Class 8 Science Notes Chapter 8 (Chapter 8) – Examplar Problems Book

Alright class, let's focus on Chapter 8, 'Cell – Structure and Functions', from your Science Exemplar book. This chapter is fundamental, not just for your class exams but also forms the basis for many questions in competitive government exams. Pay close attention as we break down the key concepts.

Chapter 8: Cell – Structure and Functions: Detailed Notes for Competitive Exams

1. Introduction: The Basic Unit of Life

• Discovery: Robert Hooke (1665) observed cork slices under a self-designed microscope and saw box-like compartments, which he termed "cells" (Latin: 'cellula' meaning 'little room'). He observed dead cells. Antonie van Leeuwenhoek (later) observed living cells (like bacteria, protozoa) for the first time using improved microscopes.
• Cell Theory (Basic Idea): All living organisms are composed of cells, and the cell is the basic structural and functional unit of life. (Note: Formal cell theory by Schleiden & Schwann is typically detailed in higher classes, but the core concept is relevant).
• Definition: A cell is the smallest structural and functional unit of an organism, capable of independent existence and performing the essential functions of life.

2. Diversity in Cells

• Number:
  • Unicellular Organisms: Made of a single cell that performs all life functions (e.g., Amoeba, Paramecium, Bacteria, Yeast).
  • Multicellular Organisms: Made of many cells (millions to trillions) grouped into tissues, organs, and organ systems (e.g., Humans, Mango Tree, Rose Plant, Insects). Cell specialization occurs, meaning different cells perform specific functions.
• Shape: Cells exhibit a variety of shapes related to their specific functions.
  • Amoeba: Irregular shape (due to pseudopodia).
  • Nerve Cell (Neuron): Long and branched (to transmit messages).
  • Muscle Cell: Spindle-shaped (for contraction and relaxation).
  • Red Blood Cells (RBCs): Biconcave disc shape (to carry oxygen efficiently and pass through narrow capillaries).
  • Plant Cells (e.g., Onion peel): Often rectangular or polygonal (due to cell wall).
• Size: Cells vary greatly in size.
  • Smallest Cell: Bacteria (e.g., Mycoplasma) - about 0.1 to 0.5 micrometres (µm). (1 µm = 1/1,000,000 metre).
  • Largest Cell: Ostrich egg (approx. 170 mm x 130 mm).
  • Longest Cell: Nerve cell (can be up to a metre long in humans).
  • Most cells are microscopic and not visible to the naked eye.

3. Cell Structure and Components
Every cell has at least three main features: Cell Membrane, Cytoplasm, and Nucleus (in Eukaryotes).

• (a) Cell Membrane (Plasma Membrane):

  • Function: Outer boundary of animal cells; lies inside the cell wall in plant cells. It controls the passage of substances into and out of the cell (selectively permeable). Provides shape (in animal cells) and protection.
  • Composition: Mainly lipids and proteins.

• (b) Cell Wall:

  • Presence: Found in plant cells, bacteria, fungi, and algae. Absent in animal cells.
  • Location: Outermost layer in plant cells, outside the cell membrane.
  • Function: Provides rigidity, definite shape, and protection to the plant cell against environmental stress (temperature variations, high wind speed, osmotic pressure).
  • Composition: Mainly Cellulose in plants.

• (c) Cytoplasm:

  • Description: Jelly-like substance filling the cell, enclosed by the cell membrane.
  • Function: Site of many metabolic activities. Contains various specialized structures called cell organelles.
  • Components: Cytosol (the fluid portion) and Cell Organelles.

• (d) Nucleus:

  • Description: Generally spherical, centrally located (in animal cells) or peripheral (in mature plant cells due to large vacuole). It's the 'Control Centre' or 'Brain' of the cell.
  • Components:
    • Nuclear Membrane (Nuclear Envelope): Double-layered membrane separating the nucleus from the cytoplasm. It is porous, allowing exchange of materials.
    • Nucleoplasm: Dense fluid inside the nucleus.
    • Nucleolus: Small, dense spherical body within the nucleus; involved in ribosome synthesis.
    • Chromosomes: Thread-like structures visible only during cell division. They carry genes.
      • Genes: Units of inheritance, responsible for transferring characters from parents to offspring. Composed of DNA (Deoxyribonucleic Acid).
      • Chromatin: In a non-dividing cell, the chromosome material exists as a tangled mass called chromatin.

