📘 Chapter 2: The Cell – The Unit of Life

Detailed Quick Reference Notes (Exam Ready)

2.1 What is a Cell?

Cell = structural and functional unit of life.
Discovered by Robert Hooke (1665) in cork slices using a simple microscope.
Leeuwenhoek (1674): First to see living cells (bacteria, protozoa).
Every living thing = made of cells.

2.2 Invention of Microscope & Discovery of Cell

Early microscopes by Leeuwenhoek revealed single-celled organisms.
Better microscopes → details of nucleus, cytoplasm, organelles discovered.
Two important scientists:
- Schleiden (1838): All plants are made of cells.
- Schwann (1839): All animals are made of cells.

2.3 Cell Theory

All living organisms are made of one or more cells.
The cell is the basic unit of structure and function.
All cells arise from pre-existing cells (added by Rudolf Virchow, 1858).
- Basis of modern biology.

2.4 Cells – How Many?

Unicellular organisms: Made of one cell (Amoeba, Paramecium, Chlamydomonas, Bacteria).
Multicellular organisms: Made of many cells (Humans, Trees, Animals).
Human body → trillions of cells.

2.5 Cells – How Small?

Smallest cells: Bacteria (0.3–5.0 μm).
Human RBCs: ~7 μm in diameter.
Longest cells: Nerve cells (up to 1 metre in humans, even longer in animals like whales).
Largest cells: Ostrich egg (a single cell, before development).
Importance → size is related to function.

2.6 Smallness of Cells = Greater Efficiency

Small size = larger surface area : volume ratio.
Benefits:
- Faster absorption of food & oxygen.
- Quicker removal of waste.
- Faster exchange of materials.
Reason why cells remain microscopic instead of growing endlessly.

2.7 Cell Shapes

Shape depends on function.
Examples:
- RBCs: Round, biconcave → easy oxygen transport.
- Nerve cell: Long, branched → transmits impulses.
- Muscle cell: Elongated → contraction.
- Amoeba: Irregular → pseudopodia for movement.

2.8 Structure of a Cell

2.8.1 Boundaries

Cell Wall (plants only):
- Non-living, rigid, made of cellulose.
- Freely permeable.
- Gives shape, support, protection.
- Middle lamella → cementing layer between adjacent plant cells.
Cell Membrane (Plasma Membrane):
- Thin, living boundary in both plant & animal cells.
- Selectively permeable → controls movement of substances.

2.8.2 Cytoplasm & Organelles

Cytoplasm:
- Jelly-like fluid inside cell.
- Site of many chemical reactions (enzymes present).
- Contains organelles and inclusions.
Ribosomes:
- Tiny, spherical structures.
- Function: Protein synthesis.
- Found free in cytoplasm or attached to Rough ER.
Endoplasmic Reticulum (ER):
- Network of membranes inside cytoplasm.
- Rough ER (RER): Ribosomes attached; transports proteins.
- Smooth ER (SER): No ribosomes; synthesizes lipids, detoxifies.
Golgi Apparatus:
- Stack of flattened sacs.
- Modifies, sorts, packages proteins into vesicles.
- Called Dictyosomes in plant cells.
Mitochondria:
- Double membrane; inner folded into cristae.
- Site of cellular respiration → makes ATP (energy currency).
- Has its own DNA & ribosomes → semi-autonomous.
- Called “Powerhouse of the cell”.
Lysosomes (animals):
- Membrane-bound sacs with digestive enzymes.
- Digest unwanted materials, worn-out organelles, or invaders.
- Called “Suicide bags” (burst = digest own cell).
Centrosome (animals):
- Near nucleus; contains two centrioles.
- Organizes spindle fibres during cell division.
Plastids (plants only):
- Triple types:
  - Chloroplasts: Green (chlorophyll), photosynthesis.
  - Chromoplasts: Coloured (carotene, xanthophyll), give colour to fruits/flowers.
  - Leucoplasts: Colourless, storage of starch, fats, proteins.
Vacuoles:
- Membrane-bound sacs.
- Large in plants (filled with cell sap).
- Store water, salts, waste, pigments.
- Small and temporary in animals.
Granules (Inclusions):
- Stored food (starch in plants, glycogen in animals, fat droplets).
- Non-living inclusions.

