Cell Organelles Structure and their Functions

What is a cell?

Your body is currently doing a lot of things simultaneously. Your body is sending electrical impulses, pumping blood and filtering urine. It also makes protein, digests food, and stores fat. And that’s not all! All of this is possible because cells are tiny units of life, which are full of machinery and machinery that help you do your job. Every living thing is made up of cells, including blue whales and the archaebacteria found in volcanoes. Cells can take on many shapes and sizes, just like the organisms that they make up. Giant squids’ nerve cells can grow to as long as 12m (39 feet) in length. Human eggs, which are the largest cells in the body, measure only 0.1mm in width. Plant cells have protective walls made out of cellulose, which also makes celery stringy and difficult to eat. Fungal cell walls are made out of the same material as lobster shells. All these factories share the same basic machinery, despite their vast differences in size, function, and shape.

There are two types of cells: prokaryotic or eukaryotic. Eukaryotes have membrane-bound nuclei. Prokaryotes don’t. We will be focusing on eukaryotes for the rest of this discussion. Consider what a factory requires to be able to function properly. A factory must have a building, a product and a method to make it. Every cell has membranes (the building), DNA and ribosomes. They can also make proteins (the product, let’s say toys). Since eukaryotes are the type of cell that has organelles, this article will be focused on them.

What’s found inside a cell

An organelle, also known as an internal organ of a cell, is a membrane-bound structure within a cell. These mini-organs, just like the cells that have membranes to keep everything in, are also bound in double layers of phospholipids to protect their small compartments in larger cells. 

Organelles can be thought of as small rooms in a factory with special conditions that allow them to perform their task. This could include a break area with snacks or a research space with cool gadgets and an air filter. The cytoplasm is the liquid that carries the organelles within cells. It is a viscous liquid inside the cell membrane. This table shows the organelles that are found in the basic human cells. It will be our guide for the discussion.

Organelle	Function	Factory part
Nucleus	DNA Storage	Room where the blueprints are kept
Mitochondrion	Energy production	Powerplant
Smooth Endoplasmic Reticulum (SER)	Lipid production; Detoxification	Accessory production – makes decorations for the toy, etc.
Rough Endoplasmic Reticulum (RER)	Protein production; in particular for export out of the cell	Primary production line – makes the toys
Golgi apparatus	Protein modification and export	Shipping department
Peroxisome	Lipid Destruction; contains oxidative enzymes	Security and waste removal
Lysosome	Protein destruction	Recycling and security

What are Cell Organelles?

Cell organelles are the cellular components. These cell organelles can be found in both membrane-bound organelles and non-membrane organelles. They are different in their functions and structures. They are essential for cell function and coordination. Some of them provide shape and support while others are involved in the movement and reproduction of cells.

Organelle is derived from the notion that organelles are part of cells. The suffix -elle refers to a diminutive. Organelles can be either enclosed in their own lipid bilayers (also known as membrane-bound organelles), or they are functionally distinct units that are not bound by a surrounding bilayer (nonmembrane bound organelles). While most organelles function within cells, there are some functional units outside cells that are called organelles. These include the flagellum, archaellum and cilia.

Microscopy can identify organelles and can be purified using cell fractionation. Organelles come in many forms, especially in eukaryotic cells. These structures include the internal endomembrane systems (such as nuclear envelope, endoplasmic retina, and Golgi apparatus) and other structures like mitochondria and plastids.

Although prokaryotes don’t have eukaryotic organelles but some prokaryotes contain protein-shelled bacteria microcompartments that are thought to act like primitive prokaryotic organismelles. There is also evidence for other membrane-bounded structures. Organelles are also often referred to as the prokaryotic flagellum, which protrudes from the cell, and its motor.

Examples of Cell Organelles

Major eukaryotic organelles

Organelle	Main function	Organisms
cell membrane	separates the interior of all cells from the outside environment (the extracellular space) which protects the cell from its environment.	all eukaryotes
cell wall	The cell wall is a rigid structure composed of cellulose that provides shape to the cell, helps keep the organelles inside the cell, and does not let the cell burst from osmotic pressure.	plants, protists, rare kleptoplastic organisms
chloroplast (plastid)	photosynthesis, traps energy from sunlight	plants, protists, rare kleptoplastic organisms
endoplasmic reticulum	translation and folding of new proteins (rough endoplasmic reticulum), expression of lipids (smooth endoplasmic reticulum)	all eukaryotes
flagellum	locomotion, sensory	some eukaryotes
Golgi apparatus	sorting, packaging, processing and modification of proteins	all eukaryotes
mitochondrion	energy production from the oxidation of glucose substances and the release of adenosine triphosphate	most eukaryotes
nucleus	DNA maintenance, controls all activities of the cell, RNA transcription	all eukaryotes
vacuole	storage, transportation, helps maintain homeostasis	eukaryotes

