# Eukaryotic Cell: Definition, Structure, Organelles, Functions, and Examples > Learn what a eukaryotic cell is, how its organelles work together, how plant, animal, fungal and unicellular cells vary, and how eukaryotes differ from... Canonical URL: https://biologynotesonline.com/eukaryotic-cell/ Author: Sourav Pan Last updated: September 13, 2026 ![Eukaryotic Cell: Definition, Structure, Organelles, Functions, and Examples](https://biologynotesonline.com/wp-content/uploads/2023/09/Eukaryotic-Cell-Definition-Structure-Functions-Examples.jpg) A eukaryotic cell is the characteristic cell type of eukaryotes, with genetic material normally enclosed within a membrane-bound nucleus and cellular activities organized among specialized internal compartments. Typical eukaryotic cells contain organelles such as mitochondria, the endoplasmic reticulum and Golgi apparatus, together with ribosomes and a cytoskeleton. Animals, plants, fungi and many unicellular organisms are eukaryotic, although specialized cells can modify or lose some of these structures during development. ## What is Eukaryotic Cell? A eukaryotic cell is a type of cell which has a membrane-bound nucleus and different membrane-bound cell organelles. The genetic material is mainly present enclosed inside the nucleus. Different organelles such as mitochondria, endoplasmic reticulum (ER), and Golgi apparatus are present in the cell, performing various specific functions. This type of internal compartmentalization is one of the major characteristics that separate eukaryotic cells from the [prokaryotic cells](https://biologynotesonline.com/prokaryotic-cells/). The term “eukaryotic cell” is used for this type of cellular organization. Whereas, a eukaryote is an organism that is made up of one or more eukaryotic cells. Animals, plants, fungi, and many unicellular organisms are eukaryotes. However, a generalized eukaryotic cell only represents the common cellular organization of these cells. It does not indicate that every specialized eukaryotic cell will contain or retain all the cell organelles. For example, mature mammalian red blood cells lose their nucleus, mitochondria, and several other organelles during the process of maturation. ## Characteristics of Eukaryotic Cell ![Generalized eukaryotic cell showing the nucleus, ER, Golgi apparatus, mitochondria, ribosomes, peroxisome, cytoskeleton and plasma membrane.](https://biologynotesonline.com/wp-content/uploads/2024/04/Generalized-Eukaryotic-Cell-Structure-and-Organelles-1024x768.webp)Generalized eukaryotic cell showing the nucleus, ER, Golgi apparatus, mitochondria, ribosomes, peroxisome, cytoskeleton and plasma membrane. Some of the important characteristics of eukaryotic cells are as follows- - True nucleus- A well-defined nucleus is present. The genetic material remains separated from cytoplasm by nuclear envelope. DNA replication and transcription mainly take place inside nucleus. - Membrane-bound organelles- Different membrane-bound organelles are present in the cytoplasm. Mitochondria, endoplasmic reticulum, Golgi apparatus and peroxisomes are some of them. - Linear chromosomes- Nuclear DNA is arranged into linear chromosomes. The DNA remains associated with histone and other proteins. - Compartmentalization- Different cellular reactions take place in separate regions of the cell. This is referred to as cellular compartmentalization. - Ribosomes- Cytoplasmic ribosomes are 80S, made up of 60S and 40S subunits. Ribosomes present inside mitochondria and chloroplasts are generally 70S. - Cytoskeleton- It is a network of protein filaments present throughout cytoplasm. It gives support and shape to the cell. It is also involved in intracellular movement. - Plasma membrane- The cell is surrounded by a plasma membrane mainly made up of phospholipid bilayer and proteins. It separates the internal part of cell from outside and controls movement of substances. - Cell division- Eukaryotic cells divide through mitosis and meiosis. Mitosis is mainly used for growth and replacement of cells, while meiosis takes part in sexual life cycle. - Cell wall- Cell wall is not present in all eukaryotic cells. Plant cells contain a cellulose-rich wall and fungal cells contain a wall mainly made up of chitin and other polysaccharides. Animal cells do not contain cell wall. - Cellular forms- Eukaryotes may be unicellular or multicellular. Protozoans, many algae and yeasts are unicellular forms, while plants and animals are multicellular. ## Why Compartmentalization Matters in Eukaryotic Cell? Compartmentalization divides the cytoplasm into different membrane-bound regions. As a result, different cellular reactions can take place in different regions of the same cell. Each organelle contains particular enzymes and other molecules required for its function. This keeps many metabolic activities localized within a particular region instead of taking place throughout the cytoplasm. Different chemical conditions can also be maintained inside different organelles. Reactions which need a particular environment are