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CELL – THE BASIC UNIT OF LIFE





Cell organelles:

  • A cell has to perform different functions with the help of its various membrane- bound cell organelles.
  • Cell organelles are “small organs” of the cell and are found embedded in the cytosol. They form living part of the cell and each of them has a definite shape, structure and function.
 
 

NAME OF THE ORGANELLE

STRUCTURE

FUNCTONS

ENDOPLASMIC  RETICULUM

It is absent in prokaryotic cells and matured RBCs of mammals.
ER occurs in three forms:
Cisternae (i.e., closed, fluid-filled sacs), vesicles and tubules.
ER is of two types:
1.Rough endoplasmic reticulum (RER) with ribosomes attached on its surface.
2.Smooth endoplasmic reticulum (SER) which is without ribosomes.

  1. It forms intracellular transporting system.
  2. SER takes part in the synthesis of lipids.
  3. RER is concerned with the transport of proteins which are synthesized by ribosomes on their surface.
 

GOLGI COMPLEX (GOLGI APPARATUS OR GOLGI BODY)

Golgi complex was discovered by Caomillo Golgi (1898).
It occurs in almost all eukaryotic cells. It is not found in prokaryotic cells.
Golgi apparatus consists of a set of membrane-bounded, fluid-filled vesicls, vacuoles and flattened cisternae.
1. It is known as packaging and dispatching unit of the cell.
2.It is involved in the synthesis of cell wall, plasma membrane and lysosomes.
3.It is involved in the formation of cell plate during cell dividion.
 

RIBOSOMES

They are present both in prokaryotic and eukaryotic cells.
Ribosomes are dense, spherical and granular particles which occur freely in cytoplasm or remain attached to the endoplasmic reticulum (RER).
Chemically ribosomes are made up of ribonucleic acid (RNA) and proteins.
Ribosomes are of two types: 70S and 80S
 
Ribosome is the site of protein synthesis so it is known as protein factory of the cell.

MITOCHONDRIA

They are present in eukaryotic cell but absent in prokaryotic cells.
Mitochondria exist in variable shapes.
It is a double membranous organelle. Outer membrane is smooth and inner membrane forms folds like structure called cristae.
The cristae consist of F1 particles on its surface.
The matrix inside it contains ribosomes, repiratory enzymes and a circular DNA.
1. Mitochondria generate energy in the form of ATP (Adenosine Tri Phosphate), that is why it is called powerhouse of the cell.
2. Mitochondria have their own ribosomes and DNA, they can synthesize their own proteins, and they are self-duplicating units. So, they are regarded as semiautonomous organelles.
 

LYSOSOMES

Lysosomes are dark, spherical, single membrane bound sacs containing several digestive enzymes. These enzymes are capable of digesting or breaking down all organic materials.
 
Lysosomes are formed by Golgi complex.They are found in the cytoplasm of eukaryotic cell, mostly in animal cells.
 
  1. Lysosomes are involved in intracellular digestion.
  2. During starvation or aging, the cell digests its own organelles through lysosomal enzymes, a process called autophagy.
  3. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn out cell organelles.
  4. When a cell is destined to die, the lysosomal enzymes digest the whole cell, a process called autolysis. Therefore lysosomes are also known as ‘suicide bags’ of the cell.
 

PLASTIDS (The term ‘plastid’ was given by Haeckel in 1866. Plastids occur in most plant cells and are absent in animal cells.

CHLOROPLAST

They are spherical or discoidal in shape and are enclosed in double membrane. Inside a plastid, two definite regions are clearly visible- Grana and Stroma.
Grana are stacks of membrane-bounded, flatterened discoid sacs containing the molecules of chlorophyll.
Stroma is the homogeneous matrix in which grana are embedded.
Plastids contain their own DNA and ribosomes i.e., they have their own protein synthesizing machinery. They are also self-replicating organelles.
On the basis of colour, plastids are of following three types:
  1. Chloroplasts: Green-coloured plastids containing chlorophyll.
  2. Leucoplasts: Colourless plastids.
  3. Chromoplasts: Coloured plastids (except green colour).
 
 
  1. Chloroplasts are photosynthetic organelles. The chlorophyll present in them trap solar energy for the purpose of synthesizing food for the plant. So chloroplasts are the ‘Kitchens of the cells’.
  2. Leucoplasts help in storing food products like starch, protein and lipids.
  3. Chromoplasts provide colour to flower which in turn attracts insects for pollination.
 
 
 

 

CENTROSOMES

Centrosome is found only in animal cells.
 
It is not bounded by any membrane but consists of two granule-like centrioles. Centrioles are hollow cylindrical structures which are made up of microtubules.
 
1.      Centrosome helps in cell division in animal cells. During cell division centrioles migrate to the poles of animal cells and are involved in the formation of the spindle.
2.      They produce basal bodies from which cilia and flagella arise.
 

