Chapter 5 of 9

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Andhra Pradesh (BIEAP)Class 9 Biological Science★ 8–10 marks · AP Board Class 9 Biological Science

Cell — The Fundamental Unit of Life

Complete AP Class 9 Cell Biology: CELL THEORY. PROKARYOTIC vs EUKARYOTIC. PLANT vs ANIMAL cells. Every ORGANELLE — nucleus, mitochondria (powerhouse), ER, Golgi, lysosomes (suicide bags), chloroplasts, vacuoles. ENDOSYMBIOSIS theory. OSMOSIS and diffusion. Cell division intro — mitosis vs meiosis.

⏱ 18 min readMedium difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1State the three tenets of cell theory (Schleiden, Schwann, Virchow)
2Distinguish prokaryotic from eukaryotic cells using at least 4 features
3Compare plant and animal cells — identify 4 key differences
4Name and describe the function of all major organelles (nucleus, mitochondria, ER, Golgi, lysosomes, chloroplasts, vacuoles)
5Explain osmosis, diffusion, plasmolysis with diagrams
6Distinguish mitosis from meiosis in terms of product and purpose

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Why this chapter matters

The Cell is the most foundational chapter in AP Class 9 Biology. Every subsequent topic (tissues, physiology, genetics) builds on cell biology. The prokaryotic vs eukaryotic comparison, the plant vs animal cell comparison, and the organelles-with-functions table are all standard diagram and short-answer questions. Osmosis and diffusion appear in 3-4 mark explanation questions. The endosymbiotic theory is a conceptual question that distinguishes good answers. Diagram questions — draw and label a plant/animal cell — appear almost every year.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

🔗

Basic Cell Structure (Class 8 Science)

Basic organelle names — prerequisite for detailed organelle functions

Cell — The Fundamental Unit of Life

"The cell is the basic unit of LIFE. From bacteria to blue whales — everything alive is made of cells."

1. Discovery of the Cell

Robert Hooke (1665) : Observed thin slices of CORK under a microscope. Saw tiny compartments like 'little rooms' and called them CELLS. 'Hooke saw DEAD cells — the cell walls of cork. He did NOT see living cells.' Anton van Leeuwenhoek (1674) : First to observe LIVING cells (bacteria, protozoa, sperm) using hand-ground lenses. Robert Brown (1831) : Discovered the NUCLEUS in orchid cells.

2. Cell Theory (Schleiden, Schwann, Virchow)

All living organisms are composed of CELLS and CELL PRODUCTS (Schleiden, 1838 — botanist; Schwann, 1839 — zoologist). 2. The cell is the BASIC STRUCTURAL and FUNCTIONAL UNIT of all organisms. 3. All cells arise from PRE-EXISTING CELLS (Rudolf Virchow, 1855: 'Omnis cellula e cellula'). Exception: Viruses are NOT cellular — they are acellular infectious particles at the border of living/non-living.

3. Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic	Eukaryotic
Nucleus	NO nuclear membrane. Nucleoid.	TRUE nucleus, double membrane.
Membrane-bound organelles	ABSENT	Present (mitochondria, ER, Golgi, lysosomes)
Ribosomes	70S	80S
Size	1-10 μm	10-100 μm
Cell division	Binary fission	Mitosis and Meiosis
Examples	Bacteria, Cyanobacteria, Mycoplasma	Plants, Animals, Fungi, Protists

The Endosymbiotic Theory (Lynn Margulis, 1967)

Mitochondria and chloroplasts were once FREE-LIVING prokaryotes. A larger prokaryote ENGULFED them — and instead of digesting them, kept them as symbionts. Evidence: Both have their OWN circular DNA (like bacteria). Both have 70S ribosomes. Both divide by BINARY FISSION. Both have DOUBLE membranes. 'This is one of the most BEAUTIFUL theories in biology — it explains how complex eukaryotic cells evolved from simple prokaryotic ancestors.'