carried out within such compartments. Some reactions require membrane surfaces for their functioning. For example, oxidative phosphorylation takes place on the inner mitochondrial membrane. During this process, movement of H+ is coupled with the formation of ATP. Enzymes and their substrates can be kept together within a particular compartment. This makes many cellular reactions more efficient. It also provides another way for controlling the activity of enzymes. Compartmentalization also keeps apart the reactions which should not freely occur in the cytoplasm. Lysosomes, for example, contain enzymes involved in the digestion of macromolecules within a membrane-bound compartment. Different substances are transported from one compartment to another in a controlled manner. Organelle membranes contain different transport proteins which help in the movement of specific molecules across these membranes. Eukaryotic cells are generally much larger than prokaryotic cells. Their internal membrane system provides additional membrane surface and separates different cellular functions. This helps such large cells to carry out their functions efficiently. ## Eukaryotic Cell Structures and Functions A eukaryotic cell is made up of different cellular structures and organelles that work together to perform normal cellular functions. Some of the important structures and their functions are as follows- ### Cell Boundary and Cytoplasm of Eukaryotic Cell The cell boundary and cytoplasm form the outer functional and internal living region of the eukaryotic cell. Plasma membrane separates the cellular contents from surrounding environment. Inside it, the cytoplasm contains cytosol, cytoskeleton and different cell organelles. - Plasma membrane- It is the thin membrane surrounding every eukaryotic cell and forms the actual cellular boundary. It separates the cytoplasm from the external environment. In plant and fungal cells, a cell wall is also present outside the plasma membrane. - Phospholipid bilayer- The basic structure of plasma membrane is formed by a phospholipid bilayer. Hydrophilic heads of the phospholipids remain towards the aqueous regions, whereas the hydrophobic tails face towards inside. Different membrane proteins are present within or associated with this bilayer. - Membrane proteins- Several types of proteins are present in the plasma membrane. Some pass through the complete bilayer, while others remain attached with its surface. These proteins function as channels, carriers, receptors and enzymes and perform different activities of the membrane. - Selective permeability- Plasma membrane is selectively permeable, hence it controls the movement of water, ions and different organic molecules between cell and surrounding environment. By this process, the composition of cytoplasm can remain different from the extracellular region. - Cytoplasm- Cytoplasm is the entire region present between the plasma membrane and nuclear envelope. It contains cytosol, cytoplasmic organelles, cytoskeleton and different dissolved or suspended substances. Thus, the terms cytoplasm and cytosol do not refer to the same region. - Cytosol- It is the fluid portion of cytoplasm in which different membrane-bound organelles remain distributed. Cytosol mainly contains water along with ions, soluble proteins and different small organic molecules. Many reactions of intermediary metabolism are carried out in this region. - Cell organelles- Different membrane-bound organelles are present in the cytoplasm of eukaryotic cells such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes and peroxisomes. These organelles form separate compartments inside the cell. As a result, particular cellular reactions can take place in particular regions. Plant cells also contain plastids such as chloroplasts. - Cytoskeleton- A network of protein filaments is present throughout the cytoplasm, which is referred to as the cytoskeleton. Its major components are microfilaments, microtubules and intermediate filaments. It provides shape to the cell and also takes part in movement and organization of cellular components. - Cellular reactions- Several important metabolic reactions take place in the cytoplasmic region. Protein synthesis on free ribosomes and many reactions involving small cellular metabolites occur in the cytosol. Other reactions are carried out inside specific membrane-bound organelles. This type of separation or compartmentalization is one of the major characteristics of eukaryotic cells. - Internal environment- Plasma membrane and cytoplasm together maintain the internal condition required for different cell activities. The membrane controls what moves in and out of the cell. Whereas, cytoplasm provides the region containing organelles and where many cellular reactions take place. ### Nucleus and Genetic Compartment of Eukaryotic Cell The nucleus is the major genetic compartment of a eukaryotic cell which contains most of the cellular DNA. The DNA is present in the form of chromatin. DNA replication, transcription and different RNA