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Cilia are thin, hair-like structures present on the surface of the cell. They are tiny tube-like structures on the surface of all eukaryotic cells in most animals. They differ in size and number. They provide locomotion and defence for the cells. They can also send signals from and to the body regarding the surroundings. Understanding the structure of cilia will help us know the functions it performs. This article will clarify how the structure of cilia relates to its function.
Cilia Definition
Cilium or plural cilia refers to the Latin term tiny eyelash-like filaments. They are an important locomotory organ in protozoans of the phylum Ciliophora. This phylum has these cilia as its special feature. Even the organ tissues of higher grade vertebrates have cilia. Tissues have definite functions for the cilia filaments to perform. The nucleus must be present for cilia to function; thus, the cilia are only present in eukaryotes. Prokaryotes don’t have cilia; instead, they have pili as their locomotory organ. The structure of cilia works together to create a movement. Depending on the cell type, they also help in feeding and mechanoreception. There are two types of cilia, depending on their function and structural properties. They are as follows;
- Motile cilia
- Non-motile cilia
Motile cilia are active cilia filaments that undergo movements. These are the locomotory organs for eukaryotic cells. The filament moves in unison back and forth to create a repeated motion that propels the organism in the desired direction.
Non-motile cells are primary cilia or sensory cilia responsible for the sensory reception of the cell. They transfer signals from the outside to the cell and vice versa.
Structure of cilia
There are three main layers in the structure of cilia: the cilia membrane layer, matrix layer, and axoneme layer. The structure of cilia with diagrams and labelling will help to better understand these layers. The cilia membrane layer is the outer layer of the cell, protecting the inner axoneme and the cilia core layers. The axoneme layer is the most important layer with the microtubules and proteins. These components are the basis for the cilia to move.
The matrix layer makes up the watery medium of the cilia to have its movement.
The cilia have longitudinal structures called microtubules coated in a plasma membrane. Each microtubule is a hollow tube of protein molecules. Every cilium will have nine pairs of these microtubules, creating a peripheral ring. There are two microtubules in the centre. This structure of microtubule arrangement forms an axoneme. The arrangement is also referred to as a 9+2 array. These microtubules are further kept together by linking proteins. Dynein is a motor protein present between these nine microtubules.
Cilia attaches to the cell membrane at the basal body. This basal body has nine triplets of microtubules. The doublets of the cilia become triplets when the cell provides extra microtubules. The two microtubules in the centre of the cilia end at the basal body before the cilia enter the cell.
The dynein, being a motor protein, helps the cilia to move. The large protein molecules of dynein get activated and allow complex movements. Dynein crawls in the plasma membrane along the microtubules. It drags other nearby microtubule doublets to create a link of movements. Since the doublets have cross-linking protein attached to them, it limits movement, thus, making the cilia bend. The cilia move together back and forth to create movement. The microtubules explain the cilia’s movement, but it is simpler to understand the structure of cilia with diagrams and cross-sectional drawings of cilia.
Functions of cilia
Now that we know the structure of cilia, let’s know more about how the structure of cilia relates to its functions. Different cells have different functions in different animals. These cells also have different cilia patterns. The cilia differ in number and size in different cells. The reasons below help us understand how cilia’s structure relates to its functions.
- Cilia is a basic locomotory organ in the protozoans of phylum Ciliophora. The movement in cilia moves the organism about its environmental changes.
- The cilia move in a rhythmic pattern to avoid unwanted substances on the cell surface.
- The movable cilia on the human body help to regulate movement in the ovaries. It also helps in the nasal passage to avoid unwanted substances reaching the lungs.
- Non-motile cilia in animals help them to be aware of environmental changes and send signals. In humans, the kidney and eyeballs have these non-motile cilia for sending signals on urinating and photoreceptions.
- Celia also helps in cell cycle maintenance and development.
Conclusion
The movement in any organism is essential for its survival. Organisms move for food, protection, and safety from predators. The cilia are the basic unit for all such movements. At times, the non-motile cilia are also useful with their sensory receptors. The cilia structure is well composed to help it regulate all the functions. Studying the cilia will help us understand the pattern of movement in organisms. The motile and non-motile cilia are both useful for the living organism to perform its functions. Humans also have cilia in their body in the kidneys and nasal tract for sensory purposes.
