🦠 Prokaryotic Cells

Prokaryotic cells are the oldest and simplest cellular forms.They are represented by Bacteria, Blue-Green Algae (Cyanobacteria), Mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).

They are highly successful because they multiply fast, adapt quickly, and can live in diverse habitats.

1️⃣ Core Definition

A prokaryotic cell is a cell that does NOT have:

  • a membrane-bound nucleus (no nuclear membrane)
  • membrane-bound organelles (no mitochondria, ER, Golgi, lysosomes, chloroplast)

🔑 Key Point:

DNA is present in the cytoplasm in a region called NUCLEOID (not a true nucleus).

2️⃣ Examples & Size

Examples

  • Bacteria
  • Cyanobacteria (Blue-green algae)
  • Mycoplasma (also called PPLO in many texts)

Size

  • Generally 1–10 µm (smaller than eukaryotes)

🔑 Key Point:

Prokaryotes are small but reproduce very fast (mostly by binary fission).

3️⃣ Basic Structure of a Prokaryotic Cell

A typical bacterial cell has:

  1. Cell envelope (outer protective covering)
  2. Cytoplasm (metabolic arena)
  3. Nucleoid (DNA region)
  4. Ribosomes (70S)
  5. Inclusion bodies (storage)
  6. Optional appendages (flagella/pili/fimbriae)

🧬 Cell Envelope

The cell envelope has three layers (outside → inside):

(a) Glycocalyx (Outermost)

  • If loose → slime layer
  • If thick and tough → capsule

Functions (easy to remember):

  • Protection (desiccation / immune attack)
  • Attachment to surfaces

🔑 Key Point:

Capsule = thick glycocalyx; slime layer = loose glycocalyx.

(b) Cell Wall (Middle)

  • Usually made of peptidoglycan (murein)
  • Absent in Mycoplasma

Functions:

  • Gives shape and rigidity
  • Prevents bursting/collapsing

Gram staining (concept):

  • Gram +: thick peptidoglycan → retains purple stain
  • Gram −: thin peptidoglycan + outer membrane → appears pink

🔑 Key Point:

Peptidoglycan is the major reason for bacterial rigidity.

(c) Plasma Membrane (Innermost)

  • Selectively permeable like eukaryotes
  • Also performs many metabolic roles because organelles are absent

🔑 Key Point:

In prokaryotes, important enzymes for respiration/photosynthesis may be on the plasma membrane.

🔹 Mesosome (Membrane Infolding)

Mesosome = infoldings of plasma membrane into the cytoplasm (vesicles/tubules/lamellae).

Common exam functions:

  • Cell wall formation
  • DNA replication & distribution
  • Respiration (helps because mitochondria are absent)
  • Secretion and increased surface area

🔑 Key Point (Mnemonic):

Mesosome = W-D-R-S → Wall formation, DNA, Respiration, Secretion.

🧪 Cytoplasm + Ribosomes + Polysome

Cytoplasm

  • No membrane-bound organelles
  • Contains enzymes, ribosomes, inclusion bodies

Ribosomes

  • 70S type = 50S + 30S
  • Site of protein synthesis

Polysome (Polyribosome)

  • Many ribosomes on one mRNA → polysome

🔑 Key Points:

  • Ribosomes are present in all cells (prokaryotes + eukaryotes).
  • 70S is typical of prokaryotes (also found inside mitochondria/chloroplast of eukaryotes).

🧩 Genetic Material: Nucleoid & Plasmids

Nucleoid

  • Single, circular, naked DNA (no nuclear membrane)

Plasmids

  • Small circular extra-chromosomal DNA
  • Carry accessory genes, often antibiotic resistance
  • Replicate independently

🔑 Key Point:

Plasmid = extra DNA that can give bacteria “extra powers” (like antibiotic resistance).

🧠 Inclusion Bodies (Storage Structures)

Inclusion bodies are non-living storage materials and are NOT membrane-bound.

Examples:

  • Phosphate granules (reserve phosphate)
  • Cyanophycean granules (food reserve in cyanobacteria)
  • Glycogen granules
  • Gas vacuoles (buoyancy in aquatic photosynthetic prokaryotes)

🔑 Key Point:

Inclusion bodies are free in cytoplasm (no membrane).

📜 Appendages (Surface Structures)

Flagella (motility)

  • Made of protein flagellin
  • Parts: filament (longest), hook, basal body

Pili

  • Elongated tubular structures
  • Help in conjugation (DNA transfer) and attachment

Fimbriae

  • Small bristle-like fibres
  • Help in attachment to surfaces/host tissues

🔑 Key Point:

Pili = DNA transfer (conjugation), Fimbriae = attachment, Flagella = movement.

📜 Quick One-Page Revision

  • No true nucleus → DNA in NUCLEOID
  • No membrane organelles → respiration happens on plasma membrane/mesosomes
  • Cell wall mostly peptidoglycan (except Mycoplasma)
  • 70S ribosomes → protein synthesis (50S + 30S)
  • Plasmids → extra DNA (antibiotic resistance)
  • Flagella → motility; Pili → conjugation; Fimbriae → attachment

💡 Questions & Answers

Q1. What are the key characteristics of prokaryotic cells?

A1. Prokaryotic cells are simple cells that do not have a true nucleus and do not have membrane-bound organelles. Their DNA lies in a region called the nucleoid, and they usually have 70S ribosomes.

🔑 Key Points:

  • No nuclear membrane → DNA in nucleoid
  • No mitochondria/ER/Golgi
  • 70S ribosomes present
  • Usually peptidoglycan cell wall

Q2. What is the prokaryotic cell envelope? Name its layers.

A2. The cell envelope is the outer protective covering of bacteria. It has three layers: glycocalyx, cell wall, and plasma membrane.

🔑 Key Points:

  • Layers (outer → inner): Glycocalyx → Cell wall → Plasma membrane
  • Provides protection + shape + control of transport

Q3. What is the difference between slime layer and capsule?

A3. Both are types of glycocalyx. A slime layer is loose and thin, while a capsule is thick and tough.

🔑 Key Points:

  • Slime layer = loose
  • Capsule = thick & tough
  • Helps in protection and attachment

Q4. What is a mesosome and why is it important?

A4. A mesosome is an infolding of the plasma membrane. It helps in respiration, DNA replication, and cell wall formation.

🔑 Key Points:

  • Mesosome = plasma membrane infolding
  • Functions: Wall formation, DNA replication, Respiration, Secretion

Q5. What are plasmids? Why are they important?

A5. Plasmids are small circular DNA pieces found in many bacteria, separate from the main DNA. They often carry useful genes like antibiotic resistance.

🔑 Key Points:

  • Extra-chromosomal circular DNA
  • Replicate independently
  • Give advantages like antibiotic resistance

Q6. What are inclusion bodies? Give examples.

Answer (Student’s words): Inclusion bodies are storage materials present in bacterial cytoplasm. They are not membrane-bound.

🔑 Key Points:

  • Stored materials, no membrane
  • Examples: phosphate granules, cyanophycean granules, glycogen granules, gas vacuoles

Q7. Differentiate between pili and fimbriae.

A7. Pili are longer structures mainly used for conjugation (DNA transfer). Fimbriae are smaller and mainly help in attachment.

🔑 Key Points:

  • Pili → conjugation/DNA transfer
  • Fimbriae → attachment

Q8. Why is Mycoplasma special among prokaryotes?

A8. Mycoplasma is special because it does not have a cell wall, so it is flexible and unique compared to most bacteria.

🔑 Key Points:

  • Cell wall absent in Mycoplasma
  • Still prokaryotic (no true nucleus)