CBSE Class 9

On this page, you will find Diversity in Living Organisms Class 9 Notes Science Chapter 7 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 7 Diversity in Living Organisms will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 7 Notes Diversity in Living Organisms

Diversity in Living Organisms Class 9 Notes Understanding the Lesson

1. What is classification?
Grouping the organisms on the basis of their similarities and differences is called classification.

2. Need for classification

• To provide information regarding diversity of plants and animals on the Earth.
• Understand the interrelationship between different groups of plants and animals.
• To find similarities or dissimilarities in their characteristic features.
• To identify the organism.
• To indicate evolutionary trends.

3. Characteristics or criteria of classification

• Complexity of structure: Prokaryotes or Eukaryotes
• Body organisation: Unicellular or Multicellular
• Mode of obtaining Nutrition: Autotrophic or Heterotrophic
• Evolutionary relationship
• Presence or absence of cell wall

4. Kingdom
It is the highest category of classification. Each kingdom has some similar fundamental characteristics in all organisms grouped under that kingdom. The five-kingdom classification was given by R.H. Whittaker.

Differentiate between:

5. Characteristic features of different Phyla of Kingdom Animalia Porifera

• Porifera means organisms with holes.
• Non-motile animals attached to solid support
• Holes or pores all over body
• Have canal system that helps in circulating water throughout the body to bring in food and oxygen.
• Body covered with hard outside layer or skeleton
• Minimal differentiation of body and division into tissues
• Commonly called sponges found in marine habitats
• Acoelomate (without body cavity)
  Examples: Euplectella, Sycon, Spongilla

6. Coelenterata

• Animals living in water
• More body design differentiation
• Diploblastic body
• Some species live in colonies (corals) while others have a solitary life span (Hydra)
  Examples: Jelly fish, Sea Anemone, Hydra

7. Platyhelminthes

• Body is complexly designed
• Bilaterally symmetrical body
• Triploblastic body
• Acoelomate
• Body flattened dorsi-ventrally, so called flatworms
• Free living or parasitic
  Examples: Planarian (free living) liver flukes, tapeworms (Parasitic)

8. Nematodes

• Bilaterally symmetrical
• Triploblastic body
• Cylindrical body
• Tissues present but no real organs
• Presence of pseudo coelom, a sort of body cavity
• Familiar as parasitic worms causing diseases, present in intestines
  Examples: Ascaris, Wucheraria

9. Annelida

• Bilaterally symmetrical
• Triploblastic
• Coelomate (having a body cavity or coelon)
• Extensive organ differentiation
• Segmented body (Metamerism)
• Found in fresh water, marine and on land
  Examples: Earthworms, Leech, Nereis

10. Arthropoda

• Largest group of animals
• Bilaterally symmetrical
• Segmented body
• Open circulatory system
• Coelomate
• Arthropoda means jointed legs
  Examples: Prawns, Butterflies, Housefly, Cockroach

11. Mollusca

• Bilaterally symmetrical
• Coelomic cavity is reduced
• Little segmentation
• Open circulatory system
• Kidney like organ for excretion
• There is a foot like structure for moving around
  Examples: Snails, mussels, Chiton, Octopus, Unio, Pila

12. Echinodermata

• They are spiny skinned organisms
• Echinos’ means hedgehog and ‘Derma’ means skin.
• Exclusively free living marine animals
• Triploblastic
• Acoelomate
• Peculiar water driven tube system
• Hard calcium carbonate structures as skeleton
  Examples: Star fish, sea urchin, feather star, sea cucumber

13. Protochordate

• Bilaterally symmetrical
• Triploblastic
• Coelomate
• Notochord present during larval stage
• Provides place for muscles to attach for easy movement
• Marine animals
  Examples: Balanoglossus, Herdmania, Amphioxu

14. Vertebrata

• Have a true vertebral column and internal skeleton
• Bilaterally symmetrical
• Triploblastic
• Coelomate and segmented
• Complex differentiation of body tissues and organs

15. All chordates possess the following features:

• Have a notochord
• Have a dorsal nerve cord
• Are triploblastic
• Have paired gill pouches in some stage of their life cycle
• Are coelomate

16. Vertebrates are grouped into 5 classes
Pisces

• Exclusively aquatic animals
• Skin covered with scales or plates
• Obtain oxygen dissolved in water
• Streamlined body and muscular tail for movement in water
• Cold-blooded
• Two-chambered heart
• Lays eggs in water

17. Are of two types:

• Cartilaginous fish (skeleton made entirely of cartilage), e.g., Shark
• Bony fish (skeleton made of both cartilages and bones), e.g., Rohu, Tima

18. Amphibia

• Lack scales
• Have mucus glands in skin
• Cold-blooded
• Three-chambered heart
• Respiration through gills, lungs or skin
• Lay eggs in water
• Live both on land and in water
  Examples: Frogs, toads, salamander, etc.

19. Reptilia

• Have scales
• Three-chambered heart but crocodiles have four chambered heart
• Cold-blooded
• Breathe through lungs
• Lay eggs with tough coverings so they do not lay eggs in water
  Examples: Snakes, turtles, lizards, crocodiles, chameleon

20. Aves

• Outside covering of feathers
• Four-chambered heart
• Warm-blooded
• Breathe through lungs
• Lay eggs
• Two forelimbs modified into wings for flight
  Examples: All birds like crow, pigeon, peacock, etc.