processing also takes place inside the nucleus. ![Cutaway of a eukaryotic nucleus showing the double nuclear envelope, nuclear pores, chromatin, nucleolus, rough ER continuity, mRNA export and protein import.](https://biologynotesonline.com/wp-content/uploads/2024/04/Eukaryotic-Nucleus-Structure-and-Nuclear-Pore-Transport-1024x768.webp)Cutaway of a eukaryotic nucleus showing the double nuclear envelope, nuclear pores, chromatin, nucleolus, rough ER continuity, mRNA export and protein import. - [Nucleus](https://biologynotesonline.com/nucleus-structure-and-functions/)- It is a membrane-bound organelle containing most of the genetic material of eukaryotic cell. The DNA is arranged into linear chromosomes. Presence of a true nucleus is one of the characteristic features of eukaryotic cells. - Nuclear envelope- The nucleus is covered by a double membrane called nuclear envelope. It has an inner nuclear membrane and outer nuclear membrane with a space between them. The outer membrane is continuous with the endoplasmic reticulum (ER). - Nuclear pores- These are openings present in the nuclear envelope containing nuclear pore complexes (NPCs). They regulate the movement of molecules between nucleus and cytoplasm. RNAs move out through these pores, whereas different nuclear proteins are transported inside. - Nucleoplasm- It is the semi-fluid material present inside the nucleus. Chromatin and nucleolus are present in this region. - Chromatin- It is made up of DNA and associated proteins. During most of the cell cycle, chromatin remains less condensed inside the nucleus. During cell division, it becomes highly condensed forming recognizable chromosomes. - Histones- Eukaryotic DNA is associated with proteins called histones. DNA wraps around these proteins and forms repeating units called nucleosomes. This arrangement helps in packing the long DNA molecules inside the nucleus. - Euchromatin- It is a comparatively less condensed chromatin and is generally associated with the DNA regions which are more accessible for transcription. Under electron microscope, it appears relatively lighter. - Heterochromatin- It is a more tightly packed form of chromatin. The transcriptional activity is generally lower than euchromatin. It can also be present near the nuclear periphery. - Chromosomes- These are linear DNA-protein structures carrying hereditary information. Individual chromosomes become clearly visible when chromatin gets condensed during cell division. The chromosome number is generally characteristic for a particular species. - Nucleolus- It is a prominent region present inside the nucleus and does not have a surrounding membrane. Ribosomal RNA (rRNA) is produced and processed here. Ribosomal proteins combine with rRNA forming ribosomal subunits, which later move to the cytoplasm through nuclear pores. - Genetic activity- DNA replication and transcription take place inside the nuclear compartment. Different RNAs are formed here and selected RNAs move into the cytoplasm. Proteins needed for replication, transcription and chromatin organization are first brought into the nucleus. - DNA organization- DNA is not kept randomly inside the nucleus. It remains organized as chromatin and chromosomes within the nuclear space. The nucleus is the major region for storage, expression and maintenance of genetic material in eukaryotic cell. ### Protein and Lipid Processing System of Eukaryotic Cell The protein and lipid processing system of eukaryotic cell mainly consists of the endoplasmic reticulum (ER), Golgi apparatus and transport vesicles. Proteins and lipids formed in the ER are processed and transported to different cellular locations through this system. - [Endoplasmic reticulum (ER)](https://biologynotesonline.com/endoplasmic-reticulum/)- It is an extensive network of membrane-bound tubules and flattened sacs present in the cytoplasm. Its membrane is continuous with the outer membrane of nuclear envelope. ER occurs in two forms, rough ER and smooth ER. - Rough endoplasmic reticulum (RER)- The surface of rough ER contains attached ribosomes, giving it a rough appearance. It is mainly involved in the synthesis of proteins that are secreted, inserted into membranes or sent into the endomembrane system. Newly formed proteins enter into the ER lumen during their synthesis. - Protein folding- In the rough ER, newly synthesized proteins are folded into their proper structure. Some proteins also undergo early modification before leaving the ER. Incorrectly folded proteins are generally retained rather than being sent forward. - Glycosylation in ER- Addition of carbohydrate groups to proteins begins in the endoplasmic reticulum. N-linked glycosylation involves attachment of an oligosaccharide to the asparagine residue of a protein. Further modification of these carbohydrate groups takes place in Golgi apparatus. - Smooth endoplasmic reticulum (SER)- It does not contain ribosomes on its surface. The smooth ER is a major site for synthesis of cellular lipids. Phospholipids and many other membrane lipids are produced in