Class 8 Science Notes Chapter 8 (Cell: Structure and Functions) – Science Book

Okay, here are detailed notes for Chapter 8, "Cell: Structure and Functions," from the NCERT Class 8 Science textbook, tailored for government exam preparation.

Chapter 8: Cell - Structure and Functions

1. Introduction & Discovery

• Cell: The basic structural and functional unit of all living organisms. Just as bricks are the building blocks of a building, cells are the building blocks of life.
• Discovery:
  • Robert Hooke (1665): Observed thin slices of cork (dead plant tissue) under a self-designed microscope. He saw box-like compartments separated by walls and termed them "cells" (Latin: cellula = little room).
  • Antonie van Leeuwenhoek (Improved Microscope): First observed living cells like bacteria, protozoa, RBCs, and sperm in pond water.
• Cell Theory (Basic Idea): All living organisms are composed of cells, and the cell is the fundamental unit of life. (Though formally proposed later by Schleiden and Schwann, the concept originates here).

2. Organisms Show Variety in Cell Number, Shape, and Size

• Cell Number:
  • Unicellular Organisms: Made up of a single cell that performs all life functions (e.g., Amoeba, Paramecium, Bacteria, Yeast).
  • Multicellular Organisms: Made up of many cells (millions to trillions). These cells often group together to form tissues, organs, and organ systems, showing division of labour (e.g., Humans, Trees, Insects).
• Cell Shape:
  • Cells exhibit a variety of shapes related to their specific functions.
  • Examples:
    • Amoeba: Irregular shape, changes shape using pseudopodia (false feet) for movement and capturing food.
    • White Blood Cells (WBCs) in humans: Can change shape (similar to Amoeba).
    • Nerve Cell (Neuron): Long and branched to transmit messages over distances.
    • Muscle Cell: Spindle-shaped (pointed at both ends) to facilitate contraction and relaxation.
    • Red Blood Cells (RBCs): Round, biconcave shape to maximize surface area for oxygen transport and fit through narrow capillaries.
    • Plant cells often have a more fixed, somewhat rectangular shape due to the cell wall.
• Cell Size:
  • Cells vary greatly in size. Most cells are microscopic and measured in micrometres (µm) or microns (1 µm = 1/1,000,000 metre).
  • Smallest Cell: Bacteria (e.g., Mycoplasma) - about 0.1 to 0.5 µm.
  • Largest Cell: Egg of an Ostrich (approx. 170 mm x 130 mm).
  • Longest Cell: Nerve cell (can be over a metre long in humans/large animals).
  • Note: The size of a cell is generally not related to the size of the organism. For example, nerve cells in a rat and an elephant perform the same function and are structurally similar, though lengths might vary.

3. Cell Structure and Function - Parts of the Cell

All cells, despite variations, have three main parts:
a. Cell Membrane (Plasma Membrane)
b. Cytoplasm
c. Nucleus

• a) Cell Membrane (Plasma Membrane):
  • The outer boundary of the animal cell and located just inside the cell wall in plant cells.
  • Structure: Thin, delicate, living, flexible membrane.
  • Function:
    • Gives shape to the cell (especially animal cells).
    • Protects the inner contents of the cell.
    • Selectively Permeable: Controls the entry and exit of substances into and out of the cell ("porous" in NCERT terms). Allows necessary substances in and waste products out.
• b) Cytoplasm:
  • Jelly-like substance filling the cell, enclosed by the cell membrane.
  • Located between the cell membrane and the nucleus.
  • Function:
    • Site of many metabolic activities (chemical reactions).
    • Contains various specialized structures called Cell Organelles.
  • Cell Organelles: Smaller structures suspended in the cytoplasm, each performing specific functions (e.g., Mitochondria, Ribosomes, Golgi bodies, Endoplasmic Reticulum, Vacuoles, Plastids). Note: Class 8 focuses mainly on Nucleus, Vacuoles, Plastids, and Mitochondria (implied).
• c) Nucleus:
  • Generally spherical, dense body, usually located in the centre of the cell (can be peripheral in mature plant cells due to large vacuole).
  • Often called the "Control Centre" or "Brain" of the cell.
  • Parts of the Nucleus:
    • Nuclear Membrane (Nuclear Envelope): A double-layered membrane separating the nucleus from the cytoplasm. It is porous (contains nuclear pores) to allow the exchange of materials between the nucleus and cytoplasm.
    • Nucleoplasm: The dense fluid inside the nuclear membrane.
    • Nucleolus: A smaller, dense spherical body within the nucleoplasm. Involved in ribosome synthesis.
    • Chromosomes: Thread-like structures present in the nucleus. They are visible only during cell division.
      • Composition: Made of DNA (Deoxyribonucleic Acid) and proteins.
      • Function: Carry Genes.
      • Genes: Units of inheritance. They control the transfer of hereditary characteristics from parents to offspring.
• Protoplasm: The entire living substance of the cell, including the cytoplasm and the nucleus, enclosed within the cell membrane. It is the "living matter" of the cell.

