Cell – Structure and Functions – Case-based Questions with Answers
CBSE Class 8 Science — Chapter 8: Cell — Structure and Functions
- Carefully read NCERT text and practise labelled diagrams.
- Use these case-based questions to apply concepts to real-life and experiment scenarios.
- Memorise key organelle functions, transport processes and practical steps for microscopy.
20 Case-Based Questions & Model Answers
Case 1 — Blank Microscope Slide
During a practical, a student sees nothing under the microscope after placing the onion peel slide.
Questions:
- List two likely technical errors the student made.
- Suggest two corrective steps.
Answer: Errors: (1) No specimen or specimen placed off the stage; (2) Cover slip with air bubbles or objective not in focus. Corrections: (1) Re-check and position onion epidermis thinly on slide and add stain (iodine) if allowed; (2) Remove air bubbles by gently lowering the cover slip and focus using coarse then fine adjustment.
Case 2 — Wilting Potted Plant
A potted plant wilts overnight after being watered with very salty water.
Questions:
- Explain why the plant wilted after salty water was added.
- Offer two remedies to help the plant recover.
Answer: Salty water makes soil hypertonic; water moves out of plant cells by osmosis causing loss of turgor and wilting (plasmolysis). Remedies: (1) Flush soil with fresh water to remove excess salts; (2) Repot with fresh soil and avoid saline irrigation; provide shade until recovery.
Case 3 — Pale Leaves in a Greenhouse
Several greenhouse plants have pale leaves and poor growth despite regular watering.
Questions:
- Suggest two cellular-level reasons for pale leaves.
- Recommend diagnostic steps.
Answer: Causes: (1) Chlorophyll deficiency due to nutrient lack (magnesium) or disease; (2) Damaged chloroplasts from light stress or chemical exposure. Diagnostics: soil nutrient test, check for pests or herbicide exposure, inspect leaves under microscope for chloroplast presence, and adjust fertilisation/light accordingly.
Case 4 — Reduced Fruit Set After Pesticide Use
Farmers report fewer bees and lower fruit set after spraying pesticides.
Questions:
- Explain the link between pesticides and lower fruit production.
- Suggest two pollinator-friendly practices.
Answer: Pesticides can kill pollinators or impair foraging, reducing pollination and fruit set. Practices: adopt integrated pest management (IPM) to reduce chemical use; create floral strips and avoid spraying during bloom or when pollinators are active to protect bee populations.
Case 5 — Bacterial Infection and Antibiotics
A doctor prescribes antibiotics for a bacterial infection but the patient questions how antibiotics work.
Questions:
- Briefly explain why antibiotics can target bacteria but not human cells.
- Why is it important to complete the full antibiotic course?
Answer: Many antibiotics target structures or processes unique to bacteria (e.g., cell wall synthesis, bacterial ribosomes) and thus harm bacteria but not human cells. Completing the full course prevents survival of partially resistant bacteria and reduces the chance of developing antibiotic resistance in the population.
Case 6 — Rapid Muscle Fatigue
An athlete feels muscle fatigue during intense exercise and wonders about the cellular cause.
Questions:
- Which organelle is central to producing energy in muscle cells?
- How does limited oxygen affect cellular energy production?
Answer: Mitochondria produce ATP via cellular respiration. Limited oxygen forces cells to rely more on anaerobic respiration (glycolysis), producing less ATP and lactic acid buildup, causing fatigue. Training increases mitochondrial efficiency and oxygen delivery, improving endurance.
Case 7 — Seedling Kept in Dark
A seedling grown in darkness remains pale and elongated compared to one grown under light.
Questions:
- Explain at the cellular level why the seedling is pale.
- What structural changes occur in its cells?
Answer: Without light, chloroplast development is limited and chlorophyll synthesis is reduced, making seedlings pale (etiolation). Cells elongate (long internodes) as the plant searches for light; chloroplasts are underdeveloped and photosynthetic capacity is low. Exposing to light induces chloroplast maturation and green colour.
Case 8 — Root Hairs After Drought
After a dry spell, root hairs of some plants appear shorter and less dense.
Questions:
- How do root hair cells help in water uptake normally?
- Why would drought affect root hair development?
Answer: Root hair cells increase surface area for absorption, aiding water/mineral uptake. Drought can reduce cell turgor and inhibit growth, limiting root hair formation; plants may prioritize deeper root growth or conserve resources, reducing investment in root hairs during stress.
Case 9 — Cells Under Extreme Heat
Leaves exposed to extreme heat show brown patches and wilting.
Questions:
- What cellular structures are most likely damaged by heat?
- Suggest two measures to protect plants from heat stress.
Answer: Heat can denature proteins and damage membranes and chloroplasts, impairing photosynthesis. Measures: provide shade/netting and adequate irrigation to reduce heat stress; use mulch to retain soil moisture and protect roots.
Case 10 — Poor Germination in Stored Seeds
A farmer finds stored seeds have low germination after a humid season.
Questions:
- Explain why humidity affects seed viability.
- How do seed banks preserve seed viability?
Answer: High humidity increases seed moisture, promoting fungal growth and metabolic activity that reduce viability. Seed banks dry seeds and store them at low temperatures to slow metabolism and prevent spoilage, preserving genetic material for future sowing or restoration.
