Prokaryotic Cell Structure: Videos & Practice Problems

Prokaryotes, including bacteria and archaea, are essential to Earth's biomass, comprising 60% of all living organisms. They lack a nucleus and membrane-bound organelles, featuring a nucleoid with circular DNA and often plasmids. Bacteria are classified as gram-positive or gram-negative based on their peptidoglycan cell wall structure. Some possess capsules for protection and can form resilient endospores. Movement is facilitated by fimbriae and flagella, while archaea differ biochemically from bacteria, lacking peptidoglycan and thriving in diverse environments, including extreme conditions.

Introduction to Bacterial Cell Walls

Bacterial cells are characterized by a unique structure known as the cell wall, which serves as a protective layer against the higher internal water pressure compared to their external environment. This pressure differential necessitates a robust structure to prevent cell rupture, making the cell wall essential for bacterial survival.

The cell wall is defined as a semi-rigid structural layer located outside the cell membrane and is a key component of the cell envelope. The cell envelope encompasses all layers surrounding the bacterial cell, including the cell wall, the cell membrane, and, in some bacteria, an outer membrane. This outer membrane may also be accompanied by a capsule, further enhancing the protective capabilities of the cell.

At the core of the bacterial cell wall is a molecule called peptidoglycan, which is crucial for maintaining the integrity of the cell under high internal pressure. The peptidoglycan layer is depicted as a light blue structure in diagrams, illustrating its position between the outer membrane (or capsule) and the inner cell membrane. The inner membrane, often referred to as the cell membrane, is vital for regulating the movement of substances in and out of the cell.

Understanding the structure and function of the bacterial cell wall, particularly the role of peptidoglycan, is fundamental as we explore the complexities of bacterial biology in greater detail throughout the course.

Which is (are) true concerning the cell wall of prokaryotes?

• A. It determines the shape of the bacteria.
• B. It is part of the cell envelope.
• C. It prevents the bacteria from bursting.
• D. All of the choices are true.

Answer: (D)

Peptidoglycan

Peptidoglycan is a crucial component of bacterial cell walls, characterized as a rigid mesh-like structure composed of polysaccharides and proteins. This unique molecule is specific to bacteria, distinguishing them from archaea and eukarya, which do not contain peptidoglycan. Its primary function is to provide structural support and maintain the rigidity of the bacterial cell, ensuring the integrity of the cell wall.

In the context of bacterial cells, the peptidoglycan layer appears as a light blue layer surrounding the cell. Upon closer examination, this layer reveals a complex structure made up of polysaccharide components linked by protein connections. This combination of polysaccharides and proteins forms a robust network that is essential for the survival and stability of bacterial cells.

Understanding the structure and function of peptidoglycan is vital, as it plays a significant role in the biology of bacteria. Future discussions will delve deeper into the specific structural characteristics of peptidoglycan, enhancing our comprehension of its importance in microbiology.

The peptidoglycan molecule is responsible for the:

• A. Entry and exit of molecules into and from the cell.
• B. Flexibility of the cytoplasmic membrane.
• C. Motility of the bacterial cell.
• D. Genetic characteristics of the bacterial cell.
• E. Semi-rigid cell wall structure of prokaryotes.

Answer: (E)

Gram-Positive & Gram-Negative Bacteria

Bacterial cells can be classified into two main categories based on their response to the Gram stain, a crucial staining technique that highlights differences in bacterial cell walls. Understanding the distinction between gram positive and gram negative bacteria is essential in microbiology.

Gram positive bacteria are characterized by their ability to absorb the Gram stain, resulting in a purple coloration. This absorption occurs due to the thick peptidoglycan layer present in their cell walls, which retains the stain effectively. In contrast, gram negative bacteria do not absorb the Gram stain and therefore do not turn purple. This lack of absorption is attributed to their thinner peptidoglycan layer and the presence of an outer membrane that prevents the stain from penetrating.

To summarize, the Gram stain serves as a vital tool in differentiating bacterial types based on their cell wall structure. Gram positive bacteria appear purple after staining, while gram negative bacteria remain unstained. This fundamental difference in cell wall composition is crucial for understanding bacterial behavior, treatment options, and their role in various environments.

