Why Are Eukaryotic Cells So Complex?
Here are several cells from the body. Clearly, the interior of these cells is more complex than that of the prokaryotes you have studied. You will soon see that the larger size of eukaryotic cells requires a complex network of membranes and other structures to perform the necessary life functions. Just as a business needs more equipment and workers as it grows, a large cell needs more components and better organization to produce its products efficiently. Also note that the cells shown here look different from one another. Because they are usually part of a multi-celled organism, eukaryotic cells can specialize, with each cell performing a specific function for the organism as a whole. You will see many dramatic differences among cell types later in the course, as you study animal cells in detail and compare them to plant cells, fungal cells and the single-celled protista.
There are a few giant prokaryotes. One recently discovered giant is shown here. This cell is a bacterium that lives in the gut of a fish. It reaches a length of 230 micrometers (100X that of an average-sized bacterium), and is so large that it may need two nucleoid regions to serve the needs of its large volume of cytoplasm. These nucleoids are visible at the arrowheads. At first glance, this cell seems to exceed the minimum surface to volume requirement discussed earlier. However, it is thought that the cell just barely meets this requirement because its environment is very rich in nutrients (food the fish has eaten). Thus, the high concentration of nutrients surrounding the cell may facilitate diffusion of adequate nutrient molecules into the bacterium. Additionally, these cells contain tubules and vesicles that may form an internal transport system to enhance the distribution of materials within the large cell volume.
Here is a true giant! This bacterium has cells with an average diameter of 180 micrometers, but which sometimes reach diameters of 750 micrometers. The bacteria grow as a string of beads large enough to be visible to the naked eye.
In this case though, increased size is not accompanied by complex internal structure. These bacterial cells are almost entirely filled with a liquid vacuole that contains nitrate taken up from seawater and stored for use in oxidizing sulfur. Thus, most of the cell volume is a storage site, not living matter.
We see a similar situation in some eukaryotic cells. An extreme example is the yolk of a bird egg. The yolk is technically a single cell and certainly violates the rules that limit cell size. But almost all of the cell volume is stored food to be used by the growing chick embryo if the egg is fertilized. The living matter of the cell is limited to a thin disk that can be seen atop the yolk. Even this small spot would be too large for a viable cell if it were spherical, but its thin, flat shape gives a sufficiently small volume of cytoplasm relative to the overlying plasma membrane.
The size of eukaryotic cells requires a complex internal structure. For one thing, specific functions need to be compartmentalized within the cytoplasm to increase their efficiency. Surrounding specialized structures with membranes enables enzymes and other needed molecules to be concentrated in a small space where they can work together to produce a product. Thus the eukaryotic cell can be compared to a factory where specialist workers and their equipment are set up in different parts of the building. Using this analogy, the cell has membrane-bound structures that serve as powerhouses to produce energy. It has a sophisticated assembly line for protein production that includes the workbenches and a distribution center. It has specialized structures that act as cleaning crews and a central control center to co-ordinate all of these activities. Eukaryotic cells even have a structural support system to prevent the “factory” from collapsing. The membrane-bound structures that perform most of the above functions are called organelles and the support system is called a cytoskeleton. The following pages will describe the organelles and cytoskeleton of animal cells in detail.
A major advantage of organelles is that they form compartments within the eukaryotic cell. The mitochondrion is a good example, as illustrated in this animated model. Looking at a whole mitochondrion, we see the outer membrane that separates this organelle from the cytosol. If a section of the membrane is removed, we find a second membrane beneath. The two membranes create a sub-compartment between them that is used to sequester hydrogen ions during the production of ATP. If we now cut the mitochondrion in half, we find that the inner membrane is highly folded. This creates additional sub-compartments and provides more membrane surface for attachment and organization of the enzymes required in energy conversion.
The use of membranes to create internal compartments is not entirely limited to eukaryotic cells. As you learned earlier, the cyanobacteria possess internal membranes on which photosynthetic reactions occur.
Additionally, some heterotrophic bacteria utilize membranes to compartmentalize metabolic reactions. In one of the nitrogen-fixing bacteria, shown here, the key enzyme in ammonia oxidation is located on these membranes.
At higher magnification, it can be seen that the bacterium also has vesicle-like structures within the cytosol. These serve as compartments to concentrate metabolic enzymes. So intracellular membranes in both eukaryotic cells and some prokaryotes provide the cell with compartments and an increased membrane surface area, resulting in enhanced metabolic activity. Note, however, that most of the internal membranes of prokaryotes are infoldings of the plasma membrane and thus different from the organelles of eukaryotic cells.
Organelles of Animal Cells: The Nucleus
The nucleus is the most prominent organelle within animal cells. Even at the rather low magnification of this image, we can see a nucleus within most of the cells. It is stained dark blue in contrast to the light blue of the cytoplasm. Although it looks like a simple structure when viewed by light microscopy, we shall soon see that the nucleus has a fair amount of complexity.
