How to Use This Page
Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Cell — The Unit of Life covers cell theory, prokaryotic and eukaryotic cells, cell organelles, and the differences between plant and animal cells — focus on structure-function relationships.
Key Concepts (Quick Recap)
Cell Theory
| Scientist | Contribution |
|---|---|
| Robert Hooke (1665) | Coined the term 'cell'; observed cork cells under a microscope |
| Anton van Leeuwenhoek | First to observe living cells (bacteria, protozoa) |
| Schleiden (1838) | All plants are composed of cells |
| Schwann (1839) | All animals are composed of cells |
| Virchow (1855) | Omnis cellula e cellula — all cells arise from pre-existing cells |
Prokaryotic vs Eukaryotic Cell
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Size | Small (0.1–5 µm) | Large (10–100 µm) |
| Nucleus | Nucleoid (no nuclear membrane) | True nucleus with nuclear membrane |
| Nucleolus | Absent | Present |
| Chromosomes | Single, circular, naked DNA | Multiple, linear DNA with histones |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, Golgi, etc.) |
| Ribosomes | 70S (smaller) | 80S (cytoplasm); 70S (organelles) |
| Cell wall | Present (peptidoglycan in bacteria) | Present in plants (cellulose); absent in animals |
| Flagella | Simple (no 9+2 arrangement) | Complex (9+2 arrangement) |
| Examples | Bacteria, Cyanobacteria (Blue-green algae) | Plant cells, Animal cells, Fungi, Protists |
Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Plastids | Present (chloroplasts) | Absent |
| Vacuole | Large central vacuole | Small or absent |
| Centrioles | Absent | Present |
| Shape | Rectangular (fixed) | Round/irregular |
| Lysosomes | Rare | Present |
| Glyoxysomes | Present | Absent |
| Starch granules | Present | Glycogen granules |
| Plasmodesmata | Present | Absent |
Cell Organelles — Structure and Function
| Organelle | Structure | Function |
|---|---|---|
| Cell Wall | Rigid, outermost; cellulose, hemicellulose, pectin, lignin | Protection, shape, prevents bursting in hypotonic solution |
| Cell Membrane (PM) | Lipid bilayer with proteins (fluid mosaic model) | Selectively permeable; transport, cell signalling |
| Nucleus | Double membrane (nuclear envelope) with nuclear pores; nucleolus; chromatin | Control centre; DNA replication; transcription; ribosome assembly |
| Mitochondria | Double membrane; inner folded into cristae; matrix | Powerhouse — ATP production (Krebs cycle, ETC) |
| Chloroplast | Double membrane; thylakoids (grana); stroma | Photosynthesis — light reaction (grana) + dark reaction (stroma) |
| Endoplasmic Reticulum | Network of membranes; Rough (with ribosomes) + Smooth | RER: protein synthesis & transport; SER: lipid synthesis, detoxification |
| Golgi Apparatus | Stack of flattened cisternae with vesicles | Packaging, modification, sorting of proteins; forms lysosomes |
| Ribosomes | 80S (cytoplasm) or 70S (organelles); rRNA + protein | Protein synthesis (translation) |
| Lysosomes | Single membrane; hydrolytic enzymes | Intracellular digestion; suicide bags of the cell |
| Vacuoles | Membrane-bound (tonoplast); cell sap | Storage, turgor pressure, waste disposal |
| Peroxisomes | Single membrane; catalase, oxidase | Photorespiration; breakdown of H₂O₂ |
| Glyoxysomes | Single membrane; enzymes for glyoxylate cycle | Conversion of fats to carbohydrates (in germinating seeds) |
Special Structures in Prokaryotes
- Mesosomes: Infoldings of plasma membrane; help in DNA replication, cell wall formation, respiration
- Plasmids: Small, circular, extrachromosomal DNA; carry antibiotic resistance genes
- Pili: Short, tubular structures; help in attachment and conjugation
- Capsule/Slime layer: Gelatinous covering; protection from desiccation and phagocytosis
I. Choose the Best Answer (MCQs)
Q1. Who coined the term 'cell'? (a) Leeuwenhoek (b) Schleiden (c) Robert Hooke (d) Schwann
Q2. The powerhouses of the cell are (a) Chloroplasts (b) Ribosomes (c) Mitochondria (d) Lysosomes
Q3. Which organelle is called the 'suicide bag' of the cell? (a) Mitochondria (b) Golgi apparatus (c) Lysosome (d) Ribosome
