Cell Organelles: Major Types, Functions, and Examples

Cell organelles are specialized structures present within cells that perform particular cellular functions. An organelle is a part of the cell. It is not a separate cell. Different organelles carry out different activities required for normal cell function.

There is no single fixed number of cell organelles used for every cell. Common study lists may contain different numbers depending on the terminology used, structures included in the list and the type of cell being described. The cell organelles discussed in introductory biology are mainly those of eukaryotic cells. These cells contain a nucleus and several membrane-bound organelles that form specialized compartments inside the cell.

The nucleus, mitochondria, endoplasmic reticulum (ER) and Golgi apparatus are some major membrane-bound organelles. Ribosomes have no surrounding membrane, but these structures are also commonly included while studying cell organelles. The following sections describe the major cell organelles and their functions.

How Many Cell Organelles Are There?

There is no single fixed number of cell organelles for all eukaryotic cells. Some study lists describe 12 organelles, while other lists contain 20 or another number. The number depends on which cellular structures are included in the list. Some structures are also counted together in one source and separately in another. Plant and animal cells do not contain exactly same set of organelles.

For example, rough and smooth endoplasmic reticulum (ER) may be taken together as ER. They can also be written as two separate organelles. Vesicles and vacuoles are treated in similar way in some lists.

The centrosome, centrioles, cytoskeleton, cilia, flagella, nucleolus and ribosomes do not have the same membrane-bound organization as structures such as mitochondria or Golgi apparatus. Still, these structures are commonly included in cell organelle lists. Plasma membrane, cytoplasm and cell wall may also be found in a “parts of a cell” list. They should not automatically be counted as organelles.

What Are the 12 Cell Organelles?

A 12-organelles list generally represents a smaller study list of major organelles and commonly included non-membranous structures. One such list contains:

1. Nucleus
2. Nucleolus
3. Ribosomes
4. Rough ER
5. Smooth ER
6. Golgi apparatus
7. Mitochondria
8. Lysosomes
9. Peroxisomes
10. Vesicles
11. Vacuoles
12. Centrosome

The exact 12 is not same in every book or study resource.

Why Some Lists Contain 20 Organelles?

A list of 20 cell organelles usually separates more cellular structures into individual entries. Rough ER and smooth ER are counted separately. Endosomes, cytoskeleton, centrioles, cilia and flagella may also be added. In plant-cell lists, chloroplasts and other plant structures can increase the number.

A larger list may contain nucleus, nucleolus, ribosomes, rough ER, smooth ER, Golgi apparatus, mitochondria, lysosomes, peroxisomes, endosomes, vesicles, vacuoles, cytoskeleton, centrosome, centrioles, cilia, flagella and chloroplasts. Some lists further add structures such as plasma membrane or cell wall. In this case the list becomes more like an expanded “parts of a cell” list, because all these structures are not equivalent under a strict organelle classification.

In this article, the membrane-bound organelles are treated as the main organelles. Nucleus, ER, Golgi apparatus, mitochondria, lysosomes, peroxisomes, endosomes, vesicles and vacuoles are included in this group. Chloroplasts and other plastids are included where plant or algal cells are discussed.

Ribosomes, cytoskeleton, centrosome, centrioles, cilia, flagella and nucleolus are discussed as commonly included non-membrane-bound cellular structures. Plasma membrane, cytoplasm and cell wall are kept as other cell structures rather than adding them only to make a fixed organelle number.

Major Cell Organelles and Their Functions

The major cell organelles perform different functions inside a cell. Some are surrounded by membrane, while structures such as ribosomes, nucleolus and cytoskeleton have no surrounding membrane. The following table gives the major organelles and commonly included cellular structures of eukaryotic cells.

Nucleus

Nucleus is a membrane-bound organelle of eukaryotic cells that contains most of the genetic material of the cell and controls its use.

Structure of Nucleus

• The nucleus is enclosed by a nuclear envelope. It is made up of two membranes, the inner and outer nuclear membrane. A narrow space occurs between these two membranes.
• The outer nuclear membrane is continuous with the endoplasmic reticulum (ER). Ribosomes may be attached on its cytoplasmic surface.
• Nuclear pores are present in the nuclear envelope. These pores contain nuclear pore complexes. They regulate the movement of proteins, ribonucleic acid (RNA) and other molecules between the nucleus and cytoplasm.
• The inner part of the nucleus contains a semi-fluid material called nucleoplasm.
• Chromatin is present in the nucleoplasm and is mainly formed of deoxyribonucleic acid (DNA) associated with proteins. During cell division, chromatin becomes highly condensed. It forms the visible chromosomes.
• One or more nucleoli may be present within the nucleus. The nucleolus has no surrounding membrane. Ribosomal RNA (rRNA) is synthesized and processed here, and early assembly of ribosomal subunits also takes place in the nucleolus.
• A protein network known as the nuclear lamina is found along the inner surface of nuclear envelope. It provides structural support to the nucleus and is associated with organization of chromatin.