association with the ER membrane. - Lipid synthesis- Most membrane phospholipids are synthesized in the ER. The newly formed lipids become part of the ER membrane and can later be transferred to other membranes. Lipids are carried either through vesicles or by lipid-transfer mechanisms. - Transport vesicles- Proteins and lipids which have to leave the ER are packed into small membrane-bound vesicles. These vesicles bud off from the ER and carry their materials towards the Golgi apparatus. Transport does not occur randomly. Specific cargo is selected during this process. - [Golgi apparatus](https://biologynotesonline.com/golgi-apparatus/)- The Golgi apparatus consists of stacks of flattened membrane-bound sacs called cisternae. Proteins and lipids coming from the ER enter through the cis side and move towards the trans side. During this movement, different modifications are carried out. - Golgi processing- Proteins received from ER undergo further carbohydrate modification in Golgi apparatus. N-linked carbohydrate chains are modified and O-linked glycosylation of many proteins also takes place here. Golgi also participates in formation of glycolipids and sphingomyelin. - Sorting and packaging- The trans Golgi network acts as a major sorting region. Proteins and lipids are packed into different transport vesicles depending upon their destination. They may be transported to the plasma membrane, lysosomes, secretory vesicles or other parts of the endomembrane system. - Secretory pathway- Proteins meant for secretion generally pass through the sequence, rough ER → Golgi apparatus → secretory vesicle → plasma membrane. The vesicle finally fuses with the plasma membrane and releases its contents outside the cell. - Membrane supply- ER and Golgi also supply new proteins and lipids to cellular membranes. Vesicles carrying these components fuse with their target membrane. In this way, membrane components are processed, moved and distributed within the eukaryotic cell. ### Energy and Metabolic Organelles of Eukaryotic Cell Eukaryotic cells contain different specialized organelles where energy production and various metabolic reactions takes place. Some of the important organelles are mitochondria, chloroplasts and peroxisomes. ![Comparative diagram of a mitochondrion, chloroplast and peroxisome showing membrane compartments, proton-driven ATP synthesis and peroxisomal hydrogen peroxide breakdown.](https://biologynotesonline.com/wp-content/uploads/2024/04/Mitochondria-Chloroplasts-and-Peroxisomes-in-Eukaryotic-Cells-1024x427.webp)Comparative diagram of a mitochondrion, chloroplast and peroxisome showing membrane compartments, proton-driven ATP synthesis and peroxisomal hydrogen peroxide breakdown. - [Mitochondria](https://biologynotesonline.com/mitochondria/)- These are double membrane-bound organelles and the major site for aerobic energy production in eukaryotic cells. Energy obtained from carbohydrates and fatty acids is mainly converted into ATP (Adenosine Triphosphate) in mitochondria. - Matrix- The mitochondrial matrix is the internal compartment enclosed by the inner mitochondrial membrane. Pyruvate is converted into acetyl-CoA in this region. The enzymes of Krebs cycle (TCA cycle) are also mainly present here. - Cristae- The inner mitochondrial membrane is folded into numerous structures called cristae. These folds increase the membrane surface area. Electron transport chain and ATP synthase are present in this membrane. - Oxidative phosphorylation- During this process, electrons from NADH and FADH₂ pass through the electron transport chain and finally reach oxygen. A proton gradient is produced in the process. This gradient is then used by ATP synthase for the formation of ATP. - Mitochondrial DNA- Mitochondria possess their own DNA and ribosomes. However, most of the mitochondrial proteins are encoded by nuclear genes. These proteins are later transported into mitochondria. - [Chloroplasts](https://biologynotesonline.com/chloroplast/)- These are energy-converting plastids found in plants and algae. Chloroplasts contain chlorophyll and carry out photosynthesis. During this process, light energy is converted into chemical energy. - Thylakoids- These are flattened membrane structures present inside chloroplasts. Chlorophyll, photosystems and components of photosynthetic electron transport are located in the thylakoid membrane. Light reactions take place here. - Stroma- It is the fluid region surrounding the thylakoids. The enzymes involved in carbon fixation are present in this region. ATP and NADPH formed during light reactions are used here for synthesis of carbohydrates from CO₂. - [Peroxisomes](https://biologynotesonline.com/peroxisomes-structure-enzymes-and-functions/)- These are small single membrane-bound organelles involved in different metabolic reactions. They contain enzymes for several oxidation reactions, including breakdown of fatty acids. During some of these reactions, hydrogen peroxide (H₂O₂) is also produced. - Catalase- Peroxisomes contain catalase and other enzymes which