CBSE Class 8 Science Cell Chapter Notes

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Definition :- The structural & functional unit of living beings is called cell.

O R

A mass of protoplasm bounded by a plasma membrane.

OR

An unit of biological activity, delimited by a differentially permeable membrane and capable of self

reproduction.

DISCOVERY OF CELL

1. Robert Hooke (1665) :– An English man and first curator of

Cork section shown in Robert Hooke's Microscope

Royal society of London.

Observed a thin transverse section of bark of a tree under selfdesigned microscope.

He noticed honey - comb like compartments.

He coined the term cell.

He wrote a book - Micrographia.

He actually observed dead cells.

2. Antony Van Leeuwenhoek (1674) was first to observe living cellslike bacteria [from tartar of teeth]

erythrocytes [fish], sperms and protozoans [eg. Vorticella]

3. N. Grew (1682) :– Proposed cell concept which states that cell isunit of structure of organisms.

4. Rudolf Virchow (1858) :– Proposed that new cells formed fromthe pre-existing cells.

MICROSCOPE

A microscope is an instrument to view small objects by magnifying them. It enables us to see the different typesof living cells and the structures they contain.

Ocular eye piece

Body tube

Revolving nose piece

High power objective

Cover slip

Slide

Condenser

Mirror

Base

Low power objective

Coarse adjustment

Fine adjustment

ARM

Stage clip

Stage

Inclination joint

Parts of an ordinary compound microscope

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TYPES OF MICROSCOPES

There are mainly three types of microscopes. They are :

• • • • • Light microscope : The light microscope uses light to produce images.

• • • • • Transmission Electron Microscope (TEM) : The electron microscope was designed by Knoll & Ruska(1932). A TEM makes use of a beam of highly energetic electrons to examine objects. The image producedis of a very fine scale.

• • • • • Scanning Electron Microscope (SEM) : Like the TEM, the SEM also uses electrons to produce images. Inthe case of a SEM, electrons are reflected off the surface of the specimen, because of which SEM imagesusually manage to capture the physical features of a cell in great detail.

How to make a Microscopic Slide?

Most of the specimens are examined under the microscope by preparing a wet mount using water. The basicsteps involved in preparing a wet mount are given below :

1. Clean the glass slide and place on a flat surface of the table.

2. Place a drop of water in the centre of the slide.

3. Place a thin piece of the specimen to be viewed on the water drop on the slide.

4. A drop of stain or dye may be added with a dropper on the slide. Staining highlights biological tisues and specificregions in the cells which makes it easier for us to view the details. Iodine, methylene blue and crystal violetstains are commonly used.

5. Hold the cover slip over the object in such a manner that it touches the edge of the drop of water. Gently lower thecover slip so that it spreads out the water and no air bubble is trapped. The coverslip serves the following purposes.

(i) It protects the microscope's objective lens from contacting the specimen sample.

(ii) It helps to create an even thickness for the sample.

6. Dry the extra water that comes out of the cover slip with the help of a blotting paper.

7. Take care that the slide thus prepared is clean and dry.

8. Now place the slide under the microscope and examine the specimen.

Basic criteria for defining the cell –

(i) Presence of genetic material. (ii) Presence of limiting plasma membrane.

(iii) Presence of a metabolic machinery.

• Cell is called structural unit of living being, because body of all living organisms is made up of one or morecells.

• Cell is called functional unit of all organisms, because all the vital activities or physiological activities[i.e. respiration, digestion, excretion, circulation, etc are performed at the level of cell].

Procedure

1. Cut an onion into small pieces. Take a fleshy leaf and break it fromconcave to convex side.

2. Peel off the thin epithelial membrane with the help of forceps andtransfer it into a watch glass containing methylene blue solution for

a few seconds.

3. Transfer the stained peel into another watch glass containing cleanwater to wash off the extra stain.

4. With the help of a brush and needle, place this peel in the centre ofthe glass slide in such a way that the membrane is not folded.

Immediately put a drop of glycerine on the peel.

5. Cover the peel gently with the coverslip to avoid the entry of air bubbles.

6. Gently press the coverslip with a needle so as to spread the glycerine evenly.

7. Remove excess glycerine from the edges of the coverslip using a blotting paper.

8. Examine the slide under low power of a microscope.

Structure of onion peel cells



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Observations :

• There are a large number of brick-shaped (rectangular) cells lying side by side in membrane.

• Each cell has a distinct cell wall.

• A distinct darkly stained nucleus is present in each cell which is spherical or oval shaped dot-like structure.

• A prominent vacuole is seen in the centre, and cytoplasm is present in every cell.

Inference : The cells observed under microscope are plant cells as each has a distinct cell wall and a large

vacuole is present in the centre of each cell.

Experiment : Preparation of a temporary stained mount of human cheek cells and observe it under a microscope.

Procedure

1. Rinse your mouth with warm water.

2. With the help of a toothpick/ice-cream spoon, gently scrape the inner surface of the cheek.

3. Put the scraping on a clean glass slide having a drop of water.

4. Spread out the scraping, and separate the cells with the help of a needle.

5. Remove the excess water and put a drop of methylene blue stain or iodine solution on the scraping.

6. Place a clean coverslip gently over it to avoid the entry of air bubbles.

7. Gently press the coverslip with the needle so as to spread the cells uniformly under the coverslip.

8. Examine the slide under the microscope.

Observations :

• A large number of cells are observed. Each cell is bounded by a thin cell membrane. These cells are of

squamous epithelium.

