CELL ORGANELLES

Cell Organelles:

Organisms are composed of cells and these cells have special structure in them that allow them to carry out their function. These structures are called cell organelles.

 

NUCLEUS:

               Occurrence:

Ø  It is found only in Eukaryotes.

Ø  Nucleus is also absent in mammalian erythrocytes so erythrocyte are known as enucleated.

Ø  Prokaryotic cells lacks a nucleus, in these organisms genetic material is found free floating in cytoplasm in the form of nucleoid.

 

Structure:

 

Ø  It is called as command centre.

Ø  Nucleus is the largest organelle in the cell. It contains dense structure called nucleolus, surrounded by membrane called nuclear envelope. Nuclear envelope is composed of two membranes separated by fluid which contain a number of nuclear pores that can allow relatively large molecule pass through membrane.

Ø  The nucleus contains nearly all of the cell’s genetic material. Nucleolus contains RNA and ribosomes which then travel out of the nucleus through nuclear pores to the cytoplasm where they are involved in protein synthesis.

Ø  Nucleus is composed primarily of histones protein and DNA. Except RBC, all cells are nucleated in human body.

Function:    

Ø  Nucleus controls all the cell’s activities such as growth and metabolism using the DNAs genetic information.

Ø  Nucleolus is the site of ribosome formation.

 

 

ENDOPLASMIC RETICULUM:

             Occurrence

Ø  It is absent in prokaryotes and present in eukaryotic cell.

 

Structure:

Ø  It is found near the nucleus and is made up of number of flattened sacs called cisternae, which are continuous with the nuclear envelope.

Ø  There are two types of ER found in a cell:  Rough ER- It is so called because it has lots of ribosome on its outer surface. SER (Smooth endoplasmic reticulum) does not have ribosomes on its outer surface. ER is a network of interconnected membrane consisting of sacs and tubules.

Ø  ER is a membranous organelle that shares part of its membrane with nucleus.

 

Function

 

Ø  RER transports proteins synthesize in ribosome. Ribosomes make proteins that are scheduled to leave the cell. The ER transports these proteins to another organelle that will package them up and ship them out.

Ø  SER helps in lipid synthesis.

 

GOLGI APPARATUS:

It is the only organelle to be named after a scientist Camillo Golgi on 19 Sep 1898, at a meeting of the medical society of Paria.

              Occurrence:

Ø  Golgi apparatus is found in all plants and animal cells.

Ø  It is absent in prokaryotes.

 

Structure:

Ø  Golgi apparatus is a stack of membrane bound flattened sacs and responsible for the modification of proteins received from ER.

 

 

 

 

Function:

 

Ø  Function of golgi body is to package protein molecule for secretion  and they also help in origin of lysosomes.

Ø  If proteins from RER require further modifications they are transported to the golgi complex.

Ø  Processed proteins stored in golgi and packaged in vesciles to be shipped elsewhere in cells.

Ø  Golgi complex change, sort and package the proteins as they leave the ER, so they are ready to leave the cell

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LYSOSOMES

       Occurrence:

Name lysosomes derived from Greek word lysis, which means dissolution or destruction and soma means body. They are frequently called as suicidal bags due to their role in autolysis. Lysosomes were discovered by Belgian cytologist Christian de duve in 1955.

Ø  Lysosomes occur in most animal cells and in few plant cell as vacuoles.

Ø  They are most abundant in cells which are related with enzymatic reaction such as liver cells, pancreatic cells, kidney cells, spleen cells, leucocytes, macrophages etc.

 

Structure:

 

Ø  It is a membrane bound organelle that has spherical sacs which contain digestive enzyme used to break down molecule such as non- self micro-organisms engulfed by phagocytes.

Ø  Cells apart, wear out and die. These pieces of garbages need to be disposed off somehow, this is performed by lysosomes.

Ø  Lysosomes are filled with digestive enzymes that break down those items that would become toxic if they were left in the cells.

Ø  Lysosomes also known as suicidal bag because it contains strong digestive enzymes which when released can break down worn out organelle or food.

 

Function:

Ø  Its function is to digest worn out cell organelle or micro- organisms.

Ø  It also breaks protein, lipid and carbohydrate into usable form for cell.

MITOCHONDRIA:

              Occurrence:

Ø  Found in cytoplasm of all eukaryotic cells. Absent in prokaryotic cell.

Ø  Erythrocytes do not contain mitochondria.

Ø  Substantial number of mitochondria are found in the liver with about 1000-2000 mitochondria per cell making up 1/5th of the cell volume.

