We're going to start with eukaryotic cells even though they tend to be more complex than prokaryotes. There's a method to our madness, though. You are a eukaryote and have eukaryotic cells, so we thought you might relate better. "Eukaryotic" also comes before "prokaryotic" in the dictionary.
Honestly, we could come up with reasons all day, but the simple fact is that eukaryotic cells are up first. C'est la vie.
A cell is defined as eukaryotic if it has a membrane-bound nucleus. Any organism composed of eukaryotic cells is also considered a eukaryotic organism. Case in point: you. Oh, and all other people, too.
Biologists do not know of any organism that is composed of both eukaryotic and prokaryotic cells. However, many different types of prokaryotic cells, usually bacteria, can live inside larger eukaryotic organisms. Creepy, but true.
We humans, for example, have trillions of bacteria living in our colons, not to mention in our mouths and stomachs and small intestines and…you get the picture. Despite the fact that we have gobs of prokaryotic cells living inside and on us, humans are still categorically eukaryotic organisms. This means that all human cells—including those found in the brain, the heart, the muscles, and so on—are also eukaryotic.
Here is what one of these little guys looks like:
Okay, we are impressed. That is a lot of stuff jam-packed into something so small.
All of the organisms we can see with the naked eye are composed of one or more eukaryotic cells, with most having many more than one. This means that most of the organisms we are familiar with are eukaryotic. However, most of the organisms on earth, by number, are actually prokaryotic.
Here are some examples of eukaryotes:
Most plants, animals, and fungi are composed of many cells and are aptly classified as multicellular. Most protists consist of a single cell and are classified as unicellular. Funny how that works.
All eukaryotic cells have the following:
Most eukaryotic cells also have other membrane-bound internal structures called organelles. Organelles include:
There are a few major differences between animal, plant, fungal, and protistan cells. And guess what? We've got some handy-dandy lists to help you learn those differences.
All plant cells have the following:
Some animal and protistan cells have:
But all animal cells have:
All fungal cells have:
Huh, that one was also quite short. Have you had enough lists? Us, too.
The nucleus in the cell is analogous to the brain in the body. It is the control center for a cell.
Presenting—dun, dun, DUN—the nucleus:
The nucleus stores all the information a cell needs to grow, reproduce, and function. This information is contained in long but thin molecules of deoxyribonucleic acid, or DNA. One of the functions of the nucleus is to protect the cell’s DNA from damage, but that is not all it does. The nucleus is basically a large membranous sac. Like your face. Ohhhh, snap!
(Sorry, that was mean. By way of an apology, have a video of a banana driving a car. It's on us.)
The nucleus also contains a small, round body called a nucleolus, which is like DNA's apartment building. It's where the DNA and all of its attendant proteins hang out all day.
The nuclear membrane has pores through which the contents of the nucleus communicate with the rest of the cell. The nuclear membrane tightly controls what gets into the nucleus and what gets out. This regulation of communication by the nuclear membrane has a great effect on what a cell looks like and what it does.
Chromosomes are also located in the nucleus and are basically organized structures of DNA and proteins. In eukaryotes, the chromosomal DNA is packaged and organized into a condensed structure called chromatin. Chromosomes are single pieces of DNA along with genes, proteins, and nucleotides, while chromatin is a condensed package of chromosomes that basically allows all the necessary DNA to fit inside the nucleus.
We will dive deeper into the world of chromosomes in another section, but for now, just remember that eukaryotic and prokaryotic cells each have genomes, the name for the entire set of an organism's genetic and hereditary information. Genomes are entirely encoded in either the DNA or the RNA. In the case of eukaryotes, multiple linear pieces of DNA comprise its genome.
In eukaryotic organisms, the DNA inside the nucleus is also closely associated with large protein complexes called histones. Along with the nuclear membrane, histones help control which messages get sent from the DNA to the rest of the cell.
The information stored in DNA gets transferred to the rest of the cell by a very elegant process—a process so common and so important to life that it is called the central dogma of biology. No, really.
In eukaryotic cells, the first stage of this process takes place in the nucleus and consists of specific portions of the DNA, called genes, being copied, or transcribed, into small strands of ribonucleic acid, or RNA. RNA containing a copy, or transcript, of DNA is called messenger RNA, or mRNA. These mRNA molecules are then physically transported out of the nucleus through the pores (holes) in the nuclear membrane and into the cytoplasm, where they are eventually translated into proteins by ribosomes.
Therefore, the central dogma of biology is simply:
DNA → RNA → Protein
And it all starts in the nucleus! (Warning: any discussion of the nucleus excludes prokaryotes. The nucleus is a eukaryotic song and dance number, and don't you forget it.)
Most eukaryotic cells have a nucleus throughout their entire life cycles, but there are a few notable exceptions. Human red blood cells (the good ol' RBCs), for example, get rid of their nuclei as they mature. Rebels without a cause. Scratch that: rebels with a cause. With their nuclei removed, red blood cells have more space to carry oxygen throughout the body.
The plasma membrane in eukaryotic cells is responsible for controlling what gets into and out of the cell. A series of proteins stuck in the membrane help the cell communicate with the surrounding environment. Among other things, this communication can include
Keep in mind that the plasma membrane is universal to all cells, prokaryotic and eukaryotic. Because this cellular component is so important and so common, it is addressed in greater detail further on in the In Depth section.
Ribsomes are small cellular machines made of proteins and ribosomal RNA. All cells, both eukaryotic and prokaryotic, have ribosomes.
Presenting, the ribosome:
Is it just us, or does that thing look like a pantsless Patrick Star? You may have to squint a little.
Eukaryotic ribosomes are larger and have a slightly different shape and composition than those found in prokaryotic cells. Eukaryotic ribosomes, for instance, have about twice the amount of ribosomal RNA (rRNA) and one-third more ribosomal proteins (~83 vs. 53) than prokaryotic ribosomes have.
Despite these differences, the function of the eukaryotic ribosome is virtually identical to the prokaryotic version. This is a remarkable example of what we call evolutionary unity. Ribosomes translate mRNA into protein, or the last step in the central dogma of biology described earlier. It all comes together.
The cytoplasm in eukaryotic cells is a gel-like, yet fluid, substance in which all of the other cellular components are suspended, including all of the organelles. The underlying structure and function of the cytoplasm, and of the cell itself, is largely determined by the cytoskeleton, a protein framework along which particles in the cell, including proteins, ribosomes, and organelles, move around.
You can think of the cytoskeleton as a type of 3D "highway system" with roads running in every direction, including up and down. The cytoplasm is the thick fluid in which the "highway system" is suspended and through which cellular materials are transported.
Helpful tip: whenever you see cyto– as part of a word, think "inside the cell."
The central dogma is central to life, but there are exceptions! Reverse transcriptase is an enzyme found in viruses that can take RNA and make DNA. We know; we just blew your mind.