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TNPSC BIOLOGY (ENGLISH) 

WHY CELL IS THE BASIC UNIT OF LIFE?

A cell is the smallest and most basic form of life. Robert Hooke an optic seller, observed a piece of cork through his hand made lens and found many identical small chambers. He discovered cell in 1665. In Latin word, 'cellula' means "a small chamber". Following him, Robert Brown, a school teacher continued and invented an advanced microscope. He discovered the nucleus.

The Cell theory incorporates four principles:

          Cells are the most basic building units of life.

          All living things are composed of cells.

          New cells are made from pre-existing cells which divide into two.

          Cells can be cultured to produce more cells.

 

Types of Cell

✦ Prokaryotic cell

✦ Eukaryotic cell

      Cells of plants and animals are not similar. Bacteria and some algae are made up of single cell. Their cells lack membrane bound organelles.

    ⋆ Cells that do not contain membrane bound organelles and possess “incipient nucleus “ are called prokaryotic cells, i.e., simple cells Eg.:Bacteria

    ⋆ Cells that contain a well-defined nucleus, nuclear membrane and all the cell organelles are called eukaryotic cell.i.e.,complete cells Eg.Cells of plants and animals

              

 

 

Prokaryotic Cell

Eukaryotic Cell

It is generally smaller (0.2-2.0 micro metre in size)

It is comparatively larger (10-100 micro metre) in size

It lacks a well organized nucleus as its nuclear material is not surrounded by a nuclear membrane

It contains a well organized nucleus as its nuclear material is surrounded by a nuclear membrane

It has a single circular chromosome; lacks histones.

Multiple linear chromosomes with histones arrangement.

It lacks membrane bound cell organelles

It posses membrane bound cell organelles

Cell division occurs by fission or budding. Mitotic and meiotic divisions are absent

Cell division takes place by mitosis and meiosis

Cell division takes place through binary fission

Cell division takes place through Mitosis

Ribosomes are smaller

Ribosome are larger

 
 
Animal and Plant cells

Animal cells

All animals are multicellular. Human body contains trillions of cells. All animal cells are eukaryotic, they are surrounded by cell membrane and do not have a cell wall. The components of animal cells are centrioles, cilia and flagella, endoplasmic reticulum, golgi apparatus, lysosomes, microfilaments, microtubules, mitochondria, nucleus, peroxisomes, plasma membrane and ribosomes.

 

Plant Cells

Plant cells are eukaryotic cells having membrane bound organelles. They are surrounded by a rigid cell wall.

Generally, plant cells are larger than animal cells and are mostly similar in size and are rectangular or cube shaped. A plant cell contains a few distinct features such as a cell wall, large vacuoles and plastids.

The three main components of ant plant cell or animal cell are,

 

Plasma Membrane or Cell Membrane

A bilipid membranous layer composed of proteins and carbohydrates. It is fluid like.

 

Functions

✦ The cell membrane separates the cell from its external environment, and is selectively permeable. It protects the cell and provides stability.

✦ Proteins are found embedded within the plasma membrane with some extending all the way through in order to transport materials.

✦ Carbohydrates are attached to proteins and lipids on the outer layer.

 

2. Cytoplasm

The jelly like substance compose dof mainly water and found between the cell membrane and nucleus.  The cytoplasm makes up most of the body of the cell and is constantly streaming.

Functions of Cytoplasm

✦ Cytoplasm helps in intracellular distribution of enzymes, nutrients andother biomolecules within the cell.

✦ Synthesis of different types of biomolecules such as proteins, nucleotides,fatty acids etc., takes place in the cytoplasm.

 

3. Nucleus

Nucleus is the largest organelle in the cell. It is dark and round, and is surrounded by a double membrane called the nuclear envelope/membrane. It is a dense spherical structure embedded in the cytoplasm. Nuclear envelope encloses a ground substance called nucleoplasm or karyolymph.

The nucleoplasm has two types of nuclear structures

  ⋆ the nucleolus and

 ⋆ Chromatin Reticulum.

The nucleolus is a spherical body rich in protein and RNA. It is the site of ribosome formation. There may be one or more nucleoli in the nucleoplasm. The chromatin is a network of fine threads composed of genetic material DNA (Deoxy ribo nucleic acid) and proteins. During cell division chromatin is condensed into thick cord like structures called Chromosomes.

The chromosomes contain genes and each gene is responsible for one hereditary character of the organism. Genes contain information for inheritance of features from parents to next generation in the form' of DNA molecule.

Functions

          Nucleus controls all the metabolic activities of the cell

          It controls the inheritance of character from  off-springs

          It controls Cell division

The following cell organelles are found in both plant and animal cells:

 

Endoplasmic Reticulum

    ✦ Endoplasmic reticulum is a complicated interconnected system of membrane bound channels and tubules. It is spread throughout the cytoplasm. .

    ✦ There are two types of Endoplasmic Reticulum

                Rough Endoplasmic Reticulum (RER)

                Smooth Endoplasmic Reticulum(SER)

Rough Endoplasmic reticulum (Granular endoplasmic reticulum)

                        This type of endoplasmic reticulum possesses rough walls because the ribosomes remain attached.

Smooth endoplasmic reticulum (Agranular endoplasmic reticulum)

                       The walls are smooth and ribsomes are not attached to its membrane.

