Centriole – Definition, Function, Structure Of Plant/Animal Cells

What is a centriole? Are you looking to learn more, boost your understanding, and enhance your knowledge of microbiology?

In short, centrioles are tiny structures found at the center of cell nuclei. They play a role in mitosis or cell division. 

We took a closer look to discover all that you need to know about centriole, as well as the function and structure in both plant and animal cells.

What Is A Centriole?

A centriole is a small cylindrical organelle with nine microtubules (MTs) radiating from its surface.

It is located near the middle of the nucleus in eukaryotic cells and plays an important role in cell division by organizing spindle fibers during mitosis and meiosis. 

The structure of a centriole has been well studied in animal cells. In plant cells, however, it was not until recently that the detailed structure of this organelle has begun to be elucidated. 

Centrioles are usually associated with microtubules (the protein fibers that form the backbone of the cytoskeleton), and they play a role during mitosis or cell reproduction.

Microtubules are important because they provide structural support for the cell and allow for the movement of materials within the cell.

Structure Of Plant Cells

Before we take a closer look at the purpose and function of centrioles in plant cells, it is important to establish the basic structure of these.

Plant cells are much simpler than animal cells, and contain a single round nucleus surrounded by a thin layer of cytoplasm.

Cytoskeleton

Plant cells have a structure called the cytoskeleton. The cytoskeleton consists of three different kinds of fibers: actin filaments, intermediate filaments, and microtubules. Actin filaments are thin threads that make up most of the cytoplasmic skeleton. 

Intermediate filaments are bundles of strings of protein that connect the nucleus with other parts of the cell. Finally, microtubules are long, cylindrical structures that consist mostly of tubulin.

Actin Filament

Actin filaments are very flexible and bend easily. They play an important role in organizing the shape of the cell. They also help move organelles around inside the cell.

Intermediate Filament

Intermediate filaments are much stiffer than actin filaments. They provide structural support for the cell. They’re often used to create a scaffold for proteins like vimentin or keratin.

Microtubule

The microtubules are usually arranged in parallel arrays. They’re responsible for transporting cargo throughout the cell.

Nucleus

The nucleus is located at the center of the cell. Inside the nucleus is a mass of DNA. Ribosomes and other structures are found inside the nucleus.

Cytoplasm

The cytoplasm is the fluid-like substance surrounding the nucleus. It contains all the organelles and other components of the cell.

Cell Membrane

A membrane surrounds each individual cell. Cell membranes are composed of lipids (fatty acids) and proteins.

Lipids are the main component of the plasma membrane. Proteins are essential for maintaining the integrity of the membrane.

Organelles

Each plant cell has many different organelles. Some of these include:

Chloroplast

A chloroplast is a specialized organelle where photosynthesis takes place. Chloroplasts are green because they contain large amounts of chlorophyll.

Mitochondria

Mitochondria are energy factories. They produce ATP, which is a molecule that provides energy to the cell.

Vacuoles

Vacuoles store water and nutrients. They are filled with liquid.

Endoplasmic Reticulum

The endoplasmic reticulum is a network of tubes within the cytoplasm. This part of the cell processes materials before they are sent out of the cell.

Plant Cell Structures And Function Of Centrioles

In plants, centrioles have a different purpose than they do in animals. In plants, centrioles are called “centrosomes” and they act like little mini-nuclei. 

The centrosome contains two centrioles that are surrounded by pericentriolar material (PCM). 

PCM is made up of proteins and other substances that help organize the centrioles into functional units.

The centrosome also has an outer wall, which helps it maintain its shape and size. 

This wall is composed of a layer of the plasma membrane, followed by a layer of cytoplasmic matrix, then another layer of the plasma membrane. 

At the very top of this wall is a layer of electron-dense material called the crystalloid.

The centrosome acts as a kind of factory where the cell’s chromosomes are organized and replicated. It also plays a role in organizing the microtubule network. 

During mitosis, when the cell divides, the centrosome duplicates itself and each new copy becomes the parent centrosome for one daughter cell.

The centrosomes also contain many small vesicles called centriolar satellites. These vesicles contain proteins that help regulate the activity of the centrosomes.

Structure Of Animal Cells

The structure of animal cells is very similar to plant cells. However, there are some differences, and these include:

Nucleus

Animal cells have a single nucleus, whereas plant cells have multiple nuclei.

Plasma Membrane

The plasma membrane of animal cells is much thicker than the plasma membrane of plant cells.

Endoplasmic Reticulum

The endoplasmic reticulum of animal cells is larger than the endoplasmic reticular system of plants.

Mitochondria

Mitochondria are more abundant in animal cells than in plant cells.

Cytoskeleton

The cytoskeleton of animal cells is made up of actin filaments, microtubules, intermediate filaments, and other proteins.

