NCERT Science Notes - Class 9
Chapter 5 - The Fundamental Unit of Life

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 9 | Science | Chapter 5 - The Fundamental Unit of Life. In this chapter, students are introduced to the cell, the basic structural and functional unit of life. They learn about the discovery of the cell, the cell theory, and the different types of cells—prokaryotic and eukaryotic. The chapter covers the structure and functions of various cell organelles like the nucleus, mitochondria, ribosomes, endoplasmic reticulum, and more. Understanding cell division through processes like mitosis and meiosis is also highlighted. This foundational knowledge helps students grasp how life is organized at the cellular level and the importance of cells in the functioning of living organisms.

Class 9 | Science | Chapter 5 - The Fundamental Unit of Life notes ajs, cbse notes class 10 ajslearning, cbse notes ajs, ajs notes class 10, ajslearning, ajs chalo seekhen

NCERT Science Notes - Class 9
Chapter 5 - The Fundamental Unit of Life

Introduction to Cells

• Historical Context:
  • In 1665, Robert Hooke observed a thin slice of cork through a self-designed microscope.
  • He noted that the cork resembled a honeycomb structure made up of small compartments, which he termed cells.
• Etymology:
  • The term "cell" comes from the Latin word cellula, meaning "a little room."
• Significance:
  • This observation marked a crucial milestone in biology as it was the first time living organisms were recognized as consisting of separate, individual units—cells.

1. Definition of a Cell:
   • A cell is the basic structural and functional unit of all living organisms.
2. Importance in Biology:
   • The concept of cells provides a foundational framework for understanding life, as all living things are composed of cells.
3. Applications:
   • The study of cells, known as cell biology or cytology, helps in understanding various biological processes, disease mechanisms, and the functions of different types of cells.

Questions and Answers:

1. Who first observed cells, and what substance did he examine?
   • Robert Hooke first observed cells while examining a thin slice of cork.
2. What did Hooke compare the structure of cork to?
   • Hooke compared the structure of cork to a honeycomb, consisting of many little compartments.
3. What does the word "cell" mean?
   • The word "cell" is derived from the Latin word cellula, which means "a little room."
4. Why is the observation of cells significant in the history of science?
   • It was the first indication that living organisms are composed of individual units, laying the groundwork for the study of biology and cellular science.

5.1 - What Are Living Organisms Made Up Of?

Activity 5.1 

• Activity Overview:
  • The activity involves preparing a temporary mount of onion peel to observe its cellular structure under a microscope.

1. Preparation of the Peel:
   • Take a small piece from an onion bulb.
   • Peel off the epidermis (the outer layer) from the concave side of the onion using forceps.
   • Place the peel in a watch glass with water to prevent it from drying or folding.
2. Mounting the Peel:
   • Transfer a small piece of the peel to a glass slide with a drop of water.
   • Ensure the peel is flat on the slide, using a thin camel hair brush if necessary.
   • Add a drop of safranin solution on the peel and cover it with a cover slip, avoiding air bubbles.
3. Observation:
   • Observe the prepared slide first under low power and then under high power using a compound microscope.
4. Drawing Observations:
   • Draw the structures observed through the microscope on an observation sheet.

Key Observations:

• The structures observed in the onion peel are cells, which appear similar regardless of the onion's size.
• These cells are the basic building units of the onion bulb.
• All living organisms, whether multicellular (like onions) or unicellular (single-celled organisms), are composed of cells.

1. Definition of Cells:
   • Cells are the smallest structural and functional units of life, responsible for carrying out all essential processes of living organisms.
2. Types of Organisms:
   • Multicellular Organisms: Organisms made up of many cells (e.g., plants, animals).
   • Unicellular Organisms: Organisms consisting of a single cell (e.g., bacteria, amoeba).
3. Uniformity of Cells:
   • Cells from different onions of varying sizes show similar structures, indicating a consistent cellular makeup across the same type of organism.

