Key Concept: Invalid: Out of Syllabus

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
This question is invalid as it lacks syllabus context.

Key Concept: Digestion in Humans

c) It starts tasting sweet as starch converts to sugar
[Solution Description]
Saliva contains an enzyme that breaks down starch into simpler sugars. When starchy food like chapati or rice is chewed for a long time, the saliva mixes thoroughly with it and converts starch into sugar. This is why such food starts tasting sweet after prolonged chewing.

Key Concept: Nutrition in Animals, Digestion in Human Beings

d) Initial starch breakdown is reduced, affecting digestion efficiency
[Solution Description]
- Saliva helps break down starch into sugar via its enzyme amylase.
- Without enough saliva, the initial digestion of starch in the mouth is impaired, leading to slower overall digestion.

Key Concept: Respiration

b) To convert nutrients into usable energy
[Solution Description]
Respiration converts nutrients like glucose into usable energy (ATP). Oxygen is used to break down glucose, releasing carbon dioxide, water, and energy. This process sustains essential life functions.

Key Concept: Nutrition

c) Obtaining and utilizing food for energy, growth, and repair
[Solution Description]
Nutrition is the process by which organisms obtain and utilize food for energy, growth, and repair. It involves ingestion, digestion, absorption, assimilation, and egestion.

Key Concept: Respiration

d) Carbon dioxide and water
[Solution Description]
Aerobic respiration breaks down glucose into carbon dioxide and water, releasing energy. The chemical equation is:
$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy$. So, the products are carbon dioxide and water.

Key Concept: Respiration, Science and Society

c) Alveolar membranes thickened, impairing gas exchange.
[Solution Description]
Severe COVID-19 infections caused inflammation and fluid accumulation in the alveoli, reducing their ability to exchange oxygen and carbon dioxide efficiently. Ventilators were used to assist breathing by maintaining air pressure and ensuring sufficient oxygen reached the bloodstream despite damaged alveolar function.

Key Concept: Ruminants

c) To chew it thoroughly after partial digestion
[Solution Description]
Ruminants have a specialized digestive process called rumination. They partially chew grass, swallow it into their stomach for initial digestion, then bring it back to the mouth (regurgitation) to chew it thoroughly (cud-chewing). This ensures efficient breakdown of tough plant material like cellulose, which other options do not explain.

Key Concept: Respiration

c) Exchange of gases
[Solution Description]
Alveoli are tiny air sacs in the lungs where the exchange of oxygen and carbon dioxide occurs between the blood and inhaled air.

Key Concept: Digestion in ruminants, Mechanical digestion

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that ruminants like cows have a four-chambered stomach for digesting cellulose-rich food, which is true. Their stomach includes chambers like the rumen, reticulum, omasum, and abomasum. The reason explains that the rumen contains bacteria aiding cellulose breakdown, which is also correct and directly explains why ruminants can digest cellulose. Thus, both statements are true, and the reason correctly explains the assertion.

Key Concept: Comparative digestion, Avian adaptation

c) The grit acts as substitute teeth to mechanically grind food in the gizzard.
[Solution Description]
Birds lack teeth, so their muscular gizzard uses grit (small stones) to grind food mechanically. This compensates for the absence of mastication, whereas humans rely on teeth for mechanical digestion.

Key Concept: Digestion in ruminants, Mechanical digestion

b) Prolonged chewing increases saliva production to prevent dehydration.
[Solution Description]
The cow is a ruminant, and it regurgitates partially digested food (cud) from the rumen back to the mouth for thorough chewing. This helps break down tough plant cellulose with the help of symbiotic bacteria, which humans cannot digest directly due to lack of such adaptations.

Key Concept: Digestion in Stomach, Role of Bile

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that stomach acid aids protein digestion but does not break down fats, which is correct as the stomach primarily digests proteins via hydrochloric acid and pepsin. Fats are digested later in the small intestine with bile's help. The Reason correctly explains that bile emulsifies fats, facilitating enzyme action. Both statements are true, and the Reason logically explains the Assertion.

Key Concept: Peristalsis and Digestive Process

c) It moves food through the digestive tract via muscular contractions.
[Solution Description]
Peristalsis refers to the wave-like muscle contractions in the walls of the oesophagus and intestines. These contractions help push food forward through the digestive tract without relying on gravity, ensuring smooth movement from the mouth to the stomach and beyond.

