Describe the process of gaseous exchange in a mammal.

Here are the answers to your biology questions: 12. What is meant by the term carrying capacity? (1 mark) Carrying capacity* is the maximum population size of a biological species that can be sustained indefinitely by a given environment, given the available resources. 13. Give two reasons why cross-pollination is considered advantageous over self-pollination. (2 marks) Cross-pollination leads to genetic variation* in offspring, which increases the chances of survival in changing environments. It often results in more vigorous offspring* with improved characteristics due to hybrid vigour. SECTION B (40 MARKS) 14. a) Describe the process of gaseous exchange in a mammal. (10 marks) Gaseous exchange in mammals occurs primarily in the alveoli of the lungs. Air rich in oxygen is inhaled into the lungs, reaching the alveoli. The alveoli have several adaptations for efficient gaseous exchange: they are numerous, providing a large surface area; they have thin walls (one cell thick); and they are richly supplied with blood capillaries, also with thin walls. Oxygen diffuses from the alveoli, where its partial pressure is high, across the alveolar and capillary walls into the blood plasma and then into red blood cells, where it binds to haemoglobin to form oxyhaemoglobin. Simultaneously, carbon dioxide, which is in high partial pressure in the blood (a waste product of metabolism), diffuses from the blood plasma and red blood cells across the capillary and alveolar walls into the alveolar air. This carbon dioxide-rich air is then exhaled from the lungs. This process relies on the partial pressure gradient of oxygen and carbon dioxide between the alveolar air and the blood. b) Explain how the human skin functions in thermoregulation. (10 marks) The human skin plays a crucial role in thermoregulation, maintaining a constant internal body temperature. Sweating: When body temperature rises, sweat glands in the skin produce sweat. As sweat evaporates from the skin surface, it absorbs latent heat from the body, leading to a cooling effect. Vasodilation: In hot conditions, arterioles supplying blood to the skin capillaries dilate (widen). This increases blood flow to the skin surface, allowing more heat to radiate away from the body. Vasoconstriction: In cold conditions, arterioles constrict (narrow), reducing blood flow to the skin surface. This minimizes heat loss from the body. Hair erection (piloerection): Tiny muscles attached to hair follicles contract, causing hairs to stand upright. This traps a layer of insulating air close to the skin, reducing heat loss. This is more effective in furry mammals but less so in humans. Subcutaneous fat: The layer of fat beneath the skin acts as an insulator, reducing heat loss from the body to the environment. 15. a) Describe the nitrogen cycle and explain the role of microorganisms in it. (12 marks) The nitrogen cycle describes the processes by which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. Microorganisms are central to this cycle. 1. Nitrogen Fixation: Atmospheric nitrogen (N_2), which is unusable by most organisms, is converted into ammonia (NH_3) or ammonium ions (NH_4^+). This is primarily done by nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules, or free-living bacteria like Azotobacter and cyanobacteria). 2. Nitrification: Ammonia/ammonium is converted into nitrites (NO_2^-) and then nitrates (NO_3^-). This two-step process is carried out by nitrifying bacteria. Nitrosomonas bacteria convert ammonium to nitrites, and Nitrobacter bacteria convert nitrites to nitrates. Nitrates are readily absorbed by plants. 3. Assimilation: Plants absorb nitrates or ammonium ions from the soil and incorporate them into organic compounds like proteins and nucleic acids. Animals obtain nitrogen by consuming plants or other animals. 4. Ammonification: When plants and animals die, or animals excrete waste, decomposers (bacteria and fungi) break down the organic nitrogen compounds into ammonia or ammonium ions. This process is called ammonification. 5. Denitrification: Under anaerobic conditions, denitrifying bacteria (e.g., Pseudomonas) convert nitrates back into gaseous nitrogen (N_2), which is then released back into the atmosphere, completing the cycle. b) Explain how water and mineral salts are absorbed by the root hairs. (8 marks) Water Absorption: Root hair cells are specialized for water absorption. They have a large surface area and a thin cell wall and membrane. The cytoplasm of root hair cells has a lower water potential (higher solute concentration) than the soil water due to dissolved sugars and mineral salts. This creates a water potential gradient. Water moves from the soil, through the partially permeable cell membrane of the root hair cell, into the cell by osmosis*. This water then moves across the root cortex cells, eventually reaching the xylem vessels. Mineral Salts Absorption: Mineral salts are absorbed from the soil in dissolved ionic forms. Diffusion: If the concentration of a particular mineral ion is higher in the soil water than inside the root hair cell, the ions can move into the cell by diffusion*, down their concentration gradient. Active Transport: Often, the concentration of mineral ions in the soil is lower than inside the root hair cells. In such cases, the root hair cells absorb these ions against their concentration gradient using active transport*. This process requires energy (ATP) produced by respiration in the root cells and involves specific carrier proteins in the cell membrane. 