NCERT Textbook Questions & Answers + Important Extra Questions
Section A: In-Text Questions
Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms?
Answer
In multicellular organisms, not all cells are in direct contact with the external environment. The body surface is covered with dead cells that prevent direct oxygen exchange. Additionally, diffusion is an extremely slow process over large distances — oxygen would take an impractically long time to reach deeper tissues just by diffusing from cell to cell. Therefore, multicellular organisms need specialised respiratory and circulatory systems to efficiently transport oxygen to every cell.
What criteria do we use to decide whether something is alive?
Answer
The most visible sign of life is movement — this includes locomotion, growth, breathing, and internal molecular movements. However, the fundamental criterion is the presence of life processes such as nutrition, respiration, transportation of materials, excretion, reproduction, and response to stimuli. Even when organisms appear to be stationary (like plants or sleeping animals), these internal processes continue, maintaining life.
What are the differences between autotrophic nutrition and heterotrophic nutrition?
Answer
Autotrophic nutrition
• Organisms prepare their own food from simple inorganic substances (CO₂ and H₂O) using energy from sunlight or chemicals.
• No digestion is required since food is synthesised directly.
• Examples: Green plants, certain bacteria, algae.
Heterotrophic nutrition
• Organisms cannot make their own food and depend on other organisms (directly or indirectly) for nutrition.
• Complex food must be digested (broken down) into simpler forms before absorption.
• Examples: All animals, fungi, most bacteria.
Where do plants get each of the raw materials required for photosynthesis?
Answer
• Carbon dioxide: Absorbed from the atmosphere through tiny pores called stomata on leaf surfaces.
• Water: Absorbed from the soil by roots and transported upward through xylem vessels to the leaves.
• Sunlight: Captured by the green pigment chlorophyll present in chloroplasts of leaf cells.
• Minerals: Absorbed from the soil along with water by the roots.
What is the role of the acid in our stomach?
Answer
The stomach secretes hydrochloric acid (HCl) which performs several crucial functions:
(i) Creates an acidic medium (pH 1.5–3.5) necessary for the enzyme pepsin to work optimally.
(ii) Activates pepsinogen (inactive enzyme) into pepsin (active enzyme) which digests proteins.
(iii) Kills harmful bacteria and other pathogens that enter with food.
(iv) Softens food and helps break it down mechanically.
(v) Stops salivary amylase action (which works only in alkaline/neutral pH).
What is the function of digestive enzymes?
Answer
Digestive enzymes are biological catalysts that break down complex, insoluble food molecules into simple, soluble, and absorbable forms. Each enzyme acts on a specific type of nutrient:
• Amylase breaks starch
⇒ maltose (sugars)
• Pepsin/Trypsin break proteins
⇒ amino acids
• Lipase breaks fats
⇒ fatty acids + glycerol
Without these enzymes, the body cannot extract nutrients from the food we eat.
How is the small intestine designed to absorb digested food?
Answer
The inner lining of the small intestine has millions of tiny finger-like projections called villi (singular: villus). These greatly increase the surface area for absorption. Each villus has:
• A thin wall (single cell layer) for easy diffusion of nutrients
• A dense network of blood capillaries to carry absorbed glucose and amino acids to the bloodstream
• Lymph vessels (lacteals) to absorb fatty acids and glycerol
The villi and microvilli together provide approximately 250 m² of absorptive surface.
What advantage over an aquatic organism does a terrestrial organism have with regard to obtaining oxygen for respiration?
Answer
Terrestrial organisms have a significant advantage because air contains about 21% oxygen, while water contains less than 1% dissolved oxygen. This means land-dwelling organisms have access to a much richer oxygen supply. Aquatic organisms must process large volumes of water through their gills to extract sufficient oxygen, expending more energy in the process. Terrestrial organisms can simply breathe in air to get abundant oxygen.
What are the different ways in which glucose is oxidised to provide energy in various organisms?
Answer
Glucose oxidation occurs through different pathways:
Step 1 (common to all): Glucose (6C) is broken into pyruvate (3C) in the cytoplasm — this step is called glycolysis and releases a small amount of energy.
