🧬 1. Ribosomes — Protein Factories of the Cell

Ribosomes are tiny, granular, non-membrane-bound organelles responsible for protein synthesis (translation). They were first observed as dense particles by George Palade (1953).

🔍 Key Features

  • Composition: Made of rRNA + proteins.
  • Membrane: Absent (free particles).
  • Location:
  • Free in cytoplasm → synthesize cytosolic proteins.
  • Bound to RER → synthesize proteins for secretion/membranes.

🔑 Memory Line:

Ribosome = rRNA + proteins → protein synthesis.

🧱 Types of Ribosomes

Type Where Found Subunits
80S (Eukaryotic) Cytoplasm (free/RER-bound) 60S + 40S
70S (Prokaryotic) Prokaryotes 50S + 30S
70S (Organellar) Mitochondria & Chloroplasts 50S + 30S

🔑 Exam Tip:

70S ribosomes inside mitochondria & chloroplasts support endosymbiotic theory.

📐 ‘S’ — Svedberg Unit

  • Indicates sedimentation coefficient (density & shape), not size.

🔑 One-liner:

80S ≠ bigger than 70S (it’s a sedimentation value).

🧱 2. Cytoskeleton — Internal Framework of the Cell

The cytoskeleton is an interconnected network of protein filaments spread through the cytoplasm. It gives the cell shape, strength, and movement.

🧩 Components of Cytoskeleton

1️⃣ Microtubules — thick, hollow tubes (tubulin)
2️⃣ Microfilaments — thin actin filaments
3️⃣ Intermediate filaments — rope-like fibers

⚙️ Functions

  • Mechanical support & maintenance of cell shape
  • Motility (cell movement, muscle contraction)
  • Intracellular transport (vesicle movement)
  • Cell division (spindle formation)

🔑 Memory Line:

Cytoskeleton = shape + support + movement.

🌀 3. Cilia and 🚩 Flagella — Motile Cell Appendages

Cilia and flagella are hair-like extensions of the plasma membrane that help in movement.

🔍 Differences at a Glance

  • Cilia: Short, numerous; move fluid over cell surface (oar-like motion).
  • Flagella: Long, few; move the entire cell.

🔑 Key Point:

Eukaryotic flagella ≠ prokaryotic flagella (structurally different).

🧠 Structure of Eukaryotic Cilia/Flagella

🔵 Axoneme (Core Structure)
  • Microtubule arrangement: 9 + 2
  • 9 peripheral doublets
  • 2 central singlets
🔗 Supporting Structures
  • Central sheath: Surrounds central pair
  • Radial spokes: Connect central sheath to peripheral doublets (9 spokes)
  • Linkers: Connect adjacent peripheral doublets
📍 Basal Body
  • Anchors cilium/flagellum to the cell
  • Structurally similar to a centriole

🔑 Diagram Line to Remember:

Axoneme = 9+2 | Centriole/Basal body = 9+0.

⚙️ Function

  • Coordinated bending of microtubules causes movement.
  • Important in locomotion, feeding, and circulation of fluids.

🧠 One-Page Memory Capsule

  • Ribosomes: Protein synthesis; 80S (eukaryotes), 70S (prokaryotes & organelles)
  • Cytoskeleton: Shape, support, movement
  • Cilia: Short, many; move fluid
  • Flagella: Long, few; move cell
  • Axoneme: 9+2 arrangement
  • Basal body/Centriole: 9+0 arrangement

💡 Questions & Answers

Q1. What are ribosomes and why are they important?

A1. Ribosomes are small, non-membrane-bound particles made of RNA and proteins. They are important because they are the sites of protein synthesis.

🔑 Key Points:

  • rRNA + proteins
  • No membrane
  • Protein synthesis

Q2. Differentiate between 80S and 70S ribosomes.

A2. 80S ribosomes are found in the cytoplasm of eukaryotic cells, while 70S ribosomes are found in prokaryotes and inside mitochondria and chloroplasts.

🔑 Key Points:

  • 80S → 60S + 40S (eukaryotic cytoplasm)
  • 70S → 50S + 30S (prokaryotes & organelles)

Q3. What is the cytoskeleton? State its functions.

A3. The cytoskeleton is a network of protein filaments in the cytoplasm that maintains cell shape, provides support, and helps in movement.

🔑 Key Points:

  • Structural framework
  • Shape & strength
  • Motility and transport

Q4. What is the difference between cilia and flagella?

A4. Cilia are short and numerous and move fluid over the cell surface, while flagella are long and few and help in movement of the entire cell.

🔑 Key Points:

  • Cilia → short, many, fluid movement
  • Flagella → long, few, cell movement

Q5. Describe the 9+2 arrangement in cilia and flagella.

A5. The axoneme of cilia and flagella has nine peripheral microtubule doublets surrounding two central microtubules. This arrangement helps in their bending movement.

🔑 Key Points:

  • Axoneme structure
  • 9 peripheral doublets
  • 2 central singlets

Q6. How does the basal body differ from the axoneme?

A6. The basal body has a 9+0 arrangement of microtubules and anchors the cilium or flagellum, while the axoneme has a 9+2 arrangement and is responsible for movement.

🔑 Key Points:

  • Basal body → 9+0, anchorage
  • Axoneme → 9+2, movement

Q7. Compare microtubule arrangement in axoneme and centriole.

A7.

  • Axoneme (cilia/flagella): 9+2 arrangement — nine peripheral doublets and two central singlets.
  • Centriole/Basal body: 9+0 arrangement — nine peripheral triplets and no central microtubules.

🔑 Key Points:

  • Axoneme → movement
  • Centriole → anchorage & spindle formation

Q8. State two functions of centrioles.

A8. Centrioles help in forming basal bodies for cilia/flagella and organize the mitotic spindle during cell division.

🔑 Key Points:

  • Basal body formation
  • Spindle organization