Key Concept: Peristalsis, Digestive Tract Coordination

c) Impaired movement of food to the stomach
[Solution Description]
Peristalsis is the wave-like muscular contraction that propels food through the esophagus to the stomach. If these contractions weaken, food movement slows down, potentially causing difficulty swallowing (dysphagia) or regurgitation. Mechanical digestion continues in the stomach, but delayed food entry disrupts timing for chemical digestion.
Step 1: Peristaltic waves push food downward via coordinated muscle contractions.
Step 2: Weakened contractions delay food transit, leading to incomplete digestion progression.
Step 3: This may cause discomfort or aspiration if food remains lodged in the esophagus.

Key Concept: Function of Bile

b) Neutralizing acids and emulsifying fats
[Solution Description]
The liver secretes bile, which is stored in the gallbladder and released into the small intestine. Bile emulsifies fats, breaking them into tiny droplets, and neutralizes the acidic chyme from the stomach due to its mildly basic nature.

Key Concept: Small Intestine, Villi Function

d) To efficiently absorb digested nutrients into the bloodstream.
[Solution Description]
Villi are specialized for efficient nutrient absorption. Their structure ensures:
Step 1: Large surface area (due to finger-like projections) maximizes contact with digested food.
Step 2: Thin epithelium allows quick diffusion of nutrients into blood capillaries.
Step 3: Rich blood supply transports absorbed sugars and amino acids to the liver via the hepatic portal vein.
Step 4: Lymph vessels (lacteals) in villi absorb fatty acids and glycerol.

Key Concept: Mechanical and Chemical Digestion, Role of Saliva

c) Partial chemical digestion of starch occurred due to enzymatic action of saliva.
[Solution Description]
The action of saliva breaks down starch into simpler sugars during chewing. Iodine turns blue-black in the presence of starch. If the color is lighter after chewing, it means some starch has been digested by salivary enzymes.
Step 1: Boiled rice contains starch which gives a dark blue-black color with iodine.
Step 2: Chewing mixes rice with saliva, which contains amylase enzyme.
Step 3: Amylase breaks down starch into maltose (a sugar), reducing the amount of starch.
Step 4: Less starch means a lighter blue-black color with iodine solution.

Key Concept: Action of Saliva on Starch

c) Starch
[Solution Description]
Saliva contains an enzyme that breaks down starch (a carbohydrate) into sugar. For example, chewing chapati or boiled rice results in a sweet taste due to this conversion.

Key Concept: Mechanical Digestion

c) To break down food into smaller pieces
[Solution Description]
The teeth break down food into smaller pieces through chewing, which is called mechanical digestion. This makes it easier to swallow and digest food.

Key Concept: Mechanical Digestion and Saliva Action

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that chewing breaks food into smaller pieces, which is true as mechanical digestion involves physical breakdown of food. The Reason states saliva contains enzymes that convert starch into sugar, which is also true as salivary amylase breaks down starch into simpler sugars. However, the Reason does not correctly explain the Assertion because the enzymatic action does not depend on the food being mechanically broken down; enzymes act even if the food is not finely chewed. Thus, both statements are true, but the Reason is not the correct explanation for the Assertion.

Key Concept: Oesophagus Function, Peristalsis Mechanism

c) Swallowing would become extremely difficult
[Solution Description]
The oesophagus must remain flexible to allow peristaltic contractions and relaxations. If stiff, it cannot generate the wave-like motions needed to push food downward, leading to swallowing difficulties and potential choking hazards.

Key Concept: Peristalsis, Digestion Process

c) It ensures food moves sequentially for efficient digestion
[Solution Description]
Peristalsis ensures continuous movement of food along the entire alimentary canal, preventing blockages and allowing sequential digestion in different organs. Without it, food would stagnate, leading to inefficient digestion and absorption.

Key Concept: Role of Saliva

c) Moistens the food for easy swallowing
[Solution Description]
Saliva moistens the chewed food, making it soft and easier to swallow. This helps in smooth movement of the food bolus through the oesophagus by peristalsis.

Key Concept: Acid function, Bacterial action, Protein breakdown

c) Reduced protein digestion and increased bacterial survival
[Solution Description]
Stomach acid has two main functions: activating pepsinogen to pepsin for protein digestion and killing harmful bacteria. If the pH increases (less acidic), both these functions would be impaired. Proteins wouldn't digest as efficiently, and more bacteria could survive passage through the stomach.

