In short: The Class 12 Biology practical exam is worth 30 marks and is arguably the easiest practical to score full marks in — if you prepare the slides, spotting specimens, and viva questions. Unlike Physics or Chemistry practicals where calculations can go wrong, Biology practicals are largely identification-based. This guide covers all experiments, spotting specimens, top 25 viva questions with answers, slide identification tips, and the record format.
Marks Distribution — CBSE Class 12 Biology Practical (30 Marks)
| Component | Marks | Details |
|---|---|---|
| One major experiment | 5 | From the prescribed list |
| One minor experiment | 4 | From the prescribed list |
| Slide preparation + identification | 5 | Temporary mount + identification |
| Spotting (8 spots) | 7 | Identify specimens, slides, charts |
| Practical Record + Viva | 4 | Lab record evaluation + oral |
| Investigatory Project + Viva | 5 | Project file + presentation + oral |
| Total | 30 |
Complete List of Experiments
Major Experiments
| # | Experiment | Key Skills |
|---|---|---|
| 1 | Prepare a temporary mount of onion root tip and study stages of mitosis | Staining with acetocarmine, identifying prophase/metaphase/anaphase/telophase |
| 2 | Study pollen germination on a slide | Preparing sugar solution, observing pollen tube |
| 3 | Study meiosis in onion bud cells / grasshopper testis (prepared slides) | Identifying stages of meiosis I and II |
| 4 | Collect and study soil from different sites for texture, moisture, pH | Soil analysis techniques |
Minor Experiments
| # | Experiment | Key Skills |
|---|---|---|
| 1 | Study of flowers adapted for pollination by wind and insects | Identifying features — colour, nectar, pollen type |
| 2 | Study of pollen grain germination on stigma through permanent slide | Observing pollen tube growth |
| 3 | Preparing temporary mounts of T.S. of stem and root (monocot/dicot) | Section cutting, staining, identification |
| 4 | Study of osmosis by potato osmometer | Understanding osmotic pressure, water movement |
| 5 | Study of plasmolysis in epidermal peel of Rhoeo/onion | Observing cell shrinkage in hypertonic solution |
Spotting — What You Need to Identify
A. Morphology/Anatomy Specimens:
| Spot | Specimen | Key Identification Features |
|---|---|---|
| 1 | Flower — insect pollinated (Hibiscus) | Colourful petals, nectar, large pollen |
| 2 | Flower — wind pollinated (Grass/Maize) | Small, no petals, feathery stigma, light pollen |
| 3 | Emasculated flower | Stamens removed, bag covered for cross-pollination |
| 4 | T.S. of mature anther | Four pollen sacs, pollen grains visible |
| 5 | Seed germination stages | Radicle, plumule, cotyledons visible |
B. Prepared Slides:
| Slide | What to Identify |
|---|---|
| T.S. of ovary showing ovules | Ovary wall, ovules attached to placenta |
| Blastula / Morula stage | Ball of cells, blastocoel |
| L.S. of embryo (dicot seed) | Radicle, plumule, cotyledon, seed coat |
| Human blood smear | RBC, WBC (types), platelets |
| Mitosis stages | Prophase (condensed chromosomes), Metaphase (aligned), Anaphase (separating), Telophase (two groups) |
C. Ecology/Environment Specimens:
| Spot | Specimen | Identification |
|---|---|---|
| Lichen | Symbiotic association | Algae + fungus, found on rocks/trees |
| Cuscuta (Amarbel) | Parasitic plant | Orange/yellow stem, no leaves, haustoria |
| Agaricus (Mushroom) | Saprophyte | Cap, gills, stipe — grows on dead matter |
| Sweet potato | Adventitious root modification | Tuberous root — stores food |
| Onion | Modified stem (bulb) | Fleshy scale leaves, reduced stem |
D. Common Models/Charts:
| Model/Chart | What to Know |
|---|---|
| Human evolution stages | Dryopithecus → Ramapithecus → Australopithecus → Homo erectus → Homo sapiens |
| DNA double helix model | Sugar-phosphate backbone, complementary base pairs (A-T, G-C) |
| Ecological pyramid (energy) | Always upright, 10% energy transfer rule |
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Slide Identification Tips
How to Score Full Marks in Slide Identification:
- Focus the microscope properly — start with low power, then shift to high power
- Scan the entire slide — don't fixate on one corner
- Look for key features:
- Cell wall present → plant cell
- No cell wall, irregular shape → animal cell
- Stained dark bodies → chromosomes (mitosis)
- Tube-like growth from round body → pollen germination
- Draw what you see — not what you think it should look like
- Label clearly — arrow marks pointing to structures, neat text
- Write the title — "T.S. of dicot stem as observed under microscope"
Common Slide Recognition Clues:
| Slide | Quick Recognition |
|---|---|
| Onion root tip (mitosis) | Rows of cells, some with dark condensed chromosomes |
| Pollen germination | Round pollen grains with tube growing out |
| Blood smear | Scattered small circles (RBC), few larger stained cells (WBC) |
| T.S. dicot stem | Vascular bundles arranged in ring, pith in centre |
| T.S. monocot stem | Vascular bundles scattered throughout |
| T.S. dicot root | Xylem in centre (star-shaped), no pith |
Top 25 Viva Questions with Answers
Q1: What is pollination? What are its types? Transfer of pollen grains from anther to stigma. Self-pollination (same flower/plant) and cross-pollination (different plant). Cross-pollination increases genetic diversity.