Minor eukaryotic organelles and cell components

Organelle/Macromolecule	Main function	Organisms
acrosome	helps spermatozoa fuse with ovum	most animals
autophagosome	vesicle that sequesters cytoplasmic material and organelles for degradation	all eukaryotes
centriole	anchor for cytoskeleton, organizes cell division by forming spindle fibers	animals
cilium	movement in or of external medium; “critical developmental signaling pathway”.	animals, protists, few plants
cnidocyst	stinging	cnidarians
eyespot apparatus	detects light, allowing phototaxis to take place	green algae and other unicellular photosynthetic organisms such as euglenids
glycosome	carries out glycolysis	Some protozoa, such as Trypanosomes.
glyoxysome	conversion of fat into sugars	plants
hydrogenosome	energy & hydrogen production	a few unicellular eukaryotes
lysosome	breakdown of large molecules (e.g., proteins + polysaccharides)	animals
melanosome	pigment storage	animals
mitosome	probably plays a role in Iron-sulfur cluster (Fe-S) assembly	a few unicellular eukaryotes that lack mitochondria
myofibril	myocyte contraction	animals
nucleolus	pre-ribosome production	most eukaryotes
ocelloid	detects light and possibly shapes, allowing phototaxis to take place	members of the family Warnowiaceae
parenthesome	not characterized	fungi
peroxisome	breakdown of metabolic hydrogen peroxide	all eukaryotes
porosome	secretory portal	all eukaryotes
proteasome	degradation of unneeded or damaged proteins by proteolysis	all eukaryotes, all archaea, and some bacteria
ribosome (80S)	translation of RNA into proteins	all eukaryotes
stress granule	mRNA storage	most eukaryotes
TIGER domain	mRNA encoding proteins	most organisms
vesicle	material transport	all eukaryotes

Prokaryotic organelles and cell components

Organelle/macromolecule	Main function	Organisms
anammoxosome	anaerobic ammonium oxidation	“Candidatus” bacteria within Planctomycetes
carboxysome	carbon fixation	some bacteria
chlorosome	photosynthesis	green sulfur bacteria
flagellum	movement in external medium	some prokaryotes
magnetosome	magnetic orientation	magnetotactic bacteria
nucleoid	DNA maintenance, transcription to RNA	prokaryotes
pilus	Adhesion to other cells for conjugation or to a solid substrate to create motile forces.	prokaryotic cells
plasmid	DNA exchange	some bacteria
ribosome (70S)	translation of RNA into proteins	bacteria and archaea
thylakoid membranes	photosynthesis	mostly cyanobacteria

Types of Cell Organelles

Classification of Cell Organelles based on presence or absence of membrane

There are many organelles within the cell. They are divided into three groups based on whether or not there is membrane.

1. Organelles without membrane: Cell wall, Ribosomes and Cytoskeleton all are organelles that are not bound by membranes. They can be found in both prokaryotic cells and eukaryotic cells.
2. Single membrane-bound organelles: Single membrane-bound organelles are Vacuole and Lysosome, Golgi Apparatus and Endoplasmic Reticulum. These organelles can only be found in a eukaryotic cells.
3. Double membrane-bound organelles: Nucleus, mitochondria and chloroplast are double membrane-bound organelles present only in a eukaryotic cell.

Classification of Cell Organelles based on location

Cell organelles are classified into these following three groups;

1. General cell organelles: General cell organelles are found in all cells, including animal and plant. They include cell membrane, cytosol and cytoplasm, nucleus and mitochondrion.
2. Temporal cell organelles: Temporal cell organelles are found only at certain stages of a cell’s life cycle: chromosomes, autophagosomes, chromosomes, and endosomes.
3. Cell type specific cell organelles: Cell type-specific cell organelles are only found in plant cells: chloroplast, central vacuumole and cell wall.

Many unique cell organelles/structures only exist in specific cell types. The food vacuoles of amoeba or the paramecia trichocysts are examples of unique cell organelles/structures that can only be found in specific cell types. However, there are some organelles in human cells that cannot be found elsewhere, such as the Weibel-Palade body in blood vessel cells.

The cell membrane (Plasma membrane/ Plasmalemma)

The plasma membrane is made up of proteins and lipids. It can fluctuate depending on fluidity and external environment.

