The Fundamental Unit of Life (Class 9 CBSE Notes)
1. Introduction
Cell is the basic structural and functional unit of life.
Every living organism is made up of one or more cells.
Cells perform all life processes such as respiration, nutrition, excretion, growth and reproduction.
2. Cell Theory
Cell Theory was proposed by Matthias Schleiden in 1838 and Theodor Schwann in 1839.
In 1855, Rudolf Virchow modified the Cell Theory by stating that all new cells arise from pre-existing cells.
Cell Theory States
All living organisms are made up of one or more cells.
The cell is the basic structural and functional unit of life.
Every new cell is formed from a pre-existing cell.
3. Discovery of Cell
Robert Hooke discovered the cell in 1665 by observing a thin slice of cork under a microscope. He observed small box-like compartments and named them "Cells."
Antonie van Leeuwenhoek was the first scientist to observe living cells.
Robert Brown discovered the nucleus in 1831.
Schleiden stated that all plants are made up of cells.
Schwann concluded that all animals are also made up of cells.
Virchow proposed that all cells arise from pre-existing cells.
4. Types of Organisms
Unicellular Organisms
Unicellular organisms consist of only one cell which performs all life activities.
Examples include Amoeba, Paramecium and Bacteria.
Multicellular Organisms
Multicellular organisms are made up of many cells.
Different cells perform different functions.
Examples include Human beings, Mango tree and Dog.
5. Shape of Cells
Cells have different shapes according to the functions they perform.
Nerve cells are long and branched.
Muscle cells are spindle-shaped.
Red Blood Cells are biconcave.
White Blood Cells have an irregular shape.
Plant cells are generally rectangular.
6. Size of Cells
The smallest known cell is Mycoplasma.
The largest animal cell is the Ostrich egg.
The longest cell in the human body is the nerve cell.
7. Cell Structure
Every cell has three main parts.
Plasma Membrane
Cytoplasm
Nucleus
8. Plasma Membrane
The plasma membrane is the outer covering of the cell.
It protects the cell from external damage.
It gives shape to the cell.
It controls the movement of substances in and out of the cell.
It is a selectively permeable membrane.
9. Diffusion
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Oxygen enters the cell by diffusion.
Carbon dioxide leaves the cell by diffusion.
10. Osmosis
Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
Hypotonic Solution
Water enters the cell.
The cell swells.
Hypertonic Solution
Water moves out of the cell.
The cell shrinks.
Isotonic Solution
No net movement of water occurs.
The cell remains unchanged.
11. Cell Wall
The cell wall is present only in plant cells.
It is made of cellulose.
It provides strength and protection.
It gives a definite shape to the cell.
It prevents the cell from bursting.
12. Nucleus
The nucleus is called the control centre of the cell.
It controls all activities of the cell.
It stores hereditary information.
It controls cell division.
Parts of the Nucleus
Nuclear Membrane
Nucleoplasm
Nucleolus
Chromatin
13. Chromosomes
Chromosomes are thread-like structures present inside the nucleus.
They are made up of DNA and proteins.
They carry genes.
Genes are responsible for the inheritance of characters from parents to offspring.
14. Cytoplasm
Cytoplasm is a jelly-like substance present between the plasma membrane and nucleus.
It contains various cell organelles.
Many metabolic reactions take place in the cytoplasm.
Cell Organelles
Endoplasmic Reticulum (ER)
The Endoplasmic Reticulum is a network of membranes inside the cell.
Rough Endoplasmic Reticulum (RER)
Ribosomes are present on its surface.
It helps in protein synthesis.
Smooth Endoplasmic Reticulum (SER)
Ribosomes are absent.
It helps in fat synthesis.
It detoxifies harmful chemicals.
Ribosomes
Ribosomes are called the protein factories of the cell.
They help in the synthesis of proteins.
Golgi Apparatus
Golgi Apparatus modifies proteins.
It packages proteins.
It stores materials.
It secretes useful substances.
It forms lysosomes.
Lysosomes
Lysosomes contain digestive enzymes.
They digest waste materials.
They destroy damaged cell organelles.
They are known as the suicidal bags of the cell.
Mitochondria
Mitochondria are known as the powerhouse of the cell.
They carry out cellular respiration.
They produce ATP.
ATP is known as the energy currency of the cell.
Plastids
Plastids are present only in plant cells.