• A dark stained distinct nucleus is observed in each cell.

Structure of cheek cells

• Cytoplasm is granular.

• Cells lack cell wall, large vacuoles and plastids.

Inference : The cells observed under microscope are animal cell as each cell has a cell membrane only as

outer boundary. Cell wall, central prominent vacuole and plastids are absent.

CELL THEORY

Cell theory was presented by Schleiden (Botanist) (1838) and Schwann (Zoologist) (1839). Accordingly, all

plants and animals are composed of cells and the cell is the basic unit of life.

Later discoveries led to the modification of cell theory. Modified cell theory is termed as cell principle or

modern cell theory.

Modern cell theory postulates that :-

1. All living beings are composed of minute units, the cells ;

2. A cell is a mass of protoplasm containing a nucleus and bounded by a cell membrane, and in many cases by a

cell wall also;

3. All cells are basically alike in structure and metabolic activities;

4. The functions of an organism as a whole is the result of the activities and interactions of the constituent cells; and

5. All living cells arise from the pre-existing cells.



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(A) Size of cell – Normal size in human 20 µm to 30 µm in diametre.

(i) Largest cell – In animals – Ostrich egg [15 cm is diametre]

In plants – Acetabularia [6-10 cm]

(ii) Longest cell – In animals – Nerve cell [upto 1mt]

In plants – Hemp fibre.

(iii) Smallest cell – PPLO – Pleuro Pneumonia Like Organism [Mycoplasma – 0.1 to 0.5 µm.]

ChromatinRibosome

A. PPLO B. ACETABULARIA C. HEN'S EGG

Cap

Stalk

Variation in cell size

(B) Shape of cell – Shape of cell mainly depends upon the specific function it performs.

(i) Elongated – Nerve cell (ii) Discoidal/saucer shaped – RBC

(iii) Spindal shaped – Muscle cell (iv) Spherical shaped – Eggs.

(vi) Slipper shaped – Paramecium

Diversity of cell shape

PARAMECIUM NEURON

Amoeba

LIVER CELL

COLUMNAR CELLS

HEN'S EGG

FAT CELL

SMOOTH MUSCLES

CHEEK CELLS



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(C) Number of cells – Living organisms are made up of one or many cells. On the basis of number of cells the

organisms can be categorised as –

(i) Unicellular – Organisms made up of a single cell are called unicellular organisms. eg – Amoeba,

Paramecium, Euglena, Plasmodium – (Malarial parasite), Chlamydomonas.

(ii) Multicellular – Organisms made up of many cells are called multicellular organisms. Most plants and animals

that we see around us are multicellular.

(D) Levels of Organisation : From Cell to organism

Units of Measurement Used in Cell Biology :

1 mm (millimetre) = 10–3 metre

1 μm (micrometre) = 10–3 mm

1 nm or 1 mμ (millimicrometre) = 10–3 μm or 10–6 mm

1 Å (Angstrom) = 10–1 nm or 10–1 mμ or 10–7 mm.

Differences between prokaryotic & Eukaryotic cells

Fe ature Prokaryo tic cell Eukaryo tic ce ll Cell size Average diameter 0.5-5 μm Diameter varies between. 1 μm-40 μm Nucleus Lacks true nucleus; Nucle olus and nuclear

membrane are absent and contains circula r DNA.

True nucleus bound by nuclear membrane contains linear DNA nucleolus and nuclear membra ne present.

Organelles Membrane-bound organelles like Golgi bodies, plastids, mitochondria and endoplasmic reticulum (ER) are absent.

Membrane-bound organelles present.

Ribosomes Smaller and randomly scattered in the cytopla sm

Bigger, can be free or attached to the ER

Photosynthesis No organized chloroplast; photosynthesis takes place on photosynthetic membranes which lie freely in the cytoplasm.

Organized chloroplasts (conta ining stacked membra nes called grana) take part in photosynthesis

Examples Bacteria and cyanobacteria (blue-green algae)

All other organisms.



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Bacterial cell

Animal cell

Plant cell

Endoplasmicreticulum Cell wall

Plasmamembrane

Tonoplast

Ribosome

Cytoplasm

Nucleolus

Nucleus

Mitochondrion

Plastids

Centralvacuole

Differences between Plant cell & animal cell

PLANT CELL ANIMAL CELL 1. Plant cells are usually larger than animal cells Animal cells are generally small in size. 2. The plasma membrane of a plant cell is

surrounded by a rigid cell wall made up of cellulose.

Cell wall is absent.

3. Plastids (leucoplasts, chloroplasts, chromoplasts) are present in plant cells.

Plastids are absent.

4 . Vacuoles are present in abundance. They are larger in size.

Vacuoles are less in number and smaller in size.

5. Plant cells have many simpler units of Golgi complex, called dictyosomes .

Animal cells have a single highly elaborate Golgi complex.

6. Centrioles have not been found in plant cells (except in a few lower plants.

Animal cells possess centrioles.

7 . Cytokinesis takes place by cell-plate formation. Cytokinesis takes place by constriction during cell division.

