Eukaryotic Cell

The term ‘eukaryote’ is derived from Greek words, ‘eu’ meaning true and ‘karyon’ meaning ‘nucleus’. Eukaryotic cells are defined as cells that contain an organized nucleus and membrane-bound organelles. They have a more advanced structural organization that is large and more complex than a prokaryotic cell. However, they share a few common features, including the cytoplasm.

Eukaryotic cells are located in plants, animals including humans, fungi, and protozoa. They are together classified under the kingdom Eukaryota.

The first eukaryotic cells probably evolved about 2 billion years ago. The endosymbiotic theory explains their evolution. According to which, large cells engulfed small cells but are not digested by them. Instead, the small cells lived within them and evolved into organelles. The large and small cells formed a symbiotic relationship where both cells benefited from each other.  

Some of the small cells were able to break down the large cell’s wastes and, in return, generate energy for them and the large cell. Those cells became the mitochondria. Other small cells were able to utilize sunlight for making food. They became the chloroplast.

The basic characteristic features of a eukaryotic cell are:

• They contain membrane-bound organelles and a well-defined nucleus. A complex nuclear membrane surrounds the nucleus
• Their cell wall consists of cellulose and some other carbohydrates
• The genetic material is DNA, which is linear and has multiple origins of replication. The DNA is complexed with histone proteins
• They contain cytoskeletal structural elements (microtubules, microfilaments, and intermediate filaments) that provide structural support to the cell
• Cilia and flagella are the locomotory organs
• Reproduce either asexually by mitosis or fission or sexually, involving two partners

The four main components that make up a eukaryotic cell are:

1. Cell (Plasma) Membrane
2. True Nucleus
3. Membrane-bound organelles like mitochondria, chloroplast, endoplasmic reticulum, and Golgi apparatus
4. Rod-shaped chromosomes

How Big are Eukaryotic Cells

Their size is significantly larger than prokaryotic cells, with an average of 10 to 100 µm in diameter.

Shapes

The shape of eukaryotic cells varies significantly with the type of cell. Some common shapes include spheroid, ovoid, cuboidal, lenticular, cylindrical, flat, fusiform, discoidal, and polygonal.

What Parts Do they Have with Functions

The two major parts of a typical eukaryotic cell are the nucleus and the cytoplasm. The cytoplasm contains all other organelles suspended in it. Given below are all the organelles found in eukaryotic cells.

1) Cell (Plasma) Membrane: It is a semipermeable membrane that separates a cell inside from outside. The cell membrane is made of proteins, carbohydrates, and phospholipid bilayer. The phospholipids are arranged with the polar, hydrophilic heads facing outwards and inside the cell. They interact with aqueous environments. The non-polar hydrophobic tails are found between the heads that remain aloof from the watery environment.

Functions

• Controlling the entry and exit of substances by selectively allowing certain substances to pass through
• Protecting the cell from shock and injury
• Allowing interaction of molecules and helps in secretion, growth, and division of cells
• Performing cell transport, either with the help of energy (active transport) or without it (passive transport)

2) Cell Wall: It is a non-living part, forming a rigid structure outside the cell membrane. It is made of cellulose, hemicellulose, proteins, and pectin in plants. In fungi, it consists of cellulose, galactan, mannan, and calcium carbonate. Structurally, a cell wall is divided into three layers: a) the outer, middle lamella, made of calcium pectate, b) the middle, primary wall, made of cellulose and hemicelluloses, and c) the inner, secondary wall, having a similar composition to the middle lamella. It is absent in animal cells.

Functions

• Providing shape to the cell
• Helping in the cell-to-cell interaction
• Protecting the cell from external injury and shock

3) Nucleus: Unique to eukaryotic cells, it is a double-membrane bound organelle that contains all the genetic information of the cell. It is the most prominent and essential part, called the ‘brain of the cell’. They are found in all eukaryotic cells except for red blood cells (RBCs) in animals and sieve cells in plants. A nucleus has four main parts: a) nuclear envelope, b) nucleoplasm, c) nucleolus, and d) chromatin.

