We're going to start with eukaryotic cells even though they tend to be more complex than prokaryotic. But, there is a method to our madness: you are a eukaryote and have eukaryotic cells, so we thought you might relate better. And, eukaryotic comes before prokaryotic alphabetically. Come up with whatever reasons you want for it, but eukaryotes are up first. Tough cookies.
Eukaryotic Cell Structure and Function
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. Biologists do not know of any single organism on Earth 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. Deal with it. 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:
OK, we are slightly impressed. That is a lot of stuff jam-packed into something we can't see too well with the naked eye.
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:
- Fungi (mushrooms, etc.)
- Protists (algae, plankton, etc.)
Most plants, animals, and fungi are composed of many cells and are, for that reason, aptly classified as multicellular
, while most protists consist of a single cell and are classified as unicellular
. Funny how that works.
All eukaryotic cells have
- A nucleus
- Genetic material
- A plasma membrane
- Cytoplasm, including the cytoskeleton
Most eukaryotic cells also have other membrane-bound internal structures called organelles
- Golgi bodies
- Endoplasmic reticulum
There are a few major differences between animal, plant, fungal, and protistan cells, and guess what? Here they are: All
plant cells have
- A cell wall made of cellulose
- A large central vacuole
animal and protistan cells have
animal cells have
Well, that was short.
All fungal cells have
- A cell wall made of chitin.
Also, quite short. Enough lists?
Structures Found in All Eukaryotic Cells
The Nucleus and Eukaryotic Genetic Material
The nucleus in the cell is analogous to the brain in the body. It is a control center for a cell.
Presenting, the nucleus:
The nucleus stores all the information the 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 that it does. The nucleus is basically a large membranous sac. Like your face. Ohhhh snap. Sorry, that was mean. Here, have a video of a banana driving a car
. It'll make you feel better.
The nucleus also contains a small round body called a nucleolus that holds nucleic acids and proteins. 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, and 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 just know that eukaryotic and prokaryotic cells each have genomes, which is what we call 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 on Earth 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: This does NOT apply to prokaryotes. You are in eukaryote-only territory, 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. Or actually, with a cause, because, with their nuclei removed, red blood cells have more space to carry oxygen throughout the body.
Eukaryotic Plasma Membrane
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
- Sending and receiving chemical signals from other eukaryotic cells
- Interacting with the cells of prokaryotic organisms during the process of infection.
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
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.3
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
Eukaryotic Cytoplasm and Cytoskeleton
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."Brain Snack
The central dogma is pretty 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.