8. Plant cells usually have a regular shape. Animal cells are usually irregular in shape.

REVIEW QUESTIONS

(A) Very Short answer type questions :

1. What is cell?

2. What is the structural and functional unit of life?

3. What are three basic criteria for defining the cell?

4. Give any three examples of unicellular organisms.

5. Give any three examples of multicellular organisms.



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(B) Short answer type Questions :

1. Differentiate a plant cell and an animal cell.

2. Differentiate prokaryotic cell and eukaryotic cell.

3. Describe cell theory.

4. "Cell is the structural and functional unit of life" Justify it.

5. Write a short note on size of cell.

(C) Fill in the blanks :

1. Prokaryotic cells are found in ...............and .................

2. Membrane-less nuclear area found in prokaryotic cells is called ..............

3. Cell organelles are well developed in ......................cells.

4. Typical chromosomes are found in ............... cells.

5. The cell theory was given by .........and ............

(D) Mark the following statements as True (T) or False (F):

1. In eukaryotic cells, the genetic material is not surrounded by a nuclear membrane.

2. Well-defined organelles are found in prokaryotic cells.

3. Plant cells are bounded by a wall composed of cellulose.

4. Amoeba is a multicellular organism.

5. Plasma membrane is present in all the cells.

STRUCTURE OF A GENERALIZED CELL

Though different types of cells differ in shape and size, all of them have a basic structure, referred to as the

generalized cell structure. A cells has both living and non-living parts. The living parts of the cell that have a

definite shape, structure, and function are called organelles. The main parts of a generalized cell are (a) cell

membrane, (b) cytoplasm, and (c) nucleus.

CELL MEMBRANE OR PLASMA MEMBRANE OR PLASMALEMMA

Each cell (prokaryotic as well as eukaryotic) is surrounded by a covering called plasma membrane or

plasmalemma or cell membrane. Most cell organelles in eukaryotic cells (e.g., Mitochondria, Plastids,

Golgi apparatus, Lysosomes, Endoplasmic reticulum, Peroxisomes, Vacuoles etc). are enclosed by subcellular

unit membranes. These membranes, thus, compartmentalise the cell.

Fluid Mosaic Model :- In 1972, S.J. Singer and G. Nicolson proposed fluid mosaic model to explain the

structure and functions of plasma membrane. According to this model, the plasma membrane is made up

of a phospholipid bilayer and two types of protein molecules 'floating about' in the fluid phospholipid

bilayer. The two types of proteins are (i) Intrinsic proteins which are embeded in the phospholipid matrix

incompletely or completely, and (ii) Extrinsic proteins which occur superficially either on the outer surface or

on the inner surface of the phospholipid layer. In other words, the membrane is a viscous fluid with phospholipids

and protein molecules arranged as a mosaic.

Oligosaccharide molecules are present on the exposed surface of the plasma membrane. They are associated

with proteins as well as lipid molecules forming glycoproteins and glycolipids respectively. Cholesterol molecules

are inserted between the phospholipid molecules of plasma membrane of animal cells to stabilize the membrane.

Presence of lipids and proteins provides flexibility to the plasma membrane.



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Fluid mosaic structure of the plasma membrane

Types of membranes :

(i) Impermeable membrane : If the membrane does not allow passage of substances (solvent and solute) through it.

(ii) Permeable membrane : If the membrane allows free passage of solute and solvent through it.

(iii) Semipermeable membrane : If the membrane allows passage to solvents but prevents the passing of solutes.

(iv) Selectively permeable membrane : If the membrane allows the passage of solvent and few selected solutes.

Advantage of Semipermeability of membrane :- Semipermeability ensures that

1. The useful molecules enter the cell,

2. The metabolic intermediates remain within the cell and

3. The secretions and wastes leave the cell.

MECHANISMS INVOLVED IN THE SEMIPERMEABILITY OF PM

(i) Diffusion :– It is passive movement of materials.

Movement of solutes or ions from higher concentration to lower concentration is called diffusion.

OR

Movement of materials [solutes or ions] from higher diffusion pressure to lower diffusion pressure.

(ii) Osmosis:– It is passive movement. The movement of solvent or water from its higher concentration to

lower concentration through semi-permeable membrane is called osmosis.

It is of two types :

(a) Endosmosis:– Movement of solvent or water into the cell is called endosmosis.

(b) Exosmosis:– Movement of solvent or water outside the cell is called exosmosis.



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Initial level ofsugar solution

Water

Thistle funnel

Semi-Permeable membrane

Final level ofsugar solution

1

2

3 Se mipermeable membrane is not required. Semipermeable membrane is required.

Osmo sis

The diffusing molecule s may be solids,liquids or gases.

Diffus io n

It involves movement of solvent molecules only.

Diffusion can occur in any me dium. It occurs only in liquid medium.

(iii) Facilitated or mediated diffusion :– This type of diffusion occurs according to concentration gradient but it

involves carriers like Permeases, Translocases etc. Expenditure of metabolic energy or ATP is not involved in

diffusion.

(iv) Endocytosis (Bulk transport) :– The intaking of large sized liquid or solid biomolecules through plasma

membrane is called endocytosis.

