Cell : The basic unit of life (11th Biology)

              CELL

The cell is defines as living unit of life or basic unit of life or structural and functional unit of life or fundamental unit of life or autonomous and self contained unit of the life.

In 1665, Robert Hook discovered cell. In his experiment, he cut the cork pieces. Then he observed under his microscope and found many small compartments in it. He named each compartment Cell.

In 1838, German Botanist Schleiden occluded that the plant body is also composed of cells. In addition, in 1839 German Zoologist Theodor concluded that the animal body is also composed of cells.

Why cell is considered as basic unit of life or why cell is called self contained or autonomous unit?

The cell is smallest unit of life. A single cell can perform different life processes as it can digest food, respire, excrete, reproduce, and grow. The cell carries different metabolic activities. The body function is out come of the different activities of the cell. So the cell is considered as basic unit or self contained or autonomous unit of life.

Cell Theory

·         Schleiden in 1838 and Schwann proposed cell theory in 1839. It states that

·         Cell is a mass of protoplasm containing nucleolus and is bounded by a membrane.

·         All the cells are similar in chemical composition and metabolic activities.

·         Living things are composed of cells

·         The function of organisms is the outcomes of cell activities and interaction of cell components.

Objection of cell theory

·         Bacteria and cyan bacteria do no contain nucleus

·         Some fungi contain many nuclei in branched body

·         There is no cellular organization in virus

To include all the living organisms the cell theory proposed by Scleidan and Schwann can be modified which is called cell principle

·         Cell is a mass of protoplasm containing nucleus or in lack of nucleus nuclear materials is taken

·         Living things are made up of cells

·         Cell can store genetic information and express from generation to generation

·         Cell sometimes assume forms, which no longer have all the characteristics of the cell.

What is cellular totipotency

Even a single cell of a plant body has a power to regenerate entire plant body. This is called cellular totipotency.

In 1950, Steward conducted an experiment on carrot.

In this experiment, He removed small piece of carrot phloem, kept the piece in liquid medium. Phloem cells divided producing cell clusters. The clusters are kept on solid medium, after few days a plant grew in it.

Difference between unicellular and multicellular organisms

In unicellular organisms the body is single cell, single cell perform all the life activities, Cells are larger in size, easily infected and susceptible for damage, Amoeba, Paramecium

In multicellular organisms, number of different types of cells forms body, the body function is outcome of the cellular activities of different cells, cells are usually smaller is size, not easily susceptible to damage, higher plants and animals

Types of cells

Prokaryotic cell

Eukaryotic cell

Small in size (range from 100-10000nm)        Usually gelatinous sheath is present        Cell wall is made up of amino sugar and muramic acid.        Mesosome is present, the power house of the cell        Nuclear membrane and nucleolus is absent        DNA is found freely in cytoplasm        Histone protein is absent in DNA, So called naked DNA        Cell organelles are absent        Flagella is simple.        Pili are present        Ribosomes are small and 70 S type.        Plasmid may be present        Cyclosis is absent        Cell division is amitosis.        Cell cycle is short        Spindle fibers are absent Bacteria and Cyanobacteria

Larger in size (range from 10000-100000nm)  Usually gelatinous sheath absent  In animal cell, cell wall is absent but in plant cell it is made up of cellulose  Mesosome is absent. Mitochondria are power house of the cell  Nuclear membrane and nucleolus are present  DNA is bounded by nuclear membrane.  Histone protein is present in DNA so called closed DNA  Cell organelles are present  Flagella are compound (9+2 pattern)  Pili absent  Ribosomes are large and 80S type  Plasmid is absent  Cyclosis is present  Cell division is mitosis and meiosis  Cell cycle is long  Spindle fibers are present Higher plants and animal cell

Structure of Eukaryotic cell

Shape

The shape is variable like round oval irregular spherical elongated or tubular etc

Size

The size is also different in unicellular organisms it range from 1µm to 1 mm. in multicellular organisms it range from 1- 100µm

Cell Components

Cell wall

It is outermost layer of plant cell. It is a rigid and protective layer. It is composed of different layer

Middle lamella: It is present between the walls of adjacent cells. It acts as cementing materials between two cells. It is made up of carbohydrate, Ca and Mg

Primary cell wall: It is found on either side of middle lamella. It is made up of middle lamella. It is made up of cellulose, hemi cellulose. It is thin elastic and permeable.

