Lesson #5: Centrioles, Cilia, and Flagella

Centriole

What is a Centriole?

Centrioles are barrel-shaped organelles found in the cells of animals and most protists (unicellular organisms like Amoeba and Paramecium). They are absent in prokaryotes (bacteria), higher plants, and fungi.

Structure of Centrioles

• A centriole pair consists of two centrioles arranged at right angles to each other.
• In animal cells, this pair is called a centrosome and is located near the nucleus.
• Each centriole is made up of 9 triplets of microtubules (tiny hollow tubes) arranged in a circular pattern. These microtubules are made of a protein called tubulin.

💡 What are Microtubules?
Microtubules are thin, tube-like structures that form the skeleton of the cell (cytoskeleton) and help in cell division and movement.

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Functions of Centrioles

1. Role in Cell Division:
   • Before cell division begins, the centriole pair duplicates (makes a copy of itself).
   • The new pairs move to opposite sides (poles) of the cell.
   • They help in the formation of spindle fibers, which are needed for chromosome separation during mitosis and meiosis.

🔹 Example: During mitosis (cell division in animals), spindle fibers pull the chromosomes apart to form two new cells.

2. Formation of Cilia and Flagella:
   • In cells that have cilia or flagella, special centrioles called basal bodies are present near the cell membrane.
   • Basal bodies are responsible for the growth and function of cilia and flagella.

🔹 Example: The basal body in sperm cells forms the flagellum, which helps in movement.

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Cilia and Flagella

What are Cilia and Flagella?

Cilia and flagella are hair-like projections that help cells move or transport substances.

• Cilia (Singular: Cilium) → Short, numerous, and hair-like.
• Flagella (Singular: Flagellum) → Longer, fewer in number, and whip-like.

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Structure of Eukaryotic Cilia and Flagella

Cilia and flagella in eukaryotic cells (cells with a nucleus) have a “9 + 2 arrangement” of microtubules:

• Nine pairs of microtubules surround a single central pair of microtubules.
• These microtubules are connected to a basal body (a modified centriole).

💡 What is a Basal Body?
A basal body is a centriole located at the base of cilia or flagella. It controls their movement.

🔹 Example: The cilia in the human respiratory tract beat in a coordinated way to remove dust and mucus from the lungs.

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Functions of Cilia and Flagella

1. Movement (Locomotion)
   • Single-celled organisms use cilia and flagella to move.
   • Flagella move in a whip-like motion, while cilia move in a coordinated wave-like motion.

🔹 Example:

1. Euglena (a protist) moves using a flagellum.
2. Paramecium moves using cilia.
3. Transport of Substances
   • In multicellular organisms, cilia help transport substances across cell surfaces.

🔹 Example:

2. The cilia in our respiratory tract push mucus and dust particles out of the lungs.
3. In the female reproductive system, cilia in the fallopian tubes help move the egg toward the uterus.

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Prokaryotic Flagella vs. Eukaryotic Flagella

Feature	Eukaryotic Flagella	Prokaryotic Flagella
Made of	Microtubules (tubulin protein)	Flagellin protein
Arrangement	“9 + 2” microtubule structure	Simple, no microtubules
Type of movement	Whip-like motion	Rotational motion
Example	Sperm cells, Euglena	Bacteria (e.g., E. coli)

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Key Takeaways

✔ Centrioles are barrel-shaped organelles that help in cell division and in forming cilia and flagella.
✔ Centrosomes (pairs of centrioles) organize spindle fibers for chromosome separation.
✔ Basal bodies are modified centrioles that give rise to cilia and flagella.
✔ Cilia and flagella help in movement and transport of substances.
✔ Prokaryotic flagella (bacteria) and eukaryotic flagella (animals, protists) have different structures and movement patterns.