Ribosomes, Cytoskeleton, Cilia and Flagella Class 11 Notes (NCERT + Diagram + MCQ + NEET PYQ)

Ribosomes, Cytoskeleton, Cilia and Flagella are important components of eukaryotic cells. They help in protein synthesis, maintaining cell shape, and cell movement. These Class 11 Biology notes are based on NCERT and are useful for NEET preparation.

 

1. Ribosomes: The Protein Factories

Ribosomes
"George Palade first observed these dense particles in 1953."

• Discovery: First observed by George Palade (1953) as dense particles under the electron microscope.

• Composition: They are composed of ribonucleic acid (RNA) and proteins. They are not surrounded by any membrane.

• Types in Eukaryotes:

  • 80S Ribosomes: Found in the cytoplasm. (S = Svedberg’s Unit, which stands for sedimentation coefficient; it is a measure of density and size).

  • 70S Ribosomes: Found in mitochondria and plastids (just like prokaryotes!).

• Structure: Each ribosome has two subunits—large and small.

  • 80S has 60S and 40S subunits.

  • 70S has 50S and 30S subunits.

Functions Summary (Important for NEET)

• Ribosomes → Protein synthesis
• Cytoskeleton → Cell shape and support
• Cilia → Movement of fluid
• Flagella → Cell movement

2. Cytoskeleton

Cytoskeleton
"Functions of Cytoskeleton: 1. Mechanical Support, 2. Motility, 3. Maintenance of Cell Shape."

It is an elaborate network of filamentous proteinaceous structures present in the cytoplasm.

• Functions: Mechanical support, motility, and maintenance of the shape of the cell.

3. Cilia and Flagella

• They are hair-like outgrowths of the cell membrane.

• Cilia – The Oars of the Cell

• 1. Definition & Nature

  • Cilia (singular: cilium) are small, hair-like outgrowths of the cell membrane.

  • They work like oars, causing the movement of either the cell or the surrounding fluid.

  2. Internal Structure (The Axoneme)

  • Cilia are covered with a plasma membrane.

  • Their core, called the axoneme, contains a number of microtubules running parallel to the long axis.

  • The 9+2 Arrangement: The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules.

  • The central tubules are connected by bridges and enclosed by a central sheath.

  3. Connection & Movement

  • The central sheath is connected to one of the tubules of each peripheral doublet by a radial spoke. Thus, there are nine radial spokes.

  • The peripheral doublets are also interconnected by linkers.

  • Origin: Both cilia and flagella emerge from centriole-like structures called the Basal Bodies.

  

  Cilia
  
  

  
  

• Flagella: Comparatively longer and responsible for cell movement.

  

  Flagella

  
  

• Core Structure: Both are covered with a plasma membrane. Their core is called the axoneme, which possesses a 9+2 array of microtubules.

Cilia vs. Flagella – At a Glance

 

Feature	Cilia	Flagella
Size	Small, hair-like structures.	Comparatively much longer.
Number	Usually present in large numbers per cell.	Fewer in number (often only 1 to 4).
Function	Work like oars, causing movement of the cell or the surrounding fluid.	Primarily responsible for cell movement (locomotion).
Action	Show coordinated movement (beat together).	Move independently.

Conclusion

Ribosomes, cytoskeleton, cilia and flagella are essential for cell structure, movement and protein synthesis. These concepts are very important for NEET and board exams. 

Practice Quiz: Eukaryotic Cells Part 5

Q1. Ribosomes were first observed under an electron microscope by:

A) Robert Brown

B) George Palade

C) Camillo Golgi

D) Rudolf Virchow

Q2. An 80S eukaryote ribosome is composed of which two subunits?

A) 60S and 40S

B) 50S and 30S

C) 60S and 20S

D) 40S and 40S

Q3. Which of the following organelles is NOT surrounded by any membrane?

A) Lysosome

B) Vacuole

C) Ribosome

D) Chloroplast

Q4. The central core of cilia and flagella, which possesses a 9+2 array of microtubules, is called:

A) Axoneme

B) Basal body

C) Centromere

D) Cytoskeleton

Q5. The function of the cytoskeleton includes:

A) Mechanical support

B) Motility

C) Maintenance of the shape of the cell

D) All of the above

Q6. Both cilia and flagella emerge from centriole-like structures called:

A) Radial spokes

B) Basal bodies

C) Central sheath

D) Doublets

Q7. In the 9+2 arrangement of the axoneme, '9' refers to:

A) Central microtubules

B) Peripheral doublets

C) Radial spokes

D) Central sheath bridges

Q8. 70S ribosomes are found in which of the following eukaryotic organelles?

A) Mitochondria

B) Chloroplasts

C) Both A and B

D) Nucleus

Q9. The central microtubules of the axoneme are connected by:

A) Radial spokes

B) Bridges

C) Doublets

D) Linkers

Q10. Cilia differ from flagella in that cilia are:

A) Longer and fewer

B) Smaller and work like oars

C) Only found in prokaryotes

D) Responsible for protein synthesis

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Answer Key & Explanations :

Q.No	Answer	Quick Explanation
1	B	George Palade discovered them in 1953.
2	A	80S = 60S + 40S (S = Svedberg Unit).
3	C	Ribosomes are non-membranous particles.
4	A	Axoneme is the core containing microtubules.
5	D	It provides support, shape, and movement.
6	B	Basal bodies are the foundation of cilia/flagella.
7	B	9 pairs of peripheral doublets.
8	C	Organellar ribosomes are 70S (Endosymbiotic theory).
9	B	Central tubules are connected by bridges.
10	B	Cilia are shorter and show coordinated oar-like movement.

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