4. Plant Cell vs Animal Cell

Feature	Plant Cell	Animal Cell
Cell Wall (cellulose)	Present	Absent
Chloroplasts	Present	Absent
Vacuole	LARGE central	Small, temporary
Centrioles	Absent	Present
Shape	Usually REGULAR	IRREGULAR

5. Cell Organelles — Structure and Function

Nucleus — Control Centre

Double membrane with PORES. Contains CHROMATIN (DNA + proteins). Contains NUCLEOLUS (rRNA synthesis). 'Red blood cells in mammals lose their nucleus to carry more haemoglobin.'

Mitochondria — Powerhouse

Double membrane. Inner folds = CRISTAE (increase surface area). Site of AEROBIC RESPIRATION: glucose + O₂ → CO₂ + H₂O + ATP. Cells needing MORE energy have MANY more mitochondria.

Endoplasmic Reticulum (ER)

Rough ER (with ribosomes): Protein synthesis. Smooth ER: Lipid synthesis. Detoxification.

Ribosomes — Protein Factories

80S in eukaryotes. 70S in prokaryotes, mitochondria, chloroplasts.

Golgi Apparatus

Stack of cisternae. Modifies, packages, and ships proteins. Forms LYSOSOMES.

Lysosomes — Suicide Bags

Contain powerful digestive enzymes (acid hydrolases). Digest foreign material, worn-out organelles. During METAMORPHOSIS (tadpole → frog), lysosomes digest the tail.

Chloroplasts (Plant Only)

Double membrane. Internal thylakoids (GRANA) contain chlorophyll. STROMA = site of dark reaction/Calvin cycle. Own DNA + 70S ribosomes.

Vacuoles

Plants: LARGE central vacuole — maintains TURGOR pressure. In Amoeba: CONTRACTILE vacuole (osmoregulation). FOOD vacuole (digestion).

6. Osmosis and Diffusion

Diffusion: Movement from HIGH to LOW concentration. Passive (no energy). Osmosis: Diffusion of WATER across a SELECTIVELY PERMEABLE membrane. Hypotonic (lower solute outside): Water ENTERS → cell SWELLS. Isotonic (same): No net movement. Hypertonic (higher solute outside): Water LEAVES → cell SHRINKS.

Plasmolysis: When a plant cell is in HYPERTONIC solution → water leaves → cell membrane pulls away from cell wall. 'This is why plants WILT when not watered — the cells lose turgor.'

7. Cell Division — An Introduction

Mitosis: Produces 2 DIPLOID (2n) daughter cells — genetically IDENTICAL to parent. For GROWTH and REPAIR. Meiosis: Produces 4 HAPLOID (n) cells — genetically DIFFERENT. For GAMETE formation (sperm/egg). 'Mitosis occurs in SOMATIC (body) cells. Meiosis occurs only in GERM cells (gonads).'

8. Common Mistakes

'All cells have a nucleus' — Mature RBCs have NO nucleus. Prokaryotes have nucleoid (no membrane).
'Mitochondria and chloroplasts are like other organelles' — They have OWN DNA + ribosomes. SEMI-AUTONOMOUS.
'Cell wall = cell membrane' — Cell WALL (cellulose, outer, rigid). Cell MEMBRANE (inner, selectively permeable). Plant cells have BOTH.

9. AP SSC Exam Focus

Topic	Marks
Prokaryotic vs Eukaryotic	3-4
Plant vs Animal cell	3-4
Organelles and functions	4-5
Osmosis	3-4
Endosymbiotic theory	2-3

Key Diagrams

Draw and label: TYPICAL PLANT CELL and ANIMAL CELL with all organelles.