1. What was the main purpose of the activity with the onion peel?
   • The main purpose was to observe the cellular structure of the onion peel under a microscope.
2. What structures are observed in the onion peel under the microscope?
   • The observed structures are cells, which are the basic building units of the onion bulb.
3. Do cells from onions of different sizes look the same?
   • Yes, the cells appear similar regardless of the size of the onion they came from.
4. What are the two types of organisms based on cell number?
   • Organisms can be classified as multicellular (made of many cells) or unicellular (made of a single cell).

More to Know

1. Historical Discoveries in Cell Biology:
   • Robert Hooke (1665): Discovered cells by observing a cork slice under a primitive microscope, identifying them as small compartments that he named "cells."
   • Antonie van Leeuwenhoek (1674): Using an improved microscope, he discovered free-living cells in pond water for the first time.
   • Robert Brown (1831): Identified the nucleus as a distinct structure within the cell.
   • Jan Evangelista Purkinje (1839): Coined the term "protoplasm" to describe the fluid substance within cells.
   • Schleiden and Schwann (1838-1839): Formulated the cell theory, stating that all plants and animals are composed of cells and that the cell is the basic unit of life.
   • Rudolf Virchow (1855): Expanded the cell theory by suggesting that all cells arise from pre-existing cells.
   • Electron Microscope (1940): Enabled scientists to observe and understand the complex structures of cells and their organelles in greater detail.
2. Types of Organisms:
   • Unicellular Organisms: Organisms composed of a single cell, such as:
     • Amoeba
     • Chlamydomonas
     • Paramecium
     • Bacteria
   • Multicellular Organisms: Organisms made up of many cells that group together and perform various functions, forming different body parts. Examples include:
     • Fungi
     • Plants
     • Animals
3. Cell Division and Origin of Multicellular Organisms:
   • Every multicellular organism originates from a single cell. This occurs through the process of cell division, where a cell divides to produce new cells of its kind. This concept reinforces the idea that all cells come from pre-existing cells, forming the foundation for growth and development in multicellular organisms.

• Cell Theory: Fundamental principles of cell biology that describe the properties and functions of cells as the basic units of life.
• Unicellular vs. Multicellular: Differentiation between organisms that consist of one cell versus those composed of multiple cells that work together.
• Cell Division: The process by which a cell replicates and divides, allowing for growth, development, and repair in multicellular organisms.

Activity 5.2: Observing Different Types of Cells

1. Preparation:
   • Prepare temporary mounts of leaf peels, the tip of onion roots, or onion peels of different sizes.
2. Questions for Discussion:(a) Do all cells look alike in terms of shape and size?
   • Answer: No, cells can vary significantly in shape and size. For example, plant cells are often rectangular, while animal cells can be more rounded or irregular.
   (b) Do all cells look alike in structure?
   • Answer: No, different types of cells have varying structures. For example, muscle cells have a different structure compared to nerve cells or plant cells, reflecting their specific functions.
   (c) Could we find differences among cells from different parts of a plant body?
   • Answer: Yes, cells from different parts of a plant can show differences. For instance, leaf cells (mesophyll) are designed for photosynthesis, while root cells are adapted for water and nutrient absorption.
   (d) What similarities could we find?
   • Answer: Despite differences, all plant cells share some similarities, such as having a cell wall, chloroplasts, and a central vacuole, which serve common functions within the plant structure.
3. Human Cells:
   • Some organisms, like humans, have various types of cells, each specialized for different functions, such as muscle cells for movement, nerve cells for signaling, and blood cells for transport.