Structure of Cell membrane

• It is structurally composed of a phospholipid bilayer and two types of proteins. There are two types of proteins: embedded proteins and peripheral proteins. These proteins provide shape and allow particles to move in and out of cells.
• A phospholipid, which has a polar head and two hydrophobic tails, is the most abundant lipid found in cell membrane.
• The cell’s cells are able to transfer particles through embedded proteins, with some proteins acting like receptors for different components.
• These proteins are used to support the cell’s structure and provide fluidity.

Functions of Cell membrane

• Cell membranes provide mechanical support to the cell, allowing it to take on the desired shape. They also protect the cell from the outside environment.
• It controls what can enter and exit the cells through channels.
• It transmits and generates signals inside and outside the cell to ensure proper functioning of all organelles and cells.
• It facilitates the interaction of cells necessary for tissue formation and cell-to-cell fusion.

Cell Wall

The cell wall is an additional layer that exists outside of the cell membrane. It provides structure, protection, and filtering mechanisms to the cells.

Structure of Cell Wall

• The cell wall of a plant cell is made up of proteins, cellulose, and hemicellulose. In a fungal cell it is made of chitin.
• Multilayered cell walls contain a middle layer, a primary and secondary cell wall.
• The middle lamina is made up of polysaccharides, which provide adhesion and allow cells to be bound together.
• The primary cell wall is made of cellulose and is located after the middle lamina. The secondary cell wall is composed of hemicellulose and cellulose. It is not always present.

Functions of Cell Wall

• Cell wall serves a critical role in maintaining and protecting the cell’s shape. It helps to withstand the turgor stress of the cell.
• It activates cell division by sending signals to the cells and allows the passage of certain molecules into the cells while blocking others.

Centriole

Centrioles, which are tubular structures found mostly in eukaryotic cells, are mainly composed of the protein tubulin.

Structure of Centriole

• A centriole is a cylindrical structure with nine triplets of microtubules. It surrounds the centriole’s periphery, while the central portion has a linker that forms a Y shape and a barrel-like structure to stabilize it.
• A centriole also contains a structure called a cartwheel. It is composed of a central hub and nine spokes/filaments radiating out it. Through a pinhead, each of these filaments/spokes are connected to the microtubules.

Functions of Centriole

• Centrioles play a critical role during cell division in the formation of spindle fibers that aid the movement of chromatids to their respective sides.
• They play an important role in the formation flagella and cilia.

Cilia and Flagella

Flagella and Cilia are microtubule-based projections of the cell that look like hairs. They are covered by the plasma membrane.

Structure of Cilia and Flagella

• Cilia are hair-like projections with a 9+2 arrangement microtubules. It has a radial pattern that includes 9 outer microtubules doublets and two singlet microtubules. This arrangement is attached to a base body.
• Flagella is a filamentous organelle that is found in both prokaryotes as well as eukaryotes. Its structure is distinct in both.
• It is composed of flagellin, a protein wrapped in helical fashion creating a hollow structure at its center.
• However, in eukaryotes the protein is absent, and the structure is replaced by microtubules.

Functions

• Movement is the most important role of flagella and cilia. They are responsible for movement of organisms and for movement of particles around them.
• One function of cilia in certain organs is sense. One example is the cilium found in blood vessels which aids in controlling blood flow.

Chloroplast

A chloroplast, a type of plastic, is involved in photosynthesis in plants or algae. Chloroplast has a vital pigment called chlorophyll that traps sunlight to produce glucose.

Structure of Chloroplast

• It’s a double-membraned structure that has its own DNA, which it inherits from the previous chloroplast.
• They are typically lens-shaped, with the number and shape of cells varying depending on their size. They are composed of an outer, inner, and a Thylakoid membrane. The inner membrane contained the gel-like matrix called the Stroma.
• Both the outer and inner membranes are porous, allowing materials to be transported. The stroma is home to DNA, starch granules, proteins, and chloroplast-ribosomes.

Functions of Chloroplast

• The primary center of photosynthesis light-dependent and light independent reactions is the chloroplast.
• Photorespiration is controlled by different proteins found in chlorophyll.

Cytoplasm

The cytoplasm is everything inside a cell, except for the nucleus. The cytoplasm is found in animal and plant cells. They are jelly-like compounds, located between the cell membrane and the nucleus. They’re mostly made up of organic and inorganic substances. The cytoplasm is among the fundamental cells in which all organelles within the cell reside. Cell organelles are home to enzymes that are responsible for controlling the metabolic activity that takes in the cell. They are the location for the majority reaction reactions that occur within cells.

