Notes Class 9 Science Chapter 5 The Fundamental Unit of Life

Notes For All Chapters Science Class 9 CBSE

1. Discovery of the Cell

Robert Hooke’s Observation (1665)

Robert Hooke examined a thin slice of cork (the bark of a tree) using a self-designed microscope.

He saw it resembled a honeycomb structure, made up of many tiny compartments.

He called these compartments cells (from Latin cellula, meaning “small room”).

This was the first observation that living things consist of small units.

Further Discoveries

2. What Are Living Organisms Made Up Of?

Unicellular and Multicellular Organisms

Unicellular organisms: Consist of a single cell that performs all life functions.

• Examples – Amoeba, Paramoecium, Chlamydomonas, Bacteria.

Multicellular organisms: Made up of many cells.

Each group of cells performs specific functions (division of labour).

• Examples – Plants, animals, fungi.

Function of Cells

• Each cell performs certain basic functions like respiration, nutrition, excretion, etc.
• In multicellular organisms, cells are specialized — e.g., nerve cells transmit signals, muscle cells contract.
• In unicellular organisms, a single cell performs all necessary life functions.

Types of Cells (based on shape)

3. Structure of a Cell

Cells show three main parts visible under a microscope:

1. Plasma Membrane (Cell Membrane)
2. Nucleus
3. Cytoplasm

3.1 Plasma Membrane or Cell Membrane

Structure

• Outermost covering of the cell (in plants, just inside the cell wall).
• Made of lipids and proteins.
• Flexible and selectively permeable – allows some substances to pass through but not others.

Functions

1. Separates the internal cell content from the external environment.
2. Controls the entry and exit of materials.
3. Allows gases like oxygen and carbon dioxide to move by diffusion.
4. Regulates water movement by osmosis.

Diffusion

Movement of molecules from higher concentration to lower concentration.

Examples:

• CO₂ formed during cellular respiration diffuses out of the cell.
• O₂ required for respiration diffuses into the cell.

Osmosis

Movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a lower one.

Three Conditions:

Activities

• Egg in pure water: swells (osmosis, water enters).
• Egg in salt solution: shrinks (water leaves by osmosis).
• Raisins in water: swell.
• Raisins in sugar solution: shrink.

Endocytosis

The cell membrane can engulf solid food or liquid materials from outside.
Example: Amoeba engulfs food through endocytosis.

3.2 Cell Wall (in Plants)

• Rigid outer layer outside the plasma membrane.
• Made of cellulose.
• Provides strength, shape, and protection.

Function

• Prevents bursting when cell absorbs water.
• Allows plants to survive in hypotonic environments.
• Plasmolysis: Shrinkage of cell contents away from the cell wall when water leaves the cell through osmosis.

Only living cells show plasmolysis; dead cells do not.

3.3 Nucleus

Structure

• Spherical or oval dark body near the centre.
• Surrounded by double-layered nuclear membrane with pores.
• Contains:
  • Nucleoplasm
  • Chromatin Material (thread-like, becomes chromosomes during division)
  • Nucleolus

Functions

1. Controls all cell activities (the “control centre”).
2. Contains DNA – carries genetic information.
3. Functional units of DNA are genes.
4. Helps in cell division and inheritance.

3.4 Cytoplasm

• Jelly-like fluid between nucleus and cell membrane.
• Contains all cell organelles suspended in it.
• Allows movement of materials and chemical reactions.
• In prokaryotes, cytoplasm lacks membrane-bound organelles.
• In eukaryotes, cytoplasm contains well-developed organelles.

Difference Between Prokaryotic and Eukaryotic Cells

Difference between Animal cell and Plant cell

Animal cell:

Plant Cell

3.5 Cell Organelles and Their Functions

(i) Endoplasmic Reticulum (ER)

• Network of membrane-bound tubes and sheets.
• Two types:
  • Rough ER (RER): with ribosomes – makes proteins.
  • Smooth ER (SER): no ribosomes – makes fats (lipids), detoxifies drugs and poisons.
• Functions:
  • Transports materials within cell.
  • Membrane biogenesis (helps make new membranes).
  • Provides a surface for biochemical reactions.

(ii) Golgi Apparatus

• Stack of flattened sacs called cisterns.
• Connected to ER.
• Functions:
  • Modifies, stores, and packages materials from ER.
  • Forms lysosomes.
  • Involved in making complex sugars from simple ones.

(iii) Lysosomes

• Small, membrane-bound sacs with digestive enzymes.
• Formed by Golgi apparatus and RER.
• Functions:
  • Digest worn-out organelles, bacteria, and waste materials.
  • Destroy the entire cell if it is damaged → hence called “Suicide Bags”.

(iv) Mitochondria

• Known as “Powerhouse of the Cell”.
• Double membrane: outer porous, inner folded (cristae).
• Releases energy in form of ATP (Adenosine Triphosphate) during respiration.
• ATP – energy currency of the cell.
• Have their own DNA and ribosomes, so they can make some proteins themselves.

(v) Plastids (Present only in plant cells)Two main types:

• Chromoplasts: coloured plastids (contain pigments).
• Leucoplasts: colourless, store food (starch, oils, proteins).

Chloroplasts: green plastids with chlorophyll, perform photosynthesis.

Like mitochondria, plastids also have their own DNA and ribosomes.

(vi) VacuolesMembrane-bound sacs for storage.

Plant Cells: one large central vacuole (up to 90% of cell volume).

Animal Cells: small and temporary vacuoles.

Functions:

• Store water, sugars, amino acids, organic acids.
• Maintain turgidity and rigidity in plants.
• In Amoeba, food vacuole stores ingested food.
• In some unicellular organisms, helps expel excess water.

4. Cell Division

Purpose:

Growth, repair, and reproduction in living organisms.Types:

• Mitosis
• Meiosis

Mitosis Fig:-

Meiosis Fig:-