21. Mammalia

• Skin has hairs, oil and sweat glands
• Four-chambered heart
• Warm-blooded.
• Most of them give birth to young ones
• Few like Platypus and Echidna lay eggs
• Some like kangaroos give birth to poorly developed young ones
• Mammary glands present for production of milk to nourish their young ones
  Examples: Cat, human, rat, bat, whale, etc.

21. Nomenclature: System of assigning names or terms to the organisms is called as nomenclature. The names given to the organism can be

• Common name or
• Scientific name.

Common names cannot be used in the same way by the scientist world over and can often result in confusion. To avoid this, a system of scientific names has been proposed.

Binomial System of Nomenclature: The binomial system of nomenclature assigns two names to the organism in order to identify it first is the generic name (genus) and the second is the specific epithet (species). This system of nomenclature was given by Carolus Linnaeus.

22. Convention for writing the scientific names:

• The name of the genus begins with a capital letter.
• The name of the species begins with a small letter.
• When printed, the scientific name is given in italics.
• When written by hand, the genus name and the species name have to be underlined separately.

23. Scientific names of some organisms:

• Tiger – Panthera tigris
• Peacock – Pauo cristatus
• Mango – Mangifera indica
• Lotus – Nelumbo nucifera
• Neem – Azadiraehta indica
• Potato – Solanum tuberosum
• Ant – Hymenopetrous formicidae
• Frog – Rana tigrina
• Rose – Rosa indica
• Pea – Pisum sativum

Class 9 Science Chapter 7 Notes Important Terms

Prokaryotes: Organisms which do not have a clearly demarcated nucleus and other organelles.

Eukaryotes: Organisms having membrane bound cell organelles and a well-defined nucleus.

Unicellular: Organisms having only one cell in their body.

Multicellular: Organisms having many cells in their body.

Autotrophs: Organisms synthesising their own food by photosynthesis.

Heterotrophs: Organisms which depend on other organisms for their food.

Bilateral symmetry: The body organisation in which the left and right halves have same body design.

Radial symmetry: Arrangement of similar parts around a central body axis as in a wheel.

Diploblastic: Animals having a body made up of two layers of cells i.e., ectoderm and endoderm.

On this page, you will find Tissues Class 9 Notes Science Chapter 6 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 6 Tissues will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 6 Notes Tissues

Tissues Class 9 Notes Understanding the Lesson

1. A single cell performs all the basic functions like digestion, respiration, excretion, etc. in order to sustain life in the unicellular organisms like Amoeba. In multicellular organisms like human beings, each specialised function to sustain life is taken up by a different group of cells.

2. Division of labour: The multicellular organisms show division of labour as each function is carried out by a cluster of specialised cells at a definite place in the body.
For example:
In human beings, muscle cells contract and relax to cause movement, nerve cells carry messages, blood flows to transport oxygen, etc. In plants, vascular tissues called xylem and phloem conduct water and food respectively from one part of the plant to the other parts.

3. Tissue: A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue. Example: Blood, phloem and muscle. Plant tissues are classified as growing or meristematic tissue and permanent tissue.

4. Meristematic tissue: This tissue consists of cells which continuously divide to produce new cells. The cells of this tissue are very active, lack vacuoles, have dense cytoplasm, thin cellulosic cell walls and prominent nuclei.

5. Location of meristematic tissue: This tissue is present only at specific regions of the plant like the root tip, shoot tip and at the base of intemodes and leaves.

6. Types of meristematic tissue: They are classified as apical, lateral and intercalary meristematic tissue based on the region where they are present.

• Apical meristem: It is present at the growing tips of stems and roots and results in increase in the length of the stem and the root.
• Lateral meristem (cambium): It helps to increase the girth of the stem or root.
• Intercalary meristem: It is present at the base of the leaves or intemodes.

5. Permanent tissue: Consists of cells which have taken up a specific role and lost the ability to divide.
It is of two types:

• Simple tissue: It is made up of only one type of cells. Its three types are: parenchyma, collenchyma and sclerenchyma.
• Complex tissue: It is made up of more than one type of cells. They are the conducting tissues called xylem and phloem.

6. Differentiation: The process of taking up a permanent shape, size, and a function by the cells is called differentiation.

7. Types of simple tissue:
(a) Parenchyma: They are loosely packed living cells, with thin cell walls and large intercellular spaces. They provide support to plants and store food. It is called chlorenchyma if it contains chlorophyll and performs photosynthesis. The parenchyma of aquatic plants have large cavities to provide buoyancy to the plants to help them float. Such type of parenchyma is called aerenchyma.

(b) Collenchyma: It consists of living, elongated cells that are irregularly thickened at the corners and have a very little intercellular space. It allows easy bending in various parts of a plant (leaf, stem) without breaking. It also provides mechanical support to plants like in the leaf stalks below the epidermis.

(c) Sclerenchyma: This tissue consists of dead cells which makes the plant hard and stiff. The cells are long and narrow as the walls are thickened (often so thick that there is no internal space inside the cell) due to lignin (a chemical substance which acts as cement and hardens them). This tissue provides strength to the plants and is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts.

8. Epidermis: The outermost layer of cells covering an organism is called epidermis. It is usually made up of a single layer of cells and gives protection. The epidermis may be thicker in some plants living in dry habitats or often secrete a waxy, water-resistant layer on their outer surface called cutin (chemical substance with waterproof quality) to prevent water loss.

The epidermis of leaves have small pores called stomata which are enclosed by two kidney shaped cells called guard cells. Stomata help in gaseous exchange and transpiration. The epidermal cells of roots bear root hairs that greatly increase the total absorptive surface area of the roots for absorption of water.