Structure of Cytoplasm

• The cytoplasm is composed of a cytosol, a gel-like substance that includes other matter; cell organelles; smaller cells-like bodies that are bound by separate membranes; cytoplasmic inclusions; and insoluble molecules that store energy but are not surrounded by any layers.
• The cytoplasm, which is colorless, contains about 80% water and various nutrients.
• It has both the properties of viscous matter and elastic matter. Cytoplasmic streaming is a process that allows materials to move inside cells because of its elastic nature.

Functions of Cytoplasm

• The cytoplasm is where most of the essential cellular and enzymatic reactions, such as cellular respiration or translation of mRNA to proteins, take place.
• It acts as a buffer, protecting genetic material and other organelles against damage from collision or changes in the pH of cytosol.
• Cytoplasmic streaming is a process that aids in the distribution and movement of nutrients within cells.

Cytoskeleton

The cytosol contains a number of fibrous structures that help give shape to cells and support cellular transport. It is a continuous system of filamentous proteinsaceous structures that extend throughout the cytoplasm starting from the nucleus and ending at the plasma membrane. It is present in all living cells, and is particularly present in eukaryotes. 

The cytoskeleton matrix comprises of a variety of proteins that are able to divide quickly or break down depending on the needs of cells. The main function of the cytoskeleton matrix is providing the form and resistance of cells against deformation. The contractile nature of the filaments aids in cytokinesis as well as in motility.

Structure of Cytoskeleton

• The cytoskeleton is made up of three types of fibers: microtubules,  intermediate filaments and microfilaments.
• They are separated according to the protein they contain.

Functions of Cytoskeleton

• The cytoskeleton’s critical function is to give shape and mechanical support to cells against deformation.
• It allows for expansion and contraction, which aids in cell movement.
• It also plays a role in the extracellular and intracellular transport of materials.

Endoplasmic Reticulum (ER)

Endoplasmic means inside (endo) the cytoplasm(plasm). The Latin word for the net is reticulum. An endoplasmic reticulum is a plasma membrane that forms inside a cell. It folds inwards to create an inner space called the lumen. The lumen is actually continuous and is connected to the perinuclear area.

The Endoplasmic Reticulum, a network membranous canals that are filled with fluid, is the largest of these. They act as the transport system for the cell and transport materials within the cell.

1. Rough Endoplasmic Reticulum – These are made up of cisternae and tubules and vesicles that are present throughout the cell and involved in protein production.
2. Smooth Endoplasmic Reticulum – These are the storage organelles, which are associated with the production and use of steroids, lipids, and detoxification.

Rough Endoplasmic Reticulum

Because its surface is covered with ribosomes (molecules responsible for protein production), the rough endoplasmic retina is named so. A ribosome may find a particular RNA segment and tell it to go to the rough endoplasmic retina to embed itself. The protein that is created from this segment will be found inside the lumen in the rough endoplasmic retina. There it will fold and be tagged with a (usually carbohydrate-based) molecule. This marks the protein for transport into the Golgi apparatus. The rough endoplasmic retina is continuous with the nuclear membrane and appears like a series canals close to the nucleus. The rough endoplasmic retina is destined for proteins to be either part of a cell membrane or secreted out of the cell. It would be much more difficult to tell the difference between proteins that should be left and those that should stay in the cell without a rough endoplasmic retina. The rough endoplasmic retina allows cells to specialize, and the organism to be more complex.

Smooth Endoplasmic Reticulum

Instead of being involved with protein synthesis, the smooth endoplasmic retina makes lipids. These fat-based molecules are vital in energy storage, membrane structure and communication. (Steroids can also act as hormones). The cell’s detoxification is also done by the smooth endoplasmic retina. It is tubularer than the rough endoplasmic retinal, and it is not always continuous with the nuclear membrane. Each cell has a smooth, endoplasmic retinalum. However, the amount of this reticulum will vary depending on how it is used. The liver, which is responsible primarily for detoxification in the body, has a greater amount of smooth endoplasmic retinaum.

Structure of Endoplasmic Reticulum (ER)

• Endoplasmic Reticulum can be found in three forms: Cisternae, Vesicles, or Tubules.
• Cisternae are flattened structures with no branches that look like sacs and can be stacked one upon the other.
• Vesicles are spherical structures which carry proteins throughout the cell.
• Tubules can be described as branched tubular structures that connect cisternae to vesicles.