The Relationship Between Cell Structure and Function

1. Shape and Size:
   • The shape and size of cells are closely related to the specific functions they perform.
   • For example:
     • Amoeba: This unicellular organism exhibits changing shapes due to its ability to move and capture food.
     • Nerve Cells: Nerve cells (neurons) have a fixed and distinct shape that aids in transmitting signals efficiently.
2. Basic Functions of Living Cells:
   • Every living cell has the capacity to perform certain basic functions that characterize all living forms.
   • These functions include:
     • Metabolism (chemical processes),
     • Growth and repair,
     • Response to stimuli, and
     • Reproduction.
3. Division of Labour in Multicellular Organisms:
   • In multicellular organisms, such as humans, there is a division of labor, meaning different organs and systems are specialized to perform specific functions:
     • Heart: Pumps blood throughout the body.
     • Stomach: Digests food.
4. Division of Labour Within Cells:
   • Similar to multicellular organisms, division of labor occurs within single cells.
   • Each cell contains specialized components known as cell organelles.
   • Functions of organelles include:
     • Nucleus: Controls cell activities and contains genetic material.
     • Mitochondria: Produce energy through cellular respiration.
     • Ribosomes: Synthesize proteins.
     • Lysosomes: Break down waste materials and cellular debris.
5. Importance of Organelles:
   • Organelles are crucial for the survival and functionality of the cell.
   • They enable the cell to live and perform its functions by working together, ensuring that various cellular processes are efficiently carried out.
6. Commonality Among Cells:
   • Interestingly, despite their different functions and the organisms they belong to, all cells possess similar organelles. This reflects a common structural and functional framework essential for life.

5.2 - What is a Cell Made Up of?
What is the Structural Organisation of a Cell?

What is a Cell Made Up Of?

A cell is primarily composed of three key features:

1. Plasma Membrane:
   • Structure: The plasma membrane is a thin, flexible barrier surrounding the cell.
   • Function: It regulates the movement of substances in and out of the cell, maintaining the internal environment (homeostasis). It is selectively permeable, allowing certain molecules to pass while blocking others.
2. Nucleus:
   • Structure: The nucleus is a membrane-bound organelle that contains the cell's genetic material (DNA).
   • Function: It acts as the control center of the cell, coordinating activities such as growth, metabolism, and reproduction. The nucleus is responsible for gene expression and the synthesis of ribosomes (in the nucleolus).
3. Cytoplasm:
   • Structure: The cytoplasm is the gel-like substance that fills the cell, lying between the plasma membrane and the nucleus.
   • Function: It contains various organelles and is the site of many metabolic processes. The cytoplasm provides a medium for the movement of materials within the cell and supports the organelles.

Structural Organisation of a Cell

The structural organisation of a cell can be described as follows:

1. Cell Membrane:
   • Composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
   • Proteins act as channels or receptors, facilitating communication and transport.
2. Nucleus:
   • Surrounded by a double membrane called the nuclear envelope, which has pores allowing exchange of materials (RNA and proteins) between the nucleus and cytoplasm.
   • Contains chromatin (DNA and proteins) that condenses to form chromosomes during cell division.
3. Cytoplasm:
   • Contains organelles, each performing specific functions. Some key organelles include:
     • Mitochondria: Produce energy (ATP) through cellular respiration.
     • Endoplasmic Reticulum (ER):
       • Rough ER: Studded with ribosomes; synthesizes proteins.
       • Smooth ER: Synthesizes lipids and detoxifies harmful substances.
     • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.
     • Lysosomes: Contain digestive enzymes for breaking down waste materials.
     • Ribosomes: Sites of protein synthesis, either free-floating in the cytoplasm or attached to the rough ER.
     • Centrioles: Involved in cell division (found in animal cells).

5.2.1 - PLASMA MEMBRANE OR CELL MEMBRANE

The plasma membrane (or cell membrane) is the outermost covering of the cell that separates its contents from the external environment. It serves several important functions:

• Selective Permeability: The plasma membrane allows certain materials to enter and exit the cell while preventing the movement of others. Because of this property, it is termed a selectively permeable membrane.