Q2: How do you identify a wind-pollinated flower? Small, inconspicuous, no petals or fragrance, light and smooth pollen, feathery stigma to catch windborne pollen, long stamens hanging outside (e.g., grass, maize, wheat).
Q3: What is emasculation? Why is it done? Removal of anthers from a bisexual flower before they mature, to prevent self-pollination in cross-breeding experiments. The emasculated flower is then covered with a bag until the stigma is receptive.
Q4: What stain is used for observing mitosis? Acetocarmine or aceto-orcein. These stains specifically bind to chromosomes, making them dark and visible against the light cytoplasm.
Q5: Name the stages of mitosis and their key features. Prophase (chromosomes condense, visible), Metaphase (chromosomes align at equatorial plate), Anaphase (chromatids separate, move to poles), Telophase (two nuclei form, cytokinesis begins).
Q6: What is the difference between mitosis and meiosis? Mitosis: one division, 2 identical daughter cells (2n), growth and repair. Meiosis: two divisions, 4 genetically different cells (n), gamete formation. Meiosis involves crossing over.
Q7: What is pollen germination? What solution is used? Growth of a pollen tube from a pollen grain when it lands on a suitable medium. We use 10% sucrose solution on a slide to simulate the stigmatic surface.
Q8: What is plasmolysis? Where does it occur? Shrinkage of the cell membrane away from the cell wall when placed in a hypertonic solution. The cell loses water by exosmosis. We observe it in Rhoeo leaf epidermal peel placed in concentrated salt/sugar solution.
Q9: What is osmosis? Movement of water molecules from a region of higher water potential to lower water potential through a semipermeable membrane. It is a special case of diffusion.
Q10: What is a temporary mount? How do you prepare one? A slide preparation that is not permanent — made for immediate observation. Steps: take a thin section/peel → place on clean slide → add water/stain → place coverslip gently (avoid air bubbles) → observe.
Q11: What is the role of safranin stain? Safranin stains lignified cells (xylem) red/pink, helping distinguish xylem from other tissues in stem and root sections.
Q12: How do you identify a dicot root from a dicot stem in T.S.? Stem: vascular bundles in ring, pith present, open bundles (with cambium). Root: radial arrangement, xylem in centre (star-shaped), no pith, endodermis prominent.
Q13: What is the significance of DNA's double helix structure? The double helix allows: (1) stable storage of genetic information, (2) accurate replication (complementary base pairing), (3) expression of genes through transcription. Discovered by Watson and Crick (1953).
Q14: What are the components of a DNA nucleotide? A phosphate group, a deoxyribose sugar (5-carbon), and a nitrogenous base (Adenine, Guanine, Cytosine, or Thymine).
Q15: What is lichen? What type of relationship does it represent? Lichen is a symbiotic association between an alga (photosynthetic partner) and a fungus (provides shelter and moisture). This is mutualism — both organisms benefit.
Q16: What is Cuscuta? How does it obtain nutrition? Cuscuta (dodder/amarbel) is a parasitic plant. It has no chlorophyll and obtains nutrition from the host plant through haustoria (specialized absorbing structures that penetrate the host's vascular tissue).
