11. The function of bile juice in animals is to emulsify fats in the small intestine. This breaks down large fat globules into smaller ones, increasing the surface area for the action of lipase enzymes, thus aiding in fat digestion. Bile also helps in the absorption of fat-soluble vitamins. 12. Energy is decreased as it moves into successive trophic levels because a significant portion of energy is lost at each transfer. Organisms use most of the energy they consume for their own metabolic processes, such as respiration, movement, and maintaining body temperature, which releases energy as heat. Only about 10% of the energy from one trophic level is typically incorporated into the biomass of the next trophic level. Some energy is also lost in undigested food or waste products. 13. Three methods of population estimation in an ecosystem are: • Quadrat method: Used for sessile or slow-moving organisms, where a defined area is sampled and individuals are counted. • Mark-recapture method: Used for mobile animals, involving capturing, marking, releasing, and then recapturing individuals to estimate population size. • Transect method: Used to study the distribution and abundance of organisms along a line or gradient. 14. Three adaptive characteristics of Ascaris lumbricoides are: • Thick cuticle: Provides protection against the host's digestive enzymes and immune responses. • High reproductive capacity: Produces a large number of eggs to increase the chances of survival and transmission to new hosts. • Anaerobic respiration: Allows the parasite to survive in the low-oxygen environment of the host's intestine. Section B. Answer Any Two 15. Describe the nitrogen cycle (20mks) The nitrogen cycle is the biogeochemical process by which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. It involves several key stages: Nitrogen Fixation: This is the process where atmospheric nitrogen ($$\text{N}_2$$), which is largely unusable by most organisms, is converted into ammonia ($$\text{NH}_3$$) or ammonium ($$\text{NH}_4^+$$). Biological Nitrogen Fixation: Carried out by nitrogen-fixing bacteria (e.g., Rhizobium in the root nodules of legumes, and free-living bacteria like Azotobacter*). These bacteria possess the enzyme nitrogenase, which converts $$\text{N}_2$$ into $$\text{NH}_3$$. Atmospheric Nitrogen Fixation*: Lightning provides the high energy needed to break the triple bond in $$\text{N}_2$$, allowing it to react with oxygen to form nitrogen oxides, which dissolve in rain to form nitrates. Industrial Nitrogen Fixation*: The Haber-Bosch process converts $$\text{N}_2$$ and hydrogen into ammonia for fertilizers. Nitrification: This is a two-step process where ammonia ($$\text{NH}_3$$) or ammonium ($$\text{NH}_4^+$$) is converted into nitrites ($$\text{NO}_2^-$$) and then into nitrates ($$\text{NO}_3^-$$). Ammonia to Nitrite: Performed by nitrifying bacteria such as Nitrosomonas*. Nitrite to Nitrate: Performed by nitrifying bacteria such as Nitrobacter*. Nitrates are the primary form of nitrogen absorbed by plants. Assimilation: Plants absorb nitrates ($$\text{NO}_3^-$$) or ammonium ($$\text{NH}_4^+$$) from the soil through their roots and incorporate them into organic molecules like proteins, nucleic acids, and chlorophyll. Animals obtain nitrogen by consuming plants or other animals. Ammonification: When plants and animals die, or when animals excrete waste products, decomposers (bacteria and fungi) break down the complex organic nitrogen compounds (e.g., proteins, nucleic acids) into simpler inorganic forms, primarily ammonia ($$\text{NH}_3$$) or ammonium ($$\text{NH}_4^+$$). This process returns nitrogen to the soil. Denitrification: This is the process where nitrates ($$\text{NO}_3^-$$) in the soil are converted back into gaseous atmospheric nitrogen ($$\text{N}_2$$) by denitrifying bacteria (e.g., Pseudomonas*) under anaerobic conditions. This completes the cycle by returning nitrogen to the atmosphere. 16. Describe the adaptations of the skin to its functions. (20mks) The skin is the largest organ of the body and is highly adapted to perform several vital functions: Protection: Physical Barrier: The outermost layer, the epidermis*, is composed of stratified squamous epithelium, with its most superficial layer (stratum corneum) consisting of dead, flattened, keratinized cells. This tough, durable layer provides a robust physical barrier against mechanical injury, water loss, and the entry of pathogens. Chemical Barrier: The skin secretes sebum* from sebaceous glands, which is slightly acidic and contains antimicrobial properties, inhibiting bacterial growth. Sweat also contributes to this acidic mantle. UV Protection: Melanocytes in the epidermis produce melanin*, a pigment that absorbs harmful ultraviolet (UV) radiation, protecting underlying tissues from damage and reducing the risk of skin cancer. Waterproofing*: The keratin and lipids in the stratum corneum make the skin largely impermeable to water, preventing excessive water loss from the body and preventing water absorption from the environment. Temperature Regulation: Sweat Glands: Eccrine sweat glands* produce sweat, which evaporates from the skin surface, carrying away heat and cooling the body. Blood Vessels: The dermis contains an extensive network of blood vessels. When the body is hot, these vessels undergo vasodilation (widen), increasing blood flow to the skin surface, allowing more heat to radiate away. When the body is cold, vasoconstriction* (narrowing) reduces blood flow to the skin, conserving heat. Adipose Tissue: The subcutaneous layer (hypodermis) contains a layer of adipose tissue* (fat), which acts as an insulator, reducing heat loss from the body. Hair: While less significant in humans than in other mammals, hair can trap a layer of air close to the skin, providing some insulation. Arrector pili muscles* contract to make hairs stand erect, increasing this trapped air layer. Sensation: The skin is richly supplied with various nerve endings and sensory receptors that detect touch, pressure, pain, and temperature. Examples include Meissner's corpuscles for light touch, Pacinian corpuscles for deep pressure and vibration, and free nerve endings* for pain and temperature. This allows the body to respond to environmental stimuli. Excretion: Sweat glands* excrete small amounts of metabolic waste products such as urea, salts, and lactic acid along with water. Vitamin D Synthesis: The skin contains a precursor molecule (a cholesterol derivative) that, upon exposure to UV radiation from sunlight, is converted into Vitamin D*. Vitamin D is crucial for calcium absorption and bone health.