Biology · Class 9-10 · CBSE
The Fundamental Unit of Life: Cell Structure and Organelles
Explore the fundamental unit of life: prokaryotic vs eukaryotic cells, plant vs animal cells, plasma membrane & osmosis, nucleus, mitochondria (powerhouse), endoplasmic reticulum, Golgi apparatus, lysosomes (suicide bags), and vacuoles.
1Cell Theory and Cell Architecture
Robert Hooke (1665) discovered cells in cork slices. Schleiden (1838) and Schwann (1839) formulated theCell Theory: all living organisms are composed of cells, and the cell is the basic structural and functional unit of life. Rudolf Virchow (1855) added:Omnis cellula-e cellula(all cells arise from pre-existing cells).
Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic Cell (Bacteria, Blue-green algae) | Eukaryotic Cell (Plants, Animals, Fungi) |
|---|---|---|
| Size | Generally small (1 – 10 μm) | Generally large (5 – 100 μm) |
| Nuclear Region | Poorly defined, lacks nuclear membrane (called Nucleoid) | Well-defined, surrounded by double nuclear membrane |
| Chromosomes | Single circular DNA chromosome | Multiple linear chromosomes with histone proteins |
| Membrane-bound Organelles | Absent (no mitochondria, plastids, endoplasmic reticulum) | Present (mitochondria, Golgi, ER, lysosomes, plastids) |
| Cell Division | Fission or budding (no mitotic spindle) | Mitosis or Meiosis |
Size
- Prokaryotic Cell (Bacteria, Blue-green algae)
- Generally small (1 – 10 μm)
- Eukaryotic Cell (Plants, Animals, Fungi)
- Generally large (5 – 100 μm)
Nuclear Region
- Prokaryotic Cell (Bacteria, Blue-green algae)
- Poorly defined, lacks nuclear membrane (called Nucleoid)
- Eukaryotic Cell (Plants, Animals, Fungi)
- Well-defined, surrounded by double nuclear membrane
Chromosomes
- Prokaryotic Cell (Bacteria, Blue-green algae)
- Single circular DNA chromosome
- Eukaryotic Cell (Plants, Animals, Fungi)
- Multiple linear chromosomes with histone proteins
Membrane-bound Organelles
- Prokaryotic Cell (Bacteria, Blue-green algae)
- Absent (no mitochondria, plastids, endoplasmic reticulum)
- Eukaryotic Cell (Plants, Animals, Fungi)
- Present (mitochondria, Golgi, ER, lysosomes, plastids)
Cell Division
- Prokaryotic Cell (Bacteria, Blue-green algae)
- Fission or budding (no mitotic spindle)
- Eukaryotic Cell (Plants, Animals, Fungi)
- Mitosis or Meiosis
2Plasma Membrane, Osmosis and Cell Wall
ThePlasma Membraneis a selectively permeable phospholipid bilayer with embedded proteins that regulates material transport. The movement of water across a semi-permeable membrane along its concentration gradient is calledOsmosis.
Behavior of Cells in Different Solutions
| Solution Type | Medium Concentration vs Cell Cytoplasm | Water Movement Direction | Effect on Animal Cell | Effect on Plant Cell |
|---|---|---|---|---|
| Hypotonic Solution | Surrounding medium has higher water (more dilute) | Water enters into the cell (Endosmosis) | Cell swells and bursts (lysis) | Cell swells and becomes turgid (cell wall prevents bursting) |
| Isotonic Solution | Equal water concentration on both sides | No net movement of water | Cell maintains constant size and shape | Cell remains unchanged (flaccid) |
| Hypertonic Solution | Surrounding medium has lower water (more concentrated) | Water leaves the cell (Exosmosis) | Cell shrinks (crenation) | Cytoplasm shrinks away from cell wall (Plasmolysis) |
Hypotonic Solution
- Medium Concentration vs Cell Cytoplasm
- Surrounding medium has higher water (more dilute)
- Water Movement Direction
- Water enters into the cell (Endosmosis)
- Effect on Animal Cell
- Cell swells and bursts (lysis)
- Effect on Plant Cell
- Cell swells and becomes turgid (cell wall prevents bursting)
Isotonic Solution
- Medium Concentration vs Cell Cytoplasm
- Equal water concentration on both sides
- Water Movement Direction
- No net movement of water
- Effect on Animal Cell
- Cell maintains constant size and shape
- Effect on Plant Cell
- Cell remains unchanged (flaccid)
Hypertonic Solution
- Medium Concentration vs Cell Cytoplasm
- Surrounding medium has lower water (more concentrated)
- Water Movement Direction
- Water leaves the cell (Exosmosis)
- Effect on Animal Cell
- Cell shrinks (crenation)
- Effect on Plant Cell
- Cytoplasm shrinks away from cell wall (Plasmolysis)
3Specialized Cell Organelles and Their Functions
