NECTA ACSEE Advanced Biology 2033 Questions & Marking Scheme Solutions

Introduction: A crab belongs to the Phylum Arthropoda. The phylum Arthropoda is the largest and most biodiverse phylum in the animal kingdom, characterized by a chitinous exoskeleton, metameric segmentation, and paired jointed appendages. Identification of Phylum and Four Classes (4 Marks): - Phylum: Arthropoda (1 Mark) - Four Classes of Phylum Arthropoda (3 Marks): 1. Class Crustacea (e.g., crabs, lobsters, prawns, woodlice) 2. Class Insecta / Hexapoda (e.g., grasshoppers, houseflies, bees, butterflies) 3. Class Arachnida (e.g., spiders, scorpions, ticks, mites) 4. Class Chilopoda (centipedes) OR Class Diplopoda (millipedes) [or subphylum Myriapoda] Analysis of Four Characteristics for Each Class (16 Marks - 4 Marks per Class): 1. Class Crustacea: - Body Tagmatization: The body is divided into two distinct tagmata: cephalothorax (fused head and thorax, covered dorsally by a protective carapace) and an abdomen. - Appendages: Organisms possess two pairs of sensory antennae (antennules and antennae) and typically biramous (forked/branched) jointed appendages specialized for walking, swimming, feeding, and defense (chelipeds). - Respiration: Gaseous exchange is carried out by specialized vascularized gills (branchiae) housed in branchial chambers beneath the carapace, or across the thin body surface. - Exoskeleton and Excretion: The chitinous exoskeleton is heavily calcified and impregnated with calcium carbonate crystals; excretion is mediated by paired antennal glands (green glands) or maxillary glands. 2. Class Insecta (Hexapoda): - Body Tagmatization: The body is distinctly divided into three tagmata: head, thorax (pro-, meso-, and metathorax), and abdomen. - Locomotor Appendages: Organisms possess three pairs of jointed walking legs (hexapod condition) attached to the thoracic segments, and typically one or two pairs of wings on the thorax (in pterygotes). - Cephalic Structures: The head bears a single pair of unbranched antennae, a pair of compound eyes (often with simple ocelli), and specialized mouthparts adapted for biting, chewing, sucking, or siphoning. - Respiration and Excretion: Gaseous exchange occurs via an extensive internal tracheal system opening externally through paired lateral spiracles; excretion is accomplished by Malpighian tubules that void nitrogen as insoluble uric acid crystals. 3. Class Arachnida: - Body Tagmatization: The body is divided into two tagmata: prosoma (cephalothorax) and opisthosoma (abdomen); true head and antennae are completely absent. - Appendages: Organisms bear four pairs of jointed walking legs (eight legs) arising from the prosoma; also possess one pair of chelicerae (clawed/fanged mouthparts, often with venom glands) and one pair of pedipalps (sensory, grasping, or copulatory). - Respiratory Organs: Respiration occurs via internal book lungs, tracheae, or a combination of both; spiracles open on the ventral surface of the abdomen. - Sensory and Excretory Structures: Excretion is performed by Malpighian tubules and/or paired coxal glands; simple eyes (ocelli, usually 6 to 8) are present, while compound eyes are absent. 4. Class Chilopoda (Centipedes) [or Class Diplopoda]: - Body Organization: Elongated, dorsoventrally flattened body composed of a distinct head and an elongated trunk with numerous similar, repeating segments (15 to 170+ segments). - Locomotor Appendages: Each trunk segment bears exactly one pair of jointed walking legs positioned laterally. - Predatory Adaptations: The first pair of trunk appendages immediately behind the head are modified into robust, venomous poison claws (maxillipeds / forcipules) used to inject toxins and immobilize prey. - Respiration and Excretion: Respiration is through a tracheal system opening by spiracles; excretion is carried out by Malpighian tubules; organisms are carnivorous, nocturnal, and fast-moving. *(Alternative: Class Diplopoda: Cylindrical body, two pairs of walking legs per diplosegment, short antennae, absence of poison claws, presence of repugnatorial stink glands, herbivorous/detritivorous).*

