CBSE Class 9 Science The Fundamental Unit Of Life Notes

FUNDAMENTAL UNIT OF LIFE

CELL

1. All living forms are composed of microscopic units called as “Cells”.
2. A cell is the basic structural and functional unit of all life forms.
3. Study of structure and composition of cell is called as “Cytology”.
4. Cell was first observed by “Robert Hooke” in a dead cork slice in the year 1665. He described about

This in his book “Micrographic”.

1. The word cell was derived from a Greek word “Cellulae” which means small room.
2. First living cell was discovered by A.V. Leeuwenhoek.
3. The term protoplasm was coined by purkinje in 1839.
4. Protoplasm was discovered by “Felix Dujardin” and named as sarcode.
5. It’s consistency differs under different condition. It exists in sol-gel states.
6. Protoplasm is an aggregate of various chemicals such as water, ions, salts and other organic molecules  like proteins, carbohydrates, fats, nucleic acids, vitamins etc.

CELL THEORY

Two biologists, “Schleiden and Schwann” gave the “Cell theory” which was later on expanded by “Rudolf Virchow”. Cell theory states that

(i) All plants and animals are composed of cells.

(ii)  Cell is the basic unit of life.

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

Viruses are the exceptions of cell theory.

TYPES OF CELL & ORGANISM

(a) On the Basis of Number of Cells Organisms can be categorized as:

1. Unicellular organisms: These are organisms which made up of single cell only. This single cell  Performs all the vital body functions of an organism. e.g. Amoeba 
2. Multicellular organisms: These are the organisms which made up of numerous cells. These cells then combine to form an organ and group of organs performing different functions forms an organ system which further forms an organism. e.g. plants and animals

(b)   On the basis of type of organization, cells are two types:

1. Prokaryotic cells: these are primitive and incomplete cells. They have less developed nucleus without nuclear membrane & nucleolus. g. Bacteria.
2. Eukaryotic cells: these are well developed cells. They have advanced nucleus with unclear membrane and nucleolus. eg. Plants & animals.

Cells are of variable shapes and sizes. Their shape is according to the function. Generally cells are spherical but they may be elongated (nerve cell), branched (pigmented), discoidal (RBC), spindle shaped (muscle cell) etc.

CELL SIZE

Size of cell is variable depending upon the type of organism. Some are microscopic while some are visible with naked eyes.

Their size may vary from 0.2 µm to18 cm.

1. Size of a typical cell in a Multicellular organism ranges from 20-30 mn.
2. The largest cell is ostrich egg(15 cm. in dia with shell & 8 cm. in dia without shell)
3. The longest cell is nerve cell.(upto 1m. or more)
4. Smallest cells so far known are PPLOs e.g. mycoplasma (0.1 µm in dia.)
5. Human egg is 0.1 mm. in dia.

COMPONENTS OF CELL

There is an occurrence of division of labour within a cell as they all got certain specific components called “Cell organelles” each of them perform a specific function.

The three basic components of all the cells are

(i)  1M (Plasma Membrane)  (ii) Nucleus   (iii) Cytoplasm

CELL MEMBRANE

(a) Cell Membrane:

1. Cell membrane is also called as plasma Membrane or Plasma lemma.
2. It is the limiting boundary of each cell which separates the cytoplasm from its surroundings.
3. It is found in both plant as well as animal cells.
4. It is the outer most covering of a cell in case of animals and lies below the cell wall in case of plants.
5. It is made up of proteins and lipids where proteins are sandwiched between bilayer of lipids.
6. Plasma membrane name was given by Nageli.
7. Plasma membrane is selectively permeable in nature. It allows or permits the entry and exit of some materials in and out of the cell.
8. Singer and Nicholson gave the fluid mosaic model of plasma membrane according to him it consists of a protein layer sandwiched between two layers of lipids. It is in quasifluid state. It is 75A thick.
9. It is flexible and can be folded, broken and reunited.