Organelles of the Eukaryotic Cell
| Organelle | Structure & Characteristic Feature | Primary Cellular Function |
|---|---|---|
| Mitochondria | Double-membrane; deeply folded inner membrane cristae; has own DNA & ribosomes | Powerhouse of the cell: synthesizes ATP via cellular respiration |
| Chloroplast (Plastid) | Double-membrane with stroma and thylakoid grana stacks; has own circular DNA | Kitchen of the cell: site of photosynthesis in green plants |
| Endoplasmic Reticulum (RER & SER) | Network of tubular membranes; RER has ribosomes; SER lacks ribosomes | RER synthesizes proteins; SER synthesizes lipids/steroids and detoxifies poisons in liver |
| Golgi Apparatus | Parallel flattened cisternae discovered by Camillo Golgi | Packaging, modification, and dispatch of proteins/lipids from ER |
| Lysosomes | Membrane-bound sacs filled with digestive enzymes (from RER) | Suicide bags: digest cellular wastes, foreign microbes, and worn-out dying organelles |
| Vacuoles | Membrane-bound (tonoplast in plants); large central vacuole in plants | Maintains turgidity and rigidity in plant cells; stores cell sap and amino acids |
Mitochondria
- Structure & Characteristic Feature
- Double-membrane; deeply folded inner membrane cristae; has own DNA & ribosomes
- Primary Cellular Function
- Powerhouse of the cell: synthesizes ATP via cellular respiration
Chloroplast (Plastid)
- Structure & Characteristic Feature
- Double-membrane with stroma and thylakoid grana stacks; has own circular DNA
- Primary Cellular Function
- Kitchen of the cell: site of photosynthesis in green plants
Endoplasmic Reticulum (RER & SER)
- Structure & Characteristic Feature
- Network of tubular membranes; RER has ribosomes; SER lacks ribosomes
- Primary Cellular Function
- RER synthesizes proteins; SER synthesizes lipids/steroids and detoxifies poisons in liver
Golgi Apparatus
- Structure & Characteristic Feature
- Parallel flattened cisternae discovered by Camillo Golgi
- Primary Cellular Function
- Packaging, modification, and dispatch of proteins/lipids from ER
Lysosomes
- Structure & Characteristic Feature
- Membrane-bound sacs filled with digestive enzymes (from RER)
- Primary Cellular Function
- Suicide bags: digest cellular wastes, foreign microbes, and worn-out dying organelles
Vacuoles
- Structure & Characteristic Feature
- Membrane-bound (tonoplast in plants); large central vacuole in plants
- Primary Cellular Function
- Maintains turgidity and rigidity in plant cells; stores cell sap and amino acids
4Exam Corner: Tips, Mistakes & Board Questions
High-Yield Board Revision Takeaways
- Mitochondria and Plastids are semi-autonomous organelles because they possess their own circular DNA and 70S ribosomes, enabling them to synthesize some of their own proteins.
- Plant cells possess a rigid outer cell wall made of cellulose that prevents the cell from bursting in hypotonic (dilute) environments.
- Lysosomes are known as suicide bags because during cellular disturbance or injury, they burst and release hydrolytic digestive enzymes that digest their own cell.
Exam question · CBSE Class 9/10 Board
Why are lysosomes called the "suicide bags" of a cell?
- A.Because they produce cellular energy
- B.Because when cell gets damaged, lysosomes burst and their hydrolytic digestive enzymes digest their own cell
- C.Because they store metabolic waste permanently
- D.Because they poison nearby healthy cells
- Lysosomes contain powerful hydrolytic enzymes capable of digesting all organic material.
- When the cell is damaged or dies, lysosomes burst and their enzymes digest their own cellular components (autolysis).
- Hence, they are popularly called suicide bags.
- Correct answer is option B.
Answer: B. Because when cell gets damaged, lysosomes burst and their hydrolytic digestive enzymes digest their own cell
Remember
- Cellis the fundamental structural and functional unit of all living organisms.
- Osmosismoves water across semi-permeable membranes; causes turgidity or plasmolysis.
- MitochondriaandChloroplastspossess their own DNA and synthesize ATP.
- Lysosomesact as suicide bags by digesting cellular debris with hydrolytic enzymes.
Check yourself
Question 1
Which cell organelle plays a crucial role in detoxifying many poisons and drugs in the liver cells of vertebrates?