Functions of Nucleus

• The nucleus stores most of the genetic information of eukaryotic cell in the form of DNA.
• It forms a separate compartment for the genetic material. Many nuclear activities are separated from those taking place in the cytoplasm.
• DNA replication takes place inside the nucleus before cell division.
• The nucleus is the main site for transcription. Here, the information present in DNA is copied into RNA. Several RNA-processing events also take place within the nucleus before RNA is transported into the cytoplasm.
• Nuclear pores regulate movement of molecules between the nucleus and cytoplasm. Proteins required in nucleus are transported inward, while different RNA molecules and assembled ribosomal subunits are carried out.
• The nucleolus takes part in formation of ribosomes. rRNA is produced and combined with ribosomal proteins to form ribosomal subunits. These subunits then leave the nucleus through nuclear pores.

Ribosomes

Ribosomes are non-membranous cellular structures made up of ribosomal RNA (rRNA) and proteins, and are the site for protein synthesis in cells.

Structure of Ribosomes

• Ribosomes have no surrounding membrane. They are small ribonucleoprotein particles found in all cells.
• Each ribosome is made up of two unequal parts, a small subunit and a large subunit. The two subunits come together during protein synthesis.
• Eukaryotic cells contain 80S ribosomes in the cytosol. They consist of a 40S small subunit and a 60S large subunit. The “S” represents the Svedberg (S) sedimentation unit and these values are not added arithmetically.
• The 40S subunit contains 18S rRNA along with ribosomal proteins. The 60S subunit contains 28S, 5.8S and 5S rRNAs and a larger group of proteins.
• Ribosomes occur freely in the cytosol or remain attached to the cytosolic surface of rough endoplasmic reticulum (ER) and outer nuclear membrane.
• Mitochondria and chloroplasts also contain their own ribosomes. These ribosomes differ from the 80S ribosomes present in eukaryotic cytosol and have several features related to bacterial ribosomes.
• Three major transfer RNA (tRNA)-binding sites are present in a functioning ribosome, called A, P and E sites. The A site accepts incoming aminoacyl-tRNA, the P site holds the tRNA carrying growing polypeptide chain and the E site is used for exit of the uncharged tRNA.

Functions of Ribosomes

• Ribosomes are responsible for translation, where the nucleotide sequence carried by messenger RNA (mRNA) is used for formation of a polypeptide chain.
• The small ribosomal subunit binds the mRNA and takes part in decoding its codons. It also helps to position the corresponding tRNA molecules during translation.
• The large subunit is involved in formation of peptide bonds between amino acids. Its rRNA forms the catalytic center for this reaction.
• Free ribosomes mainly synthesize proteins that remain in the cytosol or are transported to structures such as nucleus, mitochondria and peroxisomes after synthesis.
• Ribosomes associated with rough ER synthesize proteins entering the ER. Many of these proteins are used for secretion, inserted into cellular membranes or carried further through the endomembrane system to other locations such as Golgi apparatus and lysosomes.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a continuous membrane system of eukaryotic cells involved mainly in protein processing, lipid synthesis and intracellular transport.

Structure of Endoplasmic Reticulum

• The ER is formed of an interconnected network of membrane-bound tubules and flattened sacs called cisternae. These membranes extend throughout the cytoplasm.
• The internal space enclosed by ER membrane is known as the ER lumen or cisternal space.
• ER membrane is continuous with the nuclear envelope. This forms a continuous membrane system between the nuclear envelope and ER.
• The ER occurs in two commonly described forms, rough ER and smooth ER. They are parts of the same membrane system but differ in their appearance and functions.
• Rough ER has ribosomes attached to its cytosolic surface. It generally appears as flattened membrane sheets or cisternae, and is especially well developed in cells producing large amounts of secreted or membrane proteins.
• Smooth ER lacks attached ribosomes. It is more tubular in structure and becomes abundant in some cells which are highly active in lipid and steroid metabolism.

Functions of Endoplasmic Reticulum

• Rough ER takes part in synthesis of proteins that are secreted from the cell, inserted into cellular membranes or transported through the endomembrane system. These proteins enter the ER during their synthesis.
• Newly synthesized proteins undergo folding and some initial modifications within the ER. Proteins which are to move further through the secretory pathway are packed into transport vesicles and carried toward the Golgi apparatus.
• Smooth ER is mainly involved in lipid synthesis. Many membrane lipids and their precursors are produced in the ER.
• In steroid-producing cells, smooth ER contains enzymes required for synthesis and modification of steroid molecules. Such cells may contain a large amount of smooth ER.
• Smooth ER of liver cells participates in metabolism and detoxification of several lipid-soluble compounds and drugs.
• The ER also stores calcium ions (Ca²⁺) and regulates their release into cytosol. In muscle cells, a specialized form of smooth ER called the sarcoplasmic reticulum stores and releases Ca²⁺ during muscle contraction and relaxation.

