B. CELLULAR ORGANIZATION: PROKARYOTIC AND EUKARYOTIC CELLS
Fundamental Statements for this Learning Object:
1. There are two basic types of cells in nature: prokaryotic and eukaryotic.
2. Prokaryotic cells are structurally simpler than eukaryotic cells.
3. The smaller a cell, the greater its surface to volume ratio.
4. The smaller the surface to volume ratio, the more structurally complex (compartmentalized) a cell needs to be in order to carry out life functions.
5. There are fundamental differences between prokaryotic and eukaryotic cells.
6. Bacteria are prokaryotic cells; fungi, protozoa, algae, plants, and animals are composed of eukaryotic cells.
7. Viruses are not cells so they are neither prokaryotic nor eukaryotic. They can replicate only inside a living cell.
LEARNING OBJECTIVES FOR THIS SECTION
According to the cell theory, the cell is the basic unit of life. All living organisms are composed of one or more cells. Based on the organization of their cellular structures, all living cells can be divided into two groups: prokaryotic and eukaryotic (also spelled procaryotic and eucaryotic). Animals, plants, fungi, protozoans, and algae all possess eukaryotic cell types. Only bacteria have prokaryotic cell types.
Prokaryotic cells are generally much smaller and more simple than eukaryotic (see Fig. 1). Prokaryotic cells are, in fact, able to be structurally more simple because of their small size. The smaller a cell, the greater is its surface-to-volume ratio (the surface area of a cell compared to its volume).
The surface area of a spherical object can be calculated using the following formula:
S = 4 π r 2
The volume of a spherical object can be calculated using the formula:
V = 4/3 π r 3
For example, a spherical cell 1 micrometer (µm) in diameter - the average size of a coccus-shaped bacterium - has a surface-to-volume ratio of approximately 6:1, while a spherical cell having a diameter of 20 µm has a surface-to-volume ratio of approximately 0.3:1.
A large surface-to-volume ratio, as seen in smaller prokaryotic cells, means that nutrients can easily and rapidly reach any part of the cells interior. However, in the larger eukaryotic cell, the limited surface area when compared to its volume means nutrients cannot rapidly diffuse to all interior parts of the cell. That is why eukaryotic cells require a variety of specialized internal organelles to carry out metabolism, provide energy, and transport chemicals throughout the cell. Both, however, must carry out the same life processes. Some features distinguishing prokaryotic and eukaryotic cells are shown in Table 1. All of these features will be discussed in detail later in Unit 1.
1. nuclear body
a. The nuclear body is bounded by a nuclear membrane having pores connecting it with the endoplasmic reticulum (see Fig. 2 and Fig. 3).
b. It contains one or more paired, linear chromosomes (def) composed of deoxyribonucleic acid (DNA) associated with histone proteins (def)).
c. A nucleolus (def) is present. Ribosomal RNA (rRNA) is transcribed and assembled in the nucleolus.
d. The nuclear body is called a nucleus (def).
a. The nuclear body is not bounded by a nuclear membrane (see Fig. 4).
b. It usually contains one circular chromosome (def) composed of deoxyribonucleic acid (DNA) associated with histone-like proteins.
c. There is no nucleolus.
d. The nuclear body is called a nucleoid (def).
2. cell division
a. The nucleus divides by mitosis (def).
b. Haploid (1N) sex cells in diploid (def) or 2N organisms are produced through meiosis (def).
a. The cell usually divides by binary fission (def). There is no mitosis.
b. Prokaryotic cells are haploid (def). Meiosis is not needed.
3. cytoplasmic membrane - also known as a cell membrane or plasma membrane
a. The cytoplasmic membrane (see Fig. 2 and Fig. 3) is a fluid phospholipid bilayer (see Fig. 5) containing sterols (see Fig. 6) (def).
b. The membrane is capable of endocytosis (def) (phagocytosis and pinocytosis) and exocytosis (def).
a. The cytoplasmic membrane (see Fig. 4); is a fluid phospholipid bilayer (see Fig. 5) usually lacking sterols . Bacteria generally contain sterol-like molecules called hopanoids (see Fig. 7).
b.The membrane is incapable of endocytosis and exocytosis.
4. cytoplasmic structures
a. The ribosomes (def) are composed of a 60S and a 40S subunit that come together during protein synthesis to form an 80S ribosome (def).