Function of plasma membrane:

(A) It regulates the movement of molecules inside and outside the cell.

(B) It helps in maintaining the distinct composition of the cell.

(ii) Transportation of molecules across  the plasma membrane: This can be done by following ways:

(A)  Diffusion: Movement of solutes or ions from higher concentration to lower  concentration is called as diffusion. It does not require energy therefore it is called as passive transport.

(B) Osmosis: The movement of solvent or water from higher concentration (solvent) to lower concentration (solvent) through a semipermeable membrane is called as osmosis. Or The movement of solvent or water from lower concentration to higher concentration of solution through a semipermeable membrane is called as osmosis. Osmosis can also be called as diffusion of solvents”.

1. Endosmosis: Movement of solvent into the cell is called as Endosmosis.
2. Endosmosis: Movement of solvent outside the cell is called as Endosmosis.

(iii) Types of solution on the basis of concentration:

(A)Isotonic solution: When the concent, ation of the solution outside the equal to the  

Concentration of cytoplasm of the cell it is called as isotonic solution.

(B) Hypertonic solution: When the of concentration of the solution outside the cell is more than that inside the cell. Due to this cell looses water and becomes plasmolysed.

(C) Hypotonic solution: When the of concentration of the solution outside the cell is lesser than that of cytoplasm of cell. Due to this cell swells up and bursts.

(b) Cell Wall:

1. It is the outermost covering of the plant cells.
2. It is absent in animal cells.
3. Cell wall is rigid, strong, thick, porous and non living structure. It is made up of cellulose and hemicelluloses. Cell walls of two adjacent cells are joined by a layer called middle lamellae. It is made up of calcium and magnesium pectate.
4. Functions of cell wall:
5. It provides definite shape to the cell.
6. It provides strength to the cell.
7. It is permeable and allows entry of molecules of different sizes.
8. It is antigen specific.
9. It has the characteristics of repair and regeneration.

Nucleus

Figure: Nucleus

1. Nucleus is the most important cell organelle which directs and controls all its cellular activities.
2. It is called as “Headquarter of the cell”.
3. It was discovered by “Robert Brown in 1831”.
4. In eukaryotes a well defined nucleus is present while in prokaryotes a well defined nucleus is absent.
5. Prokaryotes contain a primitive nucleus.
6. It has double layered covering called as nuclear membrane.
7. Nuclear membrane has pores which regulate the movement of materials of materials in & out of the cell.
8. Besides nuclear membrane nucleus also contains nucleolus and chromatin material and the substance filled inside the nucleus is nucleolus or karyolymph.
9. Chromosomes or chromatin material consists of DNA which stores and transmits hereditary information for the cell to function, grow and reproduce.

(i) Function of the nucleus:

(A) It controls all the metabolic activities of the cell and regulates the cell cycle.

(B) It helps in transmission of hereditary characters from parents to off springs

CYTOPLASM           

1. Cytoplasm was discovered by Kolliker in 1862.
2. It is the site of both biosynthetic and catabolic pathways.
3. It can be divided into two parts:

(i)   Cytosol: Aqueous soluble part contains various fibrous proteins forming cytoskeleton.

(ii)  Cell organelles: Living part of the cells having definite shape, structure and function bounded By Plasma membrane.

ENDOPLASMIC RETICULUM

1. It is the network of membranes present in the cytoplasm.
2. It was discovered by Porter, Claude and Fullam.
3. These are present in all cells except prokaryotes and mammalian erythrocytes.
4. They are made up of three components:
5. Cistemae: These are long, flattened, parallely arranged, unbranched tubules. These from Successive layers of nucleus. These are found in cells which are active in protein synthesis and are 40-50 µm in diameter.
6. Vesicles: These are around or spherical they are founded in synthetically active cells.

