Cell Organelles: Definition, Types & Their Functions

Organelles are the functional structures contained inside the cell. Every single species is composed of cells including both single celled and multicellular organisms. Apart from providing shape and structure to an organism, the cell performs different functions in order to keep the entire system active. So, the functional structures called organelles inside the cell are responsible to keep the entire system active.

Key Terms:- Cell organelles, Membranes, Multicellular organisms, Cytoplasm, Nucleus, Cell wall.

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Definition of organelle

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The cellular components responsible for the functioning of a cell are called cell organelles . Different organelles present within the cell are classified into three categories based on the presence or absence of the membrane.

• Organelles without membrane: include cell wall, ribosomes and cytoskeleton. These organelles are present in both Prokaryotic and Eukaryotic Cells.
• Single membrane bound organelles: include vacuole, Golgi apparatus, endoplasmic reticulum and lysosome and are present only in eukaryotic cells.
• Double membrane bound organelles: include nucleus, chloroplast and mitochondria. These organelles are present only in eukaryotic cells.

These organelles also differ in their structures and functions performed by them. For normal functioning of the cells, these organelles coordinate and function efficiently. Few organelles function by providing shape and support whereas some are responsible for locomotion and reproduction of the cell.

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Cell Organelles Detailed Video Explanation:

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Types of organelles and their functions

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Below are listed the types of organelles with their functions.

• Cell Membrane

Cell membrane is also called plasma membrane or cytoplasmic membrane. It is present in both animal and plant cells. Cell membrane is composed of lipid bilayer and proteins. Plasma membrane is a selectively permeable membrane i.e., it permits the entry of selective material inside and outside of the cell depending on the requirement. The function of the cell membrane is that it provides shape and protects the internal content of the cell. Model for plasma membrane was proposed called a fluid mosaic model. According to this model, the lateral movement of proteins within the overall bilayer is due to the quasi-fluid nature of lipids and this ability to move within the membrane is measured as fluidity.

• Cell Wall

Cell wall is a non-living structure forming the outer covering for the plasma membrane of fungi and plants. Cell wall gives shape to the cell and protects the cell from damage and infection. It also helps in cell-to-cell interaction and provides a barrier to undesirable macromolecules. The cell wall of algae is composed of cellulose, galactans, mannans and minerals like calcium carbonate. And the cell wall of plants has a composition of cellulose, hemicellulose, pectin and proteins. The cell wall of young plants is capable of growth which is gradually diminished while the cell matures and then forms the secondary wall on the inner side of the cell. The middle lamella layer mainly consists of calcium pectate which holds different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata connecting the cytoplasm of neighbouring cells.

• Cytoplasm

Cytoplasm is a jelly-like substance found between cell membrane and nucleus. All the cell organelles are embedded in the cytoplasm. It is composed of water, organic and inorganic compounds. Cytoplasm is one of the essential components of the cell that is present in both plant and animal cells. Cytoplasm functions by controlling all the metabolic activities taking place within the cell and most of the chemical reactions are carried within it.

Also Read: Difference Between Cytoplasm and Protoplasm

• Nucleus

Nucleus is a double membraned and the largest organelle present in all eukaryotic cells. It acts as the storehouse of the cell`s DNA and functions as the control centre of the cellular activities. This round nucleus is surrounded by a nuclear membrane and is dark in colour. The nuclear Membrane is a porous membrane and forms a wall between cytoplasm and nucleus. Nucleus contains tiny spherical bodies called nucleolus. It also carries other essential structure called Chromosomes. Chromosomes are a thread like structures which carry other important component called gene.

Gene is a hereditary unit of organisms that helps transfer traits from parents to offspring. Therefore, nucleus controls the functions and characters of the cell in organisms. The primary function of the nucleus is to monitor cellular activities including metabolism and growth. Protein and RNA is also synthesized in the nucleus with the help of nucleoli.

