Cells and Microorganisms
Cells and Microorganisms – Common to All Living Beings
Cell – The basic unit of life
Core idea: All living organisms – plants, animals and microorganisms – are made of cells. Cell is the smallest unit that can carry out all life processes like respiration, nutrition, excretion and reproduction.
| Aspect | Key Points | Exam Focus |
|---|---|---|
| Definition of cell | Structural and functional unit of life | Usually asked as direct definition or statement-based MCQs |
| Discovery | Robert Hooke (1665) observed cork cells using a simple microscope | Name of scientist, year, and material observed are common questions |
| Observation of living cells | Antonie van Leeuwenhoek first saw living cells (microorganisms) in pond water | Linked to “animalcules” word in some questions |
| Cell theory – originators | Schleiden (plants), Schwann (animals), later Virchow | “Who proposed cell theory?” and matching type questions |
| Cell theory – main ideas | All living organisms are made of cells; cell is basic unit; new cells arise from pre-existing cells | The three main statements are often mixed as right/wrong statements |
| Level | Example | Relation to Cell |
|---|---|---|
| Cell | Muscle cell, nerve cell, guard cell | Basic living unit |
| Tissue | Muscle tissue, xylem, phloem | Group of similar cells performing same function |
| Organ | Heart, leaf, root | Different tissues combine for a specific major function |
| Organ system | Digestive system, circulatory system | Group of organs working together |
| Organism | Human, neem tree, cow | Complete living individual |
Memory Line: “Hooke saw cells in cork; Schleiden & Schwann gave cell theory; Virchow said cells come from cells.”
Exam Tip: When a question focuses on “basic unit” of structure or function or heredity, read carefully: structure & function → cell, heredity → gene / chromosome.
Types of cells
Core idea: Cells differ in size, shape and internal organisation depending on the function. Two broad internal types are prokaryotic and eukaryotic. There are also plant and animal cells.
| Type | Key Features | Examples / Exam Points |
|---|---|---|
| Prokaryotic cell | No true nucleus, genetic material not enclosed by nuclear membrane, few organelles, no membrane-bound organelles like mitochondria | Bacteria, blue-green algae (cyanobacteria). Often asked: prokaryote vs eukaryote differences. |
| Eukaryotic cell | True nucleus with nuclear membrane, many membrane-bound organelles | Plant, animal, fungal cells. More complex and larger than prokaryotic. |
| Plant cell | Cell wall, large central vacuole, chloroplasts, generally fixed shape | Questions on presence of cell wall, chloroplast, shape and vacuole size. |
| Animal cell | No cell wall, small or temporary vacuoles, no chloroplasts, more flexible shape | Lysosomes more common in animal cells; centrioles present in animal cells. |
| Variation | Example | Functional Importance |
|---|---|---|
| Cell size | Nerve cell is very long; RBC is small and disc-shaped | Large size of nerve cell helps in long-distance impulse conduction. |
| Cell shape | Muscle cell (spindle shaped), nerve cell (branched), guard cells (bean-shaped) | Shape is related to function – exam questions often connect specific shape with role. |
| Number of cells in organism | Unicellular – Amoeba, Paramecium; Multicellular – mango tree, man | Unicellular organism performs all life activities with a single cell. |
Memory Line: “Pro = primitive, Eu = true.” Prokaryotic = primitive nucleus (no membrane), Eukaryotic = true nucleus with membrane.
Exam Tip: If a question says “no membrane-bound organelles, nucleoid present”, the answer must be prokaryotic cell / bacteria, even if other options look attractive.