After this, three different pathways exist:
(i) Aerobic respiration (in presence of O₂): Pyruvate is completely broken down in mitochondria into CO₂ + H₂O + 38 ATP (large energy yield). Occurs in most organisms.
(ii) Anaerobic respiration (in yeast): Pyruvate is converted to ethanol + CO₂ + 2 ATP. This is called fermentation.
(iii) Anaerobic respiration (in muscles): During vigorous exercise when O₂ supply is insufficient, pyruvate is converted to lactic acid + 2 ATP. This causes muscle cramps.
How is oxygen and carbon dioxide transported in human beings?
Answer
Oxygen transport: About 97% of oxygen binds with haemoglobin (a protein in red blood cells) to form oxyhaemoglobin. The remaining 3% dissolves directly in blood plasma. At the tissues, oxyhaemoglobin releases oxygen due to lower O₂ concentration.
Carbon dioxide transport: CO₂ is transported in three ways:
• About 70% as bicarbonate ions (HCO₃⁻) dissolved in plasma
• About 23% bound to haemoglobin as carbaminohaemoglobin
• About 7% dissolved directly in plasma
At the lungs, CO₂ is released and exhaled out.
How are the lungs designed in human beings to maximise the area for exchange of gases?
Answer
The respiratory passage branches extensively within the lungs. The trachea divides into two bronchi, which further divide into smaller bronchioles. Each bronchiole ends in a cluster of tiny, balloon-like air sacs called alveoli. There are approximately 300 million alveoli in both lungs, providing a total respiratory surface area of about 80 m². Each alveolus has extremely thin walls (one cell thick) surrounded by a network of blood capillaries, allowing rapid and efficient exchange of O₂ and CO₂ by diffusion.
What are the components of the transport system in human beings? What are the functions of these components?
Answer
The human transport system consists of:
(i) Heart: A muscular pump with four chambers (2 atria + 2 ventricles) that pumps blood throughout the body. The right side handles deoxygenated blood; the left side handles oxygenated blood.
(ii) Blood vessels:
• Arteries — carry oxygenated blood away from the heart (thick, elastic walls)
• Veins — carry deoxygenated blood back to the heart (thinner walls, have valves)
• Capillaries — tiny vessels where exchange of materials occurs between blood and tissues
(iii) Blood: Contains plasma (liquid part carrying dissolved substances), red blood cells (carry O₂), white blood cells (fight infections), and platelets (help in clotting).
Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds?
Answer
Mammals and birds are warm-blooded (homeothermic) animals that maintain a constant body temperature. This requires a high metabolic rate, which in turn demands a continuous and efficient supply of oxygen to all body cells. If oxygenated and deoxygenated blood were to mix (as in amphibians), the oxygen concentration in the blood reaching the tissues would be reduced, lowering energy production. The four-chambered heart in mammals and birds ensures complete separation, delivering fully oxygenated blood to body tissues for maximum energy output.
What are the components of the transport system in highly organised plants?
Answer
Plants have two types of conducting (vascular) tissues:
(i) Xylem: Transports water and dissolved minerals from roots to all parts of the plant. It consists of tracheids and vessels — both are dead, hollow tubes that form continuous pipelines.
(ii) Phloem: Transports prepared food (sucrose and other organic molecules) from the leaves (source) to other parts of the plant (sink — roots, growing tips, fruits, seeds). It consists of sieve tubes and companion cells — these are living cells.
How are water and minerals transported in plants?
Answer
Water and minerals are absorbed by the root hairs from the soil through osmosis and active transport respectively. They enter the xylem vessels in the root and travel upward through the stem to the leaves. The main driving force is transpiration pull — as water evaporates from leaf surfaces through stomata (transpiration), it creates a suction force that pulls water upward through the xylem. Additionally, root pressure pushes water upward from below. The cohesion between water molecules and adhesion to xylem walls help maintain a continuous column of water.
How is food transported in plants?
Answer
Food (mainly sucrose) is transported through phloem from the site of synthesis (mainly leaves) to storage organs and growing parts. This process is called translocation. Unlike xylem transport, phloem transport requires metabolic energy (ATP). Sucrose is loaded into phloem sieve tubes using energy, which increases the osmotic pressure. Water enters the phloem by osmosis, creating pressure that pushes the sap in both directions — upward and downward — to wherever it is needed.
