The Fundamental Unit of Life – Case-based Questions with Answers
Class 9 • Biology
Chapter 5: The Fundamental Unit of Life
Instructions: The 20 case-based questions below are grouped by topic within Chapter 5 (The Fundamental Unit of Life). Each case presents a short situation followed by one or more questions. Answers include concise explanations that follow NCERT concepts to aid understanding and revision.
Topic: Discovery of Cell & Microscopy
Case 1
A student observes a thin slice of onion epidermis under a light microscope. He notices many regular, brick-shaped units separated by lines.
Q1: Identify the structures the student is seeing and explain why they appear brick-shaped.
A1: The student is observing plant cells (onion epidermal cells). They appear brick-shaped because plant cells are often rectangular due to rigid cell walls that maintain a fixed shape and pack tightly in a tissue.
Explanation: Plant cells have cellulose-rich cell walls that give definite shapes and allow regular packing. In onion epidermis the cells are large and regular, making them easy to see under a light microscope.
Case 2
A biology teacher demonstrates the use of a compound light microscope. She asks students why they cannot see very small details such as ribosomes using this microscope.
Q1: Why are ribosomes not visible under a compound light microscope?
A1: Ribosomes are too small (about 20–30 nm) and below the resolving power of a light microscope (≈200 nm), so they cannot be seen with an ordinary light microscope.
Explanation: Resolution limit of light is set by wavelength of visible light; electron microscopes are required to view ribosomes and other ultrastructures.
Topic: Cell Theory & Types of Cells
Case 3
A lab report states that new cells form only by division of pre-existing cells. A student asks how this idea fits with older theories that organisms spontaneously arose from non-living matter.
Q1: Which cell theory principle does this statement support, and who is credited with establishing the idea?
A1: This supports the principle "Omnis cellula e cellula" — every cell arises from a pre-existing cell. Rudolf Virchow is credited with popularizing this idea.
Explanation: Modern cell theory includes that cells are the basic structural and functional unit and arise from existing cells by division.
Case 4
Two bacteria under a microscope look simple compared to an animal cell. One student says bacteria have no nucleus and are called prokaryotes.
Q1: Define prokaryotic and eukaryotic cells and give two differences.
A1: Prokaryotic cells lack a true nucleus and membrane-bound organelles (example: bacteria). Eukaryotic cells have a well-defined nucleus and membrane-bound organelles (example: plant and animal cells). Key differences: (1) Nucleus — absent in prokaryotes, present in eukaryotes. (2) Size — prokaryotic cells are generally smaller than eukaryotic cells.
Explanation: Prokaryotes have DNA in the nucleoid region; eukaryotes enclose DNA within a nuclear membrane and have organelles like mitochondria, ER.
Topic: Cell Structure — Cell Wall, Cell Membrane, Nucleus
Case 5
A gardener notices that cut stems of two plants differ: one wilts quickly while the other stays turgid for longer after cutting. He wonders if cell wall composition could matter.
Q1: How does the presence of a cell wall help plant cells retain structure compared to animal cells?
A1: The rigid cell wall (made mainly of cellulose) provides mechanical strength and prevents excessive water uptake, helping plant cells maintain turgor and structural integrity even when external conditions change. Animal cells lack a cell wall and depend on extracellular matrix and cytoskeleton for shape.
Explanation: Cell wall prevents bursting in hypotonic conditions and gives plants upright support; cutting disrupts xylem but wall remains, slowing collapse in some species.
Case 6
In a laboratory experiment, a student's cheek cell preparation shows a prominent nucleus while the plant cell slide shows an even larger central vacuole and chloroplasts.
Q1: Why is the nucleus important and what are two functions it carries out?
A1: The nucleus contains genetic material (DNA) and controls cell activities. Two functions: (1) It stores hereditary information (genes) and (2) coordinates processes like transcription and cell division by regulating gene expression.
Explanation: The nucleus is bound by nuclear membrane and contains nucleolus where ribosomal RNA is made.