Golgi Apparatus

The Golgi apparatus is a membrane-bound organelle found in eukaryotic cells that is involved in modification, sorting and transport of proteins and lipids. It is also known as the Golgi complex or Golgi bodies.

Structure of Golgi Apparatus

• The Golgi apparatus is made up of flattened membrane-bound sacs known as cisternae. These sacs are arranged one above another forming a stack.
• Each cisterna is surrounded by a single membrane and contains an internal space called lumen.
• The Golgi apparatus is generally located near the nucleus and endoplasmic reticulum (ER) in many animal cells. In plant cells, several Golgi stacks are distributed throughout the cytoplasm.
• It has two distinct faces, the cis face and trans face. The cis face is the receiving side of Golgi apparatus, which receives materials coming from ER. The trans face is involved in sorting and sending out the processed materials.
• The middle part of the Golgi stack contains medial cisternae. Different enzymes are present in the cis, medial and trans regions, which carry out different processing reactions.
• Small transport vesicles are found around the Golgi stacks. They help in movement of materials to and from the Golgi apparatus.

Functions of Golgi Apparatus

• The Golgi apparatus receives proteins and lipids coming from ER. These materials undergo further modification before being transported to other cellular compartments.
• It is involved in glycosylation of proteins and lipids. The carbohydrate chains attached to these molecules are modified by the addition or removal of specific sugar residues.
• The Golgi apparatus also takes part in synthesis of certain glycolipids and proteoglycans.
• Proteins and lipids are sorted at the trans-Golgi network (TGN) according to their destination. They are transported toward the plasma membrane, secretory vesicles or other compartments such as endosomes and lysosomes.
• Proteins which are to be secreted outside the cell are packed into transport vesicles. These vesicles move toward the plasma membrane and release their contents by fusion with the membrane.
• In plant cells, Golgi apparatus is involved in synthesis of cell wall polysaccharides, mainly pectins and hemicelluloses. These materials are transported to the cell surface through secretory vesicles.
• During plant cell division, Golgi-derived vesicles also take part in formation of the cell plate, which develops into the new partition between daughter cells.

Mitochondria

Mitochondria are double-membrane organelles of eukaryotic cells where aerobic energy metabolism and most adenosine triphosphate (ATP) production take place.

Structure of Mitochondria

• Mitochondria are surrounded by two membranes, an outer mitochondrial membrane and an inner mitochondrial membrane. The space present between them is called the intermembrane space.
• The outer membrane is relatively permeable to small molecules due to the presence of channel proteins called porins.
• The inner membrane is highly selective and much less permeable to ions and most small molecules. It contains the protein complexes required for electron transport and ATP synthesis.
• The inner membrane folds inward at many places. These folds are known as cristae. They increase the surface area of inner membrane where oxidative phosphorylation takes place.
• The internal space enclosed by the inner membrane is the mitochondrial matrix. It contains enzymes involved in the citric acid cycle and several other reactions of oxidative metabolism.
• Mitochondria contain their own deoxyribonucleic acid (DNA) and ribosomes. The mitochondrial genome encodes some of the proteins and RNA molecules required by the organelle, while most mitochondrial proteins are encoded by nuclear genes and imported from cytosol.

Functions of Mitochondria

• The major function of mitochondria is production of ATP by oxidative phosphorylation. A large part of ATP required by aerobic animal cells is produced in this process.
• The citric acid cycle takes place mainly in the mitochondrial matrix. Acetyl coenzyme A is oxidized here, producing reduced electron carriers that supply electrons to the respiratory chain.
• The electron transport chain is present in the inner mitochondrial membrane. Electron transfer through this chain pumps protons from the matrix and forms an electrochemical proton gradient across the inner membrane.
• Protons return to the matrix through ATP synthase. The energy released by this movement is used for formation of ATP from adenosine diphosphate (ADP) and inorganic phosphate.
• Mitochondria take up and release calcium ions (Ca²⁺) and participate in cellular calcium signaling. Mitochondrial Ca²⁺ can also regulate mitochondrial metabolism according to cellular activity.
• They also take part in regulation of apoptosis. During the mitochondrial pathway of apoptosis, mitochondrial changes can result in release of proteins that participate in the cell-death process.
• Mitochondrial DNA and ribosomes allow synthesis of a small number of proteins encoded by the mitochondrial genome. Most other mitochondrial proteins are synthesized on cytosolic ribosomes and then transported into mitochondria.