Key Concept: Digestive Juice Function

b) It breaks down proteins
[Solution Description]
The digestive juice in stomach contains enzymes that break down proteins into simpler substances. This is its primary function as per the syllabus.

Key Concept: Stomach Acid Purpose

b) To help breakdown proteins and kill bacteria
[Solution Description]
The stomach acid serves two main purposes: helps in protein breakdown and kills harmful bacteria. Both functions are mentioned in the syllabus.

Key Concept: Disorder Related to Small Intestine

d) Avoid gluten-containing foods
[Solution Description]
Celiac disease is managed by avoiding gluten-containing foods like wheat, barley, and rye. Millets such as jowar, bajra, and ragi are safe alternatives as they are naturally gluten-free.

Key Concept: Small Intestine, Digestive Secretions, Celiac Disease

c) The absorption decreases due to damage to the intestinal lining.
[Solution Description]
Celiac disease damages the inner lining of the small intestine, specifically the finger-like projections called villi. These villi are essential for increasing the surface area for nutrient absorption. When damaged, the intestine cannot efficiently absorb nutrients, leading to malabsorption. The pancreas and liver continue to secrete their respective juices, but the structural damage in the small intestine hinders nutrient uptake.

Key Concept: Function of Bile

c) Neutralises acids and breaks down fats
[Solution Description]
Bile neutralises acids from the stomach and breaks down fats into tiny droplets for easier digestion, as mentioned in the syllabus.

Key Concept: Small Intestine Absorption

c) By increasing surface area
[Solution Description]
Finger-like projections (villi) increase the surface area of the small intestine, allowing more efficient absorption of digested nutrients into the bloodstream. The correct answer is by increasing surface area.

Key Concept: Small Intestine Absorption

c) To increase surface area for absorption
[Solution Description]
The inner lining of the small intestine has thousands of finger-like projections to increase the surface area for efficient nutrient absorption. From the syllabus, "The inner lining of the small intestine is thin and has thousands of finger-like projections... that increase the surface area for efficient nutrient absorption."

Key Concept: Absorption efficiency, Villi function

c) The absorption of nutrients significantly decreases because villi are essential for increasing surface area.
[Solution Description]
Celiac disease damages the villi, reducing the surface area available for nutrient absorption. Since villi are finger-like projections that increase the surface area for efficient absorption, their damage leads to decreased nutrient uptake into the bloodstream.

Key Concept: Comparative physiology, Villi vs. large intestine

c) The small intestine has villi for high-efficiency nutrient absorption, whereas the large intestine has no villi and focuses on water absorption.
[Solution Description]
The small intestine has thousands of villi (finger-like projections) that greatly increase its surface area for efficient nutrient absorption. In contrast, the large intestine lacks villi and primarily absorbs water and salts, not nutrients, making it less efficient for nutrient absorption.

Key Concept: Large Intestine Function

c) Absorb water and salts from undigested food
[Solution Description]
The large intestine primarily absorbs water and salts from undigested food, converting it into semi-solid waste called stool. It does not digest food or produce enzymes but helps in waste removal.

Key Concept: Large Intestine, Fiber-rich food

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Fiber-rich foods play a crucial role in digestive health by absorbing water in the large intestine, which increases the bulk of the stool. This bulking effect helps in stimulating peristaltic movements (intestinal contractions), thereby facilitating easier passage and preventing constipation. The Reason correctly explains why fiber helps in healthy stool formation and smooth expulsion. Both Assertion and Reason are true, and the Reason provides the correct explanation.

Key Concept: Absorption in the Large Intestine

b) Stool
[Solution Description]
The large intestine absorbs water and some salts from undigested food, forming semi-solid waste known as stool.

Key Concept: Function of Bacteria in Large Intestine

b) Breaking down undigested food and producing nutrients
[Solution Description]
Bacteria in the large intestine help break down undigested food, especially fibre, and produce essential nutrients for digestive health.

Key Concept: Formation of semi-solid waste in the large intestine

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the large intestine absorbs water from undigested food, converting it into semi-solid waste (stool). This is true as per the syllabus. The reason explains that this absorption aids in stool formation and its eventual expulsion through the anus (egestion), which is also correct. Additionally, the reason correctly justifies why the assertion is true because the absorbed water directly contributes to the semi-solid state of stool.