9. Cork: A strip of secondary meristem replaces the epidermis of the older stem and cuts off cells towards outside to form a several-layer thick cork or the bark of the tree. Cells of cork are dead, compactly arranged without intercellular spaces and have a chemical called suberin in their walls that makes them impervious to gases and water.

10. Complex Permanent Tissue: These tissues are made of more than one type of cells which coordinate to perform a common function, e.g., Xylem and phloem. They are mainly conducting tissues and constitute a vascular bundle.

(a) Xylem: Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres. All the cells of xylem except the xylem parenchyma are dead. Xylem helps to transport water and minerals. Tracheids and vessels help in vertical transport whereas the parenchyma stores food and helps in the sideways conduction of water. Fibres are mainly supportive in function.

(b) Phloem: Phloem has four elements called sieve tubes, companion cells, phloem fibres and the phloem parenchyma. All cells of phloem are living except the phloem fibres. Phloem transports food from leaves to other parts of the plant.

11. Animal Tissues: The animal tissues are of four types: epithelial tissue, connective tissue, muscular tissue and nervous tissue.

12. Epithelial Tissue: They are the covering or protective tissues and cover most organs and cavities in the animal body. These cells are tightly packed, form a continuous sheet and are almost without any intercellular spaces between them. e.g., skin, the lining of the mouth, the lining of blood vessels, lung alveoli and kidney tubules are all made of epithelial tissue. All epithelium is usually separated from the underlying tissue by an extracellular fibrous basement membrane. The types of epithelium on the basis of their structure and functions are:

(a) Squamous epithelium: Consists of flattened cells. Present in oesophagus and lining of the mouth. Skin epithelial cells are arranged in many layers to prevent wear and tear and are called as stratified squamous epithelium.

(b) Columnar epithelium: Has tall or ‘pillar-like’ cells. It forms inner lining of the intestine.

(c) Cuboidal epithelium: Has cube-shaped cells. It forms the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support.

(d) Ciliated epithelium: Have cilia on the outer surfaces of epithelial cells. The cilia can move and their movement pushes the mucus in the respiratory tract forward to clear it.

(e) Glandular epithelium: Has gland cells which secrete substances at the epithelial surface.

13. Connective Tissue: The cells of connective tissue are loosely spaced and embedded in an intercellular matrix which may be jelly like, fluid, dense or rigid, e.g., blood, bone, cartilage, etc.

(a) Blood: It has a fluid (liquid) matrix called plasma having red blood cells (RBCs), white blood cells (WBCs) and platelets. Blood helps in the transport of gases, digested food, hormones and waste materials to different parts of the body.

(b) Bone: It has bone cells embedded in a hard matrix composed of calcium and phosphorus compounds. It is a strong and non-flexible tissue which forms a framework that supports the body, anchors the muscles and supports the main organs of the body.

(c) Cartilage: It has widely spaced cells and a solid matrix composed of proteins and sugars. It helps to smoothen bone surfaces at joints and is also present in the nose, ear, trachea and larynx.

(d) Areolar connective tissue: It fills the space inside the organs, supports internal organs and helps in repair of tissues. It is found between the skin and muscles, around blood vessels and nerves and in the bone marrow.

(e) Adipose tissue: It is a fat storing tissue having cells filled with fat globules. It is found below the skin and between the internal organs.

(f) Ligament: It is the connective tissue which connects two bones. This tissue has very little matrix, is very elastic and has considerable strength.

(g) Tendon: It is the connective tissue which connects muscles to bones. It is a fibrous tissue with great strength but limited flexibility.

14. Muscular Tissue: This tissue is responsible for movement in our body and consists of elongated cells, also called muscle fibres. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement.

(а) Striated Muscles: These muscles are also called skeletal muscles as they are mostly attached to bones and help in body movement. These muscles show alternate light and dark bands. These are long, cylindrical, unbranched and multinucleate. These are voluntary muscles as we can move them by conscious will, e.g., muscles of our limbs.

(b) Unstriated Muscles: They are also called smooth muscles or unstriated muscles as they do not have light and dark bands. The cells are long, uninucleate, involuntary in nature and spindle shaped. They are present in iris of the eye, ureters, blood vessels, alimentary canal and bronchi of lungs.

(c) Cardiac Muscles: These are the muscles of the heart which show rhythmic contraction and relaxation throughout life. They are involuntary, cylindrical, branched and uninucleate.

15. Nervous Tissue: The cells of this tissue are called nerve cells or neurons. Each neuron consists of a cell body with a nucleus and cytoplasm, a single long part called the axon, and many short branched parts called dendrites. The cells of the nervous tissue are highly specialised for transmitting the stimulus from one place to another within the body on being stimulated. The brain, spinal cord and nerves are composed of the nervous tissue. A nerve consists of many nerve fibres bound together by connective tissue.

Class 9 Science Chapter 6 Notes Important Terms

Tissue: A group of cells that are similar in structure and/or work together to achieve a particular function.

Meristematic tissue: This tissue that consists of cells which continuously divide to produce new cells.

Permanent tissue: The tissue which consists of cells which have taken up a specific role and lost the ability to divide.

Differentiation: The process of taking up a permanent shape, size, and a function by the cells is called differentiation.

Chlorenchyma: Parenchyma which contains chlorophyll and performs photosynthesis.

Aerenchyma: Parenchyma which contains large cavities to provide buoyancy to the aquatic plants to help them float.

Cambium: It is a lateral meristem which helps in increasing the girth of the stem or root.

Lignin: A chemical substance which acts as cement and hardens the cells of sclerenchyma.

