Eukaryotic Cells: Cytoskeleton, Cilia, Flagella, Centrosome and Centrioles - Ultrastructure and Function

Can you imagine the human body without its skeletal framework? No, right? It will be a wobbly mass of skin and muscles with no bones to support them.

What do you think provides a supportive framework to the cell? Yes, just like the human body, the cell has a cytoskeleton to support it.

The cytoskeleton is a complex mesh of microtubules, microfilaments and intermediate filaments in the cells of eukaryotes and prokaryotes which are made up of proteins.

Centrosomes, centrioles, and the locomotory structures like-cilia and flagella are made up of cytoskeletal elements. Centrosomes are found in animal cells only whereas locomotory structures cilia and flagella are found in plants and animals both.

Table of contents

• Cytoskeleton and its elements
• Cilia and flagella
• Centrosome and centrioles
• Practice problems 
• FAQs

Cytoskeleton and its elements

The cytoskeleton is the network of fibres found in eukaryotic cells, prokaryotic cells and archaea.

Cytoskeleton is not permanent like our skeleton because its components can disassemble and reform. Cytoskeleton is made up of three kinds of elements - microtubules, microfilaments and intermediate filaments.

Fig : Arrangement of the cytoskeletal filaments

Microfilaments

Microfilaments are thin, long and narrow cylindrical rods that are primarily composed of globular actin protein molecules. These filaments also consist of some filamentous myosin protein molecules, which in association with the actin filaments help in the contraction of microfilaments. Microfilaments are found in the cells of microvilli, epithelial cells, muscle fibres, etc. Pseudopodia and plasma membranes of fibroblasts also possess microfilaments. They are contractile and help in cell movement. A microfilament is a helical polymer of dumb-bell shaped monomeric subunits. The free actin molecules can polymerise to form microfilaments when needed or can disintegrate as per the cell’s requirement.

Fig: Microfilament

Microtubules

Microtubules are unbranched, hollow, tubules made of tubulin proteins. They are several micrometres in length with a lumen of diameter 15 nm and a total diameter of 25 nm.

The wall of the microtubules are 10 nm thick and made up of a linear series of 13 protofilaments. Each protofilament is composed of α-tubulin and β-tubulin molecules arranged alternatively in a helical fashion. Found in the cytoplasm of all eukaryotic cells but not found in Amoeba, slime moulds and prokaryotes. They increase or decrease in length by assembling or disassembling the tubulin monomers, respectively. They help in several cellular functions such as support, motility, conduction of impulses in nerve cells, formation of mitotic spindle, etc.

Fig: Microtubule

Intermediate filaments

These are nearly hollow, non-contractile, unbranched filaments.The filaments are composed of 8 sub fibrils composed of structural proteins such as keratin, vimentin, desmin. They are not directly involved in the cell movement and only contribute to mechanical strength, maintenance of cell shape, etc. The cytoplasm of plant cells is devoid of intermediate filaments but they occur prominently in the cell junctions and around the nucleus in animal cells.

Fig: Intermediate filaments

Functions of cytoskeleton

• Provides mechanical support to the cell
• Keeps cell organelles separate from each other
• Maintains the cell shape too.
• Helps in cell movement.

Cilia and Flagella 

In eukaryotes, cilia and flagella are the cell membrane extensions present on certain cells that help in locomotion. Prokaryotic bacteria also contain flagella and cilia but prokaryotic flagella is made up of flagellin protein whereas eukaryotic flagella is made up of tubulin protein.

Fig: Cilia and flagella

Difference between flagella and cilia

Structural organisation

Cilia and flagella are entirely covered by the plasma membrane which is extended out from the cell. They arise from centriole-like structures known as basal bodies. The core structure running through the centre of the cilia or flagella is called ‘axoneme’.

This axoneme contains a number of microtubules arranged parallel to the long axis and each other. The axoneme usually has nine doublets of radially arranged peripheral microtubules, and a pair (two) of centrally located microtubules. In eukaryotes the 9+2 arrangement of microtubules is found in both cilia and flagella. The two singlet microtubules present in the centre are connected to each other by double bridges and surrounded by a common central sheath. Central sheath is connected to the nine peripheral tubules by nine radial spokes.