Functions of Endoplasmic Reticulum (ER)

• Many of the enzymes needed for many metabolic processes are found in ER. The ER’s surface is also essential for operations such as diffusion, osmosis and active transport.
• The synthesis of lipids such as cholesterol and steroids is one of the most important functions of ER.
• Rough ER permits modification of polypeptides that emerge from ribosomes in order to create secondary and tertiary structure of the protein.
• ER is also responsible for the production of various membrane proteins. It plays a critical role in preparing nuclear envelopes after cells are divided.

Endosomes

Endosomes are membrane-bound cells within cells that are derived from the Golgi network

Structure of Endosomes

• There are several kinds of endosomes, based on their morphology as well as the length of duration it takes for endocytosed substances to get to them.
• Early endosomes were constructed of tubular-vesicular networks. the endosomes of late do not have tubules, however, they contain a number of tightly packed intraluminal vessels. Recycling endosomes are made with microtubules and are comprised of tubular structures.

Functions of Endosomes

• Endosomes facilitate the sorting and distribution of internalized material from the cell’s surface as well as transportation of the materials to Golgi or Lysosomes.

Golgi Apparatus/ Golgi Complex/ Golgi Body

The Golgi Apparatus is the main organelle in cells of eukaryotrd that helps in the packing of macromolecules into vesicles that can be transported out to the site of action.

Golgi Apparatus is also known by its name Golgi Complex. This is an elastomeric organelle made up of a set of pouches flattened, stacked and known as Cisternae. The cell organelle is principally involved in transporting, changing and packaging proteins and lipids to specific locations. Golgi Apparatus is located within the cell’s cytoplasm and is present in animal and plant cells.

Structure of Golgi Apparatus/ Golgi Complex/ Golgi Body

• It has a pleomorphic structure; However, it is typically found in three forms i.e. cisternae, vesicles and tubules.
• The cisternae which is the smallest component of the Golgi Complex has a sac-like structure that is flattened. It is placed in bundles in a similar fashion.
• Tubules appear in the form of branched and tubular structures that extend from the cisternae. They’re located at the edges of the cisternae.
• Vesicles are spherical body parts that can be classified into three categories as secretory vesicles, and clathrin-coated.

Functions of Golgi Apparatus/ Golgi Complex/ Golgi Body

• Golgi Complex has an essential function of directing proteins and lipids to their destinations and thus acting as the “traffic police” of the cell.
• They play a role in the exocytosis process of various products and proteins, including mucus, zymogen, lactoprotein, as well as parts of thyroid hormone.
• Golgi Complex is involved in the synthesis of different organelles in the cell, such as cells, membranes and lysozymes among others.
• They also play a role in the sulfation of a variety of molecules.

Intermediate filaments

The third filament class that makes up the cytoskeleton is the intermediate filaments. They are classified as intermediate filaments due to the diameter intermediate of the filaments compared to myosin and microfilaments.

Structure of Intermediate filaments

• Intermediate filaments comprise a family of proteins that are related.
• The filaments are coiled around each other to form a helical structure known as the coiled-coil.

Functions of Intermediate filaments

• Intermediate filaments aid in the structural integrity of cells while also playing an important role in the holding of tissues of different organs such as the skin.

Lysozyme

Lysozymes are membrane-bound organelles which are found in the cytoplasm of animal cells. These organelles are home to a range of hydrolytic enzymes that are required for the degradation of different macromolecules. There are two kinds of lysozymes.

1. Primary lysosome, which contains hydrolytic enzymes such as lipases, proteases, amylases, and nucleases.
2. Secondary lysozyme created by the combination of primary lysozymes that contain organelles or molecules that have been engulfed.

Structure of Lysozyme

• Lysozymes’ shape is not uniform or pleomorphic, however the majority of them are located in the spherical or the granular form.
• Lysozymes are protected by a lysosomal membrane which is home to the enzymes in the lysosome. It also shields the cytosol along with the cells from the destructive actions that the enzymes can exert.

Functions of Lysozyme

• Organelles are responsible for digestion within the cell where macromolecules larger than the cell are reduced into smaller molecules through the use of the enzymes found in them.
• Lysozymes are also responsible for the crucial function of autolysis of undesirable organelles in the cells.
• In addition, the lysosome plays a role in a variety of cellular processes including repair of the plasma membrane cell signaling, as well as energy metabolism.

Microfilaments

Microfilaments form part of the cytoskeleton in cells, made of actin proteins as parallel polymers. They are the smallest filaments in the cytoskeleton that have the highest rigidity and flexibility giving strength and flexibility to the cells.

Structure of Microfilaments

• The filaments appear in networks of cross-linked proteins or in bundles. Protein chains remain wrapped around one another in the form of a helical structure.
• The polar edges of the filament is barbed and positively charged and pointed, while the other is charged negatively and pointed.

