CBSE CLASS 9 CHAPTER 5 NOTES

CHAPTER 5

CBSE SCIENCE
CLASS 9

THE FUNDAMENTAL UNIT OF LIFE

DISCOVERY OF CELL

·        Robert Hooke was the first person to see the cell in a slice of cork in 1665.

·        He named it as ‘Cell’ which means – little room in Latin.

·        Anton Leeuwenhoek (1674) discovered free living cells in pond water.

·        Robert Brown (1831) discovered nucleus.

·        Purkinje (1839) coined the term Protoplasm for the fluid substance in the cell.

·        Schleiden (1838) & Schwann (1839) proposed the Cell Theory.

·        Virchow (1855) expanded the Cell Theory.

 

CELL THEORY

·        All the plants & animals are made up of cells & the cells are the basic unit of life (Schleiden & Schwann)

·        All cells arise from pre-existing cells (Virchow)

 

CELL – ITS SIZE & SHAPE

·        Organisms are classified into 2 types based on the number of cells they possess.

o    (i) Unicellular Organisms – that are made of only one cell

o    Example: Amoeba, Chlamydomonas, Paramoecium & bacteria

o    (ii) Multicellular Organisms – that are made of more than one cell.

o    Example: Plants, Animals, Humans, etc.,

·        The shape & size of the cell differ from each other according to the function they perform.

·        Each cell has certain specific components within it known as Cell organelles.

·        Each cell organelle performs a special function.

 

WHAT IS A CELL MADE UP OF?

·        Each cell has 3 major parts

    o   (i) Plasma membrane

o   (ii) Nucleus &

o   (iii) Cytoplasm

 

PLASMA MEMBRANE OR CELL MEMBRANE

·        This is the outermost covering of the cell

·        It separates the cell contents from its environment

·        It allows only certain materials to pass in & out of the cell & so called as selectively permeable membrane

 

MOVEMENT OF SUBSTANCES ACROSS PLASMA MEMBRANE

·        DIFFUSION – the movement of gases from a region of higher concentration towards a region of lower concentration

·        Example: Exchange of gases (like O₂ & CO₂) during respiration

·        OSMOSIS – the movement of water molecules from a region of higher concentration towards a region of lower concentration through a semi-permeable membrane

·        The amount of substance dissolved in water is a main factor that affects Osmosis

 

OSMOSIS IN THREE DIFFERENT MEDIUM

(i) HYPOTONIC SOLUTION

·        Solution with less solute (sugar/salt) & more solvent (water)

·        If cell is kept in such a medium, the cell will gain water (as the water from the medium i.e., higher concentration move towards the cell i.e., in lower concentration)

·        As a result, the cell swells up

(ii) HYPERTONIC SOLUTION

·        Solution with more solute (sugar/salt) & less solvent (water)

·        If cell is kept in such a medium, the cell loses water (as the water from the cell i.e., in higher concentration moves out towards the medium i.e., in lower concentration)

·        As a result, the cell shrinks in size

(iii) ISOTONIC SOLUTION

·        Solution which has the same water concentration as that of the cell

·        If the cell is kept in such a medium, the movement of water inside the cell is equal to the movement of water outside the cell

·        As a result, the cell size remains the same

OSMOSIS IN LIVING BEINGS

·        Unicellular freshwater organisms & most plants gain water through osmosis

·        Osmosis occurs only in live cells

·        Plasma membrane is flexible & is made up of lipids & proteins

·        In Amoeba, the flexibility of the plasma membrane allows it to engulf food material. This process is called Endocytosis

 

CELL WALL

·        Plant cells in addition to the plasma membrane have a rigid outer covering called the cell wall

·        Cell wall is absent in animal cells

·        Cell wall lies outside the plasma membrane in plant cells

·        It is made up of cellulose

·        Cellulose provides strength to plants

·        During osmosis, the cell loses water & its contents shrinks away from cell wall. This phenomenon is called Plasmolysis

·        During osmosis, when cell absorbs water the cell swells, building up pressure against cell wall

·        The wall exerts an equal pressure against the swollen cell & prevents the cell from bursting

 

NUCLEUS

·        Nucleus is present at the center of the cell

·        Nucleus has a double layered covering called nuclear membrane

·        The nuclear membrane has pores which allow the transfer of materials in & out of the nucleus

·        Nucleus contains chromosomes which are visible as rod shaped structures only at the time of cell division

·        Chromosomes contain DNA (Deoxyribo Nucleic Acid) which carries inherited characteristics from the parents to the next generation

·        Chromosomes are made of DNA & protein

·        DNA also contains information necessary for constructing & organising cells

·        Functional segments of DNA are called Genes

·        When the cell is not dividing, DNA is present as chromatin material which is present as entangled mass of thread

·        During cell division chromatin gets organised into chromosomes

·        The nucleus plays a central role in cellular reproduction

·        It also plays an important role in determining the way the cell develops & exhibits maturity

·        Based on the nucleus the organisms are classified into

·        Prokaryotes &

·        Eukaryotes

 

(I) PROKARYOTES

·        Pro – primitive; karyote – nucleus

·        Organisms that have a poorly defined nuclear region

·        This undefined nuclear region contains only nucleic acids called nucleoid

·        They do not have nuclear membrane

·        Prokaryotes also lack other cytoplasmic organelles

·        Example: Bacteria

(II) EUKARYOTES

·        Eu – true; karyote – nucleus

·        Organisms that have a well defined nucleus

·        Nucleus is covered by nuclear membrane

·        Eukaryotes have cytoplasmic organelles

·        Example: Plants, Animals

 

CYTOPLASM

·        Cytoplasm is the fluid content inside the cell

·        It contains many cell organelles

·        Each cell organelle performs a specific function

·        Cell organelles are enclosed by membrane (in case of eukaryotic cells)