Movement of Substances

The movement of substances across the plasma membrane can occur through various processes:

• Diffusion: This is the spontaneous movement of substances from a region of higher concentration to a region of lower concentration. For example:
  • Carbon Dioxide (CO₂): When CO₂ accumulates inside the cell (high concentration), it moves out to the external environment (low concentration) through diffusion. This is necessary for the excretion of cellular waste.
  • Oxygen (O₂): Conversely, when the concentration of O₂ inside the cell decreases, O₂ enters the cell from the external environment, again via diffusion.
• Gaseous Exchange: Diffusion plays a critical role in the exchange of gases (O₂ and CO₂) between the cells and their surroundings.
• Osmosis: This is a specific type of diffusion that involves the movement of water molecules through a selectively permeable membrane. Water moves from an area of higher water concentration to an area of lower water concentration, which can also affect the cell's internal environment.

Osmosis and Cell Behavior in Different Solutions

The movement of water across the plasma membrane is influenced by the concentration of solutes dissolved in water. Osmosis is defined as the net diffusion of water across a selectively permeable membrane toward a region of higher solute concentration.

Effects of Different Solutions on Cells

When animal or plant cells are placed in solutions of sugar or salt, one of the following three scenarios can occur:

1. Hypotonic Solution:
   • Definition: A solution where the concentration of water outside the cell is higher than inside the cell (very dilute).
   • Outcome: The cell will gain water by osmosis because more water will enter the cell than will leave. This can cause the cell to swell, and in extreme cases, it may burst.
2. Isotonic Solution:
   • Definition: A solution where the concentration of water is equal inside and outside the cell.
   • Outcome: There is no net movement of water across the cell membrane; water molecules move in and out at equal rates. The cell will maintain its size.
3. Hypertonic Solution:
   • Definition: A solution where the concentration of water outside the cell is lower than inside the cell (very concentrated).
   • Outcome: The cell will lose water by osmosis, leading to more water exiting the cell than entering it. As a result, the cell will shrink.

Activity 5.3: Osmosis with an Egg

Objective: To observe osmosis using an egg as a model.

Materials:

• An egg
• Dilute hydrochloric acid
• Pure water
• Concentrated salt solution

• Remove the shell of the egg by placing it in dilute hydrochloric acid, which dissolves the calcium carbonate shell. You will be left with a thin outer skin (the plasma membrane) enclosing the egg.
• Observation in Pure Water: Place the de-shelled egg in pure water for 5 minutes.
  • Expected Result: The egg will swell as water passes into it by osmosis.

• Place another de-shelled egg in a concentrated salt solution for 5 minutes.
• Observation:
  • Expected Result: The egg will shrink as water exits the egg and moves into the salt solution, which has a higher concentration of solute.

Activity 5.4: Observing Osmosis in Dried Fruits

Objective: To observe the effects of osmosis on dried raisins or apricots when placed in different solutions.

Materials:

• Dried raisins or apricots
• Plain water
• Concentrated sugar or salt solution

1. Soaking in Water:
   • Place dried raisins or apricots in plain water and leave them for some time.
   • Observation:
     • (a) The dried fruits will absorb water and swell due to osmosis.
2. Soaking in Concentrated Solution:
   • After swelling, transfer the soaked fruits to a concentrated solution of sugar or salt.
   • Observation:
     • (b) The fruits will lose water and consequently shrink when placed in the concentrated solution.

• Unicellular Organisms: Freshwater unicellular organisms, such as Amoeba, gain water through osmosis, as the concentration of water outside their cells is higher than inside.
• Plant Cells: Most plant cells also gain water via osmosis, especially through the absorption of water by their roots. This process is crucial for maintaining turgor pressure, which helps keep plants upright.

• Gas and Water Exchange: Diffusion plays a vital role in the exchange of gases (like oxygen and carbon dioxide) and water within the life of a cell. This is crucial for cellular respiration and other metabolic processes.
• Nutrient Absorption: In addition to osmosis and diffusion, cells obtain nutrients from their environment through active transport mechanisms, which require energy. This allows cells to take in necessary substances even when they are in lower concentrations outside the cell.