Most of the genetic material of eukaryotic cells is housed within the nucleus. This material is, of course, DNA and as in prokaryotes it takes the form of chromosomes. However, eukaryotic cells have much more DNA than prokaryotes. Whereas prokaryotic cells have one small, circular chromosome, eukaryotic cells have many linear chromosomes. Chromosome number depends on the species: for example, humans have 46 chromosomes in each cell (except the sex cells which have 23), and fruit flies have 4 chromosomes (with 2 in the sex cells). Eukaryotic cells must make a much large number of different proteins, more than prokaryotes; hence they have greater number of genes and longer chromosomes. Besides needing more proteins because their cells are larger, most eukaryotes are multi-celled organisms with each cell type requiring a different set of proteins.
If we look at a nucleus using electron microscopy, we find that the interior is filled with light and dark substance that cannot be clearly seen. This material contains the chromosomes which are extremely long and thin, so cannot be seen as distinct structures. Each chromosome is composed of a double strand of DNA plus a larger number of proteins. The DNA plus protein is called chromatin.
It is difficult to realize just how much DNA is in the nucleus. If a typical chromosome were removed from the nucleus and extended to its full length, it would be between 2-8 cm long. So how is all of this genetic material packed into the nucleus? Well, it certainly is not fully extended, but rather is coiled and looped into a very compact form.
The first level of coiling occurs due to the attachment of proteins called histones at regular intervals along the DNA molecule. As you can see from the model on the left, the DNA is wrapped twice around each histone. The complex of wrapped DNA and protein is called a nucleosome. If chromatin is removed from the nucleus, spread out and photographed at very high magnification, the nucleosomes appear as beads along the DNA strand. This can be seen in the image on the right.
One further word about histones. You may remember that the main differences between domains bacteria and archaea were biochemical. We can now add that histones are one of these differences. Histones are present in the archaea, but absent in all bacteria. So surprisingly, the archaea resemble eukaryotes more than bacteria regarding types of chromosomal proteins.
Before we leave the subject of chromosomes, we should probably see some in their most highly compacted form. If you view the nucleus of a cell that is not dividing, the chromosomes will not be seen as distinct entities. Most of the chromatin will look like the euchromatin shown here. However, when a cell divides, it must compact the chromosomes as tightly as possible to assure that complete chromosomes are distributed to the new cells.
Here are some chromosomes removed from dividing cells. One way of stating the difference in the two images is to call the chromatin in the first image uncondensed as opposed to condensed in the chromosomes shown here. This is just another way of saying that the chromatin is loosely packed vs. tightly packed.
Finally, we see a light micrograph that is probably familiar to you, since almost everyone has seen dividing cells under a microscope. The cell in the center is dividing and its chromosomes are clearly visible. Most of the other cells are not dividing and their chromosomes are in the euchromatin (or uncondensed) state.
This is the nucleus of a living cell preparing to divide. Watch as the chromatin condenses into visible chromosomes.
Organelles of Animal Cells: The Endomembrane System
We are looking at an electron micrograph of an animal cell. Like most eukaryotic cells, it is filled with internal membranes. These membranes consist of the rough and smooth endoplasmic reticulum, the Golgi apparatus, lysosomes, and the outer nuclear membrane. They are collectively called the endomembrane system. Additionally, the plasma membrane is closely associated with endomembranes as we shall see shortly.
The endomembrane system has several important functions, one of which is to assist in the synthesis of proteins and their transport throughout the cell. Many of these proteins are needed as components of growing membranes within the cell, whereas others are cell products that will be secreted and used elsewhere in the animal’s body. To assure that each of the many kinds of proteins produced arrive at the correct place, endomembranes form a sort of traffic control and transport system. This animation shows proteins leaving the RER in vesicles. The vesicles carry the proteins into the Golgi where they move from sac to sac, finally leaving in vesicles for their final destination. In this case, the proteins are secreted from the cell when the vesicle fuses with the plasma membrane. You will learn more about each of these steps farther down the page.
Now that you have seen diagrams of RER, it is time to look at the real thing. This colorized electron micrograph gives a close-up view of RER within the cytoplasm of a liver cell. Note that the RER consists of a series of thin channels formed by endomembranes. The interior of the channels has been colored green. The ribosomes are visible as red dots attached to the outer surface of the membranes.
The RER provides most of the membranes in the endomembrane system. The RER itself grows in place. It makes membrane phospholipids from precursors in the cytoplasm and inserts some of the proteins synthesized on bound ribosomes into its own membrane structure. When vesicles bud from the endoplasmic reticulum, they carry the surrounding membrane as well as a cargo to the Golgi. The vesicle membranes fuse with those of the Golgi thus bringing new membranes to this organelle. Some of the Golgi membranes bud off and form lysosomes, and others fuse to the plasma membrane to secrete their product. So, the source of most cellular membranes can be traced back to the RER.
This diagram gives a good 3-dimensional view of the endoplasmic reticulum. Note how the membrane channels are interconnected. The rough and smooth endoplasmic reticulum are similar and are, in fact, continuous with one another, but the smooth ER does not bind ribosomes.
