PART 1 Introduction
2.1. ARE ALL CELLS THE SAME?
2.1.5. Eucaryotes
The archaebacteria usually live in extreme environments and possess unusual me- tabolism. Methanogens, which are methane-producing bacteria, belong to this group, as well as the thermoacidophiles. The thermoacidophiles can grow at high temperatures and low pH values. The halobacteria, which can live only in very strong salt solutions, are members of this group. These organisms are important sources for catalytically active proteins (enzymes) with novel properties.
The nucleus of eucaryotic cells contains chromosomes as nuclear material (DNA molecules with some closely associated small proteins), surrounded by a membrane. The nuclear membrane consists of a pair of concentric and porous membranes. The nucleolus is an area in the nucleus that stains differently and is the site of ribosome synthesis. How- ever, many chromosomes contain small amounts of RNA and basic proteins called his- tonesattached to the DNA. Each chromosome contains a single linear DNA molecule on which the histones are attached.
Cell division (asexual) in eucaryotes involves several major steps, such as DNA synthesis, nuclear division, cell division, and cell separation. Sexual reproduction in eu- caryotic cells involves the conjugation of two cells called gametes(egg and sperm cells).
The single cell formed from the conjugation of gametes is called a zygote. The zygote has twice as many chromosomes as does the gamete. Gametes are haploidcells, while zygotes are diploid. For humans, a haploid cell contains 23 chromosomes, and diploid cells have 46. The cell-division cycle (asexual reproduction) in a eucaryotic cell is depicted in Fig. 2.4.
The cell-division cycle is divided into four phases. The M phase consists of mitosis where the nucleous divides, and cytokinesis where the cell splits into separate daughter cells. All of the phases between one M phase and the next are known collectively as the interphase. The interphase is divided into three phases: G1, S, and G2. The cell increases in size during the interphase period. In the S phase the cell replicates its nuclear DNA.
There are key checkpoints in the cycle when the cell machinery must commit to entry to the next phase. Checkpoints exist for entry into the S and M phases and exit from M phase. Cells may also be in a G0state, which is a resting state where there is no growth.
The mitochondriaare the powerhouses of a eucaryotic cell, where respiration and oxidative phosphorylation take place. Mitochondria have a nearly cylindrical shape 1 mm in diameter and 2 to 3 mm in length. The typical structure of a mitochondrion is shown in
20 An Overview of Biological Basics Chap. 2
Figure 2.4. Schematic of cell division cycle in an eucaryote. (See text for details.)
Fig. 2.5. The external membrane is made of a phospholipid bilayer with proteins embed- ded in the lipid matrix. The mitochondria contain a complex system of inner membranes called cristae. A gellike matrix containing large amounts of protein fills the space inside the cristae. Some enzymes of oxidative respiration are bound to the cristae. A mitochon- drion has its own DNA and protein-synthesizing machinery and reproduces indepen- dently.
The endoplasmic reticulum is a complex, convoluted membrane system leading from the cell membrane into the cell. The rough endoplasmic reticulum contains ribo- somes on the inner surfaces and is the site of protein synthesis and modifications of pro- tein structure after synthesis. The smooth endoplasmic reticulum is more involved with lipid synthesis.
Lysosomesare very small membrane-bound particles that contain and release diges- tive enzymes. Lysosomes contribute to the digestion of nutrients and invading substances.
Peroxisomesare similar to lysosomes in their structure, but not in function. Peroxi- somes carry out oxidative reactions that produce hydrogen peroxide.
Glyoxysomes are also very small membrane-bound particles that contain the en- zymes of the glyoxylate cycle.
Golgibodies are very small particles composed of membrane aggregates and are re- sponsible for the secretion of certain proteins. Golgi bodies are sites where proteins are modified by the addition of various sugars in a process called glycosylation. Such modifi- cations are important to protein function in the body.
Vacuoles are membrane-bound organelles of low density and are responsible for food digestion, osmotic regulation, and waste-product storage. Vacuoles may occupy a large fraction of cell volume (up to 90% in plant cells).