VACUOLES

Vacuoles are fluid-filled or solid-filled and membrane bound spaces in the cytoplasm.
Vacuoles are small sized in animal cells while plant cells have very large vacuoles.
The vacuole is bounded by a membrane, called tonoplast. The vacuole is filled with cell sap which is watery solution rich in sugar, amino acids, proteins, mineralsand metabolic wastes.
 
 
  1. Vacuoles are meant for the storage of food, water and other substances.
  2. Vacuoles help to maintain the osmotic pressure in a cell (osmoregulation).
  3. Vacuoles provide turgidity and rigidity to the plant cells.
 

PEROXISOMES

Peroxisomes are small and spherical organelles containing powerful oxidative enzymes.
They are bounded by a single membrane.
 
Peroxisomes are specialized for carrying out some oxidative reactions, such as detoxification or removal of toxic substances from the cell.
 
 

Nucleus:

  • The nucleus is a major, centrally located spherical cellular component. It is the centre from where all cellular activities are controlled. It is the carrier of hereditary material in the cell.
  • It is bounded by two membranes, both forming a nuclear envelope. The nuclear envelope contains many pores known as nuclear pores and encloses the liquid ground substance, the nucleoplasm.
  • The nucleopores allow transfer of materials between the nucleoplasm and the cytoplasm. Within nucleoplasm are embedded two types of nuclear structures- nucleolus and chromatin material. 
Nucleolus:
  • The nucleolus may be one or more in number ad is not bounded by any membrane.
  • Nucleolus consists of DNA, RNA and protein.
  • It is the site of ribosome formation. Nucleolus is known as factory of ribosomes.
Chromatin material:
  • The chromatin is a thin, thread-like intermingled mass of chromosome material and composed of the genetic substance DNA (deoxyribonucleic acid) and proteins (i.e., histones).
  •  DNA stores all the information necessary for the cell to function (metabolism), to grow and to reproduce further cells of the next generation.
  • The chromatin is condensed into two or more thick ribbon-like chromosomes during the division of cell.
  • Functional segments of DNA are called genes.
  • Genes carry information for protein synthesis, which in turn control the development of characters. Since genes are present on chromosomes and chromosomes pass from one generation to the next through, genes are called hereditary units.


 Functions of nucleus:
  1. The nucleus controls all metabolic activities of the cell. If the nucleus is removed from the cell, the protoplasm ultimately dries up and dies.
  2. It regulates the cell cycle.
  3. It is the storehouse of genes which are concerned with the development and manifestation of all the body characters.
  4. Nucleus helps in the formation of ribosomes and RNA.
  5. It helps in the transmission of characters from one generation to the next.
 

Test your Knowlegde

Fill in the blanks 1) ______________are “small organs” of the cell and are found embedded in the cytosol.
2) ______ discovered Golgi complex in _____
3) ______________ apparatus consists of a set of membrane-bounded, fluid-filled vesicls, vacuoles and flattened cisternae.
4)_____________ are present both in prokaryotic and eukaryotic cells.
5) Chemically ribosomes consists of _______ and ________
6)_______________ is the site of protein synthesis so it is known as protein factory of the cell.
7) ____ is a double membranous organelle
8)) Mitochondria have their own ribosomes and __________________, they can synthesize their own proteins, and they are self-duplicating units.
9) ______________ are dark, spherical, single membrane bound sacs containing several digestive enzymes.
10) _____________ helps in the formation of ribosomes and RNA.
11) When a cell is destined to die, the lysosomal enzymes digest the whole cell.This process is known as __________
12) PLASTIDS term is given by __________________ in _____
13) ____________are spherical or discoidal in  shape   and are enclosed in double membrane.
14) ______________ is the homogeneous matrix in which grana are embedded.
15) Chloroplasts are Green-coloured plastids containing __________________.
16) The chlorophyll present in Chloroplasts trap solar energy for the purpose of synthesizing food for the __________________.
17) _____________ carry information for protein synthesis, which in turn control the development of characters.
18) _________ are small and spherical organelles containing powerful oxidative enzymes and are specialized for carrying out some oxidative reactions, such as detoxification or removal of toxic substances from the cells
19) The _______________ is a major, centrally located spherical cellular component.
20) The nuclear envelope contains many pores known as nuclear pores and encloses the liquid ground substance, the __________________
21) Nucleolus made of __________________, RNA and protein.
22) Nucleolus is called as __________________ of ribosomes.
23) The __________________ is a thin, thread-like intermingled mass of chromosome material and composed of the genetic substance DNA (deoxyribonucleic acid) and proteins
24) Functional segments of __________________ are called genes.
25)______________are fluid-filled or solid-filled and membrane bound spaces in the cytoplasm.
26) Lysosomes are formed by __________________ complex.They are found in the cytoplasm of eukaryotic cell, mostly in animal cells.

  






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