Cell Structure

A living cell is a complex, multi-functional unit. Even the simplest of cells performs a large array of different tasks and functions. Despite our size prejudice, which makes us view cells as very tiny, they are very large places at the level that matters, which is the chemical level.

Cells come in two basic types. Read the Prokaryotic and Eukaryotic Cells essay for a discussion of the differences between these cells. Prokaryotic cells are found in bacteria, including both Archaebacteria and Eubacteria, and including the blue-green algae. Eukaryotic cells are found in animals, plants, Fungi and protists.

In this essay we are looking at straightforward structure issues.

Here's a labeled diagram of a simple prokaryotic cell:

The structures shown here are:

  1. The plasma membrane, which serves as a diffusion barrier between the cell and its environment. All living cells have plasma membranes. A "diffusion barrier" prevents the loss of cellular materials by interfering with the physical tendency of molecules to spread out. It also prevents many substances from diffusing into the cell. And it also controls the movements of various substances and objects into and out of the cell.

  2. The cell wall, which is very different from the plasma membrane. Cells with cell walls also have plasma membranes; the wall can't perform the diffusion barrier tasks of the plasma membrane. One of the primary functions of the cell wall is physical support. Some kinds of bacterial cell walls also have other functions. Prokaryotic cell walls are composed at least partially of an interesting substance called peptidoglycan, which is a kind of hybrid between polysaccharide and protein.

  3. The capsule, which is not found in all prokaryotic cells. The capsule is composed of rather amorphous polysaccharide material similar to mucous. This material is hygroscopic, meaning that it has a great capacity to retain water. Water is a vital component of any living cell, and this layer around the cell can be very advantageous for the cell.

  4. The nucleoid, which is an essentially imaginary "structure." This is the central region of the cell, where the DNA is largely located. There is no physical structure enclosing the nucleoid. The name means "imitating a nucleus," implying that this is the region in the cell in which the functions of a eukaryotic cell's nucleus occur.

  5. The naked, circular DNA, which is characteristic of prokaryotic cells, mitochondria and chloroplasts. This DNA is structurally identical to the DNA found in a eukaryotic cell's nucleus, except that instead of being like a long string, with two ends, it's formed into a closed circle. This DNA molecule, which is not properly called a chromosome, carries all of the essential genes for the cell.

    Not pictured here are plasmids. These are much smaller circles of DNA, carrying only a very few genes each. The genes on plasmids are "luxury" genes--the cell doesn't need any of them for normal functions of life. Genes carried on plasmids are often things like antibiotic resistance genes. The F factor which determines the role of the cell in sexual reproduction is also often a plasmid gene. Prokaryotic cells trade plasmids easily and frequently, and the population of plasmids in any prokaryotic cell is pretty much constantly changing.

  6. Ribosomes are another essential component for any living cell. Each cell contains many, many ribosomes. The function of a ribosome is to make protein, following instructions sent from the DNA's genes.

  7. Cytoplasm (not labeled) is the fluid substance that fills the interior of the cell. Cytoplasm is often described as a "rich, organic soup." This analogy seeks to honor the complexity of cytoplasm, which is mostly water, but contains a large assortment of molecular and structural components.

  8. There are a couple of additional structures found in some, but not all, prokaryotic cells, but which are not included in this diagram. Both of these structures are actually elaborations of the plasma membrane, not separate, independent structures.

    1. Mesosomes are rosette-like clusters of folds in the plasma membrane, protruding toward the interior of the cell. These structures are important in the performance of the aerobic parts of aerobic cellular respiration. A key part of this process requires a lot of membrane surface, and mesosomes greatly increase the membrane surface of the cell.

    2. Thylakoids are long, strap-like invaginations of the plasma membrane. They're so long the typically wrap multiple times around in the interior of the cell. Thylakoids are found only in photosynthetic prokaryotic cells, such as the cells of blue-green algae and the purple photosynthetic bacteria. Like aerobic respiration, photosynthesis contains a vital function which requires a lot of membrane surface, and the long, thin thylakoid surfaces provide that area.

Eukaryotic cells, while very similar in function to prokaryotic cells, are much larger and more complex. They are typically characterized by the presense of a prominent, more or less central nucleus. Viewed through a light microscope, this nucleus is by far the most striking visible feature of the cell. However, there is a lot more inside a eukaryotic cell than what can be seen through a light microscope. Most serious study of cell structure utilizes an electron microscope, which is capable of much higher resolution (and thus supports much higher magnification) than a light microscope.

