Cell Organelles: Pumping blood, filtering urine, digesting food, synthesising protein, storing fat, and so on are all possible because of different organs like the heart, kidney, liver etc. These organs are, in turn, made up of cells and tissues. Cells are the smallest units of life that function like specialised factories, full of machinery designed to complete life’s tasks. Every living thing, from blue whales to the archaebacteria that dwell inside volcanoes, is made up of cells.

Cells come in various forms and sizes. Regardless of their size, form, or function, these little manufacturers use the same fundamental technology. It is because eukaryotes are the cell type that contains almost all the types of organelles that this article will concentrate on them.

Cell Organelles: Definition

Fig: Eukaryotic Cell with the Cell Organelles

A cell organelle is a specialised unit that performs a specific function inside a certain type of cell. There are numerous cell organelles, some of which are found in all types of cells, such as cell membranes, nuclei, and cytoplasm. However, some organelles are specific to one particular type of cell-like plastids and cell walls in plant cells.

Fig: Cell Wall, Cell Membrane and Cytoplasm

Structure	Definition	Functions
Cell Membrane	The cell membrane is a thin layer of protein and fat that surrounds the cell. It is semipermeable, which means it lets certain chemicals into the cell while inhibiting others.	The cell membrane gives mechanical support for the cell’s structure while also shielding the cell and its components from the outside world. It acts as a semipermeable membrane that regulates what can enter and exit the cell through channels, facilitating the interchange of vital molecules required for the cell’s survival.
Cell Wall	The cell wall is a non-living layer that exists outside of the cell membrane in some cells and provides structure, protection, and a filtering mechanism for the cell.	The cell wall’s most important role is to defend and maintain the cell’s form. It also aids the cell’s resistance to turgor pressure. It triggers cell division by sending signals to the cell, and it is fully permeable to all molecules and supports the cell and stops it from bursting when it gains water by osmosis.
Cytoplasm	Outside the nucleus, where the cell organelles are found, there is a jelly-like substance called cytoplasm. Protein synthesis and a variety of metabolic processes take place here. Many enzymes for general metabolism can be found in the cytoplasm. It contains cytoskeletal system fibres that organise the cytoplasmic structure.	The cytoplasm is where most critical cellular and enzymatic activities take place, such as cellular respiration and mRNA translation into proteins. Cytoplasmic streaming is a process that aids in the distribution of various nutrients and the movement of cell organelles inside the cell. It works as a buffer, protecting genetic materials and other organelles from harm caused by cytosolic collisions or changes in pH.

Mitochondria

Fig: Mitochondria

These are organelles that are spherical to rod-shaped and have a double membrane. The inner membrane is repeatedly folded inward, generating a sequence of projections (called Cristae). The mitochondrion is known as the cell’s powerhouse because it produces ATP (Adenosine Triphosphate), the cell’s energy currency.

Functions

1. The generation of energy in the form of ATP, which is essential for the appropriate functioning of all cell organelles, is the major function of Mitochondria.
2. Mitochondria also aid in the process of apoptosis by balancing the amount of Ca+ ions in the cell.

Nucleus

Fig: Nucleus

The nucleus is a double membrane-bound structure that controls all cellular activity and serves as a storage and transfer centre for genetic resources. It is one of the big cell organelles, taking up 10% of the cell’s total space. It is known as the “brain of the cell” because it sends commands to other cell organelles to ensure that they function properly. In eukaryotic cells, the nucleus is well delineated; nevertheless, in prokaryotic species, the genetic material is spread throughout the cytoplasm.

Functions

1. The nucleus is in charge of both storing and transmitting genetic elements in the form of DNA or RNA.
2. It facilitates transcription by assisting the production of mRNA molecules.
3. The nucleus regulates the activity of all other organelles while promoting processes such as cell division, cell development, and protein synthesis.

Endoplasmic Reticulum

Fig: Endoplasmic Reticulum

In 1948, Porter used the term endoplasmic reticulum (ER) to characterise a fine reticulum in endoplasmic cells. It refers to the cytoplasm’s extensive network of membrane-enclosed tubules, vesicles, and sacs. Differentiated cells have Endoplasmic Reticulum. Prokaryotic cells and undifferentiated cells lack it. ER appears as a double membrane structure with varying gaps under an electron microscope. The ER is a continuous system. On one side, it is attached to the nuclear envelope, while on the other, it is attached to the cell membrane.

