There are two kinds of cells: Prokaryotes and Eukaryotes.
These two different cells have many differences but many similarities at the same time.
One very prominent difference between prokaryotes and eukaryotes are the size. Prokaryotes are between 1-10µm in size while eukaryotes can go up to 100µm! There are differences in the organelles as well. Let’s look at each of the organelles in prokaryotes and eukaryotes, and their functions.
Prokaryotes:
An easier term to define prokaryotes would be bacteria: yes, those puny little (sometimes) disease causing microbes.
Prokaryotes, firstly, are small. The majority of them are between 1-10µm. Prokaryotes are very simple; they are made up of only a cell wall, plasma membrane, cytoplasm, membrane and a nucleoid. Some, though, have two more: pili and flagella.
The cell wall in a prokaryote is not much different from the cell wall in a plant cell (a eukaryote). The cell wall, however, is not made up of cellulose, but rather out of peptidoglycan. The cell wall has the following attributes: “always present, composed of peptidoglycan, protects the cell, maintains its shape, prevents cell from bursting”. The last attribute is interesting; the peptidoglycan cell wall prevents the cell from bursting. Without a cell wall, water would get into the cell via osmosis; the process of water diffusion. And this exact exploit is what doctors use in antibiotics such as penicillin.
Penicillin was discovered by Alexander Fleming. He was cleaning up his lab one day when he found a plate with mold growing in it. In that plate, he was growing a bacteria called staphyloccus.He found that around the mold growing, there was a ring of no bacteria. And thus the discovery of penicillin took place.
Penicillin is an antibiotic; anti meaning against, biotic referring to bacteria (against bacteria). It works by penetrating the cell wall by disabling an enzyme, transpeptidase. The penicillin is accepted by the enzyme as a substrate, but the penicillin instead messes up the enzyme by taking electrons from oxygen molecules that make up transpeptidase. Yes, everybody, betrayal exists in the cellular level too. As the enzyme is destroyed, the cell wall develops holes in it. As these holes are made, water diffuses into the cell via osmosis, and once enough water is inside the bacteria, the bacteria start to pop. Quite literally, actually. They just pop.
However, there are different kinds of bacteria which have more complex cell walls. They are called gram positive and gram negative bacteria. Bacteria can be categorized as gram positive or gram negative through a test called the “gram stain”. Doctors, scientists or whoever wants to know the gram state of the bacteria would inject the “stain” into the bacteria. The “stain” attaches to the peptidoglycan in the cell wall and thus “stains” the bacteria purple.
Now let me explain the whole point of this. There are evolutionarily “smarter” types of bacteria as well. These bacteria have evolved membrane on top of their cell wall, causing the peptidoglycan in the bacteria’s cell wall to be unreachable. Without access to the peptidoglycan in the cell wall, penicillin has virtually no effect, and thus different types of antibiotics have to be used. The smarter bacteria with the second membrane are called gram negative, as the fail the gram stain. How do they fail? Well, since the stain has no access to the peptidoglycan, the bacteria does not stain purple. On the other hand, gram positive bacteria, ones that pass the gram stain, have the cell wall which gives access to the peptidoglycan, therefore they are easier to pop.
Anyway, so moving on, now comes the plasma membrane of the prokaryote. This too, is not much different from a plasma membrane of a eukaryotic cell. It is a layer of phospholipids pushed up against the cell wall. The key difference here is the fact that while the prokaryotic cell has a layer of phospholipids, eukaryotic cells have a bilayer, or two layers. This makes the membrane harder to get across from. The membrane in the prokaryote is partially permeable, which is no different from the eukaryote.
The main difference between the prokaryotic plasma membrane and the eukaryotic plasma membrane is the fact that the prokaryotic cell membrane produces ATP, adenosine triphosphate. ATP is like the petrol, or if you’re environmentally conscious, electricity, of the cells. It is the power. ATP is like little high-power packets of energy. Like.. Redbull or Monster. Or Burn (Energy drinks). When enzymes break ATP, energy is released, and this energy powers the cell. ATP is naturally made in your body as it oxidizes (be chemically combined with oxygen) your food.
In eukaryotic cells, the mitochondrion are the generators of ATP. However, in prokaryotes, the plasma membrane generates the ATP.