Cutin: A chemical substance with waterproof quality present in epidermis of leaves to prevent water loss by transpiration.

Suberin: A chemical present in the walls of cork cells that makes them impervious to gases and water.

Adipose tissue: It is a fat storing tissue having cells filled with fat globules.

Ligament: It is the connective tissue which connects two bones. This tissue has very little matrix, is very elastic and has considerable strength.

Tendon: It is the connective tissue which connects muscles to bones. It is a fibrous tissue with great strength but limited flexibility.

Contractile proteins: Special proteins present in muscles which contract and relax to cause movement of body parts.

Nerve: A nerve consists of many nerve fibres bound together by connective tissue.

On this page, you will find The Fundamental Unit of Life Class 9 Notes Science Chapter 5 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 5 The Fundamental Unit of Life will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 5 The Fundamental Unit of Life

The Fundamental Unit of Life Class 9 Notes Understanding the Lesson

1. Robert Hooke: In 1665, he discovered cell in a thin slice of cork (bark of cork tree) by using a self designed microscope. The structure consisted of many little compartments which resembled the structure of a honeycomb. He called boxes as ‘cell’ which is Latin word for ‘a little room’.

2. Leeuwenhoek: In 1674, discovered the free-living cells in pond water for the first time by using an improved microscope.

3. Robert Brown: In 1831, discovered the nucleus in the cell.

4. Piirkinje: In 1839, coined the term ‘protoplasm’ for the fluid substance of the cell.

5. Schleiden (1838) and Schwann (1839): Put forth the cell theory, which said that:

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

6. Virchow: In 1855, expanded the cell theory by suggesting ‘Omni cellula-e-cellula’ which means all cells arise from pre-existing cells.

7. Unicellular organisms: Organisms which have only a single cell, e.gAmoeba, Paramecium, Chlamydomonas, bacteria, etc.

8. Multicellular organisms: Organisms which consist of more than one cell e.g., Plants, animals, fungi, etc.

9. Cell division: It is the process by which a cell divides to form new cells. This supports the fact that, all cells arise from the existing cells.

10. The shape and size of cells are related to the specific function they perform: Amoeba can change its shape as per the conditions or its need whereas for most of the other cases, the cell shape is more or less fixed, e.g., nerve cells have a typical shape.

11. Division of Labour: In all multicellular organisms there is a division of labour. This means that different parts of the body perform different functions. For example, the stomach helps in digestion; blood is pumped by the heart, etc. Division of labour can also be seen within a single cell.

12. Three main parts of a cell: Most cells (excluding bacteria) have three main parts:

• Plasma membrane/Cell Membrane
• Nucleus
• Cytoplasm

13. Plasma Membrane or Cell Membrane

• It is the outermost covering of the cell.
• It separates the contents of the cell from its external environment.
• It is mainly composed of lipids and proteins.
• It is called selectively permeable as it permits the entry and exit of only some materials in and out of the cell.

14. Diffusion: The movement of a substance from a region of its high concentration to the region of its low concentration is called diffusion. Diffusion helps in gaseous exchange between the cells as well as the cell and its external environment.

15. Osmosis: The spontaneous movement of water molecules from a region of its high concentration to the region of its low concentration through a selectively permeable membrane is called osmosis.

Effect on animal cell or a plant cell put into a solution of sugar or salt in water

16. Cell Wall

• It is a rigid outer covering which lies outside the plasma membrane.
• It is made of cellulose which provides structural strength to plants.
• The shrinkage or contraction of the contents of the cell away from the cell wall when a living plant cell loses water through osmosis is known as plasmolysis.

17. Nucleus

• It is a dark coloured, spherical or oval, dot-like structure near the centre of each cell.
• It is the control centre of the cell as it controls all the activities of the cell.
• It has a double-layered covering called nuclear membrane.
• The nuclear membrane has pores which allow the transfer of materials from inside the nucleus to its outside, that is, to the cytoplasm.
• The nucleus plays a central role in cellular reproduction (process by which a single cell divides and forms two new cells).
•  Nucleus along with the environment directs the chemical activities of the cell to determine the way the cell will develop and the form it will exhibit at maturity.
• Nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane in some organisms like bacteria. Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

18. Chromosomes

• The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
• Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.
• Chromosomes are composed of DNA and protein.

19. Genes
Functional segments of DNA are called genes.
Chromatin Material:

• DNA is present as part of chromatin material in the cells which are not dividing.
• Chromatin material is visible as entangled mass of thread-like structures.
• Chromatin material gets organised into chromosomes, when the cell is about to divide.

20. Types of organisms on the basis of the nature of nucleus and nuclear membrane

20. Cytoplasm

• It is the fluid content enclosed by the plasma membrane.
• It contains many specialised cell organelles.

21. Cell Organelles
Cell organelles are parts of the cell which are specialised for carrying out one or more vital functions, analogous to the organs of the human body.

22. A. Endoplasmic Reticulum (ER)
(i) Consists of a large network of membrane-bound tubes and sheets which appear as long tubules or round or oblong bags (vesicles).

(ii) ER serves as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.

(iii) It also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.

(iv) ER are of two types:
Rough endoplasmic reticulum (RER) and
Smooth endoplasmic reticulum (SER).

(v) Rough endoplasmic reticulum

• It looks rough as it has particles called ribosomes attached to its surface. Ribosomes are the site of protein synthesis.