16. a) Describe the structure and functions of the mammalian heart. (10 marks) The mammalian heart is a muscular, four-chambered organ located in the chest cavity, slightly to the left. Its primary function is to pump blood throughout the body. Structure: Four Chambers: It consists of two upper atria (singular: atrium) and two lower ventricles. The right atrium receives deoxygenated blood from the body, and the left atrium receives oxygenated blood from the lungs. The right ventricle pumps deoxygenated blood to the lungs, and the left ventricle* pumps oxygenated blood to the rest of the body. Septum: A muscular wall called the septum* divides the heart longitudinally, preventing the mixing of oxygenated and deoxygenated blood. Valves: The heart has four valves that ensure unidirectional blood flow: Tricuspid valve:* Between the right atrium and right ventricle. Bicuspid (mitral) valve:* Between the left atrium and left ventricle. Pulmonary valve:* At the exit of the right ventricle into the pulmonary artery. Aortic valve:* At the exit of the left ventricle into the aorta. Major Blood Vessels: Vena Cava (superior and inferior):* Bring deoxygenated blood from the body to the right atrium. Pulmonary Artery:* Carries deoxygenated blood from the right ventricle to the lungs. Pulmonary Veins:* Bring oxygenated blood from the lungs to the left atrium. Aorta:* Carries oxygenated blood from the left ventricle to the rest of the body. Functions: Pumping Blood: The heart acts as a double pump. The right side pumps deoxygenated blood to the lungs for oxygenation (pulmonary circulation), while the left side pumps oxygenated blood to all parts of the body (systemic circulation). Maintaining Blood Pressure: The rhythmic contractions and relaxations of the heart chambers generate the pressure needed to circulate blood throughout the vast network of blood vessels. Separation of Blood: The four-chambered structure and septum ensure complete separation of oxygenated and deoxygenated blood, allowing for efficient transport of oxygen to tissues. Regulation of Blood Flow: The heart's pumping rate can be adjusted to meet the body's varying demands for oxygen and nutrients, such as during exercise. b) Explain the various adaptations of xerophytes to their environment. (10 marks) Xerophytes are plants adapted to survive in dry environments with limited water availability. Their adaptations aim to reduce water loss and/or increase water uptake. Reduced Leaf Surface Area: Many xerophytes have small leaves, needle-like leaves (e.g., pines), or no leaves at all (e.g., cacti, where stems perform photosynthesis). This minimizes the surface area for transpiration. Thick, Waxy Cuticle: A thick, impermeable waxy layer on the leaf epidermis reduces water loss through evaporation from the leaf surface. Sunken Stomata: Stomata (pores for gas exchange) are often located in pits or depressions on the leaf surface. This creates a humid microenvironment around the stomata, reducing the water potential gradient and thus transpiration. Hairy Leaves: A dense covering of hairs (trichomes) on the leaf surface traps a layer of moist air, reducing air movement over the stomata and decreasing transpiration. Rolled Leaves: Some plants (e.g., marram grass) roll their leaves inwards, enclosing the stomata within a humid chamber, further reducing water loss. Extensive Root Systems: Xerophytes often have very long taproots to reach deep groundwater, or widespread, shallow fibrous roots to quickly absorb surface water after rainfall. Succulent Stems/Leaves: Many xerophytes (e.g., cacti, aloes) have fleshy stems or leaves that store large quantities of water, allowing them to survive long periods of drought. Reduced Number of Stomata: Fewer stomata on the leaf surface directly limit the number of pores through which water can be lost. Modified Photosynthesis (CAM pathway): Some xerophytes use Crassulacean Acid Metabolism (CAM) photosynthesis, where stomata open only at night to absorb CO_2 (when temperatures are cooler and humidity is higher), and close during the day to conserve water. 3 done, 2 left today. You're making progress.