CBSE Class 11 Biology Cell The Unit Of Life Notes

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Revision Notes for Class 11 Biology Cell The Unit Of Life

Class 11 Biology students should refer to the following concepts and notes for Cell The Unit Of Life in standard 11. These exam notes for Grade 11 Biology will be very useful for upcoming class tests and examinations and help you to score good marks

Cell The Unit Of Life Notes Class 11 Biology

cell the unit of life class 11 notes



• Schieiden and Schwann together formulated the cell theory.

• They proposed that the body of animal and plant are composed of cells and product of cells.

• Rudolf Virchow (1855) he modified and gives the final shape to thecell theory.

o All living organisms are composed of cells and products of cells.

o All cells arise from pre-existing cells.


• Represented by bacteria, blue-green algae, mycoplasma and PPLO.

• Smaller and multiply more rapidly than eukaryotic cells.

• Four basic shape of bacteria are

o Bacillus (rod like)

o Coccus (spherical)

o Vibrio (comma shaped)

o Sprillum (spiral)

• All prokaryotes have a cell wall surrounds the plasma membrane.

• There is no well defined nucleus.

• Chromosome is single and circular, not enveloped by nuclear membrane.

• Presence of many small, circular, extra chromosomal and self replicable DNA called plasmid.

• Plasmid provides unique characteristic to the bacteria.

• One of the characteristic is antibiotic resistant.

• No membrane bound cell organelles.

• A specialized differentiated form of cell membrane called mesosome is present.

Cell envelope and its Modifications :

• Most prokaryotic cell have characteristic complex cell envelope.

• The cell envelops consists of tightly bound three layer structure;

o The outermost glycocalyx.

o Middle cell wall

o Inner plasma membrane.

• According to the nature of the cell wall and behavior towards the stain developed by Gram, bacteria are Gram positiveor Gram negative.

• Glycocalyx may for a loose sheath called slime layer.

• Glycocalyx may be thick and tough called the capsule.

• The cell wall determines the shape of the cell, strong structural support to prevent the bacterium from bursting and collapsing.

• Plasma membrane is semi-permeable in nature and interacts with outside world.

• A special membranous structure is the mesosome which is formed by extensions of plasma membrane into the cell.

o These extensions are in the form of vesicles, tubules and lamellae.

o They help in cell wall formation, DNA replication and distribution to the daughter cell.

o They also help in respiration and secretion.

• Some bacteria have filamentous extensions called flagella.

• Bacterial flagellum has three parts – filament, hook and basal body.

• Pili are elongated tubular structures made of a special protein.

• Fimbriae are small bristle like fibres sprouting out of the cell. Play role in adhesion.

Ribosome and inclusion bodies :

 • Ribosomes are associated with the plasma membrane of the cell.

• Each ribosome (70 S) has two sub units; smaller (30 S ) and large (50 S).

• Ribosomes are the site of protein synthesis.

• Several ribosomes attached to a single mRNA to form a chain called polyribosome or polysome.

• Reserve materials are stored in the form of inclusion bodies.

• Phosphate granules, cyanophycean granules and glycogen granules are the inclusion bodies.


• Eukaryotes include all the Protists, plants, animals and fungi.

• Extensive compartmentalization due to membrane bound cell organelles.

• Possess an organized nucleus with nuclear envelope.

• Have a variety of complex locomotory and cytoskeletal structure.

• Genetic materials are organized into chromosomes.

• The plant cell possess cell walls, plastids and a large central vacuole, which are absent in animal cell.

• Animal cells have centrioles which absent in plant cells.

Cell membrane :

• The cell membrane composed of lipids that arranged in bilayer.

• Lipids are arranged within the membrane with the hydrophilic polar head towards the outer sides and thehydrophobic tails towards the inner part.

• Non polar tail of saturated hydrocarbons is protected from the aqueous environment.

• The lipid component of the membrane mainly consists of phophoglycerides.

• Cell membrane also possesses protein and carbohydrates.

• Ratio of protein and lipids varies from cell to cell.

• Human erythrocyte plasma membrane contains 52 % protein and 40 % lipids.

• Membrane protein may be integral or peripheral.

• Peripheral protein lie on the surface and integral proteins are partially or totally buried in the membrane.

• The improved model of the structure of plasma membrane was proposed by singer and Nicolson (1972) widely accepted as fluid mosaic model.

• According to this the quasi fluid nature of lipid enables the lateral movement of proteins within the overall bilayer.

Function :
• Passive transport.
o Simple diffusion
o Facilitated diffusion.
• Active transport.
• Phagocytosis
• Exocytosis.
• Pinocytosis.