Topic: Cell Organelles — Structure & Function
Case 7
A plant is kept in darkness for several days and its leaves become pale. A student suggests chloroplasts are affected.
Q1: Explain how chloroplasts are involved in the leaf's green colour and why leaves turn pale in darkness.
A1: Chloroplasts contain chlorophyll, the green pigment that captures light for photosynthesis. In darkness, chlorophyll production decreases and chloroplast activity reduces, causing leaves to become pale (chlorosis).
Explanation: Chlorophyll synthesis requires light; prolonged dark conditions reduce pigment levels and photosynthetic activity.
Case 8
A muscle cell requires a lot of energy for contraction. Under study, these cells show many mitochondria.
Q1: Why do muscle cells have numerous mitochondria? State the main function of mitochondria.
A1: Mitochondria generate ATP via cellular respiration, supplying energy required for muscle contraction. Cells with high energy demands (muscle, liver) thus contain many mitochondria.
Explanation: Mitochondria are the ‘powerhouses’ where glucose/food molecules are oxidised to produce ATP used in energy-consuming processes.
Case 9
A cell biologist finds many ribosomes free in the cytoplasm of a liver cell sample and some attached to membranes.
Q1: Distinguish between proteins synthesised by free ribosomes and those made by membrane-bound ribosomes.
A1: Free ribosomes synthesise proteins used within the cytoplasm (eg. enzymes for metabolism). Membrane-bound ribosomes (on rough ER) synthesise proteins destined for secretion, insertion into membranes, or for organelles like lysosomes.
Explanation: The location of ribosomes dictates the initial targeting of the polypeptide (signal peptide directs to ER).
Topic: Cell Membrane & Transport
Case 10
A freshwater fish placed in seawater initially loses water from its body cells and becomes dehydrated. A student links this to osmosis and cell membranes.
Q1: Describe osmosis and explain why the fish's cells lose water when moved to seawater.
A1: Osmosis is the movement of water across a semipermeable membrane from higher water potential (dilute) to lower water potential (concentrated). Sea water has higher solute concentration (lower water potential) than the fish's body fluids, so water moves out of the fish's cells into the seawater, causing dehydration.
Explanation: Cell membrane is selectively permeable; water moves to balance solute concentrations leading to cell shrinkage in hypertonic external solution.
Case 11
A plant placed in a 10% salt solution wilts rapidly. After placing in freshwater, it regains turgidity slowly.
Q1: Explain the changes in guard cells and plant cells during these transfers using the concept of turgor pressure.
A1: In 10% salt solution (hypertonic), plant cells lose water by osmosis, decreasing turgor pressure and causing wilting. Guard cells lose turgor and stomata close. In freshwater (hypotonic relative to cell sap), water enters cells, turgor pressure increases, cells become firm and stomata can open, restoring normal appearance.
Explanation: Turgor pressure against the cell wall keeps plants upright; loss of water reduces pressure and causes limpness.
Topic: Vacuoles, Lysosomes, Golgi Apparatus & Endoplasmic Reticulum
Case 12
An experiment shows that animal cells have many small vacuoles while plant cells have a single large central vacuole.
Q1: What roles do vacuoles play in plant cells?
A1: Vacuoles store water, ions, pigments and waste; maintain turgor pressure; help in cell growth by enlarging; and can store defensive compounds. The large central vacuole is key to plant cell rigidity.
Explanation: Central vacuole is enclosed by tonoplast and contributes to cell volume and osmotic balance.
Case 13
A cell secretes digestive enzymes packaged in vesicles. A student suggests the Golgi complex is involved.
Q1: Describe the role of the Golgi apparatus in processing and packaging proteins.
A1: The Golgi apparatus modifies proteins (eg. glycosylation), sorts them, and packages them into vesicles for secretion or transport to other organelles. It acts as the cell's packaging and dispatch center.
