically just means DNA is attached to the plasma membrane. It kind of stays in that
general area the whole time. Prokaryotic cells also have one organelle and it is a ribosome.
They don’t have anything else. Eukaryotic cells, on the other hand, have their DNA located
inside the nucleus, which is a membrane bound organelle. Then they have other membrane bound
organelles like mitochondria, chloroplasts, rough ER, smooth ER, Golgi apparatus, lysosomes,
on and on and on. First we are going to look at the difference between these two on the
next slide and then we are going to go into the different organelles found in eukaryotic
cells. Prokaryotic cells, like I said, have their
circular DNA. They have a single strand of DNA with no associated proteins attached to
it which turn it into chromosomes and things like that. Occasionally they will have some
small pieces of DNA called plasmids. They just really have one piece of circular DNA.
They have ribosomes. They have cell walls made from peptidoglycans. The last one I have
not told you yet. If they have flagella, there are no microtubules inside the flagella.
Eukaryotic cells, on the other hand, have double helix DNA, like what you are used to
seeing. They have larger ribosomes. If they have a cell wall, it is made from cellulose
in plants, chitin in fungus, and the protists vary. Again, if they have flagella, they do
have microtubules in it. Prokaryotes belong to two different domains.
Domains are a classification above Kingdoms. First you have Life. Then you have three different
Domains. Then you have Kingdoms. There are two domains that prokaryotes belong to. Archaea--which
are very ancient forms of bacteria: things that live in high methane areas, inside volcanoes,
inside places that are salty, very stange environments. Then the domain Eubacteria which
are your common every day bacteria. Within the Eubacteria there is a kingdom Bacteria.
These are things we already know. They have no nuclei, single piece DNA, occasionally
some plasmids. They do have ribosomes but no other organelles. Eukarya is the domain
that eukaryotes belong in. There are four Kingdoms: Fungi, Plantae, Animalia, or Protista.
They have a nucleus with a double helix inside it, and they do have organelles.
Again the cell walls: peptidoglycan or no peptidoglycan.
Bacteria are much smaller in size than eukaryotic cells. As you can see inside the bacteria,
you got some DNA, you got some ribosomes and that is about it. Inside the eukaryotes you
have all kinds of organelles and stuff inside it.
Here is a picture of some bacteria. This is a real picture from a scanning electron microscope.
This is a bacteria called streptococcus, it causes strep throat. What you see are little
tiny round balls and that is the bacteria. This particular strain grows in lines like
this. Special features with prokaryotes: most have
cell walls. These are three questions that I want you to answer tonight. This is your
homework. 1. What way are the peptidoglycan cell walls different from plant cell walls?
How is cellulose different from peptidoglycan? 2. Some bacteria have a capsule that surrounds
the cell wall. What is the purpose of this capsule? 3. Some bacteria have an internal
membrane. Why could this have been an evolutionary advantage to those types of bacteria? The
bacteria you are going to want to cyanobacteria. Go ahead and look those three things up tonight
and we will carry on with eukaryotes. Eukaryotes are things like animals, plants,
protists or fungus. That is a cool fungus. Once you get past the plasma membrane (and
we are going to work our way inside the cell), the first thing you are going to find is a
semi-fluid material which surrounds all the organelles. That material is called cytosol.
When you were in elementary school and middle school, they told it was cytoplasm. That is
wrong. They have told you incorrectly. Cytoplasm is the region that the cytosol is found in.
The material itself is called cytosol. So please clear that up in your brains.
Cytoplasm has two components: the cytosol and free ribosomes.
So let’s look at ribosomes. Ribosomes, they are made from 1 or 3 special ribosomal molecules
called rRNA. They have multiple proteins which combine with the three rRNA molecules to create
a ribosome. There are two halves to it: there is a large subunit and a small subunit. The
small subunit has three slots in it called A, P and E. I will help you remember those.