But why this difference? Why doesn’t the eukaryotic cell also have its plasma membrane create ATP? Well, I’ll tell you that later, but however, just remember this: the plasma membrane “produces ATP by aerobic cell respiration” (bio book, page 22)
Moving on, next you have ribosomes. Ribosomes mainly synthesize (make) protein. They are small bead-like structures. The main difference between ribosomes in prokaryotes and in eukaryotes is the fact that the ribosomes in prokaryotes are smaller than the ribosomes in eukaryotes. Prokaryotic ribosomes come in at 70S, while eukaryotic ribosomes at 80S. What is that? What does the S stand for? S is a unit called sedimentation rate.
Sedimentation rate is measured by how far down a centrifuge an item goes. When a centrifuge spins, the contents go a certain distance down the centrifuge according to their mass and size. There is a formula including the length and the force of the centrifuge that when solved for, will give you the sedimentation rate. In this case, the prokaryotic DNA moved 70 units, therefore a sedimentation rate of 70S is given. On the other hand, the eukaryotic DNA moved 80 units, therefore a sedimentation rate of 80S is given.
Then, you have the cytoplasm. The cytoplasm is no different from the cytoplasm of the eukaryote. Its basically water with many substances dissolved in it. The cytoplasm, however, unlike eukaryotes, has no membrane-bound organelles. Membrane bound organelles include organelles such as the rough endoplasmic reticulum (rER) and lysosomes. These are organelles with a phospholipid bilayer surrounding them.
Finally, you have the nucleoid. The prokaryote is a very unique type of cell; it has no nucleus! Rather, it just has a loop of DNA floating around in the cytoplasm, called the nucleoid. The DNA in a nucleoid and in a eukaryote is very different; prokaryotic DNA is just a single circular strand of DNA, while eukaryotic DNA is complex and in chromatin. Eukaryotic DNA also have histones, a protein which the DNA coil around ever so tightly. Prokaryotic DNA do not have histones. Also, prokaryotic DNA is much more simple compared to eukaryotic DNA. Finally, the nucleoid is less dense than the eukaryotic nucleus as there are fewer ribosomes and less protein inside the nucleoid.
Now, while all prokaryotes have the components listed above, there are a few prokaryotes that have two extra components: Pili and flagella.
Pili are like small little hairs that come out of a prokaryote’s cell wall. These hairs are used for cell adhesion; you can think of it like Velcro. Pili are like Velcro on the prokaryotic cells. They are also used during sexual reproduction of prokaryotic cells. They are made out of protein filaments that stick out of the cell wall, and they can be retracted when necessary.
Then, you have the flagella. Flagella are like little propellers for the prokaryote; it helps them move. They are made with a cockscrew shape, and unlike the pili, the base is on the cell wall. The pili protrude out of the cell wall. Also, these flagella, unlike eukaryotic flagella, are solid.
Now, moving on:
Eukaryotes
Eukaryotes are much more complex compared to the prokaryotes. Eukaryotes are pretty much all other cells that are not bacteria. They have a nucleus, a cytoplasm and other membrane bound organelles.
Eukaryotes, unlike a prokaryote, have a nucleus. Now, while in prokaryotes, there is just a nucleoid where there is free-floating, uncoiled DNA, such is not the case in the eukaryote.
The DNA in a nucleus is first of all protected from the extranuclear environment. That protection is achieved from the nuclear envelope, which consists of two layers, the outer and the inner membrane. The nuclear membrane is porous and selectively permeable. Not everything can go inside the nuclear membrane, and not everything can come out. Inside the nuclear membrane, you have the chromatin. Chromatin is basically the DNA that is inside the nucleus, but not only the DNA.
In prokaryotes, there is just a single ‘ring’ of DNA. It is like a small bracelet which is conveniently located in one area of the cell. In eukaryotes, it is a bit of a different story.
It is said that if you line up the DNA in a cell, it will roughly be 3-6 feet long (that is 0.9-1.8 meters). How do you think something THIS long can fit into a cell 100µm in diameter?
The answer is coiling; tight, tight coiling.
But what does the DNA coil around? The answer is a protein called histone.
The chromatin is a combination of DNA and proteins such as histone.
Anyway, inside the chromatin you have the nucleolus. This is where ribosomes are made.