Key Concept: Role of Fiber

b) It makes the stool softer and easier to pass
[Solution Description]
Fiber adds bulk to stool, making it easier to pass through the intestines. It does not directly help absorb nutrients or increase stomach acid production.

Key Concept: Role of Fiber

c) Adds bulk to stool for easy passage
[Solution Description]
Fiber adds bulk to the stool, making it easier to pass through the digestive tract. It helps prevent constipation and ensures smooth bowel movements.

Key Concept: Role of Gut Bacteria, Fiber Digestion

c) They convert fiber into absorbable nutrients and improve stool consistency
[Solution Description]
Gut bacteria break down undigested fiber into beneficial nutrients and short-chain fatty acids, aiding digestion. Fermented foods introduce probiotics that enhance gut flora, improving stool consistency and overall digestive efficiency, ensuring smoother egestion through the anus.

Key Concept: Fermented foods and gut health

c) Curd
[Solution Description]
Fermented foods like curd, buttermilk, pickles, and kanji contain beneficial bacteria that aid digestion by breaking down undigested food and producing essential nutrients, thus promoting a healthy gut.

Key Concept: Role of fiber in digestion

d) It adds bulk to stool and makes it easier to pass
[Solution Description]
Fiber-rich foods, such as fruits, vegetables, and whole grains, help add bulk to the stool, making it easier to pass through the digestive system. This prevents constipation and ensures smooth bowel movements.

Key Concept: Human vs. Snake Digestion, Alimentary Canal Adaptations

c) Humans digest food faster because they chew it, unlike snakes.
[Solution Description]
Snakes swallow prey whole due to flexible jaws and stretchable skin, relying on powerful stomach acids and slow digestion over days or weeks. Humans chew food using teeth, mixing it with saliva containing enzymes (like amylase) for initial starch breakdown before it reaches the stomach. The snake's adaptation allows consumption of large, infrequent meals, while humans require frequent, processed meals.

Key Concept: Birds' Digestion

b) A gizzard with swallowed grit to grind food.
[Solution Description]
Birds have a muscular chamber called a gizzard that contracts and relaxes to mechanically break down food. They swallow grit (small stones) that aids in grinding food inside the gizzard, compensating for the absence of teeth.

Key Concept: Digestive Adaptation

c) It ensures efficient breakdown of cellulose-rich food.
[Solution Description]
The rumination process allows ruminants to efficiently break down tough plant material like grass. By regurgitating and re-chewing the partially digested food, they maximize nutrient extraction from cellulose-rich plants, as explained in the syllabus under "Key Points" and "FASCINATING FACTS."

Key Concept: Comparative Anatomy, Digestive Adaptations

b) It facilitates fermentation of plant material by symbiotic microbes
[Solution Description]
The four-chambered stomach (rumen, reticulum, omasum, abomasum) enables ruminants to digest cellulose via microbial fermentation in the rumen, whereas carnivores rely on enzymatic digestion in a single chamber. This adaptation allows ruminants to extract nutrients from plant matter, which is otherwise indigestible by most animals.

Key Concept: Role of Microbes, Rumination Efficiency

b) Undigested grass would accumulate, impairing cud formation and nutrient extraction.
[Solution Description]
Rumen bacteria play a crucial role in breaking down cellulose into fermentable sugars. Without these bacteria, cellulose digestion would halt, leading to undigested cud accumulation. The cow would struggle to regurgitate and re-chew effectively, causing reduced nutrient absorption and potential bloating due to unmetabolized plant matter. This imbalance directly impacts rumination efficiency and overall health.

Key Concept: Rumination Process

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that grass-eating animals regurgitate partially digested food for re-chewing, which is true as per the syllabus. The reason explains that this process aids in digesting cellulose-rich grass, which is also correct and directly supports the assertion by providing the purpose of rumination.
Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Comparative digestive systems, Rumen adaptation

b) Cows can extract more nutrients from low-quality plant material.
[Solution Description]
The rumen allows cows to break down complex cellulose through bacterial fermentation before enzymatic digestion, enabling them to extract more nutrients from plant material than non-ruminants.

Key Concept: Ruminant Characteristics

c) Ruminants
[Solution Description]
Animals like cows that ruminate (chew cud) are called ruminants. Their four-chambered stomach allows them to efficiently digest tough plant material.