Describe the structure and functioning of nephrons.
Answer
The nephron is the structural and functional unit of the kidney. Each kidney contains about one million nephrons. A nephron consists of:
(i) Bowman's capsule: A cup-shaped structure that encloses a network of blood capillaries called the glomerulus. Blood enters under high pressure, and water, glucose, salts, urea, and other small molecules are filtered out — this is ultrafiltration. Large molecules and blood cells remain in the blood.
(ii) Proximal convoluted tubule (PCT): Most of the useful substances (glucose, amino acids, salts, water) are reabsorbed back into the blood here.
(iii) Loop of Henle: Helps in concentrating the urine by reabsorbing more water.
(iv) Distal convoluted tubule (DCT): Some substances are actively secreted from blood into the tubule. Fine-tunes the composition of urine.
(v) Collecting duct: Collects urine from multiple nephrons and carries it to the renal pelvis, then to the ureter, and finally to the urinary bladder.
What are the methods used by plants to get rid of excretory products?
Answer
Plants use several methods to dispose of their waste:
(i) Gaseous exchange through stomata: CO₂ (from respiration) and O₂ (from photosynthesis) are released through stomatal pores.
(ii) Transpiration: Excess water is lost as vapour through stomata — this also helps in cooling and maintaining water flow.
(iii) Falling of leaves: Waste products are stored in leaves; shedding leaves removes these wastes.
(iv) Storage in vacuoles and bark: Some wastes (resins, gums, tannins) are stored in old xylem, bark, or cellular vacuoles where they do not interfere with metabolism.
(v) Excretion into soil: Some waste products are released into the surrounding soil through roots.
How is the amount of urine produced regulated?
Answer
Urine production is regulated by antidiuretic hormone (ADH), also called vasopressin, secreted by the pituitary gland:
• When the body has excess water, ADH secretion decreases. Less water is reabsorbed from the kidney tubules, resulting in large volumes of dilute urine.
• When the body is dehydrated, ADH secretion increases. More water is reabsorbed from the tubules back into the blood, producing small volumes of concentrated urine.
Other factors include blood pressure, blood volume, and the concentration of dissolved substances in the blood.
Section B: NCERT Exercise Questions
Q1. The kidneys in human beings are a part of the system for
(a) nutrition (b) respiration (c) excretion (d) transportation
Ans. (c) Excretion. Kidneys filter waste products (urea, uric acid, excess salts, water) from the blood and excrete them as urine.
Q2. The xylem in plants is responsible for
(a) transport of water (b) transport of food (c) transport of amino acids (d) transport of oxygen
Ans. (a) Transport of water (and dissolved minerals from roots to the rest of the plant).
Q3. The autotrophic mode of nutrition requires
(a) carbon dioxide and water (b) chlorophyll (c) sunlight (d) all of the above
Ans. (d) All of the above. Photosynthesis requires CO₂, water, chlorophyll, and sunlight to produce glucose and oxygen.
The breakdown of pyruvate to give carbon dioxide, water and energy takes place in:
Answer
(a) cytoplasm (b) mitochondria (c) chloroplast (d) nucleus
Ans. (b) Mitochondria. The aerobic breakdown of pyruvate (produced during glycolysis in the cytoplasm) occurs in the mitochondria, releasing CO₂, water, and a large amount of energy (ATP).
How are fats digested in our bodies? Where does this process take place?
Answer
Fat digestion occurs mainly in the small intestine in two steps:
(i) Emulsification: Bile salts (produced by the liver, stored in the gall bladder) break down large fat globules into tiny droplets — this increases the surface area for enzyme action.
(ii) Enzymatic digestion: Pancreatic lipase and intestinal lipase act on these small fat droplets and convert them into fatty acids and glycerol, which are then absorbed by the intestinal villi.
What is the role of saliva in the digestion of food?
Answer
Saliva plays multiple roles:
(i) It contains the enzyme salivary amylase (ptyalin) which begins the digestion of starch, converting it into maltose (a sugar) in the mouth.
(ii) It moistens and softens the food, making it easier to chew and swallow.
(iii) It helps form a smooth, rounded mass called a bolus that can slide easily down the oesophagus.