It is of two types :–

(a) Phagocytosis:– Cell eating

The intaking of large sized solid bio-molecules by plasma membrane is called phagocytosis. OR

Ingestion of solid complex materials by plasmalemma in the form of vesicles (Phagosome) is called phagocytosis.

(b) Pinocytosis:– Cell drinking

The intaking of liquid biomolecules by plasma membrane is called pinocytosis. OR

Ingestion of liquid material by plasmalemma in the form of vesicles or bag like structure (pinosome) is called

pinocytosis.

S.No. Pino cyto s is Phago cyto sis

1 It is the intake of extracellular fluid drop lets. It is the intake of e xtracellular particles

2 Cell membrane invaginate s to take up the materia l.

Ce ll membrane grows around the pa rticle as pseudopodia.

3 It is a nutritive process. It is a nutritive and defensive process.

Differe nces Between Pino cyto sis And Phago cyto sis

(v) Active transport:– The transport of ions or microbio-molecules across a semipermeable membrane against

the concentration gradient in the presence of energy provided by ATP and specific carrier molecules.



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(vi) Exocytosis/Emiocytosis/Cell vomitting :– Egestion of waste materials from cell through plasma membrane.

Functions of plasma membrane

(a) Provides definite shape to the cell.

(b) Protects various cell organelles.

(c) Differential permeability or selective permeability.

Type of solutions on the basis of concentration

(a) Isotonic solution : When the concentration of the solution outside the cell is equal to the concentration of

solution inside the cell is called isotonic solution. [No effect on size of cell.]

(b) Hypertonic solution : When the concentration of the solution outside the cell is more than that of inside

the cell is called hypertonic solution. [Cell become plasmolysed.]

(c) Hypotonic solution : When the concentration of the solution outside the cell is less than that of inside the

cell is called hypotonic solution. [Cell swelled up]

CELL WALL

Discovered by Robert Hooke

• The outermost covering of the plant cell is called cell wall.

• It is absent in animal cell.

• It is rigid, thick, porous and non-living structure.

• Middle lamella is consist of Ca & Mg pectates (Plant cement). Fruits becomes soft and juicy due to dissolve of

middle lamella.

Composition of cell wall : (i) Cellulose + Hemicellulose-in plants

(ii) Chitin – in fungi

(iii) Peptidoglycan – in bacteria.

Functions of cell wall : (i) It provides a definite shape to cell.

(ii) It is a protective and supportive coat.

(iii)It is permeable and allows entry of molecule of different size.

DO YOU KNOW?

• The fluid present inside the nucleus is called the nucleoplasm and the fluid present outside the nucleus is calledcytoplasm. The cytoplasm and nucleoplasm together from a semi solid substance called the protoplasm.

• 90% of protoplasm by weight is water. Rest of the fluid is composed of proteins, carbohydrates, fats, andminerals.

REVIEW QUESTIONS

(A) Very short answer type questions :

1. What is plasma membrane?

2. Give the names of three basic molecules of the plasma membrane.

3. Mention the constituent of cell wall in bacteria & Plants.

4. What is diffusion?

5. Define osmosis

6. What is active transport?

7. Is the plant cell wall living or dead?

8. What will happen if a few drops of ink are added to pure water?

9. What will happen to plant cell when placed in concentrated sugar solution?

10. What will happen to resins when placed in pure water?



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(B) Short Answer type questions :

1. Mention the functions of plasma membrane.

2. Describe the type of solutions on the basis of concentration.

3. Give difference between diffusion and osmosis.

4. Describe the structure of cell membrane.

5. What is endocytosis? Mention its types.

(C) Reasoning type questions :

1. Why plasma membrane is called a selectively permeable membrane?

2. Give the reason of liquid nature of plasma membrane.

3. Why integral proteins cannot be separated easily from plasma membrane?

4. How cell identifies each other?

5. Why cell wall allows entry of different sized molecules?

(D) Fill in the blanks :

1. Middle lamella is made up of .....................

2. In Fungus Cell wall is made up of .....................

3. The intaking of large sized solid bio-molecules by plasma membrane is called .....................

4. Diffusion can occur in any medium, while osmosis occurs only in ..................... medium.

5. Movement of solutes or ions from ..................... to ..................... is called diffusion.

A N W E R S

1. Calcium and magnesium pectate. 2. Chitin 3. Phagocytosis 4. Liquid

5. Higher concentration, Lower concentration

Protoplasm [Essence of life] – The entire matter found inner to the cell membrane is called as protoplasm.

Huxley : Physical basis of life. Protoplasm – term was proposed by Purkinje.

It can be divided into 2 parts :– (a) Cytoplasm (b) Nucleus

Cell organelles/Cytoplasmic organs:– The living structures present in cytoplasm are called cell organelles.

(i) Golgibody (ii) Endoplasmic reticulum (iii) Mitochondria (iv) Ribosome (v) Lysosome (vi) Plastids

(vii) Centrosome (viii) Vacuole (ix) Peroxisome

Cell inclusions : The cell inclusions are non living materials present in the cytoplasm. They are often called

deutoplasmic substances. They may be organic or inorganic compounds, or both. The common cell inclusions

are stored organic food materials, secretions and excretions, and inorganic crystals.



