Prokaryotic Cell: Introduction, its Features and Size and Shape

You all know that air contains lots of germs. Can you recollect a few examples?

Yes, bacteria. Have you ever wondered why we are not able to see bacteria? Why are they small and we are large? Who came first on Earth? Animals, plants or bacteria.

Do these questions also come to your mind? If yes then how to find an answer to them? Let's go through the article and understand more about bacteria.

Table of Contents

• Introduction to prokaryotes
• Unique features of prokaryotes
• Size and shape of prokaryotic cell
• Practice Problems
• FAQs

Introduction to Prokaryotes

Cell is considered as the basic unit of life. There are two basic types of cells as follows based on the nuclear complexity and presence of cell organelles:

• Prokaryotic cell
• Eukaryotic cell

Prokaryon

The Greek term ‘prokaryon’ is made up of two words, where ‘pro’ means ‘before’ and ‘karyon’ means ‘nucleus’. Thus ‘prokaryon’ means ‘primitive nucleus’. The prokaryotic cells are the most primitive cells ever known on Earth. Their genetic material is not surrounded by any membrane, so the term given is ‘primitive nucleus’. They lack membrane-bound cell organelles. These cells are small and rapidly dividing in nature. Examples include bacteria, blue green algae (cyanobacteria), Mycoplasma, Rickettsia, spirochaetes etc. 

Unique Features of Prokaryotes

The distinguishing features of prokaryotes are as follows:

• The nuclear envelope is absent, thus genetic material is in direct contact with cytoplasm. It is localised in a region called nucleoid.
• Double membrane-bound cell organelles such as Mitochondria, Chloroplast etc. are absent. 
• Ribosomes are present and they are of the 70S type.
• DNA is double-stranded, circular which is similar to naked, single chromosome. Hence, it is also called a Pro chromosome or Genophore.
• Histone proteins, the essential constituents of eukaryotic chromosomes, are absent in them.
• The cell wall in a prokaryotic cell is made up of Peptidoglycan. It is also called as Mureins.
• The plasma membrane contains respiratory enzymes, hence takes part in respiration.
• Reproduction is generally asexual involving binary fission in most bacteria for rapid multiplication. 
• Parasexual mode of reproduction is also observed which involves conjugation, transformation and transduction.
• In many prokaryotes some small, double stranded circular DNA molecules are present lying in the cytoplasm in addition to their prochromosome. These are called plasmids. Plasmids are responsible for the presence of various phenotypic features in bacteria such as antibiotic resistance. They also monitor bacterial transformation with foreign DNA. 

Size and Shape

Size

Prokaryotic cell size ranges within a few micrometres. Mycoplasma are the smallest known prokaryotes, being 0.1 to 1µm. The small size of prokaryotic cells allows the ions and organic molecules to diffuse easily and rapidly throughout the other parts of the cell without involving a complex transportation machinery.

Shape

Bacteria may occur as single cells or can also form colonies. Some bacteria assume different forms or shapes during their lifecycle, thus are called ‘Pleomorphic’. Prokaryotic cells can be commonly found in the following shapes:

• Spherical (Cocci)
• Rod-shaped (Bacilli)
• Comma-shaped (Vibrio) 
• Spiral (Spirochaetes)

Practice Problems of Prokaryotic Cell

Question1. Which of these structures play an important role in bacterial transformation?

A. Plasmid
B. Cell membrane
C. Ribosomes
D. Genomic DNA

Answer: In many prokaryotes some small, circular, double stranded DNA molecules are present lying in the cytoplasm in addition to their prochromosome. These are called plasmids. Plasmids are responsible for the presence of various phenotypic features in bacteria such as antibiotic resistance. They also monitor bacterial transformation with foreign DNA. Hence option a is correct. 

Question2. Identify the cell organelle which is present in both prokaryotic and eukaryotic cells.

A. Mitochondria
B. Endoplasmic reticulum
C. Nucleus
D. Ribosomes

Answer: Prokaryotes and eukaryotes are the two different types of cells based on the complexity and presence of cell organelles. Ribosomes are present in both prokaryotes and eukaryotes. In prokaryotes they are of 70S type whereas in eukaryotes they are of 80S type. Both prokaryotic and eukaryotic ribosomes perform protein synthesis.

Question3. Which of these is not a basic shape of bacteria?

A. Vibrio
B. Spirillum
C. Triangular
D. Coccus

Answer: Triangular shape is not basically found in any prokaryotes. Prokaryotic cells can be commonly found in the following shapes:

• Spherical (Cocci)
• Rod-shaped (Bacilli)
• Comma-shaped (Vibrio) 
• Spiral (Spirochaetes)

Question4. Why are prokaryotes important?

Answer: Prokaryotes play a critical role in the recycling of nutrients. They help in decomposing dead organisms and excreta. In this way they allow the nutrients to be reused. They play a role in many metabolic processes like carbohydrate digestion. Bacteria in human or animal gut and mouth help in the digestion of food by breaking down complex carbohydrates like cellulose and other compounds. They help in the industries for fermenting many food items like yoghourt, cheese, pickles, sauerkraut etc. 

FAQs of Prokaryotic Cell

Question1. How are prokaryotes able to live without a nucleus?

Answer: In prokaryotes there is no nuclear membrane, because there is no proper nucleus. The DNA concentrates in the middle of the cytoplasm in a region called a nucleoid. Therefore, it is possible for prokaryotes to live without a nucleus.

Question2. What qualities make it possible for prokaryotic cells to survive?

Answer: All prokaryotes have some specialised features which help them to survive. These are enlisted below: 

•  A single cell performs all the necessary activities required for survival. 
• Rigid cell wall - It is made up of peptidoglycan. It acts as a protective structure that allows them to survive in both hypertonic and hypotonic conditions.
•  Formation of endospores that resist natural calamities such as heat and drought, thus helping them to survive until favourable conditions return.

Question3. Do prokaryotic cells possess genetic material as DNA?

Answer: Yes, prokaryotic cells have a single circular DNA which is in direct contact with the cytoplasm. The genetic material is concentrated in a region called nucleoid.

Question4.  What advantage do prokaryotes have by being small?

Answer: The size of Prokaryotes ranges from 0.1 - 5 μm in diameter. Their small size benefits them in many ways. It allows the quick entrance and diffusion of molecules and ions. It allows easy elimination of waste products from the cell. It allows them to remain in the air or attached to body surfaces without notice.

Related Topics to Prokaryotic Cells in Biology

• Differences between Prokaryotic and Eukaryotic Cell Difference
• Cell
• Eukaryotic cells 
• Mitochondria
• What is a Cell
• Animal Cell
• Plant Cell
• Cell Envelopes and Cell Membranes
• Types of Cells
• Endomembrane System

NCERT Class 11 Biology Chapters