CONCEPTS IN BIOLOGY

PART II. CORNERSTONES: CHEMISTRY, CELLS, AND METABOLISM

4. Cell Structure and Function

4.8. Prokaryotic and Eukaryotic Cells Revisited

Now that you have an idea of how cells are constructed, we can look at the great diversity of the kinds of cells that exist. You already know that there are significant differences between prokaryotic and eukaryotic cells.

Because prokaryotic (noneukaryotic) and eukaryotic cells are so different and prokaryotic cells show up in the fossil records much earlier, the differences between the two kinds of cells are used to classify organisms. Thus, biologists have classified organisms into three large categories, called domains. The following diagram illustrates how living things are classified:

The Domain Bacteria contains most of the microorganisms and can be found in a wide variety of environments. The Domain Archaea contains many kinds of microorganisms that have significant biochemical differences from the Bacteria. Many of the Archaea have special metabolic abilities and live in extreme environments of high temperature or extreme saltiness. Although only a few thousand Bacteria and only about 200 Archaea have been described, recent DNA studies of seawater and soil suggest that there are millions of undescribed species. In all likelihood, these noneukaryotic organisms far outnumber all the species of eukaryotic organisms combined. All other living things are comprised of eukaryotic cells.

Prokaryotic Cell Structure

Prokaryotic cells, the Bacteria and Archaea, do not have a typical nucleus bound by a nuclear membrane, nor do they contain mitochondria, chloroplasts, Golgi, or extensive networks of endoplasmic reticula. However, prokaryotic cells contain DNA and enzymes and are able to reproduce and engage in metabolism. They perform all of the basic functions of living things with fewer and simpler organelles. Although some Eubacteria have a type of green photosynthetic pigment and carry on photosynthesis, they do so without chloroplasts and use somewhat different chemical reactions.

Most Bacteria are surrounded by a capsule, or slime layer, which is composed of a variety of compounds. In certain bacteria, this layer is responsible for their ability to stick to surfaces, forming biofilms (e.g., the film of bacteria on teeth), and to resist phagocytosis. Many bacteria also have fimbriae, hairlike protein structures, which help the cell stick to objects. Those with flagella are capable of propelling themselves through the environment. Below the capsule is the rigid cell wall, comprised of a unique protein/carbohydrate complex called peptidoglycan. This gives the cell the strength to resist osmotic pressure changes and gives it shape. Just beneath the wall is the plasma membrane. Thinner and with a slightly different chemical composition from that of eukaryotes, the plasma membrane carries out the same functions as the plasma membrane in eukaryotes. Most bacteria are either rod-shaped (bacilli), spherical (cocci), corkscrew-shaped (spirilla), or comma-shaped (vibrio). The genetic material within the cytoplasm is DNA in the form of a loop.

The Archaea share many characteristics with the Bacteria. Many have a rod or spherical shape, although some are square or triangular. Some have flagella and have cell walls, but the cell walls are made of a different material than that of bacteria.

One significant difference between the cells of Bacteria and Archaea is in the chemical makeup of their ribosomes. The ribosomes of Bacteria contain different proteins from those found in the cells of Eucarya or Archaea. Bacterial ribosomes are also smaller. This discovery was important to medicine, because many cellular forms of life that cause common diseases are bacterial. As soon as differences in the ribosomes were noted, researchers began to look for ways in which to interfere with the bacterial ribosome’s function, but not interfere with the ribosomes of eukaryotic cells. Antibiotics, such as streptomycin, are the result of this research. This drug combines with bacterial ribosomes and causes bacteria to die because it prevents production of the proteins essential to survival of bacteria. Because eukaryotic ribosomes differ from bacterial ribosomes, streptomycin does not interfere with the normal function of the ribosomes in human cells.

Eukaryotic Cell Structure

Eukaryotic cells contain a true nucleus and most of the membranous organelles described earlier. Eukaryotic organisms can be further divided into several categories, based on the specific combination of organelles they contain. The cells of plants, fungi, protozoa and algae, and animals are all eukaryotic. The most obvious characteristic that sets plants and algae apart from other organisms is their green color, which indicates that the cells contain chlorophyll in chloroplasts. Chlorophyll is necessary for photosynthesis—the conversion of light energy into chemical-bond energy in food molecules. Another distinguishing characteristic of plant and algal cells is that their cell walls are made of cellulose (table 4.2).

The fungi are a distinct group of organisms that lack chloroplasts but have a cell wall. However, the cell wall is made from a polysaccharide, called chitin, rather than cellulose. Organisms that belong in this category of eukaryotic cells include yeasts, molds, mushrooms, and the fungi that cause such human diseases as athlete’s foot, jungle rot, and ringworm.

Eukaryotic organisms that lack cell walls and chloroplasts are placed in separate groups. Organisms that consist of only one cell are called protozoans—examples are Amoeba and Paramecium. They have all the cellular organelles described in this chapter except the chloroplast; therefore, protozoans must consume food as do fungi and multicellular animals.