(a) Features of a mitochondrion and their roles in cellular respiration: (i) Folded inner membrane (Cristae): Provides a vast surface area for the attachment of electron transport chain (ETC) protein complexes, electron carriers (cytochromes), and stalked particles (ATP synthase/F0-F1 ATPases), maximizing oxidative phosphorylation and ATP generation. (ii) Matrix: Contains a concentrated aqueous mixture of soluble respiratory enzymes (e.g., citrate synthase, dehydrogenase) required for the Krebs cycle (TCA cycle) and beta-oxidation of fatty acids, as well as NAD+ and FAD coenzymes to accept hydrogen atoms. (iii) Circular DNA (mtDNA): Encodes essential mitochondrial respiratory proteins and enzyme subunits (including components of the electron transport chain and ATP synthase), enabling the mitochondrion to synthesize its own respiratory machinery semi-autonomously and replicate when metabolic demands increase. (iv) Ribosomes (70S Ribosomes): Translate mitochondrial mRNA into polypeptide chains and structural enzymes within the organelle, allowing direct local protein synthesis without total reliance on the cell's nucleus and cytoplasm. (b) Importance of the specified organelles (two points each): (i) Microtubules: 1. Spindle Apparatus Formation: Form the spindle fibers during mitosis and meiosis that attach to kinetochores and ensure precise segregation of sister chromatids and homologous chromosomes. 2. Cytoskeletal Support and Motility: Maintain eukaryotic cell shape, act as intracellular tracks for organelle and vesicle transport (via motor proteins kinesin and dynein), and form the 9+2 structural core (axoneme) of cilia and flagella for cellular movement. (ii) Golgi body: 1. Modification and Sorting: Chemically modifies proteins and lipids received from the rough and smooth endoplasmic reticulum (e.g., glycosylation to form glycoproteins and glycolipids) and packages them for targeted delivery. 2. Secretion and Lysosome Formation: Packages secretory products into vesicles for exocytosis across the cell membrane, synthesizes plant cell wall pectins, and forms primary lysosomes packed with acid hydrolases. (iii) Lysosomes: 1. Heterophagy and Autophagy: Fuse with phagocytic vacuoles to digest engulfed pathogens (e.g., bacteria in macrophages) and degrade worn-out intracellular organelles to recycle vital macromolecular constituents. 2. Autolysis (Programmed Cell Death): Release hydrolytic enzymes to break down the entire cell during metamorphosis, developmental tissue resorption (such as tadpole tail regression), or removal of damaged/senescent cells.

Introduction: Nitrogenous wastes originate from the metabolic deamination of excess dietary and cellular amino acids, as well as the catabolism of purine and pyrimidine nucleic acids. In animals, nitrogen is eliminated primarily in three chemical forms: ammonia (ammonotelism), urea (ureotelism), and uric acid (uricotelism). The evolutionary selection of which nitrogenous excretory product is produced is governed by critical ecological, physiological, and biochemical trade-offs. Four Factors Determining the Form of Nitrogenous Waste Elimination: 1. Availability of Water in the External Environment (Habitat & Water Economy) (4.5 Marks): - Aquatic Environments (Ammonotelism): Animals living in abundant freshwater or marine environments (e.g., teleost fish, aquatic amphibians, aquatic invertebrates) have continuous access to water. Because ammonia is extremely soluble, it can be flushed out across the gills or body surface continuously. However, ammonia requires a massive volume of water for safe excretion (approximately 500 mL of water per 1 g of nitrogen excreted). - Terrestrial Environments with Moderate Water (Ureotelism): Terrestrial mammals and adult amphibians face periodic water scarcity. They convert ammonia into urea, which is 100,000 times less toxic than ammonia and requires substantially less water for excretion (approximately 50 mL of water per 1 g of nitrogen). - Arid, Desert, and Aerial Environments (Uricotelism): Birds, desert reptiles, insects, and land snails face severe risks of dehydration. They eliminate nitrogen as uric acid, which is virtually insoluble in water. It requires minimal water (approximately 1 to 10 mL of water per 1 g of nitrogen) and precipitates as a thick, semi-solid paste or dry pellet, preserving critical body water. 