Key Concept: Grit in Digestion

d) To aid in the mechanical grinding of food in the gizzard
[Solution Description]
Birds swallow grit to assist in the mechanical breakdown of food inside the gizzard. The grit acts like tiny grinding stones, helping to crush and grind the food as the gizzard muscles contract and relax.

Key Concept: Gizzard function, Adaptations in birds

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the gizzard in birds functions similarly to teeth in mammals by mechanically breaking down food. This is true as the gizzard uses muscular contractions and swallowed grit to grind food, a role analogous to chewing with teeth. The reason explains that birds swallow grit to assist the gizzard's muscular contractions, which compensates for the lack of teeth. This is also true, and furthermore, it correctly explains why the gizzard functions like teeth—because the grit aids mechanical breakdown just as teeth do. Thus, both statements are true, and the reason properly explains the assertion.

Key Concept: Gizzard in birds

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion is true because birds indeed lack teeth and instead rely on a muscular chamber called the gizzard for breaking down food. The reason is also true because the gizzard uses swallowed grit (small stones) to mechanically grind the food, aiding in digestion. Additionally, the reason correctly explains why birds have a gizzard, as it compensates for the absence of teeth.

Key Concept: Comparison with Teeth

d) Birds use a gizzard and grit instead of teeth to grind food
[Solution Description] Mammals use teeth to chew and mechanically break down food, while birds rely on their gizzard and swallowed grit for grinding, as they lack teeth.

Key Concept: Animal Respiration

c) Through their gills
[Solution Description]
Fish primarily use gills to extract oxygen dissolved in water. Gills are richly supplied with blood vessels, allowing efficient gas exchange between the water and the fish's bloodstream.

Key Concept: Respiratory Adaptations, Aquatic vs Terrestrial

c) Fish optimize unidirectional water flow while humans enhance tidal lung ventilation.
[Solution Description]
Fish increase gill ventilation rate to maximize contact between water and gill surfaces for gas exchange when oxygen is scarce. Humans respond to low oxygen by increasing breathing rate (hyperventilation), but rely on lung alveoli rather than countercurrent flow like fish gills. Gills exploit continuous water flow whereas human lungs depend on bidirectional air movement.

Key Concept: Respiratory System Pathway

d) Nostrils
[Solution Description]
The nostrils have tiny hair and mucus to trap dust and dirt from inhaled air. This is the first line of defense in the respiratory system.

Key Concept: Gas Exchange in Alveoli

b) Thin walls and a rich blood supply
[Solution Description]
The alveoli have thin walls surrounded by a network of blood capillaries. This structure allows oxygen to diffuse into the blood and carbon dioxide to diffuse out efficiently. Additionally, the large surface area provided by numerous alveoli enhances gas exchange.

Key Concept: Nature of Process

a) Physical process vs. chemical process
[Solution Description]
Breathing is a physical process involving the movement of air into and out of the lungs, while respiration is a chemical process where glucose is broken down to release energy.

Key Concept: Function of Alveoli

c) To facilitate the exchange of oxygen and carbon dioxide between air and blood.
[Solution Description]
The alveoli are tiny sacs in the lungs where the exchange of oxygen and carbon dioxide takes place between the air and blood. Their thin walls allow efficient diffusion of gases.

Key Concept: Gas Exchange

c) Oxygen
[Solution Description]
In the alveoli, oxygen from the inhaled air passes into the blood, and carbon dioxide from the blood is released into the exhaled air. Oxygen is the primary gas needed for respiration.

Key Concept: Human Respiratory System - Alveoli

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that gas exchange occurs primarily in the alveoli, which is true because the alveoli are the tiny air sacs in the lungs where this exchange happens. The reason explains why this is possible: the alveoli's thin walls and surrounding blood capillaries allow for efficient diffusion of oxygen into the blood and carbon dioxide out of the blood. Thus, both statements are true, and the reason correctly explains the assertion.

Key Concept: Breathing through Nose

b) The nose filters dust and dirt while the mouth does not
[Solution Description]
Breathing through the nose allows the tiny hairs and mucus in the nostrils to filter out dust and dirt, which does not happen when breathing through the mouth.

