BIOL307 - Cell Structure

What is the difference between prokaryotic and eukaryotic cells?

Prokaryotic cells (Bacteria and Archaea) do not have a true nucleus or membrane-bound organelles.

Eukaryotic cells have a nucleus and other membrane-bound organelles.

What are the nucleoid and bacterial chromosome?

The nucleoid is the region where bacterial DNA is located.

Most Bacteria and Archaea have one main circular chromosome containing most or all of their genes.

Unlike eukaryotic chromosomes, it is not enclosed in a nucleus.

What are plasmids and why are they important?

Plasmids are small, usually circular, double-stranded DNA molecules that replicate independently of the chromosome.

They are not always essential for survival but can carry genes that provide advantages under particular conditions and can sometimes be transferred between bacteria.

What is the bacterial cytoplasmic membrane, and how is it different from the cell wall?

The cytoplasmic membrane surrounds the bacterial cytoplasm, while the cell wall is a rigid structural layer outside the membrane.

The membrane and cell wall are separate structures with different roles.

What are the major functions of the bacterial cell wall?

The cell wall gives the bacterial cell rigidity and helps it withstand intracellular pressure, including pressure caused by water entering the cell.

Most bacterial cell walls are made primarily of peptidoglycan.

What is peptidoglycan made of?

Peptidoglycan consists of alternating modified sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), forming glycan chains.

Short chains of amino acids attached to NAM cross-link the glycan chains and strengthen the wall.

How is peptidoglycan synthesized?

Peptidoglycan precursors are made in the cytosol, attached to the membrane carrier bactoprenol, and transported across the cytoplasmic membrane.

They are then incorporated into growing peptidoglycan chains, which are cross-linked by transpeptidase.

What are bactoprenol, transglycosylase, and transpeptidase?

Bactoprenol is a membrane carrier that transports peptidoglycan precursors across the cytoplasmic membrane.

Transglycosylase links sugar components into glycan chains.

Transpeptidase cross-links the peptide portions of peptidoglycan, and are a target of antibiotics.

What is the basic structure of a Gram-positive cell envelope?

Gram-positive bacteria have a thick, multilayered peptidoglycan wall, often up to about 40 layers, reinforced with teichoic acids.

They do not have the outer membrane found in Gram-negative bacteria.

A capsule or S-layer may also be present.

What is the basic structure of a Gram-negative cell envelope?

Gram-negative bacteria have only about 1–2 layers of peptidoglycan.

Outside the peptidoglycan is a periplasm and an outer membrane containing LPS and porins.

A capsule or S-layer may also be present.

How do Gram-positive and Gram-negative cell envelopes differ?

Gram-positive bacteria have a much thicker peptidoglycan layer and teichoic acids.

Gram-negative bacteria have a much thinner peptidoglycan layer plus an outer membrane, periplasm, and LPS.

What are teichoic acids and what do they do?

Teichoic acids are negatively charged molecules associated with the thick peptidoglycan of Gram-positive bacteria.

They help retain the Gram stain and can also help pathogens attach to host cells.

What are the periplasm and outer membrane of Gram-negative bacteria?

The periplasm is the space between the cytoplasmic membrane and outer membrane and contains proteins that can protect the cell from dangerous oxidants.

The outer membrane contains LPS and porins and helps regulate nutrient uptake and exclude toxins.

What is LPS and what are its major parts?

LPS (lipopolysaccharide) is found in the outer leaflet of the Gram-negative outer membrane.

It contains Lipid A, a core polysaccharide, and the O antigen.

Lipid A is the endotoxin portion, while the O antigen can help resist phagocytosis.

What is the purpose of the Gram stain?

The Gram stain separates bacteria into Gram-positive and Gram-negative groups based on differences in their cell-envelope structures.

What are the four main reagents used in the Gram stain?

Crystal violet is the primary stain, Gram's iodine is the mordant, alcohol or acetone is the decolorizer, and safranin is the secondary/counterstain.

What color are bacteria at each stage of the Gram stain?

After crystal violet: both are purple.

After iodine: both remain purple.

After decolorization: Gram-positive remains purple while Gram-negative becomes colorless.

After safranin: Gram-positive remains purple while Gram-negative becomes red/pink.

Why do Gram-positive bacteria remain purple while Gram-negative bacteria become pink?

Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet-iodine complex during decolorization.

Gram-negative bacteria have a thin peptidoglycan layer and lose the crystal violet during decolorization, then take up the safranin counterstain.

Why can bacterial structures be targets for selective antimicrobial drugs?

Bacterial cells have structures that human cells lack, such as peptidoglycan cell walls.

Targeting bacterial-specific structures can damage or kill bacteria without directly targeting an equivalent structure in human cells.

What are capsules and what are their functions?

A capsule is a polysaccharide layer outside the cell wall.

It helps retain moisture, protects the cell, prevents dehydration, helps bacteria adhere to surfaces and acquire nutrients, and can inhibit phagocytosis by white blood cells.

What are S-layers?

S-layers are surface layers made of proteins or glycoproteins.

Their subunits contain pores that allow a wide range of molecules to pass through.

In some bacteria, S-layers can contribute to virulence.

How do capsules and S-layers affect bacterial interactions with the environment or host?

Both are surface structures that can help bacteria interact with their surroundings.

Capsules can help with moisture retention, adhesion, protection, and avoidance of phagocytosis, while S-layers can provide surface protection and contribute to virulence.

What are fimbriae, pili, and flagella?

Fimbriae are short protein appendages mainly involved in attachment.

Pili are protein appendages involved in attachment and specialized functions such as twitching movement, pathogenesis, and transformation.

Flagella are long, thin appendages primarily used for movement.

How are fimbriae and pili related?

Fimbriae are shorter pili.

Both are made of protein and can help bacteria attach to surfaces, while some types of pili have additional functions.

What are Type IV pili?

Type IV pili are a common class of pili involved in adhesion, twitching movement, pathogenesis, and transformation.

What are bacterial flagella?

Flagella are long, thin appendages that are anchored to the bacterial cell at one end and free at the other.

They allow bacteria to move through their environment.

What is chemotaxis?

Chemotaxis is the ability of bacteria to respond to changes in chemical concentrations in their environment.

Bacteria respond to temporal, or changes over time, rather than simply sensing the concentration at one location.

What are endospores and why are they advantageous?

Endospores are inert, heat-resistant, dormant structures that can remain viable for very long periods.

They allow bacteria to survive unfavorable environmental conditions without actively growing or replicating.

Why are endospores important in healthcare, food safety, and microbial control?

Endospores can resist desiccation, high-energy radiation, freezing, and chemical disinfectants. Because they are highly resistant and can survive harsh conditions, they can be difficult to eliminate during disinfection, food processing, and other microbial-control procedures.

What is the difference between an endospore and a vegetative bacterial cell?

An endospore is a dormant, non-growing form that is highly resistant to environmental stress.

A vegetative bacterial cell is the actively growing form.

When conditions become favorable, an endospore can return to a vegetative cell.

How would you identify a bacterium as Gram-positive from its cell-envelope structure?

A Gram-positive bacterium has a thick, multilayered peptidoglycan wall with teichoic acids and lacks the outer membrane characteristic of Gram-negative bacteria.

How would you identify a bacterium as Gram-negative from its cell-envelope structure?

A Gram-negative bacterium has a thin peptidoglycan layer, a periplasm, and an outer membrane containing LPS and porins.

If a bacterium is purple after the Gram stain is complete, what does that tell you?

It is Gram-positive because it retained the crystal violet stain during decolorization.

If a bacterium is pink/red after the Gram stain is complete, what does that tell you?

It is Gram-negative because it lost crystal violet during decolorization and then took up safranin.

Why is peptidoglycan a useful antimicrobial target?

Bacteria have peptidoglycan cell walls, while human cells do not.

This difference allows drugs to target bacterial cell-wall synthesis selectively.

A bacterium has a capsule. What advantages could this provide?

The capsule can prevent dehydration, help the bacterium adhere to surfaces, help acquire nutrients, protect the cell, and inhibit phagocytosis.

A bacterium has flagella. What advantage does this provide?

Flagella allow the bacterium to move through its environment, which can help it respond to changing environmental conditions.

Why are endospores difficult to eliminate?

Endospores are highly resistant to desiccation, high-energy radiation, freezing, and chemical disinfectants, allowing them to survive conditions that would kill actively growing cells.

What happens to an endospore when environmental conditions become favorable?

It can return to a vegetative bacterial cell and resume growth.