Cell: The Building Block of Life Class 9 Notes Part - 1 PDF
On this page, you will find comprehensive Cell: The Building Block of Life Class 9 Notes, including Class 9 Science Chapter 2 Cell Notes Handwritten, easy-to-understand explanations, labelled diagrams, definitions, important terms, and quick revision points. These notes follow the new NCERT syllabus and are ideal for school exams, unit tests, annual examinations, and competitive foundation preparation.Cell: The Building Block of Life Notes PDF, Cell: The Building Blocks of Life Class 9 Question Answer, Important Questions Class 9 Science Chapter 2, and Chapter 2 Cell: The Building Block of Life Worksheet PDF to strengthen your preparation. Every topic is presented in a student-friendly format so that learning becomes easier and more effective. At Info Edu Story, our goal is to provide accurate, updated, and exam-focused educational content. Bookmark this page and explore our complete collection of Class 9 Science Chapter 2 Cell: The Building Block of Life Notes, printable PDFs, handwritten notes, worksheets, and important questions to score excellent marks in your examinations.
Cell: The Building Block of Life Class 9 Question Answer, Worksheet PDF & Notes
Cell – The Fundamental Unit of Life
➨ The organisms living in hot springs are mostly heat-loving bacteria called thermophiles.
➨ Thermophiles are unicellular organisms, which means they are made up of a single cell.
➨ All living organisms are made up of cells.
➨ A cell is the smallest structural and functional unit of life.
➨ A cell is called the structural unit because all living organisms are composed of cells.
➨ A cell is called the functional unit because it is capable of performing all the basic life processes.
➨ A group of similar cells performing the same function is called a tissue.
➨ Different tissues combine together to form an organ.
➨ Several organs work together to form an organ system.
➨ For example, the nasal pores, nasal cavity, trachea, and lungs together form the respiratory system.
➨ The respiratory system helps in the process of breathing.
➨ According to the Cell Theory, all organisms are made up of cells.
➨ The Cell Theory also states that all cells arise from pre-existing cells.
➨ Therefore, the cell is the basic building block of all living organisms.
Other discoveries about cell are:
◉ Cell was discovered by Robert Hooke in 1665.
◉ Robert Brown –Nucleus of the cell in 1831
◉ Anton Van Leeuwenhoek – Free living cells in pond water (bacteria) in 1674
◉ Purkinje – Fluid substance of cell (Protoplasm) in 1839
◉ J.E.Virchow- Expanded cell theory (all cells arise from pre-existing cells) in 1855
◉ M.J. Schleiden and T. Schwann – Gave cell theory (cell is basic unit of life) in 1838-39
◉ The discovery of electron microscope in 1940 helped to understand complex structure of cell and its organelles.
Shapes of Cells
➨ Some cells are round (spherical), while others are oval or irregular.
➨ A nerve cell is long and branched, which helps in the transmission of nerve impulses throughout the body.
➨ A muscle cell is spindle-shaped, which helps in the movement of the body.
➨ Animal cells are generally round (spherical) or irregular in shape.
➨ Plant cells are usually rectangular because they have a rigid cell wall.
Size of a cell : -
◉ The size of cells varies greatly among different organisms.
◉ The smallest known cell is Mycoplasma, which is about 0.0001 mm in size.
◉ The largest known single cell is Caulerpa taxifolia, which can grow up to 6–12 inches (15–30 cm) long.
◉ Although cells differ greatly in size, all cells perform the basic functions of life.
Types of Cells : –
Cells can be classified into two main types based on their internal structure:
1. Prokaryotic Cells
2. Eukaryotic Cells
1. Prokaryotic Cells : -
➨ Prokaryotic cells are simple cells that do not have a true nucleus.
➨ Their genetic material is present in a region called the nucleoid, instead of being enclosed by a nuclear membrane.
➨ These cells are usually unicellular (single-celled) organisms.
➨ They are very small, generally measuring 0.1–0.5 µm in diameter.
➨ Examples include bacteria, archaea, and cyanobacteria.
➨ They reproduce mainly through binary fission, which is a type of asexual reproduction.
➨ Some prokaryotes also exchange genetic material through conjugation, but this is not considered true sexual reproduction.
Characteristics of Prokaryotic Cells
◉ No true nucleus.
◉ No membrane-bound organelles.
◉ Small and simple in structure.
◉ Mostly unicellular.
◉ Reproduce by binary fission.
2. Eukaryotic Cells
➨ Eukaryotic cells are more complex and contain a well-defined nucleus enclosed by a nuclear membrane.
➨ They possess several membrane-bound organelles that perform different functions.
➨ Their size usually ranges from 10–100 µm in diameter.
➨ Eukaryotic cells are found in plants, animals, fungi, and protozoa.
➨ The plasma membrane controls the movement of substances into and out of the cell and also helps in communication between cells.
➨ These cells can reproduce by both sexual and asexual methods.
Characteristics of Eukaryotic Cells
◉ True nucleus present.
◉ Membrane-bound organelles present.
◉ Larger and more complex than prokaryotic cells.
◉ Found in both unicellular and multicellular organisms.