Key Concept: Nostrils and Nasal Passages

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that tiny hair and mucus in the nasal passages filter dust and dirt from inhaled air, which is correct as per the syllabus. The reason explains why breathing through the nose is healthier, attributing it to the presence of these specialized structures. Since the reason correctly explains the assertion, both statements are true, and the reason provides the correct explanation.

Key Concept: Respiration, Gas Exchange in Alveoli

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that the decrease in oxygen percentage in exhaled air is due to its usage in cellular respiration, which is true as oxygen is consumed in the process of breaking down glucose to release energy. The reason explains the role of alveoli in gas exchange, which is also correct. However, while both statements are true, the reason does not directly explain why the oxygen percentage decreases; it describes the mechanism of gas exchange. Therefore, the reason is not the correct explanation of the assertion.

Key Concept: Mechanism of Breathing

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion states that during inhalation, the diaphragm moves upward. However, this is incorrect because during inhalation, the diaphragm contracts and moves downward, increasing the volume of the chest cavity and reducing the pressure inside the lungs, which draws air in. The reason states that the upward movement of the diaphragm increases the space in the chest cavity, which is also incorrect as it is the downward movement that achieves this. Therefore, both the assertion and reason are false. But the option stating "Assertion is false, but Reason is true" does not match the solution since both are false. Hence, the correct answer must be that Assertion is false, but Reason is true. However, reviewing again: The Reason itself is factually wrong (diaphragm moves down, not up), so neither Assertion nor Reason is true. Thus, none of the provided options perfectly fit since there should ideally be an option where both are false. But among given choices, the closest is that Assertion is false but Reason is true, though strictly speaking Reason is also false.

Key Concept: Gas exchange, Respiratory adaptations

c) 3.16 atm
[Solution Description]
At sea level, the partial pressure of nitrogen ($P_{N_2}$) in inhaled air is 79\% of 1 atm = 0.79 atm. At 30 meters depth, total pressure is 4 atm. The partial pressure of nitrogen in inhaled air becomes $0.79 \times 4 = 3.16$ atm. Since alveolar gas composition remains the same, the partial pressure of nitrogen in the alveoli is also 3.16 atm.
Calculation:
$P_{N_2} = 0.79 \times 4 = 3.16$ atm.

Key Concept: Alveolar diffusion, Gas percentages

c) 30 mL
[Solution Description]
Inhaled oxygen volume = $500 \, \text{mL} \times 0.21 = 105 \, \text{mL}$.
Exhaled oxygen volume = $500 \, \text{mL} \times 0.15 = 75 \, \text{mL}$.
Oxygen absorbed by blood = Inhaled oxygen - Exhaled oxygen = $105 - 75 = 30 \, \text{mL}$.
Calculation:
$105 \, \text{mL} - 75 \, \text{mL} = 30 \, \text{mL}$.

Key Concept: Inhaled vs. Exhaled Air

d) Because exhaled air contains more carbon dioxide
[Solution Description]
Inhaled air contains only about 0.04\% carbon dioxide, which is insufficient to cause a visible reaction with limewater (calcium hydroxide solution). However, exhaled air has around 4-5\% carbon dioxide, which reacts with limewater to form insoluble calcium carbonate, turning it milky. This indicates the presence of higher CO\textsubscript{2} in exhaled air.

Key Concept: Mechanism of Breathing

c) Diaphragm moves downward, ribs move upward and outward
[Solution Description]
During inhalation, the diaphragm contracts and moves downward, increasing the volume of the chest cavity. Simultaneously, the ribs move upward and outward due to the contraction of intercostal muscles, further expanding the thoracic space. This creates a lower pressure inside the lungs compared to the atmospheric pressure, allowing air to rush in.

Key Concept: Breathing vs. respiration, Oxygen utilization

d) Because oxygen is absorbed by the blood for cellular processes.
[Solution Description]
During respiration, oxygen from the inhaled air diffuses into the blood through the alveoli to be transported to body cells for cellular respiration. As a result, the exhaled air has less oxygen because some of it has been absorbed by the body.

Key Concept: Gas Exchange Site

c) In the alveoli
[Solution Description]
Gas exchange occurs in the alveoli, which are tiny balloon-like sacs at the ends of bronchioles. Their thin walls allow oxygen to pass into the blood and carbon dioxide to be released.

Key Concept: Ribcage Function

b) To protect the lungs
[Solution Description]
The ribcage protects the lungs and helps in the movement of air by expanding during inhalation and contracting during exhalation.