Types :

Function of ER:

1. It is the only organelle which can move within a cell so it serves as a channel for the transport of materials between various regions of cytoplasm and between cytoplasm and nucleus.
2. It also function as a cytoplasmic framework to provide space for some of the biochemical activities. It forms endoskeleton of cell.
3. It helps in synthesis of fats, steroids, cholesterol etc.
4. It contains secretory proteins.
5. SER plays a crucial role in detoxification of drugs and poisonous by-products.

 GOLGI APPARATUS

1. Golgi apparatus consists of a system of membrane bounded vesicles arranged parallel to each other in stacks called Cisternae along with some large and spherical vacuoles.
2. It was discovered by Camilo Golgi.
3. In plants Golgi membrane bounded.
4. It is single membrane bounded.
5. It is absent in prokaryotes, mammalian RBC’s & sieve cells.

(a)Functions:

1. It helps in formation of lipids
2. It helps in formation of middle lamellae
3. It is secretary in nature.
4. It helps in melanin synthesis
5. Lipids and proteins synthesized in endoplasmic reticulum are packed at Golgi complex. They provide the site for assembly of new membrane material.

MITOCHONDRIA 

1. It is a rod shaped structure found in cytoplasm of all eukaryotic cells except mammalian RBC’s.
2. These are also absent in prokaryotes.
3. It was first seen by Kolliker in insect cells.
4. Maximum mitochondria are found in metabolically active cells.
5. It is also called as “Power House of the Cell” or the “Storage Battery”.
6. It is double membranous structure where outer membrane has specific proteins While inner membrane is folded inside to from chambers called Cristae. ”Cristae” are the infoldings of inner mitochondrial membrane that possess enzymes for respiratory cycles like Kreb Cycle. ATP synthesizing units are called Oxysomes or F₁ particles.
7. Space between inner and outer mitochondrial membranes is called as Perimitochondrial space. The fluid present in mitochondrial is called as matrix. 

 (a) Functions:

1. Its main function is to produce and store the energy in the form of ATP.
2. It is the site of Kreb cycle of respiration.
3. Oxysome contains enzymes for ATP production.
4. Matrix contains enzymes for Kreb cycle.

RIBOSOMES

Ribosomes are the sites of protein synthesis.

All structural and functional proteins (enzymes) coded by the nuclear DNA, are synthesized upon cytoplasmic ribosomes. The DNA codes are transcripted into messenger RNA (mRNA) molecules in the chromosomes of the nucleus. mRNA molecules diffuse out  into the cytoplasm and each becomes attached to several ribosomes which thus from a group called polyribosome or polyribosomes. In this way each mRNA molecule brings about polymerization of specific protein molecules, with the help of ribosomes from amino acid molecules found in the Cytosol.

PLASTID

1. It is double membranous discoidal structure, fond only in plant cells.
2. Term plastid was given by Haeckel.
3. Chloroplast was discount by A.V. Leeuwenhoek and named by Schimper.
4. Besides being discoidal of rhombic in plant cells they occur in variable shapes like in algae they can be ‘U’ shaped , spiral , coiled , ribbon shaped etc.

Depending upon the type of pigment present in them they are of following three types.

1. Grana: It constitutes the lamellar system. These are found layered on top of each other, these stacks are called as Grana. Each granum of the chloroplast is formed by superimposed closed compartments called Thylakoids.
2. Chloroplast have Following Two Parts :
3. Function: They are the sites of light reaction of photosynthesis as they contain photosynthetic pigment chlorophyll. In each thylakoid Quantasomes are present which are called as Photosynthetic units. Each quantansome possesses 230 chlorophyll molecules.
4. Stroma: It is a granular transparent substance also called as matrix. Grana are embedded in it. Besides Grana they also contain lipid droplets, starch grains, ribosomes etc.

Function: This is the site of dark reaction of photosynthesis. Also helps in protein synthesis due to presence of ribosomes.

VACUOLES

1. These are membrane bounded regions in the cytoplasm containing water and other substances.
2. They are bounded by a single membrane called Tonoplast.
3. In animal cells vacuoles are smaller in size and numerous while in plant cells a single large vacuole Is found which occupies about 90% of the volume of cell.