Secondary cell wall: it is found on inner side of primary cell wall. It is thick non-elastic and rigid it is made up of cellulose lignin and pectin

Function of cell wall

It provides rigidity to the cell. It protects the inner mass of the cell. Maintain the shape of the cell. Balance the osmotic pressure. It helps in absorption of water.

Cell membrane

The most important structure of the cell is its membrane, for it is the cell membrane that controls and regulates what enters and leaves the cell. The cell membrane is made of two layers of lipids, called a lipid bilayer. The lipid bilayer is made of lipids (fats) and phosphates, normally called phospholipids Phospholipids have a polar end, and a non-polar end. In making membranes, the non-polar ends of both layers face each other, while the polar end faces the outside and inside of the cell. There are two theories for model of cell membrane

Sandwich Model

In this, there are two layers of protein outer and inner. In between two protein layers, there are two phospholipids layers.

The heads of the phospholipids molecules face towards protein layer and the tails toward the centre.

Fluid Mosaic Model

In this model, there are 3 layers, two layers of protein and one layer of phospholipids. The phospholipids are liquid in nature. Therefore, molecules can exchange their position. This is called transition movement. The molecules can also interchange between two layers this is called flip-flop movement. The Proteins are of two types

Peripheral protein (extrinsic protein): They are present either inner or outer surface.

Intrinsic protein: It is partially or completely embedded in phospholipids. 

Function of cell membrane

It is the outer covering membrane of animal cell, which maintain cell shape, helps in diffusion, helps in osmosis endocytosis (intake of large molecules in to the cell) and exocytosis Expelling of large molecules of liquid or solid from the cell.

Protoplasm

It is found inside cell membrane. It is a liquid mass viscous in nature. It contains several living and non-living things. It is divided into two parts

Cytoplasm: It is liquid mass present between plasma membrane and nuclear membrane. it is differentiated into two layers The outer ectoplasm which is non granular and inner endoplasm which is granular containing cell inclusions and cell organelles.

Nucleolasm:  It is liquid material within nuclear membrane.

Cell Organelles

They are living sub cellular structures which are embedded within cytoplasm.

a) Mitochondria (Mitochondrion)

Its shape is variable and the size range from 2-6 micrometer.  It is double membrane structure. Its outer membrane is smooth and regular. Its inner membrane gives finger like projections towards inner side called cristae. The surface of the cristae contains number of stalked and globular proteins called oxysomes. The cavity bounded by membrane is filled with a liquid mass called matrix. The matrix contains DNA RNA, inorganic and organic molecules. The mitochondria have its own DNA so it expresses its nature and characters own self. It is distributed in the cytoplasm.

 Function of mitochondria

It stores various types of enzymes used in aerobic respiration. It releases energy during oxidation of food material in aerobic respiration and stores energy in the form of ATPs so it is considered as the powerhouse of the cell. It can synthesize some proteins and it is carrier of genetic material.

b) Plastids

Plastids are photosynthetic pigments containing bodies found in the cytoplasm of plant cell. There are three types of plastids

      Chloroplasts

Plastids, which contain green colored photosynthetic pigments, are called chloroplast. It is double membraned. A kind of fluid is enclosed by membrane is called stroma. In the stroma number of sac like membranes found called thyllakoid or lamellae. The group of thyllakoid is called granum. The thyllakoid that connects one granum to another granum is called fret. The stroma also caontains DNA RNA and other organic and inorganic substances.

Function of chloroplast

The main function of chloroplast is photosynthesis. The green colored pigments, the chlorophylls traps solar energy and the energy is then converted into chemical energy C₆H₁₂O₆ with the combination of CO₂ and H₂O. It is also carrier of genetic materials, as it possesses its own DNA.