The Gram stain works because of differences in the ________ of bacteria.

• A. Genetic characteristics
• B. Cell membranes
• C. Cell walls
• D. Capsules

Answer: (C)

Types of Bacterial Cell Walls

Bacterial cell walls are categorized into two main types: gram-positive and gram-negative, based on their structural differences. Gram-positive bacteria are characterized by a thick layer of peptidoglycan, which allows them to absorb the gram stain, appearing purple under a microscope. In contrast, gram-negative bacteria possess a much thinner peptidoglycan layer and do not retain the gram stain, appearing pink instead.

In addition to the peptidoglycan layer, gram-negative bacteria have a complex outer membrane that gram-positive bacteria lack. This outer membrane serves as an additional barrier, contributing to the overall structural integrity and protection of the cell. The presence of this outer membrane is a key distinguishing feature between the two types of bacteria.

To summarize, gram-positive bacteria have:

• A thick peptidoglycan layer (shown in blue in diagrams).
• An inner plasma membrane.
• No outer membrane.

Conversely, gram-negative bacteria have:

• A thin peptidoglycan layer.
• An inner plasma membrane.
• An outer membrane that surrounds the peptidoglycan layer.

Understanding these differences is crucial for studying bacterial physiology and the implications for antibiotic treatment, as the structural variations can influence the effectiveness of certain drugs. As the course progresses, further exploration of these bacterial types will enhance comprehension of their roles in health and disease.

The Glycocalyx: Capsules & Slime Layers

The glycocalyx is a crucial component found in most bacteria, consisting of a sticky, gel-like substance made up of complex polysaccharides. This layer envelops the bacterial cell, forming a protective and adhesive barrier. It can be observed as a yellow border surrounding the cells in illustrative representations, applicable to both gram-negative and gram-positive bacteria.

One of the primary functions of the glycocalyx is to promote adhesion, allowing bacterial cells to attach to solid surfaces and to one another. This adhesion is vital for forming biofilms, which are communities of microorganisms that adhere to surfaces. Additionally, the glycocalyx plays a significant protective role by preventing dehydration, as it helps retain water around the cell. Its sticky nature also enables the collection of nutrients from the environment, facilitating energy acquisition for the bacteria.

In summary, the glycocalyx serves multiple essential functions, including adhesion, protection from dehydration, and nutrient collection. Understanding its structure and role is fundamental in microbiology, as it sets the stage for exploring its various forms, such as capsules and slime layers, which will be discussed in further detail in subsequent lessons.

What is the function of the glycocalyx in bacterial cells?

• A. Attachment of cells to surfaces and other cells
• B. Protein synthesis
• C. Phagocytosis of other cells
• D. DNA replication

Answer: (A)

__________ is the slime-like layer of polysaccharides on the outer surface of bacterial cells.

• A. The outer membrane
• B. The cytoplasm
• C. The glycocalyx
• D. The periplasm
• E. The S-layer

Answer: (C)

Capsules vs. Slime Layers

The glycocalyx is a crucial component of many cells, existing primarily in two forms: capsules and slime layers. Capsules are characterized by their highly organized and dense structure, composed of polysaccharides that are tightly anchored to the cell membrane. This strong attachment provides a protective barrier and plays a significant role in the cell's ability to evade the immune system and adhere to surfaces.

In contrast, slime layers are less organized and consist of polysaccharides that are loosely associated with the cell. This makes slime layers easily removable, allowing for greater flexibility in their function. Both structures can be visually represented as a yellow border surrounding the cell, with capsules appearing more structured and compact compared to the more diffuse and disordered slime layers.

Regardless of their form, both capsules and slime layers contribute to the formation of biofilms, which are communities of microorganisms that adhere to surfaces and are encased in a protective matrix. Understanding the differences between these two forms of glycocalyx is essential for studying microbial behavior and interactions in various environments.