 (a) Functions:

1. It helps in maintaining osmotic pressure in a cell.
2. It stores toxic metabolic products of plant cell.
3. It contains various Coloured pigments like anthocyanins.

 LYSOSMES

 (Discovery: Christian de Duve) (Lyso = digestive, some = body)

1. These are tiny sac like granules containing enzymes of intracellular digestion.
2. They are bounded by a single membrane.
3. They occur in animal cells and a few plant cells.
4. They do not have a definite shape or size.
5. They contain hydrolyzing enzymes called acid hydrolyses.

(a) FUNCTION:

1. Their main function is phagy = digestion
2. They are kind of waste disposal system.
3. They help in digesting foreign materials and wom out cells.
4. During disturbances in cellular metabolism i.e. in case of cell damage lysosomes burst and their enzymes are released into the cytoplasm and they digest their own cell so they are also called as “Suicidal Bags”.

PEROXISOMES

1. These structures were first described from liver and kidney cells by Rodhin(1954)
2. In plant cells, they were first observed in germinating seeds by Tolbert(1969)
3. The term ‘peroxysomes’ was first used by de Duve and also called as uricosomes.
4. Peroxysomes are ovoid or granular structures, limited by a single unit membrane and have a diameter of 0.5 to µm.
5. In green leaves of C₃ plants, peroxysomes carry out photorespiration.
6. In animal cells they carry out lipid metabolism.
7. They contain important enzymes as oxidases (peroxide producing enzyme), peroxidases and catalases (which break down toxic peroxides to water and oxygen).

GLYOXYSOMES

1. A beaver (1961) was the first person to discover these organelles and were described later by R.W. Briedenbach (1967).
2. They are about 0.5 to 1 µm in size and are surrounded by a single unit membrane. 
3. They are found in plant cells, particularly, in germinating fatty seeds e.g. Ricinus (castor) and groundnut where fat is being converted into carbohydrates by a process called glyoxylate cycle.
4. Glyoxysomes contain important enzymes, isocitrate, lyase, maltase and Synthelast along with several others.
5. Structure of glyoxysomes is similar to peroxisome.

DIFFERENCES BETWEENA PLANT CELL AND ANANIMAL CELL

SOME IMPORTANT POINTS

1. Centrosome :
2. It is found in animal cell.
3. It consists of two centrioles.
4. It is non membranous.
5. It is made up of microtubules.
6. It helps in cell division and spindle formation.
7. Cilia and flagella: These are thread like appendages used for locomotion and emerge from basa body.
8. Chromosomes: These are thread like structures containing hereditary information in from of genes.
9. There are four different types of membranes on the basis of permeability:

(A)Permeable: They allow diffusion of both solvent and solute molecules and icon through them. e.g. Cell wall of plant cell.

(B) Impermeable: They prohibit the diffusion of both solvent and solute particles through them. e.g. Cutinized cell wall, Lignified cell wall.

(C) Semipermeable: Allows diffusion of solvent molecules but do not allow the passage of solute molecules. e.g. Parchment paper, Kidney membrane.

(D) Differentially Permeable: It allow some solutes to pass through them along with the solvent molecules. E.g. Plasmalemma, Tonoplast.

