Long Answer Questions — Cells, Tissues and Division of Labour - (For CBSE Class 9 Science — Biology)
Medium Level (Application & Explanation)
Q1. Explain how the concept “all living organisms are made of cells” is supported by the examples of both unicellular and multicellular organisms.
Answer:
The idea that all living organisms are made of cells is shown clearly by examples from both unicellular and multicellular life.
In unicellular organisms like Amoeba, a single cell carries out every life process — movement, feeding, gaseous exchange, growth and excretion. This shows that a single cell can be a complete living unit.
In multicellular organisms such as humans and plants, many specialized cells work together. For example, muscle cells help movement, nerve cells carry messages, and vascular tissues in plants move water and food.
Thus, whether life is at the level of one cell or many cells, cells are the basic structural and functional unit of life.
This unity across diversity supports the cell theory and shows why studying cells is essential to understand all living things.
Q2. Describe how a single-celled organism like Amoeba performs the functions of movement, feeding, gaseous exchange and excretion.
Answer:
An Amoeba is a unicellular organism that performs all life functions within one cell.
For movement, it forms temporary projections called pseudopodia by reshaping its cytoplasm; this allows it to crawl along surfaces.
For feeding, Amoeba engulfs food particles through phagocytosis, surrounding food with its cell membrane to form a food vacuole where digestion occurs.
Gaseous exchange happens directly across the cell membrane by diffusion; oxygen enters and carbon dioxide leaves because of differences in concentration.
For excretion, waste materials are removed either by diffusion across the membrane or expelled through contractile vacuoles that collect and push wastes out.
These processes show how a single cell maintains life by using special structures and simple transport mechanisms inside it.
Q3. Explain the importance of division of labour in multicellular organisms and give examples from human body and plants.
Answer:
Division of labour means different cells or groups of cells perform specific tasks, which makes the whole organism more efficient.
In humans, muscle cells are specialized to contract and bring about movement, while nerve cells are specialized to transmit messages quickly across the body. Blood cells (red blood cells) transport oxygen, while white blood cells defend against infection. Each cell type performs its role efficiently because of its unique structure.
In plants, vascular tissues — xylem and phloem — show division of labour: xylem conducts water and minerals upward from roots, while phloem transports food (sugars) from leaves to other parts.
This specialization allows organisms to grow larger, maintain internal balance, respond to the environment, and survive better than single-celled organisms in complex habitats.
Q4. Define tissue and describe how animal tissues differ from plant tissues with suitable examples.
Answer:
A tissue is a group of similar cells that work together to perform a specific function.
In animals, tissues include muscle tissue (cells that contract to produce movement), nervous tissue (cells called neurons that carry messages), epithelial tissue (covers body surfaces), and blood (a fluid tissue that transports substances). Animal tissues are often flexible and involved in rapid response and movement.
In plants, tissues include meristematic tissue (for growth), permanent tissues like parenchyma (storage and photosynthesis), collenchyma (support), and vascular tissues — xylem and phloem (transport). Plant tissues are typically rigid because of cell walls and are specialized for support, transport and photosynthesis.
Thus, while both kingdoms have tissues formed by similar cells, their structure and functions reflect different life strategies.
Q5. How does blood qualify as a tissue? Explain its components and their functions.
Answer:
Blood is considered a tissue because it is made of different types of cells working together to perform common functions.
Its main components are:
Red blood cells (RBCs): contain a pigment hemoglobin and transport oxygen from lungs to body tissues and carry some carbon dioxide back.
White blood cells (WBCs): part of the immune system; they protect the body by fighting infections and foreign particles.
Platelets: small cell fragments that help in blood clotting, preventing bleeding.
Plasma: the liquid part that carries nutrients, hormones, wastes, and dissolved substances throughout the body.
Together, these components maintain transport, defense, and homeostasis, showing that blood is a specialized tissue with multiple coordinated functions.
High Complexity (Analytical & Scenario-Based)
Q6. A student observes that a plant’s leaves have started wilting while soil moisture is adequate. Analyze possible cellular and tissue-level reasons for wilting despite sufficient water in soil.
Answer:
Wilting despite adequate soil moisture suggests problems beyond water availability in soil; it points to issues in water uptake or transport at the cellular and tissue levels. Possible causes:
Blocked or damaged xylem: If xylem vessels are broken, clogged by air bubbles (embolism), or blocked by pathogens, water cannot move upward to leaves, causing wilting.
Root damage: Even with moist soil, if root cells are injured by pests, disease or physical damage they cannot absorb water effectively.
Poor root pressure or transpiration imbalance: If stomata are excessively open or cuticle is damaged, transpiration may exceed the plant’s capacity to replace water, leading to wilting.
Cellular loss of turgor: If leaf parenchyma cells lose water or have membrane damage causing leakage, cells collapse and leaves droop.
Phloem dysfunction: Though phloem transports food, its impairment can affect overall plant health and indirectly reduce water uptake.
Investigating xylem integrity, root health, stomatal behavior, and signs of disease will help determine the exact problem and remedy.
Q7. Design an experiment a student could perform to demonstrate that specialized cells are grouped together to form tissues, using easily available materials or microscopic observation.
Answer:
Objective: Show that similar cells form tissues and have a common function.
Peel a thin layer of onion epidermis and place it on a slide with one drop of water.
Add a drop of iodine or methylene blue to stain cell walls and nuclei for better visibility.
Cover and observe under the microscope at low and high magnification.
Observations and conclusion:
You will see many similar, regularly arranged cells forming a continuous layer — these are epidermal tissue cells.
Note common features (shape, size, cell wall) and collective function (covering and protection).
Draw labeled diagrams and state that grouping of similar cells with the same structure indicates formation of a tissue with a shared role.
This simple experiment demonstrates how cells of the same type form tissues that perform a specific function.
Q8. Compare and contrast the roles of phloem and xylem in plants and explain what would happen if phloem is damaged while xylem remains intact.
Answer:
Xylem and phloem are vascular tissues with complementary roles:
Xylem transports water and mineral salts from roots to stems and leaves; it provides structural support due to thick, lignified walls.
Phloem transports organic food (sugars made by photosynthesis) from leaves to other parts for growth and storage. Phloem cells are living and help distribute nutrients.
If phloem is damaged but xylem is intact:
Water and minerals will still reach the leaves, so leaves may not immediately wilt from lack of water.
However, sugars cannot be transported from leaves to roots and growing tissues. This results in starvation of roots and storage organs, poor growth, and eventual root death. Over time the plant cannot maintain respiration and synthesis of essential compounds, causing gradually declining health and possible death.
Symptoms include stunted growth, reduced root vigor, and accumulation of sugars in leaves leading to osmotic imbalances.
Thus, both tissues are essential and damage to phloem has severe long-term effects even if xylem supply continues.
Q9. Given that nerve cells transmit messages and muscle cells contract for movement, analyze how these two cell types must be structurally and functionally adapted to work together efficiently in a human limb.
Answer:
For coordinated movement in a limb, nerve cells (neurons) and muscle cells (fibers) must have complementary structural and functional adaptations:
Nerve cells have long axons and branching dendrites to send fast electrical signals (nerve impulses) over distances. Their endings form synapses at specific contact points on muscle cells. They release neurotransmitters to trigger muscle response.
Muscle cells are elongated and packed with contractile proteins (actin and myosin) organized into fibers that shorten efficiently. They have special receptors at the neuromuscular junction to receive chemical signals.
Functionally, a neuron’s impulse causes release of neurotransmitter, which binds to receptors on the musc...