Functions

1. The synthesis of lipids such as cholesterol and steroids is one of the most important tasks of the ER.
2. Rough ER facilitates polypeptides emerging from ribosomes to be modified in order to prepare secondary and tertiary protein structures.
3. The ER also produces numerous membrane proteins and is involved in preparing the nuclear envelope during cell division.

Ribosome

Fig: Ribosome

Protein synthesis takes place in RNA-rich cytoplasmic granules found in small organelles. The size of ribosomes is measured in Svedberg (S) units, which are generated from ultracentrifuge sedimentation (used before electron microscopes were available).

Functions

1. In all living organisms, ribosomes are the site of biological protein production.
2. They help protein synthesis by arranging amino acids in the order specified by tRNA.

Lysosomes

Fig: Lysosome

Lysosomes are cellular components that contain a variety of digesting enzymes. Dave coined the term “lysosome” back in 1955. Lysosomes are commonly seen in animal cells. However, they have recently been discovered in fungi such as Neurospora. Because lysosomes come in such a wide range of shapes and sizes, identifying them can be difficult. They range in size from 400 to 800 mm. Only the presence of digestive enzymes can be used to identify them.

Functions

1. These organelles are in charge of intracellular digestion, in which larger macromolecules are broken down into smaller ones by enzymes found within them.
2. Lysozymes are also responsible for the autolysis of undesirable organelles within the cytoplasm. Hence, these are called suicidal bags.

Golgi Apparatus

Fig: Golgi Apparatus

The Golgi Apparatus is a cell organelle found mostly in eukaryotic cells that is responsible for packaging macromolecules into vesicles and transporting them to their active sites.

Functions

1. The Golgi Complex is responsible for directing proteins and lipids to their proper destinations, acting as the cell’s “traffic cop.”
2. They have a role in the exocytosis of a variety of products and proteins, including zymogen, mucus, lactoprotein, and thyroid hormone fragments.
3. Other cell organelles, such as the cell membrane and lysozymes, are synthesised by the Golgi Complex.
4. They also participate in the sulfation of a wide range of compounds.

Centrioles

Fig: Centriole

Centrioles can only be present in animal cells. Centrioles are cellular entities that are cylindrical and always occur in pairs. They have a diameter of 3000-5000 and a length of 1500-1800. The centrosphere is the cytoplasm found at the spindle’s poles. Centrosomes are made up of the centrioles and the centrosphere.

Functions

1. Centrioles play a critical function in cell division by producing spindle fibres that aid in transporting chromatids to their respective sides.
2. They play a role in the development of Cilia and Flagella.

Cilia and Flagella

Cilia are tiny hair-like projections found outside the cell membrane that act like oars to propel the cell or the ECF along. Flagella are slightly larger and are responsible for cell movement. The eukaryotic flagellum differs structurally from its bacterial counterpart. An axoneme is the part of the cilium and flagellum that has nine sets of well-organised peripheral microtubules and a cluster of central microtubules that run parallel to the axis. A scaffold connects the core tubules and encases them in a central sheath. In addition, a radial spoke connects one of the peripheral microtubular pairs to the central sheath. Hence, there is a complete set of 9 radial spokes. The cilia and flagella rise out from centriole-like forms known as basal bodies.

• Microfilaments

Actin (a highly conserved protein that is the most abundant protein in most eukaryotic cells) is used to make microfilaments, which are the thinnest component of the cytoskeleton. Actin is a useful protein in cell motility since it is both flexible and strong. The actin-myosin system mediates contractions in the heart.

Functions

1. It generates the strength for the structure and movement of the cell in association with myosin protein.
2. They help in cell division and are involved in the products of various cell surface projections.

• Microtubules

Fig: Microtubules

Functions

1. Microvilli increase the cell’s surface area, which improves absorption and secretion processes.
2. The membrane of microvilli is densely packed with enzymes that allow bigger molecules to be broken down into smaller molecules, allowing for more effective absorption.