1. Cell organelles can be separated by the method of differential centrifugation. 
2. First cell that developed in laboratory by Sydney Fox and A.I. Oparin was called as Coacervate.
3. Smallest cell is Mycoplasma (PPLO)(0.1 µ dia)
4. Largest cell is Ostrich egg (15 cm.dia.)
5. Longest Animal Cell is Nerve fibre (1m.)
6. Lysosomes exhibit polymorphism i.e. they occur in different forms.
7. Mitochondria, plastid and centrioles have their own DNA molecules so they are called as “Semiautonomous Cell Organelles ”.
8. Connection through which cells communicate chemically with each other through their  thick walls are called as “Plasmodesmata”.
9. Cell wall of fungi is made up of chitin instead of cellulose.
10. Protoplasm was called as “Physical Basis of Life” by Huxley
11. DNA is called as “Chemical Basis of life”.
12. Euglena is the connecting link between plants and animals as it lacks cell wall but has plastids.
13. Mesosomes in bacteria are analogous to mitochondria as they both help in cellular respiration.
14. Centre for cellular and molecular biology is situated at Hyderabad.
15. In animal cell the ‘Cell Coat’ is present instead of cell wall which provides them  protection. It is made up to glycocalyx.
16. ATP (Adenosine Triphosphate) is the energy currency of the cell i.e. energy in cell is stored in from of ATP molecules.

“The Fundamental Unit Of Life Summary”

1. All the living organisms are made up of fundamental unit of life called” cell”.

2. The cell is a Latin word for “a little room”.

3. The scientist Robert Hooke saw a little room in the cork (the bark of a tree) resembled the structure of a honeycomb. The use of the word “Cell” to describe these units is used till this day in Biology as” Cell Biology”.

4. The Compound Microscope consist eye piece,objective lens and condenser to observe a cell after putting a drop of Safranin (for plant cell) and methylene blue (for animal cell). 

5. The scientist Leeuwenhoek saw free living cells in the pond water for the first time. ( father of microbiology)

6. The scientist Robert Brown discovered the nucleus in the cell.

7. The cell theory states that all the plants and animals are composed of cells, it was proposed by Schleiden and Schwann.

8. The cell theory was further expanded by Virchow by suggesting that “all cells arise from the pre-existing cells”.

9. The cells differ in size, shape, structure ( Onion peel/Various cells in Human body ) :Types of cells: Onion cells, Smooth muscle cell, Blood cells, Bone cell, Fat cell, Nerve cell, Ovum, Sperm etc. Each kind of cell performs specific function.

10. A single cell may constitute a whole organism as in Amoeba, Chlamydomonas, Paramecium and Bacteria; these are called as unicellular organisms. Whereas in multi-cellular organisms (Human beings) division of labor is seen.

11. The feature in almost every cell is same: Plasma membrane, nucleus and cytoplasm.

12. Plasma membrane: It is the outermost covering of the cell.

- It is called as selective permeable membrane (because it prevents movement of some materials).

- It helps in diffusion and osmosis

- Diffusion: movement of substance from high concentration to low concentration.

Eg; exchange of carbon dioxide or oxygen with external environment.

osmosis: it is the passage of water from the region of high water concentration to a region of low water concentration through a selective permeable membrane.

a) The cell gains water, if the medium surrounding the cell has a higher water concentration (Hypotonic solution) than the cell.

b) The cell maintains the same water concentration as the cell (Isotonic solution), water crosses the cell membrane in both directions.

c)The cell loses water, if the medium has lower water concentration (Hypertonic solution) than the cell.

Note - The cell drinking is endosmosis;

- omission of water is called ex-osmosis.

13. The cell engulfs food is called endocytosis and ejects solid is called exocytosis. Amoeba acquires food through endocytosis and excretion of solid is called exocytosis.

14. The cell wall is a rigid outer covering composed of cellulose. It provides structural strength to plant c When a living cell loses water, there is shrinkage of contents of a cell away from the cell wall. This phenomenon is called as plasmolysis. The cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (Hypotonic) external media without bursting.

15. The Nucleus: It is a dark colored, spherical or oval, dot-like structure near the center of a cell called Nucleus. The nucleus plays a central role in cellular activities/ reproduction. The chromatin material gets organized into chromosomes. The chromosomes contain information for inheritance of features from parents to next generations in the form of DNA( Deoxyribo Nucleic Acid ) and protein molecules. The functional segments of DNA are called genes.

16. In some organisms like Bacteria nucleus is not covered by nuclear membran Hence it is called as prokaryote. (Pro= primitive; karyote = karyon = nucleus.) The organisms with cells having a nuclear membrane are called eukaryotes.

