Types of Cell

Cells are known as the basic fundamental units of life on Earth. If any organism is broken apart as per the cellular structure, then the smallest independent components will be the cells. In short, cells are the smallest known particles responsible for performing all life's functions. Each living organism is either a single-cell organism or a multicellular organism consisting of various cells functioning simultaneously.

Therefore, it is necessary to know what the different types of cells are. Here, we are discussing cell types and their characteristics. Before discussing the different cell types, let us first briefly explore the definition and main functions of cells.

What is a Cell?

By definition, "a cell refers to the smallest known basic unit of life primarily responsible for all of life's processes".

In simple words, a cell is the biological, structural, and basic functional unit of life in all living beings or organisms. Moreover, cells tend to replicate themselves. That is the reason they are referred to as the building blocks of all living beings. The cells typically build up the structure to the body elements and convert the nutrients carried out by foods into energy.

All organisms have cells, either single cell (unicellular) or multiple cells (multicellular). They are complex and are found in different sizes and shapes. The number of cells usually varies from one organism to another. Mycoplasmas are by far the smallest type of cells. Biologist Robert Hooke first discovered the cells.

Functions of Cells

Some of the major functions of cells are discussed below:

Facilitate Growth Mitosis: The parent cells are sub-divided into the daughter cells in the mitosis process. By doing so, the cells multiply and facilitate further growth in organisms.
Provide Support and Structure: The basic structure of all organisms is made up of cells. The cell membrane and cell wall are the primary components in organisms that work and include support and structure. For instance, our skin is composed of using a large number of cells.
Allow Transport of Substances: Different nutrients are imported by the cells to perform chemical processes occurring within the cells. Additionally, the waste carried out by chemical processes is eliminated from the cells using the active and passive transport systems.
Energy Production: Cells use energy to perform various chemical processes. The energy is typically produced by the cells using photosynthesis in plants and respiration in other organisms.
Aids in Reproduction: Using the processes called mitosis and meiosis, the cells aid in reproduction. Mitosis is the asexual reproduction process primarily responsible for producing daughter cells from the parent cells. Besides, meiosis mainly plays a vital role in making daughter cells genetically different from the parent cells.

Classification of Cell

There are over trillions of cells, and each has a special structure and function. In particular, different cells produce different tissues in organisms. Every cell is equally important. We can generally classify cells under the following two broad categories:

Prokaryotic Cells
Eukaryotic Cells

Prokaryotic Cells

Prokaryotic cells are known as the smallest and earliest types of cells. They are single-celled microorganisms. Examples of prokaryotic include Bacteria and archaebacteria. Despite this, the photosynthetic prokaryotic includes cyanobacteria that help perform the photosynthesis process.

Prokaryotic cells consist of a single membrane and one or more layers of protection from the outer forces and environments. They usually have a plasma membrane made up using phospholipids, surrounded by a cell wall made up primarily using rigid sugar. In some prokaryotic, the cell wall is further enclosed by the thick 'capsule' made up using sugars. Since they have a single membrane, all metabolic functions performed by prokaryotic occur in the plasma membrane or cytosol.

Most of the prokaryotic cells also have cilia, tails, or other ways by which the cells control their movement. The prokaryotic cells can either be free-living or parasites.

Some of the major characteristics of the prokaryotic cells are listed below:

Prokaryotic don't contain a membrane-bound nucleus and membrane-bound organelles.
They don't have chloroplast, Golgi bodies, mitochondria, and lysosomes.
One of the essential constituents of eukaryotic chromosomes, the histone protein, is not found in prokaryotic.
In prokaryotic, the genetic material exists on a single chromosome.
The cell wall present in prokaryotic is composed of amino acids and carbohydrates.
Prokaryotic have a plasma membrane, which mainly acts as a mitochondrial membrane and carries respiratory enzymes.
Prokaryotic reproduce asexually due to binary fission. Besides, the sexual mode of reproduction consists of conjugation.

