Endomembrane System | Biology for ACT PDF Download

Golgi Complex

Discovered by C. Golgi (1898) -  In nerve cells of owl and named "internal reticular apparatus" (Golgi body first observed by L.S. George) (Impregnated with silver nitrate).

Golgi Body

The cytoplasm surrounding Golgi body have fewer or no other organelles. It is called Golgi ground substance or zone of exclusion. Golgi bodies are pleomorphic structures because component of golgi body are differ in structure & shape in different cells.

Structure

Golgi complex is made up of four parts:

• Cisternae: These are unbranched saccules likes smooth E.R., many cistenae are arranged in a stack. Dense opaque material inside cisternae is called nodes.
  (i) Convex surface of cisternae which is towards the nucleus is called cis face or forming face.
  (ii) Concave surface of cisternae which is 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 to tubules.
• Vesicles: Spherical structures arise by budding from tubules. Vesicles are filled with secretory materials. 

•  Golgibody is single membrane bound cell organelle.
• About 60% proteins and 40% phospholipid occur in golgi body.

Functions

• Cell Secretion: Chief function of golgi body is secretion (export) of macromolecules.
• Secretion involve three steps:
  (i) Golgi body recieves the materials from E.R. through it's cis - face.
  (ii) These materials are chemically modified by golgi body. (For e.g. glycosidation (glycosylation) of proteins and lipids takes place in golgi body and it yields glycoprotiens and glycolipids).
  (iii) After chemical modifications materials are packed in vesicles. These vesicles are pinched off from trans face of golgi body and discharged out side the cell (Reverse pinocytosis)
• Golgi complex involves secretion of zymogen granules from pancreas, secretion of lactoprotein from mammary Glands.
• The secretion of hormone by endocrine glands is mediated through golgibodies.
• All the macromolecules which are to be sent out side the cell, move through the golgi body. So golgi body is termed as “Director of macromolecular traffic in cell” or middle men of cell.
• Synthesis of cell wall Material (Polysaccharide synthesis)
• Cell plate formation (Phragmoplast) during cell formation.
• Formation of acrosome during spermiogenesis. (formation of male gametes)
• Vitelline membrane of egg is secreted by golgi body. 
• Formation of Lysosome = It is collective function of golgi body and E.R.
• Mucilage secretion by root cells for lubrication to soil.
• Secretion of hormones by cells of glands.

Lysosome

• Christian De Duve (1955) discovered lysosome as cell organelle and also named Lysosomes. (Lysosome were first observed by Novikoff)
• With the exception of mammalian RBC they were reported from all cells.
• In plant cells large central vacuole functions as Lysosome. So in higher plants lysosomes are less frequent. But number of lysosomes is high in fungi. 
• Periplasmic Space :– space between cell wall and cell membrane in bacteria,may play similar role.
• Lysosomes are spherical bag like structures  (0.1-0.8 mm) which is covered by single unit membrane. They are larger in Phagocytes (WBC) (0.8 to 2mm). 
• Lysosomes are filled with 50 different type of digestive enzymes termed as Acid hydrolases. These acid hydrolases function in acidic medium ( pH=5).Membrane of lysosome has  an active H+ pump mechanism which produce acidic pH in lumen of lysosome.
• Lysosomes are highly polymorphic cell organelle. Because, during functioning, lysosomes have different morphological and physiological states.

Types of Lysosomes

• Primary Lysosomes or Storage Granules: These lysosomes store enzyme Acid Hydrolases in the inactive form. (Enzymes  synthesized on ribosomes in cytoplasm) these are newly formed lysosome.
• 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 exocytosis. These are also called as Telolysosomes. (Tertiary lysosomes) 
• Autophagic Lysosomes or Cytolysosomes or Autophagosomes: Lysosomes containing cell organelles to be digested are known as Autophagosomes.

Functions

• Heterophagy :– This is digestion of foreign materials received  in cell by phagocytosis and pinocytosis.
• Autophagy :– Digestion of old or dead cell organelles. Autophagy also takes place during starvation of cell. [Ambilysosomes :– Lysosomes which perform both heterophagy and autophagy.]
• Extracellular digestion :– Lysosomes of osteoclast (bone eating cells) dissolve unwanted part of bones. (Extracellular digestion also occurs by fungal lysosomes.)
• Crinophagy :– Excessive secretory granules of hormone in endocrine gland may be digested by lysosomes. This event is called crinophagy. Thyroglobulin stores in thyroid gland with its follicles and after crinophagy by proteases itproduces thyroxine.
• Cellular digestion (Autolysis) :– Sometimes all lysosomes of a cell burst to dissolve the cell completely. Old cells are removed by autolysis. unwanted organs of embryo are destroyed by autolysis Cathepsin of lysosome digests the tail of tadpole of frog during metamorphosis.
• Lysosomes are helpful in digestion of egg membrane to assist fertilisation.
• Lysosome also trigger the cell division or mitosis.
  (i) Membrane stabilizers are substances, which stabilize the lysosome membrane and stop its rupture, thus prevents autolysis. e.g. cholesterol, chloroquine, cortisone etc.
  (ii) Membrane labilizers are substances which make the lysosome membrane fragile and increase the chance of autolysis e.g. Progesterone, testosterone, Vitamin    A, D, E, K, U.V. radiations, bile salts etc.
  Sometime Lysosomes burst it's whole cell so Lysosome called as suicidal bags of cell.
  (iii) Biogenesis of Lysosome Lyosomes originates from G E R L - (Golgi associated Endoplasmic Reticulum from which Lysosomes arise).
  E.R. → Golgi body → Lysosome

Endoplasmic Reticulum

"Garnier" (1897) first observed them and called Ergastoplasm. E. R. name proposed by "Porter" (1961). (Credit for discovery of ER goes toPorter) Components of E.R.