• Composition: The plasma membrane is flexible and primarily composed of organic molecules called lipids and proteins. Its structure can only be observed through an electron microscope.
• Functionality: The flexibility of the plasma membrane allows cells to perform endocytosis, a process where the cell engulfs food and other materials from its external environment. For example, Amoeba acquires its food using this method.

Activity 5.5: Exploring Electron Microscopes

Objective: To learn about electron microscopes and their significance in studying cell structures.

Steps to Perform the Activity:

1. Research:
   • Use resources available in the school library or reliable online sources to gather information about electron microscopes.
   • Focus on the following aspects:
     • What is an electron microscope?
     • How does it work?
     • Types of electron microscopes (e.g., Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM)).
     • Advantages of electron microscopes over light microscopes.
     • Applications of electron microscopy in biology and materials science.
     • Historical significance and key scientists involved in its development.
2. Discussion:
   • After gathering the information, discuss your findings with your teacher and classmates.
   • Share interesting facts or insights that you discovered during your research.
     
     

Expected Details about Electron Microscopes

• Definition: An electron microscope is a type of microscope that uses a beam of electrons to create an image of a specimen. It has a much higher resolution than light microscopes, allowing for the observation of very small structures, such as organelles within cells.
• How It Works:
  • Instead of visible light, electron microscopes use electrons, which have much shorter wavelengths, allowing for higher resolution imaging.
  • The specimen is often coated with a thin layer of metal to enhance contrast.
• Types:
  • Transmission Electron Microscope (TEM):
    • Electrons pass through a thin specimen, providing detailed images of internal structures.
  • Scanning Electron Microscope (SEM):
    • Electrons are scattered off the surface of a specimen, providing 3D images of the surface structure.
• Advantages:
  • Higher resolution than light microscopes, allowing for the observation of structures at the molecular and atomic levels.
  • Can visualize the surface and internal structures of cells.
• Applications:
  • Used in biology to study cell structures, organelles, and viruses.
  • Used in materials science to analyze the properties of materials at the microscopic level.
• Historical Significance:
  • Developed in the 1930s, with contributions from scientists such as Ernst Ruska, who was awarded the Nobel Prize for his work on electron optics.

5.2.2 - Cell Wall

Activity 5.6 : Observing Plasmolysis

Objective: To observe the phenomenon of plasmolysis in plant cells.

Materials Needed:

• Fresh onion or elodea (water plant) leaf
• Salt solution (high concentration)
• Microscope
• Glass slides and cover slips
• Dropper or pipette
• Forceps

1. Preparation of Specimen:
   • Take a small piece of the onion or elodea leaf.
   • Using forceps, peel off a thin layer from the leaf (the epidermis) and place it in a watch glass with a few drops of water to keep it moist.
2. Mounting the Specimen:
   • Place a drop of salt solution on a glass slide.
   • Transfer the peel onto the slide, ensuring it is flat.
   • Place a cover slip gently over the specimen, avoiding air bubbles.
3. Observation:
   • Observe the specimen under a microscope.
   • Initially, note the appearance of the cells in the normal (water) condition.
4. Inducing Plasmolysis:
   • Now, using a dropper, carefully add a few drops of salt solution around the cover slip to change the environment of the cells.
   • Wait a few minutes and observe the changes in the cells again under the microscope.
5. Record Observations:
   • Note any shrinkage of the cell contents away from the cell wall, indicating plasmolysis.
   • Sketch the appearance of the cells before and after adding the salt solution.

5.2.3 - Nucleus

The nucleus is a key component of the cell and plays an important role in controlling its activities.