• (e) Cell Organelles (Structures within Cytoplasm):

  • Mitochondria: (Singular: Mitochondrion)
    • Known as the 'Powerhouse' of the cell.
    • Function: Site of cellular respiration; they use oxygen and glucose to produce energy in the form of ATP (Adenosine Triphosphate).
  • Plastids:
    • Found only in plant cells.
    • Types:
      • Chloroplasts: Contain green pigment chlorophyll. Site of photosynthesis (food production).
      • Chromoplasts: Contain pigments other than green (e.g., red, orange, yellow). Give colour to fruits and flowers.
      • Leucoplasts: Colourless plastids. Function: Storage of food (starch, oils, proteins).
  • Vacuoles:
    • Membrane-bound sacs used for storage (water, nutrients, waste products).
    • Plant Cells: Usually have one large central vacuole, occupying a significant portion of the cell volume. Helps maintain turgor pressure.
    • Animal Cells: Have small, temporary vacuoles or may lack them altogether.
  • Ribosomes: Tiny granules responsible for protein synthesis. (Often mentioned in relation to ER or free in cytoplasm).
  • Endoplasmic Reticulum (ER) & Golgi Apparatus (Golgi Complex/Body): Involved in synthesis, modification, packaging, and transport of materials within and outside the cell. (Basic awareness is useful).

4. Prokaryotic vs. Eukaryotic Cells
This is a very important distinction for competitive exams.

• Prokaryotic Cells: ('Pro' = Primitive, 'Karyon' = Nucleus)
  • Lack a well-defined nucleus; nuclear material (DNA) is not enclosed by a nuclear membrane and lies directly in the cytoplasm (region called nucleoid).
  • Lack membrane-bound organelles (like mitochondria, plastids, ER, Golgi).
  • Ribosomes are present.
  • Examples: Bacteria, Blue-Green Algae (Cyanobacteria).
• Eukaryotic Cells: ('Eu' = True, 'Karyon' = Nucleus)
  • Have a well-organized nucleus enclosed by a nuclear membrane.
  • Contain membrane-bound organelles.
  • Examples: All organisms other than bacteria and blue-green algae (Plants, Animals, Fungi, Protists like Amoeba).

5. Comparison: Plant Cell vs. Animal Cell

6. Levels of Organisation in Multicellular Organisms
Cell → Tissue (group of similar cells performing a specific function, e.g., muscle tissue) → Organ (structure made of different tissues working together, e.g., stomach, leaf) → Organ System (group of organs working together, e.g., digestive system, respiratory system) → Organism.

Multiple Choice Questions (MCQs)

1. Who first observed dead cork cells and coined the term 'cell'?
   (a) Antonie van Leeuwenhoek
   (b) Robert Hooke
   (c) Robert Brown
   (d) Schleiden

2. Which of the following is NOT found in an animal cell?
   (a) Cell Membrane
   (b) Nucleus
   (c) Mitochondria
   (d) Cell Wall

3. The control centre of a eukaryotic cell is the:
   (a) Cytoplasm
   (b) Nucleus
   (c) Mitochondrion
   (d) Vacuole

4. Which organelle is known as the 'powerhouse' of the cell?
   (a) Plastid
   (b) Ribosome
   (c) Mitochondrion
   (d) Nucleolus

5. Organisms lacking a well-defined nucleus and membrane-bound organelles are called:
   (a) Eukaryotes
   (b) Multicellular
   (c) Prokaryotes
   (d) Unicellular

6. Chromosomes, which carry genes, are located within the:
   (a) Cytoplasm
   (b) Vacuole
   (c) Nucleus
   (d) Cell Membrane

7. Which plastid is responsible for photosynthesis in plant cells?
   (a) Leucoplast
   (b) Chromoplast
   (c) Chloroplast
   (d) Tonoplast

8. Amoeba moves and captures food using:
   (a) Cilia
   (b) Flagella
   (c) Pseudopodia
   (d) Tentacles

9. The jelly-like substance present between the cell membrane and the nucleus is called:
   (a) Nucleoplasm
   (b) Protoplasm
   (c) Cytoplasm
   (d) Ectoplasm

10. Which of the following represents the correct sequence of levels of organisation in multicellular organisms?
    (a) Tissue → Cell → Organ → Organ System → Organism
    (b) Cell → Tissue → Organ → Organ System → Organism
    (c) Cell → Organ → Tissue → Organ System → Organism
    (d) Organism → Organ System → Organ → Tissue → Cell

Answer Key for MCQs:

1. (b)
2. (d)
3. (b)
4. (c)
5. (c)
6. (c)
7. (c)
8. (c)
9. (c) (Note: Protoplasm includes cytoplasm + nucleoplasm, but cytoplasm is the specific term for the substance between the nucleus and cell membrane).
10. (b)

Revise these notes thoroughly. Understanding the structure and function of each cell part, the differences between cell types, and the levels of organisation is crucial. Good luck with your preparation!