             Structure:

Ø  Mitochondria is round double membrane organelle whose inner membrane is folded inside to form cristae which are folded in the matrix( central part of mitochondria).

Ø  Mitochondria are known as power house of the cell because it is responsible for ATP synthesis which supplies energy to the cell.

 

Function:

 

Ø  Its function is that ATP fuels cellular processes by breaking its high energy chemical bonds.

Ø  It is the site where energy is released from food molecule and transformed into usable form.

Ø  Mitochondria are most plentiful in cells that require significant amount of energy to function properly such as liver and muscle cells.

Ø  As the site of cellular respiration mitochondria serves to transform molecule such as glucose into an energy molecule such as ATP.

CHLOROPLASTS:

Occurrence:

Ø  Chloroplast is found only in plants and some protists.

 

Structure:

Ø  Chloroplasts contain two fluid separated membranes and the inner membrane is folded into a network of flattened sacs called thylakoids that are stacked into grana. The thylakoid contain chlorophyll in which the process of photosynthesis occurs.

Ø  Chloroplasts allow autotrophic organisms to meet their energy needs without consuming other organisms.

Ø  Thylakoid is the third internal membrane system contains membrane bound photosynthetic pigments.

 

Function:

 

Ø  Thylakoid  is the site of oxygen generation and photochemistry

Ø  Stroma is soluble portion of chloroplast. It is the site of CO2 fixation and sugar (glucose) synthesis.

RIBOSOME:

Occurrence:

Ribosomes are found in two states: Free State and Bound state. In Free State it is found in the cytoplasm and in bound state it is found attached to ER. It is also found in mitochondria and chloroplast.

 

Structure:

Ø  They are small spherical organelles composed of two subunits : one is smaller subunit and another one is larger subunit.

Ø  In prokaryotes 70S type of ribosome is present which is composed of two subunits, smaller subunit is 30S and larger subunit is 50S (S stands for swedberg unit).

Ø  In eukaryotes 80S type is present and is composed of two subunits, smaller subunit is 40S and larger subunit is 60S.

Ø  Ribosome is found in two forms in a cell, Bound state i.e. found attached to ER and in Free State i.e. found in cytoplasm.

Function:

Ø  Facilitates the specific coupling of t RNA anticodon with m RNA codons during protein synthesis.

Ø  Translate encoded information from the cell nucleus provided by m RNA.

Ø  Linked together amino acid selected and collected from the cytoplasm by t RNA.

CENTRIOLES:

Occurrence:

Ø  Centrioles are microtubules found next to the nucleus of animal cells and some protists.

 

Structure:

Ø  Centrioles are made up of protein strands called microtubules, made up of nine triplets of microtubules arranged in cylinder .

Ø  A centriole is small set of microtubules arranged in a specific way. There are nine groups of microtubule, when two centrioles are found next to each other they are usually at right angles. The centrioles are found in pairs and move towards the opposite pole of the nucleus at the time of cell division. During division group of threads attached to the centrioles. These threads are called mitotic spindle.

 

Function:

Ø  Helps in organizing and assembly of microtubule during the process of cell division

Ø  Centrioles are called basal bodies which help in the formation cilia and flagella.

 

CYTOSKELETON:

Occurrence:

It is found in eukaryotic cell while absent in prokaryotic cell.

Structure:

Ø  It is flexible tubular scaffold of microfilaments.

Ø  Cytoskeleton basically composed of two components: Microfilaments, they are solid proteins which is assembled at one end and disassembled at other end. Intermediate filament, rope like fibrous protein. Microtubules, they are hollow tubes of tubulin.

Function:

Ø  Microtubules maintain cell shape and provide support to the cell.

Ø  Microtubules anchor organelles and enzymes to specific regions of the cell.

Ø  Helps in contraction and movement.

Ø  Also involve in intracellular transport.

Ø  Intermediate filaments provide structure reinforcement.

Ø  Intermediate filaments help to anchor organelle.

 

MICROBODIES:

Occurrence:

Microbodies are found in protists, plants and animal cells. In vertebrates, it is especially prevalent in liver and kidney organs. In higher plants, leaves of C4 plants contain such structures. They are mostly associated with plastids and mitochondria. Glyoxysomes are found in fat rich seeds, fungi and other higher plants.

Structure:

Ø  Microbodies are single membrane bound organelle.

Ø  Microbodies are basically of two types: Peroxisomes and Glyoxysomes.

Ø  Peroxisomes are self duplicating spherical or dumb-bell shaped, fluid filled and membrane bound bodies.

Ø  Peroxisomes are 0.6mm-0.8mm in size.