Key Concept: Breathing vs. Respiration

d) Breathing is physical, respiration is chemical
[Solution Description]
Breathing is a physical process involving inhalation and exhalation, while respiration is a chemical process where cells use oxygen to produce energy and release carbon dioxide.

Key Concept: Gas Exchange in Alveoli

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that gas exchange occurs in the alveoli, which is true as per the syllabus. The reason explains that alveoli have thin walls and are surrounded by blood capillaries, allowing efficient gas exchange. Both statements are true, and the reason correctly explains the assertion.

Key Concept: Gas Exchange in Alveoli

c) Alveoli
[Solution Description]
The exchange of gases in humans occurs in the alveoli, which are tiny air sacs in the lungs surrounded by blood capillaries. Oxygen from inhaled air diffuses into the blood, and carbon dioxide from the blood diffuses into the alveoli to be exhaled.

Key Concept: Respiration Equation

c) Glucose + Oxygen $\rightarrow$ Carbon dioxide + Water + Energy
[Solution Description]
During respiration, glucose and oxygen react to produce carbon dioxide, water, and energy. The word equation is: $Glucose + Oxygen \rightarrow Carbon\ dioxide + Water + Energy$.

Key Concept: Breathing vs Respiration

c) Both breathing and respiration
[Solution Description]
Breathing is a physical process that involves inhaling oxygen-rich air and exhaling carbon dioxide-rich air. Respiration is a chemical process where cells use oxygen to break down glucose to release energy. Both involve gas exchange but at different levels.

Key Concept: Gas exchange in alveoli

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The syllabus states that blood carries carbon dioxide from the body to the alveoli, where it is released into the air. This confirms Assertion (A). The Reason (R) correctly explains why carbon dioxide is transported to the alveoli: because it is produced as a waste product during respiration and needs to be removed. Thus, both Assertion and Reason are true, and Reason explains Assertion correctly.

Key Concept: Respiration equation, Energy production

b) 6
[Solution Description]
The balanced chemical equation for respiration is:
$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + Energy$
According to the equation, 6 molecules of O$_2$ produce 6 molecules of CO$_2$.

Key Concept: Purpose of Respiration

c) To release energy from food
[Solution Description]
Respiration releases energy from glucose, which is essential for bodily functions like movement, growth, and thinking. It does not directly produce oxygen or remove nitrogen.

Key Concept: Respiration equation, Gas exchange

b) 105 mL
[Solution Description]
First, find the volume of oxygen in the inhaled air: $500 \, \text{mL} \times 0.21 = 105 \, \text{mL}$ of oxygen.
The balanced chemical reaction for respiration is:
$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy$.
For every 6 volumes of oxygen consumed, 6 volumes of carbon dioxide are produced. Therefore, 105 mL of oxygen will produce 105 mL of carbon dioxide.

Key Concept: Difference between breathing and respiration

c) Breathing
[Solution Description]
Breathing is defined as the physical process of inhaling and exhaling air, which involves movement but no chemical change. Respiration is the chemical process where energy is produced.

Key Concept: Difference between Breathing and Respiration

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that breathing is a physical process involving air movement, which is correct as per the syllabus. The reason states that respiration is a chemical process where glucose is broken down using oxygen to release energy, which is also accurate. However, while both statements are true, the reason does not explain why breathing is a physical process; it merely describes respiration separately. Thus, they are independently true but not causally linked.

Key Concept: Respiration Equation

b) $Glucose + Oxygen \rightarrow Carbon\ dioxide + Water + Energy$
[Solution Description]
The chemical equation for respiration is: $Glucose + Oxygen \rightarrow Carbon\ dioxide + Water + Energy$. This shows how glucose and oxygen combine to produce energy, carbon dioxide, and water.

Key Concept: Respiration Equation

a) It turns into carbon dioxide and water directly
[Solution Description]
Glucose reacts with oxygen during respiration to produce carbon dioxide, water, and energy. This process occurs in cells where glucose is broken down to release energy.

Key Concept: Gills, Aquatic adaptations

c) More efficient oxygen extraction
[Solution Description]
Deep-sea environments have lower oxygen availability due to high pressure and lack of photosynthetic activity. To survive in such conditions, deep-sea fish have evolved specialized gills with a higher concentration of oxygen-binding pigments (like hemoglobin). This allows them to extract more oxygen from the water efficiently. Increased buoyancy, reduced metabolic rate, or temperature regulation are not directly related to gill adaptations for oxygen uptake.