2. Toxicity of the End Product to Cells and Tissues (4.5 Marks): - Ammonia (High Toxicity): Ammonia (NH3/NH4+) is highly cytotoxic because it is strongly alkaline, rapidly crosses biological membranes, alters blood and cellular pH, and replaces potassium ions (K+) in transmembrane ion pumps. It disrupts mitochondrial ATP synthesis and neurotransmitter function in the central nervous system. Because of this extreme toxicity, ammonia cannot be stored in tissues or bodily fluids even transiently. - Urea (Moderate Toxicity): Urea is far less toxic and can be tolerated in animal body fluids at moderate concentrations without causing cellular injury. This allows mammals to concentrate urea in renal tubules and temporarily store it in the urinary bladder prior to micturition. - Uric Acid (Non-Toxic): Uric acid is biologically inert and non-toxic. Because it precipitates out of solution into crystals, it exerts virtually zero osmotic pressure on cellular or extracellular fluids. It can safely accumulate in the cloaca, tissues, or eggs without disrupting osmotic balance or poisoning cells. 3. Energetic Cost of Biosynthesis (ATP Expenditure) (4.5 Marks): - Ammonia (Zero Energy Cost): Ammonia is the direct, unmediated byproduct of oxidative deamination (e.g., conversion of glutamate to alpha-ketoglutarate). No ATP is consumed in synthesizing ammonia, making it energetically the cheapest excretory pathway for aquatic animals. - Urea (Moderate Energy Investment): Synthesizing urea from toxic ammonia and carbon dioxide requires the ornithine-urea cycle located in liver hepatocytes. Forming each molecule of urea consumes 3 molecules of ATP (cleaving 4 high-energy phosphate bonds). Mammals and amphibians invest this metabolic energy to avoid ammonia toxicity while conserving water. - Uric Acid (Highest Energy Investment): The de novo synthesis of uric acid involves a complex, multi-step enzymatic pathway starting from glutamine, glycine, aspartate, and carbon dioxide. It consumes substantial amounts of ATP per nitrogen atom excreted. Arid animals and birds pay this heavy metabolic energy cost because water retention is the primary limiting factor for survival. 4. Mode of Reproduction and Embryonic Development (Cleidoic vs. Non-Cleidoic Eggs) (4.5 Marks): - Cleidoic (Shelled) Eggs: Terrestrial birds, reptiles, and insects lay closed, shelled eggs on land. The developing embryo is confined within an impermeable eggshell containing a fixed, finite volume of water. If the embryo excreted soluble ammonia or urea, the toxic wastes would dissolve in the amniotic and allantoic fluids, rapidly reaching lethal concentrations or creating steep osmotic gradients that dehydrate the embryo. Instead, the embryo synthesizes insoluble uric acid, which precipitates harmlessly as crystals and is sequestered inside the allantoic sac until hatching. - Non-Cleidoic Eggs: Aquatic amphibians and fish lay soft, shell-less eggs directly in water. Toxic ammonia freely diffuses out through the permeable egg membranes into the surrounding aquatic environment without accumulating. - Viviparous Placental Mammals: In mammalian embryos, soluble urea diffuses across the placenta into maternal blood, where it is excreted by the mother's kidneys, eliminating the need for embryonic uric acid precipitation. Conclusion (2 Marks): In conclusion, the form in which nitrogenous wastes are eliminated represents an evolutionary compromise between water conservation, toxicity tolerance, metabolic energy investment, and embryonic constraints. Ammonotelism conserves energy where water is unlimited; ureotelism balances water economy and energy in terrestrial environments; and uricotelism maximizes water retention and embryonic survival in arid and aerial habitats.