Q1. Compare and contrast aerobic and anaerobic respiration in terms of site, steps, products, and energy yield with suitable examples.
Answer:
Aerobic respiration happens in the presence of oxygen and largely occurs in the mitochondria. It completely breaks down glucose (C₆H₁₂O₆) into carbon dioxide (CO₂) and water (H₂O), releasing a large amount of ATP. The overall equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP). This pathway is used by humans and most animals, especially during prolonged activities.
Anaerobic respiration occurs in the absence of oxygen. It is partial breakdown of glucose and occurs in the cytoplasm. In yeast, it produces ethanol and CO₂ (fermentation). In muscle cells during intense exercise, it produces lactic acid. It yields much less ATP compared to aerobic respiration.
In summary, aerobic respiration is highly efficient and cleaner in products, whereas anaerobic respiration is a quick backup for energy but forms ethanol or lactic acid and yields less energy.
Q2. Explain why ATP is called the “energy currency of the cell.” Describe how ATP is produced in respiration and how cells use it.
Answer:
ATP (adenosine triphosphate) is called the energy currency because it stores energy in its phosphate bonds and releases it quickly when needed. Cells convert ADP + Pi → ATP during respiration.
In aerobic respiration, ATP is produced mainly in the mitochondria through oxidative phosphorylation, giving a high yield. In anaerobic respiration, ATP comes from glycolysis in the cytoplasm, giving a low yield.
Cells use ATP for:
Muscle contraction (sliding of actin and myosin).
Active transport across cell membranes (ion pumps).
Protein synthesis and other biosynthetic reactions.
Nerve impulse conduction, including restoring ion gradients using Na⁺/K⁺ pumps.
ATP’s quick release and universal use in all cells make it the most convenient form of energy for immediate cellular work.
Q3. Describe how gas exchange occurs in plants and humans.
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meaning of word here
meaning of word here
the structures involved and the gases exchanged during day and night.
Answer:
In plants, gas exchange occurs mainly through stomata on leaves and lenticels on stems. During daytime, plants take in CO₂ for photosynthesis and release O₂. At night, photosynthesis stops, and plants respire, taking in O₂ and releasing CO₂. Guard cells control stomatal opening to balance gas exchange and water loss.
In humans, gas exchange occurs in the lungs, particularly at the alveoli. O₂ from inhaled air diffuses into the blood, binding to haemoglobin, while CO₂ diffuses from blood into the alveoli to be exhaled. The process is supported by a large surface area, thin alveolar walls, and a rich blood supply.
Thus, both plants and humans exchange O₂ and CO₂, but the purpose and timing differ due to photosynthesis in plants and continuous respiration in humans.
Q4. How do haemoglobin and myoglobin help in oxygen transport and storage? Also explain how most carbon dioxide is carried in blood.
Answer:
Haemoglobin (Hb) in red blood cells binds oxygen in the lungs to form oxyhaemoglobin. It then releases O₂ in tissues where oxygen levels are low, enabling aerobic respiration. Its ability to bind and release oxygen depends on partial pressure of O₂, pH, and temperature.
Myoglobin, present in muscle cells, acts as an oxygen store. It has a higher affinity for O₂ than haemoglobin, helping muscles access oxygen quickly during intense activity.
Carbon dioxide (CO₂) is more soluble in water than oxygen. In blood, most CO₂ is carried as bicarbonate ions (HCO₃⁻) after reacting with water; a small amount is dissolved in plasma or bound to haemoglobin as carbaminohaemoglobin. This efficient transport supports continuous cellular respiration and removal of metabolic waste.
Q5. Smoking affects the respiratory system in many ways. Explain the damage caused by tobacco smoke and link it to reduced respiratory efficiency.
Answer:
Smoking destroys cilia in the respiratory tract. Without cilia, dust and germs are not cleared well, increasing the risk of infections and inflammation.
Tar from smoke irritates airways and contributes to chronic bronchitis. Nicotine increases heart rate and causes addiction, making quitting harder.
Carbon monoxide in smoke binds to haemoglobin to form carboxyhaemoglobin, reducing the oxygen-carrying capacity of blood. This causes shortness of breath and fatigue.
Long-term smoking leads to COPD (Chronic Obstructive Pulmonary Disease) and increases the risk of lung cancer.