___________ are an organized layer of polysaccharides tightly anchored to the cell wall.

• A. Capsules
• B. Periplasms
• C. Slime-layers
• D. Teichoic acids
• E. Lipopolysaccharides

Answer: (A)

Which of the following is not a function of slime layers and capsules:

• A. Protection against dehydration.
• B. Collect nutrients from its surroundings.
• C. Attachment to other cells & solid surfaces.
• D. Anchors the outer membrane to peptidoglycan.

Answer: (D)

Pili

Pili are long, filamentous protein structures found on the surface of bacterial cells, with the term "pili" being the plural form and "pilus" the singular. These structures extend from the cell surface and serve various functions depending on the type of bacteria. For instance, pili can play crucial roles in adhesion, allowing bacteria to attach to surfaces or other cells, which is essential for colonization and infection. Additionally, some pili are involved in the process of conjugation, where genetic material is transferred between bacterial cells, contributing to genetic diversity and the spread of antibiotic resistance. Understanding the structure and function of pili is vital for comprehending bacterial behavior and interactions in various environments.

Functions of Pili

Pili are hair-like structures found on the surface of bacterial cells, typically numbering one to two per cell. They serve two primary functions that are essential for bacterial survival and interaction with their environment.

The first function of pili is cell motility, which refers to the ability of an organism to move through its environment. Bacteria utilize pili to propel themselves, allowing for movement from one location to another. This motility is crucial for bacteria to navigate toward nutrients or away from harmful substances.

The second primary function of pili is DNA transfer. Pili act as a bridge or tunnel that connects one bacterial cell to another, facilitating the exchange of genetic material. This process, often referred to as conjugation, allows bacteria to share DNA, which can enhance their adaptability and survival by introducing new traits, such as antibiotic resistance.

In summary, pili play a vital role in both the movement of bacterial cells and the transfer of genetic information, contributing to the dynamic nature of bacterial populations and their ability to thrive in various environments.

Which of these are true about pili?

• A. Pili are short filaments of pilin on the surface of bacterial cells that allow neighboring cells to adhere to one another.
• B. Pili allow cells to 'crawl' across a surface.
• C. Pili are short filaments on the surface of archaea cells that allow neighboring cells to adhere to one another.
• D. Pili allow cells to share genetic information through a process called DNA transfer or conjugation.
• E. A and B
• F. B and C
• G. A and C
• H. B and D

Answer: (H)

Fimbriae

Fimbriae are short, filamentous structures made of pilin protein that extend from the surface of many bacteria. Unlike pili, which are longer, fimbriae play a crucial role in the adhesion of bacterial cells to one another and to various surfaces. This adhesion is essential for the formation of biofilms, which are communities of microorganisms encased in an extracellular polymeric substance (EPS).

In a biofilm, fimbriae facilitate the connection between bacterial cells, allowing them to form a cohesive community. The image of a biofilm illustrates how these structures enable the bacteria to adhere to each other and to the surfaces they colonize. The fimbriae project from the bacterial cell surface, enhancing their ability to interact and stick together, which is vital for the stability and functionality of the biofilm.

Understanding the role of fimbriae in biofilm formation is important for comprehending how bacteria thrive in various environments, as these structures significantly contribute to microbial community dynamics and resilience.

The presence of fimbriae on a bacterial cell is most likely to have a critical role in

• A. Conjugation
• B. Chemotaxis
• C. Biofilm formation
• D. DNA replication

Answer: (C)

Endospores

Endospores are specialized, dormant cells produced by certain bacteria, discovered by scientist John Tyndall. These structures are highly resistant to extreme conditions, including heat, toxic chemicals, and nutrient depletion, allowing them to survive environments that would be lethal to typical bacterial cells. Unlike vegetative cells, which are active and capable of replication, endospores do not replicate; instead, they serve as a survival mechanism for the bacteria.

When nutrients are plentiful, bacteria exist in a vegetative state, actively growing and reproducing. However, when nutrients become scarce, these vegetative cells can undergo a process known as sporulation, leading to the formation of endospores. This process allows the bacteria to endure harsh conditions by entering a dormant state, significantly reducing their metabolic activity.

