What is a Eukaryotic Cell
Where are they Found
How did Eukaryotic Cells Evolve
Characteristics of Eukaryotic Cells
Components
Eukaryotic Cell Structure
How Do Eukaryotic Cells Reproduce: Cell Cycle
Examples of Eukaryotic Cells
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.
They contain membrane-bound organelles and a well-defined nucleus. A complex nuclear membrane surrounds the nucleusTheir cell wall consists of cellulose and some other carbohydratesThe genetic material is DNA, which is linear and has multiple origins of replication. The DNA is complexed with histone proteinsThey contain cytoskeletal structural elements (microtubules, microfilaments, and intermediate filaments) that provide structural support to the cellCilia and flagella are the locomotory organsReproduce either asexually by mitosis or fission or sexually, involving two partners
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.
- 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 throughProtecting the cell from shock and injuryAllowing interaction of molecules and helps in secretion, growth, and division of cellsPerforming cell transport, either with the help of energy (active transport) or without it (passive transport)
- 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 cellHelping in the cell-to-cell interactionProtecting the cell from external injury and shock
- 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 reproductionContaining information for protein synthesis and other cellular functions
- 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 cellRegulating cell metabolismPerforming protein synthesis for itself
- 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)
- 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
- 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 cellModifying and maturing of proteinsProducing glycoproteins (proteins with carbohydrates) and glycolipids (lipids with carbohydrate)
- 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 acidsPreventing the entry of foreign particles such as bacteria and viruses and destroying them once they enter the cell through phagocytosis
- 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)
- 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 colorStoring 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 cellMaintaining cell shapeHelping in cellular motility
- 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)
- 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 cellTransporting ions and molecules in and out of the cell through endocytosis and exocytosis, respectivelyStoring of reserve food, water, and wastes
- 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
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.
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.