Ø  Glyoxysomes are found in greater number in storage organs rich in oil globules. When membrane is loaded with oil globules called sphaerosomes which are 0.5-1.5 nm in size.

Ø  Sphaerosomes are surrounded by single layer of lipids instead of lipid bilayer.

Ø  In some plants, small glyoxysomes are found in the cotyledons of developing seeds.

Ø  Each of them consists of enzyme rich granular matrix or crystalline core.                    

Function:

Ø  Peroxisomes are mainly responsible for photorespiration in C3 plants (dicots), when CO2 level in the leaves drop and oxygen level increases.

Ø  Certain enzymes within peroxisomes detoxify various toxic substances that enter the blood.

Ø  Helps in breakdown of fatty acid molecule by β-oxidation.

Ø  Helps in biosynthesis of phospholipids and plasmalogens.

Ø  Peroxisomes carried out thermogenesis in animals.

Ø  Glyoxysomes are responsible for converting fats and fatty acid into sucrose.

Ø  Glyoxysomes contain host of enzymes which are responsible for fatty acid oxidation, glyoxylate cycle and gluconeogenesis.

 

CHROMATIN:

Occurrence:

Chromatin is found in both eukaryotes and prokaryotes. It is located in the nucleus of cell.

Structure:

Ø  Chromatin is a mass of genetic material which is composed of DNA and proteins, which condense to form chromosome during eukaryotic cell division.

Ø  Basic structure of chromatin is nucleosome that is 147 base pairs of DNA wound in two turns around the outside of disc like complex of eight proteins called histone.

Ø  Nucleosome consists of core particles and linker DNA.

Ø  Histones help to organize DNA into structure and provide a base on which DNA wound around.

Ø  Histone core contains two copies each of four histones, H2A, H2B, H3, H4. Histone octamer is wrapped by very close two turns of DNA named linker DNA and H1 histone join together the nucleosome.

Ø  Histones(chromatin interacting proteins), Transcription factors, DNA repair machinery play a role in shaping chromatin structure.

Ø  Chromatin remodeling complexes can change chromatin architecture by modulating the interaction between nucleosome and DNA.

Ø  Chromtin is mainly categorized into two types: Heterochromatin and Euchromatin.

Ø  Heterochromatin is tightly packed form of chromatin or most condensed form of chromatin that can silence gene transcription. It constitutes telomeres, pericentric regions and repetitive sequences.

Ø  Heterochromatin further divided into Facultative heterochromatin and Constitutive heterochromatin. Facultative heterochromatin is present in nucleus of some cells. For example, in animals, gene encoding β-globin is condensed in cells that are not precursors to blood cells.

Ø  Euchromatin is less condensed and contains most actively transcribed genes.

Ø  Constitutive heterochromatin is DNA sequences that are condensed in all cells of an organism. It is associated with highly reiterated DNA. It includes DNA of centromeres and telomeres.

Ø  Eukaryotic chromatin consists of one part of DNA and two parts of proteins.

Function:

·         Chromatin helps in efficiently packaging of DNA into small volume to fit into the nucleus of a cell and protect the DNA structure and sequences.

·         Packaging DNA into chromatin allows for mitosis and meiosis prevents chromosome breakage and controls gene expression and DNA replication.

·         Chromatin consists of complexes of small proteins called as histones and DNA. Histones help to organize DNA into structure called as nucleosome, which provides a base on which DNA can be wrapped around.

·         Non- histone regulates the condensation and decondensation of chromatin.

NUCLEOLUS:

Occurrence:

·         It is found in eukaryotic cells, it is located at the centre of the nucleus of a cell.

·         Estable and Sotelo (1951) described the structure of nucleolus under light microscope.

·         First description of nucleolus ultrastructure was given by Borysko and Bang (1951) and Berhard (1952).

Structure:

Ø  Nucleolus is the non-membranous structure.

Ø  It is a dynamic structure that assembles around clusters of r RNA genes repeat during late telophase.

Ø  It consists of DNA, RNA and proteins.

Ø  It consists of three main components:

        I.            Fibrillar centres consisting RNA, 80-100A° in diameter

      II.            Dense fibrillar components

    III.            Granular components consist of ribonucleoparticle granules which is 150-200 A° in diameter.

Ø  The nucleolar organizer region of chromosomes, which contains the genes for pre (r RNA) serves as the foundation for nucleolar structure.

Function:

Ø  It is the site of r RNA (ribosomal) transcription, pre- r RNA processing.

Ø  Ribosome subunit assembly i.e. production of subunits which then together form ribosome.

Ø  Synthesis and storage of RNA.

Ø  Biogenesis or synthesis of ribosomes.

 

 

 

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