First of all, the A P, which class are you in? and E is where the material is going to
Exit. The three slots function to build a protein. When we get to protein synthesis
in a later chapter we are going to learn how this happens. The DNA for a gene, first of
all, is copied and it creates a messenger RNA (mRNA) molecule. That messenger RNA is
sent out of the nucleus and it goes to a ribosome. At the ribosome, the messenger RNA is going
to pair up with a transfer RNA (a tRNA). If they pair up, tRNA will leave its amino acid
behind on top of the ribosome. A chain of amino acids left behind is called a protein.
The whole function of ribosomes is to make a protein.
Here is a picture of a ribosome. Here is a messenger RNA molecule coming in and here
is a chain of amino acids coming out. That would be your protein. And you can see there
are two halves: there is one half here and there is a bigger half up here.
Let’s move into the nucleus now. The nucleus contains most of the genes of the eukaryotic
cell. Did you realize I said most? Some genes are found in the mitochondria and in the chloroplast.
Those two organelles have their very own DNA. That is kind of cool. We will find out why
at the end of this power point. The nucleus has a double layered phospholipid bilayer.
That means it has four layers of phospholipids, not just two like the plasma membrane. It
has basically two plasma membranes surrounding it which is kind of unique. The membrane is
perforated by little tiny holes called nuclear pores. Those nuclear pores are the perfect
size. Messenger RNA is half as big as a DNA. It is basically one side of a DNA. DNA has
two sides to the ladder. The holes are too tiny to let a whole DNA molecule out because
the ladder is too big. The messenger RNA, because it is half a ladder, is exactly the
right size to fit through those nuclear pores. So messenger RNA can take the message of how
to build proteins out of the nucleus and go to the ribosome and build the protein. The
DNA can be safe and protected inside the nucleus. So here is your nucleus. If you look at that
membrane, each one of these lines is actually a phospholipid bilayer; you have a bilayer
here and a bilayer here. Each one of these has a bilayer so it is four thick. This is
your nuclear pore. It is exactly the right size to let things in an out. And of course,
embedded on the outside, there are ribosomes are embedded in your nucleus. There are a
bunch of them there. The nucleus is where DNA is located and it
is also where DNA replication occurs. Of course, if DNA is there, that is where you are going
to replicate the DNA. It is also where DNA is going to be transcribed into messenger
RNA (mRNA). It is the location of the nucleolus. The nucleolus is actually the location where
ribosomes are going to be assembled. That is where they are put together.
DNA There are two forms of DNA inside your nucleus:
one is called chromosomes and one is called chromatin. Chromatin is when the DNA is all
unwound. It is like pile of spaghetti or an unwound bunch of yarn. When the DNA is condensed,
it is called chromosome. The reason the DNA is going to be condensed is because it is
a whole lot easier to move a bunch of yarn that is condensed into that spool, than it
is to move a big pile of yarn that is just kind of like all willy nilly in a pile, especially
if you have a whole bunch of DNA that needs to be moved around. In your cell you have
46 chromosomes. We want to move those things around in an organized fashion. The cell does
that by creating chromosomes. Chromosomes are created by taking your chromatin and winding
it around special proteins called histones. There is a specific pattern that they wind
around. It is going to wind around histones creating bigger and bigger globs and by doing
that you create your chromosome. You can see chromosome is made up of all those little
globs like this. I mentioned the nucleolus. Again it is where
the ribosomes are going to be assembled. Basically what happens, the nucleus creates rRNA. The
rRNA molecules are copied from the DNA. Specific proteins are going to be synthesized in the
cytoplasm. The rRNA, three of them are going to be put together (assembled) in the nucleolus.
Then they are going to leave, go out into the cytoplasm, and combine with these specific
proteins which build the ribosome. Endoplasmic Reticulum is a network of fluid
filled membranes that have a primary function of being able to move material throughout
the cell. There are two types: rough ER and smooth ER. The difference between them is
the rough ER has ribosomes sitting on top of it and smooth ER does not. The function
of ER is when the ribosomes make proteins or something, those proteins are transported
to the inside of the rough ER and then the rough ER transports those towards the Golgi.