remove harmful hydrogen peroxide. H₂O₂ is converted into water and oxygen, preventing its accumulation inside the cell. - Metabolic compartments- Different metabolic reactions are kept within separate regions by these organelles. Mitochondria mainly carry out oxidative energy metabolism. Chloroplasts perform photosynthetic energy conversion, while different oxidative reactions takes place in peroxisomes. ### Cellular Machinery Without Internal Membranes of Eukaryotic Cell Some cellular structures of eukaryotic cells are not surrounded by membrane. Ribosomes, cytoskeleton, centrosome and proteasomes are some of the important non-membranous cellular structures. ![Eukaryotic cell cytoskeleton showing actin filaments, intermediate filaments, microtubules radiating from the centrosome and motor proteins transporting cargo.](https://biologynotesonline.com/wp-content/uploads/2024/04/Cytoskeleton-Structure-and-Intracellular-Transport-in-Eukaryotic-Cells-1024x936.webp)Eukaryotic cell cytoskeleton showing actin filaments, intermediate filaments, microtubules radiating from the centrosome and motor proteins transporting cargo. - Ribosomes- These are small non-membranous structures involved in protein synthesis. Eukaryotic cytoplasmic ribosome is 80S, made up of 60S large subunit and 40S small subunit. During translation, amino acids are joined according to information present in mRNA. - Free ribosomes- These ribosomes remain freely present in the cytoplasm. They mainly form proteins that are used in cytosol and some other parts of the cell. - [Cytoskeleton](https://biologynotesonline.com/cytoskeleton/)- It is a network of protein fibres present throughout the cytoplasm. It provides shape and support to the cell. Cytoskeleton is also involved in cell movement, intracellular transport and cell division. - Microfilaments- These are thin filaments mainly formed of actin protein. They help in movement of cells and change in cell shape. During cytokinesis, actin filaments also take part in separation of daughter cells. - Intermediate filaments- These are rope-like protein fibres which provide mechanical strength to the cell. They help in maintaining the position of different cellular structures. Their protein composition may differ in different cell types. - Microtubules- These are hollow structures formed from tubulin proteins. Microtubules are used in movement of cell organelles, intracellular transport and formation of mitotic spindle. They can assemble and break down according to the requirement of cell. - Centrosome- It is the major microtubule-organizing center (MTOC) in most animal cells. It is generally located close to the nucleus. During cell division, centrosomes are involved in formation of spindle poles. - Centrioles- A centrosome usually contains two centrioles. Each centriole is a cylindrical structure having nine sets of microtubule triplets. They are also involved in formation of basal bodies of cilia and flagella. - Motor proteins- These proteins work along with cytoskeletal filaments. They move different materials and cellular components along microtubules or actin filaments. This helps in intracellular transport and movement. - Proteasomes- These are large non-membranous protein complexes involved in degradation of proteins. Proteins marked with ubiquitin are recognized and broken down by the 26S proteasome. Damaged, faulty and short-lived proteins are removed by this system. ## How Eukaryotic Cell Compartments Work Together Different compartments of eukaryotic cell are connected through movement of proteins, lipids and other materials. A material formed in one region may be processed or used in another region. ![Protein-trafficking pathway showing mRNA export, rough ER synthesis, vesicular transport through the Golgi and delivery to the plasma membrane, lysosome or extracellular space.](https://biologynotesonline.com/wp-content/uploads/2024/04/Protein-Synthesis-and-Trafficking-in-a-Eukaryotic-Cell-1024x768.webp)Protein-trafficking pathway showing mRNA export, rough ER synthesis, vesicular transport through the Golgi and delivery to the plasma membrane, lysosome or extracellular space. ### Protein Synthesis and Trafficking - DNA to RNA- Genetic information present in DNA is first transcribed into RNA inside nucleus. The processed mRNA then moves out through nuclear pores. - RNA to ribosome- In cytoplasm, ribosome reads the information present in mRNA and protein synthesis takes place. But all newly formed proteins do not follow the same route. - Entry into rough ER- Proteins meant for secretion, plasma membrane and different parts of endomembrane system are directed towards rough ER. During their synthesis, these proteins enter the ER or become inserted into its membrane. - ER to Golgi- After initial processing in ER, proteins are packed into transport vesicles. These vesicles move towards Golgi apparatus and deliver their contents there. - Golgi to final site- In Golgi, the proteins are further processed and sorted out. From here they may be sent to plasma membrane, lysosome or secretory