Q17: What is a saprophyte? Give an example. An organism that feeds on dead and decaying organic matter. Example: Agaricus (mushroom), Mucor (bread mould). They secrete enzymes externally and absorb digested nutrients.
Q18: What is the ecological significance of decomposers? Decomposers break down dead organic matter, releasing nutrients back into the soil (nutrient cycling). Without them, dead matter would accumulate and nutrients would be locked up.
Q19: Name the types of WBCs visible in a blood smear. Neutrophils (most common, multilobed nucleus), Lymphocytes (large nucleus, thin cytoplasm), Monocytes (largest, kidney-shaped nucleus), Eosinophils (bilobed, red granules), Basophils (rare, dark granules).
Q20: What is the function of RBCs and WBCs? RBCs: transport oxygen (via haemoglobin) and CO₂. WBCs: immune defence — fight infections through phagocytosis (neutrophils) and antibody production (lymphocytes).
Q21: What is the 10% law in ecology? Only 10% of energy is transferred from one trophic level to the next. The rest is lost as heat during respiration. This limits the number of trophic levels in a food chain (usually 4-5).
Q22: What is biodiversity? Why is it important? The variety of life forms (species, genetic, ecosystem diversity) in an area. Important for: ecosystem stability, food security, medicine (many drugs from plants), ecological balance, aesthetic value.
Q23: Differentiate between homologous and analogous organs. Homologous: same embryonic origin, different function (human arm, whale flipper). Analogous: different origin, similar function (bird wing, insect wing). Homology indicates divergent evolution.
Q24: What is the investigatory project you did? Explain briefly. Be ready to explain your project in 2-3 sentences — the objective, method, and conclusion. Common projects: study of drug resistance, pollinator diversity, effect of fertilizers on plant growth, water quality analysis.
Q25: What is the difference between a monocot and dicot seed? Monocot: one cotyledon, endosperm present, e.g., maize. Dicot: two cotyledons, endosperm may be absent (food stored in cotyledons), e.g., gram, pea.
Spotting Preparation Strategy
2-Week Preparation Plan:
| Week | Activity |
|---|---|
| Week 1 | Study all specimens — real specimens preferred over photos. Visit the lab, handle specimens. Learn key identification features for each. |
| Week 2 | Practice identification — ask a friend to place specimens and test you. Practice drawing and labeling. Review viva questions. |
Quick-Reference Identification Chart:
For each specimen, memorize:
- Name (scientific if possible)
- Type (plant/animal/microscopic)
- Key feature (one distinguishing characteristic)
- Biological significance (why it's studied)
Record Format
Lab Record Should Include:
| Section | Content |
|---|---|
| Title Page | Name, class, section, roll number, school, year |
| Certificate | Signed by teacher and principal |
| Index | List of experiments with page numbers |
| Each Experiment | Date, aim, requirements, theory (brief), procedure, observation, result, diagrams |
| Project Report | Separate section — introduction, method, observations, conclusion, bibliography |
Record Maintenance Tips:
- Write in neat handwriting — blue pen, pencil for diagrams
- Get each experiment signed by the teacher after completion
- Don't leave blank pages — complete all experiments
- Add coloured diagrams where possible (especially for morphology)
- Keep the record clean and intact — no torn pages
Frequently Asked Questions
Is the Biology practical exam easier than Physics/Chemistry?
Many students find it easier because it's largely identification-based rather than calculation-based. However, you still need to know the theory behind each specimen and experiment. Viva questions can be tricky if you only memorize without understanding.
What if I can't identify a slide under the microscope?
Focus properly first — adjust coarse and fine knobs. If still unclear, ask the examiner for help with focusing (this is allowed). Look for key features systematically. Even if you're unsure, draw what you see and write your best guess with reasoning.
How much time do I get for spotting?
Typically 1-2 minutes per spot (8 spots = 10-16 minutes total). You rotate through the spots. Write quickly but clearly — name, one identification feature, labeled diagram if required.
Should I draw coloured diagrams in the practical exam?
Colour is not mandatory but can enhance your answer. Use pencil for the diagram outline and colour pencils for details if time permits. Clear labeling is more important than colouring.
How important is the investigatory project?
It carries 5 marks — significant. Choose a relevant topic, conduct it genuinely, and prepare a proper report. The examiner will ask 1-2 questions about your project during viva. Saying "my tuition teacher did it" is obvious and loses marks.
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