Prokaryotes versus Eukaryotes Help
Introduction to Prokaryotes versus Eukaryotes
The modern electron microscope has been especially valuable in viewing the organelles of the prokaryote cell, such as a bacterial cell. Such cells are only about 1/10 the size of a typical eukaryote cell. Further, they are much simpler in their structural design. As you examine Figure 5.2, note that the prokaryote bacterial cell lacks a true nucleus. You may recall (Chapter 3) that the prokaryote cells appeared “before” (pro-) nuclei had evolved. Instead, the bacterium (bak- TEER -ee-um) holds a central, oval, nucleoid ( NEW -klee-oyd) region that is “kernel-like,” but not surrounded by its own individual membrane. The nucleoid region contains a complex collection of coiled DNA molecules. These DNA molecules use RNA molecules to help it direct and control the activities of the other organelles.
Prominent among these are the ribosomes ( RYE -buh- sohms ) or “5-carbon sugar” ( rib ) “bodies” (- somes ). The ribosomes are tiny black bodies containing the 5-carbon sugar, ribose ( RYE -bohs). These black bodies are the main locations for protein synthesis in the bacterial cell.
Like most other cells, the bacterial cell is surrounded by a soft cell membrane (also called plasma membrane ). This cell (plasma) membrane encloses the cytoplasm and most other organelles. In addition, however, bacteria have a rigid cell wall, a protective barrier outside the soft cell membrane. Most external of all is the bacterial capsule, a sticky outermost coat that helps glue some types of bacteria firmly to particular surfaces, such as those on human or animal cells.
Two types of projections often extend from the bacterial surface: pili ( PIE -lee) or short, “hair”-like strands, as well as flagella (flah- JELL -ah) or long, “whip”-like strands. The pili help the bacterium attach itself to other objects, while each flagellum (flah- JELL -um) acts like a whip to push the cell through its watery surroundings.
Eukaryote Cells: Plant And Animal Cells
Most plant and animal cells have a much more complex structure than does a bacterium. As eukaryotes, both types of cells have their own well-defined nucleus, surrounded by a nuclear ( NEW -klee-ar) membrane.
Of the two eukaryote types, the plant cell has more in common with the bacterial cell. Note from Figure 5.3, A, for instance, that the plant cell, like the bacterium, is surrounded by a rigid cell wall. Likewise, plant cells have a plasma (cell) membrane immediately deep to the cell wall. This is quite different from the typical animal cell (Figure 5.3, B), which has only a plasma membrane surrounding it.
The plant cell has its green chloroplasts filled with chlorophyll, and a large central vacuole ( VAK -yew-ohl). The central vacuole lies near the center of the cell, and it appears to be “empty” (vacu) or clear when viewed through a light microscope. In reality, however, it is a storage sac for various digestive enzymes. This makes the central vacuole in the plant cell the rough equivalent of the lysosome ( LIE -soh- zohm ) or “breakdown” (lys) “body” (som). Like the central vacuole, the lysosome contains digestive enzymes which, when released, digest or break down foodstuffs and other materials within the cell.
Both types of eukaryote cells have numerous mitochondria. These unique-looking organelles are frequently nicknamed the “powerhouse” of the cell, because they contain the chemicals necessary for aerobic respiration and ATP production. Also present in both cell types is an endoplasmic ( en -doh- PLAZ -mik) reticulum ( reh - TIK -yoo-lum). The endoplasmic reticulum (frequently abbreviated as ER ), is literally a “tiny network present within the cytoplasm.” The ER is a complex network of flattened sacs that serves to carry things around in the cell, much like a miniature circulation or highway system. The rough ER gets its name from the fact that its “rough” surface is studded with many ribosomes. The smooth ER, in marked contrast, does not have any ribosomes attached to its surface. The ribosomes on the rough ER engage in protein synthesis.
The synthesized proteins are often then circulated to the Golgi ( GOAL -jee) body or apparatus. The Golgi body/apparatus is named after its discoverer, Camillo Golgi, an histologist (hiss- TAHL -oh- jist ) or “one who specializes in the study of tissues.” The Golgi body consists of a series of tightly stacked, flattened sacs. It mainly serves to package the proteins, lipids, hormones, and various other products of the cell.
Finally, both plant and animal cells contain a cytoskeleton ( sigh -toh- SKEL -eh-ton). The cytoskeleton is literally the “skeleton” of the “cell,” giving it some rigidity and support. The cytoskeleton consists of both hollow microtubules ( my -kroh- TWO -byools) and solid microfilaments ( my -kroh- FILL -ah- ments ). The microtubules are “tiny tubes or tubules,” while the microfilaments are just “tiny threads.”