17. Differences between prokaryotes and eukaryotes

18. Cell organelles: Every cell has fluid matrix (other than nucleus) is called cytoplasm. The nucleus and cytoplasm is together called as protoplasm. The protoplasm term was coined by Purkinje. It has important cell organelles: Endoplasmic Reticulum (ER), Golgi apparatus, Lysosomes, Mitochondria, Plastids, and vacuoles.

19. Endoplasmic Reticulum (ER): It is a large network of membrane –bound tubules and vesicles.

- There are two types of Endoplasmic Reticulum

- Rough Endoplasmic Reticulum (RER) (It looks rough because Ribosomes are attached to its surface. They are the sites of protein synthesis).

- Smooth Endoplasmic Reticulum (SER) (It looks smooth because Ribosomes are not attached to its surface. They are the sites of fat molecules synthesis).

1. SER ; help in the functioning of enzymes and hormones to carryout biochemical activities.
2. SER detoxifies many poisons and drugs from the cell

2. ER serves as channel for the transport of material between various regions of the cytoplasm and the nucleus

3. Proteins and fat molecules produced by ER helps in membrane biogenesis

20. Golgi apparatus: It was first described by a scientist Camillo It is a system of membrane bound vesicles called cisterns. It functions include the storage, modification and package of cell products. The complex sugars are made from simple sugars in the Golgi apparatus. It is also involved in the formation of lysosomes.

21. Lysosomes: They contain membrane-bound sacs with powerful digestive enzymes (enzymes are made by RER) to digest the worn-out cell organelle When the cell gets damaged, lysosomes may burst and the enzymes digest their own cell, hence called as “Suicidal bags of a cell”. It is a waste disposal system of the cell.

22. Mitochondria: It is covered by a double membran Outer membrane is very porous and the inner membrane is deeply folded. These folds create a large surface area for ATP (Adenosine Triphosphate) molecule synthesis. ATP is the energy currency of a cell; hence the Mitochondria are called as Power House of a Cell. Mitochondria have their own DNA and Ribosomes; therefore they can make their own proteins.

23. Plastids: They are present only in plant c They are two types.

1. Chromoplasts (Colored Plastids: Chloroplasts – Green pigmented and useful in Photosynthesis and also contains various other pigments like yellow or orange)

2. Leucoplasts (White or colorless plastids; stores materials such as oils, proteins, fats etc. ) Plastids are also covered by a double membran The matrix is called Stroma, seat for enzymatic actions. Plastids have their own DNA and Ribosomes; therefore they can make their own proteins.

24. Vacuoles: Storage sacs for solid or liquid content They are small in size in animals while plants have large, may occupy 50-90 % of the cell volume. Helps to provide turgidity and rigidity to the cell. Many substances like amino acids, sugars, organic acids and proteins are stored in vacuoles. In Amoeba food vacuole is specialized to play an important role.

25. Cell: It is the fundamental structural unit of living organisms, helps in respiration, obtaining nutrition and clearing waste material or forming a new protein. Differences between Plant cell and Animal Cell

Key learnings:

1) In 1665, Robert Hooke first discovered and named the cells.

2) Cell is the structural and functional unit of all living organisms.

3) Organisms may be unicellular or multicellular. A single cell constitutes the unicellular organism whereas many cells coordinately function in case of multicellular organism.

4) The size, shape and volume of the cell are related to the specific function that they perform.

5) A cell generally shows plasma membrane, nucleus and cytoplasm

6) Plasma membrane is a thin, selectively permeable membrane, covering the cell and is made up of lipids and proteins.

7) Functions of plasma membrane :

i) It separates the contents of a cell from its outside environment.

ii) It regulates the flow of substances to and from the cell through diffusion, facilitated diffusion, active transport and endocytosis.

8) Osmosis is diffusion of water through a selectively permeable membrane.