2.8.3 Nucleus

Surrounded by double nuclear membrane with pores.
Contains:
- Nucleoplasm: Fluid inside.
- Nucleolus: Makes ribosomes.
- Chromatin: DNA + proteins; condenses into chromosomes during division.
Functions:
- Controls all activities.
- Stores genetic information.
- Transmits hereditary traits.

2.9 Plant vs Animal Cells

Feature	Plant Cells	Animal Cells
Cell wall	Present (cellulose)	Absent
Cell membrane	Present	Present
Plastids	Present (3 types)	Absent
Centrosome	Absent	Present
Lysosomes	Rare	Many
Vacuoles	Large, central, permanent	Small, temporary
Shape	Fixed (rectangular)	Variable (round/irregular)

2.10 Protoplasm

Definition: Total living substance inside cell = Cytoplasm + Nucleus.
Characteristics:
- Colourless, jelly-like.
- Made of water, proteins, carbohydrates, fats, salts.
- Described as “Physical basis of life”.

2.11 Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic (Bacteria)	Eukaryotic (Plants/Animals)
Nucleus	Not well defined (nucleoid)	True nucleus with membrane
DNA	Naked, circular	Linear, with proteins
Ribosomes	Small (70S)	Larger (80S)
Organelles	Absent (no ER, mitochondria, Golgi)	Present
Size	Small (1–10 μm)	Larger (10–100 μm)
Examples	Bacteria, Cyanobacteria	Amoeba, Plants, Animals

2.12 Cellular Activities

Growth: Increase in size and number of cells.
Repair: Replacement of old/injured cells (e.g., lizard tail regeneration).
Movement: Muscle contraction; plants bend toward light.
Nutrition: Digestive enzymes, absorption of food.
Circulation: Heart cells pump blood.
Respiration: Mitochondria release energy.
Protection: WBCs destroy germs.
Senses: Sensory cells in eyes, ears, tongue, skin.
Excretion: Vacuoles in plants; kidney cells in animals.
Reproduction: Germ cells (sperm, egg).
Photosynthesis: Chloroplasts trap sunlight.

🌱 Extra Information – Stem Cells

Stem cells: Undifferentiated cells that can multiply and form new cell types.
Types:
1. Embryonic stem cells: Can form any tissue (pluripotent).
2. Tissue-specific stem cells: Restricted to certain types.
3. Induced pluripotent stem cells (iPS): Normal cells reprogrammed into stem cells.
Medical uses: Treatment of thalassemia, type-1 diabetes, spinal injuries.

📌 Points to Remember

Cell = basic unit of life.
Cell theory = 3 main principles.
Protoplasm = living substance.
Prokaryote vs Eukaryote differences.
Plant vs Animal cell differences.
Organelles and their functions (RER, SER, Golgi, Mitochondria, Lysosomes, Plastids, Vacuoles, Nucleus).

📝 Mnemonics

Plastids:
- C → Chloroplast → Chlorophyll → Photosynthesis.
- Ch → Chromoplast → Colours.
- L → Leucoplast → Storage.
Protein Pathway:
Naughty Rats Run Go Mad
Nucleus → Ribosome → Rough ER → Golgi → Membrane.
Prokaryote vs Eukaryote key:
“Pro = No” → Prokaryote = No true nucleus/organelles.

❓ Review Q&A (Exam Practice)

Who discovered cells and when?
→ Robert Hooke, 1665, in cork.
What are the three parts of cell theory?
→ All living things made of cells, Cell = unit of structure & function, Cells from pre-existing cells.
Why are cells small?
→ Higher surface area to volume ratio → faster exchange of substances.
Give one example each: smallest cell, largest cell, longest cell.
→ Smallest = Bacteria, Largest = Ostrich egg, Longest = Nerve cell.
List two differences between plant and animal cells.
→ Plant: cell wall, plastids, large vacuole. Animal: lysosomes, centrosome.
What is protoplasm?
→ Living content of cell = Cytoplasm + Nucleus.
Differentiate between prokaryotic and eukaryotic cells.
→ Nucleus absent vs true nucleus, organelles absent vs present.
Why are mitochondria called “Powerhouse of cell”?
→ Release ATP through respiration.
Why are lysosomes called “Suicide bags”?
→ Can burst and digest cell itself.
Which organelles have their own DNA?
→ Mitochondria & Chloroplasts.