vesicles. Some are released outside the cell by secretion. - Other protein routes- Not all proteins pass through ER and Golgi. Proteins which remain in cytosol, or are required in nucleus, mitochondria, chloroplasts and peroxisomes are generally synthesized on free ribosomes. They are later directed to their required location. ### Intracellular Transport and Membrane Exchange - Vesicle transport- Vesicles carry selected proteins and lipids between different membrane compartments. They bud off from one membrane and fuse with the required target membrane. - Cytoskeletal transport- Vesicles and other cellular materials can be moved along cytoskeletal tracks. Motor proteins carry the cargo by using energy from ATP. - Endocytosis- Materials from outside the cell are taken in by folding of plasma membrane and formation of vesicles. These materials can then pass through endocytic compartments and some are delivered towards lysosomes for degradation. - Exocytosis- Vesicles moving towards plasma membrane fuse with it and release their contents outside the cell. During this process, membrane proteins and lipids can also become part of plasma membrane. - Membrane exchange- Membrane is continuously moved between ER, Golgi, vesicles, plasma membrane and endocytic compartments. Forward transport and recycling together maintain the different cellular compartments. - Cellular coordination- One compartment forms a product, another modifies it and another may transport or use it. In this way, different compartments remain separate but their activities work together as one cellular system. ## Variation in Eukaryotic Cells There is no single fixed structure for all eukaryotic cells. The basic eukaryotic organization is common, but cell shape, size and some cellular components change according to the group of organism and function of the cell. ### Plant and Animal Cells [Plant and animal cells](https://biologynotesonline.com/plant-cell-vs-animal-cell/) contain many common structures such as nucleus, mitochondria, ER, Golgi apparatus, ribosomes and cytoskeleton. Some major differences are as follows- CharacterPlant cellAnimal cellCell wallCellulose-containing cell wall is present outside plasma membrane.Cell wall is absent.Plastids/chloroplastsPlastids are present. Chloroplasts occur in photosynthetic cells.Plastids and chloroplasts are absent.VacuolesA large central vacuole is generally prominent in mature plant cells.Large central vacuole is absent. Smaller vesicular compartments may be present.Cell shapeCell wall often gives a more fixed or regular shape.Shape is usually more variable because rigid cell wall is absent.Centrosomal organizationTypical centrosome with centrioles is absent in most higher plant cells. Microtubules are organized by other sites.Centrosome with centrioles is typically present.Outside the plasma membraneA rigid cell wall forms the major extracellular covering.Cells commonly interact with an extracellular matrix (ECM) instead of a cell wall. ### Fungal Cells Cell wall- Fungal cells are surrounded by a cell wall, but it is different from plant cell wall. Chitin and glucans are important components of many fungal cell walls. Chloroplasts- Fungi are non-photosynthetic eukaryotes and chloroplasts are absent. Vacuoles- Fungal cells contain vacuoles. These are involved in degradation and storage of different cellular materials, and also take part in ion and pH balance. Their size and number can change with cellular conditions. Yeast and hyphae- Fungal body does not always have the same form. Yeasts such as Saccharomyces cerevisiae are unicellular, while many fungi grow as long filamentous structures called hyphae. A mass of hyphae forms the mycelium. ### Unicellular Eukaryotes Eukaryotic does not mean that the organism must be multicellular. Many eukaryotes complete their life as a single cell. Amoebae- These are unicellular eukaryotes, and many forms move or capture food with temporary cytoplasmic extensions called pseudopodia. Ciliates- They are single-celled eukaryotes having numerous cilia in many forms. Cilia are used for movement and can also help in movement of food. Yeasts- These are unicellular fungi. They contain a nucleus and other eukaryotic cell components even though the whole organism consists of a single cell. Unicellular algae- Many algae also occur as single cells and carry out photosynthesis. Other algal forms may be colonial or multicellular. The term "protist" is still used for convenience for many diverse eukaryotes. It is a broad conventional grouping, not one uniform structural type or a single natural taxonomic group. ### Specialized Eukaryotic Cells In multicellular organisms, the basic eukaryotic cell structure can be highly modified according to its function. - Neurons- These cells develop long processes such as axons and branched dendrites. Their shape is associated with receiving and carrying cellular signals. - Skeletal muscle cells- These are very long cells formed by fusion of many developing muscle cells. Hence, a mature skeletal muscle fibre contains