Cell Organelles Definition
Cell organelles are specialised structures that are found within a given kind of cell and carry out a particular function. There are many different cell organelles, some of which, like cell membranes, nuclei, and cytoplasm, are present in the majority of cell types. However, certain organelles, like plastids and cell walls in plant cells, are unique to a certain kind of cell.
List of Cell Organelle
- Cell membrane (plasma membrane/plasmalemma)
- Cell Wall
- Cilia and Flagella
- Endoplasmic Reticulum (ER)
- Golgi Apparatus/ Golgi Complex/ Golgi Body
- Intermediate filaments
- Storage granules
Cell membrane (plasma membrane/plasmalemma)
The lipids and proteins that make up a plasma membrane are subject to variation depending on the fluidity, external environment, and stage of cell growth.
Structure of Cell Membrane
- A phospholipid bilayer serves as the cell’s structural foundation, and two different kinds of proteins—embedded proteins and peripheral proteins—serve to give the cell its form and permit the passage of particles into and out of the cell.
- The most prevalent lipid in a cell membrane is phospholipid, which has two hydrophobic fatty acid tails and a polar head group.
- With certain proteins serving as receptors for the binding of different components, the embedded proteins function as channels for the transport of particles throughout the cell.
- The purpose of the peripheral proteins is to provide the cell’s structure with both mechanical support and fluidity.
Functions of Cell Membrane
- The cell membrane supports the form of the cell mechanically and protects it and its constituent parts from the outside environment.
- It acts as a semi-permeable membrane that controls what may pass through channels and into and out of the cell, facilitating the interchange of vital substances needed for the cell to survive.
- For the correct operation of the cell and all of the organelles, it produces and distributes signals both within and outside of the cell.
- It permits the cell contact necessary for tissue development and cell fusion.
The cell wall is an extra, non-living layer that is present outside the cell membrane in certain cells and serves as the cell’s structure, defence, and filtration system.
Structure of Cell Wall
- In a fungal cell, the cell wall is constructed of chitin, while in a plant cell, it is composed of cellulose, hemicellulose, and proteins.
- A cell wall is multilayered with a central lamina, a primary cell wall, and a secondary cell wall.
- Polysaccharides in the middle lamina provide adhesion and permit cell attachment to one another.
- The major cell wall, which is made of cellulose, comes after the middle lamina. The secondary cell wall, which is comprised of cellulose and hemicellulose, is the last layer and is not always present.
Functions of Cell Wall
- The protection and maintenance of the cell’s form is the primary purpose of the cell wall. Additionally, it increases the cell’s ability to endure turgor pressure.
- By sending signals to the cell, it triggers cell division and permits certain molecules to enter the cell while preventing others.
Centrioles are tubular organelles that are mostly present in eukaryotic cells and made of the protein tubulin.
Structure of Centriole
- The core of a centriole contains a Y-shaped linker and a barrel-like structure that stabilises the centriole, while the perimeter is made up of a cylindrical structure comprised of nine triplet microtubules.
- A centriole contains a second kind of structure known as a cartwheel, which consists of a central hub and nine radiating spokes or filaments. A pinhead connects each of these filaments/spokes to the microtubules.
Functions of the Centriole
- Centrioles play a critical part in the formation of spindle fibres during cell division, which facilitates the migration of chromatids towards their respective sides.
- They contribute to the development of flagella and cilia.
Cilia and Flagella
Microtubule-based projections from the cell called cilia and flagella that are shielded by the plasma membrane resemble small hairs.
Structure of Cilia and Flagella
- Microtubules in cilia are arranged in a 9+2 configuration with a radial pattern of 9 outer doublet microtubules around 2 singlet microtubules. A basal body holds this arrangement to the ground.
- Prokaryotes and eukaryotes have differing structures for their flagella, which are filamentous organelles.
- In prokaryotes, it is composed of the flagellin protein, which is helical-wrapped around to form a hollow structure in the centre along the length.
- But in eukaryotes, the protein is missing, and microtubules take its place in the structure.
Functions of Cilia and Flagella
- Movement is the cilia’s and flagella’s most important function. These control how the numerous particles that are present surrounding the creatures move as well as how the organisms themselves move.
- Some of the cilia seen in some organs may have a sensory role. As an example, consider the cilium in blood vessels, which aids in regulating blood flow.