Figure 2.5. Diagram of a mitochondrion. Respiratory enzymes that make ATP are lo- cated on the surfaces of the inner membrane and the cristae, which are infoldings of the inner membrane. (With permission, from J. G. Black, Microbiology Principles and Appli- cations, 3d ed., 1996, p. 94. This material is used by permission of John Wiley & Sons, Inc.)
Chloroplastsare relatively large, chlorophyll-containing, green organelles that are responsible for photosynthesis in photosynthetic eucaryotes, such as algae and plant cells.
Every chloroplast contains an outer membrane and a large number of inner membranes called thylakoids. Chlorophyll molecules are associated with thylakoids, which have a regular membrane structure with lipid bilayers. Chloroplasts are autonomous units con- taining their own DNA and protein-synthesizing machinery.
Certain procaryotic and eucaryotic organisms contain flagella—long, filamentous structures that are attached to one end of the cell and are responsible for the motion of the cell. Eucaryotic flagella contain two central fibers surrounded by eighteen peripheral fibers, which exist in doublets. Fibers are in a tube structure called a microtubuleand are composed of proteins called tubulin. The whole fiber assembly is embedded in an organic matrix and is surrounded by a membrane.
The cytoskeleton (in eucaryotic cells) refers to filaments that provide an internal framework to organize the cell’s internal activities and control its shape. These filaments are critical in cell movement, transduction of mechanical forces into biological responses, and separation of chromosomes into the two daughter cells during cell division. Three types of fibers are present: actin filaments, intermediate filaments, and microtubules.
Ciliaare flagellalike structures, but are numerous and shorter. Only one group of protozoa, called ciliates,contains cilia. Paramecium species contain nearly 104cilia per cell. Ciliated organisms move much faster than flagellated ones.
This completes our summary of eucaryotic cell structure. Now let us turn our atten- tion to the microscopic eucaryotes.
Fungiare heterotrophs that are widespread in nature. Fungal cells are larger than bacterial cells, and their typical internal structures, such as nucleus and vacuoles, can be seen easily with a light microscope. Two major groups of fungi are yeasts and molds.
Yeastsare single small cells of 5- to 10-mm size. Yeast cells are usually spherical, cylindrical, or oval. Yeasts can reproduce by asexual or sexual means. Asexual reproduc- tion is by either budding or fission. In budding, a small bud cell forms on the cell, which gradually enlarges and separates from the mother cell. Asexual reproduction by fissionis similar to that of bacteria. Only a few species of yeast can reproduce by fission. In fission, cells grow to a certain size and divide into two equal cells. Sexual reproduction of yeasts involves the formation of a zygote(a diploid cell) from fusion of two haploid cells, each having a single set of chromosomes. The nucleus of the diploid cells divides several times to form ascospores. Each ascospore eventually becomes a new haploid cell and may re- produce by budding and fission. The life cycle of a typical yeast cell is presented in Fig. 2.6.
The classification of yeasts is based on reproductive modes (e.g., buddingor fission) and the nutritional requirements of cells. The most widely used yeast, Saccharomyces cerevisiae, is used in alcohol formation under anaerobic conditions (e.g., in wine, beer and whiskey making) and also for baker’s yeast production under aerobic conditions.
Moldsare filamentous fungi and have a mycelial structure. The myceliumis a highly branched system of tubes that contains mobil cytoplasm with many nuclei. Long, thin fila- ments on the mycelium are called hyphae. Certain branches of mycelium may grow in the air, and asexual spores called conidia are formed on these aerial branches. Conidia are nearly spherical in structure and are often pigmented. Some molds reproduce by sexual means and form sexual spores. These spores provide resistance against heat, freezing,
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drying, and some chemical agents. Both sexual and asexual spores of molds can germi- nate and form new hyphae. Figure 2.7 describes the structure and asexual reproduction of molds.