many nuclei. - Sperm cells- Their structure is strongly modified for reproduction. A sperm contains a compact head and a flagellum, while mitochondria are concentrated near the base of flagellum to support movement. - Mature mammalian erythrocytes- These provide an important exception to the usual eukaryotic cell structure. During maturation, the nucleus is removed and other organelles including mitochondria, ER, Golgi apparatus and ribosomes are also lost. The mature cell therefore remains without a nucleus and most internal organelles. ## Cell Division in Eukaryotic Cells Cell division is required for growth, replacement of old cells and reproduction in eukaryotes. Mitosis and meiosis are the two important types of nuclear division. ![Comparison of mitosis producing two nuclei with the original chromosome-set number and meiosis producing four haploid nuclei after two divisions.](https://biologynotesonline.com/wp-content/uploads/2024/04/Mitosis-vs-Meiosis-in-Eukaryotic-Cells-974x1024.webp)Comparison of mitosis producing two nuclei with the original chromosome-set number and meiosis producing four haploid nuclei after two divisions. ### Mitosis [Mitosis](https://biologynotesonline.com/mitosis/)- It is a type of nuclear division where replicated chromosomes are separated into two daughter nuclei. The chromosome number remains same. Role- Mitosis is used for growth and replacement of cells in multicellular organisms. It also takes part in asexual reproduction of many eukaryotes. ### Meiosis [Meiosis](https://biologynotesonline.com/meiosis/)- It is a special type of nuclear division associated with sexual reproduction. One chromosome replication is followed by two divisions. Chromosome reduction- During meiosis, chromosome number is reduced from diploid to haploid. This helps in maintaining the chromosome number after fertilization. Role in sexual life cycle- Meiosis forms haploid cells required for sexual reproduction. During fertilization, two haploid cells combine and diploid chromosome number is restored. ## Origin of the Eukaryotic Cell The exact process involved in origin of eukaryotic cell is not completely known. It is considered that different changes took place over a long period of evolution. Internal compartment formation and endosymbiosis were important among them. ![Evolutionary schematic showing an Asgard-related archaeal lineage, alphaproteobacterial origin of mitochondria and later cyanobacterial origin of primary plastids.](https://biologynotesonline.com/wp-content/uploads/2024/04/Endosymbiotic-Origin-of-Mitochondria-and-Plastids-in-Eukaryotic-Cells-1024x455.webp)Evolutionary schematic showing an Asgard-related archaeal lineage, alphaproteobacterial origin of mitochondria and later cyanobacterial origin of primary plastids. - Early eukaryotic lineage- Eukaryotes show close evolutionary relationship with Archaea, particularly in many genes involved in genetic information processing. Later, a more complex cellular organization developed. - Internal compartments- Development of internal membranes separated different cellular reactions into specific regions. The nucleus and endomembrane system also developed during early eukaryotic evolution. Their exact evolutionary origin is still not completely understood. - [Endosymbiosis](https://biologynotesonline.com/endosymbiotic-theory-origin-of-the-eukaryotic-cell/)- It is a process where one cell lives inside another cell and becomes permanently associated with it. Origin of mitochondria and plastids is based on this process. - Origin of mitochondria- An ancestral host cell developed an association with an alphaproteobacterium. With time, the bacterial cell became integrated with the host and gave rise to mitochondrion. - Mitochondrial evidence- Mitochondria contain their own DNA and divide from pre-existing mitochondria. Their genetic system shows bacterial relationship. Many genes of the original endosymbiont were later transferred to the nuclear genome. - Origin of plastids- A cyanobacterium was taken up by an ancestral eukaryotic cell in the lineage leading to plants and many algae. During evolution, it became permanently integrated and gave rise to primary plastids, including chloroplasts. - Plastid evidence- Plastids contain their own DNA and divide from pre-existing plastids. Their genetic features show relationship with cyanobacteria. - Gene transfer- Many genes from the original bacterial endosymbionts were transferred to the nucleus during evolution. Therefore, present mitochondria and plastids depend on many proteins produced from nuclear genes. - Eukaryotic evolution- Modern eukaryotic cell was not formed in a single step. Different cellular changes, endosymbiosis and integration of these systems took place during its evolution. ## References - Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th ed.). Garland Science. [https://www.ncbi.nlm.nih.gov/books/NBK21054/](https://www.ncbi.nlm.nih.gov/books/NBK21054/) - Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. 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