Cell wall :
• A non-living rigid structure called cell wall present outside the plasma membrane of plant and fungal cell.
• Algae have a cell wall made of cellulose, galactans, mannans and minerals like calcium carbonate.
• Plant cell wall consists of cellulose, hemicelluloses, pectins and proteins.
• The cell wall of young plant is called primary cell wall.
• On maturity secondary cell wall formed inner to it.
• The middle lamella is a layer of calcium pectate which holds or glues the neighboring cells.
• The cell wall and middle lamella may traversed by plasmodesmata; the cytoplasmic connection between two adjacent cell.

The endomembrane system :
Endoplasmic reticulum :
• Network or reticulum of tiny tubular structures scattered in the cytoplasm, called endoplasmic reticulum.
• It divides the intracellular space into two distinct compartments:
o Luminal (inside the ER)
o Extra luminal (cytoplasm).
• The ER shows ribosomes attached to their outer surface called Rough Endoplasmic reticulum (RER).
• Endoplasmic reticulum without ribosome called SER (smooth endoplasmic reticulum)

• RER present in the cell actively involved in protein synthesis.
• SER is the site for synthesis of lipid, glycogen and steroid hormones.

Golgi apparatus:
• Camillo Golgi (1898) first observed this organelle, and named after him.
• It consists of many flat, disc shaped sacs or cisternae.
• These are staked parallel to each other.
• The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cisor the forming face and concave transor the maturing face.

• Principally responsible for packing of materials to be delivered intra-cellular target or intercellular target.
• Materials are packed in the form of vesicles, from the ER fuse with the cis face of the Golgi apparatus and move towards the trans face.
• Important site for the formation of glycoprotein and glycolipids.

• Membrane bound vesicular structure.
• Formed by Golgi body.
• Rich in all type hydrolytic enzymes, optimally active in acidic pH.
• These enzymes are capable to digesting carbohydrates, proteins, lipids and nucleic acids.

• The vacuole is the membrane-bound space found in the cytoplasm.
• It contains water, sap, excretory product.
• Vacuole is bounded by a single membrane called tonoplast.
• In plant cells the vacuole can occupy 90% of the volume.
• Tonoplast facilitates active transport of material from cytoplasm into the vacuole.
• In Amoeba the contractile vacuole is important for excretion and osmoregulation.
• In many protists, food vacuoles are formed by engulfing the food particles.

• The number of mitochondria varies according to the physiological activity of the cell.
• Each mitochondrion is a double membrane bound structure with outer and inner membrane, dividing its lumen into two aqueous compartments
o Outer compartment.
o Inner compartment.
• The inner compartment is called the matrix.
• The inner membrane forms a number of infoldings called the cristae towards the matrix.
• The cristae increase the surface area.
• Two membranes have their own specific enzymes associated with the mitochondrial function.
• Mitochondria are the site of aerobic respiration.
• They produce cellular energy in the form of ATP, hence called ‘power house’ of the cell.
• The matrix also possess single circular DNA molecule and a few RNA molecules, ribosomes (70S), they synthesize their own protein.
• Mitochondria divide by fission.
Plastids :
• Plastids are found in all plant cells and in Euglenoids.
• They bear some specific pigment, impart specific colour to the plants.
• Based on the type of pigments plastids can be classified into
o Chloroplast.
o Chromoplast
o Leucoplast.

• The chloroplasts contain chlorophyll and carotenoid pigments, traps solar energy for photosynthesis.
• In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls are present.
• The leucoplasts are colourless plastids of varied shapes and size with stored nutrients.
o Amyloplast - store carbohydrates.
o Elaioplasts – store oils and fats.
o Aleuroplast – store proteins and minerals.
• Chloroplasts are found in the mesophyll cells of the leaves.
• These are oval, spherical, discoid or even ribbon like organelles.
• Chloroplast is a double membrane organelle.
• The space limited by inner membrane is called stroma.
• A number of organized flattened membranous sacs called thylakoid are present in the stroma.
• Thylakoids are arranged like stakes of coins to form grana.
• There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.
• They thylakoids enclose a space called lumen.
• Chlorophyll pigments are located in the thylakoids.
• Chloroplast contains enzymes required for the synthesis of carbohydrates and proteins.
• Stroma contains small circular DNA and ribosomes.

Ribosomes :
• Ribosomes are granular structure first observed by George Palade (1953).
• Composed of ribonucleic acid (RNA) and proteins.
• Non-membranous cell organelles.
• Eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.
• ‘S’ stands for sedimentation coefficient; measure of density and size.
• Both 70S and 80S ribosomes consists of two subunits.
• Primary function is protein synthesis hence called protein factory of the cell.

