Some Major Cell Organelles

Cell organelles are specialised structures within cells; they have various shapes and sizes. They serve a crucial purpose because we must compartmentalise all the operations within the cell. As a result, a membrane must surround the systems within a cell that produce a different product; organelles are entirely membrane-bound. They also distinguish one function from another. We will look in detail at the cell organelles and their function chart.

Types of Cell Organelles

The cytoplasm and the nucleus are the two major sections of a cell. A nuclear envelope surrounds the nucleus, which contains The genetic code in the format of a chromosome. The cytoplasm is a liquid network surrounding the nucleus, separated by the cell’s outer membrane. Organelles are tiny structures in the cytoplasm that perform functions that keep the cell in a state of homeostasis. They are involved in various functions, including energy synthesis, protein and secretion formation, toxin destruction, and reacting to external signals.

Structure of Cell Organelles

Mitochondria

It is a rod shaped structure found in cytoplasm of all eukaryotic cells except mammalian RBCs. It is absent in prokaryotes.

•  Power house of cell or ATP-mill in cell
•  Cell within cell/ second largest organelle in plant, largest organelle in animal.
• Cell furnaces or storage batteries
•  Most busy and active organelle in cell
•  Semi autonomous cell organelle.
•  Discovered by Kolliker (1857) and termed it sarcosomes.
• Altmann (1894) renamed it bioplast.
•  Mitochondria term given by Benda (1897)
•  Higher number of mitochondria is found in metabolically active cells.

Structure

 It is double membrane bound cell organelles.The space between inner and outer mitochondrial membrane is called peri mitochondrial space.The outer membrane is smooth and straight. The inner membrane is infolded into the cavity. These finger-like infoldings are called cristae.Many electron carrier cytochromes are arranged in a definite sequence in the inner membrane of mitochondria which forms Electron transport system (ETS) for oxidative phosphorylation. Cristae is studded with pin head particles called oxysomes or elementary particles or  – F0 particles. These particles were first described by Fernandez Moran. So it is called the Fernandez Moran particle The fluid filled in mitochondrial cavity is called matrix..Head of oxysomes composed of ATPase (ATP synthase) enzymes and concerned with oxidative phosphorylation. Mitochondrial matrices have enzymes for the Krebs cycle. Beside these enzymes, matrices have a complete protein synthesis apparatus (Ribosome-70-s, DNA & RNA, enzymes) so mitochondria are called as semi autonomous cell organelle.

Function:

Its main function is to produce and store the energy in the form of ATP,

 (i) Mitochondrial matrix – Site of Krebs cycle.

(ii) Peri-mitochondrial space – Site of link reaction.

(iii) oxysomes – Site of ETS.

Endoplasmic Reticulum (ER)

First observed by Garnier (1897) – Termed as Ergastoplasm.

E.R. name proposed by K.R. Porter. [Credit for discovery of ER goes to Porter]

Components of E.R.

(i)  Cisternae – These are long, flattened and unbranched units arranged in stacks.

(ii) Vesicles – These are oval membrane bound structures.

(iii) Tubules – These are irregular, often branched tubes bounded by membranes. Tubules may be free or associated with cisternae.

The endoplasmic reticulum is present near the nucleus. 

Based on the attachment of ribosomes on the surface of the endoplasmic reticulum, we can categorise these cell organelles into two categories, the rough ER and the smooth ER. 

Function:

(i)  Mechanical support.

(ii) Rough E.R. – synthesis of protein

(iii) Smooth E.R. – Glycogen and lipid metabolism, detoxification of drugs, toxin pesticides.

(v) Circulation.

(vi) Formation of lysosomes, Golgi-body & Micro-bodies – All the organelles are formed by E.R. which have membranes except chloroplast and mitochondria.

(vii)   E.R. provides the precursor of secretory material to the Golgi body.

Membrane bio-genesis-It is the process of preparing of damaged cell membrane by lipids & protein provided by SER & RER respectively.

