Class 9 Science Chapter 2 Notes : Cell The Building Block of Life

Class 9 Science Chapter 2 Notes : Cell The Building Block of Life

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  Chapter 2: CellThe Building Block of Life Origin of Life and Importance of Cells Scientists believe that life first originated in water. Some researchers suggest that life may have begun in small water pools or hot springs where environmental conditions changed frequently. Such conditions were present on early Earth about 3.5 billion years ago . The hot springs of Puga Valley in Ladakh are examples of such environments. These hot springs contain thermophiles , which are heat - loving unicellular bacteria. Scientists found that calcium carbonate deposits around these springs may have protect ed early organic molecules from harmful radiation and helped in forming the first cell membrane. All living organisms are made up of cells , which are the basic units of life. Some organisms like bacteria and yeast have only one cell and are called unicellular organisms , while organisms like plants, animals, and humans have many cells and are called multicellular organisms . Similar cells form tissues , tissues form organs , and organs together form organ systems such as the respiratory system. NCERTGuide.in https://www.ncertguide.in/ 1
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  Cell – TheBasic Unit of Life The cell is called the structural and functional unit of life because all living activities occur within cells. Even though cells are tiny, they perform all life processes such as respiration, nutrition, growth, excretion, and reproduction. How to Study Cells? The human eye cannot see very tiny objects clearly. The minimum distance at which two points can be seen separately is called the limit of resolution , which for the human eye is 0.1 mm. Since most cells are smaller than this, we need microscopes to study them. Microscope A microscope uses convex lenses to magnify tiny objects. Robert Hooke first observed cells in 1665 using a microscope while examining cork and named the box - like structures “cells.” Types of Microscopes 1. Light Microscope – Uses visible light and can magnify objects up to about 1000 times. 2. Electron Microscope – Uses electrons instead of light and can show structures at the nanometre level with very high clarity. Important Features of a Microscope ✔ Magnification – Makes the object appear larger. ✔ Resolution – Ability to distinguish two close points clearly. ✔ Contrast – Difference in brightness between different parts. Estimating Cell Size Scientists estimate cell size by comparing the microscope field diameter with the number of cells visible across it. Example: If field diameter is 5000 µm and 25 cells fit across it, then one cell is 200 µm . Structure of a Cell Every cell has three main parts: 1. Cell membrane 2. Cytoplasm 3. Nucleus In addition, eukaryotic cells contain organelles. NCERTGuide.in https://www.ncertguide.in/ 2
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  2.2 Cell Membrane– Universal Feature of a Cell The cell membrane (or plasma membrane) is a thin protective boundary around the cell. It separates the cell from its surroundings and controls the movement of substances. Selectively Permeable The cell membrane is selectively permeable , meaning it allows some substances to pass but blocks others. Osmosis The movement of water through a selectively permeable membrane from a dilute solution to a concentrated solution is called osmosis . Example: • Potato in plain water swells because water enters the cells. • Potato in salt solution shrinks because water leaves the cells. Diffusion The movement of particles from higher concentration to lower concentration is called diffusion . Types of Solutions 1. Isotonic – Equal solute concentration inside and outside the cell. 2. Hypotonic – Lower solute concentration outside; water enters cell. 3. Hypertonic – Higher solute concentration outside; water leaves cell. Fluid Mosaic Model of Cell Membrane NCERTGuide.in https://www.ncertguide.in/ 3
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  The cell membraneis made of lipids and proteins and is only 7–10 nm thick . Features: ✔ Lipid bilayer with two layers of fats. ✔ Proteins act as gatekeepers. ✔ Molecules can move sideways, so membrane is fluid. ✔ Mosaic arrangement gives it the name “fluid mosaic model. ” 2.2.2 Cell Wall – Outer Covering of Plant Cells Plant cells have an extra rigid layer outside the cell membrane called the cell wall . Functions: ✔ Provides shape and support. ✔ Protects against environmental stress like wind and rain. ✔ Keeps plant upright. ✔ Allows water and minerals to pass. The cell wall is mainly made of cellulose , a carbohydrate made of glucose units. Cellulose in food acts as roughage . Animal cells do not have a cell wall, so they can easily change shape. Difference Between Plant and Animal Cells Plant cells: Animal cells: ✔ Cell wall present ✔ Fixed shape ✔ Large vacuole ✔ Plastids present ✔ No cell wall ✔ Flexible shape NCERTGuide.in https://www.ncertguide.in/ 4
