Eukaryotic Cell and Eukaryotic Cell Structure

Eukaryotic cells are those with a well-defined nucleus (true nucleus) and membrane-bound organelles. Organisms with eukaryotic cell structure are called eukaryotes, are typically multicellular, consisting of many cell types that form different tissues, such as those found in animals and plants.

Table of Contents

Characteristics of Eukaryotic Cell

Unlike prokaryotic cell, eukaryotic cell is consist of true nucleus and is surrounded by a double membrane layer.
Nuclear DNA is linear and is associated with proteins, while extra nuclear DNA is present without proteins.
Cytoplasm consist of organelles like centrosomes, endoplasmic reticulum, mitochondria, Golgi apparatus, microfilaments, intermediate filaments, microtubules and micro bodies.
Eukaryotic cells exist in protists, fungi, plants, and animals.
In multicellular organisms, cell numbers generally scale with body size. For instance, human blood has approximately 30 quadrillion (3 × 10¹⁵) blood cells, and a 60 kg human has around 60 × 10¹⁵ cells overall.
Cells can take on various shapes, including spherical, cuboidal, oval, disc-like, polygonal, columnar, spindle-shaped, or irregular.
These shapes vary not only between different organisms but also among the tissues within the same organism.
A cell’s shape is often closely related to its function, as seen in muscle and nerve cells, which have shapes suited to their specific roles.
Most eukaryotic cells are microscopic, generally ranging from 10 to 100 µm in size.
Some of the smallest cells include the sporozoites of the malaria parasite Plasmodium vivax, measuring about 2 µm long, while one of the largest is the ostrich egg at 175 × 120 mm.
Nerve cells are the longest, with fibers that can reach several meters, and typical human cells range from 20 to 30 µm.
In eukaryotic cell, the process of meiosis, gamete formation and true fertilization occur in most cases of sexual reproduction.

Components of Eukaryotic Cell – Eukaryotic Cell Structure

Eukaryotic cells are primarily composed of three main parts: the cell membrane, cytoplasm, and nucleus. Both the cytoplasm and nucleus contain additional components. The various cell structures are detailed below:

Cell membrane
Cytoplasm
Nucleus

Plant cell - eukaryotic cell structure — Fig: Eukaryotic cell, Plant cell [Image by LadyofHats, Wikipedia / Public Domain]

A. Cell Membrane/Cell Wall

The cell membrane, also known as the plasma membrane or plasmalemma, is a thin, flexible, living layer that encloses the cell, maintains its contents, provides shape, and regulates material movement across it.
It consists of a lipid-protein complex and lacks respiratory enzymes. In many protists and animal cells, it enables endocytosis and exocytosis.

In some protists, most fungi, and all plant cells, a thick, rigid, non-living cell wall surrounds the cell membrane, providing protection and support.
In prokaryotes, the cell wall surrounding the plasma membrane differs structurally from that in eukaryotes.

B. Cytoplasm

The cytoplasm, or cytosome, is a semi-fluid, uniform, and translucent substance called the cytoplasmic matrix or cytosol, located between the cell membrane and the nucleus.
In protozoan cells, the outer, firmer layer of cytoplasm is known as the ectoplasm, while the inner layer surrounding the central fluid area is called the endoplasm.
The cytosol exhibits “cyclosis,” or streaming movement.
The eukaryotic cytoplasm includes the following characteristic components:

Cell Organelles
Cell Inclusions

1. Cell Organelles

Organized structures within the cell that perform specific functions and, in some cases, can grow and multiply are called organelles.
Mitochondria, centrosomes, Golgi bodies, plastids, and vacuoles are visible under a light microscope, whereas the endoplasmic reticulum, ribosomes, microfilaments, microtubules, intermediate filaments, and microbodies require an electron microscope to be seen.
These organelles are often referred to as protoplasmic structures.
In cells with cilia or flagella, basal bodies are located at the base within the cytoplasm, with the remaining parts extending out of the cytoplasm.
The following are descriptions of these organelles:

Mitochondria:

These rod-like or globular structures are dispersed throughout the cytoplasm, appearing singly or in clusters.
They are enclosed by a double membrane made of lipoproteins, with the inner membrane forming finger-like projections called cristae, which partially subdivide the inner chamber.
Mushroom-like structures, known as oxysomes, are found on the cristae and play a role in phosphorylation.
The area between the membranes and the interior of the mitochondrion is filled with the mitochondrial matrix, which contains various oxidative enzymes and coenzymes.
Because mitochondria contain DNA and ribosomes, they can synthesize some proteins.
They generate energy and store it as adenosine triphosphate (ATP). With their own DNA and ability to replicate and produce proteins, mitochondria are considered semi-autonomous organelles.
The DNA in mitochondria resembles that of bacteria, which is why they are also known as endosymbiotic organelles.

Centrosomes:

The centrosome has a (9+0) structure and is characterized by a clear zone surrounding the centrioles, located near the nucleus, which contains a specialized area of cytoplasm known as the centrosphere.
Its matrix, called kinoplasm, houses two rounded bodies known as centrioles.
Each centriole is made up of nine fibrillar units, each containing three microtubules arranged in a circular pattern.
The two centrioles lie 90 degree to each other.
During cell division, centrioles help form the spindle apparatus made of microtubules.
In plant cells, centrioles are absent, and spindle formation occurs without their assistance.

