Cell — The Basic Unit of Life — AP Class 8 Biological Science
'The cell is the building block of every living organism — from a tiny bacterium to a giant banyan tree.'
1. Discovery of the Cell
Robert Hooke (1665)
- An English scientist using a self-designed microscope
- Observed a thin slice of cork (dead plant cells)
- Saw small, box-like compartments which he called 'cells' (Latin: cella = small room)
- Published his observations in the book 'Micrographia'
Antonie van Leeuwenhoek (1674)
- First to observe living cells — bacteria, protozoa, red blood cells
- Called them 'animalcules'
'Key Point: Hooke observed dead cork cells (only cell walls remained), while Leeuwenhoek was the first to observe living cells.'
2. Cell Theory
Formulated by Matthias Schleiden (botanist) and Theodor Schwann (zoologist) in 1838–1839.
| Principle | Statement |
|---|---|
| Principle 1 | All living organisms are made up of one or more cells |
| Principle 2 | The cell is the basic structural and functional unit of life |
| Principle 3 | All cells arise from pre-existing cells (added by Rudolf Virchow, 1855) |
'AP Board Fact: The cell theory is one of the FOUNDATION principles of biology. Expect a 2-mark question on who proposed it and its main points.'
3. Types of Cells — Prokaryotic vs Eukaryotic
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (nucleoid region — no nuclear membrane) | Present (true nucleus with nuclear membrane) |
| Size | Small (1–10 micrometres) | Larger (10–100 micrometres) |
| Organelles | No membrane-bound organelles | Membrane-bound organelles present |
| Cell Wall | Present (peptidoglycan in bacteria) | Present in plants (cellulose), absent in animals |
| DNA | Single, circular chromosome | Multiple, linear chromosomes |
| Ribosomes | 70S (smaller) | 80S (larger) |
| Examples | Bacteria, Blue-green algae (Cyanobacteria) | Plants, Animals, Fungi, Protists |
'Remember: 'Pro' = primitive/before, 'Eu' = true/well-defined. Prokaryotes came first on Earth.'
4. Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (cellulose) | Absent |
| Shape | Fixed, rectangular | Irregular, round |
| Chloroplast | Present (for photosynthesis) | Absent |
| Vacuole | Large central vacuole | Multiple small vacuoles |
| Stored Food | Starch | Glycogen |
| Lysosomes | Rare | Common |
| Centrioles | Absent (except in lower plants) | Present |
'Comparison questions (Plant vs Animal cell) are very common in AP Board exams — typically 2 to 4 marks.'
5. Cell Organelles — Structure and Functions
Cell Membrane (Plasma Membrane)
- Structure: Made of a phospholipid bilayer with embedded proteins
- Function: Separates the cell from its environment
- Key Property: Selectively permeable — allows some substances to pass through while blocking others
- Model: Fluid Mosaic Model (Singer and Nicolson, 1972)
'Important: The cell membrane is NOT the same as the cell wall. The membrane is present in ALL cells; the cell wall is found only in plants and bacteria.'
Cell Wall
- Location: Outside the cell membrane (plants only)
- Composition: Cellulose (a carbohydrate)
- Functions: Provides shape, structural support, and protection
- Property: Fully permeable (allows all substances to pass through)
Nucleus
- The 'control centre' or 'brain' of the cell
- Parts:
- Nuclear membrane: Double membrane with pores
- Nucleoplasm: Semi-fluid matrix inside
- Chromatin: DNA + protein (condenses into chromosomes during cell division)
- Nucleolus: Site of ribosome synthesis
- Function: Controls all cellular activities through gene expression
Cytoplasm
- Jelly-like substance between cell membrane and nucleus
- Contains all organelles
- Site of many metabolic reactions (e.g., glycolysis)
Mitochondria (Powerhouse of the Cell)
- Structure: Double membrane — outer smooth, inner folded into cristae
- Function: Cellular respiration — produces ATP (energy currency) by breaking down glucose
- Key Fact: Contains its own DNA (circular) — evidence of endosymbiotic theory
- Nicknames: 'Powerhouse of the cell', 'Energy factory'
'Remember: More mitochondria are found in cells with high energy demand — muscle cells, nerve cells, sperm cells.'
Endoplasmic Reticulum (ER)
- Network of membrane-bound tubes and sacs
- Types:
- Rough ER (with ribosomes) — protein synthesis and processing
- Smooth ER (no ribosomes) — lipid synthesis, detoxification
- Function: Transport of materials within the cell
Golgi Apparatus (Golgi Body / Dictyosome)
- Structure: Stack of flattened membrane sacs called cisternae
- Function: Packaging, modification, and sorting of proteins and lipids
- 'Think of it as the POST OFFICE of the cell — it packages materials and sends them to their destinations.'
