CH-8 (CELL : THE UNIT OF LIFE) BIOLOGY CLASS -XI, CBSE NCERT EXAM NOTES

Chapter-8
Cell : The Unit Of Life

Points to Remember

Cell Theory : Cell Theory was formulated by Schleiden and Schwann, and was modified by Rudolf Virchow. Cell theory States that :

(A) All living organisms are composed of cells and products of cells.

(B) All cells arise from pre-existing cells.

Cell : Cell is the structural and functional unit of life.

Prokaryotic Cell	Eukaryotic Cells
Generally small sized (1-10 um) Well defined nucless absent Membrane bounded cell organelles absent. DNA without histone protine e.g., Bacteria, Mycoplasma, Blue, green Algae	Generally large sized (5-10 um) Well defined nucleus present Membrane bounded cell organelles present. DNA with histone protein e.g., Amoeba, Euglena and other higher organism
Gram Positive Bacteria	Gram Negative Bacteria
Bacteria that take up gram Stain e.g, Bacillus	Bacteria do not take up gram stain e.g., Escherichia coli

PROKARYOTIC CELL :

Modification of cell envelope


• Cell enevelope consits of tightly bound 3 layered structure - Outermost Glycocalyx followed by cell well and plasma membrane.
• Glycocalyx in form of loose sheath, is called slime same layer
• Glycocalyx in form of thick and tough sheath, is called capsule
• Mesosomes : Extension of plasma memebrane. These can be in the form of vesicles, tubules and lamellae.
• Functions : Cell wall formation, DNA replication and distribution to daughter cells, repiration, secretion to increase surface area of plasma membrane and enzyme content.
• Flagella : Extension of cell wall. It is composed of three structures - filament, hook and basal body. It help in motility of bacteria.
• Pili and Fimbriae : Surface structure of some bacteria which attaches them to rocks in streams and to host tissues.

Genetic Material : It is not covererd by nuclear envelope. In addition to the genomic DNA (the single chromosome/ circular DNA), many bacteria have plasmid contain genes like antibiotic resistance.

Ribosomes : Associated with plasma memebrane of Prokaryotic cell, site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called ployribosomes or ploysomes. They translate mRNA into Proteins.

Inclusion Bodies : Stores reserve material, lie freely in cytoplasm not bound by any membrane, e.g., phossphate granules, cyanophycean granules and glycogen granules.

Eukaryotic Cells

Possess an oragnized nucleus with nuclear envelope and have a variety of complex locomotory and cytoskeletal structures.

Cell Membrane - Singer and Nicolson (1972) gave 'fluid mosaic model'. According to this the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer; two types of proteins (Peripheral and integral proteins) with cholesterol, glycolipids and glycoporteins. Erythrocyte memebrane has 52% protein and 40% lipids.

Function - It is selectively permeable and helps in transport of molecule across it.

Passive Transport	Active Transport
•Transport of molecules from higher to lower concentration. •It do not utilise energy (ATP). e.g., diffusion.	•Transport of molecules from lower to higher concentration •It utilise energy (ATP) e.g., Na+/k+ ATPase Pump.

Cells Wall is non-living rigid structure which gives shape to the cell and protects cell from mechanical damage and infection, helps in cell-to-cell interaction and provides barrier to undesirable macoromolecules.

Cell wall of algae is made of cellulose, galactans, mannans and minerals like calcium carbonate. Plant cell wall consist of cellulose, hemicellulose, pectins and proteins.

Middle lamella is made of calcium pectate which holds neighbouring cells together.

Plasmodesmata Connect the cytoplasm of neighbouring cells.

Endoplasmic Reticulum (ER)

Consist of network to tiny tubular structure. ER divides the intracellular space into tw distinct compartments- luminal (inside ER) and Extra luminal (cytoplasm).

(I) Rough Endoplasmic Reticulum (ER) :
• Ribosomes attached to outer surface.


Function : • Involved in protein synthesis and secretion.
(II) Smooth Endoplasmic Peticulum (SER) :
• Lack ribosomes function.
• Site for synthesis of lipid.

Golgi apparatus : First observed by Camillo Golgi (in 1898)

Consist of cisternae stacked parallel to each other. Two faces of te organelle are convex/ cis or forming face and concave/ trans or maturing face but inter connected.

Functions : Performs packaging of materials, to be delivered either to the intra-cellular targets or secreated outside the cell. Important site of formation of glycoporteins and glycolipids.

Lysosomes :

Membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. Contain hydrolysing enzymes (lipases, proteases, carbohydrases) which are active in acidic pH. Also called 'Suicidal Bag'.

Function : Intracellular digestion.

Vacuoles : Membrane boudn space found in the cytoplasm. Contain water, sap, excetory product, etc. In plant cell, vacoule occupies 90% of space.

Function : In plants tonoplat (single membrane of vacoule) faciliates trasnport of ions and other substances.

Contractile vacuole for excertion in Amoeba and food vacuoles formed in protists for digestion of food.

Mitochondria : Double memebraned structure. Outer memebrane smooth and inner membrane forms a number of infoldings called cristae. The inner compartment is called matrix. The cristae increase the surface area.

Function : Sites of aerboic respiration called 'power houses' of cell as produce cellular energy in the form of ATP. Matrix Possess single circular DNA molecule, a few RNA molecules, ribosomes (70S). It divides by binary fission.

Plastids : Found in plant cells and in euglenoides. Chloroplasts, chromplasts and lecucoplasts are 3 types of plastids depending on pigments contained.

Function : Site of photosynthesis, and imparts colours to fruits and flowers.

Ribosomes

Composed of RNA and protenis; without membrane. Eucaryotic ribosomes are 80S. S = Svedberg's unit)

Function : Site of protein synthesis.

Cytoskeleton : Network of filaments.

Proteinaceous structure in cytoplasm made up of micortubules and micor filaments

Function : Mechanical support, motility, maintenance of the shape of the cell.

Cilia and Flagella :

Cila are small structures which like oars which help in movement.

Flagella are longer and responsible for cell movement. They are covererd with a plasma membrane. Core is called axoneme which has 9+2 arragement of axonemal micortubules.

Centrosome and Centrioles

Centrosome contains two cylindrical structures called centrioles. Surronded by amorphous pericentriolar material. Made up of nine evenly spaced peripheral fibrils of tubulin protein (9+0). Centrioles form the basal body of cilia or flagella and spindle fibres for cell divison in animal cells. They prodcuces spindle apparatus during cell divison.

Nucleus : Double memebranous with perinuclear space and nuclear pores; has Chromatin, nuclear and nucleoli (site for rRNA synthesis). (Named by Robert Brown - 1831)


Chromatin : DNA + nonhistone proteins. (Named by Fleming)


Nucleoplasm : Nucleolus + Chromatin


Nulear memebrane : It is with perinuclear space and nucleopors.


Chromosome : DNA/ RNA + Histone portein / Nonhistone protein.


Centromere : Primary constriction - in every chromosome


Kinetochores : Disc shaped structure on the sides of centsomere.


No nucleus in Erythrocyte (RBC) of mammals and sieve tubes in vascular plants.


Chromosome (on basis of postition of centromere) :


Metacentric : Middle centromere.


Sub-metacentric : Centromere nearer to one end of chromosomes.


Acrocentric : Centromere situated close to its end.


Telocentric : Has terminal centromere.

Satellite : Some chromosomes have non-staining secondary constrictions at a constant location, which gives the apperance of small fragment called satellite.