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🦠 Prokaryotic Cells

Spec: 3.5(ii), 3.5(iii), 3.8, CP5

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Learning Objectives

✓
Know the ultrastructure of prokaryotic cells, including the cell wall, capsule, plasmid, flagellum, pili, ribosomes, and circular DNA
✓
Understand the function of each structure in prokaryotic cells
✓
Describe the differences between Gram-positive and Gram-negative bacteria
✓
Explain how the Gram stain technique works and why it's useful
✓
Classify bacteria by shape: cocci, bacilli, spirilla, and vibrios

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What Do You Already Know?

Let's test your prior knowledge before we begin!

Question 1

Which type of cell does NOT have a membrane-bound nucleus?

A Eukaryotic cell

B Prokaryotic cell

C Plant cell

D Animal cell

✓ Correct! Prokaryotic cells (bacteria) lack a membrane-bound nucleus - their DNA is found in the nucleoid region.

✗ Not quite. Remember: "Pro" means before, "karyon" means nucleus. Think about which cells evolved first!

Question 2

What is the main component of bacterial cell walls?

A Cellulose

B Chitin

C Peptidoglycan

D Phospholipid

✓ Correct! Peptidoglycan (also called murein) is the key structural component of bacterial cell walls.

✗ Cellulose is found in plant cell walls, chitin in fungi. Bacterial walls have a unique polymer!

Question 3

What size ribosomes do bacteria have?

A 70S

B 80S

C 60S

D 90S

✓ Excellent! Bacterial ribosomes are 70S (30S + 50S subunits), smaller than eukaryotic 80S ribosomes.

✗ Remember: Prokaryotes have 70S, Eukaryotes have 80S. This difference is targeted by some antibiotics!

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Bacterial Cell Structure

Bacteria, cyanobacteria, and archaebacteria are all prokaryotic organisms. They share certain features but can vary greatly between species. Click each structure to learn more!

Cell Wall Essential

Maintains shape and prevents bursting...

Click to expand

Structure:

Made of peptidoglycan - polysaccharide chains with short peptide cross-linkages
Creates a net-like structure

Function:

Prevents cell from swelling and bursting due to osmosis
Maintains cell shape
Provides support and protection

Capsule/Slime Layer Some bacteria

Protective outer coating found in some bacteria...

Click to expand

Structure:

Made from starch, gelatin, protein, or glycolipid
Thin and diffuse layer around the cell

Function:

Protects from phagocytosis by white blood cells
Covers cell membrane markers (helps avoid immune detection)
Helps bacteria survive very dry conditions
Can make bacteria pathogenic (disease-causing)

Nucleoid Central region

Region containing the bacterial chromosome...

Click to expand

Structure:

Single, circular strand of DNA
NOT contained in a membrane-bound nucleus
DNA is folded and coiled to fit
Occupies about half the cytoplasm in E. coli

Function:

Contains genetic material for all cellular functions
Key difference from eukaryotic cells!

⚠️ Don't confuse nucleoid with nucleus or nucleolus!

Plasmids Small circles

Small, independent DNA circles...

Click to expand

Structure:

Small, circular pieces of DNA
Separate from the main chromosome
Can replicate independently

Function:

Code for specific traits (e.g., antibiotic resistance, toxin production)
Can be transferred between bacteria via pili
Important in genetic engineering!

Flagella ~20nm diameter

Long, helical structures for movement...

Click to expand

Structure:

Made of protein called flagellin
Many-stranded helix structure
Can rotate at ~100 revolutions per second!

Function:

Movement through liquid environments
Enables bacteria to move towards nutrients or away from harmful substances

Pili (Fimbriae) Thread-like

Short protein projections from the surface...

Click to expand

Structure:

Thread-like protein projections
100-several hundred per cell
Found in E. coli, Salmonella, and many others

Function:

Attachment to host cells
Sexual reproduction (conjugation) - transfer of plasmids
Can act as entry points for bacteriophages

70S Ribosomes ~20nm

Smaller than eukaryotic ribosomes...

Click to expand

Structure:

70S total (30S small subunit + 50S large subunit)
Smaller than eukaryotic 80S ribosomes
No membrane-bound organelles!

Function:

Protein synthesis
Target for many antibiotics (e.g., streptomycin)

💡 Ribosomes are the ONLY organelles shared with eukaryotes!

Mesosomes Infoldings

Infoldings of the cell membrane...

Click to expand

Structure:

Infoldings of the plasma membrane
Found in some bacteria (e.g., Bacillus subtilis)
May be artefacts of preparation!

Possible Functions:

Site of respiratory enzymes (like mitochondria)
DNA separation during cell division
Cell wall formation
Photosynthesis (in some species)

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Quick Check: Structures

Question 1

Which structure allows bacteria to transfer genetic material to other bacteria?