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  ✔ Small vacuoles ✔No plastids 2.3 Cell Interior – Coordinated Working System Inside the cell is a jelly - like substance called cytoplasm . It contains organelles and cell inclusions. Types of Cells Prokaryotic Cells Eukaryotic Cells “Pro” = primitive, “karyon” = nucleus Examples: bacteria Features: ✔ No true nucleus ✔ DNA present in nucleoid ✔ No membrane - bound organelles ✔ Small (1 –10µm) “Eu” = true Examples: plants and animals Features: ✔ True nucleus ✔ Membrane - bound organelles present ✔ Larger (10 –100 µm) NCERTGuide.in https://www.ncertguide.in/ 5
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  Cell Organelles andTheir Functions 1. Nucleus – Control Centre The nucleus controls all cell activities. Parts: ✔ Nuclear membrane with pores ✔ Nucleolus – forms ribosomes ✔ Chromatin – thread - like DNA ✔ Chromosomes – visible during cell division DNA and Genes DNA stores hereditary information. Genes are functional parts of DNA responsible for traits. 2. Ribosomes – Protein Factories Ribosomes synthesize proteins. They may float freely or attach to ER. 3. Endoplasmic Reticulum (ER) – Manufacturing Unit Network - like organelle connected to nucleus. Types: Rough ER (RER) – has ribosomes; makes proteins. Smooth ER (SER) – no ribosomes; makes fats and hormones. 4. Golgi Apparatus – Packaging Centre Modifies, packages, and transports proteins and lipids. It acts like the cell’s post office. NCERTGuide.in https://www.ncertguide.in/ 6
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  5. Lysosomes –Clean - up Crew Contain digestive enzymes. Functions: ✔ Break waste materials ✔ Destroy damaged organelles ✔ Keep cell clean 6. Mitochondria – Powerhouse Produces energy through cellular respiration . Energy is stored as ATP (Adenosine Triphosphate) . Structure: ✔ Double membrane ✔ Inner folds called cristae 7. Plastids – Plant Cell Organelles Present only in plant cells. Chloroplast Contains chlorophyll and performs photosynthesis . Chromoplast Contain coloured pigments like red, yellow, and orange. Give colour to flowers and fruits. Leucoplast Colourless plastids. Store starch, oils, or proteins. NCERTGuide.in https://www.ncertguide.in/ 7
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  8. Vacuoles –Storage Organelles Store water, minerals, sugars, and waste. In plants: ✔ One large central vacuole ✔ Maintains turgidity Loss of water causes wilting. In animals: ✔ Small temporary vacuoles Synthetic Cell In 2010, scientist J. Craig Venter created a cell controlled by synthetic DNA. This proved that DNA controls cell activities , though the whole cell was not created from scratch. 2.4 Cell Division Cells grow only up to a limit, so body growth occurs mainly through cell division . Definition: Cell division is the process by which new cells are formed from existing cells. Functions: ✔ Growth ✔ Repair ✔ Replacement ✔ Reproduction Types of Cell Division 1. Mitosis Produces two identical daughter cells . Importance: ✔ Growth ✔ Tissue repair ✔ Asexual reproduction NCERTGuide.in https://www.ncertguide.in/ 8
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  parent cell →identical daughter cells 2. Meiosis Produces four daughter cells with half the chromosome number . Occurs in reproductive organs to form gametes . parent cell → gametes chromosomes} Importance: ✔ Sexual reproduction ✔ Genetic variation Errors in mitosis may cause tumours . Errors in meiosis may cause genetic disorders . Cell Culture Growing cells outside the body in controlled conditions is called cell culture . It is used for: ✔ Medicine production ✔ Vaccine production ✔ Scientific research 2.5 Cell Theory Developed by: • Matthias Schleiden (1838) – All plants are made of cells. • Theodor Schwann (1839) – All animals are made of cells. • Rudolf Virchow (1855) – New cells arise from pre - existing cells. Cell Theory States: 1. All living organisms are made of one or more cells. 2. The cell is the basic structural and functional unit of life. 3. All cells arise from pre - existing cells. Do Cells Live Forever? No. Every cell has a fixed lifespan. Old cells die and are replaced by new ones. Contact Inhibition NCERTGuide.in https://www.ncertguide.in/ 9
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  Normal animal cellsstop dividing when they touch neighboring cells. This is called contact inhibition . Cancer Cells Cancer cells lose contact inhibition and divide uncontrollably, forming tumours . Programmed Cell Death (PCD) Cells naturally die when needed. This helps maintain body balance. Example: During embryo development, cells between fingers die to form separate fingers. Important Summary ✔ Cell is the structural and functional unit of life. ✔ Cells may be unicellular or multicellular. ✔ Prokaryotic cells lack true nucleus; eukaryotic cells have true nucleus. ✔ Cell membrane is selectively permeable. ✔ Osmosis is movement of water through membrane. ✔ Plant cells have cell wall made of cellulose. ✔ Nucleus stores DNA and controls cell activities. ✔ Mitochondria produce ATP. ✔ Chloroplast performs photosynthesis. ✔ Vacuole stores water and maintains firmness. ✔ Mitosis forms 2 identical cells. ✔ Meiosis forms 4 gametes. ✔ Cell theory explains unity of life. ✔ Cancer occurs due to uncontrolled cell division. Every living organism, from tiny bacteria to giant trees and humans, is built from tiny hardworking cells. Cells truly are the building blocks of life . NCERTGuide.in https://www.ncertguide.in/ 10