The following image displays a structural view of the prokaryotic cells:

It is generally a bacterial cell. This diagram displays that a true nucleus is absent, while flagellum is present. This eventually differentiates the prokaryotic cells from the eukaryotic cells.

Components of Prokaryotic Cells

The following are the components of the prokaryotic cells:

Plasma Membrane: It is an outer covering layer of phospholipid molecules. This outer covering helps to separate the inner components of the cell from the outside environment.
Cell Wall: It is defined as the outermost layer, helping cells to get certain shapes.
Capsule: In addition to the cell wall, a capsule is another outer protective layer covering bacterial cells. This layer mainly keeps the cell protected when engulfed. Additionally, it also supports retaining moisture and in the attachment of cells to nutrients or surfaces.
Mesosome: It is formed by an extension of the plasma membrane inside the cell wall. It is present in the form of tubules, vesicles, and lamellae.
Ribosomes: They are responsible for protein synthesis. The synthesis process of proteins occurs here.
Cytoplasm: It is a jelly-like material within the cell, which is composed of using cell organelles, salts, and enzymes. All the cell organelles are also suspended here.
DNA (Nucleoid): A nucleoid is an area within the cytoplasm in which genetic material is found.
Bacterial Flagellum:

It is important to note that some prokaryotic cells may have components that other prokaryotic cells lack.

Eukaryotic Cells

Eukaryotic cells are referred to as the modern types of known cell types. These cells create complex and large organisms that are mostly based on multicellular cells. Organisms such as humans, animals, plants, fungi, and protozoa all have eukaryotic cells, and these organisms are called Eukaryota. These cells can maintain multiple environments in a single cell, allowing them to perform different metabolic reactions. This eventually helps eukaryotic cells to grow larger than prokaryotic cells.

However, the eukaryotic cells are mainly characterized by the presence of a true nucleus. The nucleus refers to a membrane-enclosed organelle consisting of DNA organized in single or multiple linear molecules. These linear molecules are known as chromosomes. Since eukaryotic cells can contain more than one chromosome, they typically have large amounts of genetic information.

Within a multicellular organism's body, various genes within these chromosomes can be "on" and "off". This ultimately allows cells to have different traits and function differently within the same organism.

Some of the major characteristics of the eukaryotic cells are listed below:

Eukaryotic cells consist of a nucleus which is enclosed by the nuclear membrane.
Eukaryotic cells have a cell wall as the outermost layer.
These cells reproduce by the process termed mitosis.
Eukaryotic cells include mitochondria.
Almost all eukaryotic cells are structured as cytoskeletal.
Flagella and cilia act as locomotory organs in eukaryotic cells.
The nucleus present in eukaryotic cells includes single, linear DNA carrying all the genetic information.

The following image displays a structural view of the eukaryotic cells:

This diagram displays that there is a presence of a nucleus, along with the membrane-bound organelles.

Components of Eukaryotic Cells

The following are the components of the eukaryotic cells:

Nucleus: It involves a single linear DNA carrying related genetic information. The production of ribosomes also occurs here.
Endoplasmic Reticulum: It acts as a network consisting of small, tabular-shaped structures that typically help divide the cell surfaces into luminal and extra-luminal.
Cytoplasm: It contains a cytoskeleton. It also includes microtubules, microfilaments, and fiber to generate the cell's shape, stimulate cell-movements, and manage cell-organelles.
Cell Membrane: It mainly helps in separating other cells from the outer environment. It also contains some specific embedded proteins that take part in the exchange of substances during the cell's in-and-out.
DNA: DNA is organized into long structures, known as chromosomes.
Lysosome: It mainly helps in processing hydrolytic enzymes to digest lipids, proteins, carbohydrates, and nucleic acids in a body.
Ribosome: It is the main region in which protein synthesis occurs. Ribosomes are usually made of proteins and ribonucleic acids.
Mitochondria: It plays a vital role in the production of energy and further helps in cell-metabolism regulation.
Golgi Apparatus: It is composed using a flat disc-shaped structure known as cisternae. It mainly plays a vital role in glycolipids and glycoproteins formation.