(vi) Smooth endoplasmic reticulum

• It helps in the manufacture of fat molecules, or lipids, important for cell function.
• Some proteins and lipids made by SER help in building the cell membrane and this process is known
  as membrane biogenesis.
• Helps in detoxifying many poisons and drugs in the liver cells of the group of vertebrates.

B. Golgi Apparatus

• First described by Camillo Golgi.
• It has membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns.
• They constitute another portion of a complex cellular membrane system as their membranes often have connections with the membranes of ER.
•  Its functions include storage, modification and packaging of products in vesicles.
• Golgi apparatus packages and dispatches the material synthesised near the ER to various targets inside and outside the cell.
• They are also involved in the formation oflysosomes.

C. Lysosomes

• They are membrane-bound sacs filled with digestive enzymes made by RER.
• They are waste disposal system of the cell as they help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles.
• Lysosomes have powerful digestive enzymes capable of breaking down all organic materials.
• Lysosomes are also known as the ‘suicide bags’ of a cell because if the cell gets damaged during disturbance in cellular metabolism, the lysosomes may burst and its enzymes digest their own cell.

D. Mitochondria

• Also known as the powerhouses of the cell as the energy required for various chemical activities needed for life is released by mitochondria in the form of Adenosine triphosphate – ATP (ATP is known as the energy currency of the cell).
• Mitochondrion is a double-membrane structure whose outer membrane is very porous while the inner
  membrane is deeply folded to form cristae. Cristae are folds which create a large surface area for ATP’ generating chemical reactions.
• Mitochondria have their own DNA and ribosomes so they can make some of their own proteins.

E. Plastids

• Plastids are present only in plant cells and are of two types – chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids).
• Chlorophyll containing plastids are known as chloroplasts and help in photosynthesis.
• Leucoplasts store starch (amyloplast), oils (elaioplasts) and protein granules (aleuroplasts).
• The plastids internally consist of numerous membrane layers embedded in a material called the stroma.
• Plastids have their own DNA and ribosomes so they can make some of their own proteins.

F. Vacuoles

• Vacuoles are storage sacs for solid or liquid contents like amino acids, sugars, various organic acids and some proteins.
• Small-sized vacuoles are present in animal cells while plant cells have very large vacuoles. A large central vacuole may occupy 50-90% of the cell volume in some plant cells.
• The vacuoles are full of cell sap and provide turgidity and rigidity to the cell in plant cells.
• Food vacuole found in Amoeba contains the food items that the Amoeba has consumed.
• Contractile vacuole found in some unicellular organisms help in expelling excess water and some wastes f from the cell.

Class 9 Science Chapter 5 Notes Important Terms

Unicellular organisms: They are single-celled organisms, e.g., Amoeba, Paramecium, Chlamydomonas, bacteria, etc.

Multicellular organisms: They are composed of more than one cell, e.g., plants, animals, fungi, etc. Diffusion: The movement of a substance from a region of its high concentration to the region of its low concentration is called diffusion.

Osmosis: The spontaneous movement of water molecules from a region of its high concentration to the region of its low concentration through a selectively permeable membrane is called osmosis.

Hypotonic solution: If the medium surrounding the cell has a higher water concentration than the cell i.e., outside solution is very dilute, then it is called a hypotonic solution.

Isotonic solution: If the medium surrounding the cell has exactly the same water concentration as the cell, then it is called isotonic solution.

Hypertonic solution: If the medium surrounding the cell has a lower concentration of water than the cell i.e., very concentrated solution, then it is called hypertonic solution.

Plasmolysis: The shrinkage or contraction of the contents of the cell away from the cell wall when a living plant cell loses water through osmosis is known as plasmolysis.

Genes: Functional segments of deoxyribonucleic acid (DNA) are called genes.

Prokaryotes: The single celled organisms which lack a well-defined nuclear membrane are called prokaryotes.

Eukaryotes: The single celled or multicellular organisms which have a well defined nuclear membrane are called eukaryotes.

Membrane biogenesis: Some proteins and lipids made by SER help in building the cell membrane and this process is known as membrane biogenesis.

Amyloplast: The starch containing leucoplasts are called amyloplast.

Elaioplast: The oil containing leucoplasts are called elaioplasts.

Aleuroplast: The protein containing leucoplasts are called aleuroplasts.

On this page, you will find Structure of the Atom Class 9 Notes Science Chapter 4 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 4 Structure of the Atom will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 4 Notes Structure of the Atom

Structure of the Atom Class 9 Notes Understanding the Lesson

1. Discovery of electron: Study of cathode rays.

• On electrical discharge through gases at very low pressure, cathode rays are produced.
• Cathode rays move in straight line.
• Cathode rays have some mechanical energy.
• Cathode rays consist of negatively charged particles, i.e., electrons.

2. Electron: An electron is the sub-atomic or fundamental particle which carries one unit negative charge.
It is represented by e^–.
Mass of one electron = 9.11 x 10^-31 kg.
Charge of one electron = – 1.6 x 10^-19 C.

3. Proton: Discovered by Goldstein (1886) anode ray or canal ray experiment.
Mass of proton = 1.67 x 10^-24 kg.
One proton is 1840 times heavier than electron.
Charge on proton = + 1.6 x 10^-19 C.

4. Thomson’s model of an atom
He proposed that:

• An atom consists of a uniform sphere of positive electricity in which the electrons are distributed more or less uniformly.
• The negative and the positive charge are equal in magnitude. Thus, the atom as a whole is electrically neutral.