      Chromoplast

The plastid, which contains colored photosynthetic pigment other than green, is called chromoplast.

Yellow coloured pigments are called Xanthophylls. Orange colored pigments is called Carotenes

Red colored pigments are called Erythrocyanine and blue colored pigments are called Phycocyanine.

The chromoplasts are also doubling membrane. In the stroma grana are absent and the thyllakoids are found degenerating. They are present in colored parts of the plant body. Their main function is to make colorful the body parts like colorful parts of the flower attracts insects.

      Leucoplast

The main function of the leucoplast is to store food materials. It is present in the stem and root. The leucoplast which store starch is called amyloplast, which stores oil, is called elaioplast and which stores protein called aleuroplast. The plastids which contain colorless pigments are called leucoplast. It is doubling membrane. The grana are absent. Developing thyllakoids are present in the stroma. In presence of light the leucoplast changes into chloroplast.

c) Endoplasmic Reticulum

It is the network of tube like structures and sac like structures. It arises from nuclear membrane and extends up to cell membrane. The endoplasmic reticulum is formed by three structuresVesicles: They are minute spherical or oval           sac like structures

Cisternae: They are long elongated flattened unbranched sac like structures lie parallel to each other.

Tubules: They are branched tube like structures they form net work with  cisternae and vesicles.

Function of ER

It provides mechanical support to the cell. Therefore, it is also known as cytoskeleton. It synthesizes proteins and hormones. It helps to transport ions, molecules and impulse or metabolites. It forms nuclear membrane. It also forms new cell membrane and components.

There are two types of ER

Rough ER	Smooth ER
It bears ribosomes on its surface It is found near nuclear membrane It is formed by nuclear membrane In it mainly cisternae are found	It bears no ribosomes It is found near cell membrane It is formed by rough ER In it mainly tubules are found

d) Golgi body

It is also known as lipochondria or dictyosome

It arises from ER and is formed by four structures. It lies near cell membrane

Cisternae: They are curved with dilated ends and are parallel to each other

Vesicles:      They lie on convex side of cisternae

Vacuoles:They are larger sac like structures lie on concave side of cisternae.

Tubules:     They formed net with cisternae vesicles and vacuoles

Functions of Golgi body

It forms new cell membrane, It forms cell wall components, It secrets hormones, It stores proteins and fats.

It forms vesicles and lysosomes

e) Ribosome

They are small dense granular particles of RNA and proteins. It is also known as ribonucleioproteins. They are found freely in cytoplasm and found on the surface of endoplasmic reticulum and within some cell organelles. It is not bounded by any membrane. Each particle of ribosome consists of two subunits i.e. One larger and another smaller cap like subunit over larger subunit. There are two types of ribosome

70S type	80 S type
It is smaller in size and the number of protein is 55 The ratio of RNA and protein is 2:1 Molecular weight is 2.7×106 Dalton When it break down then it gives 50S and 30S subunits It is found in prokaryotic cells (1 dalton = 1.6*10-24)	It is larger in size the no of proteins is 82 The ratio of RNA and protein is 1:1 Molecular weight is 4.5*106 when it breaks down it gives 60S and 40S subunits it is found in eukaryotic cells

Function of Ribosome

It is the site of protein synthesis, it also stores protein, and therefore ribosome is called protein Factory.

f) Lysosomes

The lysosomes are vesicles like structures bounded by single membrane; the lysosome contains several enzymes that digest extra cellular and intra cellular substances. Lysosomes digest cell organelles causing death of cell. Therefore, it            is called suicidal bag.

.Function of Lysosome

It digests dead cells, it digests extra and intra cellular particles, it ingest particles

g) Centrosome

The centrosome is globose body, which consists of two star shaped structures called centrioles. Each centriole consists of nine rows of triplet fibrils and remains surrounded by cytoplasmic area called centrosphere. When centrosome is present at the base of flagella or cilia then it is called basal body.

Function of Centro some

During cell division, it produces spindle fibers; the basal bodies control the movement of cilia and flagella.

h) Micro bodies

They are small vacuole like structures, which are of following types.