Cell Organelles — Quick Reference Table

Organelle	Membrane	Key Feature
Nucleus	Double + pores	Contains DNA. Control centre.
Mitochondria	Double (inner folded → cristae)	ATP production. Own DNA + 70S ribosomes.
Rough ER	Single, with ribosomes	Protein synthesis and transport
Smooth ER	Single, no ribosomes	Lipid synthesis. Detoxification.
Golgi	Single (stacked cisternae)	Packages and ships proteins. Forms lysosomes.
Lysosomes	Single	Digestive enzymes (pH~5). Suicide bags.
Chloroplasts	Double, internal thylakoids (grana)	Photosynthesis. Own DNA + 70S ribosomes.
Vacuole	Single (tonoplast)	Storage. Turgor (plants). Osmoregulation.

Quick Memory: 'NUMEG CLV'

Nucleus, Mitochondria, ER, Golgi, Chloroplasts, Lysosomes, Vacuoles — the 7 key organelles every AP student must know.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Cell Theory

Schleiden (1838, plants) + Schwann (1839, animals): all organisms are made of cells. Virchow (1855): all cells arise from pre-existing cells ('Omnis cellula e cellula').

Exception: viruses are acellular — not made of cells

Prokaryotic vs Eukaryotic

Prokaryotic: no nuclear membrane, no membrane-bound organelles, 70S ribosomes, 1-10 μm, binary fission. Eukaryotic: true nucleus, membrane-bound organelles, 80S ribosomes, 10-100 μm, mitosis/meiosis.

Examples: bacteria (prokaryotic) vs plants, animals, fungi (eukaryotic)

Plant vs Animal Cell differences

Plant only: cell wall (cellulose), chloroplasts, large central vacuole, NO centrioles. Animal only: centrioles, small/no vacuole, NO cell wall, NO chloroplasts.

Both have: nucleus, mitochondria, ER, Golgi, ribosomes, cell membrane

Mitochondria

Double membrane; inner folds = cristae. Site of aerobic respiration: glucose + O₂ → CO₂ + H₂O + ATP. Own circular DNA + 70S ribosomes.

Called 'powerhouse of the cell'

Chloroplast

Double membrane. Thylakoids (grana) contain chlorophyll. Stroma = site of Calvin cycle. Own DNA + 70S ribosomes. Site of photosynthesis.

Present only in plant cells (and algae)

Lysosome

Single membrane. Contains acid hydrolases (digestive enzymes at pH ~5). Digests worn-out organelles, foreign material, and damaged cells.

Called 'suicide bags' — can trigger cell death (apoptosis)

Golgi apparatus

Stack of cisternae. Modifies, packages, and ships proteins received from rough ER. Forms lysosomes.

Like a 'post office' — sorts and dispatches cellular proteins

Osmosis

Water movement across a selectively permeable membrane from lower solute concentration (higher water potential) to higher solute concentration (lower water potential).

Hypotonic → water in → cell swells. Hypertonic → water out → cell shrinks. Isotonic → no net movement.

Plasmolysis

Plant cell placed in hypertonic solution → water exits → cell membrane pulls AWAY from cell wall.

Explains why plants wilt without water — turgour pressure lost

Endosymbiotic theory (Lynn Margulis)

Mitochondria and chloroplasts were once free-living prokaryotes engulfed by larger cells. Evidence: own circular DNA, 70S ribosomes, binary fission, double membrane.

This explains why both organelles are called 'semi-autonomous'

Mitosis vs Meiosis

Mitosis: 2 diploid (2n) identical daughter cells; growth and repair. Meiosis: 4 haploid (n) varied cells; gamete formation.

Mitosis = somatic cells. Meiosis = germ cells (gonads)

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Confusing cell wall with cell membrane, or saying plants have no cell membrane

✓ Plant cells have BOTH: cell wall (outer, rigid, cellulose, structural support) AND cell membrane (inner, flexible, selectively permeable). Osmosis occurs across the CELL MEMBRANE, not the cell wall. The cell wall is freely permeable.

WATCH OUT

✗ Saying mitochondria and chloroplasts have 80S ribosomes like other eukaryotic organelles

✓ Mitochondria and chloroplasts have 70S ribosomes — like prokaryotes. This supports the endosymbiotic theory. Cytoplasmic ribosomes in eukaryotes = 80S.

