Cell: Tiny Units of Life Step inside the microscopic world where life begins.

What Is a Cell?

Cell

Smallest living unit that can grow, reproduce, and perform all chemical reactions needed for life.

Unicellular: one cell manages every task. Multicellular: many specialised cells share the work.

Cell Theory

1

Matthias Schleiden (1838)

Every plant is built of cells; the cell is the basic structural unit.

2

Theodor Schwann (1839)

All animals are also cellular, revealing a universal “unit of life.”

3

Rudolf Virchow (1855)

New cells originate only from pre-existing cells—Omnis cellula e cellula.

Pro Tip:

Remember these three pillars—structure, universality, continuity—to quickly recall NCERT § 8.2 and restate cell theory.

Inside a Typical Cell

Main Points

  1. 1 Plasma membrane
  2. 2 Cytoplasm
  3. 3 Nucleus

Key Highlights

  • Selective barrier controlling what enters/leaves.
  • Fluid matrix hosting organelles and reactions.
  • DNA vault; issues genetic instructions.
  • Detailed in NCERT §8.3.

Two Cell Worlds (8.4 & 8.5)

Prokaryote

Size : 1–5 µm
No membrane-bound nucleus
70S ribosomes
Rapid binary fission

Eukaryote

Size : 10–20 µm
True, membrane-bound nucleus
80S ribosomes
Membrane-bound organelles

Key Similarities

Both have a plasma membrane
Both contain DNA
Both perform metabolism

Sizing Up Cells

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Relative sizes of various cells (log scale)

Cell size spans several powers of ten

Section 8.4 shows that cells vary from sub-micrometre microbes to egg cells large enough to see.

Comparing familiar examples helps you visualise this extraordinary range.

Key Points:

  • Mycoplasma — ~0.1 µm, one of the smallest living cells.
  • Escherichia coli — ~1 µm rod-shaped bacterium.
  • Human red blood cell — ~7 µm diameter.
  • Typical plant/animal cell — 20–30 µm.
  • Ostrich egg — ~170 mm, the largest known single cell.

Meet the Endomembrane

Trace how biomolecules travel: ER → Golgi → lysosome/vacuole.

1

Rough ER

Ribosome-studded sheets fold and modify proteins for secretion.

2

Smooth ER

Synthesises lipids, stores Ca2+, and detoxifies drugs.

3

Golgi Apparatus

Sorts, tags, and packs ER cargo into vesicles for delivery.

4

Lysosome

Acidic enzymes break down waste, pathogens, and old organelles.

5

Vacuole (plants)

Stores water, ions, and pigments; keeps cells firm via turgor.

Pro Tip:

Together, these organelles form a pipeline that moves molecules from synthesis to secretion.

Quick Check

Question

Choose the correct answer (CBSE 8.5.4): Which organelle is rightly called the ‘powerhouse’ of the cell?

1
Endoplasmic reticulum
2
Golgi apparatus
3
Mitochondrion
4
Lysosome

Hint:

Think about where ATP is produced.

Key Takeaways

All life is cellular.

Cell theory unifies biology.

Membrane, cytoplasm, nucleus = core triad.

Prokaryotes vs eukaryotes differ in complexity.

Organelles divide labour inside eukaryotic cells.

Next Steps

Explore mitosis and meiosis in the upcoming lesson.

Thank You!

We hope you found this lesson informative and engaging.