• Cisternae - These are long flattened and unbranched units arranged in stacks.
• Vesicles - These are oval membrane bound structures.
• Tubules - These are irregular, often branched tubes bounded by membrane. Tubules may free or associated with cisternae.
  (i) Structure of E.R. is like the golgi body but in E.R. cisternae, vesicles and tubules are isolated in cytoplasm and these do not form complex.
  (ii) Golgi body is localised cell organelle while E.R. is widespread in cytoplasm. E.R. is often termed as “System of Membranes”.

Enzymes of E.R.

Sucrases, NADH diphosphatase, Gulcose-6-phosphatase, NADH-cytochrome-C-reductase, Mg+2 activated ATPase, Nucleotide diphosphatase, Ascorbic acid synthase are enzymes of E.R.

Functions of E.R.

• Mechanical support: Microfilaments, Microtubules and E.R. forms endoskeleton of cell.
• Intracellular exchange: E.R. forms intracellular conducting system. Transport of materials in cytoplasm from one place to another may occurs through the E.R.
  (i) At some places E.R. is also connected to P.M. So E.R. can secrete the materials outside the cell.
• Rough E.R.: Provides site for the protein synthesis, because rough E.R., has ribosomes on its surface.
• Lipid Synthesis: Lipids (cholesterol & phospholipids) synthesized by the agranular portion of E.R. (Smooth E.R.). The major lipids synthesized by S. E. R. are phospholipids and Cholesterol.
• Release of Glucose from Glycogen: Endoplasmic reticulum seems to play a role in breakdown of glycogen (glycogenolysis).(The polymerisation of glucose to form glycogen  probably occur in the cytosol not in the wall of S.E.R.)
• Cellular metabolism:The membranes of the reticulum provides an increased surface for metabolic activities within the cytoplasm.
• Formation of nuclear membrane: Fragmented vesicles of disintegrated nuclear membrane and ER elements arranged around the chromosomes to form a new nuclear membrane during cell division.
• Formation of lysosomes, Golgi–body & Micro–bodies. All the organelles are form by E.R. which have membrane except chloroplast and mitochondria (semi autonomous organelles)
• Detoxification: Smooth ER concerned with detoxification of drugs, pollutants and steroids. 
• Cytochrome P450 in E.R. act as enzyme which function in detoxification of drugs and other toxins
• E.R. provides the precursor of secretory material to golgi body.

Vacuoles

“Vacuoles are membrane-bound cell organelles present in the cytoplasm and filled with a watery fluid containing various substances.”

What are Vacuoles?

The term “vacuole” means “empty space”. They help in the storage and disposal of various substances. They can store food or other nutrients required by a cell to survive. They also store waste products and prevent the entire cell from contamination. The vacuoles in plant cells are larger than those in the animal cells. The plant vacuoles occupy more than 80% of the volume of the cell. The vacuoles may be one or more in number.

Structure of Vacuole

A vacuole is a membrane bound structure found in the cytoplasmic matrix of a cell. The membrane surrounding the vacuole is known as tonoplast. The components of the vacuole, known as the cell sap, differ from that of the surrounding cytoplasm. The membranes are composed of phospholipids. The membranes are embedded with proteins that help in transporting molecules across the membrane. Different combinations of these proteins help the vacuoles to hold different materials.

Functions of Vacuole 

The important functions of vacuole include:

Storage 

A vacuole stores salts, minerals, pigments and proteins within the cell. The solution that fills a vacuole is known as the cell sap. The vacuole is also filled with protons from the cytosol that helps in maintaining an acidic environment within the cell. A large number of lipids are also stored within the vacuoles.

Turgor Pressure 

The vacuoles are completely filled with water and exert force on the cell wall. This is known as turgor pressure. It provides shape to the cell and helps it to withstand extreme conditions.

Endocytosis and Exocytosis 

The substances are taken in by a vacuole through endocytosis and excreted through exocytosis. These substances are stored in the cells, separated from the cytosol. Lysosomes are vesicles that intake food and digest it. This is endocytosis and it varies in different cells.

Frequently Asked Questions

Why do plant cells have larger vacuoles? 
The plant cells have larger vacuoles because they require more water, organic and inorganic components for the proper functioning of the cell. 

Why are vacuoles an important cell organelle? 
Vacuoles store nutrients and water on which a cell can rely for its survival. They also store the waste from the cell and prevents the cell from contamination. Hence, it is an important organelle.

Old NCERT Syllabus

Golgi body is known by several other names:
(1/16)  Golgi body                                      
(1/16)  Dalton complex                  
(1/16)  Golgi complex
(1/16) Lipochondria ( rich in lipids)            
(1/16) Baker's body                    
(1/16) Idiosome
(1/16) Dictyosome (plant golgi body)          
 (1/16) Trophospongium

Modifications of E.R.

• Sarcoplasmic Reticulum (S.R.): These smooth E.R. occurs in skeletal and cardiac muscles. S.R. Stores Ca+2 and energy rich compounds required for muscle contraction.
• T-tubules: These are transversely arranged tubules in skeletal and cardiac muscle cells. These transmits stimulus for contraction of muscles.
• Ergastoplasm: When the ribosomes are accumulated on the small parallel cisternae of E.R., then called Ergastoplasm. Ergastoplasm of nerve cells is called as Nissl's bodies.
• Myeloid Bodies: Myeloid bodies are the specialised smooth E.R. which found in pigmented epithelial cells of the retina. Myeloid body is light sensitive structure and may be involved in pigment migration.
• Microsomes: These are pieces of E.R. with associated ribosomal particles (Claude 1951). These can be obtained by Fragementation and high speed centrifugation of cell. They do not exist as such in the living cell.