5. Rutherford’s model of an atom
Rutherford observed that:

• Most of the a-particles (nearly 99%) passed through the gold foil undeflected.
• Some of the a-particles (about one in every 20,000) were deflected by small angles.
• A few particles (1 in about 10⁶) were either deflected by very large angles or were actually reflected back along their path.

In order to explain the observation of his scattering experiment, Rutherford assumed that the solid gold foil consists of layers of individual atoms touching each other so that there is hardly any empty space between them. Rutherford explained his observation as follows:

• Most of the space inside the atom is empty because most of the a-particles passed through the gold foil without getting deflected.
• Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space
• A very small fraction of a-particles was deflected by 180°, indicating that all the positive charge and mass of the gold atom were, concentrated in a very small volume within the atom.

On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which had the following features:

• There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
• The electrons revolve around the nucleus in circular paths.
• The size of the nucleus is very small as compared to the size of the atom.

6. Drawbacks of Rutherford’s model of the atom
Rutherford’s model could not explain the stability of the atom. This is because according to Rutherford’s model, an atom consists of a small heavy positively charged nucleus in the centre and the electrons revolve around it.

However, whenever a charged particle like an electron revolves around a central force like that of a nucleus, it loses energy continuously in the form of radiations. Thus, the orbit of the revolving electron will keep on becoming smaller and smaller and ultimately the electron should fall into the nucleus.

7. Discovery of Neutrons
Neutron may be defined as subatomic particle which had no charge and a mass nearly equal to that of proton:

• Mass of neutron = 1.676 x 10^-24
• Charge of neutron = 0 (zero).

8. Bohr’s atomic model

• Electron revolves around nucleus only in certain selected circular orbits with definite energies and are called energy shells or energy levels.
• While revolving around the nucleus in an orbit, an electron does not lose energy nor does it gain energy.
• Different shells or orbits are numbered as 1, 2, 3, 4…………………… or designated as K, L, M, N……………….
• Every orbit is associated with a fixed amount of energy, so on gaining a certain amount of energy e~, jumps to the higher orbit.

9. How are electrons distributed in different orbit (shells)?
Answer:
The distribution of the electrons in the shells is known as electronic configuration. It is based on certain guide­lines or rules given by Bohr and Bury. This is known as Bohr-Bury scheme. According to this scheme,

1. The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number of energy level index, 1, 2. 3,………………… Hence the maximum number of electrons in different shells are as follows:

• First orbit or K-shell will be = 2 x 1² = 2,
• Second orbit or L-shell will be = 2 x 2² = 4,
• Third orbit or M-shell will be = 2 x 3² = 18,
• Fourth orbit or N-shell will be = 2 x 4² = 32 and so on.

3. The maximum number of electrons that can be accommodated in the outermost orbit is 8.

4. Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.
Atomic structure of first eighteen elements is shown schematically in the figure given below.

10. Valency
The electrons present in the outermost shell of an atom are known as the valence electrons.
From the Bohr-Bury scheme, the outermost shell of the an atom can accommodate a maximum of 8 electrons. The combining capacity or valency of elements having a completely filled outermost shell is zero. Inert elements, like helium atom has two electrons in its outermost shell and all other elements have atoms with eight electrons in their outermost shell.

The combining capacity of the atoms of other elements was explained in terms of their tendency to attain a fully-filled outermost shell (stable octect or dulpet). The atoms which do not have their outermost shell fully filled enter into bond formation with other atoms in order to achieve an octet (or duplet) of electrons in their outermost shells. They do so either by sharing, losing or gaining electrons.

If the number of electrons in the outermost shell of an atom is close to its full capacity, then valency is determined in a different way.

11. Atomic number (Z)
The number of unit positive charges present in the nucleus of an atom is known as atomic number of the element.
It is denoted by the symbol Z.
Atomic no. (Z) = No. of protons = No. of electrons.
For example: Hydrogen, Z = 1 because in hydrogen atom, only one proton is present in the nucleus.
Similarly, for carbon Z = 6.

12. Mass number
The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. For example, mass of carbon is 12u because it has 6 protons and 6 neutrons, 6u + 6u = 12u. Similarly, the mass of aluminium is 27u because it has 13 protons and 14 neutrons.

The atomic number, mass number and symbol of the element are to be written as:

For example: Nitrogen is written as \({ }_{7}^{14} \mathrm{N}\).

13. Isotopes
The different atoms of the same element having same atomic number but different mass numbers.
For example: Hydrogen atom, has three isotopes, namely protium \(\left({ }_{1}^{1} \mathrm{H}\right)\), deuterium and tritium \(\left({ }_{1}^{3} \mathrm{H} \text { or } \mathrm{T}\right)\)

14. Applications of isotopes:

• An isotope of Uranium (U-235) is used as a fuel in nuclear reactors.
• An isotope of cobalt (C-60) is used in treatment of cancer by radiation therapy.
• An isotope of iodine is used in the treatment of goitre.
• Ages of old wooden articles are determined by observing the radioactivity of C-14 isotope of carbon.

15. Isobars:
Atoms of different elements with different atomic numbers which have the same mass number are known as isobars.

For example: Two elements – calcium, atomic number 20, and argon, atomic number 18 are isobars. The number of electrons in these atoms is different, but the mass number of both of these elements is 40.

Class 9 Science Chapter 4 Notes Important Terms

Cathode rays: It consist of negatively charged particles (electrons) emitted by passing electricity through gases at low pressure.

Anode rays: These rays are produced along with cathode rays and move towards cathode. They consist of positively charged ions.

Electrons: Electrons are fundamental particles carrying a unit negative charge and are a common constituent of all atoms.

