Understanding the cell - The building block of all living things
A cell is the smallest structural and functional unit of life. All living organisms are made up of cells.
Imagine a building made of bricks. Just like bricks are the smallest building blocks of a house, cells are the smallest building blocks of our body! Just as many bricks together make a building, many cells together make our body.
Robert Hooke discovered the cell in 1665. He observed a thin slice of cork under a simple microscope and saw small box-like structures. He called them "cells" because they reminded him of the small rooms (cells) where monks lived.
Fun fact: The cells Robert Hooke saw were actually dead cells. He only saw the cell walls, not the living parts!
Proposed by Schleiden (1838) and Schwann (1839), later modified by Virchow (1855).
Three main points:
1. All living organisms are made up of one or more cells
2. Cell is the basic unit of life
3. All cells arise from pre-existing cells
Organisms made up of a single cell are called unicellular organisms. The single cell performs all the life processes.
Examples: Amoeba, Paramecium, Bacteria, Yeast, Euglena
Think of a one-man army! Just like one person doing all tasks - cooking, cleaning, working - a single cell in unicellular organisms does everything: eating, breathing, reproduction, movement!
Organisms made up of many cells are called multicellular organisms. Different cells perform different functions.
Examples: Humans, Animals, Plants, Fungi (like mushrooms)
Imagine a school with many people - teachers teach, cooks prepare food, guards provide security. Each person has a specific job. Similarly, in our body, different cells do different jobs: muscle cells help us move, nerve cells carry messages, blood cells transport oxygen!
Cells are very tiny and can only be seen under a microscope. Most cells are between 1-100 micrometers (Ξm) in size.
Note: 1 micrometer = 1/1000 of a millimeter =
0.001 mm
The smallest cell: Mycoplasma (0.3 Ξm)
The largest cell: Ostrich egg (single cell!)
Cells come in different shapes based on their function:
âĒ Round/Spherical: Red Blood Cells (RBCs)
âĒ Long and branched: Nerve cells (to carry
messages over long distances)
âĒ Elongated: Muscle cells (for contraction)
âĒ Irregular: White Blood Cells (to squeeze
through tissues)
âĒ Rectangular: Plant cells (for structural
support)
A cell has three main parts: Cell Membrane, Cytoplasm, and Nucleus (in eukaryotic cells).
The cell membrane is a thin, flexible outer covering of the cell that separates the cell from its surroundings.
The cell membrane is like the boundary wall of your house! It decides who can enter and who cannot. It allows useful things (like oxygen and food) to enter and lets waste materials go out. This is called selective permeability.
Diffusion: Movement of substances from high
concentration to low concentration. Example: Perfume spreading in
a room.
Osmosis: Movement of water from low concentration
of solute to high concentration of solute through a semi-permeable
membrane. Example: Raisins swelling in water.
The cell wall is a rigid outer covering present only in plant cells. It is made of cellulose (a complex carbohydrate).
If cell membrane is like a curtain (flexible), the cell wall is like a brick wall (rigid and strong)! Plant cells need this extra protection because they don't move and need to stand upright. Animal cells don't have cell walls because they need to move and be flexible.
The nucleus is the control center of the cell. It is a spherical structure present in the center of the cell, surrounded by a nuclear membrane.
The nucleus is like the principal's office in a school! Just like the principal controls all school activities, the nucleus controls all cell activities. It's the "brain" of the cell!
1. Nuclear Membrane: Double-layered membrane with
pores that controls exchange of materials between nucleus and
cytoplasm.
2. Nucleoplasm: Jelly-like substance inside the
nucleus.
3. Chromatin: Thread-like structures made of DNA
and proteins. During cell division, chromatin condenses to form
chromosomes.
4. Nucleolus: Small, spherical body inside
nucleus that helps make ribosomes.
Chromosomes: Rod-shaped structures made of DNA.
They carry genes from parents to children. Humans have 46
chromosomes (23 pairs).
Genes: Segments of DNA that carry information for
a particular trait. For example, genes decide your eye color, hair
color, height, etc.
Think of chromosomes as books and genes as chapters in those books
containing specific information!
Cytoplasm is the jelly-like substance between the cell membrane and nucleus. It contains many cell organelles floating in it.
Imagine cytoplasm as a swimming pool filled with water (cytoplasm) and various pool toys and floats (cell organelles) floating in it. All the activities happen in this pool!
Cell organelles are small structures inside the cell that perform specific functions. They are like tiny organs of the cell!
The Endoplasmic Reticulum is a network of tube-like structures spreading throughout the cytoplasm. It connects the cell membrane to the nucleus.
Think of ER as a highway system in a city! Just like highways transport goods from one place to another, ER transports materials within the cell.
1. Rough ER (RER): Has ribosomes attached on its
surface, looks rough. Makes proteins.