A Flagella

B Pili

C Capsule

D Mesosomes

✓ Correct! Pili are used for conjugation - the transfer of plasmids between bacteria.

✗ Think about which structure is involved in bacterial "mating" (conjugation).

Question 2

What is the function of a bacterial capsule?

A Movement through liquids

B Protein synthesis

C Protection from phagocytosis

D DNA replication

✓ Correct! The capsule helps bacteria evade the immune system by protecting against phagocytosis.

✗ The capsule is an outer protective layer. What might it protect against?

Question 3

Why do some antibiotics target 70S ribosomes specifically?

A They differ from human 80S ribosomes, so won't harm our cells

B 70S ribosomes are easier to destroy

C 70S ribosomes produce toxins

D Human cells don't have ribosomes

✓ Excellent! Because bacterial ribosomes (70S) differ from human ribosomes (80S), antibiotics can target bacteria without harming us!

✗ Think about why targeting bacterial ribosomes wouldn't affect human cells...

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Gram Staining & Cell Walls

💡 Why is this important?

Different types of bacteria respond differently to antibiotics based on their cell wall structure. Doctors need to know if a pathogen is Gram-positive or Gram-negative to choose the right treatment!

The Gram Staining Technique

Developed by Christian Gram in 1884, this technique distinguishes bacteria by their cell wall structure.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan layer	Thick layer	Thin layer
Teichoic acid	Present (in cell wall)	Absent
Outer membrane	Absent	Present (with lipopolysaccharides)
Stain colour	Purple/Blue	Red/Pink
Examples	MRSA, Streptococcus	E. coli, Salmonella

How Does Gram Staining Work?

Crystal violet stain applied - all bacteria turn purple
Iodine added - forms complex trapped in peptidoglycan
Alcohol/acetone wash - dehydrates the bacteria
Safranin counterstain - stains decolourised cells red

🔑 Key Point

Gram-positive bacteria retain the purple crystal violet because their thick peptidoglycan layer traps the stain. Gram-negative bacteria lose the purple stain when washed (their thin peptidoglycan can't hold it) and take up the red safranin counterstain instead.

Antibiotics & Cell Walls

Different antibiotics work better against different types of bacteria:

Beta-lactam antibiotics (penicillins, cephalosporins): Very effective against Gram-positive bacteria - inhibit peptidoglycan synthesis
Glycopeptide antibiotics (vancomycin): Effective against Gram-positive bacteria, even antibiotic-resistant strains
Polypeptide antibiotics (polymyxins): Effective against Gram-negative - target outer membrane phospholipids

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Quick Check: Gram Staining

Question 1

A bacterium appears purple after Gram staining. What can you conclude?

A It has a thick peptidoglycan layer

B It has an outer membrane

C It lacks peptidoglycan

D It is resistant to all antibiotics

✓ Correct! Purple = Gram-positive = thick peptidoglycan layer that retains the crystal violet stain.

✗ Remember: Purple bacteria are Gram-positive. What's special about their cell wall?

Question 2

Why are beta-lactam antibiotics (like penicillin) more effective against Gram-positive bacteria?

A Gram-positive bacteria have no cell wall

B Gram-positive bacteria have a thick peptidoglycan layer exposed on the surface

C Gram-negative bacteria don't have ribosomes

D Gram-positive bacteria cannot reproduce

✓ Correct! Beta-lactams inhibit peptidoglycan synthesis. Gram-positive bacteria have a thick, exposed peptidoglycan layer, making them more vulnerable.

✗ Think about what beta-lactam antibiotics target (peptidoglycan) and which type has more of it exposed.

Question 3

Which substance is found in Gram-positive cell walls but NOT in Gram-negative cell walls?

A Lipopolysaccharides

B Peptidoglycan

C Teichoic acid

D Phospholipids

✓ Correct! Teichoic acid is unique to Gram-positive bacteria - it helps trap the crystal violet/iodine complex.

✗ Both types have peptidoglycan. What chemical is unique to Gram-positive walls?

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Bacterial Classification by Shape

Bacteria can also be classified by their shape - visible under a light microscope:

Cocci

○○○

Click to reveal

Spherical bacteria

Examples: Staphylococcus, Streptococcus

Bacilli

▭ ▭ ▭

Click to reveal

Rod-shaped bacteria

Examples: E. coli, Bacillus subtilis, Salmonella

Spirilla

〰️〰️

Click to reveal

Twisted/spiral-shaped bacteria

Example: Spirillum volutans

Vibrios

〉〉〉

Click to reveal

Comma-shaped bacteria

Example: Vibrio cholerae (causes cholera)

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Practice Activities

Test your understanding with these interactive exercises!