Cell structure and function
Core idea: Each cell has common parts – cell membrane, cytoplasm and nucleus. Plant cells have additional structures like cell wall and chloroplasts. Different organelles perform specific functions.
| Part | Structure | Main Function |
|---|---|---|
| Cell wall (plants, fungi) | Rigid outer covering made of cellulose (in plants) | Provides shape and protection; prevents bursting of plant cell in hypotonic solutions |
| Cell membrane (plasma membrane) | Thin, living, selectively permeable membrane surrounding cytoplasm | Controls entry and exit of substances; maintains internal environment |
| Cytoplasm | Jelly-like substance between cell membrane and nucleus | Holds organelles; many metabolic reactions occur here |
| Nucleus | Large, often central in animal cells; surrounded by nuclear membrane; contains chromosomes and nucleolus | Controls cell activities; stores genetic material (DNA); involved in cell division |
| Mitochondria | Double membrane organelle, inner membrane folded; present in most eukaryotic cells | Site of cellular respiration; releases energy – “power house of cell” |
| Chloroplast (plants) | Green plastid with chlorophyll pigment | Site of photosynthesis; prepares food using sunlight, CO₂ and water |
| Vacuole | Large central vacuole in plant cells; small in animal cells | Storage of water, waste and other substances; maintains turgidity in plant cells |
| Endoplasmic reticulum (ER) | Network of membranes; rough ER has ribosomes, smooth ER does not | Rough ER – protein synthesis; smooth ER – lipid synthesis, detoxification |
| Golgi apparatus | Stacks of membrane-bound sacs | Packaging and secretion of substances; formation of vesicles, lysosomes etc. |
| Organelle | Important Keyword | Exam Angle |
|---|---|---|
| Mitochondria | Power house of cell; ATP production | Often used in assertion-reason questions on energy release |
| Chloroplast | Photosynthesis; chlorophyll; green | Connected to questions on autotrophs and food preparation |
| Nucleus | Control centre; hereditary material | Linked with genes, chromosomes, and cell division |
| Lysosomes (animals) | “Suicide bags” – contain digestive enzymes | MCQs test whether they are present mainly in animal cells |
Memory Line: “Membrane controls, cytoplasm works, nucleus commands, mitochondria powers, chloroplast prepares food.”
Exam Tip: When a question gives an organelle name, first recall key word (like power house, control centre, photosynthesis) and then match with the correct function to avoid confusion.
Cell division
Core idea: Cell division helps in growth, repair and reproduction. Major types are mitosis (equational division) and meiosis (reduction division).
| Type of Division | Chromosome Behaviour | Where / Why |
|---|---|---|
| Mitosis | Number of chromosomes remains same in daughter cells as in parent cell | Occurs in somatic (body) cells; growth, repair, replacement of old cells |
| Meiosis | Chromosome number becomes half in daughter cells | Occurs in reproductive organs to form gametes (sperm, ovum, pollen); important for sexual reproduction |
| Stage (Mitosis simplified) | Key Features | Exam Clue |
|---|---|---|
| Prophase | Chromosomes condense; nuclear membrane starts disappearing; spindle formation begins | “Thick, visible chromosomes” photos/diagrams usually show prophase |
| Metaphase | Chromosomes line up at equator of cell | Seen as chromosomes arranged in a line in the middle |
| Anaphase | Sister chromatids separate and move towards opposite poles | V-shaped chromatids migrating to poles |
| Telophase | Chromosomes decondense; nuclear membranes reform; division of cytoplasm (cytokinesis) | Two nuclei visible; final stage of division |
Memory Line: Names of mitosis stages in order: “PMAT” – Prophase, Metaphase, Anaphase, Telophase.
Exam Tip: If question mentions same chromosome number in parent and daughter cells, choose mitosis. If it mentions half chromosome number or gamete formation, choose meiosis.
Introduction to microorganisms
Core idea: Microorganisms are very small living organisms which cannot be seen with naked eye. They may be unicellular or multicellular. They are present everywhere – air, water, soil, inside our body and on surfaces.