Functions of Microfilaments

• It helps to strengthen the cell’s structure and the movement of cells in conjunction with the myosin protein.
• They assist in the division of cells and play a role in the creation of various cells’ surface projections.

Microtubules

Microtubules also form a part of the cytoskeleton, which differs from microfilaments because of that they contain tubulin.

Structure of Microtubules

• They are long hollow beaded tubular structures with a dimensions of around 24nm.
• The microtubule’s wall comprises globular subunits within a helical array consisting of tubulins a and.
• Like microfilaments, the ends of microtubules exhibit a distinct polarity, where one of them is positively charged, and the other is negatively charged.

Functions of Microtubules

• As part of the cytoskeleton, they give shape and motion to the cell.
• Microtubules help in the movement of organelles of the cell via binding proteins.

Microvilli

Microvilli are tiny structures resembling fingers that extend from or onto cells. They can be found on their alone or when they are in combination with villi.

Structure of Microvilli

• Microvilli are protuberances in a bundle scattered across the cell’s surface without any organelles in the cell.
• They are enclosed by plasma membrane, which encapsulates microfilaments and cytoplasm.
• They are a collection of filaments of actin that are joined by villin, fimbrin and epsin.

Functions of Microvilli

• Microvilli expand the surface of cells, thereby improving the ability to absorb and release substances.
• The microvilli membrane is filled with enzymes that facilitate the breakdown of larger molecules into smaller ones which allows for more efficient absorption.
• Microvilli are an anchoring component in white blood cells as well as in the sperms that fertilize during fertilization.

Mitochondria

Mitochondria are cells with two membranes that are responsible for the production as well as storage of fuel to the cell. The oxidation process of different substances in cells and the release of energy as a form ATP (Adenosine Triphosphate) is the main purpose of mitochondria.

Structure of Mitochondria

• The mitochondrion has two membranes, with the outer layer being smooth and the inner layer is characterized with finger-like and fold-like structures known as the cristae.
• The membrane of the mitochondrial interior contains diverse coenzymes, enzymes, and other components of many cycles, as well as pores for the transportation of substances, ATP as well as phosphate molecules.
• In the membranes is a layer of matrix that is home to various enzymes involved in metabolic processes such as Kreb’s cycle.
• Alongside the enzymes mentioned above, mitochondria also the home of single-stranded or double-stranded DNA, known as mtDNA , which can make 10 percent of the proteins found in mitochondria.

Functions of Mitochondria

• The main role of mitochondria is the formation of energy in form of ATP essential for the correct functioning of all organelles of the cell.
• Mitochondria assist in balancing the number of Ca+ ions inside cells and assist in the process of apoptosis.
• Different blood-related hormones and blood components are incorporated into mitochondria.
• Mitochondria that reside in the liver can help to detoxify ammonia.

Nucleus

Nucleus is a dual membrane-bound structure that is responsible for controlling every cell’s activity and is also a central point for genetic materials and also for transferring. It is among the cell organelles that are large, taking up 10% of the total cell space. It is commonly referred to as”the “brain of the cell” because it is the one that gives commands to the functioning of other organelles within the cell. The term “nucleus” is defined when it comes to cells that are eukaryotic, but it is not present in prokaryotic cells that have the genetic material that is distributed within the cytoplasm.

Structure of Nucleus

• The nucleus is structurally composed of a nuclear envelope, chromatin and nucleolus.
• Nuclear envelopes are comparable as the cell membrane in terms of structure and composition. It is composed of pores that permit to move proteins as well as RNA inside and outside of the nucleus. It allows interconnection with other organelles of the cell while keeping nucleoplasm and chromatin in the envelope.
• The nucleus’s chromatin is a home for DNA or RNA along with nuclear proteins, which act as genetic material, which is responsible for carrying genetic information from one generation to the next. It’s visible in a sense, and has a small structure that could be observed as a chromosome under a high magnification.
• The nucleolus functions as an inner nucleus. It is a non-membrane organelle that is responsible for production of rRNA as well as the formation of ribosomes needed for the synthesis of proteins.

Functions of Nucleus

• The nucleus is the storage organ responsible for and transfer of genetic material as DNA and the RNA.
• It helps during transcription via the production of the mRNA molecules.
• The nucleus is responsible for the function of the other organelles, while helping to facilitate processes like cell division, cell growth and the synthesis of proteins.