Explanation: Proteins synthesized in rough ER are sent to Golgi for final modifications and sorting into secretory vesicles or lysosomes.
Topic: Cell Division — Basic Concept
Case 14
A child cuts herself; the wound heals as new cells form to replace damaged ones.
Q1: Which cellular process is primarily responsible for this repair and what is its significance?
A1: Cell division (mitosis) is responsible for replacing damaged or dead cells. It ensures growth, repair, and maintenance by producing genetically identical daughter cells.
Explanation: Mitosis maintains chromosome number and produces cells for tissue repair, while meiosis is for gamete formation.
Case 15
A rapidly growing tadpole's tissues show frequent cell divisions in contrast to some adult cells which divide rarely.
Q1: Why do some cells divide frequently while others do not? Give two examples of cells with low division rates.
A1: Division rate depends on the cell's role: tissues requiring growth or repair (skin, intestinal lining) have high division rates; highly specialised cells with limited regenerative ability divide rarely—examples: mature nerve cells (neurons) and cardiac muscle cells.
Explanation: Differentiation and cell cycle regulation determine whether a cell remains active in the cycle or exits into G0 phase.
Topic: Plant vs Animal Cells
Case 16
A microscope image shows a cell with chloroplasts and a large vacuole beside another cell lacking these but containing many lysosomes.
Q1: Identify which is plant and which is animal cell and justify your answer with two reasons.
A1: The cell with chloroplasts and a large central vacuole is a plant cell. The cell with many lysosomes and no chloroplasts is an animal cell. Reasons: presence of chloroplasts (photosynthesis) and large vacuole (turgor) indicate plant cell; lysosomes and absence of cell wall suggest animal cell.
Explanation: Cell wall would also indicate plant cell; animal cells are more irregular in shape.
Case 17
A biology practical asks students to label parts of a cell and explain why plant cells are usually larger than animal cells.
Q1: Provide two reasons why plant cells are often larger than animal cells.
A1: (1) Large central vacuole occupies most of plant cell volume aiding storage and turgor-driven expansion. (2) Rigid cell wall allows cells to expand without bursting, facilitating larger cell sizes for support and storage.
Explanation: Plant cells also often contain larger plastids and storage structures contributing to larger size.
Topic: Specialised Cells & Tissues
Case 18
A root hair cell shows long protrusions increasing surface area while a red blood cell is biconcave and lacks nucleus.
Q1: Explain how structure suits function for (a) root hair cells and (b) red blood cells.
A1: (a) Root hair cells have long extensions (root hairs) to increase surface area for water and mineral absorption. (b) Red blood cells are biconcave to increase surface area for gas exchange and lack nucleus to maximise haemoglobin space and flexibility in capillaries.
Explanation: Specialisation tailors organelle content and shape to function—eg. root hair cells have many mitochondria for active uptake; RBCs lose nucleus during maturation.
Case 19
A muscle fibre is multi-nucleated whereas many epithelial cells have a single nucleus.
Q1: Why might a muscle fibre be multinucleated?
A1: Muscle fibres form by fusion of many myoblasts, resulting in long cells with multiple nuclei to support high metabolic and protein synthesis demands along the length of the fibre.
Explanation: Multiple nuclei help coordinate gene expression and protein production throughout the large cytoplasmic volume of muscle cells.
Topic: Practical Skills & Investigative Questions
Case 20
During a practical exam, a student prepares a temporary mount of a leaf and observes stomata. The examiner asks how stomatal opening is related to guard cell turgor.
Q1: Describe how guard cells control stomatal opening and name two factors that influence guard cell turgor.
A1: Guard cells swell (gain turgor) when they accumulate potassium ions and water enters by osmosis; this causes them to become turgid and the stomatal pore opens. When they lose K+ and water, they become flaccid and stomata close. Two influencing factors: light (promotes opening) and abscisic acid or water stress (promotes closing).
Explanation: Guard cell walls are unevenly thickened causing them to bow outwards on turgidity, creating an opening between them.