PROKARYOTIC AND EUKARYOTIC CELLS

PROKARYOTIC CELLS

All members from Archaea and Bacteria are prokaryotes. They are amongst smallest organisms having diameters in range from 0.5 microns to 2.0 microns. Their surface to volume ratio is 12:4. They are present in various shapes like spherical in case of coccus, rodlike in case of bacillus, comma shape in case of vibrio. Other shapes can be triangular, wavy shaped, cork-screw shaped, etc. Prokaryotic cells can be found in different arrangements. They can be present in pairs like as in Enterococcus spp.; in chains like as in Streptococcus; or in tetrads. They can also be present in grape like clusters (staphylo-).

Prokaryotic cells divide through binary fission.

The structure of prokaryotic cell can be understood by studying structure of bacteria [click here].

EUKARYOTIC CELLS

Eukaryotic cells are larger than prokaryotic cell, former having the diameter of more than 10 microns. All the members of kingdom Protista, Plantae, Fungi and Animalia bear Eukaryotic cells. Structure of eukaryotic cell is discussed in detail here:

Structure of Eukaryotic cell

the above image is taken only for educational purpose from Microbiology Principles and Explorations by J.G. Black

• Cell wall: Algal cell wall consist mainly of cellulose but may also contain polysaccharides. Fungal cell wall is made up of cellulose, chitin or both. Cell walls provides rigidity to the cell and prevents the cells from bursting when water from outer environment moves inside of the cell.

• Plasma membrane: In addition to fluid mosaic structure of membrane, some membrane bound organelles are also present in the eukaryotic cells. Membrane also contains lipids such as sterols. These sterols help in withstanding the greater stress due to lower surface to volume ratio. The surface to volume ratio is 0.3:1. Respiratory enzymes are absent in the membrane.

• Cytoplasm: It is mainly made up of water with cell organelles dissolved in it. Comparatively, percentage of cytoplasm in eukaryotic cells is lower than that of prokaryotic cells, as former has nucleus inside the cell which covers greater portion of the cell. It is semifluid in nature having fibrous supports known as cytoskeleton which provide shape and support to the cell.
• Nucleus: Presence of cell nucleus is basic difference between eukaryotic cell and prokaryotic cell. Nucleus is present in double membrane layered nuclear envelope. These membranes have nuclear pore which helps in the RNA molecules to move out into cytoplasm. The semifluid portion is called as nucleoplasm. Nucleolus is the site in nucleus where large amount of RNA is present and where assembly of ribosomes takes place. In the nucleus, chromosomes and histone proteins are present which helps in transferring the genetic materials from parent cells to daughter cells at the time of cell division.
• Mitochondria: It is the powerhouse of eukaryotic cells and may cover upto 20% of the total cell volume. It has double layered membrane. The inner membrane has folds known as cristae which extends into the matrix. Mitochondria have enzymes which can capture energy in form of ATP. ATPs are then can be used by cells for their activities. It can replicate independently.

Structure of mitochondria

The above image is taken  for educational purpose , from Microbiology Principles and Explorations by J.G. Black

• Chloroplast: Algae and green plants have chloroplasts which makes them capable for doing photosynthesis reactions. They also have double layered membrane. Chloroplast is filled with stroma which is having thylakoids which contain chlorophyll in it. Chlorophyll can capture the sunlight and generation of energy. It can replicate independently.
• Ribosomes: Eukaryotic cells have 80S ribosomes. The sedimentation rates of the subunits are 60S and 40S. Ribosome got assemble in the nucleolus. These are present in chains, forming polyribosomes which are involved in protein synthesis. Protein made by the ribosomes which are attached with the endoplasmic reticulum is secreted out from the cell, while those which are not associated with ER generate proteins which are used for the cell.
• Endoplasmic reticulum: ER are the extension in cytoplasm forming tubular and plates like organelles in cytoplasm. When ribosomes are attached on them, they appeared to have rough texture and thus, referred to as rough ER. Rough ER participates in protein synthesis. Smooth ER participates in lipid synthesis which can be used for formation of membranes.
• Golgi apparatus: These are flattened membraneous sacs which are involved in packaging and secretion of the substances. They help in formation of membranes of lysosomes and plasma membrane.
• Lysosomes: They are membrane bound organelles containing digestive enzymes. They help in digesting the substances in the vacuole.
• Peroxisomes: They are membrane bound organelles, containing enzymes. In the plant cells, these oxidizes the fat while in animal cells, these oxidizes amino acids.
• Vacuoles: It is membrane bound structure which stores energy containing materials like fat, glycogen and starch. In plant cells, rigidity to the cell is provided by the water containing vacuoles.
• Cytoskeleton: It is a network made up of protein fibers which forms microtubules and microfilaments. Its function is to provide rigidity and shape to the cell. They also help in amoeboid movement of cells.
• Flagella: It has 9+2 arrangement in cell. The fiber is made up of tubulin protein. Their movement is like a whip.

• Cilia: They are shorter but numerous than flagella. Coordinated beating of large number of cilia helps in the organism to move. Cilia are also present in human respiratory tract.
• Pseudopodia: They are temporary projections of cytoplasm which helps some cells in amoeboid movements.

MCQs

1. Surface to volume ratio of prokayotic organisms is?

a. 3:1

b. 1:4

c. 0.3:1

d. 1:2

2. The function of cell wall in eukaryotic cells is?

a. To control the entry of toxic material

b. Provide rigidity to cell

c. Synthesis of RNA

d. ATP synthesis

3. Basic difference between prokaryotic cells and eukaryotic cell is?

a. Prokaryotic cells have clear nucleus

b. Eukaryotic cells have distinct nucleus

c. Cytoplasm is absent in prokaryotes

d. Presence of mitochondria in prokaryotes

4. Function of plasma membrane?

a. Provides selective permeability for certain substances

b. Provides rigidity to the cell

c. Contains genetic material  of the cell

d. All of the above

5. Surface volume ratio of eukaryotic cells is?

a. 1: 0.3

b. 5:1

c. 0.3:1

d. 1:1

6. Which of the following statements are correct?

I. Single circular chromosome is present in prokaryotic cell

II. Nucleolus is present in Prokaryotic cell

III. Nucleus is present in eukaryotic cell

IV. Eukaryotic cell has 60S ribosomes

a. I, II

b. I, IV

c. I, III

d. III, IV

7. Correct sequence of true/false for the given statements can be?

• Cell wall of prokaryotic cells is made of cellulose, chitin and hemicellulose.
• Spindle fibers are present in prokaryotic cells.
• Prokaryotic cells have 80S ribosomes
• Flagella have 9+2 arrangement in eukaryotic cells.

a. TTFT

b. FTTT

c. TTTT

d. FFFF

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ANSWERS

1-a

2-b

3-b

4-a

5-a

6-c

7-d

REFERENCES