A form of plastic called a chloroplast is used by plants and algae during photosynthesis. Chloroplast houses the vital pigment chlorophyll, which is required to capture sunlight for the synthesis of glucose.
Structure of Chloroplast
- It is a double-membraned structure that has its own DNA and is descended from the former chloroplast.
- These are typically lens-shaped, and their size and quantity depend on the cells. They have a thylakoid membrane that encloses the gel-like matrices known as the stroma, an inner membrane, and an outer membrane.
- The stroma is made up of proteins, DNA, chloroplast ribosomes, and starch granules, whereas the outer and inner membranes are permeable and permit material transfer.
Functions of Chloroplast
- The principal hub for both light-dependent and light-independent processes during photosynthesis is the chloroplast.
- The control of photorespiration is mediated by a variety of proteins found in chlorophyll.
Everything found within a cell, except the nucleus, is referred to as “cytoplasm.”
Structure of Cytoplasm
- Cell organelles, which are tiny, cell-like structures linked by distinct membranes, cytoplasmic inclusions, which are insoluble molecules that store energy and are not surrounded by any layer, and the cytosol, a gel-like material, make up the cytoplasm.
- The cytoplasm is the whitish portion of the cell that contains several nutrients and around 80% water.
- It is generally known that it has both elastic and viscous characteristics. Cytoplasm assists in the transport of components within the cell via a process known as “cytoplasmic streaming” because of its flexibility.
Functions of Cytoplasm
- The majority of crucial cellular and enzymatic processes, such as protein translation from mRNA and cellular respiration, take place in the cytoplasm.
- It serves as a buffer, preventing genetic material and other organelles from being harmed by collision or a change in the cytosol’s pH.
- The procedure known as “cytoplasmic streaming” enables the movement of cell organelles within the cell and aids in the distribution of different nutrients.
The cytosol contains a variety of fibrous structures that aid in cellular transport and help provide the cell form.
Structure of the Cytoskeleton
- The cytoskeleton is made up of microtubules, microfilaments, and intermediate filaments, which are roughly divided into three main types.
- These are divided according to the proteins they contain.
Functions of the cytoskeleton
- The cytoskeleton’s crucial job is to provide the cell structure and mechanical support to prevent deformation.
- It enables the cell’s expansion and contraction, which helps the cell move.
- Additionally, it contributes to the movement of materials both within and outside of cells.
Reticulum Endoplasmic (ER)
In eukaryotic cells, the endoplasmic reticulum (ER) is a network of tubules attached to the nuclear membrane.
Based on whether ribosomes are present or not, there are two different kinds of ER:
- Rough ER (RER) is involved in protein synthesis because it has ribosomes attached to its cytosolic face.
- Without ribosomes, smooth ER (SER) serves a purpose in lipid synthesis.
Structure of the Endoplasmic Reticulum (ER)
- Endoplasmic There are three types of reticulum: cisternae, vesicles, and tubules.
- Cisternae are flat, unbranched, sac-like formations that are continuously layered on top of one another.
- Proteins are transported throughout the cell by spherical organelles called vesicles.
- Cisternae and vesicles are connected by tubules, which are tubular branching structures.
Functions of Endoplasmic Reticulum (ER)
- The surface of the ER is crucial for activities including diffusion, osmosis, and active transport, and it includes many of the enzymes needed for several metabolic processes.
- The creation of lipids like cholesterol and steroids is one of the essential roles of ER.
- Rough ER enables the modification of polypeptides that leave the ribosomes to create the protein’s secondary and tertiary structures.
- In addition to producing several membrane proteins, ER is essential for preparing the nuclear envelope after cell division.
Endosomes are membrane-bound cell structures that come from the Golgi network.
Structure of Endosomes
- Depending on their form and the amount of time it takes for endocytosed molecules to reach them, there are many kinds of endosomes.
- While late endosomes lack tubules but are densely packed with intraluminal vesicles, early endosomes are constructed with a tubular-vesicular network. Microtubules are present in recycling endosomes, which are mostly made of tubular components.
Functions of Endosomes
- Endosomes enable the internalisation of substances from the cell surface and the transfer of those substances to the Golgi or the lysosomes.
Golgi Apparatus/ Golgi Complex/ Golgi Body
The packaging of macromolecules into vesicles so they may be transported to their site of action is carried out by the Golgi Apparatus, a cell organelle that is mostly found in eukaryotic cells.