Lysosomes

Lysosomes are single membrane-bound organelles containing digestive enzymes used for breakdown and recycling of different cellular materials.

Structure of Lysosomes

• Lysosomes are enclosed by a single membrane. Their size and internal contents can vary depending on the material being digested and the state of the cell.
• The inner space of lysosome is acidic, generally around pH 4.5 to 5. This acidic condition is required for proper activity of lysosomal enzymes.
• A large number of acid hydrolases are present within lysosomes. These include proteases, nucleases, lipases, glycosidases, phosphatases and other enzymes which can break down proteins, nucleic acids, lipids and carbohydrates.
• The lysosomal membrane contains a proton pump known as vacuolar H⁺-adenosine triphosphatase (V-ATPase). It moves H⁺ into the lysosome and maintains its acidic internal condition. Energy for this transport is obtained from adenosine triphosphate (ATP).
• Different transport proteins are also present in lysosomal membrane. Products formed after digestion, such as amino acids, sugars and nucleotides, can be transported from lysosome back into cytosol.
• Lysosomes are connected with the endosomal pathway. Lysosomal hydrolases are synthesized through the secretory pathway and delivered through Golgi and endosomal compartments, while endosomes carrying material taken up from cell surface can mature and fuse into the lysosomal system.

Functions of Lysosomes

• The primary function of lysosomes is intracellular digestion. Proteins, lipids, polysaccharides and nucleic acids can be broken down by the hydrolytic enzymes present within them.
• Materials entering the cell by endocytosis are transported through endosomes and can finally reach lysosomes for degradation. Large particles taken up by phagocytosis are also digested after the phagocytic compartment joins with lysosomal compartments.
• Lysosomes remove worn-out or damaged parts of the cell by autophagy. Cellular components are enclosed and delivered to lysosomes, where they are digested by lysosomal enzymes.
• The products obtained after digestion can be returned to cytoplasm and used again by the cell. Amino acids, sugars and other small molecules are released through specific membrane transport proteins.
• Lysosomal degradation also helps in destruction of some microorganisms and other foreign materials taken up by the cell.
• Lysosomes also participate in cellular processes other than digestion. They are involved in nutrient sensing, membrane trafficking, membrane repair and some signaling processes.

Peroxisomes

Peroxisomes are small single membrane-bound organelles of eukaryotic cells containing enzymes mainly involved in oxidative reactions, fatty acid metabolism and control of hydrogen peroxide (H₂O₂).

Structure of Peroxisomes

• Peroxisomes are surrounded by a single membrane. Inside the membrane is a dense matrix containing different metabolic enzymes.
• They do not contain their own deoxyribonucleic acid (DNA) or ribosomes. All peroxisomal proteins are encoded by nuclear genes and proteins required inside the organelle are imported from the cytosol.
• The peroxisomal matrix contains several oxidases and the enzyme catalase. Oxidases can form H₂O₂ during their reactions, while catalase breaks down H₂O₂ or uses it in other oxidative reactions.
• Peroxisomes are not identical in every cell. Their enzyme content can differ according to cell type and the metabolic activity of the cell.
• They are dynamic organelles. Peroxisomes can change in their size, number and shape depending on cellular conditions and metabolic requirement.

Functions of Peroxisomes

• Peroxisomes carry out several oxidative reactions in which molecular oxygen is used. H₂O₂ is formed during many of these reactions.
• The catalase present in peroxisomes removes excess H₂O₂. It can convert H₂O₂ into water and oxygen, reducing its accumulation inside the cell.
• Fatty acid β-oxidation takes place in peroxisomes. In mammalian cells, peroxisomes are especially involved in breakdown and shortening of very-long-chain and some branched-chain fatty acids.
• They are also involved in lipid synthesis. In animal cells, the first reactions required for formation of plasmalogens, a group of ether phospholipids, take place in peroxisomes.
• In liver cells, peroxisomal reactions also take part in metabolism of some toxic compounds and in pathways associated with bile acid synthesis.
• Plant peroxisomes perform some additional functions. They participate in photorespiration, while peroxisomal β-oxidation is also involved in fatty acid metabolism and several metabolic processes of plant cells.

Endosomes

Endosomes are membrane-bound compartments of eukaryotic cells that receive and sort materials entering the cell by endocytosis.