Peroxisomes

Peroxisomes are membrane-bound organelles that undergo oxidation that are found in the cell cytoplasm of all Eukaryotes. Their name is attributed to their hydrogen peroxide-generating and eliminating activities.

Structure of Peroxisomes

• Peroxisome is composed of one membrane and aggregation of granular material scattered throughout the cells.
• They can be found as interconnected tubules, or as peroxisomes on their own.
• The peroxisome’s compartments provide optimal conditions for various metabolic activities.
• They comprise a variety of enzymes, the major ones including urate oxidase and D-amino acid oxidase, as well as catalase.

Functions of Peroxisomes

• Peroxisomes are involved with the removal and production of hydrogen peroxide through biochemical processes.
• The oxidation process of fatty acids takes place in peroxisomes.
• Peroxisomes are involved in the production of plasmalogens and lipids-like cholesterol.

Plasmodesmata

Plasmodesmata are tiny channels or passages which allow for the transfer of materials and communications between cells.

Structure of Plasmodesmata

• There are 103 to 105 numbers of plasmodesmata linking two cells adjacent to each other with 50-60 nm of size.
• A plasmodesma is composed of three layers.
• Plasma membranes are continuous with the cell’s plasma membrane and is composed of the same bilayer of phospholipids.
• The cytoplasmic sleeves that are in continuous contact with the cytosol permits the exchange of substances within two cell.
• Desmotubule which is part of the endoplasmic-reticulum which creates a connection between two cells. It also facilitates the transportation of molecules.

Functions of Plasmodesmata

• Plasmodesmata are the principal location for communication between two cells. It facilitates to transfer molecules such as proteins, RNA and viral genomes.

Plastids

Plastids are membrane-bound double structures found in plants as well as other eukaryotes involved with the production and storage of nutrients. Based on the kind of pigments the plastids come in three kinds:

• Chloroplasts – Chloroplasts are organelles with two membranes that typically vary in their shape – ranging from disc-like shapes to discoid-spherical, spherical oval, and ribbon. They are found in mesophyll cells found in leaves, where they contain chloroplasts as well as the other pigments of carotenoid. They help in capturing sunlight energy to enable photosynthesis. The membrane’s interior is enclosed by the stroma, a space. Flattened disc-like chlorophyll-containing structures known as thylakoids are arranged in a stacked manner like a pile of coins. Each pile is known as the granum (plural: the word grana) as well as the thylakoids from various grana are linked by membranous tubules that are flat, referred to as lamella stromal. Similar to mitochondrial matrix, the chloroplast’s stroma also has the double-stranded circular DNA 70S ribosomes and enzymes that are require the synthesis of proteins and carbohydrates.
• Chromoplasts –  The chromoplasts contain carotenoid pigments, fat-soluble like carotene, xanthophylls, etc. that provide plants with their unique color, namely red, yellow, orange, etc.
• Leucoplasts – Leucoplasts are plastids that have no color and store nutrients. Amyloplasts are able to store carbohydrate (like the starch found in potatoes) and aleuroplasts store proteins and elaioplasts store fats and oils.

Structure of Plastids

• Plastids are typically oval or spherical, with an outer and an internal membrane, between which is an intermembrane area.
• The membrane’s inner layer contained the stroma matrix, which has small structures known as”grana.
• Each granum is made up of sac-like thylakoids that are stacked one on one another and joined with lamellae of stroma.
• Plastids contain DNA and also RNA which allow it to create essential proteins needed for various processes.

Functions of Plastids

• Chloroplasts play a central role in various metabolic activities, such as photosynthesis since they contain enzymes as well as other elements required for photosynthesis.
• They also aid when it comes to the storage process of food items, particularly starch.

Ribosomes

Ribosomes are ribonucleoproteins that contain equal amounts of proteins and RNA together with a variety of other components essential to synthesize proteins. In prokaryotes they can be found freely and in eukaryotes they are either found in the form of on their own or in the endoplasmic retina.

Structure of Ribosomes

• The ribonucleoprotein is composed of two subunits.
• In the prokaryotic cell, the ribosomes belong to 70S. They have the 50S subunit that is larger and the smaller subunit of 30S.
• Eukaryotic cells are made up of 80S ribosomes that have 60S subunits larger in size and smaller subunit 40S.
• Ribosomes have a short lifespan as, after protein synthesis the subunits break up and may be recycled or left to break up.

Functions of Ribosomes

• Ribosomes are the primary site of biochemical protein synthesis in all living organisms.
• They arrange amino acids according to the order that is indicated by tRNA. They aid in the process of protein synthesis.

Storage granules

Storage granules, which are membrane-bound organelles also known as zymogen granules that store cells’ energy reserve as well as other compounds.

