Introduction:
The cell is the smallest structural and functional unit of life. Every living being, from a tiny bacterium to a large elephant, is made of cells. That’s why it is called the “basic unit of life.”
The word “cell” was first used by Robert Hooke in 1665, when he observed thin slices of cork under a microscope and saw tiny box-like structures. He named them “cells,” meaning small compartments.
Later, in the 1670s, Antonie van Leeuwenhoek improved the microscope and was the first person to observe living cells like bacteria and protozoa.
The study of cells is known as Cytology.
Cell Theory
The Cell Theory is one of the most important foundations of biology. It was proposed by:
· Matthias Schleiden (1838): said plants are made of cells.
· Theodor Schwann (1839): said animals are made of cells.
· Rudolf Virchow (1855): added that new cells arise only from pre-existing cells.
👉 Main points of Cell Theory:
1. All living organisms are made of one or more cells.
2. The cell is the basic unit of life.
3. Every cell comes from a pre-existing cell.
This means life cannot exist without cells.
Unicellular and Multicellular Organisms
Unicellular organisms –
consist of a single cell.
Example: Amoeba,
Paramecium, Bacteria.
o In Amoeba, one cell performs all life functions – movement, feeding, digestion, excretion, and reproduction.
Multicellular organisms –
consist of millions of cells that form tissues, organs, and systems.
Example: Humans,
plants, animals.
o In humans, nerve cells, muscle cells, and blood cells have specialized roles.
👉 Thus, a single cell can be an
independent organism (unicellular), or many cells can work together in harmony
(multicellular).
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Animal Cell – Structure and Functions
The animal cell is typically round or irregular in shape. It is surrounded by a plasma membrane but does not have a cell wall.
1. Cell Membrane (Plasma Membrane)
· Thin, flexible covering that separates the cell from its surroundings.
· Made of lipid bilayer and proteins.
· Controls what enters (nutrients, oxygen) and leaves (waste, carbon dioxide) the cell.
· Example: Red blood cells take in oxygen and release carbon dioxide through the membrane.
2. Nucleus – The Control Centre
· Largest organelle, usually spherical.
· Surrounded by a nuclear membrane.
· Contains DNA (genetic material) in the form of chromosomes.
· Functions as the “brain of the cell.”
· Example: DNA decides eye colour in humans or flower colour in plants.
3. Mitochondria – The Powerhouse
· Small, rod-shaped structures.
· Provide energy by breaking down glucose (cellular respiration).
· Known as “Powerhouse of the cell.”
· Example: Muscle cells have more mitochondria because they need more energy.
4. Endoplasmic Reticulum (ER)
· A network of membranes inside the cell.
· Two types:
o Rough ER – has ribosomes, makes proteins.
o Smooth ER – makes fats (lipids) and helps detoxify harmful substances.
· Example: Liver cells have more smooth ER to detoxify alcohol/drugs.
5. Golgi Body (Golgi Apparatus)
· Stacked, flat membranes like a post office.
· Modifies, packs, and transports proteins and lipids.
· Example: Enzymes made in the ER are packaged by Golgi and sent to lysosomes.
6. Lysosomes – The Suicide Bags
· Small sacs filled with digestive enzymes.
· Break down old organelles or harmful materials.
· If a cell is damaged beyond repair, lysosomes destroy it (autolysis).
7. Ribosomes
· Tiny particles found free in the cytoplasm or attached to ER.
· Known as “protein factories.”
· Example: Ribosomes make insulin hormone in humans.
8. Vacuoles
· Small, temporary sacs.
· Store water, nutrients, and waste.
· Animal cells usually have many small vacuoles.
9. Centrioles (Unique to Animals)
· Cylindrical structures near the nucleus.
· Help in cell division (mitosis).
· Example: Absent in plant cells.
Plant Cell – Structure and Functions
The plant cell
is rectangular or box-shaped. It has all organelles of animal cells but with
some unique
features.
