Class IX SEBA Science – Chapter 5: The Fundamental Unit of Life
Class IX SEBA Science – Chapter 5: The Fundamental Unit of Life
🧬 1. What are Living Organisms Made Up Of?
- All living organisms are made of cells.
- Cell is the basic structural and functional unit of life.
- Discovered by Robert Hooke in 1665 (from cork).
- Anton van Leeuwenhoek observed living cells.
🔬 2. Discovery of the Cell
- Robert Hooke coined the term “cell”.
- Cell theory by Schleiden and Schwann:
- All living things are made of cells.
- Cell is the basic unit of life.
- Rudolf Virchow added: “All cells arise from pre-existing cells.”
🔍 3. What is a Cell Made Up Of?
- Cell has:
- Plasma membrane
- Cytoplasm
- Nucleus
- Organelles perform various functions like respiration, protein synthesis, waste disposal.
🧫 4. Plasma Membrane or Cell Membrane
- Made of lipids and proteins.
- Semi-permeable: allows selective entry/exit.
- Allows diffusion (movement of substances from high to low concentration).
- In osmosis, water moves across membrane from high water concentration to low.
🧱 5. Cell Wall
- Found only in plant cells.
- Made of cellulose.
- Provides rigidity and protection.
🧠 6. Nucleus
- Controls cell activities.
- Surrounded by nuclear membrane.
- Contains:
- Nucleolus
- Chromatin material → condenses to form chromosomes.
- Chromosomes contain DNA (genetic material).
🌊 7. Cytoplasm
- Jelly-like fluid between nucleus and plasma membrane.
- Contains all cell organelles.
⚙️ 8. Cell Organelles
Mitochondria
- Site of respiration.
- Known as “powerhouse of the cell”.
Endoplasmic Reticulum (ER)
- Rough ER: Has ribosomes; helps in protein synthesis.
- Smooth ER: Synthesizes lipids and detoxifies drugs.
Golgi Apparatus
- Modifies, stores, and packages proteins.
Lysosomes
- Contain enzymes for digestion.
- Called “suicide bags” of the cell.
Plastids (only in plant cells)
- Chloroplasts → photosynthesis (contains chlorophyll)
- Chromoplasts → give color
- Leucoplasts → store starch
Vacuoles
- Store nutrients and waste.
- Large in plant cells, small in animal cells.
📏 9. Differences Between Plant Cell and Animal Cell
- Cell wall: Present in plant cell, absent in animal cell.
- Chloroplast: Present in plant cell, absent in animal cell.
- Vacuole: Large in plant, small or absent in animal cell.
- Centrosome: Present in animal, usually absent in plant cell.
📘 Exercise Questions and Answers
1. Who discovered the cell and how?
Robert Hooke discovered cells in 1665 while observing cork slices through a microscope.
2. Why is cell called the structural and functional unit of life?
Because all living organisms are made up of cells, and all life functions occur within cells.
3. How do substances like CO₂ and water move in and out of the cell?
Through the plasma membrane by the processes of diffusion and osmosis.
4. Why is plasma membrane called a selectively permeable membrane?
Because it allows only specific substances to pass in or out of the cell.
5. Fill in the blanks:
a) Golgi apparatus → storage, modification, packaging of products
b) Mitochondria → powerhouse of the cell
c) Ribosomes → protein synthesis
d) Chloroplasts → photosynthesis
6. What are the functions of the nucleus in a cell?
- Controls cell functions
- Contains DNA
- Helps in cell division
- Passes genetic material to next generation
7. Where are chromosomes found and what are they made of?
- Found inside the nucleus.
- Made of DNA and proteins.
8. Cell organelles and their functions:
- Mitochondria → Respiration
- Ribosomes → Protein synthesis
- Chloroplasts → Photosynthesis
- Golgi body → Packaging and secretion
- Lysosomes → Digestion
- Vacuole → Storage
✍️ Extra Important Questions with Answers
1. What is osmosis?
Osmosis is the movement of water through a semi-permeable membrane from high water concentration to low.
2. Define prokaryotic and eukaryotic cells.
- Prokaryotic: Cells without a true nucleus (e.g., bacteria)
- Eukaryotic: Cells with a well-defined nucleus (e.g., plants, animals)
3. What is cytoplasm?
Cytoplasm is the jelly-like fluid in cells where all organelles are suspended.
4. Why are lysosomes called suicide bags?
Because they contain digestive enzymes that can destroy the cell when damaged.
5. What are plastids?
Organelles in plant cells responsible for photosynthesis, color, and storage.
Example: Chloroplasts (green pigment, photosynthesis)
continue to Chapter 6: Tissues?