Cytoskeleton :
• An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively known ascytoskeleton.
• Cytoskeleton involved in many function such as mechanical support, motility, maintenance of the shape of the cell.

Cilia and Flagella :
• Cilia and flagella are hair-like outgrowths of the cell membrane.
• Cilia are small help in the movement of cell or surrounding fluid.
• Flagella are longer and responsible for cell movement.
• Cilia and flagella covered by plasma membrane.
• Their core called axoneme, possess a number microtubules running parallel to the long axis.
• The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules.
• The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublet by radial spoke.
• The peripheral doublets are also interconnected by linkers.

Important Questions for NCERT Class 11 Biology Cell-The Unit of Life


Ques. A major breakthrough in the studies of cells came with the development of electron microscope. This is because
(a) the electron microscope is more powerful than the light microscope as it uses a beam of electrons which has wavelength much longer than that of photons
(b) the resolving power of the electron microscope is much higher than that of the light microscope
(c) the resolving power of the electron microscope is 200 - 350 nm as compared to 0.1 - 0.2 nm for the light microscope
(d) electron beam can pass through thick materials, whereas light microscopy requires thin sections.

Answer: B


Ques. A student wishes to study the cell structure under a light microscope having 10X eyepiece and 45X objective. He should illuminate the object by which one of the following colours of light so as to get the best possible resolution?
(a) Blue       (b) Green
(c) Yellow     (d) Red

Answer: A

Ques. Electron microscope has a high resolution power. This is due to
(a) electromagnetic lenses
(b) very low wavelength of electron beam
(c) low wavelength of light source used
(d) high numerical aperture of glass lenses used.

Answer: B

Ques. Magnification of compound microscope is not connected with
(a) numerical aperture
(b) focal length of objective
(c) focal length of eye piece
(d) tube length. 

Answer: A


Ques. The concept of “Omnis cellula-e-cellula” regarding cell division was first proposed by
(a) Aristotle (b) Rudolf Virchow
(c) Theodore Schwann (d) Schleiden.

Answer: B


Ques. Which is correct about cell theory in view of current status of our knowledge about cell structure?
(a) It needs modification due to discovery of subcellular structures like chloroplasts and mitochondria.
(b) Modified cell theory means that all living being are composed of cells capable of reproducing.
(c) Cell theory does not hold good because all living beings (e.g., viruses) do not have cellular organisation.
(d) Cell theory means that all living objects consists of cells whether or not capable of reproducing.

Answer: C


Ques. Names of Schleiden and Schwann are associated with
(a) protoplasm as the physical basis of life
(b) cell theory
(c) theory of cell lineage
(d) nucleus functions as control centre of cell

Answer: B


Ques. Angstrom (Å) is equal to
(a) 0.01 mm (b) 0.001 mm
(c) 0.0001 mm (d) 0.00001 mm. 

Answer: C


Ques. Organelles can be separated from cell homogenate through
(a) chromatography (b) X-rays diffraction
(c) differential centrifugation
(d) auto-radiography. 

Answer: C


Ques. Which of the following statements about inclusion bodies is incorrect?
(a) They are not bound by any membrane.
(b) These are involved in ingestion of food particles.
(c) They lie free in the cytoplasm.
(d) These represent reserve material in cytoplasm.

Answer: B


Ques. Select the wrong statement.
(a) Bacterial cell wall is made up of peptidoglycan.
(b) Pili and fimbriae are mainly involved in motility of bacterial cells.
(c) Cyanobacteria lack flagellated cells.
(d) Mycoplasma is a wall-less microorganism.

Answer: B


Ques. Which of the following structures is not found in a prokaryotic cell?
(a) Mesosome               (b) Plasma membrane
(c) Nuclear envelope     (d) Ribosome 

Answer: C


Ques. The structures that help some bacteria to attach to rocks and/or host tissues are
(a) mesosomes            (b) holdfast
(c) rhizoids                  (d) fimbriae. 

Answer: D


Ques. Which one of the following is not an inclusion body found in prokaryotes?
(a) Glycogen granule (b) Polysome
(c) Phosphate granule
(d) Cyanophycean granule

Answer: B


Ques. Which structures perform the function of mitochondria in bacteria?
(a) Nucleoid (b) Ribosomes
(c) Cell wall (d) Mesosomes 

Answer: D


Ques. The motile bacteria are able to move by
(a) fimbriae (b) flagella
(c) cilia (d) pili. 