Here's a labeled diagram of a typical eukaryotic animal cell. Eukaryotic cells are found in animals, plants, Fungi and protists, and there are structural differences among these four groups. We'll be using the animal cell as our basic eukaryotic cell, but will discuss some of these differences down below.

As you can see from comparing these labels, there are several features which are common to both prokaryotic and eukaryotic cells. Recall that these cells perform the same kinds of functions. The eukaryotic cell is much larger than the prokaryotic cell, and this larger size means that there's a lot more space inside the cell. If you think of a prokaryotic cell as something like a studio apartment, a eukaryotic cell is like a gigantic warehouse. In order to make the huge space relatively as efficient as the small space, a lot of compartmentalization and internal specialization is required.

  1. The plasma membrane for a eukaryotic cell is the same kind of structure, and has the same function as the plasma membrane of a prokaryotic cell.

  2. Endoplasmic Reticulum (ER) is a system of membrane-enclosed channels which ramifies throughout the cytoplasm of the cell. It comes in two types--smooth and rough. The difference is that rough ER has ribosomes all over its outer surface. "Endo" means "inside," "plasmic" refers to the cytoplasm, and "reticulum" means "network." ER interconnects with the plasma membrane and the nuclear envelope. Some suspect that all of the ER within a cell is actually interconnected, but this has never been established.

    ER has several functions. It helps to compartmentalize the cell, and it serves as routes for the transport of materials from one part of the cell to another. It's associated with lipid synthesis and protein synthesis (rough ER only). And smooth ER is responsible for generating new layers for Golgi bodies.

  3. Vacuoles are small, varied membrane "bubbles" found throughout the cytoplasm. Vacuole is a very general term, and there are quite a few different kinds of vacuoles, including food vacuoles, contractile vacuoles, and the very specialized central vacuoles found only in plant cells.

  4. Each cell contains a number of Golgi bodies. "Golgi" is the name of the person who first described these structures. Golgi bodies are like little stacks of hollow membrane pancakes. Their function is to process materials manufactured by the cell, then package those products into small structures called "Golgi vesicles." The materials arrive at the Golgi bodies from the smooth endoplasmic reticulum. Golgi vesicles come in two general types--microbodies and secretory vesicles. Microbodies are fated to remain in the cell. They contain materials, usually enzymes, which the cell needs, but which must remain packaged away from the cell's other contents. The best known of these microbodies is the lysosome. "Lysis" means "breakage," and "some" means "body." Lysosomes contain digestive enzymes which, if released into the cell, would digest the vital components of the cell and kill it. "Break" it, in other words.

    The other kind of Golgi vesicle contains materials to be exported (secreted) from the cell. These materials are not waste products--they are chemicals intentionally manufactured by the cell for export, like hormones and pheromones.

  5. Mitochondria are very complex, double-membrane-bound organelles. Their function is to perform the aerobic portions of aerobic cellular respiration, the essential energy-producing process of the cell. This is the same function performed by the mesosomes in many prokaryotic cells. Mitochondria contain their own naked, circular DNA and their own ribosomes.

  6. Ribosomes perform precisely the same function in eukaryotic cells as they perform in prokaryotic cells. Eukaryotic ribosomes are larger and more complex than prokaryotic ribosomes, but they are very similar.

  7. The centrioles are a pair of structures composed of microtubules. The primary function of centrioles is to generate the cell's cytoskeleton (not shown in this diagram). The cytoskeleton is a system of microtubules and microfilaments which runs all through the cell, particularly just under the plasma membrane. Microtubules and microfilaments are responsible for all kinds of movement functions. For example, the contractile part of a muscle cell is composed of two kinds of microfilaments. And the spindle apparatus which moves chromosomes around during mitosis or meiosis is composed of microtubules.

    An interesting observation about centrioles is that there are none in the cells of higher plants, even though these plants perform mitosis and meiosis just fine. The explanation for this turns out to be pretty simple. One of the important functions that centrioles perform is the generation of cilia and flagella for cells. These are surface features that cells use for movement. The cells of higher plants have no centrioles because there's no cell anywhere throughout their life cycles which makes cilia or flagella. A cell can make a spindle without centrioles, but it can't make cilia or flagella.