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NUCLEUS-HEADQUARTER OF THE CELL

Discovered by – Robert Brown (1831)

• "Nucleus is double membrane bound dense protoplasmic body, which controls all cellular metabolism andencloses the genetic information of cell".

• Nucleus is consider as controller or director of cell.

Generally eukaryotic cell contain at least one nucleus. but some cells having more than one nucleus.

Structure :– It is made up of following four contents :

(a) Nuclear membrane/ Nuclear envelope/Karyotheca

(b) Nuclear sap/Nucleoplasm/Karyolymph.

(c) Nucleolus.

(d) Chromatin threads.

(a) Nuclear envelope:– Nucleus is surrounded by two membranes, that separates nucleoplasm from cytoplasm.The

nuclear membrane has minute pores. These are called nucleo-pores. Nucleopore takes part in exchange of

different substances between nucleoplasm and cytoplasm.

(b) Nucleoplasm:– The part of protoplasm which is enclosed by nuclear membrane is called nucleoplasm. It

contains chromatin threads and nucleolus.

(c) Nucleolus:– Discovered by Fontana. Usually one nucleolus is present in each nucleus but sometimes more

than one nucleoli are present. It is a store house of RNA & site of r-RNA transcription and processing

of ribosome assembly.Calcium is essential for maintenance of nucleolus.

(d) Chromatin Threads:– A darkly stained network of long and fine threads called chromatin threads. Chromatin

threads are intermingled with one another forming a network called chromatin reticulum.

Functions of Nucleus :–

The nucleus performs following functions :

1. It controls all the metabolic activities of the cell.

2. It brings about growth of the cell by directing the synthesis of structural proteins.

3. It takes part in the formation of ribosomes.

4. It regulates cell cycle.

5. It contains genetic information and is concerned with the transmission of hereditary traits from one generation

to another.



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C H R O M O S O M E

At the time of cell division the chromatin threads separate apart from

one another and become smaller and thicker, are called chromosome.

Chromatid

Centromere

Kinetochore

ChromatidA chromosome is a deeply staining, rodlike structure.

1. Prokaryotic chromosomes are composed of circular DNA. It lacks histone protein. There is only one

chromosome in a prokaryotic cell.

2. Eukaryotic chromosomea are composed of Linear DNA & Histone protein.

Number of Chromosomes :-

The number of chromosomes within the nucleus is constant in all individuals of a given species. For example,

there are 46 chromosomes in a human being ; 40 in a mouse ; 8 in a fruitfly (Drosophila melanogaster). As it

represents two sets of chromosomes, one set received from each parent, this number is called the diploid

number of chromosomes. The nucleus of a gamete (egg or sperm) contains half this number of chromosomes,

or the haploid number. Thus a human gamete contains 23 chromosomes. A haploid set of chromosomes is

called genome.

• Gene : The segment of DNA and act as unit of heredity.

• ATP : Adenosine triphosphate. It is also known as energy currency. It provides energy to perform bio-

synthesis & mechanical work.

• Homologous chromosomes : All chromosomes are found in pair and the chromosomes of a pair are called

homologous chromosomes.

• Non-homologous chromosomes : Chromosomes of different pair.

• Nucleus is also called director of cell as it controls most of the cellular activities.

• Nucleus is absent in sieve tubes of vascular plants & mature RBC's of mammals. Mammalian

RBC also lacks Golgibodies, mitochondria, ER, lysosomes.

MITOCHONDRIA

It is a rod shaped structure found in cytoplasm. It is absent in prokaryotes.

• Power house of cell or ATP-mill in cell.

• Discovered by Kolliker (1857) and termed it sarcosomes. Altmann (1894) renamed it bioplast.

• Mitochondria term was proposed by Benda (1897)

• Higher number of mitochondria are found in metabolically active cells.

Structure :

• It is double membrane bound cell organelles.

• The space between inner and outer mitochondrial membrane is called perimitochondrial space.

• The outer membrane is smooth and straight. The inner membrane is infolded into the cavity. These fingure-like

infoldings are called as cristae.

• The fluid filled in mitochondrial cavity is called matrix.



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Function :– • Its main function is to produce and store the energy in the form of ATP, that's why it is also

known as powerhouse of the cell.

DO YOU KNOW ?

(i) Mitochondria and plastids are also known as semi-autonomous cell organelles as they having 70 s

type of ribosome, circular DNA and many enzymes which are useful in its protein synthesis.

(ii) An egg cell is the largest human cell.

(iii) Largest cell organelle - Nucleus

Second Largest cell organelle - Chloroplast [ in plant cell], Mitochondria - [in Animal cell]

PLASTIDS

Plastid term first used by Haeckel, Chloroplast term given by Schimper.

Shape (i) Discoidal or oval – Higher plants. (ii) Girdle shaped – Ulothrix (iii) Cup-shaped Chlamydomonas.

Types of Plastids

LeucoplastFeature ChloroplastChromoplast

Green colourOther than green [Red, orange,yellow]

Store the reserve food PhotosynthesisAttract insects and animals forpollination

(1)

(2)

Pigment

Function

Structure : It is double membrane bound cell organelle and is the largest organelle of cell.

Plastids have following 2 parts :–

(a) Grana:–It constitutes of the lamellar system.These are found in layers on top of each other, these stacks are

called as grana.Each lamella is made up of unit membrane.Each granum of the chloroplast is formed by

superimposed, closed compartments called thylakoid.