TABLE 4.2. Comparison of Various Kinds of Cells

Note: Viruses are not included in this classification system, because viruses are not composed of the basic cellular structural components. They are composed of a core of nucleic acid (DNA or RNA, never both) and a surrounding coat, or capsid, composed of protein. For this reason, viruses are called acellular or noncellular.

The Cell—The Basic Unit of Life

Although the differences in these groups of organisms may seem to set them worlds apart, their similarity in cellular structure is one of the central themes unifying the field of biology. One can obtain a better understanding of how cells operate in general by studying specific examples. Because the organelles have the same general structure and function, regardless of the kind of cell in which they are found, we can learn more about how mitochondria function in plants by studying how mitochondria function in animals. There is a commonality among all living things with regard to their cellular structure and function. The fact that all eukaryotic organisms have the same cellular structures is strong evidence that they all evolved from a common ancestor.

4.8. CONCEPT REVIEW

17. List five differences in structure between prokaryotic and eukaryotic cells.

18. What two types of organisms have prokaryotic cell structure?

Summary

The concept of the cell has developed over a number of years. Initially, only two regions, the cytoplasm and the nucleus, could be identified. At present, numerous organelles are recognized as essential components of both noneukaryotic and eukaryotic cell types. The structure and function of some of these organelles are compared in table 4.3. This table also indicates whether the organelle is unique to noneukaryotic or eukaryotic cells or is found in both.

The cell is the common unit of life. Individual cells and their structures are studied to discover how they function as individual living organisms and as parts of many-celled beings. Knowing how prokaryotic and eukaryotic organisms resemble each other and differ from each other helps physicians control some organisms dangerous to humans.

There are several ways in which materials enter or leave cells. These include diffusion and osmosis, which involve the net movement of molecules from an area of high to low concentration. In addition, there are several processes that involve activities on the part of the cell to move things across the membrane. These include facilitated diffusion, which uses carrier molecules to diffuse across the membrane; active transport, which uses energy from the cell to move materials from low to high concentration; and endocytosis and exocytosis, in which membrane-enclosed packets are formed.

TABLE 4.3. Summary of the Structure and Function of the Cellular Organelles

Basic Review

1. The first structure to be distinguished within a cell was the _____.

2. Membranous structures in cells are composed of

a. phosopholipid.

b. cellulose.

c. ribosomes.

d. chromatin.

3. The Golgi apparatus produces

a. ribosomes.

b. DNA.

c. lysosomes.

d. endoplasmic reticulum.

4. If a cell has chloroplasts, it is able to carry on photosynthesis. (T/F)

5. The nucleolus is

a. where the DNA of the cell is located.

b. found only in prokaryotic cells.

c. found in the cytoplasm.

d. where ribosomes are made and stored.

6. Diffusion occurs

a. if molecules are evenly distributed.

b. because of molecular motion.

c. only in cells.

d. when cells need it.

7. Prokaryotic cells are larger than eukaryotic cells. (T/F)

8. Osmosis involves the diffusion of _____ through a selectively permeable membrane.

9. The structure of the plasma membrane contains proteins. (T/F)

10. Which one of the following have cell walls made of cellulose?

a. animals

b. protozoa

c. fungi

d. plants

11. The _____ _____ manufactures some polysaccharides and lipids and packages molecules within sacs.

12. Which is an example of a noneukaryotic cell?

a. muscle cell

b. bacterium

c. fungal cell

d. virus

13. The internal structural framework or cytoskeleton of a cell is composed of which combination of elements?

a. microtubules, microfilaments, and intermediate filaments

b. centrioles, actin, and intermediate filaments

c. ER, nuclear membrane, and Golgi

d. thylakoid, cristae, and centrioles

14. When a cell is placed in a _____ solution, it loses water and it shrivels.

15. These cell components are involved in the destruction of microbes.

a. carrier proteins

b. phagocytic vacuoles

c. centrioles

d. eucarya

Answers

1. nucleus 2. a 3. c 4. T 5. d 6. b  7. F 8. water 9. T 10. d 11. Golgi apparatus 12. b 13. a 14. hypertonic 15. B

Thinking Critically

Athletes and Osmosis

We all know that as we exercise, we sweat and, as a result, lose water and salt. These materials must be replaced. Athletes who participate in extremely long events of several hours have a special concern. They need to replace the water on a regular basis during the event. If they drink large quantities of water at one time at the end of the event, they may dilute their blood to the point that they develop hyponatremia. This condition can result in swelling of the cells of the brain and lead to mental confusion and, in extreme cases, collapse and death.

1. What is hyponatremia?

2. What is a “sports drink”?

3. How is one of these drinks supposed to help an athlete?

4. What is the point of the drinks various colors and flavors?

5. How could the kinds of liquids you drink affect your cell’s osmotic balance?

6. Why can drinking electrolyte-free water at the end of an endurance athletic event cause the brain to swell?

7. Should sports drinks be available to children in school cafeterias?