• Endoplasmic reticulum

Endoplasmic reticulum is a network of tiny tubular structures scattered in the cytoplasm. It divides the intracellular space into two distinct compartments, one is luminal (inside ER) and another is extra luminal (outside ER). They function as the transport system of a cell, involved in transporting materials throughout the cell.

Endoplasmic reticulum is divided into two types based on presence and absence of ribosomes.

• Rough endoplasmic reticulum: The endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER). It is involved in the protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus.
• Smooth endoplasmic reticulum: The endoplasmic reticulum with the absence of ribosomes on its surface is called smooth endoplasmic reticulum (SER). They are responsible for lipid syntheses. In animal cells, lipid like steroid hormones is synthesized in SER. They are also responsible for detoxifying the cell.

• Mitochondria

Mitochondrion is a double membrane bound cell organelle that are not normally visible under the microscope. This cylindrical or sausage shaped structure is also called the power house of the cell as they are the sites for aerobic respiration of the cell and produce cellular energy in the form of ATP. The outer membrane and the inner membrane divide its lumen into two compartments i.e., inner compartment called matrix possessing single circular DNA molecule and a few RNA molecules. forming a number of infoldings called cristae and the outer membrane forming the continuous limiting boundary of the organelle. Depending on the shape, size and physiological activity of the cell, the number of mitochondria per cell are variable. The structure has a diameter of 0.2 to 1µm (average 0.5 µm) and length 1.0 to 4.1 µm.

Also Read:  Difference Between Mitochondria and Chloroplast

• Plastids

Plastids are large and membrane bound organelles that are found in all plants and in euglenoids. They contain some specific pigments that are responsible for imparting specific colours to the plants. On the basis of the type of pigment, plastids can be classified into three types:

1. Chloroplasts: Chloroplasts are double membrane bound cell organelles that contain chlorophyll and carotenoid pigments that are responsible for trapping light energy and this energy is used for the purpose of photosynthesis.

The inner membrane surrounds a space called stroma. Chlorophyll containing structures called thylakoids are arranged as piles of coins and each pile is called a granum. Thylakoids are connected by flat membranous tubules known as stromal lamella.

2. Chromoplasts: These include carotenoid, fat soluble pigments like carotene, xanthophyll etc. providing characteristic colours like yellow, orange, red, etc. to the plants.
3. Leucoplasts: These types of plastids store nutrients and are colourless. They include amyloplasts storing carbohydrates, aleuroplasts storing proteins, and elaioplasts storing oils and fats.

• Ribosomes

Ribosomes are dense particles with granular structures. They have a composition of 2/3 of ribonucleic acid (RNA) and 1/3 of proteins. They are not surrounded by any membrane. Ribosomes present in eukaryotic cells is 80S while ribosomes found in prokaryotes is 70S. The letter S represents the density and size and is called Svedberg`s unit. Both 70S and 80S ribosomes are composed of two subunits. The primary function of the ribosomes is protein synthesis in all living cells ensuring the survival of the cell.

• Cytoskeleton

Cytoskeleton is a network of filamentous proteinaceous structures present in the cytoplasm. The functions of cytoskeleton include motility, mechanical support and maintenance of the shape of the cell.

• Golgi apparatus

Golgi apparatus is a membrane bound organelle composed of flattened, stacked pouches called cisternae. It is also called the Golgi complex. Golgi apparatus is responsible for transporting, modifying, and packaging proteins and lipids to their targeted destinations. These are found in both plant and animal cells.

• Microbodies

Microbodies are the membrane bound minute vesicles that contain various enzymes. They are found in both plant and animal cells. They can be visualised only under the electron microscope.

Also Read: Difference Between Cell and Tissue

• Cilia and flagella

The hair-like outgrowths or projections of the cell membrane are called cilia (sing: cilium). They work like oars to move either the cell or the outside fluid. On the other hand, flagella are bigger than cilia and are responsible for cell movements. Flagella possessed by prokaryotic bacteria are structurally different from that of eukaryotic flagella.