• Different grana are connected with the help of tubular connections, called stroma lamellae or Fret

channels or intergranum.

(b) Stroma:– It is a granular transparent substance filled in cavity of chloroplast. It is also called as matrix.

Grana are embedded in it.It contains enzymes for Calvin cycle or dark reaction.



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Functions :-

1. By trapping solar energy, green plastids manufacture food through photosynthesis.

2. Chromoplasts provide colour to various flowering parts.

3. Leucoplasts help in the storage of protein, starch and lipid (oil).

ENDOPLASMIC RETICULUM

First observed by Garnier (1897).

E.R. name proposed by K.R. Porter. [Credit for discovery of ER goes to Porter]

Components of E.R. : (i)Cisternae(ii) Vesicles (iii)Tubules

It is also known as "System of Membranes".

Nucleus LumenOuter membraneInner membrane

Nuc

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Endoplasmic reticulum

Cell membrane

Endoplasmic reticulum as seen in section

Cisternae Vesicles Tubules

Parts of Endoplasmic reticulum

Ribosomes

Lumen

Lumen

(A) (B)

(A) : Rough endoplasmic reticulum(B) : Smooth endoplasmic reticulum

Types of E.R.

Rough E.R/Granular E.RFeature Smooth E.R./Agranular E.R.

Present

Concerned with protein synthesis

Absent

Concerned with glycogen and lipid metabolism

Ribosome

Function

Functions :– The endoplasmic reticulum has the following functions.

1. It increases the surface area of the cytoplasm for various metabolic activities of the cell.

2. Rough Endoplasmic Reticulum (RER) is associated with the synthesis of proteins.

3. SER plays a crucial role in detoxifying many poisons and drugs.

GOLGI COMPLEX

Discovered by Camillo Golgi (1898) in nerve cells of owl.

Other names :

(i) Baker's body, - In fungus (ii) Dictyosomes – In plants

Position : It is located near the nucleus.

• The cytoplasm surrounding Golgi body have fewer or no other organelles. It is called Golgi ground substance

or zone of exclusion.

• Golgi bodies are pleomorphic structures, because component of golgi body are differ in structure & shape in

different cells.



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Structure:– It is formed of four types of contents.

(i) Cisternae (ii) Tubules (iii) Vacuoles (iv) Vesicles

Golgibody is single membrane bound cell organelle.

Function : (i) It involved in cell-secretion and acts as storage, modification and condensation or packaging

membrane.

(ii) It forms the lysosomes and secretory vesicles.

(iii) It is the site for formation of glycolipids and glycoproteins.

LYSOSOME

First observed and the term coined by Christian De Duve (1955)

• Lysosomes are spherical bag like structures [0.1 – 0.8 µm] which is covered by single unit membrane.With the

exception of mammalian RBC they are reported from all cells. Lysosomes are filled about 50 different types

of digestive enzymes termed as acid hydrolases.

• Lysosomes are highly polymorphic cell organelle. Because, during functioning, lysosomes have different

morphological and physiological states.

Functions : (i) It involve in digestion of foreign materials received in cell.

(ii) Digestion of old or dead cell organelles.

(iii) Sometimes all lysosomes of a cell burst to dissolve the cell completely. That's why lysosomes

are also known as suicidal bags.

RIBOSOME (Engine of cell)

Claude (1941) First observed them and called microsome.

Palade (1955) coined the term Ribosome.

• All living cells have Ribosomes. [both prokaryotes & eukaryotes]

• These are smallest and membraneless cell organelles.

Ribosome



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Ribosomes (On the basis of their sedimentation coefficient)

Prokaryotic ribosomes. Eukaryotic ribosomes

70s 80s

Larger subunit 50s

Larger sub 60s

unitSmaller subunit 30s

Smaller subunit 40s

• Magnesium ion [Mg++] is essential for binding of both the sub units of ribosome.

Functions :–

Site of protein synthesis, so these are also called protein factories.

PEROXISOMES/URICOSOMES

• Discovered by Rhodin & Tolbert.

• Peroxisome term was first used by De Duve.

It contains per-oxide forming enzymes.

Functions :–

(i) In animals peroxisomes are concerned with β-oxidation of fatty acids & peroxide metabolism.

(ii) In plants peroxisomes are concerned with βββββ-oxidation of fatty acids, peroxide metabolism and

photorespiration.

COMPETITION WINDOW

• Scattered Golgibodies in the cytoplasm of plant cells are called Dictyosomes.

• Lysosome found in four forms that's why it is also called polymorphic cell organelle.

• Chloroplasts are centres of photosynthesis to prepare the organic food so are called kitchens of the cells.

VACUOLES

• Vacuoles of animal cells arise from Golgi-complex.

• Tonoplast:– Plasma membrane that covers the vacuole is called tonoplast.

Functions :–

(i) Storage of food, water and other substances.

(ii) They help in the elimination of excess water from the cell (osmoregulation), and maintains internal pressure

of the cell



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Centrosome :– Discovered by Benden. Boveri named it as centrosome.

• Centrosome is generally found in animal cells. Only few type of a plant cells show its presence.

• It is situated near the nucleus of the cell and shaped like star.

