Back To CourseBiology 101: Intro to Biology
24 chapters | 226 lessons
Kristin has taught college Biology courses and has her doctorate in Biology.
Your body's composed of trillions of cells - lots of different types of cells that make up different organs and other parts of your body. Your body is also where 10 times that number of bacteria call 'home sweet home.' But don't be afraid - these bacteria do more good than harm to you. And besides, just in case you wanted to strike up a conversation with your tenants, you and your bacteria do have a few things in common.
All cells share some common characteristics that make them living things. All organisms are composed of cells, the basic fundamental unit of life. They contain DNA as a heritable genetic material, and they can reproduce. They transcribe DNA into RNA and translate RNA into proteins on ribosomes. They can also regulate transport across a cell membrane and require chemical energy for some cellular processes.
The number one biggest difference between the bacteria in your body and the cells making up your body are these tiny cellular components called organelles. You've actually learned a lot about organelles in other lessons without knowing it. Organelles are simply membrane-bound compartments within a cell, such as the nucleus, mitochondria, chloroplasts, golgi, and endoplasmic reticulum.
You are a eukaryote. Your cells are eukaryotic. Eukaryotic cells contain membrane-bound organelles, including a nucleus. Eukaryotes can be single-celled or multicelled, such as you, me, plants, fungi, and insects.
Bacteria are an example of prokaryotes. Prokaryotic cells do not contain a nucleus or any other membrane-bound organelle. Prokaryotes include two groups: bacteria and another group called archaea.
Having organelles is a big deal for a cell. A bacteria cell gets along just fine without organelles, but bacteria are tiny. That's why we're able to have so many of them on our body without really noticing them. Our cells, though - they're still small to the naked eye, but they're huge in comparison to bacteria. Our eukaryotic cells are bigger in size, with much more DNA. More DNA means more transcription, and more transcription means more translation, and more translation means more proteins. Bigger cells create the need for organelles.
You can think of it this way. If you only had two pairs of shoes and a few simple outfits, you could just hang up your outfits and put your shoes on the floor inside a small closet. Simple. However, let's say you have a shopping addiction, and you have seven different pairs of black pants, ten pairs of shoes in completely different shades of brown (and other colors, of course), and you hardly ever wear the same hat twice. You can imagine that you would need a walk-in closet, complete with shelving systems to organize everything, right?
Well, organelles are an efficient way to organize everything that's going on in the cell - to compartmentalize cellular functions. That's exactly what a eukaryotic cell is doing - separating cellular processes and organizing its space. But don't be fooled by the 'simplicity' of prokaryotes. Their smaller size and simplicity is an advantage to their lifestyle.
If you have a lot of shoes and a walk-in closet, the nucleus would obviously be the shoe rack - a structure to hold and organize all this important material. Having a nucleus or not is the biggest difference between eukaryotes and prokaryotes. Remember that your eukaryotic cells have linear DNA. Prokaryotic cells have a smaller, circular DNA genome. With a smaller size and a smaller genome, prokaryotes don't really need a nucleus. Their DNA is housed in a nucleoid, which isn't really a structure at all. There are no membranes around it, so it's not a separated compartment. The nucleoid is simply the area where DNA exists. This is the defining difference between prokaryotic and eukaryotic cells.
Again, don't be fooled by the lack of compartmentalization. Prokaryotic cells are less complex, but they can still perform the same processes similar to eukaryotic functions - they just do it differently. For example, DNA replication and transcription take place in this nucleoid area of the cytoplasm. While translation is still carried out on ribosomes, these are not membrane-bound organelles, and all ribosomes in a bacterium float free in the cytoplasm. There is no endoplasmic reticulum.
Prokaryotic and eukaryotic ribosomes are both made of rRNA and proteins, but the subunits are going to be different sizes. In addition, a group of bacteria can perform photosynthesis like plants. Without a chloroplast organelle, a bacterium can perform photosynthesis in the folds of its cell membrane. These cells may have been the first early organisms to ever perform photosynthesis!
Prokaryotes also have this way of keeping their structure and shape. Many of them have a cell wall, like plant cell walls. However, prokaryotic cell walls are made of a different specialized molecule. For example, bacterial cell walls can contain peptidoglycan, or a polymer of sugar and amino acids that give shape and structure to the bacterial cell wall. Prokaryotes also have this version of a cytoskeleton that contains some of the same proteins that are similar to the actin and filaments of eukaryotic cells and function in a similar, though simpler, way.
Some prokaryotes also have flagella, as do some eukaryotic cells. The function is the same - a long appendage that drives movement. However, prokaryotic and eukaryotic flagella are structurally different. While we have learned that eukaryotic flagella are composed of the cytoskeletal proteins called microtubules, flagella in bacteria are made of the protein flagellin. Bacterial flagella are also driven by motor proteins that swing the flagella around clockwise or counterclockwise, while eukaryotic flagella whip back and forth, not around.
Let's summarize the similarities and differences between prokaryotes and eukaryotes_. Both organisms are composed of cells, the basic unit of life, with each cell surrounded by a cell membrane. The biggest difference between prokaryotes and eukaryotes is that eukaryotes have a nucleus. They also have other membrane structures called organelles. Organelles are structures such as the endoplasmic reticulum and the mitochondria. The DNA in prokaryotes exists in a space called the nucleoid, and it is circular DNA, while eukaryotes have linear DNA, and it's organized into a higher-order structure.
Both eukaryotes and prokaryotes have ribosomes. Ribosomes are not a membrane-bound organelle, but in both eukaryotes and prokaryotes, they are used for translation of RNA into proteins. Eukaryotes and prokaryotes can both perform transport, DNA replication, transcription, translation, and movement. It is important to note, however, that although they share these similar processes, many are carried out within organelles in eukaryotes or use different proteins. For example, flagella and the cytoskeleton may be structured differently in prokaryotes and eukaryotes, but they have the same functions of movement and providing cell structure, respectively.
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Back To CourseBiology 101: Intro to Biology
24 chapters | 226 lessons