Prokaryotic and Eukaryotic Cells

I. INTRODUCTION

B. CELLULAR ORGANIZATION: PROKARYOTIC AND EUKARYOTIC CELLS

The overall purpose of this Learning Object is:
1) to compare prokaryotic cells with eukaryotic cells in terms of size and structure; and
2) to learn which microbes are prokaryotic and which are eukaryotic.

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.

Table 1: Eukaryotic Versus Prokaryotic Cells

1. nuclear body

eukaryotic cell

a. The nuclear body is bounded by a nuclear membrane having pores connecting it with the endoplasmic reticulum (see Fig. 1 and Fig. 2).
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.
d. The nuclear body is called a nucleus (def).

Electron micrograph of a nucleus courtesy of Dennis Kunkel's Microscopy

prokaryotic cell

a. The nuclear body is not bounded by a nuclear membrane (see Fig. 3).
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

eukaryotic cell

a. The nucleus divides by mitosis (def).
b. Haploid (1N) sex cells in diploid (def) or 2N organisms are produced through meiosis (def).

Review of Mitosis from Unit 6

prokaryotic cell

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

eukaryotic cell

a. The cytoplasmic membrane (see Fig. 1 and Fig. 2) is a fluid phospholipid bilayer (see Fig. 4) containing sterols (def).
b. The membrane is capable of endocytosis (def) (phagocytosis and pinocytosis) and exocytosis (def).

prokaryotic cell

a. The cytoplasmic membrane (see Fig. 3); is a fluid phospholipid bilayer (see Fig. 4) usually lacking sterols . Many bacteria do contain sterol-like molecules called hopanoids.
b.The membrane is incapable of endocytosis and exocytosis.

4. cytoplasmic structures

eukaryotic cell

a. The ribosomes (def) are composed of a 60S and a 40S subunit forming an 80S ribosome (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. 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

prokaryotic cell

a. The ribosomes (def) are composed of a 50S and a 30S subunit forming an 70S ribosome (def). See Fig. 5.
b. Internal membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles, and lysosomes are absent (see Fig. 3)
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. They may contains only actin-like proteins that, along with the cell wall, contribute to cell shape.

5. respiratory enzymes and electron transport chains

eukaryotic cell

- The electron transport system is located in the inner membrane of the mitochondria.

prokaryotic cell

- The electron transport system is 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. Eukaryotic cell walls are never composed of peptidoglycan (def) (see Fig. 2).
b. Animal cells and protozoans lack cell walls (see Fig. 1).

prokaryotic cell

a. With few exceptions, members of the domain Bacteria have cell walls composed of peptidoglycan (def) (see Fig. 3).
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 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. 6).

Electron micrograph of cilia showing microtubules courtesy of Dennis Kunkel's Microscopy

prokaryotic cell

- Many prokaryotes have flagella, each composed of a single, rotating fibril and usually not surrounded by a membrane (see Fig. 7). There are no cilia.

8. representative organisms

eukaryotic cell

- The domain Eukarya: animals, plants, algae, protozoans, and fungi.

prokaryotic cell

- The domain Bacteria and the domain Archae.

Since viruses are acellular and possess both living and nonliving characteristics, they are considered neither prokaryotic nor eukaryotic. Viruses will be discussed in Unit 3.

For further information on prokaryotic and eukaryotic cellular structure see the online Microbiology Web Textbook at the University of Wisconsin-Madison.

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