9) A cell placed in different solutions:

i. hypotonic solution : A cell placed in it will gain water.

ii. hypertonic solution: A cell placed in it will lose water: Also known as plasmolysis.

iii. isotonic solution: A cell placed in it will neither gain nor lose water

10) Cells of Plants, fungi & bacteria: Contain both plasma membrane & cell wall. Cell wall is rigid, non-living & outer most covering, composed mainly of cellulose.

11) When placed in hypertonic solution, a living plant cell shows plasmolysis

12) Cell wall provides mechanical strength to the cell. It permits the cell to withstand huge changes in the surrounding medium.

13) Nucleus is an important, spherical, usually centrally located constituent of the cell and is bounded by double layered nuclear envelope.

14) The nucleus of a dividing cell shows rod-shaped chromosomes, made up of DNA and proteins. In a non-dividing cell, the chromosomes elongate and take the form of thread-like chromatin.

15) DNA molecules are responsible for transmitting hereditary information from one generation to the next.

16) Nucleus controls all metabolic activities of the cell.

17) Depending on the presence or absence of nucleus, cells may be prokaryotic or eukaryotic.

i) Prokaryotic cells lack a well-defined nucleus and instead show nucleoid, an undefined nuclear region containing the genetic material.

ii) Eukaryotic cells possess a proper nucleus with nuclear membrane.

18) Cytoplasm is the fluid content of the cell, occurring between nucleus and plasma membrane. It stores several vital chemicals and is the site of certain important metabolic pathways.

19) Several specialized cell organelles are present in the cytoplasm. These organelles perform different kinds of metabolic activities and are kept separate from each other.

20) The various cell organelles include endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids, vacuoles and centrosome.

21) Endoplasmic reticulum (ER) is an extensive, interconnected, membranebound network of tubes and sheets.

22) Ribosomes are attached to the surface of Rough Endoplasmic Reticulum (RER) and are absent in Smooth Endoplasmic Reticulum (SER)

23) Functions of ER :

i) It synthesizes important proteins (RER) and lipids (SER).

ii) It provides a pathway for intracellular transport of materials.

iii) SER of liver cells is important for detoxification.

24) Golgi apparatus is a network of stacked, flattened, membrane bound sacs and vesicles.

25) Golgi apparatus carries out the storage, modification and packaging of substances manufactured in the cell and is also involved in lysosome formation.

26) The spherical, sac-like lysosomes contain powerful digestive enzymes and form the waste disposal system of the cell. They are also known as ‘suicide bags’.

27) Mitochondria and plastids are each covered by 2 membranes and possess their own DNA and ribosomes.

28) Mitochondria are the ‘powerhouses of the cell’, providing energy for various metabolic activities.

29) Chromoplasts and leucoplasts are the 2 types of plastids present in plant cells.

30) Chloroplasts are chromoplasts containing chlorophyll and carry out photosynthesis in plants.

31) Leucoplasts store starch, oil and protein granules.

32) The large central vacuole of mature plant cells provides turgidity to the cell and also stores important substances.

33) In unicellular organisms, vacuoles play important roles in nutrition and osmoregulation.

34) Ribososmes are sites of protein synthesis.

35) Centrosome is found only in animal cells and consists of 2 centrioles. Centrosome helps in cell division.

37) The membrane-bound cell organelles are absent in prokaryotic cells.

38) Differences between plant cell and animal cell.

39) The basic structural organization of the cell helps it to perform important functions like respiration, nutrition, excretion and protein synthesis.

Top definitions

1) Cell - An autonomous self-replicating structure that forms the structural, functional and biological unit of all living organisms.

2) Prokaryotic cell – A cell characterized by the absence of a distinct, membrane-bound nucleus or membrane-bound organelles, and by DNA that is not organized into chromosomes.

2) Nucleoid – An undefined nuclear region of the prokaryotic cell, containing the genetic material (nucleic acids).