It will secrete those into little tiny vesicles. Those vesicles will move to the Golgi. Also
inside the ER, specific hormones are going to be synthesized. In and of itself, rough
ER makes its own hormones too. The rough ER is connected to the nucleus.
You have all these little ribosomes connected to it. The ribosomes are going to create the
protein. Protein is going to go into this interior space. This interior space is called
the cisternal space or the cisternae. It moves inside there. Then we are going to create
a vesicle. When the vesicle gets to the Golgi, it is first going to fuse with the Golgi.
Then the Golgi is going to put the proteins into their final functioning shape. The reason
I say that is many proteins have to be combined into the quaternary structure. This is where
that is going to happen. They are going to be modified. Sometimes they are stored. Usually
they are going to be transported. The Golgi are made up of flattened series of membranes,
also called cisternae, with cisternal spacing inside them. They have the same fluid filled
sacs on the inside. So a vesicle comes in from the ER, it is going to fuse with it.
Material is going to pass around here. Then it is going to bud off little tiny vesicles.
The vesicles will then be transported out of the cell or it could be stored in the cell
in the vesicle, whatever the cell needs. One of the things that will bud off from a
Golgi is an organelle called a lysosome. So ribosome is going to make hydrolytic enzymes.
Those enzymes are going to be moved through the rough ER to the Golgi. The Golgi is then
going to bud off these hydrolytic enzymes into a lysosome. The lysosomes function is
to take those enzymes and to digest stuff. Hydrolytic means to break down. These are
the enzymes that are going to break down carbohydrates, proteins and things like that. So this is
the stomach acid, if you will, for the cell. They are not found in plants.
Here is the next picture. The rough ER made it, transported it, and created the lysosome.
There are two main functions for it. If the cell brought in food, one of the lysosomes
can fuse with the food vacuole, digest the food, and then release the food material into
the cell or waste products will then be excreted out of the cell. Or if organelles have died,
the lysosome can fuse with the vacuole surrounding the dead organelle, go through autophagy,
which means basically to eat itself. It is going to eat dead organelles. Then the material
that the organelle is made from can be used to build new organelles.
Vacuole and Vesicles There are several types of vacuoles and vesicles.
The difference between them primarily is size. Vacuoles are larger than vesicles. First of
all you have food vacuoles. Food vacuoles are formed by phagocytosis when food material
comes into the cell. The second type is called contractile vacuoles. These are found in fresh
water protists where they are surrounded by fresh water. When fresh water diffuses into
the cell, it will actually cause the cell to rupture. So these fresh water protists
have to have some way of excreting water. So the contractile vacuole will fill up with
water. Then kind of like a pore on the outside of the cell, it will squeeze the water out
and excrete it. So we are going to watch a little video right
now. It is very short. You will see them excrete it.
The contractile vacuoles of ciliads are permanent structures that periodically squeeze excess
water out through a tiny pore. In some paramecium species, the contractile vacuoles have conspicuous
feeder arms. But most ciliads, the canal system is less apparent and the contractile vacuole
appears as a simple sphere. I told you that was a short one; it was like
20 seconds. Other Types of Vesicles and Vacuoles
Plants have a central vacuole which is found, when you look at plants under a microscope
they are absolutely huge, it is found in the middle of a plant cell. They have lots of
different functions. Primarily, it is water storage. It helps to keep the plants turgent
which means firm. There are also transport vesicles found between rough ER and smooth
ER, and from rough ER and Golgi, and from the Golgi when they excrete material. Those
are transport vesicles. Then you also have storage vesicles which in plants are used
to store things like starch and pigments and toxic substances like nicotine and nasty stuff
like that. Mitochondria are a really cool organelle.
