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

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. 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

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. Ribosomal RNA (rRNA) is transcribed and assembled in the nucleolus.
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 . Bacteria generally 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 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. 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 that come together during protein synthesis to form a 70S ribosome (def). See Fig. 5.

- ribosomal subunit densities: 50S and 30S

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 contain actin-like proteins that, along with the cell wall, contribute to cell shape.

Prokaryotic cells with internal membrane-bound compartments?