Chloroplast (Plants only)
- Structure: Double membrane + internal thylakoid stacks (grana) in a fluid called stroma
- Pigment: Contains chlorophyll (green pigment that traps sunlight)
- Function: Photosynthesis — converts light energy into chemical energy (glucose)
- Key Fact: Also has its own DNA (circular) — another evidence for endosymbiosis
Vacuole
- Plant cells: One large central vacuole filled with cell sap (water, nutrients, waste)
- Functions: Storage, turgor pressure (maintains plant rigidity), waste disposal
- Animal cells: Multiple small vacuoles — mainly for storage and transport
Lysosomes (Suicidal Bags of the Cell)
- Contain digestive enzymes
- Break down waste, damaged organelles, and pathogens
- 'Called 'suicidal bags' because if they burst, they digest their own cell'
Ribosomes
- Structure: Tiny, spherical — no membrane
- Location: Free in cytoplasm or attached to Rough ER
- Function: Protein synthesis
- Composition: RNA + protein
6. Comparison Table — All Organelles at a Glance
| Organelle | Membrane? | Found in | Primary Function |
|---|---|---|---|
| Cell Membrane | Yes | All cells | Selectively permeable barrier |
| Cell Wall | No (rigid layer) | Plants, bacteria | Support and protection |
| Nucleus | Yes (double) | Eukaryotes | Control centre, contains DNA |
| Mitochondria | Yes (double) | All eukaryotes | ATP (energy) production |
| Chloroplast | Yes (double) | Plants | Photosynthesis |
| ER | Yes | Eukaryotes | Protein/lipid synthesis, transport |
| Golgi | Yes | Eukaryotes | Packaging and modification |
| Vacuole | Yes (tonoplast) | All cells | Storage, turgor (plants) |
| Lysosome | Yes | Animal cells | Digestion, waste disposal |
| Ribosome | No | All cells | Protein synthesis |
7. Worked Examples
Example 1: Identify the Organelle
Description: 'I have a double membrane, inner folds called cristae, and I produce ATP.'
Answer: Mitochondria.
Example 2: Identify the Organelle
Description: 'I am found only in plant cells. I contain a green pigment and convert sunlight into food.'
Answer: Chloroplast.
Example 3: Comparison
Question: Why are plant cells rigid while animal cells are flexible?
Answer: Plant cells have a cell wall made of cellulose outside the cell membrane, providing structural rigidity. Animal cells lack a cell wall and have only a flexible cell membrane.
Example 4: Cell Theory Application
Question: A scientist discovers a new organism that has no nucleus and no membrane-bound organelles. Which group does it belong to?
Answer: Prokaryotes (e.g., bacteria).
8. Common Mistakes
| Mistake | Fix |
|---|---|
| Confusing cell wall and cell membrane | Cell wall = rigid, cellulose, only in plants; Cell membrane = flexible, all cells |
| Saying mitochondria produce 'energy' | Mitochondria produce ATP (energy currency) — not energy itself |
| Thinking all cells are the same size | Cells vary greatly in size: RBC = 7 micrometres, nerve cells can be 1 metre long |
| Forgetting the nucleolus | Nucleolus is inside the nucleus — it makes ribosomes, not to be confused with chromatin |
| Saying plant cells have no lysosomes | They are rare in plants, not absent. Some functions are done by vacuoles instead |
| Confusing rough and smooth ER | RER = ribosomes = protein; SER = no ribosomes = lipid |
9. Exam Focus (AP Board BSEAP Class 8)
| Topic | Marks | Question Type |
|---|---|---|
| Cell discovery (Hooke, Leeuwenhoek) | 1–2 | Very short answer |
| Cell theory (Schleiden, Schwann, Virchow) | 2 | Short answer |
| Prokaryotic vs Eukaryotic (table) | 2–4 | Table / Short answer |
| Plant vs Animal cell (differences) | 2–4 | Table / Long answer |
| Organelle functions (Mitochondria, Nucleus) | 2 | Short answer |
| Cell membrane (selective permeability) | 2 | Short answer |
| Chloroplast (photosynthesis) | 1–2 | Very short answer |
| Diagram — plant cell / animal cell | 4 | Label the diagram |
10. Quick Self-Test
Q1. Who discovered the cell and in which year? A1. Robert Hooke in 1665.
Q2. What is the cell theory? A2. (1) All living things are made of cells. (2) Cell is the basic unit of life. (3) All cells arise from pre-existing cells.
Q3. Name two differences between plant and animal cells. A3. Plant cells have a cell wall and chloroplasts; animal cells do not.
Q4. Which organelle is called the 'powerhouse of the cell'? A4. Mitochondria.
Q5. What is the function of the cell membrane? A5. It is selectively permeable — controls the entry and exit of substances.
Q6. What is the green pigment in chloroplasts called? A6. Chlorophyll.
Q7. Are bacteria prokaryotic or eukaryotic? A7. Prokaryotic.
Q8. What does the Golgi apparatus do? A8. Packages, modifies, and sorts proteins and lipids.
Q9. Why are lysosomes called 'suicidal bags'? A9. They contain digestive enzymes that, if released, can digest the entire cell.
Q10. What is the function of the large central vacuole in plant cells? A10. It stores water and cell sap and provides turgor pressure that keeps the plant rigid.
11. Summary
- Robert Hooke discovered cells in 1665 looking at cork under a microscope
- Cell Theory: All organisms are made of cells; cell = basic unit; all cells from pre-existing cells
- Prokaryotes (no nucleus) vs Eukaryotes (true nucleus)
- Plant cells have cell wall, chloroplast, large vacuole; animal cells do not
- Key organelles: Nucleus (control), Mitochondria (ATP), Chloroplast (photosynthesis), ER (transport), Golgi (packaging), Vacuole (storage)
- The cell membrane is selectively permeable; the cell wall is fully permeable
'Every living thing you see — or cannot see — is made of cells. Learn the cell, and you learn life itself.'