  8. Rough ER has ribosomes attached to its outer surface. This is the source of the oft-read claim that "ER performs protein synthesis." This statement is quite misleading. ER never performs protein synthesis. Only ribosomes do that. Protein synthesis is associated with rough ER because rough ER has ribosomes all over it; the ribosomes are doing the protein synthesis, not the ER itself.

  9. The nucleus of the cell is a chamber specialized in DNA functions. It's enclosed by a double layer of membrane called the nuclear envelope. The function of the nuclear envelope is to confine the materials necessary for DNA and RNA synthesis inside the nucleus, and controlling movement into and out of the nucleus.

  10. The nucleus contains several--generally two to four--dense structures called nucleoli (singlur "nucleolus"). Assembly of ribosomes takes place in nucleoli.

  11. The nucleus of a eukaryotic cell contains a number of chromosomes, which are composed of DNA and histone proteins. The number, shape and arrangement of genes on chromosomes is characteristic of the species from which the nucleus came. Chromosomes carry the information archive of the cell. Each gene is a set of instructions for the construction of a specific protein.

  12. Nuclear pores perforate the double nuclear envelope. These pores do have structures in them which open and close to control movement through the pores. The nuclear pores are important for ribosomes to leave the nucleus. Though ribosomes are constructed in the nucleus, they must move to the cytoplasm in order to function.

  13. The nucleoplasm is the rich organic soup that fills the interior of the nucleus. This is very similar to cytoplasm. It's mostly composed of water, containing a complex assortment of materials. Nucleoplasm would be distinct from cytoplasm due to the high concentration of materials like nucleotides, which are used to make DNA and RNA, and the suite of enzymes which control the DNA and RNA construction reactions.

This diagram and list are not exhaustive. There are other structures which have been identified within cells. Some of them are mentioned in this list, though they are not included in the diagram--the cytoskeleton, cilia and flagella. There are others, but this is a pretty good introduction to the structures in animal cells

Other Eukaryotic Cells

As mentioned above, animals aren't the only kind of creature to have eukaryotic cells. Plants, Fungi and protists are also eukaryotic organisms.

There are several important differences between plant and animal cells. The description of the animal cell above points out that at least higher plants have no centrioles in their cells. There are also three kinds of structures found in plant cells and not in animal cells.

  1. Plant cells have cell walls; animal cells do not. As noted in the description of the prokaryotic cell wall, the cell must also have a plasma membrane. Plant cells have cell walls in addition to plasma membranes, not instead of plasma membranes. Plant cell walls are composed all or partially of a carbohydrate called cellulose, which is quite different from the peptidoglycan of prokaryotic cell walls. The primary function of the cell wall is support, and inconjunction with the central vacuole, to create turgor (stiffness) in plant structures like leaves.

  2. Plant cells contain a specialized vacuole called the central vacuole. This is a large, membrane bound structure filling most of the interior of the cell. The central vacuole is filled mostly with water, but always with some impurities--mineral or protein--so that the water concentration is always less than 100%. When the cell is surrounded by sufficient water, osmosis causes the central vacuole to swell, and thus causes the cell to press against the inside of the cell wall. This phenomenon in all of the cells of a leave causes the leaf's tissues to be stiff, and keeps this delicate structure spread out so it can serve its vital function as a solar panel.

  3. Plant cells contain a family of organelles called plastids. There are several kinds of plastids, all related to each other and, under appropriate conditions, capable of modifying from one type to another. The best known of these plastids is the chloroplast, which performs the function of photosynthesis. Chloroplasts are double-membrane-bound, like mitochondria. Also like mitochondria, their inner membrane is very complicated. In fact, it's formed into many thylakoid structures which perform the same function performed by the thylakoids in prokaryotic cells.

Despite common beliefs, Fungi are not plants. Biologically speaking, they are more animal-like than plant-like. They just look like plants to us. They do no photosynthesis and have no plastids. They have cell walls, but they are different from plant cell walls, and are clearly a separate evolutionary development. The cell walls of Fungi are typically composed of a material called chitin. Strangely enough, we also find chitin in one of the most successful of all animal phyla, the Arthopoda (which includes spiders, insects, crabs and lobsters). Arthropods have endoskeletons made of chitin. Again, a separate evolutionary invention.