Fill in the Blanks

Prokaryotic cells have a region called the which contains a single, circular strand of . Unlike eukaryotic cells, they have smaller ribosomes. Some bacteria have thread-like for attachment and conjugation.

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Key Vocabulary

Click a term, then click its matching definition:

Terms

Peptidoglycan

Plasmid

Bacteriophage

Obligate aerobe

Facultative anaerobe

Definitions

A virus that attacks bacteria

Net-like molecule in bacterial cell walls

Organism that can survive with or without oxygen

Small circular DNA separate from chromosome

Organism that requires oxygen for respiration

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Final Assessment

Test everything you've learned! Try to answer without looking back.

Question 1

Which of the following is NOT found in prokaryotic cells?

A Ribosomes

B Cell membrane

C Mitochondria

D Cell wall

✓ Correct! Mitochondria are membrane-bound organelles found only in eukaryotes.

✗ Remember: prokaryotes have NO membrane-bound organelles.

Question 2

A Gram-negative bacterium would have:

A Thick peptidoglycan and teichoic acid

B Thin peptidoglycan and an outer membrane with lipopolysaccharides

C No peptidoglycan

D 80S ribosomes

✓ Correct! Gram-negative = thin peptidoglycan + outer membrane with LPS.

✗ Gram-negative bacteria stain red because their thin peptidoglycan doesn't retain the purple stain.

Question 3

Which structure enables bacteria to move through their environment?

A Pili

B Flagella

C Capsule

D Plasmids

✓ Correct! Flagella are helical protein structures that rotate to propel bacteria.

✗ Pili are for attachment and conjugation. What's the whip-like structure for movement?

Question 4

Bacteria that can use oxygen if available but can also survive without it are called:

A Obligate aerobes

B Obligate anaerobes

C Facultative anaerobes

D Microaerophiles

✓ Correct! Facultative anaerobes are flexible - they can use aerobic or anaerobic respiration.

✗ "Facultative" means optional/flexible. Which type can switch between oxygen and no oxygen?

Question 5

Which statement about mesosomes is correct?

A They are found in all prokaryotic cells

B They are identical to mitochondria

C They may be infoldings associated with respiratory enzymes

D They contain the bacterial DNA

✓ Correct! Mesosomes are membrane infoldings that may house respiratory enzymes, though some may be preparation artefacts.

✗ Mesosomes are infoldings of the membrane. What might their function be?

Question 6

A bacterium is found to be resistant to penicillin. Which mechanism is MOST likely?

A The bacterium lacks ribosomes

B The bacterium produces enzymes that break down the antibiotic

C The bacterium has no cell wall

D The bacterium is Gram-negative

✓ Correct! Many resistant bacteria produce beta-lactamase enzymes that destroy penicillin. Resistance genes are often carried on plasmids.

✗ While Gram-negative bacteria are naturally less susceptible, TRUE resistance often involves enzyme production...

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Key Takeaways

🦠

No Membrane-Bound Organelles

Prokaryotes lack a nucleus and organelles like mitochondria

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Circular DNA

Single chromosome in nucleoid region + plasmids

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70S Ribosomes

Smaller than eukaryotic 80S - antibiotic target

🔬

Peptidoglycan Walls

Gram+ = thick, purple; Gram- = thin + outer membrane, red

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Motility Structures

Flagella for movement, pili for attachment & conjugation

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Shape Classification

Cocci, bacilli, spirilla, vibrios

Learning Objectives

What Do You Already Know?

Question 1

Question 2

Question 3

Bacterial Cell Structure

Cell Wall Essential

Structure:

Function:

Capsule/Slime Layer Some bacteria

Structure:

Function:

Nucleoid Central region

Structure:

Function:

Plasmids Small circles

Structure:

Function:

Flagella ~20nm diameter

Structure:

Function:

Pili (Fimbriae) Thread-like

Structure:

Function:

70S Ribosomes ~20nm

Structure:

Function:

Mesosomes Infoldings

Structure:

Possible Functions:

Quick Check: Structures

Question 1

Question 2

Question 3

Gram Staining & Cell Walls

💡 Why is this important?

The Gram Staining Technique

How Does Gram Staining Work?

🔑 Key Point

Antibiotics & Cell Walls

Quick Check: Gram Staining

Question 1

Question 2

Question 3

Bacterial Classification by Shape

Cocci

Bacilli

Spirilla

Vibrios

Practice Activities

Fill in the Blanks

Key Vocabulary

Terms

Definitions

More Practice Questions

Question 1

Question 2

Question 3

Question 4

Final Assessment

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Your Final Score

Key Takeaways

No Membrane-Bound Organelles

Circular DNA

70S Ribosomes

Peptidoglycan Walls

Motility Structures

Shape Classification