Structure of Endosomes

• Endosomes have a single surrounding membrane. Their structure is not fixed and includes vesicular as well as tubular regions.
• Early endosomes are generally located near the cell periphery. They have an irregular shape. Membrane tubules extend out from their vesicular regions.
• The endosomal lumen is acidic. Hydrogen ions (H⁺) are pumped inside by membrane proton pumps. Late endosomes are generally more acidic than early endosomes.
• Early endosomes can develop into late endosomes. During this change the position, membrane proteins and internal structure of endosome also changes. Late endosomes are commonly present nearer the nucleus.
• Many late endosomes contain small membrane vesicles inside their lumen. These are known as multivesicular bodies (MVBs). The internal vesicles are formed by inward budding of endosomal membrane.
• Recycling endosomes are another part of the endosomal system. Selected membrane proteins and lipids enter these endosomes before moving back to the plasma membrane.

Functions of Endosomes

• Endosomes mainly perform sorting of materials entering the cell through endocytosis.
• Materials reaching an early endosome do not all move in same direction. Some are returned to the plasma membrane. Others remain in the endosomal pathway and move toward late endosomes and lysosomes.
• The acidic condition inside early endosome can separate some ligands from their receptors. The receptor may return back to cell surface while the ligand moves further through the endosomal pathway.
• Recycling endosomes return membrane proteins and lipids to the plasma membrane. In polarized cells, selected materials can also be moved from one region of plasma membrane to another.
• Late endosomes carry materials mainly toward lysosomal degradation.
• Endosomes also regulate the amount of some receptors present on cell surface. The receptors may be recycled for further use or passed toward lysosomes where they are degraded.

Vacuoles

Vacuoles are single membrane-bound compartments mainly involved in storage, degradation and maintenance of water and ion balance in plant and fungal cells.

Structure of Vacuoles

• Vacuoles are surrounded by a single membrane known as the tonoplast. It separates the vacuolar contents from cytoplasm.
• Inside the vacuole is an aqueous solution containing water, ions and different dissolved substances.
• Plant vacuoles differ greatly in size. A mature plant cell commonly has one large central vacuole, which may occupy most of the cell volume.
• The tonoplast contains proton pumps, ion channels and different transport proteins. These regulate movement of ions and other substances between cytoplasm and vacuole.
• The inside of many vacuoles is acidic. Lytic vacuoles also contain hydrolytic enzymes for breakdown of cellular materials.
• Vacuoles are not same in every cell. Some mainly perform lytic functions, while storage vacuoles are specialized for accumulation of proteins and other materials.

Functions of Vacuoles

• Vacuoles store water, ions, sugars, amino acids, proteins, pigments and different metabolites. Waste and toxic substances can also be accumulated inside them.
• The central vacuole maintains turgor pressure in plant cells. Water entering the vacuole increases its volume and produces pressure against the cell wall.
• Vacuole enlargement also helps in growth of plant cells. A large part of cell volume can be increased by taking up water without producing same amount of new cytoplasm.
• Lytic vacuoles break down macromolecules and other cellular materials. Hydrolytic enzymes perform many of the degradative functions carried out by lysosomes in animal cells.
• Vacuoles take part in maintaining ion and pH balance of cell. Transport across the tonoplast controls accumulation and release of many ions and solutes.
• Some harmful compounds are isolated inside vacuoles. Pigments and different compounds involved in plant defense can also be stored there.

Vesicles

Vesicles are small membrane-bound sacs present in eukaryotic cells, mainly used for transport and movement of materials between different parts of cell.

Structure of Vesicles

• Vesicles are surrounded by a single lipid bilayer membrane which encloses their contents.
• Most transport vesicles are formed by budding from an existing cellular membrane. A small part of the membrane bends outward and finally separates, carrying selected proteins, lipids or soluble materials with it.
• The membrane and contents are not same in all vesicles. They depend on where the vesicle is formed and where it has to go.
• Many newly forming transport vesicles have a protein coat on their cytoplasmic surface. Clathrin, coat protein complex I (COPI) and coat protein complex II (COPII) form three major types of coated vesicles. The coat helps in selecting cargo and also bends the membrane during vesicle formation. It is usually removed before fusion with the target membrane.
• Specific proteins are also present on vesicle membrane. Rab proteins and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) take part in recognition, docking and fusion with the correct target membrane.

Functions of Vesicles

• Vesicles transport proteins, lipids and other cellular materials from one membrane-bound compartment to another.
• Proteins leaving the endoplasmic reticulum are carried toward the Golgi apparatus in transport vesicles. Other vesicles move materials between Golgi compartments and back toward the endoplasmic reticulum.
• Secretory vesicles carry cellular products toward the plasma membrane. Their membrane fuses with plasma membrane and the contents are released outside the cell.
• Vesicles are also formed during endocytosis. Materials from outside the cell are enclosed by the plasma membrane and carried into the cell for further sorting or processing.
• They deliver selected materials to endosomes and lysosomes. Some vesicles also return membrane components and proteins back to their original compartment.
• Vesicle fusion allows the transported cargo to enter its correct target compartment without mixing the contents directly with cytoplasm.

Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments present in the cytoplasm of eukaryotic cells that gives structural support and takes part in cell movement, transport and cell division.

Structure of Cytoskeleton

• The cytoskeleton is mainly formed of three types of protein filaments, microfilaments (actin filaments), intermediate filaments and microtubules. These filaments extend through different regions of the cytoplasm.
• Microfilaments are the thinnest filaments, about 7 nm in diameter. They are made up of actin proteins arranged into filamentous strands.
• The actin filaments are dynamic. They can assemble and disassemble rapidly, allowing the arrangement of cytoskeleton to change according to cellular activity.
• Intermediate filaments are about 8 to 10 nm in diameter. They are rope-like fibers made from different fibrous proteins, and the proteins forming these filaments differ among different cell types.
• Microtubules are hollow tubular structures about 25 nm in diameter. Their walls are formed from α-tubulin and β-tubulin dimers. They are the widest of the three major cytoskeletal filaments.
• Microtubules also undergo continuous growth and shortening. Actin filaments and microtubules are more dynamic than intermediate filaments and their arrangement can change quickly inside the cell.
• Different accessory and motor proteins remain associated with these cytoskeletal filaments. Myosin works mainly with actin filaments, while kinesin and dynein move along microtubules.

Functions of Cytoskeleton

• The cytoskeleton provides shape and mechanical support to the cell. Microfilaments and intermediate filaments resist tension, while microtubules help the cell resist compressive forces.
• It helps in positioning the nucleus and other cellular organelles. Intermediate filaments form a supporting network, and microtubules also help determine the position of membrane-bound organelles.
• Cytoskeletal filaments take part in intracellular transport. Vesicles and organelles can move along microtubules with the help of motor proteins.
• Actin filaments are involved in different types of cell movement. Actin together with myosin produces movement in many cells and also takes part in muscle contraction.
• The cytoskeleton is involved in cell division. Microtubules form the mitotic spindle which moves duplicated chromosomes, while actin filaments participate in division of cytoplasm during cytokinesis.
• In plant cells, actin filaments also take part in cytoplasmic streaming, producing movement of cytoplasm and cellular materials through the cell.
• Microtubules form the main internal structural framework of eukaryotic cilia and flagella. These structures are used for movement of cells or movement of substances over the cell surface.

Centrosome

Centrosome is a non-membranous microtubule-organizing center (MTOC) of most animal cells, mainly involved in organization of microtubules and formation of mitotic spindle.

Structure of Centrosome

• The centrosome is generally present near the nucleus. It has no surrounding membrane.
• A typical centrosome contains a pair of centrioles surrounded by a protein-rich material called pericentriolar material (PCM).
• The two centrioles are cylindrical structures, usually placed approximately at right angles to each other.
• Each centriole is formed of nine sets of microtubule triplets arranged around its circumference. The centrioles lie inside the PCM, but most cytoplasmic microtubules do not grow directly from the centriole wall.
• PCM forms the major microtubule-nucleating part of centrosome. It contains γ-tubulin complexes and many other proteins needed for formation and anchoring of microtubules.
• The centrosome duplicates once during the cell cycle. After duplication, the two centrosomes separate before and during mitosis and move toward opposite sides of the cell.

Functions of Centrosome

• Centrosome acts as the major MTOC in many animal cells. Microtubules are nucleated in its PCM and extend outward into the cytoplasm.
• It organizes the cytoplasmic microtubule network. The minus ends of many microtubules remain associated with centrosome, while the plus ends extend toward different parts of cell.
• During mitosis, the two centrosomes form the two poles of the mitotic spindle. Spindle microtubules extend from these regions and take part in separation of duplicated chromosomes.
• Centrosome also helps in organizing the position and arrangement of different cell structures through its control over microtubules.
• The centrioles of centrosome can give rise to basal bodies. Basal bodies are used for formation and anchoring of cilia and flagella in cells having these structures.

Centrioles

Centrioles are non-membranous cylindrical structures made up of microtubules, found mainly in animal cells and associated with centrosomes, cilia and flagella.