WATCH OUT

✗ Confusing mitosis (growth) with meiosis (gametes)

✓ Mitosis: body cells, 2 daughter cells, genetically identical, diploid (2n). Meiosis: gonads only, 4 daughter cells, genetically varied, haploid (n). 'MItosis = MIrror copy. MEiosis = MEiosis makes Eggs and sperm.'

Practice problems

Work through this chapter's problems as a readiness check — reveal each solution, mark yourself honestly, and get your gap report at the end.

Readiness check

Are you exam-ready for Cell — The Fundamental Unit of Life?

6 problems from this chapter. Try each one, reveal the worked solution, mark yourself honestly — get your gap report at the end.

6 questions~4 min worth ~10 marks in Andhra Pradesh (BIEAP) exams

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Cell Theory: Schleiden (plants, 1838) + Schwann (animals, 1839) + Virchow (cells from cells, 1855).
•Prokaryotic: no nuclear membrane, 70S ribosomes, binary fission. Examples: bacteria.
•Eukaryotic: true nucleus, membrane-bound organelles, 80S ribosomes. Examples: plants, animals.
•Plant only: cell wall (cellulose), chloroplasts, large central vacuole. Animal only: centrioles.
•Nucleus = control centre. Mitochondria = powerhouse (ATP). Golgi = post office (packaging). Lysosomes = suicide bags.
•Osmosis: water moves from hypotonic → hypertonic across selectively permeable membrane.
•Plasmolysis: plant cell in hypertonic solution — membrane pulls from cell wall.
•Endosymbiotic theory: mitochondria and chloroplasts were once free prokaryotes. Evidence: circular DNA, 70S ribosomes, binary fission, double membrane.
•Mitosis = 2 diploid cells (growth/repair). Meiosis = 4 haploid cells (gametes).
•NUMEG CLV: Nucleus, mitochondria, ER, Golgi, Chloroplasts, Lysosomes, Vacuoles — 7 key organelles.

Andhra Pradesh (BIEAP) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 8–10 marks

Question type	Marks each	Typical count	What it tests
Very Short Answer	1	2–3	Name, define, one-line explanation
Short Answer	2	2	Explain with one example
Long Answer	4	1	Detailed explanation with diagram

Prep strategy

AP Board biology questions reward diagram labelling — practise diagrams until you can draw from memory
Know the function of every structure you name — 'state and explain' is the most common question type
Practise 4-mark questions with full paragraph answers (4 valid points = 4 marks)

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Cancer biology

Cancer = uncontrolled cell division (mitosis gone wrong). Understanding mitosis at cell level is the foundation for understanding cancer and its treatment.

Organ transplantation

The liver can regenerate because hepatocytes (liver cells) undergo mitosis. Up to 70% of the liver can be removed and it will regrow — enabling partial liver transplants.

Food preservation (osmosis)

Pickling vegetables in salt (hypertonic solution) dehydrates bacterial cells via osmosis, preventing growth. The same principle is used in jams (sugar) and preserved foods.

Agriculture — wilting and irrigation

Crop wilting in drought-prone Rayalaseema (AP) is due to plasmolysis — cells lose water. Irrigation restores turgidity by providing hypotonic water for osmosis.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Draw plant cell and animal cell from memory — this is likely worth 4-5 marks. Label all organelles using horizontal indicator lines.

For prokaryotic vs eukaryotic: make a 4-column table (feature, prokaryotic value, eukaryotic value, example).

Osmosis explanation: always draw arrows showing water movement direction and state hypotonic/isotonic/hypertonic.

Endosymbiotic theory: memorise the 4 evidence points as a list — circular DNA, 70S ribosomes, binary fission, double membrane.

Organelle functions: use the nickname system (powerhouse = mitochondria, suicide bag = lysosome, post office = Golgi) for fast recall.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

STRETCH

The nuclear envelope is actually continuous with the rough ER — the cell has an interconnected membrane system. This is called the endomembrane system (ER → Golgi → Vesicles → Cell membrane).