Structure of the Golgi Apparatus
- The Golgi Complex has a pleomorphic structure, but it commonly takes the shapes of cisternae, vesicles, and tubules.
- The smallest component of the Golgi Complex, the cisternae, has a flattened sac-like shape that is organised into parallel bundles.
- In the form of tubular, branching structures with fenestrated edges, tubules are present and extend from the cisternae.
- Transitional vesicles, secretory vesicles, and clathrin-coated vesicles are three types of spherical structures known as vesicles.
Functions of the Golgi Apparatus
- The Golgi Complex serves as the cell’s “traffic police” by guiding proteins and lipids to their proper locations.
- They participate in the exocytosis of a variety of substances and proteins, including zymogen, mucus, lactoprotein, and thyroid hormone fragments.
- Other cell organelles, including a cell membrane and lysozymes, among others, are produced in the Golgi Complex.
- They participate in the sulfation of different compounds.
The intermediate filaments are the third kind of filament that makes up the cytoskeleton. Due to their intermediate diameter in relation to microfilaments and myosin proteins, they are known as intermediate filaments.
Structure of intermediate filaments
- A family of similar proteins may be found in intermediate filaments.
- A helical structure known as a “wrapped-coil structure” is formed when the individual filaments are coiled around one another.
Functions of Intermediate filaments
- The tissues of different organs, such as the skin, are held in place by intermediate filaments, which also contribute to the structural integrity of a cell.
Animal cells’ cytoplasm contains membrane-bound organelles called lysozymes. Numerous hydrolytic enzymes are present in these organelles, which are necessary for the breakdown of different macromolecules.
Lysozymes come in two varieties:
- Primary lysozyme:Hydrolytic enzymes such as lipases, amylases, proteases, and nucleases are present in primary lysozyme.
- Secondary lysozyme:Primary lysozymes that included molecules or organelles that were absorbed fused to produce a secondary lysozyme.
Structure of Lysozyme
- Lysozymes may have an irregular or pleomorphic form, but they are often found in spherical or granular structures.
- The lysosomal membrane that encloses lysozymes keeps the enzymes inside the lysosome and protects the cytosol and the rest of the cell from the detrimental effects of the enzymes.
Functions of Lysozyme
- Larger macromolecules are broken down into smaller molecules by the internal digestion of these organelles, which is carried out with the aid of their own enzymes.
- Additionally, lysozymes carry out the vital task of cytoplasmic autolysis of undesirable organelles.
- In addition to these, the lysosome also plays a role in energy metabolism, cell signalling, plasma membrane repair, and secretion.
The cytoskeleton of a cell is made up of microfilaments, which are parallel polymers of actin protein. The cytoskeleton’s tiniest filaments have a high degree of stiffness and elasticity, giving the cell strength and mobility.
Structure of Microfilaments
- The filaments may be found as bundles or in cross-linked networks, creating networks. Protein chains continue to be twisted around one another in a helical pattern.
- The filament has two polar ends: one is negatively charged and pointed, the other is positively charged and barbed.
Functions of Microfilaments
- Together with the myosin protein, it produces the force necessary for the cell’s structure and mobility.
- They participate in the outcome of numerous cell surface projections and aid in cell division.
The cytoskeleton also includes microtubules, which vary from microfilaments in that they include the protein tubulin.
Structure of Microtubules
- They are lengthy, hollow, beaded structures with a 24 nm diameter.
- Microtubule walls are made up of globular tubulin a and b subunits arranged in a helical pattern.
- Similar to microfilaments, microtubules have distinct polarity at their ends, with one being positively and the other negatively charged.
Functions of Microfilaments
- In collaboration with the myosin protein, it creates the force for the cell’s structure and mobility.
- They support cell division and participate in the creation of different cell surface projections.
The presence of tubulin protein distinguishes microtubules from microfilaments, which are both components of the cytoskeleton.
Structure of Microtubules
- They are lengthy, hollow, 24 nm-diameter, beaded tubular structures.
- Globulular tubulin subunits located in a helical array of a and b tubulin make up the wall of microtubules.
- The ends of microtubules have a distinct polarity, similar to that of microfilaments, with one end being positively charged and the other being negatively charged.
Functions of Microtubules
- They provide the cell structure and mobility as a component of the cytoskeleton.