They have, again, a double phospholipid bilayer membrane, four layers of phospholipids. Inside
they have a folded membrane called a cristae membrane. Then a fluid that surrounds the
cristae membrane called matrix, sometimes called fluid matrix. These mitochondria have
their very own DNA and the double phospholipid bilayer. That is unique. We are going to find
out why at the end. It is a unique process that created these mitochondria and chloroplasts.
They believe the organelle went through a process of something called endosymbiosis.
We will find out more later. Here is what you have got. You have a double phospholipid
bilayer here, the cristae, which are this folded membrane, then the fluid matrix on
the inside. Chloroplast is the second one. It also has
the double phospholipid bilayer and also has its own DNA. Inside they have an inner membrane
also. This membrane, though, is flattened small discs. Each disc is called thylakoid.
A stack of thylakoid is called a granum. They are surrounded by another fluid. This fluid
is called stroma. So you have got two phospholipid bilayers (can you see them there), stacks
of thylakoids. Then there is a fluid surrounding that. The function of chloroplasts is to go
through photosynthesis. Another organelle found in some cells but
not most cells is an organelle called peroxisomes. Peroxisomes are a specialized single membrane
bound organelle filled with enzymes called catalase which first form hydrogen peroxide
and then break down hydrogen peroxide to non-toxic substances. The reason is these specific cells
that these organelles are found in go through a very high rate of metabolism and they create
very toxic substance. Those need to be broken down very rapidly. Hydrogen peroxide kills
cells. This organelle breaks down hydrogen peroxide so the cells survive. One thing that
is very different about peroxisomes from other organelles, unlike lysosomes which bud off
from the Golgi, these do not. Peroxisomes go through binary fission which means, you
have one and it breaks into two. That is different. That is very unusual. These are only found
in humans in liver cells and kidney cells and in plants, the cells in the plant that
are going through photosynthesis very rapidly. Inside your cell you have a network of microfibers
that are called the cytoskeleton. There are three types of them. They are classified by
their size. They all function in mechanical support, transportation and to help maintain
cell shape. The largest one is called microtubules. Here is the size: 25 to 200 nanometers. They
are hollow and they are constructed from a molecule called tubulin. They function in
the shape and support of the cell. Also, they serve kind of like railroad tracks along which
organelles are going to move around the cell. When a cell is going through mitosis and it
is going to divide, it wants to make sure each half of the cell has the right amount
of mitochondria and chloroplasts and everything like that. To do that, they have to jump on
these railroad tracks and get moved to opposite sides of the cell. They also function as part
of the centriole and spindle to help separate the chromosomes during cell division, during
mitosis. Microfilaments are about 7 nanometers in diameter.
They are the smallest of the cytoskeleton. They are composed of two intertwined strands
of actin. And they function in muscle contraction, cytoplasmic streaming and the formation of
cleavage furrow during cell division. So they are very tiny.
Cytoplasmic streaming, in case you don’t know, is what some things like amoeba do.
What they do is they push against the side with the cytoplasm. By pushing against the
side, it moves that side of the cell forward so they kind of slime their way around by
pushing forward in this movement called amoeboid movement. In plant cells you have a movement
of cytoplasm throughout the cell in this circular pattern called cytoplasmic streaming also.
But they don’t push against the wall because the cell wall prevents the cell from moving
or changing shape. The last fiber is the intermediate filaments.
They are in between the microtubules and the microfilaments. They are in the middle. These
are made from protein and they function to help in cell shape, anchoring the nucleus
into the middle and moving the organelles around.
The next organelle is the centriole. Now I mentioned this just a minute ago when I was
talking about microtubules. It is made from microtubules. Centrioles, first of all are
not found in plants. In humans or in animals you have a pair of centrioles. A pair is called
a centrosome. It is made from the microtubules. The arrangements of microtubules, and this
matters, are nine sets of threes. So if you look at it, you will find nine triplets. Here
is a triplet. There are nine of them and there is nothing on the inside. Nine triplets. It
is called a nine plus zero arrangement. Cilia are also made of microtubules. Cilia
have a nine plus two arrangement. It is made of nine pairs of two around two in the middle.