2. Smooth ER (SER): No ribosomes, looks smooth.
Makes lipids (fats) and detoxifies harmful substances.
The Golgi Apparatus consists of a stack of flat, membrane-bound sacs. It looks like a stack of pancakes!
Golgi Apparatus is like a post office! It receives packages (proteins from ER), modifies them, packs them properly, labels them, and sends them to their correct destinations inside or outside the cell.
Lysosomes are small, spherical sacs containing digestive enzymes. They are also called the "suicide bags" of the cell.
Lysosomes are like the garbage disposal system or recycling unit of the cell! They break down waste materials, old cell parts, and unwanted substances. They're also like the cleanup crew!
When a cell is damaged or old, lysosomes burst and release their enzymes. These enzymes digest the cell's own components, leading to cell death. Hence, they're called "suicide bags". This is actually helpful as it removes damaged cells!
Mitochondria (singular: mitochondrion) are rod-shaped or cylindrical organelles with a double membrane. They are called the "powerhouse of the cell".
Mitochondria are like power plants or charging stations! Just as a power plant generates electricity for a city, mitochondria produce energy (ATP) for the cell to do all its work.
Mitochondria produce energy in the form of ATP (Adenosine
Triphosphate) through a process called cellular respiration. ATP
is the energy currency of the cell - it's used for all cellular
activities!
Equation: Glucose + Oxygen â Carbon dioxide +
Water + Energy (ATP)
Plastids are colored or colorless organelles found only in plant cells and some protists. They are absent in animal cells.
1. Chloroplasts: Green colored, contain
chlorophyll, perform photosynthesis (make food for plants).
2. Chromoplasts: Colored plastids (yellow,
orange, red). Give color to flowers and fruits.
3. Leucoplasts: Colorless plastids. Store food in
the form of starch, oils, or proteins.
Chloroplasts are like solar panels! Just as solar panels capture sunlight and convert it to electricity, chloroplasts capture sunlight and convert it to food (glucose) through photosynthesis. That's why they're called the "kitchen of the cell"!
Plants are green because of chloroplasts containing chlorophyll
(green pigment). Chlorophyll absorbs red and blue light but
reflects green light, making leaves appear green!
Fun fact: Like mitochondria, chloroplasts also
have their own DNA and ribosomes!
Vacuoles are fluid-filled sacs surrounded by a membrane. They store various substances.
In Plant Cells: One large central vacuole
occupying 50-90% of cell volume. Stores water, minerals, and waste
products. Helps maintain cell rigidity.
In Animal Cells: Many small vacuoles. Used for
temporary storage.
Vacuoles in plant cells are like a big water tank that stores water and also acts as a storage room for various things. In animal cells, they're like small pockets or bags used for temporary storage.
When plants don't get enough water, their vacuoles shrink. This makes the cells lose their rigidity, and the plant wilts (droops down). When you water the plant, vacuoles fill up again, cells become rigid, and the plant stands upright!
Prokaryotic cells are primitive cells that do not have a well-defined nucleus or membrane-bound organelles.
Examples: Bacteria, Blue-green algae (Cyanobacteria)
Eukaryotic cells are advanced cells with a well-defined nucleus and membrane-bound organelles.
Examples: Plants, Animals, Fungi, Protists
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | No well-defined nucleus (nucleoid region) | Well-defined nucleus with nuclear membrane |
| Nuclear Membrane | Absent | Present |
| Organelles | No membrane-bound organelles | Membrane-bound organelles present |
| Size | Smaller (1-10 Ξm) | Larger (10-100 Ξm) |
| Chromosome | Single, circular chromosome | Multiple, linear chromosomes |
| Ribosomes | Smaller (70S) | Larger (80S) |
| Cell Division | Binary fission | Mitosis and Meiosis |
| Examples | Bacteria, Blue-green algae | Plants, Animals, Fungi |
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (made of cellulose) | Absent |
| Cell Membrane | Present (inside cell wall) | Present (outermost layer) |
| Plastids | Present (chloroplasts, etc.) | Absent |
| Vacuoles | One large central vacuole | Many small vacuoles |
| Centrosome | Absent (except in lower plants) | Present |
| Shape | Fixed (rectangular/square) | Irregular shape |
| Nucleus | Present (may be at periphery) | Present (usually at center) |
| Storage | Store as starch | Store as glycogen |
Q1. Why is cell called the structural and
functional unit of life?
Answer: Because all living organisms are made of
cells (structural unit) and all life processes occur in cells
(functional unit).
Q2. Why do plant cells have cell walls but animal
cells don't?
Answer: Plant cells need cell wall for rigidity
to stand upright and bear weight. Animal cells need flexibility
for movement, so they don't have cell walls.
Q3. What would happen if the cell has no
mitochondria?
Answer: The cell would not get energy to perform
its functions, and it would die.