- Through the use of binding proteins, microtubules help other cell organelles move about inside the cell.
Microvilli are minuscule structures that resemble little fingers that protrude onto or from cells. These may be seen alone or in combination with villi.
Structure of Microvilli
- Microvilli are clusters of protuberances with few or no cellular organelles that are loosely grouped on the surface of the cell.
- A plasma membrane that encloses cytoplasm and microfilaments surrounds them.
- These are fimbrin, villin, and epsin-coated bundles of actin filaments.
Functions of Microvilli
- Microvilli expand the cell’s surface area, which improves the absorption and secretion processes.
- Enzymes that enable the breakdown of bigger molecules into smaller ones and more efficient absorption are abundant in the membrane of microvilli.
- In both sperm and white blood cells during fertilisation, microvilli serve as an anchoring agent.
The double-membrane-bound cell organelles known as mitochondria are in charge of supplying and storing energy for the cell. The main function of the mitochondria is to oxidise different substrates inside the cell to release energy in the form of ATP (Adenosine Triphosphate).
Structure of Mitochondria
- The outer layer of a mitochondrion’s two membranes is smooth, while the inner layer is distinguished by cristae, which are structures that fold and resemble fingers.
- Several enzymes, coenzymes, and elements of numerous cycles are found in the inner mitochondrial membrane, which also has holes for the transport of substrates, ATP, and phosphate molecules.
- There is a matrix inside the membranes that houses different enzymes involved in metabolic processes like the Kreb’s cycle.
- In addition to these enzymes, mitochondria also contain mtDNA, a kind of single-or double-stranded DNA that may produce 10% of the proteins found in the mitochondria.
Functions of Mitochondria
- The production of ATP, which is essential for the normal operation of all cell organelles, is the mitochondria’s main task.
- Additionally, mitochondria aid in the process of apoptosis and balance the quantity of Ca+ ions present in the cell.
- Within the mitochondria, many hormone segments and blood constituents are formed.
- The liver’s mitochondria have the capacity to detoxify ammonia.
The nucleus is a double membrane-bound structure that regulates all cellular functions and serves as a repository for genetic material. It is one of the big cell organelles, taking up 10% of the whole cell’s volume. As it issues instructions for the appropriate operation of other cell organelles, it is often referred to as the “brain of the cell.” In eukaryotic cells, the nucleus is distinct; in prokaryotic species, when the genetic material is dispersed throughout the cytoplasm, the nucleus is nonexistent.
Structure of the Nucleus
- The nuclear envelope, chromatin, and nucleolus make up the nucleus’ structural components.
- In terms of both structure and makeup, the nuclear envelope resembles the cell membrane. It contains pores that let RNA and proteins enter and exit the nucleus. While maintaining nucleoplasm and chromatin within the envelope, it permits contact with other cell organelles.
- Nuclear proteins and RNA or DNA make up the chromatin, the genetic material in the nucleus that is in charge of transmitting genetic information from one generation to the next. It has a sensation of presence and a compact shape that, with very strong magnification, may be seen as a chromosome.
- The nucleolus functions as a nucleus within a nucleus. It is a membrane-free organelle that produces rRNA and puts together the ribosomes needed for protein synthesis.
Functions of Nucleus
- The storage and transmission of genetic material, such as DNA or RNA, takes place in the nucleus.
- By producing mRNA molecules, it assists in the transcription process.
- The nucleus facilitates procedures including cell development, cell division, and protein synthesis while regulating the function of all other organelles.
All eukaryotes have peroxisomes, which are oxidative membrane-bound organelles. The reputation is owed to their production and removal of hydrogen peroxide.
Structure of Peroxisomes
- A single membrane and granular matrix dispersed throughout the cytoplasm make up a peroxisome.
- They may be found as solitary peroxisomes or as networks of linked tubules.
- Every peroxisome has compartments that let distinct metabolic functions be carried out in optimal environments.
- They are made up of a variety of enzymes, with catalase, D-amino acid oxidase, and urate oxidase being the three main groupings.
Functions of Peroxisomes
- Hydrogen peroxide is produced and eliminated during metabolic activation by peroxisomes.
- Within peroxisomes, fatty acids are oxidised.
- In addition, peroxisomes work with lipid-like cholesterol and plasmalogen production.
Plasmodesmata are minuscule passageways or tunnels that enable communication and material transfer between several cells.