See if you can distinguish the two types of endoplasmic reticulum in this electron micrograph. And keep in mind that the endoplasmic reticulum is an extensive network throughout the cytoplasm of most animal cells. Taken together, the rough and smooth ER can account for more than half of the total cellular membrane.
Functions of Smooth
The smooth endoplasmic reticulum functions in a number of metabolic processes, depending on the cell type. These include synthesis of phospholipids and metabolism of carbohydrates. In hormone-producing cells, the smooth ER synthesizes steroid hormones (which have a lipid base). The cell shown here on the right is from the adrenal gland. It synthesizes and secretes several adrenal hormones, such as cortisol. The lipid precursors of the hormone are stored in large vesicles labeled “T”. The smooth ER is too small to be seen in the cell on the right, but a portion of it is shown at much higher magnification in the image on the left.
The smooth ER membranes have an active transport system that moves calcium ions from the cytoplasm into the ER interior and stores them at high concentration. Calcium ions are then released back into the cytoplasm to trigger specific events. For example, in muscle cells the release of calcium causes muscle contraction. Thus the smooth ER plays a role in regulating cellular processes.
Animal cells can be exposed to toxic substances that have entered the body through food or water. This includes alcohol and drugs deliberately consumed by humans, but perceived by the body as poisons. Liver cells are especially active in the detoxification process and thus have large amounts of smooth ER. In this electron micrograph of a liver cell, smooth ER is visible at the arrowheads. Also, note the presence of rough ER on the left side of the image.
The enzymes that detoxify harmful substances are present within the smooth ER and its membranes. Large amounts of toxins stimulate the production of more, smooth ER. This can result in drug tolerance. For example, if barbiturates are consumed, the amount of smooth ER increases to detoxify the drug rapidly and greater quantities of barbiturates must then be consumed to have the desired effect on the body. Since the detoxifying enzymes usually affect more than one kind of drug, this can have additional side effects, such as reducing the effectiveness of antibiotics and other useful drugs.
The Golgi apparatus resembles the smooth ER in that it consists of flattened, interconnected sacs. The Golgi membranes separate the contents of sacs from the cytoplasm. Vesicles that have budded from the rough ER move to the Golgi and fuse to the closest sac. This is called the “cis face” of the Golgi apparatus. The protein contents of the vesicle are now inside of the Golgi and the vesicle membranes become part of the Golgi membrane system. Both the transported proteins and the membranes from the original vesicle will modified by the Golgi apparatus. In the case of the membranes, this is necessary because the specific composition of membranes differs in each compartment of the endomembrane system. In the case of the proteins, they will be further processed and sorted as they pass from one Golgi sac to another, finally arriving at on the opposite side of the Golgi called the “trans face”. From here, the proteins will be packaged into vesicles and released to travel to their final destinations.
This high magnification electron micrograph has been colorized to show the relationship between the rough endoplasmic reticulum and vesicle membranes. The RER is colored green. Blue vesicles containing proteins are budding from it. They move toward the Golgi as orange vesicles, some of which fuse to form a new sac on the cis face of the Golgi apparatus. Most of the Golgi is farther away and not present in the image.
Lysosomes function as digestive compartments for the cell. They are small vesicles containing acid and hydrolytic enzymes that can degrade a variety of macromolecules. The three lysosomes in this electron micrograph contain detritus in various stages of digestion. Lysosomal enzymes are made in the RER and processed in the Golgi like other proteins. Most lysosomes form as vesicles budding from the Golgi, making them part of the endomembrane system.
In a multi-celled organism, tissue debris from a cell’s environment can be taken in by some cells for disposal. Once in the cell, the vesicle containing debris fuses with a lysosome. The contents are then digested. The cells of some animals and protista take in food particles and digest them by this method. Note that the lysosome’s own membrane is not digested, probably because its membrane proteins have a 3-dimensional structure that protects them from enzyme attack.
Some cells of the human body, including certain white blood cells and the macrophages shown here, engulf and digest harmful microorganisms. These macrophages are ingesting yeast cells. The yeast at the arrowhead is contained within a vesicle to which lysosomes will fuse.
Lysosomes also degrade old organelles. Here we see a lysosome fusing with a mitochondrion. Lysosomal enzymes then digest the mitochondrion and useful molecules are returned to the cytoplasm for recycling to new mitochondria. Products of digestion that cannot be used are expelled from the cell. With the assistance of lysosomes, a cell can continuously renew itself. For example, human liver cells recycle half of their macromolecules each week.
This electron micrograph shows a lysosome close-up. It contains two damaged organelles, a mitochondrion and a peroxisome. You might wonder if lysosomal enzymes ever leak and cause damage to organelles in the cytoplasm. Well, these enzymes only function well at an acidic pH, so If leakage does occur, the neutral pH of the cytosol greatly reduces their activity. It takes massive leakage of lysosomes to cause damage within the cell.