Both cilia and flagella are covered with plasma membranes. The core of the cilium and flagellum is known as axoneme containing nine pairs of peripheral microtubules and a set of central microtubules running parallel to the axis. This arrangement of axonemal microtubules is called a 9+2 array. The central tubules are connected by bridges and are also embedded by a central sheath that is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes. Both the cilium and flagellum emerge from centriole-like structures called the basal bodies.

• Centrosome and centrioles

Centrosome is a structure formed by two mutually perpendicular structures known as centrioles. Centriole consists of equally spaced 9 peripheral fibrils of tubulin protein. Core part of the centriole is proteinaceous and is known as a hub. It connects the fibrils through radial spokes made of proteins. During the cell division, centrioles from the basal bodies of cilia and flagella give rise to spindle fibres.

• Vacuoles

Vacuoles are defined as storage bubbles of irregular shapes and fluid filled organelles enclosed by membranes. They are responsible for storage of the food or nutrients that are needed by a cell. They also store waste products and these waste products are eventually thrown out by vacuoles. So, because of the presence of vacuoles, the cell is protected from contamination. Plant Cells have larger vacuoles as compared to animal cells.

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Things to Remember

• Organelles are the cellular components responsible for the normal functioning of the cell.
• Organelles are classified into three types i.e., organelles without membrane, single membrane bound organelle and double membrane bound organelle.
• Fluidity is the measure of the movement of the proteins within the membrane.
• Cytoplasm is responsible for controlling all the metabolic activity of the cell.
• Nucleus is the largest organelle found inside the cell and is considered as the control centre of the cell.
• Nucleus contains thread-like structures called chromosomes and another structure called a gene which is responsible for transfer of traits from parents to offspring.
• Nucleus is also responsible for cell division.
• Endoplasmic reticulum is the transport system of the cell. Rough endoplasmic reticulum and smooth endoplasmic reticulum are the two types of endoplasmic reticulum.
• Mitochondria is considered as the site for aerobic respiration in the cell and it produces energy in the form of ATP and is given a second name as power house of the cell.
• Plastids are of three types, Chloroplasts, chromoplasts and leucoplasts. Chloroplasts are responsible for photosynthesis in the plants.
• Ribosomes are responsible for protein synthesis in all living cells that ensure the survival of the cell.
• Golgi Apparatus is responsible for transporting, modifying, and packaging proteins and lipids to targeted destinations.
• Cytoskeleton provides shape and mechanical support to the cell.
• Cilia and flagella are responsible for the movement of the cell.
• Vacuoles store food or different nutrients needed by the cell.

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Previous Year Questions

1. A bivalent consists of… (NEET 1989)
2. A cell organelle containing hydrolytic enzymes is​... (NEET 2016)
3. Water soluble pigments found in plant cell vacuoles are​... (NEET 2016)
4. Which of the following are not membrane - bound?... (NEET 2015)
5. Which one of the following is not an inclusion body found in prokaryotes?​... (NEET 2015)
6. Flagella of prokaryotic and eukaryotic cells differ in​... (NEET 2004)
7. A major site for synthesis of lipids is​... (NEET 2013)
8. According to fluid mosaic model, plasma membrane is composed of​...(NEET 1988)
9. Acetabularia used in Hammerling's nucleocytoplasmic experiments is​...(NEET 1988)
10. Active and passive transports across cell membrane differ in...(NEET 1993)
11. All types of plastids possess essentially the same structure because they​...(NEET 1992)
12. An outer covering membrane is absent over​...(NEET 1992)
13. Balbiani rings are the sites of​...(NEET 1993)
14. Besides giving out secretory vesicles, the Golgi apparatus is also concerned with the formation of​...(NEET 1994)
15. Biological organisation starts with​... (NEET 2007)
16. Cell recognition and adhesion occur due to biochemicals of cell membranes named​...(NEET 1993)
17. Cell wall shows​...(NEET 1991)
18. Cellular organelles with membranes are...(NEET 2015)
19. Cellular totipotency is demonstrated by​...(NEET 2003)
20. Chlorophyll in chloroplast is located in​...(NEET 2005)

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