Key Concept: Respiration in Animals, Amphibians

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion states that adult frogs exclusively use lungs for respiration, which is false because frogs also use their moist skin for gas exchange when submerged in water. The reason correctly identifies the adaptation of frogs to their dual habitat by using lungs on land and skin in water. Thus, the assertion is false, but the reason is true.

Key Concept: Lungs in Terrestrial Animals, Gizzard in Birds

b) The gizzard compensates for tooth loss, allowing lighter skulls and better flight efficiency.
[Solution Description]
Birds lack teeth but have a muscular gizzard that mechanically breaks down food using swallowed grit. This adaptation reduces head weight, enabling lighter skulls and facilitating flight. The energy saved from not having heavy jaw structures supports their high metabolic rate, which is sustained by efficient lung respiration. Thus, the absence of teeth indirectly enhances respiratory efficiency by optimizing body weight for flight and high-energy demands.

Key Concept: Respiration in Animals: Variations in Breathing Among Animals

d) Assertion is false, but Reason is true.
[Solution Description]
According to the syllabus, adult frogs use lungs on land and moist skin for gas exchange in water, while tadpoles breathe through gills. Therefore, the assertion that frogs use gills for respiration throughout their life is false because adult frogs do not use gills. The reason correctly states the respiratory adaptations of frogs at different life stages.

Key Concept: Lung Adaptations in Birds

b) Additional air sacs for continuous airflow
[Solution Description]
Birds have specialized structures called air sacs that work with their lungs to ensure a continuous unidirectional flow of air, maximizing oxygen uptake during flight. This adaptation allows them to meet high oxygen demands efficiently.

Key Concept: Gas Exchange in Mammals

c) Alveoli
[Solution Description]
In mammals, the alveoli are tiny balloon-like structures in the lungs where oxygen from inhaled air diffuses into the blood, and carbon dioxide diffuses out of the blood to be exhaled. This is the primary site for gas exchange.

Key Concept: Gas Exchange in Gills

d) To exchange gases between blood and water
[Solution Description]
The primary function of gills in fish is to facilitate the exchange of gases, specifically oxygen and carbon dioxide, between the blood and the surrounding water. Gills are richly supplied with blood vessels, allowing efficient diffusion of oxygen into the blood and carbon dioxide out of the blood.

Key Concept: Respiration in Tadpoles

c) Gills
[Solution Description]
Tadpoles, being aquatic, primarily use gills for respiration as they live in water. This adaptation allows them to extract dissolved oxygen from water, similar to fish. As they metamorphose into adult frogs, they develop lungs for breathing on land.

Key Concept: Gills, Lungs \& Skin Adaptation

b) 30\%
[Solution Description]
Adult frogs use lungs on land and moist skin underwater for gas exchange. When submerged, lungs are less efficient (reduced by 30\%). Assume the frog's oxygen uptake is 100 units on land (all via lungs). Underwater, lung efficiency drops to 70\%, contributing 70 units. The remaining 30 units must come from the skin (since gills are absent in adults). Thus, 30\% of total oxygen uptake occurs through the skin when in water.

Key Concept: Respiration in Adult Frogs on Land

c) Lungs
[Solution Description]
Adult frogs live both on land and in water. While on land, they primarily use lungs for breathing as lungs are adapted to extract oxygen from the air. The syllabus specifies that adult frogs use lungs for breathing on land.

Key Concept: Gas Exchange in Earthworms

c) Oxygen and carbon dioxide diffuse only through a wet surface
[Solution Description]
Earthworms rely on their moist skin for gas exchange because oxygen and carbon dioxide diffuse directly through it. If the skin dries out, diffusion becomes inefficient or stops, making moisture essential for survival. The moist surface allows gases to dissolve before entering or leaving the body.

Key Concept: Earthworm respiration

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that earthworms respire through their moist skin, which is true as per the syllabus. The reason explains that the moist skin facilitates the diffusion of oxygen into the bloodstream and carbon dioxide out, which is also true. Moreover, the reason correctly explains why earthworms need a moist skin for respiration, as the moisture keeps the skin permeable for gas exchange. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.