Functions

• Storing genetic information as DNA that is necessary for the development and cell reproduction
• Containing information for protein synthesis and other cellular functions

4) Mitochondria: An oval-shaped structure, it is bounded by two membranes.  The outer and inner membranes divide the mitochondrial lumen into two compartments. The outer membrane surrounds the organelle, while the inner member is semipermeable that forms folds called cristae. The region within the inner membrane is called the matrix, and between the two membranes is called the intermembrane space. Mitochondria contain DNA, RNA, and other components required for protein synthesis.

Functions

• Producing energy as ATP, thus called the ‘energy-currency’ or ‘powerhouse’ of the cell
• Regulating cell metabolism
• Performing protein synthesis for itself

5) Endoplasmic Reticulum (ER): It is a small network of tubular structures. It divides the cell cytoplasm into two parts: luminal and cytoplasm. They are of two types: a) smooth endoplasmic reticulum (SER) that is devoid of the ribosome and b) rough endoplasmic reticulum (RER), with the attached ribosome.  

Functions

• Performing lipid synthesis (SER)
• Carrying out protein-folding and transporting them to Golgi apparatus (SER)
• Performing protein synthesis (RER)

6) Ribosomes: Structures not bounded by a membrane. They are made of ribonucleic acids (RNA) and proteins. Eukaryotic ribosome is the 80S, with 60S large subunit and 40S small subunit. It has a size of between 25 and 30 nm.

Function

• Acting as the site for proteins and polypeptides synthesis

7) Golgi Apparatus: Made of many flat, disc-shaped structures called cisternae. It is found in every eukaryotic cell except human red blood cells and sieve cells of plants. The cisternae are arranged concentrically in parallel to the nucleus. It has a cis (forming) face that faces the cell membrane and the trans (maturing) face that faces the nucleus.  

Functions

• Packaging material within the cell
• Modifying and maturing of proteins
• Producing glycoproteins (proteins with carbohydrates) and glycolipids (lipids with carbohydrate)

8) Lysosomes: Membrane-bound organelles formed in the Golgi apparatus. They contain rich hydrolytic enzymes such as lipases, proteases, and peptidases. Lysosomes are thus called ‘suicidal bags’.

Functions

• Digesting lipids, proteins, carbohydrates, and nucleic acids
• Preventing the entry of foreign particles such as bacteria and viruses and destroying them once they enter the cell through phagocytosis

9) Peroxisomes: Single membrane-bound small, round-shaped structures. They contain digestive and oxidative enzymes. Peroxisomes are a group of heterogeneous organelles, and the presence of marker enzymes distinguishes them from others.

Functions

• Detoxifying poisons inside the body through various oxidation reactions (animals)
• Facilitating in photosynthesis and seed germination (plants)
• Converting stored fats into sugars (plants)
• Helping in metabolism, pathogen defense, and stress response (plants)

10) Plastids: Double-membrane bound organelle found only in plants. Based on the type of pigment present, they are three types: a) chloroplasts, containing green pigment, b) chromoplast, containing green carotene, and c) leucoplast, with no pigment.

Functions

• Helping to perform photosynthesis (chloroplast)
• Imparting flowers and fruits its yellow, red or orange color
• Storing carbohydrates (amyloplasts), oils and fats (elaioplasts), and proteins (aleuroplasts)

11) Cytoskeleton: A network of filaments present in the cell cytoplasm. The cytoskeleton is three types: a) microtubules, b) microfilaments, and c) intermediate filaments.

Functions

• Providing mechanical support to the cell
• Maintaining cell shape
• Helping in cellular motility

12) Cilia and Flagella: Cilia are short hair-like structures that cover the cell’s entire surface. Flagella are long tube-like structures that are present at one end of the cell. They are composed of microtubules.