Structure of Plasmodesmata
- Two neighbouring cells are connected by 103–105 plasmodesmata, each of which has a diameter of 50–60 nm.
- Three layers make up a plasmodesma:
- The plasma membrane shares the same phospholipid bilayer and is continuous with the cell’s plasma membrane.
- The interchange of materials between two cells is made possible by the cytoplasmic sleeve’s continuity with the cytosol.
- The endoplasmic reticulum’s desmotubule, which connects two cells and enables the movement of certain chemicals,
Functions of Plasmodesmata
- The major location for two cells to communicate is in plasmodesmata. Proteins, RNA, and viral genomes may all be transferred thanks to it.
The production and storage of food is carried out by plastids, double membrane-bound organelles found in plants and other eukaryotes.
Structure of Plastids
- The intermembrane gap is located between the outer and inner membranes of plastids, which are typically oval or spherical in shape.
- A matrix known as the stroma, which includes tiny structures known as grana, was contained by the inner membrane.
- Each granum is made up of a number of stacked, sac-like thylakoids joined by stromal lamellae.
- Plastids have DNA and RNA, which enables them to produce the essential proteins for various functions.
Functions of Plastids
- As they contain the necessary enzymes and other elements, chloroplasts serve as the focal point for numerous metabolic processes, including photosynthesis.
- Additionally, they handle the storage of food, particularly starch.
In addition to a variety of other crucial elements needed for protein synthesis, ribosomes are ribonucleoproteins that contain an equal amount of RNA and proteins. They are found either free or linked to the endoplasmic reticulum in eukaryotes but exist freely in prokaryotes.
Structure of Ribosomes
- Two subunits make up the ribonucleoprotein.
- The ribosomes in prokaryotic cells are 70S, with the bigger subunit being 50S and the smaller one being 30S.
- In eukaryotic cells, 80S ribosomes with a 60S bigger subunit and a 40S smaller subunit are seen in eukaryotic cells.
- After protein synthesis, ribosomes divide into separate subunits that may either be employed again or stay broken. This causes ribosomes to be short-lived.
Functions of Ribosomes
- All living things produce biological proteins at the ribosome level.
- They aid in the production of proteins by arranging the amino acids in the manner specified by tRNA.
Storage granules, also known as zymogen granules, are membrane-bound organelles that store a cell’s energy reserve as well as other metabolites.
Structure of storage granules
- These granules, which are mostly made of phosphorus and oxygen, are enclosed by a lipid bilayer.
- The contents of these storage granules vary according to where they are located in the body, with some even holding degradative enzymes that have not yet begun to engage in digestive processes.
Functions of storage granules
- Storage granules are a common way for prokaryotes and eukaryotes to store nutrients and reserves in the cytoplasm.
- Prokaryotes that use hydrogen sulphide as a source of energy are known to have sulphur granules.
In the cells of many species, vacuoles are membrane-bound organelles that come in various sizes.
Structure of Vacuoles
- Tonoplast is a membrane that encloses fluid containing both organic and inorganic components, including nutrients and even enzymes.
- Vacuoles have many structural similarities with vesicles since they are created by the fusion of several vesicles.
- To keep the cell from becoming poisonous, vacuoles serve as storage spaces for nutrients and waste.
- They play a crucial role in homeostasis by allowing the entry and outflow of H+ ions into the cytoplasm, which maintains the pH equilibrium of the cell.
- Enzymes that are found in vacuoles are vital to several metabolic activities.
Vesicles, also known as liposomes, are structures that exist within the cell and may either develop naturally via procedures like exocytosis, endocytosis, or the transfer of materials across the cell, or they can arise artificially.Based on their intended use, vesicles may be classified as vacuoles, secretory, or transport vesicles.
Structure of Vesicles
- A structure with a lipid bilayer enclosing a liquid or cytosol is referred to as a vesicle.
- A lamellar phase, which resembles the plasma membrane, is the term for the outer layer surrounding the liquid. The lipid bilayer has two ends, one of which is hydrophilic and the other hydrophobic.
Functions of Vesicles
- Vesicles make it easier to store and move things within and outside of the cell. Even the interchange of chemicals between two cells is made possible by it.
- Vesicles have a role in metabolism and the storage of enzymes because they are contained inside a lipid bilayer.
- They let food be temporarily stored and regulated the cell’s buoyancy.
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