4. Additional Structures

• Cell Wall:
  • A thick, rigid, non-living, fully permeable outer layer found outside the cell membrane in plant cells, fungi, and bacteria.
  • Absent in animal cells.
  • Composition (Plants): Mainly made of Cellulose.
  • Function:
    • Provides definite shape and rigidity to the plant cell.
    • Provides protection against mechanical injury and environmental stresses (temperature variations, high wind speed, atmospheric moisture).
    • Prevents excessive water uptake.
• Vacuoles:
  • Membrane-bound sacs within the cytoplasm.
  • Function: Storage of water, food, pigments, and waste products.
  • Plant Cells: Typically have a single, large central vacuole that occupies a significant portion (50-90%) of the cell volume. Helps maintain turgor pressure.
  • Animal Cells: Have small, temporary vacuoles or may lack them altogether.
• Plastids:
  • Organelles found only in plant cells. Absent in animal cells.
  • Several types, differing in colour and function.
  • Chloroplasts: Green-coloured plastids containing the pigment chlorophyll. They are the site of photosynthesis (food production using sunlight, water, and CO2).
  • Other Plastids: Chromoplasts (contain coloured pigments other than green, give colour to fruits and flowers), Leucoplasts (colourless, store food like starch).

5. Comparison of Plant and Animal Cells

6. Prokaryotic and Eukaryotic Cells

This is a fundamental classification based on nuclear organization.

• Prokaryotic Cells (Pro = Primitive, Karyon = Nucleus):
  • Cells lacking a well-defined nucleus.
  • The nuclear material (DNA) is not enclosed by a nuclear membrane and lies directly in the cytoplasm in a region called the nucleoid.
  • Lack membrane-bound organelles (like mitochondria, plastids, endoplasmic reticulum, Golgi apparatus).
  • Examples: Bacteria, Blue-Green Algae (Cyanobacteria).
• Eukaryotic Cells (Eu = True, Karyon = Nucleus):
  • Cells possessing a well-organized nucleus enclosed by a nuclear membrane.
  • Contain membrane-bound organelles suspended in the cytoplasm.
  • Examples: All organisms other than bacteria and cyanobacteria (e.g., Plant cells, Animal cells, Fungi, Protists like Amoeba).

Key Takeaways for Exams:

• Know the discoverers (Hooke, Leeuwenhoek) and what they observed.
• Understand the definitions of unicellular and multicellular organisms with examples.
• Relate cell shape to function (nerve, muscle, RBC).
• Know the relative sizes (smallest, largest, longest).
• Be very clear about the three main parts: Cell Membrane, Cytoplasm, Nucleus.
• Know the structure and function of the Cell Membrane (selective permeability) and Cell Wall (rigidity, protection, plants only).
• Understand the role of the Nucleus (control center, chromosomes, genes, inheritance).
• Know the main function of key organelles: Mitochondria (energy - implied), Chloroplasts (photosynthesis - plants), Vacuoles (storage - difference between plant/animal).
• Crucially: Be able to list the key differences between Plant and Animal cells (Cell wall, Plastids, Vacuole size).
• Be able to differentiate between Prokaryotic and Eukaryotic cells based on the nucleus and membrane-bound organelles.
• Remember diagrams are important for visualization – practice identifying parts in typical plant and animal cell diagrams.
• Define Protoplasm.

This detailed breakdown covers the essential points from NCERT Class 8 Chapter 8 relevant for competitive government exams. Focus on definitions, functions, key differences, and examples.