STRETCH

Programmed cell death (apoptosis) is carried out by lysosomes — a 'suicide bag' that systematically dismantles a cell when it receives a death signal. This is crucial for development (fingers are sculpted by apoptosis of webbing).

STRETCH

Chloroplasts likely evolved from cyanobacteria; mitochondria from alpha-proteobacteria. This means all the ATP in your body is produced by what were once independent bacteria.

STRETCH

The discovery of CRISPR-Cas9 (used to edit DNA in the nucleus) was recognised with a Nobel Prize in 2020. It was originally found in bacterial immune systems — another link between prokaryotes and eukaryotes.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

AP Board SSC (Class 10)Cell division, heredity, and genetics build directly on this chapter

NTSECell organelles, osmosis, and cell division are frequent MCQ topics

Biology Olympiad (NSEB)Endosymbiosis, organelle ultrastructure, and cell division are tested at depth

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

Yes. Plant cells have BOTH a cell wall (outer, rigid, cellulose) AND a cell membrane (inner, flexible, selectively permeable). The cell wall is freely permeable; the cell membrane controls entry and exit.

Mature red blood cells in mammals lose their nucleus to maximise space for haemoglobin — allowing each cell to carry more oxygen. This is an evolutionary adaptation.

Diffusion: movement of any substance from high to low concentration (no membrane needed). Osmosis: specifically the diffusion of WATER across a SELECTIVELY PERMEABLE membrane from lower to higher solute concentration.

They have their own DNA and ribosomes and can divide independently — hence 'autonomous.' But they still need many proteins encoded in the nuclear DNA — hence 'semi.' They cannot survive completely independently outside the cell.

No. Viruses are ACELLULAR — they are not made of cells. Cell Theory therefore does not apply to viruses, which is why viruses are considered to be at the boundary of living and non-living.

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Andhra Pradesh (BIEAP) Class 9 mock tests

Andhra Pradesh (BIEAP) Class 9 PYQs

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Cell — The Fundamental Unit of Life

By the end of this chapter you'll be able to…

Before you start — revise these

Cell — The Fundamental Unit of Life

1. Discovery of the Cell

2. Cell Theory (Schleiden, Schwann, Virchow)

3. Prokaryotic vs Eukaryotic Cells

The Endosymbiotic Theory (Lynn Margulis, 1967)

4. Plant Cell vs Animal Cell

5. Cell Organelles — Structure and Function

Nucleus — Control Centre

Mitochondria — Powerhouse

Endoplasmic Reticulum (ER)

Ribosomes — Protein Factories

Golgi Apparatus

Lysosomes — Suicide Bags

Chloroplasts (Plant Only)

Vacuoles

6. Osmosis and Diffusion

7. Cell Division — An Introduction

8. Common Mistakes

9. AP SSC Exam Focus

Key Diagrams

Cell Organelles — Quick Reference Table

Quick Memory: 'NUMEG CLV'

Key formulas & results

Common mistakes & fixes

Practice problems

Are you exam-ready for Cell — The Fundamental Unit of Life?

5-minute revision

Andhra Pradesh (BIEAP) marks blueprint

Where this shows up in the real world

Exam strategy

Going beyond the textbook

Where else this chapter is tested

AI helpers for this chapter

Generate 5 more questions

Explain at a different level

📝 My notes

Questions students ask

Do plant cells have a cell membrane?

Why do RBCs in humans have no nucleus?

What is the difference between osmosis and diffusion?

Why are mitochondria and chloroplasts called semi-autonomous?

Can viruses be classified using cell theory?

Practice more, ask doubts, find a tutor

Related chapters

Biological Science — Cell, Tissues, Diversity & Physiology (AP Class 9)

Diversity in Living Organisms — The Five Kingdoms

Plant Physiology — Photosynthesis, Transpiration and Transport

Animal Physiology — Nutrition and Digestion

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