Function :–

(i) In animal cells centrioles play important role in initiation of cell division by arranging spindle fibres between

two poles of cell.

(ii) The location of centrioles during cell division decides the plane of division.

TISSUES

A group of similar cells that perform a particular function is called a tissue.

Most plants and animals show a well-defined tissue system. Cells that make up these tissues can be seen under

a microscope. We can view them either lengthwise, called the longitudinal section (L.S.) or as a cross section

called transverse section (T.S.) view.

Plant Tissues : These are of following types :-

Epidermal Tissue : Cells forming the epidermal tissue are found as a thin outer lining on leaves, stems, roots,

flowers, and fruits. Epidermal cells are spherical, oval, or polygonal in shape. A prominent nucleus is present

and the cell wall is thin. The intracellular spaces may not be present. There is plenty of protoplasm in these cells.

Conducting Tissue : Transport of water and food to different parts of the plant is one of the main functions of

plant tissues. Tissues that help in performing this function are called conducting tissues. There are two types of

conducting tissues in plants.

(i) Xylem : Transports water and minerals absorbed by the roots to the leaves.

(ii) Phloem : Transports the food made by the leaves to the other parts of the plants.

Animal tissue : Based on the location and function, the animal tissues are classified into four types -

S .No . Type Orig in Function

1 Epithelia l tissue Ectoderm, Endoderm, Mesoderm

Protection, Secretion,Absorp tion etc.

2 Connective tissue Mesoderm Support, binding, storage p rotection, circulation

3 Muscular tissue Mesoderm Locomotion and movement4 Nervous tissue Ectoderm Control, coordination and

conduction of impulse



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EXERCISE-1 (FOR SCHOOL/BOARD EXAMS)

VERY SHORT ANSWER TYPE QUESTION

1. Why are mitochondria called the power house of the cell?

2. What name is given to the Golgi apparatus occurring in plant cells? Why is it named differently in plant cells?

3. Why are lysosomes called scavengers?

4. What are suicide bags? Why are they called so?

5. What for ATP stands?

6. What would happen if plasma membrane ruptures or breaks down?

7. Who discovered cells and how ?

8. Why is the plasma membrane called a selectively permeable membrane ?

9. Where are proteins synthesized inside the cell ?

SHORT ANSWER TYPE QUESTION

1. How does a cell act as basic structural and functional unit of an organism.

2. How is rough ER different from smooth ER? What functions do they perform in a cell?

3. What are different types of plastids? What are their functions?

4. Describe the functions of Golgi complex.

5. What would happen to the life of a cell if there was no Golgi apparatus?

LONG ANSWER TYPE QUESTION

1. Explain the structure of nucleus. What is its function?

2. What is the main function of each of the following organelles:

(i) Cell wall (ii) Plasma membrane (iii) Chromosomes (v) Mitochondria (vi) Chloroplasts

(vii) Golgi apparatus (viii) Lysosomes (ix) Centrioles (x) Vacuoles.

3. Distinguish between cell wall and cell membrane.

4. Comment of the following :

(i) Chloroplast is called "Kitchen of the cell"

(ii) Chloroplast is semi-autonomous structure

(iii) Lysosomes are Garbage disposer.

WHICH OF THE FOLLOWING ARE TRUE (T) OR FALSE (F)

1. The plasma membrane has a rigid structure.

2. Membranes are useful for compartmentalisation in cells.

3. Solid particles are ingested by pinocytosis.

4. ATP is required for active transport.

5. The ostrich egg is the largest cell known.

6. Robert Hooke discovered the nucleus.

7. Dictyosomes are found in plant cells.

8. Oxysomes are found in Golgibodies.

9. Vacuoles contain hydrolytic enzymes.

10. Golgibodies are also considered as "traffic police man of cell".



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WRITE THE TECHNICAL TERMS FOR THE FOLLOWING

(i) An organism whose body is made up of many cells.

(ii) An organism whose cells do not have well organised nucleus

(iii) The living substance present inside the cell.

COMPLETE THE FOLLOWING SENTENCES

1. Centrioles help in cell division by forming..........

2. During cell division chromatin condensed into thick cord like structures called...............

3. A compound microscope can magnify an object upto ..... times.

4. The cells having well organised nucleus are called............

5. The body of Amoeba is made up of .......cell.

6. Ribosomes are associated with the synthesis of .............

7. The cellular organelles often referred as suicide bags are .............

8. Chromosomes are made up of ..........and ..........

MATCH THE ITEMS IN COLUMN-A WITH THE ITEMS IN COLUMN-B

1. Column-A Column-B

(A) Mitochondria (p) Cell sap

(B) Lysosome (q) Cell's control centre

(C) Vacuole (r) Garbage disposer

(D) ER (s) Formation of Golgibody

& Lysosome

(E) Nucleus (t) Tonoplast

(u) Suicidal bags

(v) Having circular DNA

(w) Having Linear DNA

(x) Detoxification

(y) Energy production

COMPLETE THE FOLLOWING SENTENCES :

1. Spindle fibres 2. Chromosomes 3. 1000 4.Eukaryotic cells 5. Single 6. Protein 7. Lysosomes

8. DNA, Protein

WRITE TRUE OR FALSE FOR THE FOLLOWING :