Golgi Bodies:

These structures consist of stacks of flattened sacs and vesicles arranged in parallel, typically associated with the endoplasmic reticulum.
Their membranes are thought to be derived from the endoplasmic reticulum and are composed of lipoproteins, consisting of lamellae, tubules, vesicles, and vacuoles.
In plant cells, the Golgi complex is referred to as a dictyosome, which secretes materials needed for cell wall formation during cell division.
The Golgi bodies also contribute to forming the acrosome in sperm cells and play a role in releasing hormones, enzymes, and other synthesized substances.

Plastids:

These organelles are present in plant cells but absent in animal cells.
They can be colored, such as chloroplasts and chromoplasts, or colorless, like leucoplasts.
Leucoplasts, which store and metabolize starch and lipids, are specifically known as amyloplasts and lipoplasts respectively.
Chloroplasts contain the green pigment chlorophyll, which is essential for photosynthesis and protein storage.
They have a double outer membrane and a stroma that contains various soluble enzymes, along with a complex network of membrane-bound compartments called thylakoids, which form grana.
Similar to mitochondria, chloroplasts possess their own DNA, ribosomes, and complete protein synthesis machinery.
Therefore, they are also classified as endosymbiotic and semi-autonomous organelles.

Cytoplasmic Vacuoles:

Vacuoles are membrane-bound compartments within cells that contain organic and inorganic molecules. Their size and shape can vary and adapt according to the cell’s needs.
Filled with a watery fluid called cell sap, vacuoles play a crucial role in plant and fungal cells, though they are less prominent in animal cells.
Some of their main functions include storing waste, regulating water content in plant cells, maintaining hydrostatic pressure (or turgor), balancing pH, and storing small molecules.
Vacuoles also aid in autophagy.
In protists, vacuoles assist in storing and digesting food.

Lysosomes:

These are spherical or irregular vesicles filled with hydrolytic enzymes that break down biomolecules such as carbohydrates, lipids, and proteins.
Lysosomes play a key role in cellular waste removal and digestion of biomolecules, while also being involved in secretion, plasma membrane repair, cell signaling, and energy metabolism.
Often referred to as the “suicide bags of the cell,” lysosomes house more than 60 different enzymes and over 50 types of membrane proteins.
These enzymes are synthesized in the rough endoplasmic reticulum.

Cilia and Flagella:

Cilia are small structures that cover the surfaces of some cells.
Both cilia and flagella arise from basal bodies or blepharoplasts located in the cytoplasm.
They consist of nine outer fibrils, with two larger central fibrils, arranged in a 9+2 pattern.
Composed of microtubules made from the protein tubulin, both structures exhibit this 9+2 configuration.
They grow from their bases and function as locomotory organelles, moving through beating or undulating motions powered by the breakdown of ATP molecules.

Microtubules:

Microtubules are ultra-fine protein tubules (made of tubulin) that run throughout the cytoplasm of plant and animal cells.
They provide structural support, determine cell shape, and help organize the cytoplasm. Each microtubule consists of 13 individual filaments.
They play a crucial role in the transport of water and ions, facilitate cytoplasmic streaming (cyclosis), and are involved in forming spindles during cell division.

Basal Granules:

The spherical structures located at the base of cilia and flagella are referred to as basal bodies.
Each basal body is made up of nine fibrils, with each fibril containing three microtubules; of these, two extend into the cilia or flagella.

Ribosomes:

Ribosomes are small, spherical structures that originate in the nucleolus and can be found attached to the membrane of the endoplasmic reticulum as well as free in the cytoplasm.
They are primarily composed of ribonucleic acids (RNA) and proteins, and their main function is to synthesize proteins.

2. Cell Inclusions:

Inclusions are non-living or deutoplasmic structures that do not have the capacity for growth or reproduction.
Common cell inclusions include stored organic materials such as starch grains, glycogen granules, aleurone grains, fat droplets, pigment granules, and inorganic crystals.
The cytoplasm serves as a storage site for raw materials essential for metabolism in both the cytoplasm and the nucleus.
Many metabolic processes, including the biosynthesis of fatty acids, nucleotides, and proteins, as well as oxidation, occur in the cytoplasm.
It facilitates the distribution of nutrients, metabolites, and enzymes within the cell and enables the exchange of materials between organelles and the external environment or extracellular fluid.

C. Nucleus

The nucleus is a distinct, centrally located, and spherical cellular structure that regulates the cell’s vital processes. It consists of three main components:
Nuclear Membrane: The outermost layer of the nucleus, present in both plant and animal cells, is the nuclear membrane. Made of a double layer of lipoproteins, its primary function is to form a protective barrier that isolates the cell’s genetic material from the rest of the cell’s chemical activities.
Nucleoplasm and Chromosomes: Similar to cytoplasm, the nucleus contains a watery substance known as nucleoplasm (or karyoplasm), which fills the space between the nuclear membrane and nucleolus. This nucleoplasm houses chromosomes and chromatin granules, which hold genetic material along with numerous nucleoproteins.
Nucleolus: Within the nucleoplasm is a prominent, darkly stained, spherical body called the nucleolus. It is primarily composed of ribosomal RNA and ribosomal proteins.