- Ribosomal subunit densities: 60S and 40S
b. Internal membrane-bound organelles such as mitochondria (def), endoplasmic reticulum (def), Golgi apparatus (def) , vacuoles, and lysosomes (def) are present (see Fig. 2 and Fig. 3).
c. Chloroplasts (def) serve as organelles for photosynthesis.
d. A mitotic spindle involved in mitosis is present during cell division.
e. A cytoskeleton (def) is present. It contains microtubules, actin micofilaments, and intermediate filaments. These collectively play a role in giving shape to cells, allowing for cell movement, movement of organelles within the cell and endocytosis, and cell division.
- Electron micrograph of a cytoplasmic membrane courtesy of Dennis Kunkel's Microscopy
- Electron micrograph of mitochondria courtesy of Dennis Kunkel's Microscopy
- Electron micrograph of rough endoplasmic reticulum courtesy of Dennis Kunkel's Microscopy
- Electron micrograph of a Golgi apparatus courtesy of Dennis Kunkel's Microscopy
a. The ribosomes (def) are composed of a 50S and a 30S subunit that come together during protein synthesis to form a 70S ribosome (def). See Fig. 8.
- Ribosomal subunit densities: 50S and 30Sb. Internal membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles, and lysosomes are absent (see Fig. 4)
b. There are no chloroplasts. Photosynthesis usually takes place in infoldings or extensions derived from the cytoplasmic membrane.
c. There is no mitosis and no mitotic spindle.
d. The various structural filaments in the cytoplasm collectively make up the prokaryotic cytoskeleton. Cytoskeletal filaments play essential roles in determining the shape of a bacterium (coccus, bacillus, or spiral) and are also critical in the process of cell division by binary fission and in determining bacterial polarity.
Prokaryotic cells with internal membrane-bound compartments?
5. respiratory enzymes and electron transport chains
- The electron transport system is located in the inner membrane of the mitochondria. It contributes to the production of ATP molecules via chemiosmosis.
-Electron micrograph of a mitochondrion from the Biology Department at the University of New Mexico.
- The electron transport system is located in the cytoplasmic membrane. It contributes to the production of ATP molecules via chemiosmosis.
6. cell wall
a. Plant cells, algae, and fungi have cell walls, usually composed of cellulose or chitin. Eukaryotic cell walls are never composed of peptidoglycan (def) (see Fig. 3).
b. Animal cells and protozoans lack cell walls (see Fig. 2).
a. With few exceptions, members of the domain Bacteria have cell walls composed of peptidoglycan (def) (see Fig. 4).
b. Members of the domain Archae have cell walls composed of protein, a complex carbohydrate, or unique molecules resembling but not the same as peptidoglycan.
7. locomotor organelles
eukaryotic cellprokaryotic cell
- Eukaryotic cells may have flagella or cilia. Flagella and cilia are organelles involved in locomotion and in eukaryotic cells consist of a distinct arrangement of sliding microtubules surrounded by a membrane. The microtubule arrangement is referred to as a 2X9+2 arrangement (see Fig. 9).
- Many prokaryotes have flagella, each composed of a single, rotating fibril and usually not surrounded by a membrane (see Fig. 10). There are no cilia.
- Movie of motile Rhodobacter spheroides with fluorescent labelled-flagella. Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
8. representative organisms
- The domain Eukarya: animals, plants, algae, protozoans, and fungi (yeasts, molds, mushrooms).
- The domain Bacteria and the domain Archae.
Since viruses are acellular- they contain no cellular organelles, cannot grow and divide, and carry out no independent metabolism - they are considered neither prokaryotic nor eukaryotic. Because viruses are not cells and have no cellular organelles, they can only replicate and assemble inside a living host cell. They turn the host cell into a factory for manufacturing viral parts and viral enzymes and assembling the viral components.
Viruses, which possess both living and nonliving characteristics, will be discussed in Unit 4. Recently, viruses have been declared as living entities based on the large number of protein folds encoded by viral genomes that are shared with the genomes of cells. This indicates that viruses likely arose from multiple ancient cells.
ASSIGNMENT FOR UNIT-1, LECTURE -2
We will be doing a classroom group activity on this section so it is critical that you come prepared. I will be assuming that you have done this preparatory assignment.
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