1. F 2. T 3. F 4. T 5. T 6. F 7. T 8. F 9. F 10.T

MATCH THE FOLLOWING :

1. (A) – (v, y), (B) – (r, u), (C) – (p, t), (D) – (s, x), (E) – (q, w)

EXERCISE-1 CELLANSWER KEY



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EXERCISE-2 (FOR SCHOOL/BOARD EXAMS)

TICK THE CORRECT OPTION

1. Double membrane is absent in –

(A) Mitochondrion (B) Chloroplast (C) Nucleus (D) Lysosome

2. Animal cell is limited by–

(A) Plasma membrane (B) Shell membrane (C) Cell wall (D) Basement membrane

3. The radiant energy of sunlight is converted to chemical energy and stored as –

(A) AMP (B) ADP (C) ATP (D) APP

4. Root hair absorbs water from soil through –

(A) Osmosis (B) Active transport (C) Diffusion (D) Endocytosis

5. The barrier between the protoplasm and outer environment in a plant cell is –

(A) Cell membrane (B) Nuclear membrane (C) Cell wall (D) Tonoplast

6. An animal cell differs from a plant cell in respect of –

(A) ER (B) Cell wall (C) Ribosomes (D) Cell membrane.

7. If the nucleus is a cell's "control centre" and chloroplasts its "solar collectors". Which of the following might

be called the cell's combination "food processor" and "garbage disposer"?

(A) Lysosome (B) Ribosome (C) Golgi apparatus (D) Nucleolus

8. The longest cell in human body is –

(A) Neuron (B) Muscle fibre (C) Epithelial cell (D) Bone cell

9. Identify human cells which lack nucleus–

(A) WBC (B) RBC (C) Platelets (D) Nerve cells

10. The energy currency of a cell is –

(A) ADP (B) AMP (C) ATP (D) CTP

11. Which organelle releases oxygen?

(A) Ribosome (B) Golgi apparatus (C) Mitochondria (D) Chloroplast.

12. The term "protoplasm" to the living substance present inside the cell, was given by

(A) Robert Hooke (B) Robert Brown (C) J.E. Purkinje (D) W.Flemming

13. Ribosomes are the centre for –

(A) Respiration (B) Photosynthesis (C) Protein synthesis (D) Fat synthesis.

14. Lysosomes are the reservoirs of

(A) Fat (B) RNA

(C) Secretory glycoproteins (D) Hydrolytic enzymes.

15. The membrane surrounding the vacuole of a plant cell is called

(A) Tonoplast (B) Plasma membrane (C) Nuclear membrane (D) Cell wall

ANSWER KEYQue. 1 2 3 4 5 6 7 8 9 10 1 1 1 2 1 3 1 4 15

An s. D A C A C B A A B C D C C D A



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EXERCISE-3 (FOR COMPETITIVE EXAMS)

TICK THE CORRECT OPTION

1. Centriole is associated with –

(A) DNA synthesis (B) Reproduction (C) Spindle formation (D) Respiration

2. The cell organelle associated with cell secretion is

(A) Plastids (B) Mitochondria (C) Golgi apparatus (D) Nucleolus

3. Which of the following is an inclusion?

(A) Mitochondrion (B) Lysosome (C) Golgi complex (D) Starch grain

4. Which of the following would not be considered part of a cell's cytoplsm?

(A) Ribosome (B) Nucleus (C) Mitochondrion (D) Microtubule

5. Which of the following is called the brain of the cell?

(A) Nucleus (B) Mitochondria (C) Ribosomes (D) Plasma membrane

6. Which one is not a part of nucleus?

(A) Chromatin (B) Nucleolus (C) Centrosome (D) Nucleoplasm

7. The common feature amongst nucleus, chloroplast and mitochondrion is –

(A) DNA (B) Lamellae (C) Cristae (D) All of these

8. Nucleus is separated from surrounding cytoplasm by a nuclear envelope which is –

(A) Single and porous (B) Double and porous (C) Single and nonporous (D) Double and nonporous

9. Nucleoplasm is continuous with cytoplasm through –

(A) Centriole (B) Golgi apparatus (C) Nuclear pores (D) Endoplasmic reticulum

10. Nucleolus was discovered by

(A) Fontana (B) Schleiden (C) Altmann (D) Robert Brown

11. The function of the nucleolus in the cell is

(A) Secretory (B) Synthesis of DNA

(C) Synthesis of RNA and ribosomes (D) None of these

12. Which of the following phenomena is commonly referred as 'cell drinking'?

(A) Exocytosis (B) Pinocytosis (C) Endocytosis (D) Phagocytosis

13. The cell organelle taking part in photorespiration is

(A) Glyoxysome (B) Dictyosome (C) Peroxisome (D) Endoplasmic reticulum

14. Endoplasmic reticulum sometime contains –

(A) Ribosomes (B) Lysosomes (C) Golgi bodies (D) None of these

15. Ribosomes are composed of –

(A) 1 subunit (B) 5 subunits (C) 2 subunits (D) 4 subunits

ANSWER KEYQue. 1 2 3 4 5 6 7 8 9 10 1 1 1 2 1 3 1 4 15

An s. C C D B A C A B C A C B C A C



CBSE Class 8 Science Cell Chapter Notes

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