Overall, smoking reduces gas exchange efficiency, lowers oxygen supply to tissues, and hampers aerobic respiration, leading to poor physical performance and health complications.
High Complexity (Analytical & Scenario-Based)
Q6. You performed the lime water experiment (Activity 5.4) twice: once by blowing through a straw and once by pushing air with a syringe. Analyze why the lime water turned milky faster when you blew into it.
Answer:
Lime water contains Ca(OH)₂. When CO₂ is bubbled through, it forms CaCO₃, which is insoluble and makes the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O.
When you blow through a straw, the air you exhale contains a higher concentration of CO₂ (about 4–5%) compared to atmospheric air (~0.04%). The higher CO₂ concentration speeds up the reaction, so CaCO₃ forms faster.
Blowing also provides a warmer and often faster airflow, increasing gas–liquid contact, which improves the rate of reaction.
In contrast, air pushed by a syringe is closer to ambient composition with very low CO₂, so the reaction is slower.
Thus, the difference in CO₂ concentration, temperature, and bubble dynamics explains the faster milky appearance during exhalation.
Q7. A class sets up yeast fermentation (Activity 5.5) with sugar solution and observes rapid CO₂ production. Evaluate the factors that affect fermentation rate and relate them to real-life uses.
Answer:
Yeast performs anaerobic respiration, converting glucose into ethanol and CO₂. The rate depends on:
Sugar concentration: Moderate levels speed up fermentation; too high causes osmotic stress.
Temperature: Optimal around 30–35°C for many yeasts; too low slows enzymes, too high denatures them.
pH: Slightly acidic conditions are preferred.
Yeast viability and amount: Fresh, active yeast and adequate inoculum increase rate.
The CO₂ produced turns lime water milky, confirming anaerobic respiration. In bakeries, CO₂ makes dough rise, while ethanol evaporates on baking. In breweries, ethanol is the desired product and CO₂ may be captured for carbonation.
The experiment models how controlled conditions maximize product yield, showing why industries carefully manage temperature, sugar levels, and yeast health.
Q8. Fish show a higher breathing rate than humans in Activity 5.6. Analyze this difference by comparing oxygen availability, respiratory structures, and exchange efficiency.
Answer:
Water contains much less dissolved O₂ than air. To obtain enough oxygen, fish must process more water, leading to a faster breathing rate.
Fish use gills with thin lamellae and a counter-current exchange system where blood flows opposite to water flow. This maintains a diffusion gradient, maximizing O₂ uptake even from low-O₂ water.
Humans breathe air rich in O₂ using lungs with alveoli providing a large surface area, thin walls, and rich capillary supply. Because air has more O₂, humans need fewer breaths to meet demand.
Environmental factors like water temperature (warmer water holds less O₂) can make fish breathe even faster. By contrast, human breathing rate changes mainly with exercise, stress, or disease.
Thus, differences in medium (air vs water) and respiratory design explain the higher respiratory rate in fish.
Q9. During a sprint, a student develops leg cramps. Use your understanding of respiration to explain what happens in the muscles and how recovery occurs.
Answer:
During a sprint, ATP demand rises suddenly. Oxygen supply may not meet the demand for aerobic respiration, so muscles shift partly to anaerobic respiration.
In humans, anaerobic respiration converts glucose to lactic acid, producing a small amount of ATP quickly. Lactic acid accumulates, lowering pH, which can cause cramps and muscle fatigue.
Myoglobin in muscles helps by releasing its stored O₂ to delay fatigue, but stores are limited. After exercise, deep breathing continues (oxygen debt) to:
Supply O₂ for converting lactic acid back into pyruvate in the liver.
Restore ATP and phosphocreatine levels.
Re-oxygenate myoglobin.
Gentle stretching, hydration, and rest help clear lactic acid. Training improves circulation, mitochondria, and aerobic capacity, reducing future cramps.
Q10. A patient with low haemoglobin feels breathless while climbing stairs. Analyze how reduced haemoglobin affects respiration at the tissue level and suggest supportive measures.
Answer:
Haemoglobin (Hb) carries most of the O₂ in blood. With low Hb, even if lungs work well, the oxygen-carrying capacity of blood is reduced. Less O₂ reaches tissues, so **aerobic respira...
