I. INTRODUCTION
B. CELLULAR ORGANIZATION: PROKARYOTIC AND EUKARYOTIC CELLS
LEARNING OBJECTIVES FOR THIS SECTION
The cell is the basic unit of life. 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. 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). For example, a spherical cell 2 micrometers (µm) in diameter has a surface-to-volume ratio of approximately 3:1, while a spherical cell having a diameter of 20 µm has a surface-to-volume ratio of around 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
eukaryotic cell
a. Bounded by a nuclear membrane having pores connecting it with the endoplasmic reticulum (see Fig. 1 and Fig. 2).
b. Contains one or more paired, linear chromosomes (def) composed of deoxyribonucleic acid (DNA) associated with histone proteins (def)).
c. Nucleolus (def) present.
d. Nuclear body is called a nucleus (def).
- UC Davis Medical School
- Judith Simmon's Tour of the Cell
- Dr. Gwen Child's Cell Biology Page
- Electron micrograph of a nucleus; courtesy of Dennis Kunkel's Microscopy
prokaryotic cell
a. Not bounded by a nuclear membrane (see Fig. 3).
b. Usually contains one circular chromosome (def) composed of deoxyribonucleic acid (DNA) associated with histone-like proteins.
c. No nucleolus.
d. Nuclear body is called a nucleoid (def).
2. cell division
eukaryotic cell
a. By mitosis (def).
b. Sex cells in diploid (def) organisms are produced through meiosis (def).
prokaryotic cell
a. Usually by binary fission (def). No mitosis.
b. Organisms are haploid (def). No meiosis needed.
3. cytoplasmic membrane (cell membrane, plasma membrane)
eukaryotic cell
a. Cytoplasmic membrane (see Fig. 1 and Fig. 2) is a fluid phospholipid bilayer (see Fig. 4) containing sterols (def) as well as carbohydrates.
b. Capable of endocytosis (def) (phagocytosis and pinocytosis) and exocytosis (def).
- UC Davis Medical School
- Judith Simmon's Tour of the Cell
- Dr. Gwen Child's Cell Biology Page
prokaryotic cell
a. Cytoplasmic membrane (see Fig. 3); is a fluid phospholipid bilayer (see Fig. 4) without carbohydrates and usually lacking sterols . Many bacteria do contain sterol-like molecules called hopanoids.
b. Incapable of endocytosis and exocytosis.
4. cytoplasmic structures
eukaryotic cell
a. Ribosomes (def) composed of a 60S and a 40S subunit (def).
b. Internal membrane-bound organelles such as mitochondria (def), endoplasmic reticulum (def), Golgi apparatus (def) , vacuoles, and lysosomes (def) are present (see Fig. 1 and Fig. 2).
c. Chloroplasts (def) serve as organelles for photosynthesis.
d. Mitotic spindle involved in mitosis is present during cell division.
e. Cytoskeleton (def) present.
- UC Davis Medical School
- Judith Simmon's Tour of the Cell
- Dr. Gwen Child's Cell Biology Page
- 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
- Electron micrograph of chloroplast; University of British Columbiaprokaryotic cell
a. 70S ribosomes composed of a 50S and a 30S subunit (see Fig. 5).
b. Internal membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles, and lysosomes are absent (see Fig. 3)
b. No chloroplasts. Photosynthesis usually takes place in infoldings or extensions derived from the cytoplasmic membrane.
c. No mitotic spindle.
d. No cytoskeleton.
5. respiratory enzymes and electron transport chains
eukaryotic cell
- Located in the mitochondria.
prokaryotic cell
- Located in the cytoplasmic membrane.
6. cell wall
eukaryotic cell
a. Plant cells, algae, and fungi have cell walls, usually composed of cellulose or chitin but never containing peptidoglycan (def) (see Fig. 2).
b. Animal cells and protozoans lack cell walls (see Fig. 1).
prokaryotic cell
a. Most Eubacteria have cell walls composed of peptidoglycan (def) (see Fig. 3).
b. The Archaebacteria have cell walls composed of protein, a complex carbohydrate, or unique molecules resembling but not the same as peptidoglycan.
7. locomotor organelles
eukaryotic cell
- 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. 6).
- UC Davis Medical School
- Judith Simmon's Tour of the Cell
- Dr. Gwen Child's Cell Biology Page
- Electron micrograph of cilia showing microtubules; courtesy of Dennis Kunkel's Microscopy
prokaryotic cell
- Some have flagella, each composed of a single, rotating fibril and not surrounded by a membrane (see Fig. 7). No cilia.
8. representative organisms
eukaryotic cell
- animals, plants, algae, protozoans, and fungi
prokaryotic cell
- bacteria (Eubacteria and Archaebacteria)
Since viruses are acellular and possess both living and nonliving characteristics, they are considered neither prokaryotic nor eukaryotic. Viruses will be discussed in Unit 2.
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about the differences between prokaryotic and eukaryotic cells, see the
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Updated: Feb, 2011
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