Chloroplast
Contains chlorophyll.
Performs photosynthesis.
Chromoplast
Provides colour to flowers and fruits.
Leucoplast
Stores food.
Vacuoles
Vacuoles store food, water and waste materials.
Plant cells have one large central vacuole.
Animal cells have many small vacuoles.
They help in maintaining turgidity.
Plant Cell and Animal Cell
Plant Cell
Cell wall is present.
Plastids are present.
One large vacuole is present.
Shape is rectangular.
Animal Cell
Cell wall is absent.
Plastids are absent.
Small vacuoles are present.
Shape is round or irregular.
Prokaryotic and Eukaryotic Cells
Prokaryotic Cells
True nucleus is absent.
Membrane-bound organelles are absent.
They are smaller in size.
Example: Bacteria.
Eukaryotic Cells
True nucleus is present.
Membrane-bound organelles are present.
They are larger in size.
Examples include plant and animal cells.
Cell Division
Cell division is the process by which a parent cell divides to form new daughter cells.
It is essential for the growth, development and repair of living organisms.
Cell division also helps in reproduction.
There are two main types of cell division.
Mitosis
Mitosis is the process in which one parent cell divides to produce two genetically identical daughter cells.
It occurs in body cells.
The number of chromosomes remains the same as the parent cell.
Mitosis helps in growth, repair of damaged tissues and replacement of old cells.
Examples include skin cell division and healing of wounds.
Meiosis
Meiosis is the process in which one parent cell divides to produce four daughter cells.
Each daughter cell has half the number of chromosomes compared to the parent cell.
It occurs only in reproductive cells.
Meiosis is important for sexual reproduction and helps maintain the chromosome number in every generation.
Difference Between Mitosis and Meiosis
Mitosis produces two daughter cells.
Meiosis produces four daughter cells.
Mitosis occurs in body cells.
Meiosis occurs in reproductive cells.
Mitosis maintains the chromosome number.
Meiosis reduces the chromosome number by half.
Mitosis is responsible for growth and repair.
Meiosis is responsible for the formation of gametes.
Cancer (Basic Introduction)
Cancer is a disease in which body cells divide uncontrollably.
Normally, cell division is regulated by the body.
In cancer, cells continue dividing without control and form a mass called a tumour.
Some tumours remain in one place and are called benign tumours.
Others spread to different parts of the body and are called malignant tumours.
Early diagnosis and treatment increase the chances of recovery.
Stem Cells
Stem cells are special cells that can divide continuously.
They can also develop into different types of body cells.
Stem cells help repair damaged tissues.
Scientists are studying stem cells to treat diseases like blood disorders, spinal cord injuries and some heart diseases.
Recent Advances in Cell Biology
Modern microscopes allow scientists to observe cells in much greater detail.
Gene therapy is being developed to treat certain genetic disorders.
Stem cell therapy is helping researchers repair damaged tissues.
Cell biology has improved the development of vaccines, medicines and diagnostic techniques.
Scientists continue to discover new ways to use cells in medicine and biotechnology.
Applications of Cell Biology
Cell biology is used in medical research.
It helps doctors understand diseases.
It is used in vaccine development.
It plays an important role in tissue engineering.
It is useful in organ transplantation.
It also supports genetic research and biotechnology.
Important One-Liners
Cell division is essential for growth and repair.
Mitosis produces two identical daughter cells.
Meiosis produces four daughter cells with half the chromosome number.
Cancer is caused by uncontrolled cell division.
Stem cells can develop into different types of cells.
Gene therapy is a modern technique used to treat genetic diseases.
Cell biology has greatly improved modern medicine.
Important Definitions
Cell is the basic structural and functional unit of life.
Diffusion is the movement of molecules from higher concentration to lower concentration.
Osmosis is the movement of water through a selectively permeable membrane.
ATP is the energy currency of the cell.
Gene is the unit of heredity.
Chromosome is the structure that carries genes.
Important One-Liners
Cell is the fundamental unit of life.
Plasma membrane is selectively permeable.
Cell wall is made of cellulose.
Mitochondria are called the powerhouse of the cell.
Ribosomes are known as protein factories.
Golgi Apparatus packages proteins.
Lysosomes contain digestive enzymes.
Chloroplast performs photosynthesis.
ATP is the energy currency of the cell.
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