Structure of Storage granules

• The granules are enclosed by a bilayer of lipids and consist mostly of oxygen and phosphorus.
• The constituents of these storage granules are influenced by their position within the body, with some even having enzymes that are degradative, yet to be involved in digestion.

Functions of Storage granules

• A lot of prokaryotes and eukaryotes keep reserves and nutrients by storing granules within the cells.
• Sulfur granules can be found in prokaryotes that use hydrogen sulfide for a energy source.

Vacuole

Vacuoles are membrane-bound cells that vary in size within cells of various species of organisms.

Structure of Vacuole

• The vacuole is enclosed by a membrane, called tonoplast. It encloses the fluid that is made up of inorganic substances such as water as well as organic materials such as the nutrients and enzymes.
• They are formed through the fusion of vesicles from different types and are therefore like vesicles in their structure.

Functions of Vacuole

• Vacuoles serve as a storage facility for nutrients and substances that are waste to shield the cell from toxicity.
• They are essential to the function of homeostasis since it regulates the equilibrium of pH in cells by the inflow and expulsion of H+ions into the cell’s cytoplasm.
• Vacuoles are home to enzymes, which play an crucial roles in various metabolic processes.

Vesicles

Vesicles are the structures in cells. They can be formed naturally through processes such as exocytosis, endocytosis or transport of material within the cell, or they may be created artificially, they are referred to as liposomes. There are different kinds of vesicles such as vacuoles, transport and secretory vesicles, based on their functions.

Structure of Vesicles

• A vesicle is a form of structure with cytosol or liquid that is covered by a lipid bilayer.
• The outer layer which surrounds the liquid is known as lamellar phase, which is similar to plasma membrane. One side of the bilayer is hydrophobic while the other is hydrophilic.

Functions

• Vesicles are a great way to store and transportation of substances inside and out of cells. It also allows for the transfer of molecules between cells.
• Since vesicles are encased within a lipid bilayer are also involved in metabolism and storage of enzymes.
• They are able to store temporary food items and also regulate how buoyant the cell is.

A Brief Summary on Cell Organelles

Cell Organelles	Structure	Functions
Cell membrane	A double membrane composed of lipids and proteins. Present both in plant and animal cell.	Provides shape, protects the inner organelle of the cell and acts as a selectively permeable membrane.
Centrosomes	Composed of Centrioles and found only in the animal cells.	It plays a major role in organizing the microtubule and Cell division.
Chloroplasts	Present only in plant cells and contains a green-coloured pigment known as chlorophyll.	Sites of photosynthesis.
Cytoplasm	A jelly-like substance, which consists of water, dissolved nutrients and waste products of the cell.	Responsible for the cell’s metabolic activities.
Endoplasmic Reticulum	A network of membranous tubules, present within the cytoplasm of a cell.	Forms the skeletal framework of the cell, involved in the Detoxification, production of Lipids and proteins.
Golgi apparatus	Membrane-bound, sac-like organelles, present within the cytoplasm of the eukaryotic cells.	It is mainly involved in secretion and intracellular transport.
Lysosomes	A tiny, circular-shaped, single membrane-bound organelles,  filled with digestive enzymes.	Helps in the digestion and removes wastes and digests dead and damaged cells. Therefore, it is also called as the “suicidal bags”.
Mitochondria	An oval-shaped, membrane-bound organelle, also called as the “Power House of The Cell”.	The main sites of cellular respiration and also involved in storing energy in the form of ATP molecules.
Nucleus	A largest, double membrane-bound organelles, which contains all the cell’s genetic information.	Controls the activity of the cell, helps in cell division and controls the hereditary characters.
Peroxisome	A membrane-bound cellular organelle present in the cytoplasm, which contains the reducing enzyme.	Involved in the metabolism of lipids and catabolism of long-chain fatty acids.
Plastids	Double membrane-bound organelles. There are 3 types of plastids:Leucoplast –Colourless plastids.Chromoplast–Blue, Red, and Yellow colour plastids.Chloroplast – Green coloured plastids.	Helps in the process of photosynthesis and pollination, Imparts colour for leaves, flowers and fruits and stores starch, proteins and fats.
Ribosomes	Non-membrane organelles, found floating freely in the cell’s cytoplasm or embedded within the endoplasmic reticulum.	Involved in the Synthesis of Proteins.
Vacuoles	A membrane-bound, fluid-filled organelle found within the cytoplasm.	Provide shape and rigidity to the plant cell and helps in digestion, excretion, and storage of substances.

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