Molds usually form long, highly branched cells and easily grow on moist, solid nu- trient surfaces. The typical size of a filamentous form of mold is 5 to 20 mm. When grown in submerged culture, molds often form cell aggregates and pellets. The typical size of a mold pellet varies between 50 mm and 1 mm, depending on the type of mold and growth conditions. Pellet formation can cause some nutrient-transfer (mainly oxygen) problems inside the pellet. However, pellet formation reduces broth viscosity, which can improve bulk oxygen transfer.
On the basis of their mode of sexual reproduction, fungi are grouped in four classes.
1. The phycomycetes are algalike fungi; however, they do not possess chlorophyll and cannot photosynthesize. Aquatic and terrestrial molds belong to this category.
2. The ascomycetes form sexual spores called ascospores, which are contained within a sac (a capsule structure). Some molds of the genera Neurosporaand Aspergillus and yeasts belong to this category.
3. The basidiomycetes reproduce by basidiospores, which are extended from the stalks of specialized cells called the basidia. Mushrooms are basidiomycetes.
Figure 2.6. Cell-division cycle of a typical yeast, Saccharomyces cerevisiae. (With per- mission, from T. D. Brock, D. W. Smith, and M. T. Madigan, Biology of Microorganisms, 4th ed., Pearson Education, Upper Saddle River, NJ, 1984, p. 80.)
4. The deuteromycetes (Fungi imperfecti) cannot reproduce by sexual means. Only asexually reproducing molds belong to this category. Some pathogenic fungi, such as Trichophyton, which causes athlete’s foot, belong to the deuteromycetes.
Molds are used for the production of citric acid (Aspergillus niger) and many antibiotics, such as penicillin (Penicillium chrysogenum). Mold fermentations make up a large frac- tion of the fermentation industry.
Algae are usually unicellular organisms. However, some plantlike multicellular structures are present in marine waters. All algae are photosynthetic and contain chloro- plasts, which normally impart a green color to the organisms. The chloroplasts are the sites of chlorophyll pigments and are responsible for photosynthesis. The size of a typical unicellular alga is 10 to 30 mm. Multicellular algae sometimes form a branched or un- branched filamentous structure. Some algae contain silica or calcium carbonate in their cell wall. Diatoms containing silica in their cell wall are used as filter aids in industry.
Some algae, such as Chlorella, Scenedesmus, Spirullina, and Dunaliella, are used for waste-water treatment with simultaneous single-cell protein production. Certain gelling agents such as agar and alginic acid are obtained from marine algae and seaweeds. Some algae are brown or red because of the presence of other pigments.
Protozoaare unicellular, motile, relatively large (1 mm to 50 mm) eucaryotic cells that lack cell walls. Protozoa usually obtain food by ingesting other small organisms, such as bacteria, or other food particles. Protozoa are usually uninucleate and reproduce by sexual or asexual means. They are classified on the basis of their motion. The amoebae move by ameboid motion, whereby the cytoplasm of the cell flows forward to form a pseudopodium (false foot), and the rest of the cell flows toward this lobe. The flagellates
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Figure 2.7. Structure and asexual reproduction of molds. (With permission, from T. D.
Brock, K. M. Brock, and D. M. Ward, Basic Microbiology with Applications, 3d ed., Pear- son Education, Upper Saddle River, NJ, 1986, p. 35.)
move using their flagella. Trypanosomesmove by flagella and cause a number of diseases in humans. The ciliatesmove by motion of a large number of small appendages on the cell surface called cilia. The sporozoans are nonmotile and contain members that are human and animal parasites. These protozoa do not engulf food particles, but absorb dis- solved food components through their membranes. Protozoa cause some diseases, such as malaria and dysentery. Protozoa may have a beneficial role in removing bacteria from waste water in biological waste-water treatment processes and helping to obtain clean ef- fluents. Microscopic pictures of some protozoa are presented in Fig. 2.8.
2.2. CELL CONSTRUCTION