Observation Using Stains:

• When we prepared the onion peel slide, we added iodine solution to stain the cells. Stains like iodine, safranin, or methylene blue are used because different parts of the cell absorb the stain to varying degrees, making the structures within the cell more visible under a microscope.
• Without the stain, the internal structures of the cell would be much harder to observe clearly.
• The darkly stained area in the cell is usually the nucleus, which stands out due to the differential staining of cellular components.

• The nucleus is often referred to as the control center of the cell because it regulates all the activities taking place within the cell.
• The nucleus contains genetic material (DNA) that determines the characteristics and functioning of the cell.

1. Nuclear Membrane: The nucleus is surrounded by a double-layered membrane called the nuclear membrane, which separates it from the rest of the cell.
2. Nucleoplasm: The fluid inside the nucleus is called the nucleoplasm.
3. Chromosomes: The nucleus contains chromosomes made of DNA, which carry genetic information. Chromosomes become visible under a microscope during cell division.
4. Nucleolus: Inside the nucleus, there is often a smaller, dense region called the nucleolus, which is responsible for making ribosomes.

• The nucleus controls cell activities, such as growth, metabolism, and reproduction.
• It houses the genetic material (DNA), which contains instructions for building proteins, the molecules responsible for most cellular functions.
• During cell division, the nucleus plays a vital role in the transmission of genetic information to the next generation of cells.

Activity 5.7: Observing Cheek Cells Under the Microscope

Nucleus and Cellular Organization

5.2.4 - Cytoplasm and Comparison of Prokaryotic and Eukaryotic Cells

Cytoplasm:

• The cytoplasm is the fluid-like substance inside the cell, enclosed by the plasma membrane.
• It contains various specialized organelles that carry out specific functions to keep the cell alive.
• The cytoplasm takes up most of the cell's space and stains lightly because it is mostly fluid.
• Cell organelles like the nucleus, mitochondria, and others are suspended in the cytoplasm.
• In prokaryotic cells, the cytoplasm lacks membrane-bound organelles and a well-defined nucleus. However, eukaryotic cells have both a nuclear membrane and organelles enclosed by membranes.

• Membranes play a crucial role in protecting and organizing cell components.
• For example, viruses do not have any membranes. As a result, they do not exhibit characteristics of life until they enter a living host cell, where they use the host’s cell machinery to replicate.

Comparison Between Prokaryotic and Eukaryotic Cells

5.2.5 - (i) Endoplasmic Reticulum (ER)

5.2.5 - (ii) Golgi Apparatus

Golgi Apparatus:

• Discovery: First described by Camillo Golgi.
• Structure: It consists of a system of membrane-bound vesicles (flattened sacs) arranged in stacks called cisterns.
• Connection with ER: The membranes of the Golgi apparatus often have connections with the membranes of the endoplasmic reticulum (ER), forming part of the complex cellular membrane system.

1. Storage and Packaging:
   • The Golgi apparatus stores, modifies, and packages materials synthesized near the ER into vesicles.
   • These materials are dispatched to various targets both inside and outside the cell.
2. Complex Sugars Formation:
   • The Golgi apparatus can convert simple sugars into complex sugars.
3. Lysosome Formation:
   • It is involved in the formation of lysosomes, which are cell organelles responsible for digestion and waste processing.

• Born: 1843 in Corteno, Italy.
• Education: Studied medicine at the University of Pavia, graduating in 1865.
• Work: His early work focused on the nervous system, and he developed a method of staining nerve cells known as the ‘black reaction’, using a weak solution of silver nitrate.
• Nobel Prize: Golgi shared the Nobel Prize in 1906 with Santiago Ramón y Cajal for their work on the structure of the nervous system.

5.2.5 - (iii) Lysosomes

Lysosomes:

• Structure: Lysosomes are membrane-bound sacs filled with digestive enzymes.
• Origin of Enzymes: These enzymes are produced by the rough endoplasmic reticulum (RER).