Another odd aspect of cell structure in many Fungi is that, in some groups, the concept of a "cell" is only very loosely applicable. There are Fungi which are largely composed of a single, huge structure with many, many nuclei and no subdivisions into cellular chambers. And there are Fungi whose bodies are divided by incomplete subdivision, with continuous cytoplasm connecting all of the "cells" into one giant super-cell. Fungi are strange ;^)

Protists are very diverse--typical of a more primitive (meaning evolutionarily older) group of organisms. This group contains animal-like cells, plant-like cells, and fungus-like cells, as well as a dizzying assortment of in-betweens and oddities. The task of classifying protists is dreadfully difficult.

Speaking of Taxonomy--The Kingdoms Quandary

As we've seen so many times, we are a species driven to categorize, and biologically speaking, the mother of all classification enterprises is our Linnaean taxonomy--the classification of living things.

In principle, it seems like such a simple thing: just group things on the basis of similarity. Back when Linnaeus set out to examine, describe, classify and name all living species, he wasn't even worried about a lot of the things that are so important to us--like evolutionary relationships. He just wanted a nice, useful, organized, easy outline-like list of all those pesky, confusing organisms.

It turns out this is a lot easier to plan than to accomplish. You might want to go back and reread the essay on *content*taxonomy.htm:Essay ASLINK=taxonomy*endcontent*.

Consider the very fundamental question of Kingdoms. How many kingdoms do we need to subdivide the world of living things into the smallest possible very general categories of "different kinds of things"?

For Linnaeus, the answer to this was obvious. Clearly, there were only two kinds of creatures: animals and plants. So that's how many kingdoms he created--two. Plantae and Animalia.

Tradition is strong, even in the sciences, and Linnaeus was, after all, the "father of taxonomy." So for a long time, biologists stuck to this two-kingdom system, despite clear evidence that it didn't really work. Linnaeus didn't know anything about things like bacteria and protists (or if he knew about them, he just ignored them). And as more knowledge was gained about Fungi, which Linnaeus just shoved into the plant kingdom because they looked more like plants than animals, the more obvious it was that they were very different from plants.

Finally, the two-kingdom system had to be discarded. So then the question was, how many kingdoms do we need? Out of the variety of proposed systems, the one that emerged as the favorite for quite a while was the Whittaker five-kingdom system. Easy. Five kingdoms: Plantae, Animalia, Fungi, Protista, and Monera (bacteria). All of those annoying in-between sorts of eukaryotic organisms were jammed into the Protista, and all of the prokaryotic organisms were shoe-horned into the Monera. The biggest apparent problem with this system was the Kingdom Protista, which contains a whole lot of really different kinds of organisms. But everyone just looked the other way. To a large extent, everyone still looks the other way when it comes to the protists ;^)

But things aren't so comfortable with another one of these kingdoms. With the advent of knowledge of DNA and the great ability to compare the DNA sequences of different organisms, we've gained a marvelous kind of telescope into the biological past. Comparing the DNA sequences of different organisms gives us at least a rough idea of how closely related they are. And the better we get at it, the more precise that idea can become.

So here's the problem. DNA comparison tells us some really interesting things about these five kingdoms of organisms. The first thing it tells us is that there are two distinct groups of organisms among the Monera (bacteria). Two groups of bacteria which are, genetically speaking, very distant from each other.

The second fascinating thing DNA tells us is that the total diversity among all four of the eukaryotic kingdoms is less than the genetic difference between those two groups of bacteria.

So just what do we do with these bits of very significant information?

The first thing we have to do is recognize that, at the very least, we need a sixth kingdom for one of those groups of bacteria. So now we have added the kingdom Archaeobacteria for the group of bacteria which seem to be of a very ancient type. "Archaeo" means "ancient."

But that really isn't enough, because remember that the difference between the two kingdoms of bacteria is greater than the total difference among all of the other four kingdoms. So it has been suggested that we need to add a new, even more general level to our classification system. This new level has been called the Domain. This system names three Domains: Monera (or Eubacteria), Archaeobacteria and Eukarya. Guess where the various Kingdoms go ;^)

Even that system seems a bit lop-sided. Some folks have made a really radical suggestion that we should actually have only three kingdoms. This would "demote" all of those eukaryotic kingdoms to, at best, sub-kingdoms. Eek! This would do away with even those two original kingdoms Linnaeus devised!

So far, this suggestion hasn't gained a lot of ground. As I mentioned, tradition is strong, and this would be a big step.

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Updated 09 Sept 2004