Answer: B


Ques. Why is a capsule advantageous to a bacterium?
(a) It protects the bacterium from desiccation.
(b) It provides means of locomotion.
(c) It allows bacterium to “hide” from host’s immune system.
(d) It allows the bacterium to attach to the surface.

Answer: A


Ques. The term ‘glycocalyx’ is used for
(a) a layer present between cell wall and membrane of bacteria
(b) cell wall of bacteria
(c) bacterial cell glyco-engineered to possess N-glycosylated proteins
(d) a layer surrounding the cell wall of bacteria.

Answer: D


Ques. Chromosomes in a bacterial cell can be 1 – 3 in number and
(a) are always circular (b) are always linear
(c) can be either circular or linear, but never both within the same cell
(d) can be circular as well as linear within the same cell. 

Answer: A


Ques. Photosynthetic bacteria have pigments in
(a) chromoplasts (b) chromatophores
(c) leucoplasts (d) chloroplasts. 

Answer: B


Ques. The site of respiration in bacteria is
(a) ribosome (b) microsome
(c) episome (d) mesosome. 

Answer: D


Ques. Which is the important site of formation of glycoproteins and glycolipids in eukaryotic cells?
(a) Endoplasmic reticulum
(b) Peroxisomes (c) Golgi bodies
(d) Polysomes 

Answer: C


Ques. Which of the following statements is not correct?
(a) Lysosomes are formed by the process of packaging in the endoplasmic reticulum.
(b) Lysosomes have numerous hydrolytic enzymes.
(c) The hydrolytic enzymes of lysosomes are active under acidic pH.
(d) Lysosomes are membrane-bound structures.

Answer: A


Ques. Which of the following statements regarding mitochondria is incorrect?
(a) Mitochondrial matrix contains single circular DNA molecule and ribosomes.
(b) Outer membrane is permeable to monomers of carbohydrates, fats and proteins.
(c) Enzymes of electron transport are embedded in outer membrane.
(d) Inner membrane is convoluted with infoldings.

Answer: C


Ques. The shorter and longer arms of a submetacentric chromosome are referred to as
(a) m-arm and n-arm respectively
(b) s-arm and l-arm respectively
(c) p-arm and q-arm respectively
(d) q-arm and p-arm respectively. 

Answer: C


Ques. Which of the following pairs of organelles does not contain DNA?
(a) Nuclear envelope and Mitochondria
(b) Mitochondria and Lysosome
(c) Chloroplast and Vacuoles
(d) Lysosomes and Vacuoles 

Answer: D


Ques. Match the column I with column II.
        Column-I               Column-II
A. Golgi apparatus     (i) Synthesis of protein
B. Lysosomes            (ii) Trap waste and excretory products
C. Vacuoles               (iii) Formation of glycoproteins and glycolipids
D. Ribosomes            (iv) Digesting biomolecules
Choose the right match from options given below.
(a) A-(iii), B-(iv), C-(ii), D-(i)
(b) A-(iv), B-(iii), C-(i), D-(ii)
(c) A-(iii), B-(ii), C-(iv), D-(i)
(d) A-(i), B-(ii), C-(iv), D-(iii) 

Answer: A


Ques. Which of the following is true for nucleolus?
(a) Larger nucleoli are present in dividing cells.
(b) It is a membrane-bound structure.
(c) It takes part in spindle formation.
(d) It is a site for active ribosomal RNA synthesis.

Answer: D


Ques. The Golgi complex participates in
(a) fatty acid breakdown
(b) formation of secretory vesicles
(c) respiration in bacteria
(d) activation of amino acid.

Answer: B


Ques. Which of the following events does not occur in rough endoplasmic reticulum?
(a) Protein folding (b) Protein glycosylation
(c) Cleavage of signal peptide
(d) Phospholipid synthesis 

Answer: D


Ques. Select the incorrect match.
(a) Lampbrush – Diplotene bivalents chromosomes
(b) Allosomes – Sex chromosomes
(c) Sub-metacentric – L-shaped chromosomes chromosomes
(d) Polytene – Oocytes of chromosomes amphibians

Answer: D


Ques. Which of the following cell organelles is responsible for extracting energy from carbohydrates to form ATP?
(a) Ribosome (b) Chloroplast
(c) Mitochondrion (d) Lysosome

Answer: C


Ques. Select the mismatch.
(a) Gas vacuoles – Green bacteria
(b) Large central vacuoles – Animal cells
(c) Protists – Eukaryotes
(d) Methanogens – Prokaryotes

Answer: B


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