Peroxisomes and Spherosomes

Fig: Structure of Peroxisomes

Talbert and his colleagues discovered peroxisomes in plant cells for the first time in 1968. A single membrane surrounds peroxisomes. Glyoxylate reductase, catalase, glutamate, glyoxylate transaminase, and malate dehydrogenase are among the enzymes found in them. These organelles are involved in photosynthesis’ glycolate metabolism. Animal cells also contain these. Plant cells have spherosomes, which are absent in prokaryotes and mammals. It has a diameter of 0.5-1 mm and is surrounded by single membranes, similar to peroxisomes. These are supposed to be associated with lipid storage.

Functions

1. During biochemical activities, peroxisomes are involved in the synthesis and removal of hydrogen peroxide.
2. Fatty acid oxidation happens within peroxisomes.
3. Peroxisomes are also involved in the production of lipids such as cholesterol and plasmalogens.

Plastids

Fig: Plastids

Plastids are found in eukaryotic cells, mostly in plants, which are double membrane-bound organelles important in food synthesis and storage. Plastids are commonly oval or spherical, with an outer and inner membrane and an intermembrane space between them. Plastids are cytoplasmic organelles that self-replicate in plant cells. The storage of starch, protein, and fat is related to leucoplasts, which are colourless. Plastids contain DNA and RNA, allowing them to produce the proteins required for various functions. Plastids are not found in bacteria, fungi, or mammals. Leucoplast and chloroplast are the two types of plastids.

1. Leucoplasts

Plants’ non-photosynthetic tissues contain leucoplasts, which serve as a storage facility for protein, fat, and carbohydrate.

2. Chloroplasts

Fig: Chloroplast

Chloroplasts are stretched organelle with a phospholipid coating surrounding them. It is because the chloroplast is shaped like a disc, the stroma is the fluid within the chloroplast that contains circular DNA. Chlorophyll, a green-hued pigment, is found in every chloroplast and is used during photosynthesis. The chlorophyll takes solar energy and uses to convert CO2 and water into glucose.

Functions

1. Because it includes enzymes and other components essential for photosynthesis, chloroplasts are at the centre of numerous metabolic functions.
2. They also have a role in the preservation of food, particularly starch.

Vesicles

Fig: A Liposome and Dendrimersome

Vesicles are structures found inside cells that are synthesised naturally during processes such as exocytosis, endocytosis, or the transfer of materials across the cell, or artificially in the form of liposomes.

There are different types of vesicles like vacuoles, and secretory and transport vesicles based on their function.

Functions

1. Vesicles help to store and move materials both inside and outside the cell. It even allows molecules to be exchanged between two cells.
2. Vesicles have a role in metabolism and enzyme storage because they are encased in a lipid bilayer.
3. They provide for temporary food storage while also controlling the cell’s buoyancy.

Vacuoles.

Plant cells and animal cells have vacuoles. Two or more vacuoles can sometimes merge together to form a big vacuole. Tonoplast is a vacuolar membrane that surrounds each vacuole. Tonoplast is made up of lipoprotein and exhibits selective permeability. Cell sap is the fluid found inside the vacuole. Sugars, organic acids, inorganic salts, proteins, and pigments are among the components found in true solutions in the cell sap.

1. To protect the cell against toxicity, vacuoles store nutrients as well as waste products.
2. They play an important role in homeostasis because they allow the cell’s pH to be balanced by the inflow and outflow of H+ ions into the cytoplasm.
3. Vacuoles contain enzymes that are involved in a variety of metabolic processes.

Summary

A cell organelle is a tiny organ-like structure found inside the cell. It has a certain structural make-up and serves a specific purpose. Ribosomes, for example, are not separated from the rest of the cell by a membrane. They can be found in both bacterial and eukaryotic organisms. A single membrane separates some organelles, like, vacuole, lysosome, Golgi apparatus, Endoplasmic reticulum, and so on. They can only be found in eukaryotic cells. Double membrane-bound organelles, such as mitochondria and chloroplast, are found in cells. They can only be found in eukaryotic cells.

FAQs on Cell Organelles

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