What Are Cell Organelles: Types and Their Functions - Studynlearn

We all know that cell is not only the structural unit but also the functional unit in all living organisms. The membrane-bound specialized subunits within a cell that perform specific functions and remain suspended in the fluid of the cytoplasm are called cell organelles.


In Eukaryotic cells, the presence of a membrane around the cell organelles keeps its contents separate from the external environment. It also keeps the different types of cellular activities separate from each other. The presence of nuclear membrane and membrane-bound cell organelles in a eukaryotic cell differentiates it from a prokaryotic cell.


Some membrane-bound cell organelles found in eukaryotic cells are:


1. Nucleus


You already know that cells are the structural and functional units of living organisms. But are you aware of the most important cell organelles? It is the nucleus. Let us learn about the nucleus in this module.


The nucleus is the largest and the most conspicuous organelle in a eukaryotic cell. It is generally enclosed in a double-layered nuclear membrane and is located in the center of the cell. It is one of the structures that make up a nucleus.


The other components include nuclear lamina, nucleoplasm, and other sub-nuclear components such as the nucleolus and the cell�s genetic material organized in a DNA-protein complex known as chromatin.


The nuclear envelope or the nuclear membrane has pores that allow the transfer of material from inside the nucleus to its outside to the cytoplasm, thus enabling nuclear transport.


The nucleus contains information for the inheritance of features from parents to the next generation which is contained in the form of DNA molecules. The functional segments of DNA are called genes.


In a non-dividing cell, the DNA is present as part of chromatin material which is visible as an entangled mass of thread-like structures.


At the time of cell division, this chromatin material gets organized into rod-shaped structures called chromosomes. These are composed of DNA and protein.

Let us also learn about the sub-nuclear body- The nucleolus.


It is a dense, dark-staining structure involved mainly in the assembly of ribosomes. It has no limiting membrane and is rich in RNA and proteins.


Unlike eukaryotic cells, prokaryotic cells such as those of bacteria lack a nuclear membrane. The nuclear region containing only nucleic acids is, therefore, poorly defined. This undefined nuclear region is known as the nucleoid.


Let us now innumerate the various important functions of the nucleus. The nucleus is considered the control center of the cell because:


- It has a central role in cellular reproduction.
- It controls all the metabolic activities of the cell.
- The nucleus also possesses genetic information concerned with the transmission of traits from one generation to another.


2. Mitochondria

Mitochondria are regarded as the�powerhouse of the cell because they generate energy-rich compounds. Several organisms inherit the mitochondrial genome from their mothers. It's a sausage-shaped organelle with two membranes linked to it that are found in nearly all eukaryotic cells.


The lumen is divided into two aqueous compartments by double membranes. The inner compartment is referred to as the 'matrix,' and it is folded into cristae, while the outside membrane forms a continuous barrier with the cytoplasm. They are typically round or oval in shape and come in a variety of sizes. Mitochondria are the locations of aerobic respiration in the cell, where energy is produced in the form of ATP and molecules are transformed.


Glucose, for example, is transformed to ATP (adenosine triphosphate).


Mitochondria have circular DNA, RNA molecules, ribosomes,�and a few additional molecules that assist in protein production.

3. Chloroplast


Chloroplasts are membrane-bound cell organelles that come in a variety of shapes, including disc, spherical, discoid, oval, and ribbon. These pigments are in charge of absorbing light energy and converting it into chemical energy for photosynthesis.


4. Endoplasmic reticulum


We have already learned about the cell membrane, cell wall, and nucleus. Let us now learn about an important cell organelle- The Endoplasmic reticulum or the ER.


The endoplasmic reticulum is a eukaryotic organelle that forms an extensive network of intracellular membrane-bound tubules and vesicles occupying most of the cell cytoplasm.


The structure of ER shows similarity to that of the plasma membrane. The Endoplasmic Reticulum is of two types namely:


  1. Rough endoplasmic reticulum or the RER
  2. Smooth endoplasmic reticulum or the SER


Let us now learn about the two types one by one.


Rough Endoplasmic Reticulum


The surface of the rough endoplasmic reticulum possesses a large number of grain-like particles called ribosomes attached to it which imparts it a rough appearance under the microscope. Its membrane is continuous with the outer layer of the nuclear envelope.


The ribosomes attached to the surface of the RER are the sites of protein synthesis and the manufactured proteins are then transferred to various places within the cell depending upon the requirement. This transfer is brought about by the ER.