(iv) It contains lysozyme, an antibacterial agent that helps kill germs in food.
What are the necessary conditions for autotrophic nutrition and what are its by-products?
Answer
Necessary conditions: Sunlight (energy source), chlorophyll (pigment to capture light), carbon dioxide (from air), and water (from soil).
6CO₂ + 6H₂O
⇒ chlorophyll, sunlight C₆H₁₂O₆ + 6O₂
The main product is glucose. The by-product is oxygen gas, which is released into the atmosphere through stomata.
What are the differences between aerobic and anaerobic respiration? Name some organisms that use the anaerobic mode of respiration.
Answer
Aerobic respiration: Occurs in presence of oxygen; complete breakdown of glucose into CO₂ + H₂O; takes place in mitochondria; produces 38 ATP (large energy yield).
Anaerobic respiration: Occurs in absence of oxygen; incomplete breakdown of glucose into ethanol + CO₂ (in yeast) or lactic acid (in muscles); takes place in cytoplasm; produces only 2 ATP (low energy yield).
Organisms using anaerobic respiration: Yeast (for fermentation), certain bacteria, and parasitic worms like tapeworms and roundworms.
How are the alveoli designed to maximise the exchange of gases?
Answer
Alveoli are designed for maximum gas exchange through:
• Enormous number: About 300 million alveoli provide ~80 m² surface area
• Extremely thin walls: Just one cell thick, minimising diffusion distance
• Rich blood supply: Dense network of capillaries surrounds each alveolus
• Moist lining: Gases dissolve in the moisture layer before diffusing across
• Balloon-like shape: Maximises surface area to volume ratio
What would be the consequences of a deficiency of haemoglobin in our bodies?
Answer
Haemoglobin is the oxygen-carrying protein in red blood cells. Its deficiency leads to a condition called anaemia, resulting in:
• Reduced oxygen-carrying capacity of blood
⇒ cells receive less oxygen
• Fatigue, weakness, and breathlessness even during mild physical activity
• Pale skin and lips
• Dizziness and headaches
• Reduced energy production → poor exercise tolerance
• In severe cases, organ damage due to chronic oxygen deprivation
Describe double circulation in human beings. Why is it necessary?
Answer
In humans, blood passes through the heart twice in one complete cycle — this is called double circulation:
(i) Pulmonary circulation: Deoxygenated blood flows from the right ventricle
⇒ pulmonary artery
⇒ lungs (where it picks up O₂ and releases CO₂)
⇒ pulmonary veins
⇒ left atrium.
(ii) Systemic circulation: Oxygenated blood flows from the left ventricle
⇒ aorta
⇒ body tissues (where it delivers O₂ and picks up CO₂)
⇒ vena cava
⇒ right atrium.
Necessity: Double circulation ensures that oxygenated blood does not mix with deoxygenated blood, maintaining maximum oxygen delivery to body tissues. This is essential for warm-blooded animals like humans who need high energy levels to maintain constant body temperature.
What are the differences between the transport of materials in xylem and phloem?
Answer
Xylem transport
• Transports water and dissolved minerals
• Direction: unidirectional (roots to leaves — upward only)
• Driven by transpiration pull (physical process, no energy needed)
• Uses dead cells (tracheids and vessels)
Phloem transport
• Transports food (sucrose) and organic molecules
• Direction: bidirectional (leaves to roots or to any other part)
• Driven by osmotic pressure (requires metabolic energy/ATP)
• Uses living cells (sieve tubes and companion cells)
Compare the functioning of alveoli in the lungs and nephrons in the kidneys with respect to their structure and functioning.
Answer
Alveoli: Tiny, balloon-shaped air sacs in the lungs. Their thin walls and large surface area facilitate gas exchange (O₂ in, CO₂ out) by diffusion between air and blood capillaries.
Nephrons: Long, tube-like structures in the kidneys. They filter blood through the glomerulus (ultrafiltration) and selectively reabsorb useful substances while excreting waste as urine.
Similarities: Both have large surface areas, thin walls for efficient exchange, rich blood capillary networks, and work as the basic functional units of their respective organs.
Difference: Alveoli exchange gases; nephrons filter and process liquid waste.