Outside the nucleus you have the rough endoplasmic reticulum, or the rER. The rER is basically a bunch of flat membranes sacs called cisternae. These sacs are hollow. On the outside of these sacs you have ribosomes attached. These ribosomes, as discussed above, specialize in synthesizing protein. However, ribosomes in the rER specialize in making certain types of protein; protein that are secreted by the cell. For example, your saliva is made by ribosomes in the rER of your salivary gland cells.
As the protein to be secreted is made by the ribosomes attached to your rER, the protein go into and travel down the cisternae. As they travel down the cisternae, they reach the sER, or the smooth endoplasmic reticulum.
As you know, ribosomes are attached to the rER. The ribosomes are what give the rER the r; roughness. The sER, on the other hand, has no ribosomes attached to them, therefore they are smooth, and thus called the sER. The protein continue to travel down the inside of the cisternae.
As they reach the bottom, they are received by vesicles.
Vesicles are simply hollow membranes which are used to take items from one place to another. You can think of them as a car. You are the item that needs to be transported and the vesicle is the car. The car, in this case, is to take you from the rER to the Golgi apparatus.
The Golgi apparatus is like the protein processing plant of your cell. The Golgi apparatus, too, like the rER is composed of cisternae. The difference, however, is that the cisternae are not as long, are more curved and do not have ribosomes attached to them. As the vesicles approach the cisternae on the Golgi body, they fuse with the Golgi body, thus transferring the protein into it. Inside the Golgi body, these protein are ‘processed’. They modify and transport the protein. Once the modification is over, the protein again go into the vesicles. The vesicles then transport the protein outside the cell, where they are let go.
Then, you have lysosomes. Lysosomes are like tight, dense packs of protein. They have very dense digestive enzymes, that if not controlled, can break down anything, including a cell itself. They are mainly used to break down nutrients, however lysosomes are used widely in your white blood cell as well. The white blood cell engulfs an intruder and the lysosomes in your white blood cell are used to break it down.
Free ribosomes are a type of ribosome found inside a eukaryote’s cytoplasm. They, unlike the ribosomes on the rER, are essentially ‘free’, floating around in the cytoplasm. Their job, unlike the job of the ribosomes in the rER, is to synthesize protein that is to be used inside the cell.
Now, finally, mitochondria. Mitochondria are like the powerhouses of the cell. They are what make the cell’s supply of ATP. It is surrounded by a double membrane. The shape is usually shperical or oval, and it produced ATP by a process called….. wait for it… aerobic cellular respiration. Remember aerobic cellular respiration? When I was talking about the plasma membrane of the prokaryotes I had mentioned to keep it in mind. Well here it comes:
Mutual symbiosis is a type of relationship where two organisms depend upon each other.
It is widely hypothesized that mitochondria are, in fact, a produce of mutual symbiosis between a prokaryote and a eukaryotic cell. The eukaryote provides protection for the prokaryote while the prokaryote provides the eukaryote with power.
Interesting, right?
Resources:
“Adenosine Triphosphate.” Adenosine Triphosphate. N.p., n.d. Web. 2 Sept. 2012. <hyperphysics.phy-astr.gsu.edu/hbase/biology/atp.html>.
“Alexander Fleming and Penicillin.” History Learning Site. N.p., n.d. Web. 2 Sept. 2012. <http://www.historylearningsite.co.uk/alexander_fleming_and_penicillin.htm>.
Allott, Andrew, and David Mindorff. Biology: IB diploma course companion. 2nd ed. Oxford: Oxford University Press, 2010. Print.
“Gram Positive and Negative Bacteria.” Virgin Media – Cable broadband, TV & phone plus mobile broadband & phone. N.p., n.d. Web. 2 Sept. 2012. <http://homepage.ntlworld.com/diamonddove/04a_Gram/Gram.htm>.
“HOW DOES PENICILLIN WORK.” Bristol University | School of Chemistry. N.p., n.d. Web. 2 Sept. 2012. <http://www.chm.bris.ac.uk/webprojects2002/thornton/how_does_penicillin_work.htm>.
“Molecular Expressions Cell Biology: The Golgi Apparatus.” Molecular Expressions: Images from the Microscope. N.p., n.d. Web. 2 Sept. 2012. <http://micro.magnet.fsu.edu/cells/golgi/golgiapparatus.html>.
“Symbiosis.” Science – AllAboutScience.org. N.p., n.d. Web. 2 Sept. 2012. <http://www.allaboutscience.org/symbiosis.htm>.
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