Let us come to the Smooth Endoplasmic Reticulum now


As the name suggests, the smooth endoplasmic reticulum has a smooth surface. It helps in the synthesis of fats or lipid molecules that are important for cell function. It is also connected to the nuclear envelope. Another function of SER is to provide an increased surface area for action and storage of key enzymes and their products vital for various metabolic processes.


Some of the proteins and lipids synthesized by the ER are utilized in building the cell membrane and the process is known as membrane biogenesis.


Let us now innumerate and briefly discuss the various functions of the Endoplasmic Reticulum.


It compartmentalizes the cytoplasm of the cell, thereby enabling the cell to perform specific functions within specific chambers.


It provides mechanical support to maintain the shape of the cell. It also serves as a channel for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus. In liver cells of the vertebrates, SER plays a crucial role in drug and poison detoxification.


5. Golgi apparatus


The Golgi Apparatus, also known as the Golgi Body or the Golgi Complex is named after the Italian physician �Camillo Golgi� who first identified it in 1897. It is found in most eukaryotic cells.


The Golgi apparatus is a disc-shaped organelle that consists of a system of membrane-bound vesicles arranged approximately parallel to each other in flattened sac-like structures called cisternae. In-plant cells, an individual stack is sometimes referred to as a dictyosome.


These membranes often have connections with the membranes of ER, thus constituting a part of the complex cellular endomembranous system.


Functions of the Golgi Apparatus


It stores modify and package the materials synthesized near the ER or the Golgi body itself into vesicles. The Golgi apparatus is also involved in the packaging and dispatch of these materials to various targets inside and outside the cell.


It is also a major site for the synthesis of complex carbohydrates from simple sugars. The Golgi apparatus is also involved in the formation of lysosomes, wherein the released vesicles form


6. Lysosomes


Lysosomes are membrane-bound cell organelles filled with hydrolytic digestive enzymes made by RER. These are responsible for the intracellular digestion in eukaryotic cells. The lysosomes were discovered by the Belgian cytologist Christian De Duve in 1949.


Let us now understand the role that lysosomes have in a eukaryotic cell.
They serve several major functions. Let us discuss them one by one.
1. The digestion of organic food particles taken up by the cells in vacuoles from the environment occurs in the lysosomes.
2. The lysosomes digest any foreign particles such as viruses, bacteria, and other toxic molecules by hydrolyzing them.
3. They also dispose off the worn-out cell organelles by digesting them, thus keeping the cell clean.
4. During instances of disturbance in cellular metabolism or cell damage, lysosomes may burst releasing the enzymes that eventually digest their own cell. Hence, they are also known as the �suicide bags� of the cell.


7. Vacuoles:


Vacuoles are membrane-bound cell organelles that serve as storage sacs for solid or liquid contents. These are formed by the fusion of multiple membrane vesicles, and their structure varies according to the requirement of the cell.


In animal cells, a large number of small-sized vacuoles are present. They simply function as storage vesicles involved in the transport and disposal of the selected proteins and lipids to the extracellular environment.


They have a different role in unicellular organisms. In organisms such as Amoeba and Paramecium, the food vacuole contains the food that the animal has ingested and subsequently consumed.


Some unicellular organisms may also possess specialized vacuoles which play important roles in expelling excess water and wastes from the cell. Unlike animal cells, plant cells possess large vacuoles which are fewer in number. They are full of cell sap and provide turgidity and rigidity to the cell. Many substances important in the life of the plant cell such as amino acids, sugars, various organic acids, proteins, etc are stored in the vacuoles.


8. Plastids


Plastids are cytoplasmic cell organelles found mostly in plant cells and are absent in animal cells. They consist of numerous membrane layers embedded in a material called the stroma. Like mitochondria, they also consist of their own DNA and ribosomes.


There are mainly two types of plastids.


  1. Chromoplasts (or the colored plastids)
  2. Leucoplasts (or the colorless plastids)


Let us discuss the two types of plastids briefly.


Chromoplasts or chromoplastids are colored plastids. Plastids containing the green pigment chlorophyll are better known as chloroplasts which are important for photosynthesis in plants.


On the other hand, Leucoplasts or leucoplastids are the colorless plastids in which materials such as starch, oils, and protein granules are stored.


Read More:
What is a Cell - Structure, Functions, And Types

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