| Group | Examples | Typical Size / Nature |
|---|---|---|
| Bacteria | Lactobacillus, Rhizobium, Vibrio cholerae | Unicellular, prokaryotic, various shapes (rod, spherical, spiral) |
| Viruses | Polio virus, influenza virus, dengue virus | Smaller than bacteria; non-cellular; reproduce only inside host cell |
| Fungi (micro-fungi) | Yeast, some moulds on bread and fruits | Yeast is unicellular; many fungi are multicellular, eukaryotic |
| Protozoa | Amoeba, Plasmodium, Giardia | Unicellular, animal-like, eukaryotic; many cause diseases |
| Algae (micro-algae) | Chlorella, diatoms | Photosynthetic; important producers in aquatic ecosystems |
| Environment | Microorganism Presence | Example |
|---|---|---|
| Soil | Bacteria, fungi, actinomycetes | Rhizobium in root nodules of pulses, decomposers in litter |
| Water | Algae, protozoa, bacteria | Diatoms in ponds; Plasmodium spread by mosquitoes breeding near water |
| Human body | Normal flora + pathogens | Useful gut bacteria help digestion; some bacteria cause disease |
Memory Line: Five main groups to remember: Bacteria, Viruses, Fungi, Protozoa, Algae.
Exam Tip: Whenever the question says “can multiply only inside host cell”, answer must be virus. “Can live freely and also cause disease” – usually bacteria or protozoa.
Useful Microorganisms
Core idea: Many microorganisms are useful in food preparation, agriculture, medicine and industry. Question papers often focus on organism–use matching.
| Field | Microorganism | Use / Product (with Indian context) |
|---|---|---|
| Food – fermentation | Lactobacillus (bacteria) | Curd, buttermilk, some types of cheese; also used as probiotic |
| Food – fermentation | Yeast (fungus) | Bread, dosa/idli batter rising, bakery products; produces CO₂ and makes dough soft and spongy |
| Medicine – antibiotics | Penicillium (fungus) | Source of penicillin, first widely used antibiotic |
| Medicine – vaccines | Weakened or killed forms of microbes | Used in vaccines against diseases like tuberculosis, polio, measles etc. |
| Agriculture – nitrogen fixation | Rhizobium (bacteria) | Lives in root nodules of leguminous plants (pulses); fixes atmospheric nitrogen; reduces need of chemical fertilisers |
| Decomposition | Soil bacteria and fungi | Break down dead plants and animals; return nutrients to soil; important for natural recycling |
| Industry | Different bacteria and fungi | Production of alcohol, organic acids, enzymes, vitamins, biogas etc. |
Memory Line: “Curd–Lactobacillus, Bread–Yeast, Nodules–Rhizobium, Antibiotic–Penicillium.”
Exam Tip: Many questions give examples like pulses + root nodules or curd setting. Immediately connect to Rhizobium and Lactobacillus respectively.
Harmful microorganism
Core idea: Some microorganisms cause diseases in humans, animals and plants, and spoil food and materials. Questions usually check causative organism, mode of spread and prevention.
| Type of Microbe | Disease / Effect | Mode of Transmission / Note |
|---|---|---|
| Bacteria | Cholera, typhoid, tuberculosis | Contaminated water/food, air, close contact. Safe drinking water, sanitation and vaccination are key control measures. |
| Viruses | Common cold, influenza, polio, dengue | Air, droplets, mosquito bites (dengue). They need host cells for multiplication. |
| Protozoa | Malaria (by Plasmodium), amoebic dysentery | Spread through mosquito (malaria) or contaminated water (dysentery). |
| Fungi | Ringworm, some plant diseases | Spread by contact or spores; like damp skin, unclean surroundings. |
| Food spoilage | Milk, cooked rice, pickles, bread getting sour or mouldy | Due to bacterial and fungal growth when food is not stored properly. |
| Effect | Example | Simple Prevention Concept |
|---|---|---|
| Human diseases | Polio, measles, cholera | Vaccination, safe water, hand-washing, clean surroundings |
| Plant diseases | Fungal diseases in crops | Use of disease-resistant varieties and proper crop rotation |
| Food poisoning / spoilage | Stale food causing illness | Refrigeration, proper cooking and avoiding very old stored food |
Memory Line: “Same microbes – some are friends, some are foes.” Usefulness or harm depends on type and situation.
Exam Tip: For disease-based questions, carefully link three things – causative organism, vector (if any), and prevention. Many options mix these parts to confuse students.