Protons: (This positively charged particle was characterised in 1919). The fundamental particle which carries one unit of positive charge and has a mass nearly equal to that of an H-atom. Mass of proton = 1.6726 x 10^-24 g.

Neutron: The fundamental particle, which has a mass nearly equal to that of an H atom but has no charge.
Mass of neutron = 1.6749 x 10^-24 g.

Valency: It is the capacity of atoms of a given element to combine with, or replace atoms of hydrogen. In HCl gas, valency of chlorine is 1.

Nucleons: Protons and neutrons together are known as nucleons.

Isobars: Atoms having the same mass number but different atomic numbers.
For example, K – 40 and Ar – 40.

Isotopes: Atoms with identical atomic number but different mass numbers.
For example,\({ }_{1}^{1} \mathrm{H},{ }_{1}^{2} \mathrm{H} \text { and }{ }_{1}^{3} \mathrm{H}\)

On this page, you will find Atoms and Molecules Class 9 Notes Science Chapter 3 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 3 Atoms and Molecules will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 3 Notes Atoms and Molecules

Atoms and Molecules Class 9 Notes Understanding the Lesson

1. The structure of matter has been a subject of speculation from a very early time. The ancient Indian and Greek philosophers regarded matter to be discontinuous and made up of infinitely small particles. Around the same period, Greek philosopher, Democritus, suggested that if we go on dividing matter into smaller parts, a stage would be reached when particles obtained cannot be divided further. He called these particles ‘Atoms’ meaning indivisible.

2. After Democritu’s death, little more was done with atomic theory until the end of the 18th century, when Antoine Lavoisier introduced modern chemistry to the world. He put forward two important laws of chemical combination which formed the basis of Dalton’s atomic theory which were published in 1808.

3. Laws of Chemical Combination: Whenever reactants react together to form the products or the elements combine together to form a compound, they do so according to certain laws. These laws are called “Laws of Chemical Combination”.

4. Laws of Conservation of Mass: Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.

5. Law of Constant Proportions: This law was stated by Proust as “In a chemical substance, the elements are always present in definite proportion by mass”. For example: In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1: 8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.

6. As the law of constant proportions is true, it helps us to calculate the percentage of any element in the given compound, using the following expression:

7. Dalton’s Atomic Theory : On the basis of laws of chemical combination, John Dalton, proposed that behaviour of matter could be explained using an atomic theory.

8. Postulates of Dalton’s Atomic Theory

• All matter is made of very tiny particles called atoms.
• Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
• Atoms of a given element are identical in mass and chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in the ratio of small whole numbers to form compounds.
• The relative number and kinds of atoms are constant in a given compound.

9. Limitations of Dalton’s Atomic Theory

• It failed to explain how atoms of different elements differ from each other i.e., it did not tell anything about internal structure of the atom.
• It could not explain how and why atoms of different elements combine with each other to form compound- atoms or molecules.
• It failed to explain the nature of forces that hold together different atoms in a molecule.
• It did not make any distinction between ultimate particle of an element that takes part in reaction (atom) and ultimate particle that has independent existence (molecule).

Question 10.
Atom
Answer:
An atom is defined as the smallest particle of an element which may not be capable of free existence. However, it is the smallest particle that takes part in a chemical reaction.

Question 11.
How big are atoms?
Answer:
Atoms are very small, they are smaller than anything that we can imagine or compare with more than millions of atoms when stacked would make a layer barely as thick as a thin sheet of paper.

Atoms are very small in size. The size of an atom is expressed in terms of atomic radius. Atomic radius is measured in nanometers (nm).
\(1 \mathrm{nm}=\frac{1}{10^{9}} \mathrm{m}=10^{-9} \mathrm{m}\)

Radii of most of the atoms are the order of 0.1 nm or 10_1° m. For example, atomic radius of hydrogen is
0. 037 nm while that of gold atom is 0.144 nm.

Question 12.
Symbols used to represent atoms of different elements
Answer:
Daltons suggested symbols for the atoms of different elements. He was the first scientist to use the symbols for elements in a quantitative sense. When he used symbol for an element he meant a definite quantity of that element, that is, one atom of the element.

Question 13.
Modern Symbols of Elements
Answer:
1. The symbol of an element is the “first letter and another letter” of the English or Latin name of the element. However, in all cases, the first letter is always capital and the other letter (if added) is always a small letter.
For example: Hydrogen is represented by ‘H’, oxygen by ‘O’, carbon by ‘C’ etc.

2. The necessity of adding another letter arises only in case of elements whose names start with the same
letter. However, the other letter added is not always the second letter of the name. Further, the other – letter added may be a letter from the Latin name of the element.

For example: Carbon, Calcium, Cobalt, Chlorine and Copper all start with the first letter ‘C’ Hence carbon is represented by C and calcium is represented by Ca, cobalt by Co, chlorine by Cl and copper by Cu

Question 14.
Atomic Mass
Answer:
The atomic mass of an element is the relative mass of its atoms as compared with the mass of an atom of carbon -12 isotope taken as 12 units.
\(\text { Atomic mass }=\frac{\text { Mass of } 1 \text { atom of the element }}{\frac{1}{12} \text { of the mass of an atom of Carbon-12 }}\)

Since atomic masses are relative masses, they are pure numbers and are often given without units. In case of most of the elements, all the atoms of the elements do not have same relative mass. The atoms of an element having different relative masses are called isotopes. For example: Chlorine contains two types of atoms having relative masses 35 u and 37 u and their relative abundance is 3:1.
\(\text { Atomic mass of chlorine }=\frac{35 \times 3+37 \times 1}{4}=35.5 \mathrm{u}\)

Question 15.
What is a Molecule?
Answer:
A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.