Once conditions improve and nutrients are available again, endospores can reactivate through a process called germination, reverting back to the vegetative state where they can resume normal functions. It is important to note that endospores are primarily produced by gram-positive bacteria, such as Bacillus and Clostridium. A helpful mnemonic to remember this association is to think of endospores as dormant like winter clothes stored away during warm summer months, linking the concept of dormancy with gram-positive cells.

Understanding the formation and function of endospores is crucial in microbiology, particularly in contexts involving bacterial survival strategies and the implications for food safety and sterilization processes.

Endospores are:

• A. A dormant cell-type
• B. A type of vegetative cell
• C. A form of reproduction
• D. Sensitive to damaging environmental conditions

Answer: (A)

Formation of endospores _________.

• A. Allows bacterial reproduction.
• B. Occurs when the cell is in thriving environmental conditions.
• C. Is called germination.
• D. Can be triggered by adverse environmental conditions.
• E. Occurs in all bacterial cells.

Answer: (D)

What 2 bacterial genera produce endospores?

• A. Escherichia and Bacillus
• B. Staphylococcus and Streptococcus
• C. Clostridium and Bacillus
• D. Enterobactor and Clostridium
• E. Citrobacter and Staphylococcus

Answer: (C)

Here’s what students ask on this topic:

What are the main differences between prokaryotic and eukaryotic cells?

Prokaryotic cells, such as bacteria and archaea, lack a nucleus and membrane-bound organelles. Their DNA is circular and located in a region called the nucleoid. They also have plasmids, which are small, extra-chromosomal DNA molecules. In contrast, eukaryotic cells have a defined nucleus containing linear DNA and various membrane-bound organelles like mitochondria and the endoplasmic reticulum. Prokaryotes have a cell wall made of peptidoglycan (in bacteria) or other materials (in archaea), while eukaryotic cells may have a cell wall (in plants and fungi) made of cellulose or chitin, respectively, or lack a cell wall entirely (in animals).

What is the function of the peptidoglycan layer in bacterial cell walls?

The peptidoglycan layer in bacterial cell walls provides structural support and shape to the cell. It is a strong, resilient material composed of sugar chains cross-linked by small peptides. This structure helps maintain the cell's integrity, preventing it from collapsing and protecting it from environmental stress. In gram-positive bacteria, the peptidoglycan layer is thick and absorbs the crystal violet stain, giving the cells a purple appearance. In gram-negative bacteria, the peptidoglycan layer is thinner and located between the plasma membrane and an outer membrane of lipopolysaccharides.

How do gram-positive and gram-negative bacteria differ in their cell wall structure?

Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain, giving them a purple appearance under a microscope. This thick layer is the primary component of their cell wall. In contrast, gram-negative bacteria have a thinner peptidoglycan layer located between the plasma membrane and an outer membrane composed of lipopolysaccharides. This outer membrane does not retain the crystal violet stain, so gram-negative bacteria appear pink or red after a Gram stain procedure. The presence of the outer membrane in gram-negative bacteria also contributes to their increased resistance to certain antibiotics.

What are endospores, and why are they important for some bacteria?

Endospores are highly resilient, dormant forms of bacteria that can survive extreme environmental conditions, such as high heat, desiccation, and lack of nutrients. They form in response to harsh conditions and can remain dormant for long periods, even hundreds or thousands of years. When favorable conditions return, endospores can reactivate and return to their vegetative state. This ability to form endospores allows certain bacteria to endure and persist in environments that would otherwise be lethal, ensuring their survival and continued proliferation.

What are the unique features of archaea compared to bacteria?

Archaea differ from bacteria in several key ways. They do not use peptidoglycan in their cell walls; instead, they have unique membrane compositions, often featuring ether-linked lipids. Their genetic machinery for transcription and translation is more similar to eukaryotes than bacteria, suggesting a closer evolutionary relationship with eukaryotes. Archaea are known for thriving in diverse environments, including extreme conditions like high temperatures, high salinity, and anaerobic environments. Despite these differences, both archaea and bacteria are prokaryotes, lacking a nucleus and membrane-bound organelles.