3) Eukaryotic cell – A cell containing a membrane-bounded nucleus and membrane-bounded organelles.

4) Unicellular organism – Organism having only one cell.

5) Multicellular organism - Organism consisting of more than one cell, wherein the differentiated cells perform specialized functions in the organism.

6) Diffusion – The spontaneous movement of a substance from a region of its higher concentration to a region of its lower concentration.

7) Osmosis – The movement of water through a semi-permeable membrane from a region of high water concentration to a region of low water concentration.

8) Hypertonic solution - A solution that has a higher solute concentration than the one to which it is compared (eg - some kind of cell).

9) Hypotonic solution - A solution that has a lower solute concentration than the one to which it is compared (eg - some kind of cell).

10) Isotonic solution - A solution that has the same tonicity as another solution with which it is compared.

11) Plasmolysis - Shrinkage or contraction of the protoplasm away from the wall of a living plant or bacterial cell, caused by loss of water through osmosis. 

12) Cell organelle – A specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own membrane.

13) Genes - A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism.

14) Membrane biogenesis – The process of synthesizing the biological membranes.

15) Plasma membrane – The thin, selectively permeable membrane composed of lipids and proteins which surrounds an entire cell and regulates the flow of substances to and from the cell.

16) Cell wall – The rigid, non-living, outer covering of certain cells (like plant and bacteria), composed mainly of cellulose and providing the cell with structural support and protection.

17) Cytoplasm - The jellylike material of a cell that is enclosed within the plasma membrane, except the nucleus and contains the cell organelles.

18) Chloroplasts – The plastids containing chlorophyll which are capable of carrying out photosynthesis in plants.

Q1. Things to remember:

(a) New cells are formed from pre-existing cells.

(b) Movement of water molecules from their higher concentration to their lower concentration through a semi- permeable membrane is called Osmosis.

(c) The functional components of cell are plasma membrane, cytoplasm & nucleus.

(d) Protoplasm has two parts- cytoplasm & nucleoplasm.

(e) Nucleus, mitochondria & plastids have their own DNA & ribosomes.

(f) The shrinkage or contraction of the contents of the cell away from the cell wall is known as plasmolysis.

(g) The process by which Amoeba can engulf a food particle is endocytosis.

(h) Biogenesis is the manufacture of lipids required for making cell membrane.

(i) A cell that lacks nuclear membrane is called a prokaryotic cell & the nuclear region is called nucleoid.

(j) Movement of materials in & out of the cell takes place by diffusion & osmosis.

Q2. What is the advantage of multicellularity over unicellularity?

Ans- Division of labour.

Q3. What are the chromosomes made up of?

Ans- DNA & proteins

Q4. A cell placed in a solution swells up. What kind of solution is it? Why does it happen?

Ans- It is a hypotonic sol ution & water enters the cell by endosmosis causing the cell to swell up.

Q5. Why are lysosomes known as “suicidal bags”?

Ans- They secrete powerful digestive enzymes to digest the worn out and damaged cell organelles as well as the cell itself when it loses its functional ability.

Q6.Why is the nucleus so significant in a cell?

Ans- Nucleus has the following important functions-

(i) It controls all cell activities

(ii) It contains hereditary material that transmits hereditary information from one generation to the next

(iii) It helps in cell division

Q8. Draw neat & labelled diagrams of the following:-

i)Typical plant cell

(ii) Typical animal cell

(iii) Prokaryotic cell

Q9. Give the major functions of the following cell organelles-

(a) Endoplasmic reticulum- RER- Synthesis of proteins as it has ribosomes attached to it SER- Synthesis of lipids required for making cell membrane

(b) Golgi apparatus- Storage & packaging of various products.

(c) Mitochondria- Production of ATP(Adenosine Triphosphate) which is a source of energy.

(d) Ribosomes- Protein synthesis

(e) Golgi Apparatus-Storage & packaging of various products.

(f) Lysosomes- Digestion of worn out & damaged organelles