Q4. 70S ribosomes are found in (a) Cytoplasm of eukaryotes (b) Mitochondria and chloroplasts (c) Nucleus (d) Golgi apparatus
Q5. The fluid mosaic model of cell membrane was proposed by (a) Schleiden and Schwann (b) Singer and Nicolson (c) Robertson (d) Rudolph Virchow
Q6. Which of the following is present in plant cells but absent in animal cells? (a) Mitochondria (b) Cell wall (c) Ribosomes (d) Golgi apparatus
Q7. Plasmids are found in (a) Eukaryotic nucleus (b) Prokaryotes (c) Plant cell wall (d) Animal cell membrane
II. Fill in the Blanks
1. The term 'cell' was coined by Robert Hooke. 2. The cell theory was proposed by Schleiden and Schwann. 3. Omnis cellula e cellula (all cells arise from pre-existing cells) was stated by Virchow. 4. The rigid outermost covering of a plant cell is the cell wall. 5. The selectively permeable membrane of a cell is the plasma membrane (cell membrane). 6. The powerhouses of the cell are the mitochondria. 7. The site of photosynthesis in a plant cell is the chloroplast. 8. Ribosomes are the site of protein synthesis. 9. The 'suicide bags' of the cell are the lysosomes. 10. The membrane surrounding the vacuole is called the tonoplast. 11. 70S ribosomes are found in prokaryotes and in mitochondria/chloroplasts. 12. 80S ribosomes are found in the cytoplasm of eukaryotic cells. 13. The fluid mosaic model of the cell membrane was proposed by Singer and Nicolson. 14. The site of lipid synthesis in a cell is the smooth endoplasmic reticulum. 15. The control centre of the cell is the nucleus.
III. True or False (with corrections for false statements)
IV. Match the Following
| Column A | Column B | Answer |
|---|---|---|
| 1. Mitochondria | (a) Protein synthesis | (b) ATP production |
| 2. Ribosome | (b) ATP production | (a) Protein synthesis |
| 3. Chloroplast | (c) Intracellular digestion | (d) Photosynthesis |
| 4. Lysosome | (d) Photosynthesis | (c) Intracellular digestion |
| 5. Golgi apparatus | (e) Lipid synthesis | (f) Packaging and secretion |
| 6. Smooth ER | (f) Packaging and secretion | (e) Lipid synthesis |
V. Answer Briefly (2-mark questions)
VI. Answer in Detail (5-mark questions)
Common Mistakes to Avoid
- Confusing 70S and 80S ribosomes: 70S ribosomes are found in prokaryotes AND in mitochondria/chloroplasts of eukaryotes. 80S ribosomes are found in the cytoplasm of eukaryotic cells.
- Saying prokaryotes have no DNA: Prokaryotes DO have DNA — it is circular and found in the nucleoid region (not enclosed by a nuclear membrane).
- Confusing cell wall and cell membrane: Cell wall is the outermost rigid layer (plants only, made of cellulose). Cell membrane is the selectively permeable layer present in ALL cells.
- Forgetting mitochondria have their own DNA: Mitochondria and chloroplasts both have circular DNA and 70S ribosomes — evidence for endosymbiotic theory.
- Saying plant cells have centrioles: Plant cells generally LACK centrioles (they form spindle fibres without them). Animal cells have centrioles.
- Confusing lysosome and peroxisome: Lysosomes contain hydrolytic enzymes for intracellular digestion. Peroxisomes contain catalase and oxidase for breaking down H₂O₂ and photorespiration.
- Calling mitochondria the only site of respiration: Glycolysis occurs in the cytoplasm; only Krebs cycle and ETC occur in mitochondria.
Exam Checklist
- [ ] Can you state the cell theory and the scientists who proposed it?
- [ ] Can you differentiate prokaryotic and eukaryotic cells (at least 6 points)?
- [ ] Can you differentiate plant cell and animal cell (at least 6 points)?
- [ ] Can you describe the structure and function of mitochondria?
- [ ] Can you describe the structure and function of chloroplast?
- [ ] Can you explain the fluid mosaic model of the cell membrane?
- [ ] Can you describe the structure and function of the nucleus?
- [ ] Can you explain the functions of ER (rough and smooth), Golgi apparatus, and lysosomes?
- [ ] Can you explain what mesosomes and plasmids are (in prokaryotes)?
- [ ] Can you explain the significance of 70S vs 80S ribosomes?