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Q13(ii):
Describe the structure of the following with the help of labelled diagrams.
(ii) Centrosome
Solution :
1. Theoretical Foundation of the Centrosome
The centrosome is a non-membrane-bound organelle, primarily found in animal cells, acting as the principal Microtubule Organizing Center (MTOC). It regulates the cell cycle progression and the assembly of the mitotic spindle during cell division. [Per Cell Biology principles, centrosomes are composed of two orthogonal centrioles surrounded by a protein-dense matrix known as the pericentriolar material (PCM)].
2. Structural Anatomy of the Centrosome
The centrosome consists of two primary components:
- Centrioles: A pair of cylindrical structures arranged at right angles to each other. Each centriole exhibits a distinct "cartwheel" organization.
- Pericentriolar Material (PCM): An amorphous, electron-dense matrix containing proteins such as $\gamma$-tubulin ring complexes, which serve as nucleation sites for microtubule polymerization.
3. The Ultrastructure of the Centriole
The centriole possesses a $9+0$ arrangement of microtubules. This implies:
- Nine sets of peripheral microtubule triplets arranged in a circle.
- Each triplet consists of three microtubules designated as $A$, $B$, and $C$ (where $A$ is the innermost).
- The triplets are linked to a central hub (the hub) via radial spokes.
- There are no central microtubules present in the center, hence the $9+0$ designation. [By Structural Biology nomenclature].
4. Schematic Representation of the Centriole
5. Functional Summary Table
| Component | Composition | Function |
|---|---|---|
| Centriole | Tubulin protein microtubules | Provides structural support for spindle fibers |
| PCM | $\gamma$-tubulin, pericentrin | Nucleates microtubule growth |
| Radial Spoke | Protein filaments | Connects triplets to the hub |
Final Solution: The centrosome is an organelle composed of two centrioles positioned at right angles to each other, embedded within a pericentriolar matrix. Each centriole is a barrel-shaped structure organized in a $9+0$ microtubule triplet arrangement, functioning essentially as the cell's microtubule organizing center to facilitate cytokinesis and mitosis.
More Questions from Class 11 Biology Cell – The Unit of Life EXERCISES
- Q1: Which of the following is not correct?
- Q2: New cells generate from
- Q3: Match the following Column I Column II (a) Cristae (i) Flat membranous sacs in stroma (b) Cisternae (ii) Infoldings in mitochondria (c) Thylakoids (iii) Disc-shaped sacs in Golgi apparatus
- Q4: Which of the following is correct:
- Q5: What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
- Q6: How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
- Q7: Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
- Q8: What are the characteristics of prokaryotic cells?
- Q9: Multicellular organisms have division of labour. Explain.
- Q10: Cell is the basic unit of life. Discuss in brief.
- Q11: What are nuclear pores? State their function.
- Q12: Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
- Q13(i): Describe the structure of the following with the help of labelled diagrams. (i) Nucleus
- Q14: What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.
CBSE Solutions for Class 11 Biology Cell – The Unit of Life
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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