I. INTRODUCTION

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

eukaryotic cell

a. The nuclear body is bounded by a nuclear membrane having pores connecting it with the endoplasmic reticulum (see Fig. 2, Fig. 3, and Fig. 4).
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).

prokaryotic cell

a. The nuclear body is not bounded by a nuclear membrane (see Fig. 5 and Fig. 6).
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).

For More Information: Review of Mitosis from Unit 7

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. 2 and Fig. 3), is a fluid phospholipid bilayer (see Fig. 7) containing sterols (see Fig. 8) (def).
b. The membrane is capable of endocytosis (def) (phagocytosis and pinocytosis) and exocytosis (def).

prokaryotic cell

a. The cytoplasmic membrane (see Fig. 5 and Fig. 6); is a fluid phospholipid bilayer (see Fig. 7) usually lacking sterols . Bacteria generally contain sterol-like molecules called hopanoids (see Fig. 9).
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. A svedberg unit (S) is a non-metric unit for sedimentation rate and is actually a measure of time defined as 10^-13 seconds. In biology, the sedimentation rate, or sedimentation coefficient, refers to the rate at which a molecule or particle travels to the bottom of a test tube under the centrifugal force of an ultra-high speed centrifuge. The S value of a molecule or particle is determined by its mass, density, and shape. See the following link:

- Ribosomal subunit sedimentation rates: 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 (see Fig. 2A.).
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
• Electron micrograph of mitochondrion and a mitochondrion undergoing fission.
• Electron micrograph of rough endoplasmic reticulum and smooth endoplasmic reculum
• Electron micrograph of a Golgi apparatus
• Electron micrograph of a Chloroplast

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. See Fig. 10. A svedberg unit (S) is a non-metric unit for sedimentation rate and is actually a measure of time defined as 10^-13 seconds. In biology, the sedimentation rate, or sedimentation coefficient, refers to the rate at which a molecule or particle travels to the bottom of a test tube under the centrifugal force of an ultra-high speed centrifuge. The S value of a molecule or particle is determined by its mass, density, and shape. See the following link:

- Ribosomal subunit sedimentation rates: 50S and 30S

b. Internal membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles, and lysosomes are absent (see Fig. 5 and Fig. 6).
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?