Differentiate between cilia and flagella

Cilia and flagella are hair-like structures that extend from the surface of certain cells and play essential roles in movement and sensory functions.

Cilia:

  • Cilia are short, numerous projections found on the surface of some eukaryotic cells. They are typically used to move fluids, mucus, or particles across the surface of a tissue. For example, in the human respiratory system, cilia help move mucus out of the lungs, trapping dust and other particles. Cilia also play sensory roles, detecting changes in the environment around the cell.

Flagella:

  • Flagella are longer and fewer in number compared to cilia. They are primarily responsible for the locomotion of the entire cell. A common example is the flagellum of a sperm cell, which helps it swim toward an egg during reproduction. Flagella can also be found in certain single-celled organisms like bacteria and protozoa, where they enable the cell to move through liquid environments.

Structure of cilia and flagellumshowing the 9+2 arrangement of microtubules:

Cilia and flagella are hair-like structures found on the surfaces of eukaryotic cells, serving essential roles in movement and sensory functions. Both have a core structure called the axoneme, which has a 9+2 arrangement of microtubules. This means there are nine pairs of outer microtubules arranged in a circle around two single central microtubules.

Each of the nine outer pairs consists of an A microtubule (with 13 strands) and a B microtubule (with 10 strands). Attached to these microtubules are dynein arms, which act as motor proteins, allowing movement by sliding the microtubules against each other. This sliding action generates the bending motion that powers cilia and flagella.

Radial spokes connect the outer microtubules to the central pair, helping control and coordinate movement. At the base of the cilia and flagella is the basal body, which anchors the structure to the cell, containing nine triplet microtubules. This arrangement allows cilia to move fluids across surfaces and flagella to propel cells, like sperm.

Differentiate between cilia and flagella

Cilia and flagella share some structural similarities but they differ significantly in aspects such as length and number as well as movement type and occurrence.

Here's a detailed comparison based on several criteria.

Based on Structure

Cilia and flagella are both composed of microtubules arranged in a "9+2" structure in eukaryotic cells. This refers to nine pairs of microtubules forming a ring around two central microtubules. The motion of these structures is powered by motor proteins called dyneins that cause the microtubules to slide past one another.

However, in prokaryotic cells, the structure of flagella is different because prokaryotic flagella are composed of a protein called flagellin and lack the "9+2" arrangement, functioning more like a rigid rotating propeller. Cilia are absent in prokaryotes, which further distinguishes them from flagella in these organisms.

Based on Location

Cilia are predominantly found in eukaryotic cells, particularly in multicellular organisms. They line epithelial surfaces such as the respiratory tract or fallopian tubes in humans where they help move fluids and particles.

On the other hand flagella are found in both eukaryotic and prokaryotic cells. In eukaryotes, they are present on certain specialized cells such as sperm cells whereas in prokaryotes like bacteria, flagella are essential for motility.

Based on Length

Cilia are generally much shorter than flagella and typically measure between 5 – 10 micrometers in length. They are densely packed on the surface of the cell.

In contrast, flagella are longer and can range from 10 to 200 micrometers or more depending on the type of cell. Their greater length allows them to generate sufficient force to propel the entire cell.

Based on Number

One key distinction between cilia and flagella is the number present on a cell. Cilia are typically found in large numbers, sometimes numbering in the hundreds and covering the entire surface of the cell. 

On the other hand flagella are usually much fewer in number. Many cells only have one or two flagella although some organisms may have more depending on the species.

Based on Motion

Cilia move in a coordinated wave-like or back-and-forth manner. This motion is typically highly synchronized and allows for the movement of fluids or particles, and the creation of currents across the cell's surface.

In contrast, flagella move in a whip-like or undulating motion in eukaryotic cells such as sperm cells. In prokaryotes, flagella rotate like a propeller which allows bacteria to swim through their environment.

Based on Function

Cilia are primarily involved in moving fluids or particles along the surface of the cell. For example, in the human respiratory system, cilia help clear mucus and trapped particles out of the lungs. They are also involved in sensory functions such as detecting environmental signals.

On the other hand flagella are mainly responsible for locomotion. In single-celled organisms such as Euglena or Chlamydomonas, flagella allow the organism to swim through its environment. In multicellular organisms, flagella are vital for the movement of certain cells like sperm cells.

Based on Occurrence in Prokaryotes and Eukaryotes

Cilia are absent in prokaryotes and are only found in eukaryotes.

On the other hand, flagella are found in both types of cells although their structure differs.
  • In prokaryotes, flagella are made of flagellin and rotate to provide movement.
  • While in eukaryotes, they are more complex with microtubules and dynein-powered movement.

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SAQ

1 Fill in the blanks: 
a) The basic unit of microtubules is …………………

b) α tubulin occurs at.............end, and β tubulin is at ..............of microtubules

c) GTP-GTP tubulin cap stabilises and promotes the .......................

d) The plus end-directed motor protein of microtubules is ………….

e) The hydrolysis of GTP from β-tubulin causes …………. of microtubules.

f) The cellular function of γ-tubulins is. …………………….

g) Colchicine binds to ……………

Answers:
a) α β tubulin heterodimers
b) plus, minus
c) polymerisation of microtubules
d) kinesin
e) Depolymerisation 
f) nucleate the growth of MTs 
g) free tubulin

SAQ 2

SAQ 3 

TERMINAL QUESTIONS

6. Draw the labelled diagram of microtubules.




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