Animal Cell Structures And Function Of Centrioles

Although centrioles don’t exist in every type of animal cell, they’re still present in some types of cells. 

For example, centrioles can be found in nerve cells, muscle cells, and epithelial cells. In these cells, the centrioles are known as basal bodies. 

However, unlike the centrosomes in plants, the centrioles in animals are not surrounded by a wall. Instead, they sit on a single piece of membrane, called the plasma membrane.

In animal cells, centrioles are cylindrical bodies with nine triplets of microtubule roots (nine triplets of 9+2 microtubule organization). These microtubule roots form the cartwheel structure. 

A central pair of microtubules is attached at one end to the centriole and extends outwards at right angles to the axis of the centriole. 

At the opposite end of the centriole, another microtubule root attaches to the centriole. This second microtubule root forms an antiparallel arrangement with the first microtubule root.

The nine microtubule roots are arranged around the circumference of the centriole like spokes on a wheel. 

The ends of the microtubules are capped with protein complexes called distal and subdistal appendages. The distal appendage is located near the center of the centriole and contains nine singlet microtubules. 

The subdistal appendage is found near the periphery of the centriole where it has two singlet microtubules extending towards the centrosome. 

Both appendages attach to the pericentriolar material (PCM) of the centriole via their respective singlet microtubules.

The PCM is a ring-shaped layer of cytoplasm surrounding the centriole. It is composed of a dense core containing tubulin and other proteins. 

The PCM also contains many signaling molecules including kinases, phosphatases, GTPase activating proteins, and scaffolding proteins. The PCM is important for recruiting proteins to the centrosome.

In both plants and animals, the centriole serves as a template for forming cilia. Cilia are hair-like projections from the surface of the cell that move fluids or particles within the body. 

In some organisms, cilia have been shown to play roles in development and embryogenesis.

Cilia are formed when the centriole grows into a tube that extends through the cell. The growing tip of the cilium nucleates the formation of new microtubules. 

As the cilium elongates, the distal and subdistal appendages become incorporated into the ciliary axoneme. Eventually, the distal and subtotal appendages are lost.

What Is The Function Of The Centriole In Plants And Animals?

Centrioles serve several functions in both plant and animal cells. One of their main roles is to duplicate themselves during cell division. 

Another important function is to organize the microtubules so that they can carry out specific tasks.

During Mitosis

During mitosis, centrioles duplicate themselves and become the basis for the formation of two new centrosomes. 

Each centrosome will eventually give rise to two separate sets of microtubules.

These microtubules will ultimately form spindles that pull apart the chromosomes of the mother cell and distribute them equally between the two daughter cells.

Regulating Microtubules

Microtubules are long, hollow tubes that run through the interior of the cell. They are made from tubulin protein molecules. 

Tubulin is a major component of the centriole walls. When microtubules grow, they push against the inner surface of the centriole wall. This causes the centriole to expand and contract. 

As the centriole expands, it pulls in more tubulin molecules, causing the microtubules to lengthen.

When the centriole contracts, the opposite happens: fewer tubulin molecules get pulled into the centriole, causing the microtubule to shorten. This process allows the cell to control how far the microtubules extend.

What Role Does The Centriole Play In The Cell?

The centrioles are important structures because they help form the primary cilia.

Primary cilia are found in almost every vertebrate cell and play a role in many different cellular processes. 

They also serve as sensory organs. For example, they detect water currents, light, chemicals, and gravity.

Primary cilia are made up of nine double-membraned tubes called basal bodies. Basal bodies are located at the base of the cell and are surrounded by a ring of microtubules.

These microtubules are used to transport materials from one part of the cell to another.

Primary cilia are present in most types of cells except red blood cells. Red blood cells do not have any organelles or membranes. 

Instead, they are formed after the RBCs leave the bone marrow. They contain hemoglobin, which carries oxygen throughout the body.

How Do Primary Cilia Form?

To make a primary cilium, the centriole must first be modified. A small piece of a protein called pericentrin is added to the centriole. Pericentrin helps organize the microtubule fibers around the centriole.

Pericentrin is then removed from the centriole, leaving behind a hole.

Microtubules begin growing through the hole and eventually form the primary cilium.

When the primary cilium grows, the centriole becomes elongated. If the centriole is too long, the cell will stop dividing.

Therefore, it is important for the centriole to remain short enough so that the cell can continue dividing.

The Cell Cycle

In order to divide, cells need to go through a series of steps. First, they must replicate the DNA within the nucleus.

Next, they must separate the replicated chromosomes into two groups. 

Then, each group needs to split itself into two copies. After this occurs, the cells must reassemble all of the components back together.

DNA Replication

In order to duplicate its DNA, a cell first has to break down the original DNA strand into smaller pieces. It then uses enzymes to copy these pieces over again.