Structure of Centrioles

• Centrioles are short cylindrical structures. They have no surrounding membrane.
• A typical centriole is made up of nine microtubule triplets arranged in a circular form. This produces the characteristic ninefold symmetry of centriole.
• Each triplet contains A, B and C tubules. The A tubule is a complete microtubule, whereas B and C tubules are incomplete and share part of their wall with the neighboring tubule.
• The proximal part of a developing centriole contains a cartwheel structure. It has a central hub with nine spokes and helps in setting up the ninefold arrangement of microtubule triplets.
• In centrosome, two centrioles are commonly present. They are generally arranged approximately at right angle to each other and are surrounded by pericentriolar material (PCM).
• The two centrioles are not exactly same in age and structure. The older mother centriole develops distal and subdistal appendages, while the younger daughter centriole acquires these structures later during maturation.
• A new centriole is formed close to each existing centriole during the cell cycle. The new structure starts as a procentriole and gradually elongates and matures.

Functions of Centrioles

• Centrioles form the structural core of the centrosome. They help in organization and recruitment of PCM around them.
• The centrosome organizes many cytoplasmic microtubules. These microtubules are mainly nucleated from the surrounding PCM, not directly from centriole wall.
• During cell division, centrioles duplicate and become associated with the two centrosomes. The centrosomes move apart and help in organization of spindle poles and mitotic spindle microtubules. Centrioles themselves are not essential for spindle formation in every type of eukaryotic cell.
• A mature centriole can become a basal body. The basal body anchors at the cell surface and organizes formation of cilia or flagella.
• Centriole duplication is coordinated with the cell cycle. This maintains proper number of centrioles and centrosomes when cells divide.

Cilia and Flagella

Cilia and flagella are membrane-covered, microtubule-based projections of eukaryotic cells used for movement and, in many cilia, cellular signaling.

Structure of Cilia and Flagella

• Cilia are generally short and occur in large numbers on the cell surface. Flagella are usually longer, and one or a few are commonly present on a cell.
• The outer covering is continuous with the plasma membrane. Inside it is a microtubule core known as the axoneme.
• Most motile cilia and eukaryotic flagella have a characteristic 9 + 2 arrangement. Nine outer microtubule doublets are arranged around two single central microtubules.
• Each outer doublet contains an A tubule and B tubule. The A tubule is a complete microtubule, while B tubule is incomplete and shares part of its wall with the A tubule.
• Axonemal dynein arms are attached to the outer microtubule doublets. Radial spokes extend toward the central pair and other proteins connect the neighboring doublets. These structures take part in producing and controlling the bending movement.
• At the base, each cilium or flagellum is attached to a basal body. The basal body has nine groups of microtubule triplets and is structurally similar to a centriole. It anchors the structure to the cell and organizes formation of axonemal microtubules.
• Primary cilia usually have a 9 + 0 arrangement, with nine outer doublets but no central pair. They are generally non-motile and commonly occur as a single cilium on a cell. Some specialized 9 + 0 cilia, such as nodal cilia, are motile.

Functions of Cilia and Flagella

• Flagella are mainly used for movement of the whole cell. The flagellum of a sperm cell, for example, produces the movement required for swimming.
• Motile cilia may also move the cell itself. In unicellular organisms such as Paramecium, repeated beating of many cilia produces locomotion.
• In many tissues, the cell remains stationary and cilia move materials over its surface. Cilia of respiratory epithelium move mucus and trapped particles along the airways. Cilia of the oviduct help in movement of the egg toward the uterus.
• Movement is produced by the sliding activity of the outer microtubule doublets. Dynein uses energy from adenosine triphosphate (ATP) and acts between adjacent doublets, while their structural connections convert sliding into bending of the cilium or flagellum.
• Primary cilia mainly perform sensory and signaling functions rather than rapid beating. Their membrane contains specific receptors and signaling proteins which allow the cell to receive different chemical and physical signals.

Plastids

Plastids are double membrane-bound organelles found in plant and algal cells, involved in synthesis and storage of food, photosynthesis and formation of several cellular compounds.

Structure of Plastids

• Plastids are surrounded by an outer and inner membrane. These two membranes form the plastid envelope.
• Inside the envelope is a protein-containing internal region. The internal membrane system is not same in all plastids. Chloroplasts have a well-developed thylakoid membrane system, while many non-photosynthetic plastids have much less internal membrane.
• Plastids contain their own deoxyribonucleic acid (DNA). They also have ribosomes and part of their own protein-synthesizing machinery, although most plastid proteins are encoded by genes in the nucleus.
• Different forms of plastids occur in different tissues. Chloroplasts are green photosynthetic plastids. Chromoplasts contain large amounts of carotenoid pigments, while leucoplasts are generally colourless.
• Leucoplasts can be further specialized. Amyloplasts contain large starch granules, elaioplasts are associated with lipid synthesis and storage, and proteinoplasts accumulate proteins.
• Proplastids are small undifferentiated plastids present mainly in meristematic cells. They develop into different plastid forms according to tissue and developmental condition. One plastid type may also change into another.