Functions

       1. Provides large surface area of the metabolic activities of the cell.             

       2. Rough endoplasmic reticulum plays an important role in protein synthesis.

       3. Smooth endoplasmic reticulum is involved in the synthesis of steroid, hormones and lipids.

 

2. Ribosomes

      ✦ Ribosomes are small granular structures made up of ribo nucleic acid (RNA) and proteins.

      ✦ Each ribosome consists of two subunits - a small subunit and a large subunit

RNA and form a structure called polyribosome.

There are two types of Ribosomes

      ✦ 70S - Ribosome - 30S and 505 subunits. It is seen in Prokaryotic cells.                 

       80S - Ribosome – It is made up of 40’s and 60s subunits. It is seen in eukaryotic cells. '

Functions

Ribosomes play an important role in protein synthesis. So they are called, protein factories of the cell.

 

3. Golgi Complex or Golgi apparatus

     ✦ The Golgi apparatus was first described by Camillo Golgi.

     ✦ Golgi complex consists of saucer-like compartments called cisternae, network of interconnecting tubules.   

     ✦ At the peripheral regions, in plant cells, Golgi apparatus is referred to as dictyosomes.

Functions

 1. Formation of lysosomes.

 2. Synthesis of cell wall and cell membrane.

 3. The golgi body temporarily stores protein which can then leave the cell via vesicles pinching off from the golgi.

 

4. Lysosomes

    ✦ Small membrane bound vesicles which contain various types of digestive enzymes. These serve as intracellular digestive system, hence they are called digestive bags. `

    ✦ Lysosomes are involved in the destruction of aged and worn out cellular organelles. Therefore also called demolition squads or scavengers or cellular housekeepers (or) suicidal bag of the cell.

Functions

   ✦ Lysosomes are involved in the intracellular digestion of food particles ingested by the cell through endocytosis. The lysosomes of WBCS (white) take part in natural defense of the body.

 

5. Mitochondria

✦ Mitochondria are globular or cylindrical organelles. Each mitochondrion is bound by two membranes - an outer continuous membrane and an inner membrane thrown into folds called cristae.

✦ The inner chamber is filled with homogenous dense material called the matrix.

✦ The cristae have pin headed bodies called F1 particles or Oxysomes which play an important role in respiration. The matrix of mitochondria contains enzymes necessary for the oxidation of food during respiration and release of energy in the form of ATP molecules.

✦ The mitochondria are called power houses of the cell. The mitochondria contain proteins.

Functions

✦ Mitochondria synthesize energy rich compounds such as ATP.

✦ Mitochondria provide important intermediates for the synthesis of several biochemical’s like chlorophyll, Cytochromes, steroids, amino acids etc.

 

6. Vacuoles

✦ Vacuoles are fluid - filled sacs bound by a single membrane and arepresent in plant cells as well as in certain protozoans as food vacuoles and contractile vacuoles. In plant cells, major portion of the cell is occupied by vacuoles and are bound by the definite membrane called tonoplast.

Functions

✦ Vacuoles store materials such as food, water, sugar, minerals and waste products.

✦ They maintain proper osmotic pressure in the cell.

Animal cell organelles not found in plant cells

 

Cilia and Flagella

        These are hair like organelles which extend from the surface of many animal cells. The structure is identical in both, except that flagella are longer and whiplike and cilia are shorter. There are only a few flagella on a cell, while cilia may cover the entire surface of the cell.

         The functions of the cilia and flagella may include locomotion for one celled organisms and to move substances over cell surfaces in multi-celled organisms.

Organelles and other features found only in Plant cells

 

Cell Wall

It is a rigid organelle. Most of the plant cell walls are made of cellulose and lying just outside the cell membrane. The cell wall gives the plant cell a box-like shape. The cell wall consists of three layers namely, middle lamella, primary wall and secondary wall. The cell wall contains pores which allow materials to pass to and from the cell membrane.

 

Functions of cell wall

              It provides mechanical strength to the cell

             ✦ It protects the protoplasm against injury.

 

Plastids

Plastids are double membrane bound organelles. It is plastids that plants make and store food. Plastids are found in cytoplasm and are of two types,

Leucoplasts: Colorless organelles which stores starch or other plant nutrients.

Chromoplasts: Contain different coloured pigments. The most important type of Chromoplast is the Chloroplast, which contains the green pigment chlorophyll. This is important in the process of photosynthesis. Each chloroplast consists of a double membrane envelope and a matrix.

        ✦ The inner membrane is arranged along the length of the plastids as lamellae. At certain regions, the lamellae are thickened and appear like pile of coins. These are called the grana.

        ✦ Each granum consists of disc shaped membranous sacs called thylakoids. Inside these grana, the chlorophyll is located. The non-thylakoid portion of the matrix is called stroma. It contains a number of enzymes involved in photosynthesis.

 

Central Vacuole

  The central vacuole is a large fluid-filled vacuole found in plants.

                       Plant Cell

                                      Animal Cell

Plant cell has an outer rigid cell wall which is made up of cellulose

Animal cell lacks cell wall

Plant cell is larger than animal cell

Animla cell is comparatively amaller in size

Plant cell has large vacuoles which occupy more space in the cell

Animal cel usually lacks vacuoles .Even if they are present ,they occur in minute sizes

Centrosome is present only in the cells of some lower plants

All animal cell have centrosome

Lysosome are found only in the eukaryotic plant cells

Lysososme are found in all animal CELLS

Plant cell contains plastids

Plastids are absent

Mostly, starch  is the storage material

Glycogen is the storage material

 

 

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