◉ Reproduce sexually as well as asexually.
| Prokaryotes | Eukaryotes |
|---|---|
| 1. Do not have a well-organized (true) nucleus. | 1. Have a well-organized, membrane-bound nucleus. |
| 2. Nuclear membrane is absent. | 2. Nuclear membrane is present. |
| 3. Examples: Bacteria and Blue-green algae (Cyanobacteria). | 3. Examples: Amoeba and Onion cell. |
| 4. Cell size is small (about 1–10 µm). | 4. Cell size is larger (about 5–100 µm). |
| 5. Genetic material is present in a region called the nucleoid. | 5. Genetic material is enclosed inside a well-defined nucleus surrounded by a nuclear membrane. |
| 6. Usually have a single circular chromosome. | 6. Usually have more than one linear chromosome. |
| 7. Membrane-bound cell organelles are absent. | 7. Membrane-bound cell organelles are present. |
Structure of a Cell : –
The structure of a cell consists of different parts called cell organelles. Each organelle has a specific function that helps the cell perform its activities efficiently. These organelles are suspended in the cytoplasm.
Basic components of cell- Plasma membrane, Nucleus and cytoplasm.
Plasma Membrane (Cell Membrane) : –
The plasma membrane, also called the cell membrane, is a thin, flexible, living boundary that surrounds the cell. In animal cells, it is the outermost covering, while in plant cells, it lies just inside the cell wall. The plasma membrane protects the cell, gives it a definite shape, and separates the cell's internal contents from the external environment.
Characteristics of the Plasma Membrane
➨ The plasma membrane is the thin outer covering of the cell.
➨ It is also known as the cell membrane.
➨ In animal cells, it forms the outermost layer of the cell.
➨ It is a living, thin, and highly flexible membrane.
➨ It is mainly made up of proteins and lipids (fats).
➨ It gives the cell its shape and provides mechanical support.
➨ It protects the inner contents of the cell from external damage.
➨ It separates the inside of the cell from the outside environment.
➨ The plasma membrane is selectively permeable. This means it allows only certain substances to enter or leave the cell while preventing others from passing through.
➨ It helps maintain the internal balance (homeostasis) of the cell.
Functions of the Plasma Membrane
◉ Protects the cell and its organelles.
◉ Gives shape and support to the cell.
◉ Controls the movement of substances into and out of the cell.
◉ Maintains the internal environment of the cell.
◉ Enables the exchange of gases, nutrients, water, and waste products.
◉ Helps the cell communicate with its surroundings.
Example : - In the alveoli of the lungs, oxygen diffuses through the plasma membrane into the blood, while carbon dioxide moves out of the blood into the alveoli. This exchange is possible because the plasma membrane is thin and selectively permeable.
Diffusion : -
◉ Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration until they become evenly distributed.
◉ It occurs without the use of energy.
◉ Diffusion can take place in solids, liquids, and gases.
◉ A selectively permeable membrane is not required for diffusion.
◉ It helps in the exchange of gases such as oxygen (O₂) and carbon dioxide (CO₂) between cells and their surroundings.
Osmosis : -
◉ Osmosis is the movement of water molecules from a region of higher water concentration (or lower solute concentration) to a region of lower water concentration (or higher solute concentration) through a selectively permeable membrane.
◉ Osmosis occurs only in liquids.
◉ A selectively permeable (semipermeable) membrane is essential for osmosis.
◉ It helps plant roots absorb water from the soil.
◉ Osmosis also maintains the water balance and shape of living cells.
◉ The movement of water during osmosis depends on the difference in concentration on the two sides of the membrane.
Question : - What happens when we put an animal cell into a solution of salt or sugar?
Solute + Solvent ---------- SOLUTION
Three things can happen-
a) Higher water concentration than a cell in surrounding medium i.e. the solution is dilute- forms hypotonic solution-Cell gains water and swells up (becomes turgid)-process is Endosmosis.
b) Lower concentration of water than a cell in surrounding medium i.e. the solution is concentrated-forms Hypertonic solution- Cell loses water and shrinks (plasmolysed i.e. becomes flaccid)-process is Exosmosis.
c) Same concentration inside and outside the cell-forms Isotonic solution- No change and remains normal cell.
Conclusion: Osmosis is special kind of diffusion through plasma membrane which can be affected by the amount of substance dissolved in water.
Cell wall — The outer covering of cells
The cell wall is the outermost covering of a plant cell. It is a rigid, non-living layer present outside the plasma membrane. The cell wall provides strength, protection, and support to the plant cell.
Characteristics of the Cell Wall
➨ The cell wall is found only in plant cells and is absent in animal cells.
➨ It is the outermost layer of a plant cell.
➨ It is non-living, rigid, and fully permeable, allowing water and dissolved substances to pass freely.
➨ The cell wall is mainly composed of cellulose, a complex carbohydrate.
➨ It also contains hemicellulose and pectin, which provide additional strength and support.
➨ The cell wall surrounds and protects the plasma membrane.
Functions of the Cell Wall
◉ Provides structural strength and rigidity to the plant cell.
◉ Gives the cell its definite shape.
◉ Protects the cell from mechanical injuries and external damage.
◉ Supports the plant and helps it remain upright.
◉ Prevents the cell from bursting when excess water enters by osmosis, especially in a hypotonic solution.
◉ Allows free movement of water, gases, and dissolved substances because it is fully permeable.
