The Fundamental Unit of Life class 9 Notes

 

The Fundamental Unit of Life class 9 Notes

Introduction

• All living organisms like plants, animals, and human beings are made up of tiny microscopic structures called cells.
• The cell is the basic structural and functional unit of life.

This means:

• It forms the structure of all living bodies.
• It performs all the functions necessary for life such as respiration, nutrition, and reproduction.

Discovery of Cell

• The cell was first discovered by Robert Hooke in 1665 while observing a thin slice of cork under a microscope.
• Later, Anton van Leeuwenhoek observed living cells in pond water.

Cell Theory

The cell theory was proposed by:

• Matthias Schleiden
• Theodor Schwann

According to the Cell Theory:

     1) All living organisms are made up of cells.

     2) The cell is the basic unit of life.

     3) All cells arise from pre-existing cells.

Why Are Cells Important?

• Every plant, animal, and human body is made up of cells.

• Cells carry out important life processes like:

  • Nutrition

  • Respiration

  • Excretion

  • Growth

  • Reproduction

Without cells, life would not exist.

Types of Organisms Based on Cells

1) Unicellular Organisms

• Made up of one single cell.
• Example: Amoeba, Paramecium.
• A single cell performs all life functions.

2) Multicellular Organisms

• Made up of many cells.
• Example: Humans, animals, plants.
• Different cells perform different functions.

Size of Cells

• Most cells are microscopic (cannot be seen with naked eyes).
• Some cells are visible, like the hen’s egg (largest single cell).

Important Points for Exam

✔ All living organisms are made up of cells.
✔ The cell is the basic unit of structure and function.
✔ New cells arise from pre-existing cells.

Various Cells from the Human Body

• The human body is made up of billions of cells. These cells differ in shape, size, and function, but all help the body work properly.

Below are some important types of human cells:

1) Nerve Cells (Neurons)

• Structure: Long and branched.
• Function: Carry messages (nerve impulses) between the brain, spinal cord, and body parts.
• Special Feature: Can be very long (some over 1 meter in length).

Example: Cells in the brain and spinal cord.

2) Red Blood Cells (RBCs)

• Shape: Round and biconcave (disc-shaped).
• Function: Carry oxygen from lungs to all parts of the body.
• Special Feature: Do not have a nucleus (in humans).

They contain hemoglobin, which gives blood its red colour.

3) White Blood Cells (WBCs)

• Shape: Irregular.
• Function: Protect the body from diseases and infections.
• Role: Part of the immune system.

4) Muscle Cells

• Shape: Long and cylindrical.

• Function: Help in movement of body parts.

• Types:

  • Skeletal muscles (voluntary movement)

  • Smooth muscles (internal organs)

  • Cardiac muscles (heart)

Cardiac muscles are found only in the heart.

5) Epithelial Cells

• Function: Form the outer covering of the body (skin).
• Also line internal organs like stomach and lungs.
• Protect the body from injury and infection.

6) Bone Cells

• Function: Provide support and shape to the body.
• Make bones strong and hard.

7) Reproductive Cells

• Male gamete → Sperm cell
• Female gamete → Egg (ovum)

Function: Help in reproduction.

Key Points

✔ Different cells have different shapes and sizes.
✔ Each type of cell performs a specific function.
✔ All cells together form tissues, organs, and organ systems.

What Is a Cell Made Up Of?

Structural Organisation of a Cell

• A cell is made up of different parts called cell organelles. Each part has a specific function. The structural organisation of a cell helps it perform life processes smoothly.

Basic Structure of a Cell

A typical cell has three main parts:

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

1) Cell Membrane (Plasma Membrane)

• The outer covering of the cell.
• Protects the cell from the external environment.
• Allows only selected substances to enter or leave the cell.
• It is selectively permeable in nature.
• It helps in processes like diffusion and osmosis.

2) Cell Wall

• The cell wall is a rigid outer covering present outside the cell membrane in plant cells.
• It gives shape, strength, and protection to the cell.
• In plant cells, the cell wall is mainly made up of Cellulose (a complex carbohydrate), Hemicellulose, and Pectin.
• It Prevents the cell from bursting when water enters the cell by osmosis.

3) Cytoplasm

• Jelly-like substance present inside the cell membrane.
• Contains many cell organelles.
• Most life activities take place here.

4) Nucleus

• Control centre of the cell.
• Contains genetic material (DNA).
• Responsible for growth and reproduction.

Structure of Nucleus:

• Nuclear membrane
• Nucleoplasm
• Chromosomes
• Nucleolus

Cell Organelles and Their Functions

• Cell organelles are specialized structures inside a cell that perform specific functions.
• They are mostly found in eukaryotic cells (plant and animal cells).

1) Nucleus

Structure:

• Surrounded by a nuclear membrane
• Contains chromosomes (DNA)
• Has a nucleolus

Functions:

• Controls all activities of the cell
• Stores hereditary material (DNA)
• Helps in cell division

- Known as the “control centre” of the cell.

2) Cytoplasm

Structure:

• Jelly-like substance inside the cell membrane

Functions:

• Holds all organelles
• Site of many chemical reactions

3) Mitochondria

Structure:

• Double membrane
• Inner membrane folded (cristae)

Functions:

• Produces energy (ATP)
• Called the “Powerhouse of the cell”

4) Endoplasmic Reticulum (ER)

Two Types:

(a) Rough ER (RER)

• Has ribosomes attached

Function:

• Helps in protein synthesis

(b) Smooth ER (SER)

• No ribosomes

Function:

• Helps in fat synthesis
• Detoxifies poisons

5) Ribosomes

Structure:

• Very small particles

Function:

• Protein factory of the cell
• Present on RER or free in cytoplasm

6) Golgi Apparatus

Structure:

• Flattened sacs

Function:

• Modifies, packages and transports proteins
• Forms lysosomes

- Works like a post office of the cell.

7) Lysosomes

Structure:

• Small sac-like structures

Functions:

• Digests waste materials
• Destroys damaged organelles

- Called the “Suicide bags” of the cell.

8) Vacuoles

Structure:

• Large sac-like structure
• Very large in plant cells

Functions:

• Stores food and water
• Maintains turgor pressure in plants

9) Plastids (Only in Plant Cells)

Types:

1) Chloroplast

• Helps in photosynthesis
• Contains chlorophyll

2) Chromoplast

• Gives colour to fruits and flowers

3) ⚪ Leucoplast

• Stores starch, oil, protein

Cell Division

• Cell division is the process by which a parent cell divides to form new cells.
• It is necessary for:

                1) Growth
                2) Repair of damaged tissues
                3) Reproduction

Types of Cell Division

1) MITOSIS (Equational Division)

• Occurs in: Body cells (somatic cells)
• Produces: 2 daughter cells
• Chromosome number remains same

Stages of Mitosis:

(a) Prophase

• Chromosomes become visible
• Nuclear membrane disappears

(b) Metaphase

• Chromosomes line up at the center

(c) Anaphase

• Chromosomes separate

(d) Telophase

• New nuclear membranes form

- After this, cytokinesis (division of cytoplasm) occurs.

Importance of Mitosis:

• Growth in plants and animals
• Healing of wounds
• Replacement of old cells

2) MEIOSIS (Reduction Division)

• Occurs in: Reproductive cells
• Produces: 4 daughter cells
• Chromosome number becomes half

Importance of Meiosis:

• Formation of gametes (sperm and egg)
• Maintains chromosome number in species
• Creates variation

Important Exam Points

• Cell division helps in growth and repair.
• Mitosis → Growth
• Meiosis → Reproduction
• Chromosomes carry genetic information.

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