Prokaryotic Cells: Cell Envelopes and Cell Membrane Invagination (Mesosome)

You all have heard a lot about bacteria. Do you think that bacteria also know how to swim, walk, hangout and chill with their friends?

Yes, is it interesting? Now you also must be wondering how is it possible? Yes, it is possible because they have the cell appendages like cilia and flagella, helping them to move, swim or adhere to a surface. Fimbriae and pili even help bacteria to make sexual contact with each other by forming bridge like connections.

You know, being so small in size, their life expectancy becomes very low but thanks to the protective covering they have. Yes, the cell wall and cell membrane. They make a strong shield around them to fight off all the enemies.

Now let's know some more interesting things like this, which enable them to survive better in their habitats. 

Table of Contents

• Cell envelopes
• Cell membrane invagination
• Practice Problems
• FAQs

Cell Envelopes

It is the outer covering which protects the cytoplasm. A bacterial cell is protected by three outer coverings adhered together as a single unit. They are as follows: 

• Glycocalyx - Outer slime layer or capsule
• A middle cell wall
• An inner cell membrane

Glycocalyx

Glycocalyx is an outermost layer of the bacterial cells. It normally protects the cell and helps in adhesion also. It normally differs in thickness and composition. It can have the following compositions:

• Slime layer
• Capsule

Slime layer

The glycocalyx is in the form of loose gelatinous sheath here. It normally protects the cells from leakage of materials like water and nutrients.

Capsule

The glycocalyx is thick and tough here. It is made up of polysaccharides and proteins. It protects the cell against desiccation, antibiotics and viral attack. Gelatinous glycocalyx glues together the bacterial cells to form colonies. It provides virulence also. 

Cell wall

It is the middle layer of the cell envelope of a prokaryote which is sandwiched between the glycocalyx and cell membrane. Cell wall in bacteria is made up of peptidoglycan which contains polymers of modified sugars (such as N-acetylglucosamine and N-acetyl muramic acid) cross linked with some short peptides. Teichoic acid is also a component of the cell wall in some bacteria. Mycoplasma lacks a cell wall. Cell wall composition varies in gram negative and gram positive bacteria. Cell wall maintains shape and provides support and strength to the majority of the prokaryotic cells such as bacteria. It prevents the bacteria from bursting.

Cell membrane

Cell membrane is the innermost layer of the cell envelope. The membranes are quasi (partly) fluid in nature. It is selectively permeable to some molecules present on either side of it. Most of the bacterial cell membrane lacks sterols like-cholesterol. In some bacteria, pentacyclic molecules can be present in cell membranes termed hopanoids. They help to stabilise the molecular structure of membranes. It is made up of lipoprotein and has membrane proteins (integral and peripheral) incorporated for movement of molecules. The ‘fluid mosaic model’ is used to explain the exact structure of the cell membrane. Cell membrane controls the movement of molecules into and out of the cell. In prokaryotes, the cell membrane acts as a respiratory membrane since it contains respiratory enzymes. It infolds to form specialised structures called mesosomes. 

Cell membrane Invagination

Cell membrane has following invaginations in bacteria: 

• Mesosomes
• Chromatophores

Mesosomes

Mesosomes are formed by the infoldings of the plasma membrane. They bear respiratory enzymes. They are formed of vesicles, lamellae and tubules. 

Types of Mesosomes

They are of two types as follows:

Septal Mesosome

They are seen associated with the chromosomes or cell wall of dividing cells. It is commonly involved in the cell wall formation, chromosome replication, and distribution of chromosomes to daughter cells. 

Lateral Mesosome 

They are not seen associated with the nucleoid. It possesses respiratory enzymes. 

Functions of Mesosomes

The main functions of mesosomes are as follows:

• Cell wall formation.
• DNA replication. 
• Distribution of replicated DNA to the daughter cells.
• Increasing surface area of the plasma membrane.
• Respiration. 
• Secretory and enzymatic activity. 

Chromatophores

These are the membranous extensions containing pigment molecules such as bacteriochlorophyll. These are abundantly found in cyanobacteria or blue green algae. Their main role is photosynthesis. They may be of different shapes like lamellar, tubular or vesicular in shape. 

Practice Problems of Prokaryotic Cells

Q1. Which of these bacteria lack a cell wall?

1. Escherichia
2. Pseudomonas
3. Mycoplasma
4. Mycobacterium

Solution: Cell wall maintains the shape and provides support and strength to the majority of the prokaryotic cells such as bacteria. It prevents the bacteria from bursting. Mycoplasma lacks a cell wall. Hence option c is correct. 

Q2. Mesosome is derived from ____________.

1. Plasmid
2. Cell wall
3. Ribosome
4. Cell membrane

Solution: Mesosomes are formed by the infoldings of the plasma membrane. They bear respiratory enzymes. They are formed of vesicles, lamellae and tubules. Hence option d is correct. 

Q3. Which of these bacteria have chromatophores?

1. Escherichia
2. Mycobacterium
3. Mycoplasma
4. Cyanobacteria

Solution: Chromatophores are the membranous extensions containing pigment molecules such as bacteriochlorophyll. These are abundantly found in cyanobacteria or blue green algae. Their main role is photosynthesis. Hence option d is correct. 

Q4. The cell wall of Eubacteria is made up of _________________. 

1. Peptidoglycan
2. Cellulose 
3. Chitin 
4. Phospholipids 

Solution: Cell wall in bacteria is made up of peptidoglycan which contains polymers of modified sugars (such as N-acetylglucosamine and N-acetyl muramic acid) cross linked with some short peptides. Hence the correct option is a. 

Q5. What is the main role of the cell wall in prokaryotic cells?

Solution: In prokaryotic cells, the cell wall protects the cells against changes in osmotic pressure and allows it to maintain its shape.

FAQs of Prokaryotic Cells

Question1. Name the type of bacteria whose chromatophores are more complex and extensive?

Answer: Photosynthetic bacteria have more complex and extensive chromatophores because of their internal membrane systems. 

Question2. Do you think the composition of cell membranes is the same in prokaryotes and eukaryotes?

Answer: Both prokaryotic and eukaryotic cells have a plasma membrane, It is made up of lipoproteins and has membrane proteins (integral and peripheral) incorporated for movement of molecules. The same ‘fluid mosaic model’ is used to explain the structure of cell membranes in both prokaryotes and eukaryotes.

Question3. Bacteria cause diseases, then why can't we just wipe them out from the Earth? 

Answer: Bacteria are believed to have existed on Earth for over 3 thousand million years whereas Early man emerged 250 thousand years ago only. They perform various functions. They are involved in nutrient recycling and a part of the detritus food chain. They help in fermentation also. 

Question4. Are mesosomes similar to mitochondria?

Answer: Mitochondria are the powerhouse of the cell. They are present in eukaryotic cells. Mesosomes are the invaginations of the plasma membrane present in prokaryotic cells. They are also involved in cellular respiration. They can be considered analogous to cristae in the mitochondria. Cristae are finger-like projections and help mainly the eukaryotic cells to undergo cellular respiration.

Related Topics to Cell Envelopes and Cell Membrane in Biology

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

NCERT Class 11 Biology Chapters