So it is a nine plus two. There are nine doublets around two in the middle. The difference between
cilia and flagella is size. Both of them are used to move the cell. Cilia are short and
a cell that has cilia is probably going to have thousands and thousands of cilia. Flagella
are long hair like projections. Typically a cell that has flagella will have one to
three flagella. They don’t have a whole lot of them. They are both used for movement.
One of the differences between the cilia and flagella found in eukaryotes and prokaryotes
is this arrangement of microtubules on the inside. Prokaryotes don’t have these. Eukaryotes
do. Also in eukaryotes, the plasma membrane surrounds these.
At the base of flagella and cilia you have another organelle called a basal body. This
connects the cilia and the flagella to the cell. It is kind of like the motor that runs
the whole cilia/flagella to make it spin. This also is made from microtubules and the
arrangement of the microtubules this time is nine triplets around one central. So if
you notice each of these three are very specific about how they are constructed. It is important
that you know the difference between the structures. It is the type of question they are likely
to ask. Here are nine triplets around one central.
The central one is right there. This is the basal body. As you move from here into the
basal body you find your nine doublets around two.
To get cilia to move, it is very much like, if you have had anatomy, actin and myosin,
how the actin arms reach out to the myosin grab on and pull. It is very much like that.
Except instead of an actin arm, it is a dynein arm. It reaches out, connects pulls, reaches
out, and re-connects pulls, in the same pattern. By doing that, it makes one side of the cilia
shorter which makes it spin in that direction. It reaches out and pulls in a pattern so that
the cilia move in a circular motion. It spins. Here are those dynein arms, reaching out from
the doublets and grabbing on. This side is shorter so it is going to spin in that direction.
Then those are going to release and these are going to pull which is going to make it
spin towards here. Then those are going to shorten and release. Then these will attach
and pull which will make this side shorter. So it is going to spin in that direction.
So it is going to spin in a circle. I mentioned endosymbiosis. This is the last
part for the day. Endosymbiosis is a theory that explains how prokaryotic cells started
gathering together inside one another forming a eukaryotic cell. Now it did this by a process
of symbiosis which means both cells are going to benefit from the relationship. One provided
protection to the other cell and the other cell provided energy. The evidence for this
lies in the fact that organelles like mitochondria and chloroplasts both have their very own
DNA and they have a double phospholipid bilayer. One bilayer was from the original plasma membrane.
The outside bilayer was from when the cell was engulfed by the host cell. When it was
engulfed it was put inside a vacuole, a food vacuole. Now it has a double phospholipid
bilayer. These two organelles both have a double phospholipid bilayer. And they reproduce
by binary fission. So they have three lines of evidence that points to them having gone
through this process: their own DNA, the double phospholipid bilayer, and binary fission.
One of the objectives that you need to know for this class is something about horizontal
gene transfer, which means the transfer of genetic material across species lines. The
process of endosymbiosis does that. When you went from two individual species of cells
and then they combine to create a new type of cell, both of their DNA was combined. You
transfer the DNA. It creates a new organism. So the DNA was moved from one species of bacteria
to another species of bacteria. Even though it stayed inside that organelle, it is still
inside that bacteria cell. So it is an example of horizontal gene transfer
It is interesting that this happened many billions of years ago and all living cells,
all eukaryotic cells, evolved from the prokaryotic cells that went through endosymbiosis. Ultimately,
we are related to bacteria if you think about it.
We are going to watch this last you tube video—note, no speaking in the video.
That is it for today. Don’t forget you have three questions about prokaryotic cells to
go over. Let me scroll back real fast to find that. (Flipping through slides, no speaking.)
There you are. That is due tomorrow. Please have all three questions answered in detail.
Don’t give me one sentence for each. Have a great night. Bye.