NECTA ACSEE Advanced Biology 2020 Questions & Marking Scheme Solutions

(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: 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) Five differences between yeast cells and bacterium cells: 1. Nuclear Organization: Yeast is a eukaryote with a true membrane-bound nucleus containing linear chromosomes with histone proteins and a nucleolus; a bacterium is a prokaryote lacking a nuclear membrane, possessing a single naked circular DNA molecule in an open nucleoid. 2. Organelles: Yeast cells possess membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus; bacterium cells lack all membrane-bound organelles (cellular respiration occurs at the plasma membrane/mesosomes). 3. Ribosome Size: Yeast cells have larger 80S cytosolic ribosomes; bacterium cells have smaller 70S ribosomes. 4. Cell Wall Composition: The yeast cell wall is composed primarily of chitin and beta-glucans; the bacterial cell wall is composed of peptidoglycan (murein). 5. Cell Size: Yeast cells are significantly larger, typically 5 to 10 micrometers in diameter; bacterial cells are much smaller, typically 0.5 to 5 micrometers. (b) Justification for the need of a vacuole in a plant cell (two points): 1. Maintenance of Turgor Pressure and Mechanical Support: The central vacuole contains cell sap with a high concentration of dissolved mineral ions, organic acids, and sugars. This generates a low (more negative) osmotic potential, causing water to enter by osmosis and creating high internal hydrostatic turgor pressure against the rigid cellulose cell wall. This turgidity provides mechanical support and structural firmness to non-woody plant organs such as stems, leaves, and petals. 2. Dynamic Storage and Waste Sequestration: The vacuole serves as a primary storage reservoir for water, metabolic nutrients (sugars, amino acids), and secondary metabolites including anthocyanin pigments (which color flowers to attract pollinators). Additionally, it securely isolates potentially harmful metabolic wastes and lytic enzymes away from the cytoplasm, protecting the plant cell's metabolic machinery.