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1. Cell Wall (Unique)
· Thick, rigid layer outside the cell membrane.
· Made of cellulose.
· Provides strength and protection.
· Example: Helps plants stand upright without bones.
2. Cell Membrane
· Present below the cell wall.
· Performs same function as in animals – controls entry and exit of materials.
3. Nucleus
· Same as in animal cells, controls heredity and cell activities.
4. Mitochondria
· Provides energy for plant processes like photosynthesis and growth.
5. Endoplasmic Reticulum (ER)
· Rough ER makes proteins.
· Smooth ER makes lipids.
6. Golgi Body
· Similar function as in animal cells, but in plants it is called Dictyosome.
7. Ribosomes
· Sites of protein synthesis.
8. Vacuole (Large, Central)
· Occupies 80–90% of the cell volume.
· Filled with cell sap (water, sugar, salts).
· Maintains turgidity (firmness) of plants.
· Example: Helps plant leaves stay straight.
9. Plastids (Unique)
· Found only in plant cells.
· Three types:
o Chloroplasts – green, contain chlorophyll, do photosynthesis.
o
Chromoplasts – contain pigments like carotene
(orange), xanthophyll (yellow).
Example: Fruits like papaya, carrot.
o
Leucoplasts – colourless, store starch, proteins,
fats.
Example: Potato tuber stores starch.
Cell Division
Cell division is the process by which cells multiply. Two main types:
1. Mitosis (Equational Division)
· One parent cell divides into two identical daughter cells.
· Stages: Prophase, Metaphase, Anaphase, Telophase.
· Important for growth and repair.
· Example: Healing of wounds.
2. Meiosis (Reduction Division)
· Produces four daughter cells with half the chromosome number.
· Occurs in reproductive cells.
· Essential for sexual reproduction.
· Example: Formation of sperm and egg cells.
Plant Cell vs Animal Cell (Comparison Table)
|
Feature |
Animal Cell |
Plant Cell |
|
Cell Wall |
Absent |
Present |
|
Shape |
Round/Irregular |
Rectangular/Box-like |
|
Plastids |
Absent |
Present |
|
Vacuole |
Small, temporary |
Large, central |
|
Centrioles |
Present |
Absent |
This knowledge is not only important for competitive exams like SSC, UPSC, Railway, NEET, and State PSC but also forms the foundation of biology.
📌 FAQs
Q1. Who is called the
Father of Microscopy and what was his contribution?
Answer: Antonie van Leeuwenhoek is called the Father of
Microscopy. In 1674, he made the first simple microscope and observed living
cells like bacteria and protozoa, which laid the foundation of cell biology.
Q2. What is the difference between
unicellular and multicellular organisms?
Answer:
· Unicellular organisms (like Amoeba, Paramecium) have only one cell that performs all life functions.
· Multicellular organisms (like humans, plants) have millions of cells with division of labor among specialized cells.
Q3. What are the major differences
between Animal Cell and Plant Cell?
Answer:
· Animal Cell: Round/irregular shape, no cell wall, no chloroplast, small vacuoles.
· Plant Cell: Rectangular shape, cell wall present, chloroplast present for photosynthesis, large central vacuole.
Q4. Which cell organelle is called the
Powerhouse of the Cell and why?
Answer: Mitochondria are
called the Powerhouse of the Cell because they generate energy in the form of
ATP (Adenosine Triphosphate) through respiration.
Q5. What is the function of the Golgi
Body in a cell?
Answer: Golgi Body works like a "post office" of the
cell. It modifies, packs, and transports proteins and lipids to different parts
of the cell.
Q6. What is the difference between
Mitosis and Meiosis?
Answer:
· Mitosis: Cell division that produces two identical daughter cells, used for growth and repair.
· Meiosis: Cell division that produces four genetically different daughter cells with half chromosomes, used for sexual reproduction (gametes).
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