It is important to note that some eukaryotic cells may have components that other eukaryotic cells lack.

Types of Cells in the Human Body

There are over 37 trillion cells in a human body. Additionally, many cells (over one million) die every second in the human body and are replaced by new cells. However, there is nothing to worry about it. It is the fundamental function of living organisms. Human is highly evolved multicellular organism and is created by the eukaryotic cells. Besides, plants, animals, and fungi are also created with eukaryotic cells.

The following are the most common cells found in a human body:

Stem Cells

Before a cell is specialized, it originates unspecified and is known as a stem cell. Stem cells are pluripotent, meaning they can take the form of any cell in the human body. These cells are typically the ancestors of each specialized cell in the body, even a basic cell or the complex neurons. The process of conversion of stem cells into other cells is known as cell differentiation. The cell differentiation process is usually managed by inner genetics and several outer factors, including chemical and physical contact with other cells. Furthermore, stem cells can replicate on their own for long time-periods.

Bone Cells

Bone cells, also called bones, are the mineralized connective tissue that usually takes part in the skeletal system. Bones are typically the combination of collagen matrix and calcium phosphate minerals. There are three main types of these cells in the human body: osteoclasts, osteoblasts, and osteocytes.

Osteoclasts: They are large and help regenerate and assimilate bones during healing.
Osteoblasts: They help regulate bone mineralization and generate osteoid. Osteoid is an organic material of the bone matrix that usually mineralizes to produce bone. Furthermore, osteoblasts mature to produce osteocytes.
Osteocytes: They incorporate in the process of bone formation and take part in maintaining calcium balance.

Other typical bone cells are the lining cells that first originate as osteoblasts before becoming lining cells. The lining cells are of flat structure, lining the bone surface, and are responsible for the release of calcium.

Blood Cells

The blood cells are essential for the human body, either when transporting oxygen within the body or fighting against infections. The bone marrow typically generates these cells. There are mainly three blood cells, such as red blood cells, white blood cells, and platelets.

Red Blood Cells: They help in determining blood type and take part in oxygen transportation.
White Blood Cells: They are usually termed immune system cells and are responsible for destroying pathogens and providing immunity.
Platelets: They support clot blood to prevent excessive blood loss when blood vessels are damaged or broken.

Muscle Cells

Muscle cells, also called myocytes, produce muscle tissue that helps in various functions such as body movement, support, and various internal functions. These cells are long and tabular. There are mainly three types of muscle cells, such as skeletal, cardiac, and smooth.

Skeletal Muscle Cells: They help connect bones to perform voluntary movements. Connective tissues cover skeletal muscle cells to protect and support muscle fiber bundles.
Cardiac Muscle Cells: They help produce involuntary muscle. Such types of muscle are found within the heart and do not usually need conscious efforts in operating.
Smooth Muscle Cells: They help produce involuntary muscle, but they are neither striated like skeletal muscle nor heart muscle. Such involuntary muscles mainly help to inline the cavities of the human body. Furthermore, they also create walls of various organs, including the kidneys, blood vessels, intestines, and airways of the lungs.

Fat Cells

Fat cells are commonly known as adipocytes. They are one of the primary components of adipose tissue. These cells consist of droplets of triglycerides (stored fat) that can help fulfill energy needs. When there is fat within the cells, the cells seem round and swollen. After the fat is consumed, the cells shrink accordingly. Fat cells also take part in hormone generation, blood clotting, blood pressure regulation, cell signaling, insulin sensitivity control, etc.

Nerve Cells

Nerve cells, also called neurons, are the fundamental units of the nervous system. These cells act as the communication system and transmit signals between the brain, spinal cord, and many other organs using the nerve impulses. Typically, they conclude two parts, such as the cell body and nerve processes. Here, the cell body (central body) contains the nucleus and other organelles, while the nerve processes (such as axons or dendrites) work like long fingers and extend from the cell body to transmit signals.