Hundreds of billions of your cells are replaced every day — almost 1% of your whole body. Yet you started as just one single fertilised cell.
Key idea
Studying Cells: Microscopes
1
Robert Hooke, 1665
First observed and named "cells" while examining cork under a self-built microscope.
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Resolution vs magnification
The naked eye's resolution limit is about 0.1 mm. Light microscopes use lenses; electron microscopes reveal structures at the nanometre scale.
Key idea
The Cell Membrane and Osmosis
1
Fluid-mosaic model
A lipid bilayer (water-loving heads out, water-repelling tails in) with embedded proteins that can move — fluid and mosaic-like.
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Osmosis
Water moves across the membrane from a dilute solution to a concentrated one. A potato piece swells in plain water, and shrinks in strong salt solution.
Key idea
The Cell Wall
Plant, fungal, and bacterial cells have a rigid cell wall outside the cell membrane — mainly cellulose in plants.
Why plants don't shrink like animal cells
In strong salt solution, a plant cell's membrane pulls away from its wall — but the rigid wall holds its overall shape. An animal cell (no wall) just shrinks.
Key idea
Nucleus, Ribosomes, and the ER
1
Nucleus
Contains chromatin (DNA + protein); the nucleolus makes ribosomal parts.
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Ribosomes
Sites of protein synthesis — free in the cytoplasm or attached to the ER.
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Rough ER
Has ribosomes attached; makes proteins.
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Smooth ER
No ribosomes; makes lipids and hormones.
Key idea
Golgi, Lysosomes, and Mitochondria
1
Golgi apparatus
Modifies, sorts, and packages proteins — the cell's post office.
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2
Lysosomes
Enzyme sacs that break down waste — the cell's clean-up crew.
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Mitochondria
Site of cellular respiration, producing ATP — the powerhouse of the cell.
Key idea
Plastids and Vacuoles
1
Plastids
Chloroplasts (green, photosynthesis), chromoplasts (colour in flowers/fruits), and leucoplasts (colourless, store starch/oil).
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Vacuoles
A mature plant cell has one large central vacuole storing cell sap, keeping the cell firm — a wilted plant has lost this water.
No — genetic material in a nucleoid — No — 1–10 µm
Eukaryotic (plant/animal)
Yes — well-defined — Yes — 10–100 µm
Key idea
Mitosis and Meiosis
Why meiosis halves the countMeiosis happens only in reproductive organs, and halves the chromosome number — so when two gametes fuse at fertilisation, the offspring ends up with the normal full number again.
1 cell
2 identical daughter cells
1 cell
4 gametes, half chromosomes
Key idea
Cell Theory, and When Division Goes Wrong
1
Cell Theory
All living things are made of cells; the cell is life's basic unit; and every cell comes from a pre-existing cell.
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When control breaks down
Contact inhibition normally stops animal cells dividing once they touch neighbours. Cancer cells lose this control and divide uncontrollably, forming tumours.
Chapter · Key terms to remember
Key Terms
Osmosis
Water moving across a membrane, dilute to concentrated.
Cell wall
Rigid outer covering in plant, fungal, bacterial cells.
Nucleus
Controls the cell; holds genetic material.
Chapter · Key terms to remember
More Key Terms
Mitochondria
The cell's powerhouse; site of respiration.
Prokaryotic / Eukaryotic
No true nucleus / has a true nucleus.
Mitosis
Division into 2 identical daughter cells.
Chapter · Key terms to remember
More Key Terms
Meiosis
Division into 4 gametes, half chromosomes.
Contact inhibition
Stops cell division once cells touch neighbours.
Questions for your notebook
Write these down, then discuss
1
Differentiate: (i) Cell membrane and cell wall (permeability) (ii) RER and SER (structure) (iii) Chloroplasts and chromoplasts (pigments).
2
Which pair is correctly placed: present in plant cells, but absent in animal cells? (i) Leucoplast & cell wall (ii) Mitochondria & ribosome (iii) Cell wall & Golgi apparatus (iv) Lysosome & ER.
3
Renu says all plant parts, even roots, contain plastids. Rohit disagrees, saying roots (underground, no photosynthesis) lack plastids. Who is correct?
Project as-is — students copy the questions, then the class discusses answers together.