The peripheral doublets are also interconnected by linkers. One of the sub fibres of each doublet of the peripheral microtubules have arms of dynein protein which can catalyse ATP hydrolysis to generate energy for movement.

 Fig : Structure of the axoneme of cilia and flagella

Function 

These organelles help in locomotion in sperms, protozoans, larvae of animals etc. They help in food capture in some protozoans. Cilia of the respiratory tract remove solid particles from the windpipe.

Centrosomes and centrioles

Centrioles can be seen in both light and electron microscopy. The centrosome is made up of two perpendicularly placed centrioles, linked together by interconnecting fibres. Centrioles are made up of a complex of proteins, producing microtubules. These are embedded in a matrix, known as pericentriolar material. Centrosomes are found in animal cells. Whereas centrioles can be found in plant cells also.

Structural organisation

Each centriole is made up of nine peripheral microtubule triplets. There is no microtubule in the centre thus the arrangement of microtubules is in the ‘9+0’ pattern.The rod shaped mass of proteins in the central part of the proximal region of the centrioles is known as hub. Hub is connected with tubules of the peripheral triplets by radial spokes made of protein.

Fig: Arrangement of microtubules in centriole

Functions

Centrioles form the basal bodies that give rise to cilia and flagella. spindle fibres which are organised by centrioles,give rise to spindle apparatus during cell division in animal cells.

Practice problems

Q1. Which of the following is the correct representation of arrangement of microtubules in centrioles

A. 9+2
b. 9+0
c. 2+9
d. 0+8

Solution: Each centriole is made up of nine peripheral microtubule triplets arranged in a ‘9+0’ pattern as there is no microtubule in the centre. The rod shaped mass of proteins in the central part of the proximal region of the centrioles is known as hub. Hub is connected with tubules of the peripheral triplets by radial spokes made of protein.

Thus, the correct option is b.

Q2. Which of the following are found only in animal cells?

(a) intermediate filaments
(b) microtubules
(c) nucleus
(d) microfilaments

Solution: Of the three cytoskeletal elements, intermediate filaments are only found in the animals. Intermediate filaments are nearly hollow, non-contractile, unbranched filaments.The filaments are composed of 8 sub fibrils composed of structural proteins such as keratin, vimentin, desmin. They are not directly involved in the cell movement and only contribute to mechanical strength, maintenance of cell shape, etc.

Thus, the correct option is a. 

Q3. Outer microtubules in a cilium are generated from ______________

(a) basal body
(b) centromere
(c) centrosome
(d) nucleus

Solution: The outer microtubules in cilia are generated from the microtubules called a basal body. The basal body is present at the base of the cilium. These are the centriole-like structures.

Thus, the correct option is a.

Q4. Which of the following proteins is a structural component of a microfilament?

(a) myosin
(b) kinesin
(c) globulin
(d) vimentin

Solution : Microfilaments are thin, long and narrow cylindrical rods that are primarily composed of globular actin protein molecules. These filaments also consist of some filamentous myosin protein molecules, which in association with the actin filaments help in the contraction of microfilaments.

Thus, the correct option is a.

FAQs

Question 1. What will be the result if the cytoskeleton stops working?
Solution: The absence of a cytoskeleton in the cell will result in cell deformity as the cytoskeleton is largely responsible for maintaining the shape and integrity of the cell. The cell will also lose its ability to move and undergo cell division in the absence of the cytoskeletal elements.

Question 2. What does colchicine do to the cytoskeleton?
Solution: Colchicine binds to tubulin molecules to form colchicine-tubulin complexes which prevent the formation of tubulin microtubules.

Question 3. How does cytochalasin B affect actin?
Solution: Cytochalasin B is a mycotoxin which inhibits cell movement by preventing the formation of actin filaments. 

Question 4. What mechanism is responsible for ciliary and flagellar movement?
Solution: Afzelius (1959) proposed that cilia and flagella are capable of showing movements due to the sliding of the doublets past one another. For this to occur, energy in the form of ATP is required along with Mg2+ and Ca2+ ions. Excitation for bending is transmitted by the central fibrils to the peripheral fibrils through the central sheath and radial spokes. With the energy obtained from ATP, heads of dynein arms make connections with the sub fibres B of adjacent fibrils in such a way as to make doublet fibrils of one side slide past the fibrils of the other side.

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