Functions

• Performing rowing movement (cilia)
• Performing up and down movement (flagella)

13) Vacuoles and Vesicles: Vacuoles are found centrally in plants, making up almost 30 to 80% of the total plant cell volume. It is the largest organelle in a plant cell, filled with fluids, ions, enzymes, and other molecules. In animal cells, they are small in size compared to a plant cell. Vesicles are membrane-bound sacs that can fuse with the cell membrane or other membrane systems within the cell.  

Functions

• Maintaining turgidity of the cell
• Transporting ions and molecules in and out of the cell through endocytosis and exocytosis, respectively
• Storing of reserve food, water, and wastes

14) Centrosome: Located only in animal cells, it is the microtubule-organizing center. It contains a pair of centrioles that lie perpendicular to each other. Each centriole is cylindrical and comprised of nine microtubule triplets.

Function

• Help in cell division

Eukaryotes perform two types of cell division: mitosis and meiosis. Mitosis produces new body cells for growth and repair, while meiosis produces sex cells or gametes.

A eukaryotic cell cycle is an ordered event involving cell growth and division, producing two daughter cells through mitosis. The cell cycle length is highly variable within the different cell types. An early embryonic cell has a turnover range of a few hours. For epithelial cells in humans, it is about two to five days. Again, cells of cortical neurons or cardiac muscle cells do not divide throughout their life cycle.

Cells on the path to division proceed through a series of stages consisting of two main phases: 1) interphase and 2) mitotic (M) phase and an alternative G0-phase.

1) Interphase

In this phase, the chromosome gets duplicated as the cell prepares for division. It is the longest phase of the cell cycle and happens between one cell division (mitotic phase) to the next. It is divided into three phases:

G1-phase: The first-gap phase when the cells grow in size, synthesize cell organelles and other macromolecules.

S-phase: Synthesis phase, when the existing DNA is copied within the nucleus. This phase is also known as the DNA-replication phase.

G2-phase: Second-gap phase when the cell grows further in size, making more proteins.

End Result

It forms two independent daughter cells.

2) Mitotic (M) Phase

Also known as the cell division phase, it occurs just after the G2-phase. During this period, the cell divides its DNA (mitosis or karyokinesis) and cytoplasm to form two new cells (cytokinesis). The phase of mitosis is further divided into prophase, metaphase, anaphase, and telophase.

G0-phase

Also known as the resting phase, during which the cells do not divide. It occurs in cells such as the liver, kidney, neurons, and stomach that do not immediately enter another round of interphase following division. Thus, G0 is called an alternative phase of the cell cycle. Many cells do not enter this phase and multiply throughout their life cycle. Others such as nerve cells and cardiac cells either never divide or seldom divide and remain in G0 permanently.  

Based on the types of organisms, eukaryotic cells are of four types: 1) plant cells, 2) animal cells, 3) fungal cells, and 4) protozoa.

1) Plant Cells

They have thick cell walls consisting of cellulose that provides structural support to the cell. Every plant cell has a large central vacuole that helps them to remain turgid. They also contain chloroplast, an organelle having the pigment chlorophyll that helps plants to perform photosynthesis.

2) Animal Cells

They lack cell walls but have a plasma membrane. Due to the lack of cell wall, animal cell shapes can change widely. It helps in the ingestion of food by phagocytosis and fluids by pinocytosis. In contrast to an animal cell, plants do not have chloroplasts but contain many small vacuoles.

3) Fungal Cells

Like plant cells, they also have a cell wall, but they are made of chitin, unlike them. Some fungi have septa, holes that allow organelles and cytoplasm to pass between them. They mostly live underground or in dead and decaying organic matter that remains interconnected as a mycelial network.  

4) Protozoa

They consist of a single cell that can move around, eat other small organisms, and digest food within vacuoles. Protists use cilia or flagella for their movement. They may also have a pellicle, a thin layer that supports the cell membrane.