Case 11 — Plant with Small Leaves
A gardener notices some plants have smaller-than-normal leaves after transplanting.
Questions:
- At the cellular level, why might leaf size be reduced?
- Recommend care to restore normal growth.
Answer: Transplant shock limits water uptake and cell expansion; reduced cell division or expansion in the leaf meristem leads to smaller leaves. Care: ensure adequate watering, avoid transplanting during heat, apply gentle fertiliser and allow recovery time for cell division and expansion to resume.
Case 12 — Algae Bloom in Pond
An increase in algae cover is observed after fertilizer runoff into a pond.
Questions:
- Why does nutrient runoff promote algal growth?
- What cellular effect can dense algal blooms have on other aquatic organisms?
Answer: Nutrients (nitrate, phosphate) fuel algal growth (eutrophication). Dense blooms block light, and when algae die, decomposition consumes oxygen causing hypoxia, which suffocates fish by impairing cellular respiration. Reducing runoff and creating buffer zones prevent eutrophication.
Case 13 — Plant with Variegated Leaves
A houseplant shows patches of white and green on leaves.
Questions:
- What causes variegation at the cellular level?
- How does variegation affect photosynthesis?
Answer: Variegation results from cells lacking chlorophyll (white areas) due to genetic mosaicism or mutations affecting chloroplast development. White patches cannot photosynthesise, reducing overall photosynthetic capacity; plants may grow slower and need more light or energy to compensate.
Case 14 — Rapid Disease Spread in Plant Nursery
A nursery has seedlings with fungal spots spreading quickly.
Questions:
- Explain how fungal pathogens spread and affect cells.
- Suggest nursery hygiene measures to control spread.
Answer: Fungi spread via spores, invading tissues and secreting enzymes that break down cell walls and disrupt cells. Measures: remove infected plants, sterilise tools, improve air circulation, avoid overwatering, and apply appropriate fungicides as per guidelines to protect healthy seedlings.
Case 15 — Regenerating Lizard Tail
A lizard regenerates its tail after loss.
Questions:
- How does regeneration relate to cell division and differentiation?
- Give one example of regeneration in plants.
Answer: Regeneration involves activation of cell division (mitosis) and differentiation where new cells form tissues that rebuild the tail. In plants, regeneration occurs in cuttings where meristematic cells divide and differentiate to form roots and shoots—used in vegetative propagation (e.g., stem cuttings).
Case 16 — Red Blood Cells in High Altitude
A person moving to high altitude produces more red blood cells over weeks.
Questions:
- Which cell component/function is adapting to low oxygen?
- Briefly explain the cellular advantage.
Answer: The body increases production of red blood cells (RBCs) containing haemoglobin to carry more oxygen. At the cellular level, more RBCs and haemoglobin increase oxygen delivery per unit blood, improving oxygen-dependent cellular respiration in tissues under low-oxygen conditions.
Case 17 — Tissue Culture to Multiply Plants
A lab uses tissue culture to rapidly produce disease-free plantlets from a valuable mother plant.
Questions:
- Why does tissue culture work for producing many identical plants?
- Mention one cellular condition that must be controlled in culture.
Answer: Tissue culture exploits totipotency: many plant cells can de-differentiate and form whole plants under suitable conditions, producing clones of the mother plant. Conditions to control include sterile environment, nutrient medium composition and hormones (auxins/cytokinins) to direct cell division and differentiation.
Case 18 — Starch Test in Varied Light
Students test leaves kept in light and darkness for starch presence; light-exposed leaves turn blue-black with iodine.
Questions:
- Explain the result in terms of cellular processes.
- Why is the decolourisation step (ethanol) needed before iodine?
Answer: Light-exposed leaves perform photosynthesis, producing and storing starch; iodine stains starch blue-black. Dark-kept leaves lack photosynthesis and have little starch. Ethanol removes chlorophyll that would mask the iodine colour, allowing clear observation of starch staining.
Case 19 — Sudden Drop in Honeybee Population
A beekeeper notices a sudden decline in bees and suspects a nearby chemical spill.
Questions:
- How can chemicals affect insect cells and behaviour?
- What immediate actions should be taken to protect remaining bees?
Answer: Chemicals (pesticides) can damage nervous system cells, impair foraging behaviour or cause mortality. Immediate steps: remove bees from contaminated area if possible, provide clean water/food sources, report spill to authorities, and avoid further chemical exposure; long-term, advocate for safer pest management near apiaries.
Case 20 — Mutation Leading to Albino Plant
A mutation causes a plant to lack chlorophyll, producing albino offspring that cannot survive long.
Questions:
- Why do albino plants struggle to survive?
- How does this example illustrate the role of genes in cellular function?
Answer: Without chlorophyll, albino plants cannot perform photosynthesis and therefore cannot make their own food, leading to poor growth and survival. This shows that genes encode components (like chlorophyll-producing enzymes) essential for organelle function; mutations can disrupt cellular processes and organismal fitness.
These case-based questions and answers are aligned with NCERT Chapter 8 and designed to test application, reasoning and exam-ready responses for CBSE Class 8.