Sphaerosomes

They are small lysosomes like structures. They contain enzymes like lysosomes, which digest lipid and other substance. They may also act as lysosomes usually present in plant cells.

Glyoxisomes

They are sphaerosomes like structures, which contain rod like enzymes, which digest lipid and decompose aminoacids.

Peroxisomes

These are microbodies, which contains two types of enzymes. The enzyme Catalases catalyses the decomposition of Hydrogen Peroxide to Water and Oxygen and The enzyme Oxidases oxidize organic compounds.

i) Microtubules

Microtubules are long cylindrical hollow tube found in cytoplasm.

They are made up of tubulin protein. Each triplet fibrils of centriole is made up of one microtubules.

It provides mechanical support to cell and helps in cell division.

j) Microfilament

They are long cylindrical solid tubes found in cytoplasm.

The function is helps in cyclosis, locomotion, and cell division

k) Vacuoles

They are single membraed sac like structures, which is filled with liquid mass called vacuolar sap (cell sap). Its membrane is called tonoplast.

Function of the vacuole

It stores various organic and inorganic molecules, it balances the water in cell, it also helps in growth and elongation of cells, cell sap exerts turgor pressure, which keeps the plant cell turgid.

l) Cilia and Flagella

They are hair like structures. They rise from basal bodies; they are made up of 11 filaments and fibrils. In 11 fibrils, nine fibrils are arranged in outer ring and two fibrils remains at centre. Outer filament consists of two microtubules a and b. Microtubules are connected by radial spoke with central fibrils.

Difference between Cilia and Flagella

Cilia It is shorter and smaller in size Covers entire cell surface Cilia shows sweeping movement Helps in locomotion and to capture food

Flagella It is longer and larger in size Usually one or two flagella are found at one end of cell Flagella show undulating movement. Flagella helps in locomotion

m) Nucleus

It is the important cell organelles, which controls all the activities of the cell. It also carries the hereditary information of the cell. Its structure is spherical or oval. It is present one or more in number in a cell.

he nucleus is doubling membraned. The outer membrane is rough and inner membrane is smooth. The nuclear membrane is porous and selective permeable. The pores help in exchange of substance between cytoplasm and nucleoplasm.

The nucleoplasm is semi liquids bounded by nuclear membrane within nucleoplasm DNA RNA enzymes are found. In nucleoplasm there is round structure of ribonucleoprotein, which is called nucleolus. Nucleolus forms ribosomes. In nucleoplasm there is net like structures of fine thread, which is called chromatin network or simply called as chromatin. During cell division, this network gives fine separate thread like structures called chromosome.

                There are two types of chromatin

Heterochromatin	Euchromatin
Ø Densely packed region of chromatin Ø It remains tightly coiled and they are condensed during interphase Ø Deeply stained Ø It possesses large amount of RNA and less amount of DNA Ø Due to small amount of DNA present     they are genetically inactive	Ø Loosely packed region of chromatin Ø It remains loosely coiled and diffused during interphase Ø Loosely stained Ø It possesses large amount of DNA and less amount of RNA Ø Euchromatin is genetically active.

Cell inclusions

Non-living substances embedded in cytoplasm are called cell inclusions. Such as proteins amino compounds, oil, cellulose, sugar etc are inclusions ergastic bodies.

Cell inclusions are divided into three categories they are

Reserved materials

The metabolic products stored inn plant cell forms the reserved materials. They are either nitrogenous compounds like proteins, amino acids or non nitrogenous compounds like starch cellulose, insulin, fats, oils, glycogen etc.

Secretory materials

Certain substances formed during plant metabolism but not concerned with nutrition. Such materials are called secretory materials. They are of following types

Plant pigments: the color producing substances of plants are plant pigments

Enzymes or Hormones: These are nitrogenous secretory products and are soluble in water

Nectar: It is secreted by nectar secreting glands known as nectarines, which helps to attract the insect for pollination.

Excretory materials

These are the by products of plant metabolisms and are of no use to the plants such as resins, gums, oils, organic acid, alkaloids, latex, tannins, inorganic compounds like urea caco3 and silica.