Biology · Chapter 01

🦠 The Cell

Cell structure, organelles, plant vs animal cell.

💡 The Building Block of Life

The cell is the smallest structural and functional unit of life. Robert Hooke discovered cells in 1665 while looking at cork under a microscope. Every living thing — from bacteria to blue whales — is made of cells.

Two main types:

• Prokaryotic — no nucleus, simple. Bacteria, archaea.
• Eukaryotic — has nucleus + organelles. Plants, animals, fungi, protists.

Key organelles (Eukaryotic):

• Nucleus — control center, holds DNA
• Mitochondria — "powerhouse", makes ATP energy
• Ribosomes — protein factories
• ER (Endoplasmic Reticulum) — transport network
• Golgi apparatus — packages proteins
• Lysosomes — "digestive bags"
• Cell membrane — controls entry/exit
• Cytoplasm — jelly where everything floats

🌱 Plant cell vs Animal cell

Plant cells have 3 extra parts animal cells don't:

• Cell wall — rigid outer layer (made of cellulose)
• Chloroplasts — green, do photosynthesis
• Large central vacuole — stores water, gives plant its shape

Animal cells are usually round; plant cells are rectangular because of the cell wall.

HeLa cells under fluorescence microscope — cell nuclei visible
HeLa cells under fluorescence microscope — cell nuclei visibleWikimedia Commons / CC BY-SA 3.0
Plant cell diagram showing chloroplasts and cell wall
Plant cell diagram showing chloroplasts and cell wallWikimedia Commons / CC BY-SA 3.0
🧠 Mitochondria fun fact

Mitochondria have their own DNA (separate from nucleus)! Scientists believe mitochondria were once free-living bacteria that got swallowed by other cells billions of years ago. This is the endosymbiotic theory.

You inherit ALL your mitochondria from your mother — that's why mitochondrial DNA is used to trace maternal ancestry.

🎬

Animal Cell — Click Any Organelle

Animation
Nucleus Mito Mito Ribosomes ER Golgi Lysosome Cell membrane CLICK ANY ORGANELLE The nucleus, mitochondria, and other parts each have a specific job. Try clicking different parts of the cell above.

Click any organelle to learn what it does. The cell is a tiny factory with specialized departments.

🔬

Plant Cell vs Animal Cell

Interactive

Toggle to see the differences. Plant cells have 3 extra parts.

ShapeRound / irregular
Cell wall❌ No
Chloroplasts❌ No
VacuoleSmall (or none)
Energy sourceEats food → mitochondria
Practice (NCERT): Why are mitochondria called the "powerhouse" of the cell?
Mitochondria produce ATP (adenosine triphosphate) — the energy currency of the cell — through cellular respiration. They convert glucose and oxygen into usable energy. Every cell activity (movement, growth, repair) needs ATP, so without mitochondria, life as we know it stops.
Practice (NEET): Which organelle is absent in prokaryotic cells but found in eukaryotic cells?
Many! Prokaryotes lack a true nucleus, mitochondria, ER, Golgi apparatus, chloroplasts, and lysosomes. They only have ribosomes (smaller, 70S), DNA floating in cytoplasm, cell membrane, and cell wall. This is why prokaryotes are considered the simplest form of cellular life.