Separation Of Chromosomes

After DNA replication, the chromosomes are separated by pulling one end away from the other. This creates two identical halves of the chromosome.

Segmentation

After separation, the two halves of the chromosome are held together by a special type of fiber called chromatids.

At the same time, the cell begins to organize the newly formed nuclei into distinct compartments. These compartments will eventually become the new cells.

Reassembly

Finally, the two halves of each chromosome are put back together.

This is done by using enzymes to attach the ends of the chromosomes to the spindle fibers. Once the chromosomes are attached, the cell can begin dividing.

The Cell Cycle And Centriole Formation

The cell cycle is the series of events that occur when a cell grows and reproduces itself.

A cell goes through four phases: G1 phase, S phase, G2 phase, and M phase. This is known as the eukaryotic cell division cycle.

Growth Phase (G1 Phase)

In the G1 phase, the cell prepares itself for growth, mitosis, and reproduction. In order to grow, the cell must replicate its DNA. To do this, the cell makes copies of all of its DNA

Preparation For Mitosis (S)

When the cell reaches a certain size, it enters the S phase. During this phase, the cell duplicates its entire genome.

Mitosis (M)

After duplication, the cell separates the chromosomes into two sets. Each set is now half as large as before.

One set remains in the middle while the other moves toward the poles of the cell. When both sets reach their destination, the cell divides into 2 identical daughter cells.

Cytokinesis

Once the cell has divided, cytokinesis begins. Cytokinesis is the process where the cell splits into two. It involves the formation of an intercellular bridge between the two daughter cells. 

The intercellular bridge contains bundles of microtubules that pull on the plasma membrane.

Eventually, the plasma membrane breaks open along the line of the bridge. This allows the cytoplasm to flow out of the cell.

Key Elements And Terms

There are a number of elements that you will need to consider when learning about and understanding centriole specifically, and plant and animal cells more generally. these include:

Cytoplasm

The cytoplasm is the fluid inside the cell. Cytoplasm surrounds all the organelles and holds them together.

It is filled with ribosomes, which are tiny factories where proteins are synthesized. Ribosomes are needed for growth and repair.

The cytoplasm is divided into two regions: the cytosol and the endoplasmic reticulum (ER).

The ER is an area where proteins are folded and stored until they are ready to be transported to other areas of the cell. The cytosol is the space between the cell walls and the plasma membrane.

Microtubules

Microtubules are made up of tubulin molecules. Tubulin is a protein found in both animal cells and plant cells.

Microtubules help transport materials within the cell. They also help hold the chromosomes during mitosis.

Mitosis

During mitosis, the cell duplicates its DNA. Each new copy of the DNA is contained in a separate structure called a chromosome. During this process, the cell divides into two identical daughter cells.

Each daughter cell receives half of each chromosome. When the chromosomes are 

separated, the sister chromatids move apart. One chromatid moves toward the spindle pole while the other moves away.

Once the chromatids reach their respective poles, the cell begins to divide.

Chromosome

A chromosome is a long thread-like body containing DNA. Chromosomes are packaged into structures called nuclei. Nuclei contain most of the genetic material of the cell.

Nuclear Envelope

The nuclear envelope is a double layer of membranes surrounding the nucleus. It helps protect the nucleus from damage by chemicals or radiation.

Plasma Membrane

The plasma membrane is a thin sheet of phospholipids that surround the outside of the cell. Plasma membranes can vary in thickness depending on what part of the cell it covers.

Cell Wall

The cell wall is a tough outer shell that protects the cell from attack by bacteria and viruses. Cell walls have pores through which nutrients pass into the cell.

Organelles

An organelle is a specialized region of the cell. Organelles perform specific functions such as manufacturing enzymes, making energy, storing food, and producing hormones.

Endomembranes

Endomembranes are similar to the plasma membrane but smaller. They are enclosed by a single lipid bilayer.

Golgi Apparatus

The Golgi apparatus is a network of tubes that run throughout the cell. The Golgi apparatus sorts and packages proteins so they can be used by different parts of the cell.

Ribosomes

Ribosomes are small factories that make proteins. Ribosomes are present in every cell except red blood cells. Ribosomes are made up of RNA and proteins.

Cytoskeleton

The cytoskeleton is a complex system of filaments and fibers that provides support for the cell. Some examples include microtubules, actin filaments, intermediate filaments, and septate junctions.

Septate Junctions

Septate junctions are specialized connections between adjacent cells. These junctions allow ions and water to flow across the cell membrane.

Final Thoughts

The centrioles are a crucial element of any cell, and play a key role in a number of other processes and functions that are essential not only to the health of the cell but to the wider organism. 

By learning the basic terms, mastering cell division, and understanding the basic structure and differences between plant and animal cells, you will be able to build a strong foundation in microbiology.

Also, ensure that you have the confidence to pursue your learning and research, taking it to the next level.

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