1. Waste Disposal System:
   • Lysosomes act as the waste disposal system of the cell, responsible for digesting foreign material and worn-out cell organelles.
   • When foreign materials like bacteria or food particles enter the cell, or when old organelles become dysfunctional, they are digested by the lysosomes.
2. Breaking Down Substances:
   • Lysosomes break down complex substances into simpler substances using their powerful digestive enzymes, which can break down almost all organic material.
3. Cell Cleaning:
   • By digesting harmful substances, lysosomes help keep the cell clean and healthy.
4. Suicide Bags:
   • Lysosomes are sometimes referred to as the ‘suicide bags’ of the cell. If the cell becomes damaged or its metabolism is disturbed, the lysosomes may burst, releasing their enzymes, which then digest the cell itself.

5.2.5 - (iv) Mitochondria

Mitochondria:

• Nickname: Mitochondria are referred to as the powerhouses of the cell.
• Membranes: They have two membrane coverings:
  1. Outer membrane: Porous, allowing materials to pass through.
  2. Inner membrane: Deeply folded, which increases the surface area for chemical reactions that generate ATP.

• Energy Production:
  • Mitochondria release energy required for the various chemical activities of life in the form of ATP (Adenosine Triphosphate) molecules.
  • ATP is called the energy currency of the cell, as it stores and provides the energy needed for cellular activities like synthesizing new compounds and performing mechanical work.
• ATP Production:
  • The folds in the inner membrane, known as cristae, enhance the ability to generate ATP by increasing the surface area for the chemical reactions that produce this energy.

• DNA and Ribosomes:
  • Mitochondria are unique because they contain their own DNA and ribosomes. This allows them to produce some of their own proteins independently from the rest of the cell.

5.2.5 - (v) Plastids

Plastids:

• Presence: Plastids are found only in plant cells.
• Types of Plastids:
  1. Chromoplasts:
     • Definition: Coloured plastids that contain pigments.
     • Function: Responsible for the color of fruits and flowers, aiding in attracting pollinators.
     • Chloroplasts: A type of chromoplast containing the pigment chlorophyll, crucial for photosynthesis in plants. They may also contain yellow or orange pigments in addition to chlorophyll.
  2. Leucoplasts:
     • Definition: White or colourless plastids.
     • Function: Primarily involved in the storage of materials such as starch, oils, and protein granules.

• Internal Organization:
  • Chloroplasts have numerous membrane layers embedded in a fluid material called the stroma.
  • The structure of chloroplasts is similar to that of mitochondria.

• DNA and Ribosomes: Like mitochondria, plastids possess their own DNA and ribosomes, enabling them to synthesize some of their own proteins independently.

5.2.5 - (vi) Vacuoles

Cell Division:

• Purpose of Cell Division:
  • To facilitate growth.
  • To replace old, dead, and injured cells.
  • To produce gametes necessary for reproduction.
• Types of Cell Division:
  • Mitosis:
    • Definition: The process by which most cells divide for growth.
    • Process:
      • A mother cell divides to produce two identical daughter cells.
      • Daughter cells have the same number of chromosomes as the mother cell.
    • Function: Mitosis is essential for growth and tissue repair.
  • Meiosis:
    • Definition: The process through which specific cells in reproductive organs or tissues divide to form gametes.
    • Process:
      • Involves two consecutive divisions, resulting in four new cells.
      • Each daughter cell contains half the number of chromosomes compared to the mother cell.
    • Importance: Meiosis is crucial for sexual reproduction, ensuring that offspring have the correct number of chromosomes.

• Reason for Halving Chromosome Number:
  • The reduction in chromosome number is essential for maintaining the species' chromosome count through generations.
  • During fertilization, the fusion of two gametes restores the full chromosome number in the offspring.

NCERT Science Notes - Class 9 | Science | Chapter 5 - The Fundamental Unit of Life

NCERT Science Notes - Class 9 | Science | Chapter 5 - The Fundamental Unit of Life