Functions of Plastids

• Plastids take part in the synthesis and storage of food materials in plant cells. Starch, lipids and proteins can be synthesized or stored in different forms of plastids.
• Photosynthesis takes place in photosynthetic plastids. Light energy is used for production of chemical energy and fixation of carbon dioxide into organic compounds.
• Plastids are important sites for synthesis of fatty acids and several membrane lipids.
• Several amino acids are synthesized partly or completely inside plastids. They also take part in nitrogen and sulfur metabolism of plant cells.
• Plastids synthesize many pigments and their precursors. Chlorophylls and carotenoids are produced through metabolic pathways associated with plastids.
• They are also involved in synthesis of several isoprenoid compounds and other metabolites required for normal plant growth and metabolism.
• Plastids can change their metabolic activity according to tissue and developmental stage. Their functions are not same in all cells.

Chloroplasts

Chloroplasts are green double membrane-bound plastids found in plants and algae, where photosynthesis takes place.

Structure of Chloroplasts

• Chloroplast is enclosed by two membranes, an outer membrane and an inner membrane. A narrow intermembrane space lies between them.
• The inner membrane surrounds a fluid-filled region known as the stroma. It contains many enzymes needed for carbon metabolism. Chloroplast deoxyribonucleic acid (DNA) and ribosomes are also found here.
• A third membrane system is present inside the chloroplast. It forms flattened membrane sacs called thylakoids.
• Thylakoids are arranged into stacks at many places. Each stack is known as a granum, while several stacks are called grana. Unstacked thylakoid membranes connect different grana with each other.
• The space present inside a thylakoid is called the thylakoid lumen. Chlorophyll, photosystems, electron carriers and adenosine triphosphate (ATP) synthase are located in the thylakoid membrane.
• Chloroplasts have their own DNA and protein-synthesizing machinery, but they are not completely independent. Most chloroplast proteins are encoded by nuclear genes and are transported into the chloroplast after synthesis.

Functions of Chloroplasts

• The major function of chloroplast is photosynthesis. Light energy is trapped and converted into chemical energy.
• Light-dependent reactions take place in the thylakoid membrane. Chlorophyll absorbs light and electrons move through the photosynthetic electron transport system. ATP and reduced nicotinamide adenine dinucleotide phosphate (NADPH) are formed. Oxygen is released from water during these reactions.
• Carbon fixation takes place in the stroma. Carbon dioxide is incorporated into organic compounds by enzymes of the Calvin cycle, using ATP and NADPH produced during the light reactions.
• Chloroplasts also synthesize several important cellular compounds. Fatty acids and a number of amino acids are produced by metabolic reactions present in chloroplast.
• Nitrite is reduced to ammonia in chloroplasts. The ammonia can then be used for formation of nitrogen-containing organic compounds in plant cells.

How Cell Organelles Work Together

Cell organelles work together by exchanging materials, proteins and energy. Different cellular activities are carried out by these organelles in coordination with each other.

1. The nucleus and ribosomes work together for protein synthesis. Inside nucleus, genetic information of deoxyribonucleic acid (DNA) is transcribed into messenger ribonucleic acid (mRNA). The mRNA then moves into cytoplasm. Ribosomes use this information for making proteins.
2. Ribosomes attached with rough endoplasmic reticulum produce proteins which are secreted, inserted into membranes or sent to organelles of the endomembrane system. These proteins enter the endoplasmic reticulum where their folding and initial processing takes place.
3. From endoplasmic reticulum, proteins are carried to the Golgi apparatus through transport vesicles. Golgi modifies and sorts them. Some proteins are packed again into vesicles. They are carried to lysosomes, plasma membrane or released outside the cell.
4. Vesicles carry selected materials between different membrane-bound organelles, budding from one membrane and fusing with another membrane at the required place.
5. The cytoskeleton acts as tracks for movement of vesicles and organelles through cytoplasm. Kinesin and dynein motor proteins carry different cellular materials along microtubules. Mitochondria and secretory vesicles can move through these tracks.
6. Mitochondria produce much of the cellular adenosine triphosphate (ATP). This energy is used in different cellular activities, including movement of materials by motor proteins.
7. Endosomes and lysosomes work together during sorting and degradation. Materials entering the cell by endocytosis can pass through endosomes. Some are then carried to lysosomes and broken down.
8. Lysosomes also break down old or damaged cellular components. During autophagy, parts of cytoplasm or organelles are enclosed inside an autophagosome and delivered to lysosomes. They are degraded there. Some products return back into cytoplasm and can be used again by the cell.
9. In plant cells, chloroplasts and mitochondria are involved in cellular energy transformations. Chloroplasts use light energy during photosynthesis while mitochondria use energy stored in organic molecules for ATP production.

Cell Organelles At a Glance