Golgi Apparatus

• Discovered by Camillo Golgi (1898) in nerve cells of owls.

Other names:

• Lipochondria,  
• Idiosome,
• Baker’s body,
• Dalton complex 
• Dictyosomes – In plants

Position:

• It is located near the nucleus.
• These bodies arise from SER mainly.
• The cytoplasm surrounding the Golgi body has fewer or no other organelles. It is called Golgi ground substance or zone of exclusion.
• Golgi bodies are pleomorphic structures, because components of the golgi body differ in structure & shape in different cells.
• Main enzyme in golgi bodies is nucleoside diphosphatase.

Structure: It is formed of four types of contents.

Cisternae – These are long flattened and unbranched saccules. 4 to 8 saccules are arranged in a stack.Convex surface of cisternae (towards the nucleus) is called cis-face or forming face.Concave surface of cisternae (towards the membrane) is called Trans-face or maturing face,

Tubules – These are branched and irregular tube like structures associated with cisternae.

Vacuoles – Large spherical structures associated with tubules.

 Vesicles – Spherical structures arise by budding from tubules. Vesicles are filled with secretory      materials. Golgibody is a single membrane bound cell organelle.

Function:

1. It is involved in cell-secretion and acts as storage, modification and condensation or packaging membrane.
2. It forms the Acrosome of sperm
3. It forms the lysosomes and secretory vesicles.
4. It is the site for formation of glycolipids and glycoproteins.
5. Synthesis of cell wall material (Polysaccharide synthesis)
6. Cell plate formation (phragmoplast) during cell formation.
7. Vitelline membrane of egg is secreted by the golgi body.
8. Lacto protein in mammary gland.
9. Secretion of insulin.

Lysozyme

LYSOSOME OR SUICIDE BAG, DIGESTIVE BAG, SCAVENGER OF CELL, CELLULAR HOUSE KEEPER, GARBAGE DISPOSAL SYSTEM

• First observed and the term coined by Christian De Duve (1955) from rat liver.
• Lysosomes are spherical bag-like structures [0.1 – 0.8 urn] which are covered by a single unit membrane.
• They are larger in size and number in phagocytes (WBC) – 0.8 to 2micrometer With the exception of mammalian RBC they are reported from all cells.
• Lysosomes are filled with about 50 different types of digestive enzymes termed as acid hydrolases.
•  Lysosomes are highly polymorphic cell organelle. Because, during functioning, lysosomes have different morphological and physiological structures.

Types of Lysosomes

Primary lysosomes or storage granules – These lysosomes store enzyme Acid Hydrolases in the inactive form. (Enzymes synthesised on ribosomes in cytoplasm) these are newly formed lysosomes.

Digestive vacuoles or Heterophagosomes – These lysosome forms by the fusion of primary lysosomes and phagosomes. These are secondary lysosomes.

Residual bodies – Lysosomes containing undigested material are called residual bodies. These may be eliminated by pinocytosis. These are also called Telolysosomes. (Tertiary lysosomes)

Autophagic lysosomal or cyto lysosomes or autophagosomes – Lysosomes containing cell organelles to be digested are known as Autophagosomes.

Function:

(i)  Heterophagy: It involves digestion of foreign materials received in cells by phagocytosis and pinocytosis.

(ii) Autophagy: Digestion of old or dead cell organelles. Autophagy also takes place during starvation of cell

(iii) Cellular digestion (Autolysis): Sometimes all lysosomes of a cell burst to dissolve the cell completely.That’s why lysosomes also known as suicidal bags.

Conclusion :

Each organelle has a specific function, and if you don’t know what cell or organelle you’re talking about, the cell will most likely stop working. 

Although, as an organism evolves, some organelles may become obsolete or be replaced by others, in which case the organelles may naturally disappear. 

Cell organelles impact the function of cells. If mitochondria or other critical organelles such as the nucleus are missing, the cell would not be able to survive.