Question 16.
Molecule of an element
Answer:
Molecule of an element means one, two or more atoms of the same element existing as one species in the free state.
(i) Monoatomic molecules: Noble gases like helium, neon, etc., exist as single atom. Hence, their molecules are monoatomic.

(ii) Diatomic molecules: For example, in a molecule of hydrogen, two atoms of hydrogen exist together. Its molecule is, therefore, represented by H₂, i.e., it is a diatomic molecule.

(iii) Triatomic molecules: For example, in a molecule of ozone, three atoms of oxygen exist together as one species. Hence, it is triatomic with formula O₂

(iv) Tetratomic molecules: For example; Phosphorus P₄ is a triatomic molecule.

Molecules containing more than four atoms are generally called polyatomic.

The number of atoms present in one molecule of the substance is called its atomicity.

Molecules of a compound: Atoms of different elements join together in definite proportion to form molecules of compounds. For example, H₂O represents the molecule of a compound in which two atoms of hydrogen are combined with one atom of oxygen, or hydrogen and oxygen are combined in the fixed proportion, i.e., 1 : 8 by mass.

Molecular mass: The molecular mass of a substance (an element or a compound) may be defined as the average relative mass of a molecule of the substance as compared with mass of an atom of carbon (C-12 isotope) taken as 12 u.
\(\text { Molecular mass }=\frac{\text { Mass of } 1 \text { molecule of the substance }}{\frac{1}{12} \text { of the mass of an atom of } \mathrm{C}-12}\)

For example, molecular mass of CO₂ is 12 x 1+ 16 x 2 = 44.
Formula mass: Formula mass of an ionic compound is obtained by adding atomic masses of all the atoms in a formula unit of the compound.
For example, formula mass of potassium chloride (KCl)
= Atomic mass of potassium + atomic mass of chlorine
= 39 + 35.5
= 74.5

Question 17.
What is an ion?
Answer:
An atom or a group of atoms which contains positive or negative charge are called ions.

• A positively charged ion called ‘cation’.
• A negatively charged ion is called ‘anion’.
• The ions consisting of only single atoms are called monoatomic ions.
• The ions consisting of a group of atoms is called polyatomic ion.

Question 18.
Writing chemical formula
Answer:
Chemical formula of a compound represents the actual number of atoms of different elements present in one molecule of the compound. For example, chemical formula of water is H₂O.

Valency is defined as the combining capacity of the element. It is equal to the number of hydrogen atoms or number of chlorine atoms or double the number of oxygen atoms with which one atom of the element combines.

For example, valency of oxygen is 2. This means that one atom of oxygen can combine with two atoms of hydrogen. Hence, the formula of the compound formed is H₂O.

Rules for writing the chemical formula

• Formula of compound is given by writing the symbols of constituent elements side by side.
• Symbol of the more metallic element is written first in the formula.
• Number of atoms of each of the constituent elements present in the molecule is indicated by subscript.
• When either of the ions or both the ions are polyatomic and their valency is more than 1, we enclose the polyatomic ions in brackets. No brackets are riecessary if the valencies of polyatomic ions are 1.
• While writing formula of a compound if the valency numbers have a highest common factor [H.C.F.], divide the valency numbers by H.C.F. to get a simple ratio between the combining elements.

The simplest compounds, which are made up of two different elements are called binary compounds and formula can be written by criss crossing the valencies of elements present in a molecule of the compound. Formula of the compound can be derived by following steps

Step – 1: Write the constituent elements and their valencies as shown.

Step – 2: Reduce the valency numerals to the simplest whole numbers by dividing by some common factor, if any

Step – 3: Criss-cross the reduced valency numerals and write them as subscripts at the bottom right-hand side of the symbols.

The subscript 1 is not written. Thus, the formula of the compound is CO₂.

Mole concept: A mole of particles (atoms, ions or molecules) is defined as that amount of the substance which contains the same number of particles as there are C-12 atoms in 12 g, i.e., 0.012 kg of C-12 isotope.

Te number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value 6.022 x 10²³. This number is called avogadro’s number. Avogadro’s number (N_A or N₀) = 6.022 x 10²³

The volume occupied by one mole of molecules of a gaseous substance is called molar volume or gram molecular volume.

Class 9 Science Chapter 3 Notes Important Terms

Element: A chemical substance that cannot be decomposed by chemical means into simpler parts. It contains same kind of atoms. There are 118 elements known, out of which 94 are natural.

Compound: When two or more atoms of different elements chemically combine, the molecule of a compound is obtained. The examples of some compounds are water, ammonia, sugar, etc.

Molar mass: Molar mass is the sum of the atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together.

Ion: Any atom or molecule which has a resultant electric charge due to loss or gain of valence electrons.

Valency: It is the capacity of atoms of a given element that tend to combine with, or replace atoms of hydrogen. In HCl gas, valency of chlorine is 1.

Atomicity: It corresponds to the number of atoms present in a given molecule of an element. For example, ozone (O₃) has an atomicity of 3 and benzene (C₆H₆) has an atomicity of 12.

Mole: [Historically the word ‘mole’ was introduced about hundred years back by Wilhelm Ostwald. He derived this word from the Latin word ‘moles’ meaning ‘ heap or a pile’]. A mole is the amount of a substance that contains as many elementary entities (atoms, molecules or other particles) as there are atoms in exactly 0.012 kg of the carbon-12 isotope.