How do nitrogen-fixing bacteria in the soil contribute to the nitrogen cycle?

Step 1: Analyze Question 25. Question 25 asks how nitrogen-fixing bacteria contribute to the nitrogen cycle. 1. They change nitrogen into soil nitrates. (True - Nitrogen fixation converts atmospheric nitrogen gas into ammonia, which is then converted to nitrites and nitrates by other bacteria). 2. They return nitrogen to the atmosphere. (False - Denitrifying bacteria return nitrogen to the atmosphere). 3. They change ammonium to nitrates. (False - Nitrifying bacteria change ammonium to nitrites and then nitrates). Therefore, only statement 1 is correct regarding nitrogen-fixing bacteria. The question asks "How do nitrogen fixing bacteria in the soil contribute to the nitrogen cycle?". Statement 1 is the direct contribution. The options A, B, C, D are for questions 26-30, not 25. Question 25 is a multiple-choice question where the options are the numbered statements. The correct statement is 1. Step 2: Analyze Question 26. First statement: Glucose is completely broken down during aerobic respiration. (True - In aerobic respiration, glucose is fully oxidized to carbon dioxide and water). Second statement: The end products of aerobic respiration are more toxic. (False - The end products of aerobic respiration are carbon dioxide and water, which are not considered toxic in normal physiological amounts. Anaerobic respiration produces more toxic byproducts like lactic acid or ethanol). Since the first statement is true and the second statement is false, the correct option is C. The final answer is C. Step 3: Analyze Question 27. First statement: The female Anopheles mosquito transmits Plasmodium. (True - Female Anopheles mosquitoes are the vectors for the Plasmodium* parasite, which causes malaria). Second statement: The female Anopheles* mosquitoes feed on the blood of its host. (True - Female mosquitoes require blood meals for egg development). Both statements are true. The transmission of Plasmodium occurs because the mosquito feeds on the blood of an infected host and then transmits it to another host during a subsequent blood meal. Therefore, the second statement is a correct explanation of the first. The final answer is A. Step 4: Analyze Question 28. First statement: Habitat loss is the most frequent cause of extinction today. (True - Habitat destruction and degradation are widely recognized as the leading causes of species extinction). Second statement: Global warming is expected to cause many extinctions in the twenty-first century. (True - Climate change, driven by global warming, is a significant and growing threat to biodiversity, leading to many predicted extinctions). Both statements are true. However, global warming is a cause of extinction, while habitat loss is another cause. The second statement does not explain why habitat loss is the most frequent cause; it describes a separate, though related, threat. Therefore, the second statement is not a correct explanation of the first. The final answer is B. Step 5: Analyze Question 29. First statement: During photosynthesis light energy is converted to chemical energy. (True - Photosynthesis captures light energy and converts it into chemical energy stored in organic molecules like glucose). Second statement: Green plants combine carbon dioxide and water using solar energy to produce organic food. (True - This describes the overall process of photosynthesis, which is precisely how light energy is converted into chemical energy). Both statements are true, and the second statement correctly explains the mechanism by which light energy is converted to chemical energy. The final answer is A. Step 6: Analyze Question 30. First statement: The cell wall in a plant cell is made up of chitin. (False - Plant cell walls are primarily composed of cellulose*. Chitin is found in the cell walls of fungi and the exoskeletons of arthropods). Second statement: Cellulose provides roughage and prevents constipation in human diet. (True - Cellulose is a type of dietary fiber (roughage) that is indigestible by humans but aids in digestion and prevents constipation). Since the first statement is false and the second statement is true, the correct option is D. The final answer is D. Step 7: Analyze Question 31. The description states: "Destroys leaves of plants through the formation of acid in rain." A. Carbon dioxide: Contributes to the greenhouse effect, but not directly to acid rain that destroys leaves. B. Carbon monoxide: A toxic gas, but not a primary cause of acid rain. C. Chlorofluorocarbons: Deplete the ozone layer. D. Sulphur dioxide: A major air pollutant that reacts with water in the atmosphere to form sulfuric acid, a primary component of acid rain, which damages plant leaves. The final answer is D. Step 8: Analyze Question 32. The description states: "Causes uterine contractions during labour." A. Testosterone: A male sex hormone. B. Oxytocin: A hormone produced by the pituitary gland that stimulates uterine contractions during childbirth and milk ejection. C. Oestrogen: A female sex hormone involved in the menstrual cycle and secondary sexual characteristics. D. Progesterone: A female sex hormone that maintains pregnancy. The final answer is B. Step 9: Analyze Question 33. The description states: "An example of a social insect." A. Mosquitoes: Generally solitary, though they may gather in swarms. B. Cockroach: Solitary insect. C. House fly: Solitary insect. D. Honey bee: Lives in highly organized colonies with a complex social structure (e.g., queen, workers, drones). The final answer is D. Step 10: Analyze Question 34. The description states: "Blood plasma minus most proteins." A. Serum: Blood plasma from which clotting factors (like fibrinogen) have been removed. It still contains other proteins. B. Lymph: Fluid that circulates in the lymphatic system, derived from interstitial fluid. C. Tissue fluids: Also known as interstitial fluid, this fluid surrounds cells. It is formed from blood plasma that filters out of capillaries, and most large plasma proteins are retained in the blood, so tissue fluid has a much lower protein concentration than plasma. D. Blood: Contains plasma, blood cells, and all plasma proteins. The final answer is C. Step 11: Analyze Question 35. The description states: "A natural unit made up of all living organisms and their interaction with the non-living." A. Population: A group of individuals of the same species in a given area. B. Community: All the different populations of living organisms interacting in a given area. C. Ecosystem: A community of living organisms (biotic components) interacting with their non-living physical environment (abiotic components). This matches the description. D. Colony: A group of organisms living together, often of the same species. The final answer is C. Step 12: Analyze Question 36. The description states: "Dense fibrous connective tissue that joins bones to bones at the joints." A. Elastic Cartilage: A type of cartilage found in structures like the ear and epiglottis, known for its flexibility. B. Hyaline Cartilage: The most common type of cartilage, found at the ends of bones in joints, in the nose, trachea, etc. C. Tendons: Dense fibrous connective tissue that connects muscle to bone*. D. Ligaments: Dense fibrous connective tissue that connects bone to bone* at joints, providing stability. The final answer is D. Step 13: Analyze Question 37. The description states: "Process that mainly drives the movement of water through a plant." A. Transpiration pull: The primary mechanism for water movement in plants. Evaporation of water from the leaves (transpiration) creates a negative pressure (tension) that pulls water up through the xylem from the roots. B. Capillary action: Contributes to water movement in narrow tubes but is not the main driving force for water transport over long distances in plants. C. Root pressure: A minor force that pushes water up from the roots, but it is generally insufficient to account for water movement in tall plants. D. Diffusion: Important for water movement at the cellular level, but not the main process for bulk water transport throughout the plant. The final answer is A.