 

CELL ORGANELLES

·        Each cell organelle has a membrane covering to separate its content from its environment

·        This is a characteristic feature of an eukaryotic cell. Cell organelles include:

o   (i) Endoplasmic Reticulum

o   (ii) Golgi apparatus

o   (iii) Lysosomes

o   (iv) Mitochondria

o   (v) Plastids &

o   (vi) Vacuoles

 

(I) ENDOPLASMIC RETICULUM

·        ER is a large network of membrane bound tubes

·        The ER membrane is similar in structure to the plasma membrane

·        There are 2 types of ER

·        Rough Endoplasmic reticulum (RER)

·        Smooth Endoplasmic reticulum (SER)

·        RER is rough because of the presence of ribosomes on its surface

·        Ribosomes are the site of manufacture of proteins

·        SER manufactures fat molecules or lipids

·        Some of these proteins & lipids help in building the cell membrane

·        This process is called biogenesis

·        Other proteins & lipids functions as enzymes & hormones

·        ER acts as transport system of materials (such as proteins) to various regions of the cell

·        In the liver cells of vertebrates, SER plays a crucial role in detoxifying many  poisons & drugs

 

(II) GOLGI APPARATUS

·        It was first described by Camillo Golgi

·        It consists of membrane bound vesicles arranged parallely as stacks called as Cisterns

·        The membranes of golgi apparatus are connected with the membranes of ER

·        The materials manufactured in ER are packaged & dispatched inside & outside the cell by golgi apparatus

·        The functions of golgi apparatus includes storage, modification & packaging of products in vesicles (bags)

·        Golgi apparatus may help in the formation od complex sugars from simple sugars

·        Lysosomes are produced by golgi apparatus

 

(III) LYSOSOMES

·        Lysosomes helps in disposal of waste from the cell

·        It keeps the cell clean by digesting any foreign material that enters the cell & also the worn out cell organelles

·        Any foreign bacteria or food that enters the cell ends up in lysosomes that breaks them into smaller pieces 

·        Lysosome is a kind of a bag that contains powerful digestive enzymes

·        When the cell dies, the lysosome burst to emit the enzymes which will digest their own cell

·        Therefore lysosomes are  called ‘Suicide bags’ of the cell

·        These digestive enzymes present in lysosomes are made by RER

 

(IV) MITOCHONDRIA

·        Mitochondria are known as the power house of the cell

·        The energy required for various metabolic processed is released by mitochondria in the form of ATP

·        ATP – Adenosine Tri Phosphate  - is known as the energy currency of the cell

·        Mitochondria has two membranes

o   (i) Outer membrane – porous

o   (ii) Inner membrane – deeply folded

·        The deep folds in the inner membrane creates large surface area for ATP generation

·        Mitochondria have their own DNA & ribosomes. Therefore they can prepare their own proteins

 

(V) PLASTIDS

·        Plastids are present only plant cells

·        There are two types of plastids

o   (i) Chromoplasts (coloured plastids)

o   (ii) Leucoplasts (white or colourless plastids)

·        Plastids having green pigments (chlorophyll) are called as chloroplasts

·        Chloroplasts are important for photosynthesis in plants

·        Chloroplasts also contain yellow or orange pigments

·        Leucoplasts store oil, starch & protein granules & so they are colourless

·        The internal structure of plastids contain numerous membrane layers embedded on stroma

·        The external structure of plastids is similar to mitochondria

·        Plastids have their own DNA & ribosomes like Mitochondria

 

(VI) VACUOLES

·        Vacuoles are storage sacs for solid or liquid contents

·        They are very small in animal cells & very large in plant cells

·        In plant cells, vacuoles are full of cell sap that provides rigidity & turgidity to the cells

·        Vacuoles also store amino acids, sugars, various organic acids & some proteins

·        In Amoeba, the food vacuole contains the food that they consume

·        In some unicellular organisms, specialised vacuoles play important roles in expelling excess water & wastes

 

CELL DIVISION

·     New cells are formed in organisms

    o  To grow

    o  To replace old, dead & injured cells

    o  To form gametes – for reproduction

·     The process – by which new cells are made – called Cell Division

·     2 main types of Cell division

    o  Mitosis

    o  Meiosis

 

MITOSIS

·     Process of cell division – by which – most cells divide for growth

·     Each cell – called Mother cell – divides to form – 2 identical daughter cells

·     Daughter cells – have same number of Chromosomes as the Mother cells

·     Mitosis – helps in growth & repair of tissues in organisms

 

MEIOSIS

·     Process of cell division – by which – cells of reproductive organs or tissues of animals & plants – divide – form gametes

·     Gametes – on fertilisation – gives rise to offsprings

·     Meiosis – involves 2 consecutive divisions

·     Cell – When divides by meiosis – produce 4 new cells – instead of just 2

·     New cells – only have half the number of chromosomes (Haploid [n])– than the mother cells  (Diploid [2n])

 

WHY CHROMOSOME NUMBER REDUCE TO HALF IN DAUGHTER CELLS?

·     Male & female gametes – fuse together to make up the total number of chromosomes in an individual

·     If there is no meiotic reduction – number of chromosomes – increase every time the next generation is produced

·     If there are 2 diploid parents (2n) – gametes will be diploid (2n – egg; 2n – sperm) – child will be tetraploid (4n)

·     Results in creation of new species – everytime there is production of new generation

 

DIFFERENCE BETWEEN MITOSIS & MEIOSIS

CONCLUSION

·        Thus each cell acquires its structure & ability to function because of the organisation of its membrane & organelles

·        Thus the cell performs functions like respiration, nutrition, clearing of waste material & forming new proteins

·        Thus cell is the fundamental & structural unit of living organisms